Mutant huntingtin aggregates impair mitochondrial movement and trafficking in cortical neurons

Pathogenic mechanisms in Huntington's disease

Huntington's disease (HD) is an autosomal dominant, progressive neurodegenerative disorder presenting in midlife. Multiple pathogenic mechanisms which hypothesise how the expanded CAG repeat causes manifest disease have been suggested since the mutation was first detected. These mechanisms include events that operate at both the gene and protein levels. It has been proposed that somatic instability of the CAG repeat could underlie the striatal-specific pathology observed in HD, although how this occurs and what consequences this has in the disease state remain unknown. The form in which the Htt protein exists within the cell has been extensively studied in terms of both its role in aggregate formation and its cellular processing. Protein-protein interactions, post-translational modifications and protein cleavage have all been suggested to contribute to HD pathogenesis. The potential downstream effects of the mutant Htt protein are also noted here. In particular, the adverse effect of the mutant Htt protein on cellular protein degradation, subcellular transport and transcription are explored, and its role in energy metabolism and excitotoxicity investigated. Elucidating the mechanisms at work in HD pathogenesis and determining when they occur in relation to disease is an important step in the pathway to therapeutic interventions.

Mitochondrial bioenergetics and dynamics in Huntington's disease: tripartite synapses and selective striatal degeneration

Preferential striatal neurodegeneration is a hallmark of Huntington's disease (HD) pathogenesis, which has been associated with mitochondrial dysfunction. Evidence from genetic HD models suggest that mutant huntingtin (mHtt) compromises mitochondrial bioenergetics and dynamics, preventing efficient calcium handling and ATP generation in neuronal networks. Striatal neurons receive abundant glutamatergic input from the cortex, forming tripartite synapses with astrocytic partners. These are involved in bidirectional communication, play neuroprotective roles, and emerging evidence suggests that astrocyte dysfunction supports non-cell autonomous neurodegeneration. In addition to mHtt effects, inherent mitochondria vulnerability within striatal neurons and astrocytes may contribute for preferential neurodegeneration in HD. Dysfunctional astrocytic mitochondria in cortico-striatal tripartite synapses might be particularly relevant in the pathogenesis of juvenile/infantile HD, frequently associated with seizures and abnormally large mHtt polyglutamine expansions. This review discusses our work, primarily addressing in situ mitochondrial function in neurons and astrocytes, in the context of related work within the HD-mitochondria field.

Mitochondrial dynamics in cell death and neurodegeneration

Mitochondria are highly dynamic organelles that continuously undergo two opposite processes, fission and fusion. Mitochondrial dynamics influence not only mitochondrial morphology, but also mitochondrial biogenesis, mitochondrial distribution within the cell, cell bioenergetics, and cell injury or death. Drp1 mediates mitochondrial fission, whereas Mfn1/2 and Opa1 control mitochondrial fusion. Neurons require large amounts of energy to carry out their highly specialized functions. Thus, mitochondrial dysfunction is a prominent feature in a variety of neurodegenerative diseases. Mutations of Mfn2 and Opa1 lead to neuropathies such as Charcot-Marie-Tooth disease type 2A and autosomal dominant optic atrophy. Moreover, both Aβ peptide and mutant huntingtin protein induce mitochondrial fragmentation and neuronal cell death. In addition, mutants of Parkinson's disease-related genes also show abnormal mitochondrial morphology. This review highlights our current understanding of abnormal mitochondrial dynamics relevant to neuronal synaptic loss and cell death in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease and Huntington's disease.

Current understanding on the pathogenesis of polyglutamine diseases

Polyglutamine (polyQ) diseases are a family of neurodegenerative disorders including Huntington's disease, spinobulbar muscular atrophy, dentatorubral-pallidoluysian atrophy and several spinocerebellar ataxias. polyQ diseases are caused by abnormal expansion of CAG repeats in certain genes. The expanded CAG repeats are then translated into a series of abnormally expanded polyQ tracts. Such polyQ tracts may induce misfolding of the disease-causing proteins. The present review mainly focuses on the common characteristics of the pathogenesis of these polyQ diseases, including conformational transition of proteins and its influence on the function of these proteins, the correlation between decreased ability of proteolysis and late-onset polyQ diseases, and the relationship between wide expression of disease-causing proteins and selective neuronal death.

Corticostriatal circuit dysfunction in Huntington's disease: intersection of glutamate, dopamine and calcium

Huntington's disease (HD) is a noncurable and progressive autosomal-dominant neurodegenerative disorder that results from a polyglutamine expansion in the amino-terminal region of the huntingtin protein. The generation of rodent HD models has revealed that cellular dysfunction, rather than cell death alone, occurs early in the disease progression, appearing even before overt symptom onset. Much evidence has now established that dysfunction of the corticostriatal circuit is key to HD symptomology. In this article, we summarize the most current findings that implicate glutamate, dopamine and calcium signaling in this system and discuss how they work in concert to disrupt corticostriatal function. In addition, we highlight therapeutic strategies related to altered corticostriatal signaling in HD.

Mitochondrial loss, dysfunction and altered dynamics in Huntington's disease

Although a direct causative pathway from the gene mutation to the selective neostriatal neurodegeneration remains unclear in Huntington's disease (HD), one putative pathological mechanism reported to play a prominent role in the pathogenesis of this neurological disorder is mitochondrial dysfunction. We examined mitochondria in preferentially vulnerable striatal calbindin-positive neurons in moderate-to-severe grade HD patients, using antisera against mitochondrial markers of COX2, SOD2 and cytochrome c. Combined calbindin and mitochondrial marker immunofluorescence showed a significant and progressive grade-dependent reduction in the number of mitochondria in spiny striatal neurons, with marked alteration in size. Consistent with mitochondrial loss, there was a reduction in COX2 protein levels using western analysis that corresponded with disease severity. In addition, both mitochondrial transcription factor A, a regulator of mtDNA, and peroxisome proliferator-activated receptor-co-activator gamma-1 alpha, a key transcriptional regulator of energy metabolism and mitochondrial biogenesis, were also significantly reduced with increasing disease severity. Abnormalities in mitochondrial dynamics were observed, showing a significant increase in the fission protein Drp1 and a reduction in the expression of the fusion protein mitofusin 1. Lastly, mitochondrial PCR array profiling in HD caudate nucleus specimens showed increased mRNA expression of proteins involved in mitochondrial localization, membrane translocation and polarization and transport that paralleled mitochondrial derangement. These findings reveal that there are both mitochondrial loss and altered mitochondrial morphogenesis with increased mitochondrial fission and reduced fusion in HD. These findings provide further evidence that mitochondrial dysfunction plays a critical role in the pathogenesis of HD.

