Calcium homeostasis and mitochondrial dysfunction in striatal neurons of Huntington disease

Mitochondria targeted therapeutic approaches in Parkinson's and Huntington's diseases

Substantial evidence from both genetic and toxin induced animal and cellular models and postmortem human brain tissue indicates that mitochondrial dysfunction plays a central role in pathophysiology of the neurodegenerative disorders including Parkinson's disease (PD), and Huntington's disease (HD). This review discusses the emerging understanding of the role of mitochondrial dysfunction including bioenergetics defects, mitochondrial DNA mutations, familial nuclear DNA mutations, altered mitochondrial fusion/fission and morphology, mitochondrial transport/trafficking, altered transcription and increased interaction of pathogenic proteins with mitochondria in the pathogenesis of PD and HD. This review recapitulates some of the key therapeutic strategies applied to surmount mitochondrial dysfunction in these debilitating disorders. We discuss the therapeutic role of mitochondrial bioenergetic agents such as creatine, Coenzyme-Q10, mitochondrial targeted antioxidants and peptides, the SIRT1 activator resveratrol, and the pan-PPAR agonist bezafibrate in toxin and genetic cellular and animal models of PD and HD. We also summarize the phase II-III clinical trials conducted using some of these agents. Lastly, we discuss PGC-1α, TORC and Sirtuins as potential therapeutic targets for mitochondrial dysfunction in neurodegenerative disorders. This article is part of a Special Issue entitled 'Mitochondrial function and dysfunction in neurodegeneration'.

Glutathione redox cycle dysregulation in Huntington's disease knock-in striatal cells

Huntington's disease (HD) is a CAG repeat disorder affecting the HD gene, which encodes for huntingtin (Htt) and is characterized by prominent cell death in the striatum. Oxidative stress was previously implicated in HD neurodegeneration, but the role of the major endogenous antioxidant system, the glutathione redox cycle, has been less studied following expression of full-length mutant Htt (FL-mHtt). Thus, in this work we analyzed the glutathione system in striatal cells derived from HD knock-in mice expressing mutant Htt versus wild-type cells. Mutant cells showed increased intracellular reactive oxygen species (ROS) and caspase-3 activity, which were significantly prevented following treatment with glutathione ethyl ester. Interestingly, mutant cells exhibited an increase in intracellular levels of both reduced and oxidized forms of glutathione, and enhanced activities of glutathione peroxidase (GPx) and glutathione reductase (GRed). Furthermore, glutathione-S-transferase (GST) and γ-glutamyl transpeptidase (γ-GT) activities were also increased in mutant cells. Nevertheless, glutamate-cysteine ligase (GCL) and glutathione synthetase (GS) activities and levels of GCL catalytic subunit were decreased in cells expressing FL-mHtt, highly suggesting decreased de novo synthesis of glutathione. Enhanced intracellular total glutathione, despite decreased synthesis, could be explained by decreased extracellular glutathione in mutant cells. This occurred concomitantly with decreased mRNA expression levels and activity of the multidrug resistance protein 1 (Mrp1), a transport protein that mediates cellular export of glutathione disulfide and glutathione conjugates. Additionally, inhibition of Mrp1 enhanced intracellular GSH in wild-type cells only. These data suggest that FL-mHtt affects the export of glutathione by decreasing the expression of Mrp1. Data further suggest that boosting of GSH-related antioxidant defense mechanisms induced by FL-mHtt is insufficient to counterbalance increased ROS formation and emergent apoptotic features in HD striatal cells.

Store-operated Ca2+ entry is remodelled and controls in vitro angiogenesis in endothelial progenitor cells isolated from tumoral patients

BACKGROUND

Endothelial progenitor cells (EPCs) may be recruited from bone marrow to sustain tumor vascularisation and promote the metastatic switch. Understanding the molecular mechanisms driving EPC proliferation and tubulogenesis could outline novel targets for alternative anti-angiogenic treatments. Store-operated Ca(2+) entry (SOCE), which is activated by a depletion of the intracellular Ca(2+) pool, regulates the growth of human EPCs, where is mediated by the interaction between the endoplasmic reticulum Ca(2+)-sensor, Stim1, and the plasmalemmal Ca(2+) channel, Orai1. As oncogenesis may be associated to the capability of tumor cells to grow independently on Ca(2+) influx, it is important to assess whether SOCE regulates EPC-dependent angiogenesis also in tumor patients.