Molecular Mechanisms and Potential Therapeutical Targets in Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the gene encoding for huntingtin protein. A lot has been learned about this disease since its first description in 1872 and the identification of its causative gene and mutation in 1993. We now know that the disease is characterized by several molecular and cellular abnormalities whose precise timing and relative roles in pathogenesis have yet to be understood. HD is triggered by the mutant protein, and both gain-of-function (of the mutant protein) and loss-of-function (of the normal protein) mechanisms are involved. Here we review the data that describe the emergence of the ancient huntingtin gene and of the polyglutamine trait during the last 800 million years of evolution. We focus on the known functions of wild-type huntingtin that are fundamental for the survival and functioning of the brain neurons that predominantly degenerate in HD. We summarize data indicating how the loss of these beneficial activities reduces the ability of these neurons to survive. We also review the different mechanisms by which the mutation in huntingtin causes toxicity. This may arise both from cell-autonomous processes and dysfunction of neuronal circuitries. We then focus on novel therapeutical targets and pathways and on the attractive option to counteract HD at its primary source, i.e., by blocking the production of the mutant protein. Strategies and technologies used to screen for candidate HD biomarkers and their potential application are presented. Furthermore, we discuss the opportunities offered by intracerebral cell transplantation and the likely need for these multiple routes into therapies to converge at some point as, ideally, one would wish to stop the disease process and, at the same time, possibly replace the damaged neurons.

Mitochondrial trafficking and the provision of energy and calcium buffering at excitatory synapses

Neuronal postsynaptic currents consume most of the brain's energy supply. Delineating how neurons control the distribution, morphology and function of the energy-producing mitochondria that fuel synaptic communication is therefore important for our understanding of nervous system function and pathology. Here we review recent insights into the molecular mechanisms that control activity-dependent regulation of mitochondrial trafficking, morphology and activity at excitatory synapses. We also consider some implications of this regulation for synaptic function and plasticity and discuss how this may contribute to synaptic dysfunction and signalling in neurological disease, with a focus on Alzheimer's disease.

S-nitrosylation of Drp1 links excessive mitochondrial fission to neuronal injury in neurodegeneration

Neurons are known to use large amounts of energy for their normal function and activity. In order to meet this demand, mitochondrial fission, fusion, and movement events (mitochondrial dynamics) control mitochondrial morphology, facilitating biogenesis and proper distribution of mitochondria within neurons. In contrast, dysfunction in mitochondrial dynamics results in reduced cell bioenergetics and thus contributes to neuronal injury and death in many neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease, and Huntington's disease. We recently reported that amyloid-beta peptide, thought to be a key mediator of AD pathogenesis, engenders S-nitrosylation and thus hyperactivation of the mitochondrial fission protein Drp1. This activation leads to excessive mitochondrial fragmentation, bioenergetic compromise, and synaptic damage in models of AD. Here, we provide an extended commentary on our findings of nitric oxide-mediated abnormal mitochondrial dynamics.

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.

Abnormal mitochondrial dynamics--a novel therapeutic target for Alzheimer's disease?

Mitochondria are dynamic organelles that undergo continuous fission and fusion, which could affect all aspects of mitochondrial function. Mitochondrial dysfunction has been well documented in Alzheimer's disease (AD). In the past few years, emerging evidence indicates that an imbalance of mitochondrial dynamics is involved in the pathogenesis of AD. In this review, we discuss in detail the abnormal mitochondrial dynamics in AD and how such abnormal dynamics may impact mitochondrial and neuronal function and contribute to the course of disease. Based on this discussion, we propose that mitochondrial dynamics could be a potential therapeutic target for AD.

Impaired mitochondrial trafficking in Huntington's disease

Impaired mitochondrial function has been well documented in Huntington's disease. Mutant huntingtin is found to affect mitochondria via various mechanisms including the dysregulation of gene transcription and impairment of mitochondrial function or trafficking. The lengthy and highly branched neuronal processes constitute complex neural networks in which there is a large demand for mitochondria-generated energy. Thus, the impaired mitochondrial trafficking in neuronal cells may play an important role in the selective neuropathology of Huntington's disease. Here we discuss the evidence for the effect of the Huntington's disease protein huntingtin on the intracellular trafficking of mitochondria and the involvement of this defective trafficking in the pathogenesis of Huntington's disease.

Mitochondrial dynamics--fusion, fission, movement, and mitophagy--in neurodegenerative diseases

Neurons are metabolically active cells with high energy demands at locations distant from the cell body. As a result, these cells are particularly dependent on mitochondrial function, as reflected by the observation that diseases of mitochondrial dysfunction often have a neurodegenerative component. Recent discoveries have highlighted that neurons are reliant particularly on the dynamic properties of mitochondria. Mitochondria are dynamic organelles by several criteria. They engage in repeated cycles of fusion and fission, which serve to intermix the lipids and contents of a population of mitochondria. In addition, mitochondria are actively recruited to subcellular sites, such as the axonal and dendritic processes of neurons. Finally, the quality of a mitochondrial population is maintained through mitophagy, a form of autophagy in which defective mitochondria are selectively degraded. We review the general features of mitochondrial dynamics, incorporating recent findings on mitochondrial fusion, fission, transport and mitophagy. Defects in these key features are associated with neurodegenerative disease. Charcot-Marie-Tooth type 2A, a peripheral neuropathy, and dominant optic atrophy, an inherited optic neuropathy, result from a primary deficiency of mitochondrial fusion. Moreover, several major neurodegenerative diseases—including Parkinson's, Alzheimer's and Huntington's disease—involve disruption of mitochondrial dynamics. Remarkably, in several disease models, the manipulation of mitochondrial fusion or fission can partially rescue disease phenotypes. We review how mitochondrial dynamics is altered in these neurodegenerative diseases and discuss the reciprocal interactions between mitochondrial fusion, fission, transport and mitophagy.