METHODOLOGY/PRINCIPAL FINDINGS

The present study employed Ca(2+) imaging, recombinant sub-membranal and mitochondrial aequorin, real-time polymerase chain reaction, gene silencing techniques and western blot analysis to investigate the expression and the role of SOCE in EPCs isolated from peripheral blood of patients affected by renal cellular carcinoma (RCC; RCC-EPCs) as compared to control EPCs (N-EPCs). SOCE, activated by either pharmacological (i.e. cyclopiazonic acid) or physiological (i.e. ATP) stimulation, was significantly higher in RCC-EPCs and was selectively sensitive to BTP-2, and to the trivalent cations, La(3+) and Gd(3+). Furthermore, 2-APB enhanced thapsigargin-evoked SOCE at low concentrations, whereas higher doses caused SOCE inhibition. Conversely, the anti-angiogenic drug, carboxyamidotriazole (CAI), blocked both SOCE and the intracellular Ca(2+) release. SOCE was associated to the over-expression of Orai1, Stim1, and transient receptor potential channel 1 (TRPC1) at both mRNA and protein level The intracellular Ca(2+) buffer, BAPTA, BTP-2, and CAI inhibited RCC-EPC proliferation and tubulogenesis. The genetic suppression of Stim1, Orai1, and TRPC1 blocked CPA-evoked SOCE in RCC-EPCs.

CONCLUSIONS

SOCE is remodelled in EPCs from RCC patients and stands out as a novel molecular target to interfere with RCC vascularisation due to its ability to control proliferation and tubulogenesis.

hMTH1 expression protects mitochondria from Huntington's disease-like impairment

Huntington disease (HD) is a neurodegenerative disease caused by expansion of CAG repeats in the huntingtin (Htt) gene. The expression of hMTH1, the human hydrolase that degrades oxidized purine nucleoside triphosphates, grants protection in a chemical HD mouse model in which HD-like features are induced by the mitochondrial toxin 3-nitropropionic acid (3-NP). To further examine the relationship between oxidized dNTPs and HD-like neurodegeneration, we studied the effects of hMTH1 expression in a genetic cellular model for HD, such as striatal cells expressing mutant htt (Hdh^Q111). hMTH1 expression protected these cells from 3-NP and H₂O₂-induced killing, by counteracting the mutant htt-dependent increased vulnerability and accumulation of nuclear and mitochondrial DNA 8-hydroxyguanine levels. hMTH1 expression reverted the decreased mitochondrial membrane potential characteristic of Hdh^Q111 cells and delayed the increase in mitochondrial reactive oxygen species associated with 3-NP treatment. Further indications of hMTH1-mediated mitochondrial protection are the partial reversion of 3-NP-induced alterations in mitochondrial morphology and the modulation of DRP1 and MFN1 proteins, which control fusion/fission rates of mitochondria. Finally, in line with the in vitro findings, upon 3-NP in vivo treatment, 8-hydroxyguanine levels in mitochondrial DNA from heart, muscle and brain are significantly lower in transgenic hMTH1-expressing mice than in wild-type animals.

Human stefin B normal and patho-physiological role: molecular and cellular aspects of amyloid-type aggregation of certain EPM1 mutants

Epilepsies are characterized by abnormal electrophysiological activity of the brain. Among various types of inherited epilepsies different epilepsy syndromes, among them progressive myoclonus epilepsies with features of ataxia and neurodegeneration, are counted. The progressive myoclonus epilepsy of type 1 (EPM1), also known as Unverricht-Lundborg disease presents with features of cerebellar atrophy and increased oxidative stress. It has been found that EPM1 is caused by mutations in human cystatin B gene (human stefin B). We first describe the role of protein aggregation in other neurodegenerative conditions. Protein aggregates appear intraneurally but are also excreted, such as is the case with senile plaques of amyloid-β (Aβ) that accumulate in the brain parenchyma and vessel walls. A common characteristic of such diseases is the change of the protein conformation toward β secondary structure that accounts for the strong tendency of such proteins to aggregate and form amyloid fibrils. Second, we describe the patho-physiology of EPM1 and the normal and aberrant roles of stefin B in a mouse model of the disease. Furthermore, we discuss how the increased protein aggregation observed with some of the mutants of human stefin B may relate to the neurodegeneration that occurs in rare EPM1 patients. Our hypothesis (Ceru et al., 2005) states that some of the EPM1 mutants of human stefin B may undergo aggregation in neural cells, thus gaining additional toxic function (apart from loss of normal function). Our in vitro experiments thus far have confirmed that four mutants undergo increased aggregation relative to the wild-type protein. It has been shown that the R68X mutant forms amyloid-fibrils very rapidly, even at neutral pH and forms perinuclear inclusions, whereas the G4R mutant exhibits a prolonged lag phase, during which the toxic prefibrillar aggregates accumulate and are scattered more diffusely over the cytoplasm. Initial experiments on the G50E and Q71P missense EPM1 mutants are described.