Disruption of mitochondrial DNA replication in Drosophila increases mitochondrial fast axonal transport in vivo

Mutations in mitochondrial DNA polymerase (pol γ) cause several progressive human diseases including Parkinson's disease, Alper's syndrome, and progressive external ophthalmoplegia. At the cellular level, disruption of pol γ leads to depletion of mtDNA, disrupts the mitochondrial respiratory chain, and increases susceptibility to oxidative stress. Although recent studies have intensified focus on the role of mtDNA in neuronal diseases, the changes that take place in mitochondrial biogenesis and mitochondrial axonal transport when mtDNA replication is disrupted are unknown. Using high-speed confocal microscopy, electron microscopy and biochemical approaches, we report that mutations in pol γ deplete mtDNA levels and lead to an increase in mitochondrial density in Drosophila proximal nerves and muscles, without a noticeable increase in mitochondrial fragmentation. Furthermore, there is a rise in flux of bidirectional mitochondrial axonal transport, albeit with slower kinesin-based anterograde transport. In contrast, flux of synaptic vesicle precursors was modestly decreased in pol γ−α mutants. Our data indicate that disruption of mtDNA replication does not hinder mitochondrial biogenesis, increases mitochondrial axonal transport, and raises the question of whether high levels of circulating mtDNA-deficient mitochondria are beneficial or deleterious in mtDNA diseases.

Abnormal mitochondrial dynamics and neurodegenerative diseases

Mitochondrial dysfunction is a prominent feature of various neurodegenerative diseases. A deeper understanding of the remarkably dynamic nature of mitochondria, characterized by a delicate balance of fission and fusion, has helped to fertilize a recent wave of new studies demonstrating abnormal mitochondrial dynamics in neurodegenerative diseases. This review highlights mitochondrial dysfunction and abnormal mitochondrial dynamics in Alzheimer disease, Parkinson disease, amyotrophic lateral sclerosis, and Huntington disease and discusses how these abnormal mitochondrial dynamics may contribute to mitochondrial and neuronal dysfunction. We propose that abnormal mitochondrial dynamics represents a key common pathway that mediates or amplifies mitochondrial dysfunction and neuronal dysfunction during the course of neurodegeneration.

Mitochondrial trafficking and morphology in neuronal injury

Alterations in mitochondrial function may have a central role in the pathogenesis of many neurodegenerative diseases. The study of mitochondrial dysfunction has typically focused on ATP generation, calcium homeostasis and the production of reactive oxygen species. However, there is a growing appreciation of the dynamic nature of mitochondria within cells. Mitochondria are highly motile organelles, and also constantly undergo fission and fusion. This raises the possibility that impairment of mitochondrial dynamics could contribute to the pathogenesis of neuronal injury. In this review we describe the mechanisms that govern mitochondrial movement, fission and fusion. The key proteins that are involved in mitochondrial fission and fusion have also been linked to some inherited neurological diseases, including autosomal dominant optic atrophy and Charcot-Marie-Tooth disease 2A. We will discuss the evidence that altered movement, fission and fusion are associated with impaired neuronal viability. There is a growing collection of literature that links impaired mitochondrial dynamics to a number of disease models. Additionally, the concept that the failure to deliver a functional mitochondrion to the appropriate site within a neuron could contribute to neuronal dysfunction provides an attractive framework for understanding the mechanisms underlying neurologic disease. However, it remains difficult to clearly establish that altered mitochondrial dynamics clearly represent a cause of neuronal dysfunction.

Downstream mechanisms triggered by mitochondrial dysfunction in the basal ganglia: from experimental models to neurodegenerative diseases

Mitochondrial dysfunctions have been implicated in the cellular processes underlying several neurodegenerative disorders affecting the basal ganglia. These include Huntington's chorea and Parkinson's disease, two highly debilitating motor disorders for which recent research has also involved gene mutation linked to mitochondrial deficits. Experimental models of basal ganglia diseases have been developed by using toxins able to disrupt mitochondrial function: these molecules act by selectively inhibiting mitochondrial respiratory complexes, uncoupling cellular respiration. This in turn leads to oxidative stress and energy deficit that trigger critical downstream mechanisms, ultimately resulting in neuronal vulnerability and loss. Here we review the molecular and cellular downstream effects triggered by mitochondrial dysfunction, and the different experimental models that are obtained by the administration of selective mitochondrial toxins or by the expression of mutant genes.

The pathogenic mechanisms of polyglutamine diseases and current therapeutic strategies

Expansion of CAG trinucleotide repeat within the coding region of several genes results in the production of proteins with expanded polyglutamine (PolyQ) stretch. The expression of these pathogenic proteins leads to PolyQ diseases, such as Huntington's disease or several types of spinocerebellar ataxias. This family of neurodegenerative disorders is characterized by constant progression of the symptoms and molecularly, by the accumulation of mutant proteins inside neurons causing their dysfunction and eventually death. So far, no effective therapy actually preventing the physical and/or mental decline has been developed. Experimental therapeutic strategies either target the levels or processing of mutant proteins in an attempt to prevent cellular deterioration, or they are aimed at the downstream pathologic effects to reverse or ameliorate the caused damages. Certain pathomechanistic aspects of PolyQ disorders are discussed here. Relevance of disease models and recent knowledge of therapeutic possibilities is reviewed and updated.

HUMMR, a hypoxia- and HIF-1alpha-inducible protein, alters mitochondrial distribution and transport

Mitochondrial transport is critical for maintenance of normal neuronal function. Here, we identify a novel mitochondria protein, hypoxia up-regulated mitochondrial movement regulator (HUMMR), which is expressed in neurons and is markedly induced by hypoxia-inducible factor 1 α (HIF-1α). Interestingly, HUMMR interacts with Miro-1 and Miro-2, mitochondrial proteins that are critical for mediating mitochondrial transport. Interestingly, knockdown of HUMMR or HIF-1 function in neurons exposed to hypoxia markedly reduces mitochondrial content in axons. Because mitochondrial transport and distribution are inextricably linked, the impact of reduced HUMMR function on the direction of mitochondrial transport was also explored. Loss of HUMMR function in hypoxia diminished the percentage of motile mitochondria moving in the anterograde direction and enhanced the percentage moving in the retrograde direction. Thus, HUMMR, a novel mitochondrial protein induced by HIF-1 and hypoxia, biases mitochondria transport in the anterograde direction. These findings have broad implications for maintenance of neuronal viability and function during physiological and pathological states.

Mitochondrial kinases in Parkinson's disease: converging insights from neurotoxin and genetic models

Alterations in mitochondrial biology have long been implicated in neurotoxin, and more recently, genetic models of parkinsonian neurodegeneration. In particular, kinase regulation of mitochondrial dynamics and turnover are emerging as central mechanisms at the convergence of neurotoxin, environmental and genetic approaches to studying Parkinson's disease (PD). Kinases that localize to mitochondria during neuronal injury include mitogen activated protein kinases (MAPK) such as extracellular signal regulated protein kinases (ERK) and c-Jun N-terminal kinases (JNK), protein kinase B/Akt, and PTEN-induced kinase 1 (PINK1). Although site(s) of action within mitochondria and specific kinase targets are still unclear, these signaling pathways regulate mitochondrial respiration, transport, fission-fusion, calcium buffering, reactive oxygen species (ROS) production, mitochondrial autophagy and apoptotic cell death. In this review, we summarize accelerating experimental evidence gathered over the last decade that implicate a central role for kinase signaling at the mitochondrion in Parkinson's and related neurodegenerative disorders. Interactions involving alpha-synuclein, leucine rich repeat kinase 2 (LRRK2), DJ-1 and Parkin are discussed. Converging mechanisms from different model systems support the concept of common pathways in parkinsonian neurodegeneration that may be amenable to future therapeutic interventions.

Mitochondria and energetic depression in cell pathophysiology

Mitochondrial dysfunction is a hallmark of almost all diseases. Acquired or inherited mutations of the mitochondrial genome DNA may give rise to mitochondrial diseases. Another class of disorders, in which mitochondrial impairments are initiated by extramitochondrial factors, includes neurodegenerative diseases and syndromes resulting from typical pathological processes, such as hypoxia/ischemia, inflammation, intoxications, and carcinogenesis. Both classes of diseases lead to cellular energetic depression (CED), which is characterized by decreased cytosolic phosphorylation potential that suppresses the cell's ability to do work and control the intracellular Ca(2+) homeostasis and its redox state. If progressing, CED leads to cell death, whose type is linked to the functional status of the mitochondria. In the case of limited deterioration, when some amounts of ATP can still be generated due to oxidative phosphorylation (OXPHOS), mitochondria launch the apoptotic cell death program by release of cytochrome c. Following pronounced CED, cytoplasmic ATP levels fall below the thresholds required for processing the ATP-dependent apoptotic cascade and the cell dies from necrosis. Both types of death can be grouped together as a mitochondrial cell death (MCD). However, there exist multiple adaptive reactions aimed at protecting cells against CED. In this context, a metabolic shift characterized by suppression of OXPHOS combined with activation of aerobic glycolysis as the main pathway for ATP synthesis (Warburg effect) is of central importance. Whereas this type of adaptation is sufficiently effective to avoid CED and to control the cellular redox state, thereby ensuring the cell survival, it also favors the avoidance of apoptotic cell death. This scenario may underlie uncontrolled cellular proliferation and growth, eventually resulting in carcinogenesis.

Mitochondrial structural and functional dynamics in Huntington's disease

Huntington's disease (HD) is an autosomal, dominantly inherited neurodegenerative disorder, characterized by chorea, involuntary movements, and cognitive impairments. Tremendous progress has been made since the discovery of HD gene in 1993, in terms of developing animal models to study the disease process, unraveling the expression and function of wild-type and mutant huntingtin (Htt) proteins in the central and peripheral nervous systems, and understanding expanded CAG repeat containing mutant Htt protein interactions with CNS proteins in the disease process. HD progression has been found to involve several pathomechanisms, including expanded CAG repeat protein interaction with other CNS proteins, transcriptional dysregulation, calcium dyshomeostasis, abnormal vesicle trafficking, and defective mitochondrial bioenergetics. Recent studies have found that mutant Htt is associated with mitochondria and causes mitochondrial structural changes, decreases mitochondrial trafficking, and impairs mitochondrial dynamics in the neurons affected by HD. This article discusses recent developments in HD research, with a particular focus on intracellular and intramitochondrial calcium influx, mitochondrial DNA defects, and mitochondrial structural and functional abnormalities in HD development and progression. Further, this article outlines the current status of mitochondrial therapeutics with a special reference to Dimebon.

Mitochondria in neuroplasticity and neurological disorders

Mitochondrial electron transport generates the ATP that is essential for the excitability and survival of neurons, and the protein phosphorylation reactions that mediate synaptic signaling and related long-term changes in neuronal structure and function. Mitochondria are highly dynamic organelles that divide, fuse, and move purposefully within axons and dendrites. Major functions of mitochondria in neurons include the regulation of Ca(2+) and redox signaling, developmental and synaptic plasticity, and the arbitration of cell survival and death. The importance of mitochondria in neurons is evident in the neurological phenotypes in rare diseases caused by mutations in mitochondrial genes. Mitochondria-mediated oxidative stress, perturbed Ca(2+) homeostasis, and apoptosis may also contribute to the pathogenesis of prominent neurological diseases including Alzheimer's, Parkinson's, and Huntington's diseases; stroke; amyotrophic lateral sclerosis; and psychiatric disorders. Advances in understanding the molecular and cell biology of mitochondria are leading to novel approaches for the prevention and treatment of neurological disorders.