Molecular identity and functional properties of the mitochondrial Na+/Ca2+ exchanger

The mitochondrial membrane potential that powers the generation of ATP also facilitates mitochondrial Ca(2+) shuttling. This process is fundamental to a wide range of cellular activities, as it regulates ATP production, shapes cytosolic and endoplasmic recticulum Ca(2+) signaling, and determines cell fate. Mitochondrial Ca(2+) transport is mediated primarily by two major transporters: a Ca(2+) uniporter that mediates Ca(2+) uptake and a Na(+)/Ca(2+) exchanger that subsequently extrudes mitochondrial Ca(2+). In this minireview, we focus on the specific role of the mitochondrial Na(+)/Ca(2+) exchanger and describe its ion exchange mechanism, regulation by ions, and putative partner proteins. We discuss the recent molecular identification of the mitochondrial exchanger and how its activity is linked to physiological and pathophysiological processes.

Huntington's disease and its therapeutic target genes: a global functional profile based on the HD Research Crossroads database

Background

Huntington’s disease (HD) is a fatal progressive neurodegenerative disorder caused by the expansion of the polyglutamine repeat region in the huntingtin gene. Although the disease is triggered by the mutation of a single gene, intensive research has linked numerous other genes to its pathogenesis. To obtain a systematic overview of these genes, which may serve as therapeutic targets, CHDI Foundation has recently established the HD Research Crossroads database. With currently over 800 cataloged genes, this web-based resource constitutes the most extensive curation of genes relevant to HD. It provides us with an unprecedented opportunity to survey molecular mechanisms involved in HD in a holistic manner.

Methods

To gain a synoptic view of therapeutic targets for HD, we have carried out a variety of bioinformatical and statistical analyses to scrutinize the functional association of genes curated in the HD Research Crossroads database. In particular, enrichment analyses were performed with respect to Gene Ontology categories, KEGG signaling pathways, and Pfam protein families. For selected processes, we also analyzed differential expression, using published microarray data. Additionally, we generated a candidate set of novel genetic modifiers of HD by combining information from the HD Research Crossroads database with previous genome-wide linkage studies.

Results

Our analyses led to a comprehensive identification of molecular mechanisms associated with HD. Remarkably, we not only recovered processes and pathways, which have frequently been linked to HD (such as cytotoxicity, apoptosis, and calcium signaling), but also found strong indications for other potentially disease-relevant mechanisms that have been less intensively studied in the context of HD (such as the cell cycle and RNA splicing, as well as Wnt and ErbB signaling). For follow-up studies, we provide a regularly updated compendium of molecular mechanism, that are associated with HD, at http://hdtt.sysbiolab.eu Additionally, we derived a candidate set of 24 novel genetic modifiers, including histone deacetylase 3 (HDAC3), metabotropic glutamate receptor 1 (GRM1), CDK5 regulatory subunit 2 (CDK5R2), and coactivator 1ß of the peroxisome proliferator-activated receptor gamma (PPARGC1B).

Conclusions

The results of our study give us an intriguing picture of the molecular complexity of HD. Our analyses can be seen as a first step towards a comprehensive list of biological processes, molecular functions, and pathways involved in HD, and may provide a basis for the development of more holistic disease models and new therapeutics.

Mitochondrial Ca(2+) and neurodegeneration

Mitochondria are essential for ensuring numerous fundamental physiological processes such as cellular energy, redox balance, modulation of Ca²⁺ signaling and important biosynthetic pathways. They also govern the cell fate by participating in the apoptosis pathway.