Mitochondrial dynamics in Parkinson's disease

The unique energy demands of neurons require well-orchestrated distribution and maintenance of mitochondria. Thus, dynamic properties of mitochondria, including fission, fusion, trafficking, biogenesis, and degradation, are critical to all cells, but may be particularly important in neurons. Dysfunction in mitochondrial dynamics has been linked to neuropathies and is increasingly being linked to several neurodegenerative diseases, but the evidence is particularly strong, and continuously accumulating, in Parkinson's disease (PD). The unique characteristics of neurons that degenerate in PD may predispose those neuronal populations to susceptibility to alterations in mitochondrial dynamics. In addition, evidence from PD-related toxins supports that mitochondrial fission, fusion, and transport may be involved in pathogenesis. Furthermore, rapidly increasing evidence suggests that two proteins linked to familial forms of the disease, parkin and PINK1, interact in a common pathway to regulate mitochondrial fission/fusion. Parkin may also play a role in maintaining mitochondrial homeostasis through targeting damaged mitochondria for mitophagy. Taken together, the current data suggests that mitochondrial dynamics may play a role in PD pathogenesis, and a better understanding of mitochondrial dynamics within the neuron may lead to future therapeutic treatments for PD, potentially aimed at some of the earliest pathogenic events.

Complex II inhibition by 3-NP causes mitochondrial fragmentation and neuronal cell death via an NMDA- and ROS-dependent pathway

Mitochondrial respiratory complex II inhibition plays a central role in Huntington's disease (HD). Remarkably, 3-NP, a complex II inhibitor, recapitulates HD-like symptoms. Furthermore, decreases in mitochondrial fusion or increases in mitochondrial fission have been implicated in neurodegenerative diseases. However, the relationship between mitochondrial energy defects and mitochondrial dynamics has never been explored in detail. In addition, the mechanism of neuronal cell death by complex II inhibition remains unclear. Here, we tested the temporal and spatial relationship between energy decline, impairment of mitochondrial dynamics, and neuronal cell death in response to 3-NP using quantitative fluorescence time-lapse microscopy and cortical neurons. 3-NP caused an immediate drop in ATP. This event corresponded with a mild rise in reactive oxygen species (ROS), but mitochondrial morphology remained unaltered. Unexpectedly, several hours after this initial phase, a second dramatic rise in ROS occurred, associated with profound mitochondrial fission characterized by the conversion of filamentous to punctate mitochondria and neuronal cell death. Glutamate receptor antagonist AP5 abolishes the second peak in ROS, mitochondrial fission, and cell death. Thus, secondary excitotoxicity, mediated by glutamate receptor activation of the NMDA subtype, and consequent oxidative and nitrosative stress cause mitochondrial fission, rather than energy deficits per se. These results improve our understanding of the cellular mechanisms underlying HD pathogenesis.

A systems biology approach to investigating apoptotic stimuli as effectors of cell metabolism: practical application of top-down control analysis to attached neurons

Reduced glycolytic and mitochondrial respiration rates are common features of apoptosis that may reflect key events contributing to cell death. However, it is unclear to what extent the rate changes can be explained by direct alterations in the kinetics of the participating reactions, as changes in the concentrations of intermediates also affect reaction rates. Direct kinetic changes can be identified, ranked, and compared to the indirect effects mediated by the intermediates using top-down control analysis. Flux changes that are explained primarily by direct effects are likely to be prime targets of the pathways that signal death, and thus important contributors to apoptosis. Control analysis concepts relevant to identifying such effects are reviewed. Metabolic flux measurements are essential for this approach, but can be technically difficult, particularly when using adherent cells such as neurons. A simple method is described that renders such measurements feasible.

Enhanced sensitivity of striatal neurons to axonal transport defects induced by mutant huntingtin

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease linked to a polyQ (polyglutamine) expansion in the huntingtin protein. Although general brain atrophy is found in HD patients, the striatum is the most severely affected region. Loss or mutant forms of huntingtin were reported to disrupt fast axonal transport in Drosophila, squid, and mice. However, previous work did not resolve whether mutant huntingtin affects global axonal transport or only a subset of cargoes, nor did it resolve whether striatal neurons are preferentially sensitive to huntingtin-mediated defects. We used amyloid precursor protein (APP)-yellow fluorescent protein and brain-derived neurotrophic factor (BDNF)-mCherry fusion proteins as markers for fast axonal transport when huntingtin is altered. We found that movement of APP and BDNF is impaired in striatal and hippocampal, but not cortical, neurons from presymptomatic homozygous mutant mice carrying 150Q huntingtin knock-in mutations. In addition, loss of huntingtin disrupts APP axonal transport, whereas overexpression of wild-type, but not mutant, huntingtin enhances APP transport in all three types of neurons tested. These data suggest that a loss of wild-type huntingtin function in fast axonal transport plays important roles in the development of cell-type-specific defects in HD.

Impairing the mitochondrial fission and fusion balance: a new mechanism of neurodegeneration

Mitochondrial dysfunction is a common characteristic of all neurodegenerative diseases. However, the cause of this dysfunction remains a mystery. Here, we discuss the potential role of mitochondrial fission and fusion in the onset and progression of neurodegenerative diseases. Specifically, we propose that an imbalance in mitochondrial fission and fusion may underlie both familial and sporadic neurodegenerative disorders. There is substantial evidence that links disruption of the mitochondrial fission and fusion equilibrium, resulting in abnormally long or short mitochondria, to neurodegeneration. First, hereditary mutations in the mitochondrial fusion GTPases optic atrophy-1 and mitofusin-2 cause neuropathies in humans. In addition, recent findings report increased mitochondrial fission in Parkinson's disease (PD) models and induction of mitochondrial fission by two proteins, PTEN-induced kinase 1 and parkin, which are mutant in familial forms of PD. Furthermore, mutant huntingtin, the disease-causing protein in Huntington's disease, alters mitochondrial morphology and dynamics. Rotenone, a pesticide and inducer of PD symptoms, and amyloid-beta peptide, which is causally linked to Alzheimer's disease, initiate mitochondrial fission. Finally, mitochondrial fission is an early event in ischemic stroke and diabetic neuropathies. In sum, a growing body of research suggests that a better understanding of mitochondrial fission and fusion and the regulatory factors involved may lead to improved treatments and cures for neurodegenerative diseases.

Chronic, low-dose rotenone reproduces Lewy neurites found in early stages of Parkinson's disease, reduces mitochondrial movement and slowly kills differentiated SH-SY5Y neural cells

Background

Parkinson's disease, the most common adult neurodegenerative movement disorder, demonstrates a brain-wide pathology that begins pre-clinically with alpha-synuclein aggregates ("Lewy neurites") in processes of gut enteric and vagal motor neurons. Rostral progression into substantia nigra with death of dopamine neurons produces the motor impairment phenotype that yields a clinical diagnosis. The vast majority of Parkinson's disease occurs sporadically, and current models of sporadic Parkinson's disease (sPD) can utilize directly infused or systemic neurotoxins.

Results

We developed a differentiation protocol for human SH-SY5Y neuroblastoma that yielded non-dividing dopaminergic neural cells with long processes that we then exposed to 50 nM rotenone, a complex I inhibitor used in Parkinson's disease models. After 21 days of rotenone, ~60% of cells died. Their processes retracted and accumulated ASYN-(+) and UB-(+) aggregates that blocked organelle transport. Mitochondrial movement velocities were reduced by 8 days of rotenone and continued to decline over time. No cytoplasmic inclusions resembling Lewy bodies were observed. Gene microarray analyses showed that the majority of genes were under-expressed. qPCR analyses of 11 mtDNA-encoded and 10 nDNA-encoded mitochondrial electron transport chain RNAs' relative expressions revealed small increases in mtDNA-encoded genes and lesser regulation of nDNA-encoded ETC genes.

Conclusion

Subacute rotenone treatment of differentiated SH-SY5Y neuroblastoma cells causes process retraction and partial death over several weeks, slowed mitochondrial movement in processes and appears to reproduce the Lewy neuritic changes of early Parkinson's disease pathology but does not cause Lewy body inclusions. The overall pattern of transcriptional regulation is gene under-expression with minimal regulation of ETC genes in spite of rotenone's being a complex I toxin. This rotenone-SH-SY5Y model in a differentiated human neural cell mimics changes of early Parkinson's disease and may be useful for screening therapeutics for neuroprotection in that disease stage.

Calcium regulation of mitochondria motility and morphology

In the Fifties, electron microscopy studies on neuronal cells showed that mitochondria typically cluster at synaptic terminals, thereby introducing the concept that proper mitochondria trafficking and partitioning inside the cell could provide functional support to the execution of key physiological processes. Today, the notion that a central event in the life of every eukaryotic cell is to configure, maintain, and reorganize the mitochondrial network at sites of high energy demand in response to environmental and cellular cues is well established, and the challenge ahead is to define the underlying molecular mechanisms and regulatory pathways. Recent pioneering studies have further contributed to place mitochondria at the center of the cell biology by showing that the machinery governing remodeling of mitochondria shape and structure regulates the functional output of the organelle as the powerhouse of the cell, the gateway to programmed cell death, and the platform for Ca(2+) signaling. Thus, a raising issue is to identify the cues integrating mitochondria trafficking and dynamics into cell physiology and metabolism. Given the versatile function of calcium as a second messenger and of the role of mitochondria as a major calcium store, evidences are emerging linking Ca(2+) transients to the modulation of mitochondrial activities. This review focuses on calcium as a switch controlling mitochondria motility and morphology in steady state, stressed, and pathological conditions.

Mutant huntingtin and mitochondrial dysfunction

Huntington's disease (HD) is a fatal, inherited neurodegenerative disorder that gradually robs affected individuals of memory, cognitive skills and normal movements. Although research has identified a single faulty gene, the huntingtin gene, as the cause of the disease, a cure remains elusive. Strong evidence indicates that mitochondrial impairment plays a key part in HD pathogenesis. Here, we highlight how mutant huntingtin (mtHtt) might cause mitochondrial dysfunction by either perturbing transcription of nuclear-encoded mitochondrial proteins or by direct interaction with the organelle and modulation of respiration, mitochondrial membrane potential and Ca(2+) buffering. In addition, we propose that mtHtt might convey its neurotoxicity by evoking defects in mitochondrial dynamics, organelle trafficking and fission and fusion, which, in turn, might result in bioenergetic failure and HD-linked neuronal dysfunction and cell death. Finally, we speculate how mitochondria might dictate selective vulnerability of long projection neurons, such as medium spiny neurons, which are particularly affected in HD.

Assessing mitochondrial morphology and dynamics using fluorescence wide-field microscopy and 3D image processing

Mitochondrial morphology and length change during fission and fusion and mitochondrial movement varies dependent upon the cell type and the physiological conditions. Here, we describe fundamental wide-field fluorescence microscopy and 3D imaging techniques to assess mitochondrial shape, number and length in various cell types including cancer cell lines, motor neurons and astrocytes. Furthermore, we illustrate how to assess mitochondrial fission and fusion events by 3D time-lapse imaging and to calculate mitochondrial length and numbers as a function of time. These imaging methods provide useful tools to investigate mitochondrial dynamics in health, aging and disease.