The mitochondrial shape, volume, number and distribution within the cells are strictly controlled. The regulation of these parameters has an impact on mitochondrial function, especially in the central nervous system, where trafficking of mitochondria is critical to their strategic intracellular distribution, presumably according to local energy demands. Thus, the maintenance of a healthy mitochondrial population is essential to avoid the impairment of the processes they regulate: for this purpose, cells have developed mechanisms involving a complex system of quality control to remove damaged mitochondria, or to renew them. Defects of these processes impair mitochondrial function and lead to disordered cell function, i.e., to a disease condition. Given the standard role of mitochondria in all cells, it might be expected that their dysfunction would give rise to similar defects in all tissues. However, damaged mitochondrial function has pleiotropic effects in multicellular organisms, resulting in diverse pathological conditions, ranging from cardiac and brain ischemia, to skeletal muscle myopathies to neurodegenerative diseases. In this review, we will focus on the relationship between mitochondrial (and cellular) derangements and Ca²⁺ dysregulation in neurodegenerative diseases, emphasizing the evidence obtained in genetic models. Common patterns, that recognize the derangement of Ca²⁺ and energy control as a causative factor, have been identified: advances in the understanding of the molecular regulation of Ca²⁺ homeostasis, and on the ways in which it could become perturbed in neurological disorders, may lead to the development of therapeutic strategies that modulate neuronal Ca²⁺ signaling.

Mitochondrial diseases

Mitochondria have a crucial role in cellular bioenergetics and apoptosis, and thus are important to support cell function and in determination of cell death pathways. Inherited mitochondrial diseases can be caused by mutations of mitochondrial DNA or of nuclear genes that encode mitochondrial proteins. Although many mitochondrial disorders are multisystemic, some are tissue specific--eg, optic neuropathy, sensorineural deafness, and type 2 diabetes mellitus. In the past few years, several disorders have been associated with mutations of nuclear genes responsible for mitochondrial DNA maintenance and function, and the potential contribution of mitochondrial abnormalities to progressive neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease has been recognised. The process of mitochondrial fission-fusion has become a focus of attention in human disease. Importantly, the mitochondrion is now a target for therapeutic interventions that encompass small molecules, transcriptional regulation, and genetic manipulation, offering opportunities to treat a diverse range of diseases.

Antioxidants in Huntington's disease

Huntington's disease (HD) is a prototypical neurodegenerative disease in which there is selective neuronal degeneration, which leads to progressive disability, manifesting itself as a movement disorder, with both psychiatric and cognitive impairment. The disease is caused by a cytosine-adenine-guanine (CAG) repeat expansion in the huntingtin gene, which causes an expanded polyglutamine repeat in the huntingtin protein, resulting in a protein with a novel gain of function. The mutant huntingtin protein causes neuronal dysfunction and eventual cell death in which transcriptional impairment, excitotoxicity, oxidative damage, inflammation, apoptosis and mitochondrial dysfunction are all implicated. A critical transcriptional impairment may be impaired expression and function of peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α), a master co-regulator of mitochondrial biogenesis and expression of antioxidant enzymes. A deficiency of PGC-1α leads to increased vulnerability to oxidative stress and to striatal degeneration. The extent and severity of the oxidative damage in HD are features well recognized but perhaps under-appreciated. Oxidative damage occurs to lipids, proteins and deoxyribonucleic acid (DNA), and it has been suggested that the latter may contribute to CAG repeat expansion during DNA repair [1]. A marked elevation of oxidized DNA bases occurs in patients' plasma, which may provide a biomarker of disease progression. Antioxidants are effective in slowing disease progression in transgenic mouse models of HD, and show promise in human clinical trials. Strategies to transcriptionally increase expression of antioxidant enzymes by modulating the Nrf-2/ARE pathway, or by increasing expression of PGC-1α hold great promise for developing new treatments to slow or halt the progression of HD. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.