Effects of overexpression of huntingtin proteins on mitochondrial integrity

Huntington's disease (HD) is caused by an expansion of a CAG trinucleotide sequence that encodes a polyglutamine tract in the huntingtin (Htt) protein. Expansion of the polyglutamine tract above 35 repeats causes disease, with the age of onset inversely related to the degree of expansion above this number. Growing evidence suggests that mitochondrial function is compromised during HD pathogenesis, but how this occurs is not understood. We examined mitochondrial properties of HeLa cells that expressed green fluorescent protein (GFP)- or FLAG-tagged N-terminal portions of the Htt protein containing either, 17, 28, 74 or 138 polyglutamine repeats. Immunofluorescence staining of cells using antibodies against Tom20, a mitochondrion localized protein, revealed that cells expressing Htt proteins with 74 or 138 polyglutamine repeats were more sensitized to oxidative stress-induced mitochondria fragmentation and had reduced ATP levels compared with cells expressing Htt proteins with 17 or 28 polyglutamine repeats. By measuring changes in fluorescence of a photoactivated GFP protein targeted to mitochondria, we found that cells expressing red fluorescent protein (RFP)-tagged Htt protein containing 74 polyglutamine repeats had mitochondria that displayed reduced movement and fusion than cells expressing RFP-Htt protein with 28 polyglutamine repeats. Overexpression of Drp-1(K38A), a dominant-negative mitochondria-fission mutant, or Mfn2, a protein that promotes mitochondria fusion, suppressed polyglutamine-induced mitochondria fragmentation, the reduction of ATP levels and cell death. In a Caenorhabditis elegans model of HD, we found that reduction of Drp-1 expression by RNA interference rescued the motility defect associated with the expression of Htt proteins with polyglutamine repeats. These results suggest that the increase in cytotoxicity induced by Htt proteins containing expanded polyglutamine tracts is likely mediated, at least in part, by an alteration in normal mitochondrial dynamics, which results in increased mitochondrial fragmentation. Furthermore, our results suggest that it might be possible to reverse polyglutamine-induced cytotoxicity by preventing mitochondrial fragmentation.

Measurement of instantaneous velocity vectors of organelle transport: mitochondrial transport and bioenergetics in hippocampal neurons

Impaired transport of mitochondria, in dendrites and axons of neurons, and bioenergetic deficit are increasingly recognized to be of pathological importance in neurodegenerative diseases. To study the relationship between transport and bioenergetics, we have developed what to our knowledge is a novel technique to quantify organelle velocity in cultured cells. The aim was to combine measurement of motion and bioenergetic parameters while minimizing photodynamic oxidative artifacts evoked by fluorescence excitation. Velocity determination from sequential fluorescence images is not trivial, and here we describe an application of "optical flow", the flow of gray values in grayscale images, to this problem. Based on the principles of photon shot noise occurring in low light level fluorescence microscopy, we describe and validate here an optical flow-based, robust method to measure velocity vectors for organelles expressing fluorescent proteins. This method features instantaneous velocity determination from a pair of images by detecting motion of edges, with no assumptions about the separation or shapes of the objects in the image. Optical flow was used in combination with single mitochondrion assay of mitochondrial thiol redox status by mitochondrially targeted redox-sensitive green fluorescent protein and measurement of mitochondrial membrane potential by tetramethylrhodamine methyl ester. Mitochondrial populations of resting cultured hippocampal neurons were analyzed. It was found that mitochondria with more oxidized thiol redox status have lower membrane potentials and are smaller in size. These mitochondria are more motile than the average; however, mitochondrial motility is only slightly dependent on the observed bioenergetic parameters and is correlated the best to the size of the mitochondria.

Role of axonal transport in neurodegenerative diseases

Many major human neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS), display axonal pathologies including abnormal accumulations of proteins and organelles. Such pathologies highlight damage to the axon as part of the pathogenic process and, in particular, damage to transport of cargoes through axons. Indeed, we now know that disruption of axonal transport is an early and perhaps causative event in many of these diseases. Here, we review the role of axonal transport in neurodegenerative disease.

Mitochondrial medicine for aging and neurodegenerative diseases

Mitochondria are key cytoplasmic organelles, responsible for generating cellular energy, regulating intracellular calcium levels, altering the reduction-oxidation potential of cells, and regulating cell death. Increasing evidence suggests that mitochondria play a central role in aging and in neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and Freidriech ataxia. Further, several lines of evidence suggest that mitochondrial dysfunction is an early event in most late-onset neurodegenerative diseases. Biochemical and animal model studies of inherited neurodegenerative diseases have revealed that mutant proteins of these diseases are associated with mitochondria. Mutant proteins are reported to block the transport of nuclear-encoded mitochondrial proteins to mitochondria, interact with mitochondrial proteins and disrupt the electron transport chain, induce free radicals, cause mitochondrial dysfunction, and, ultimately, damage neurons. This article discusses critical issues of mitochondria causing dysfunction in aging and neurodegenerative diseases, and discusses the potential of developing mitochondrial medicine, particularly mitochondrially targeted antioxidants, to treat aging and neurodegenerative diseases.

Accumulation of N-terminal mutant huntingtin in mouse and monkey models implicated as a pathogenic mechanism in Huntington's disease

A number of mouse models expressing mutant huntingtin (htt) with an expanded polyglutamine (polyQ) domain are useful for studying the pathogenesis of Huntington's disease (HD) and identifying appropriate therapies. However, these models exhibit neurological phenotypes that differ in their severity and nature. Understanding how transgenic htt leads to variable neuropathology in animal models would shed light on the pathogenesis of HD and help us to choose HD models for investigation. By comparing the expression of mutant htt at the transcriptional and protein levels in transgenic mice expressing N-terminal or full-length mutant htt, we found that the accumulation and aggregation of mutant htt in the brain is determined by htt context. HD mouse models demonstrating more severe phenotypes show earlier accumulation of N-terminal mutant htt fragments, which leads to the formation of htt aggregates that are primarily present in neuronal nuclei and processes, as well as glial cells. Similarly, transgenic monkeys expressing exon-1 htt with a 147-glutamine repeat (147Q) died early and showed abundant neuropil aggregates in swelling neuronal processes. Fractionation of HD150Q knock-in mice brains revealed an age-dependent accumulation of N-terminal mutant htt fragments in the nucleus and synaptosomes, and this accumulation was most pronounced in the striatum due to decreased proteasomal activity. Our findings suggest that the neuropathological phenotypes of HD stem largely from the accumulation of N-terminal mutant htt fragments and that this accumulation is determined by htt context and cell-type-dependent clearance of mutant htt.

Sex-dependent effect of BAG1 in ameliorating motor deficits of Huntington disease transgenic mice

The pathogenesis of Huntington disease (HD) is attributed to the misfolding of huntingtin (htt) caused by an expanded polyglutamine (polyQ) domain. Considerable effort has been devoted to identifying molecules that can prevent or reduce htt misfolding and the associated neuropathology. Although overexpression of chaperones is known to reduce htt cytotoxicity in cellular models, only modest protection is seen with Hsp70 overexpression in HD mouse models. Because the activity of Hsp70 is modulated by co-chaperones, an interesting issue is whether the in vivo effects of chaperones on polyQ protein toxicity are dependent on other modulators. In the present study, we focused on BAG1, a co-chaperone that interacts with Hsp70 and regulates its activity. Of htt mice expressing the N171-82Q mutant, we found that male N171-82Q mice show a greater deficit in rotarod performance than female N171-82Q mice. This sex-dependent motor deficit was improved by crossing N171-82Q mice with transgenic mice overexpressing BAG1 in neurons. Transgenic BAG1 also reduces the levels of mutant htt in synaptosomal fraction of male HD mice. Overexpression of BAG1 augmented the effects of Hsp70 by reducing aggregation of mutant htt in cultured cells and improving neurite outgrowth in htt-transfected PC12 cells. These findings suggest that the effects of chaperones on HD pathology are influenced by both their modulators and sex-dependent factors.

Polyglutamine gene function and dysfunction in the ageing brain

The coordinated regulation of gene expression and protein interactions determines how mammalian nervous systems develop and retain function and plasticity over extended periods of time such as a human life span. By studying mutations that occur in a group of genes associated with chronic neurodegeneration, the polyglutamine (polyQ) disorders, it has emerged that CAG/glutamine stretches play important roles in transcriptional regulation and protein-protein interactions. However, it is still unclear what the many structural and functional roles of CAG and other low-complexity sequences in eukaryotic genomes are, despite being the most commonly shared peptide fragments in such proteomes. In this review we examine the function of genes responsible for at least 10 polyglutamine disorders in relation to the nervous system and how expansion mutations lead to neuronal dysfunction, by particularly focusing on Huntington's disease (HD). We argue that the molecular and cellular pathways that turn out to be dysfunctional during such diseases, as a consequence of a CAG expansion, are also involved in the ageing of the central nervous system. These are pathways that control protein degradation systems (including molecular chaperones), axonal transport, redox-homeostasis and bioenergetics. CAG expansion mutations confer novel properties on proteins that lead to a slow-progressing neuronal pathology and cell death similar to that found in other age-related conditions such as Alzheimer's and Parkinson's diseases.

Suppression of neuropil aggregates and neurological symptoms by an intracellular antibody implicates the cytoplasmic toxicity of mutant huntingtin

Mutant huntingtin accumulates in the neuronal nuclei and processes, which suggests that its subcellular localization is critical for the pathology of Huntington's disease (HD). However, the contribution of cytoplasmic mutant huntingtin and its aggregates in neuronal processes (neuropil aggregates) has not been rigorously explored. We generated an intracellular antibody (intrabody) whose binding to a unique epitope of human huntingtin is enhanced by polyglutamine expansion. This intrabody decreases the cytotoxicity of mutant huntingtin and its distribution in neuronal processes. When expressed in the striatum of HD mice via adenoviral infection, the intrabody reduces neuropil aggregate formation and ameliorates neurological symptoms. Interaction of the intrabody with mutant huntingtin increases the ubiquitination of cytoplasmic huntingtin and its degradation. These findings suggest that the intrabody reduces the specific neurotoxicity of cytoplasmic mutant huntingtin and its associated neurological symptoms by preventing the accumulation of mutant huntingtin in neuronal processes and promoting its clearance in the cytoplasm.

Huntington's disease: from pathology and genetics to potential therapies

Huntington's disease (HD) is a devastating autosomal dominant neurodegenerative disease caused by a CAG trinucleotide repeat expansion encoding an abnormally long polyglutamine tract in the huntingtin protein. Much has been learnt since the mutation was identified in 1993. We review the functions of wild-type huntingtin. Mutant huntingtin may cause toxicity via a range of different mechanisms. The primary consequence of the mutation is to confer a toxic gain of function on the mutant protein and this may be modified by certain normal activities that are impaired by the mutation. It is likely that the toxicity of mutant huntingtin is revealed after a series of cleavage events leading to the production of N-terminal huntingtin fragment(s) containing the expanded polyglutamine tract. Although aggregation of the mutant protein is a hallmark of the disease, the role of aggregation is complex and the arguments for protective roles of inclusions are discussed. Mutant huntingtin may mediate some of its toxicity in the nucleus by perturbing specific transcriptional pathways. HD may also inhibit mitochondrial function and proteasome activity. Importantly, not all of the effects of mutant huntingtin may be cell-autonomous, and it is possible that abnormalities in neighbouring neurons and glia may also have an impact on connected cells. It is likely that there is still much to learn about mutant huntingtin toxicity, and important insights have already come and may still come from chemical and genetic screens. Importantly, basic biological studies in HD have led to numerous potential therapeutic strategies.

Impaired ubiquitin-proteasome system activity in the synapses of Huntington's disease mice

Huntington's disease (HD) is caused by the expansion of a polyglutamine tract in the N-terminal region of huntingtin (htt) and is characterized by selective neurodegeneration. In addition to forming nuclear aggregates, mutant htt accumulates in neuronal processes as well as synapses and affects synaptic function. However, the mechanism for the synaptic toxicity of mutant htt remains to be investigated. We targeted fluorescent reporters for the ubiquitin–proteasome system (UPS) to presynaptic or postsynaptic terminals of neurons. Using these reporters and biochemical assays of isolated synaptosomes, we found that mutant htt decreases synaptic UPS activity in cultured neurons and in HD mouse brains that express N-terminal or full-length mutant htt. Given that the UPS is a key regulator of synaptic plasticity and function, our findings offer insight into the selective neuronal dysfunction seen in HD and also establish a method to measure synaptic UPS activity in other neurological disease models.