Mutant huntingtin, abnormal mitochondrial dynamics, defective axonal transport of mitochondria, and selective synaptic degeneration in Huntington's disease

Huntington's disease (HD) is a progressive, fatal neurodegenerative disease caused by expanded polyglutamine repeats in the HD gene. HD is characterized by chorea, seizures, involuntary movements, dystonia, cognitive decline, intellectual impairment and emotional disturbances. Research into mutant huntingtin (Htt) and mitochondria has found that mutant Htt interacts with the mitochondrial protein dynamin-related protein 1 (Drp1), enhances GTPase Drp1 enzymatic activity, and causes excessive mitochondrial fragmentation and abnormal distribution, leading to defective axonal transport of mitochondria and selective synaptic degeneration. This article summarizes latest developments in HD research and focuses on the role of abnormal mitochondrial dynamics and defective axonal transport in HD neurons. This article also discusses the therapeutic strategies that decrease mitochondrial fragmentation and neuronal damage in HD.

Cellular fura-2 manganese extraction assay (CFMEA)

Cellular manganese (Mn) uptake and transport dynamics can be measured using a cellular fura-2 manganese extraction assay (CFMEA). The assay described here uses immortalized murine striatal cell line and primary cortical astrocytes, but the method is equally adaptable to other cultured mammalian cells. An ultrasensitive fluorescent nucleic acid stain for quantification of double-stranded DNA (dsDNA) in solution, Quant-iT PicoGreen, has been utilized for normalization of Mn concentration in the cultured cells, following Mn (II) chloride (MnCl(2)) exposure. Depending on the cell type and density, other methods, e.g., protein determination assays or cell counts, may also be used for normalization. Methods are described for rapidly stopping Mn uptake and transport processes at specified times, extraction, and quantification of cellular Mn content, and normalization of Mn levels to dsDNA concentration.

Mitochondria as a therapeutic target for aging and neurodegenerative diseases

Mitochondria are cytoplasmic organelles responsible for life and death. Extensive evidence from animal models, postmortem brain studies of and clinical studies of aging and neurodegenerative diseases suggests that mitochondrial function is defective in aging and neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. Several lines of research suggest that mitochondrial abnormalities, including defects in oxidative phosphorylation, increased accumulation of mitochondrial DNA defects, impaired calcium influx, accumulation of mutant proteins in mitochondria, and mitochondrial membrane potential dissipation are important cellular changes in both early and late-onset neurodegenerative diseases. Further, emerging evidence suggests that structural changes in mitochondria, including increased mitochondrial fragmentation and decreased mitochondrial fusion, are critical factors associated with mitochondrial dysfunction and cell death in aging and neurodegenerative diseases. This paper discusses research that elucidates features of mitochondria that are associated with cellular dysfunction in aging and neurodegenerative diseases and discusses mitochondrial structural and functional changes, and abnormal mitochondrial dynamics in neurodegenerative diseases. It also outlines mitochondria-targeted therapeutics in neurodegenerative diseases.

Mitochondrial permeability transition in Ca(2+)-dependent apoptosis and necrosis

A variety of stimuli utilize an increase of cytosolic free Ca(2+) concentration as a second messenger to transmit signals, through Ca(2+) release from the endoplasmic reticulum or opening of plasma membrane Ca(2+) channels. Mitochondria contribute to the tight spatiotemporal control of this process by accumulating Ca(2+), thus shaping the return of cytosolic Ca(2+) to resting levels. The rise of mitochondrial matrix free Ca(2+) concentration stimulates oxidative metabolism; yet, in the presence of a variety of sensitizing factors of pathophysiological relevance, the matrix Ca(2+) increase can also lead to opening of the permeability transition pore (PTP), a high conductance inner membrane channel. While transient openings may serve the purpose of providing a fast Ca(2+) release mechanism, persistent PTP opening is followed by deregulated release of matrix Ca(2+), termination of oxidative phosphorylation, matrix swelling with inner membrane unfolding and eventually outer membrane rupture with release of apoptogenic proteins and cell death. Thus, a rise in mitochondrial Ca(2+) can convey both apoptotic and necrotic death signals by inducing opening of the PTP. Understanding the signalling networks that govern changes in mitochondrial free Ca(2+) concentration, their interplay with Ca(2+) signalling in other subcellular compartments, and regulation of PTP has important implications in the fine comprehension of the main biological routines of the cell and in disease pathogenesis.

Huntington's disease, calcium, and mitochondria

Huntington's disease (HD) is caused by a mutation that increases the number of CAG repeats in the gene encoding for the protein Huntingtin (Htt). The mutation results in the pathological expansion of the polyQ stretch that is normally present within the N-terminal region of Htt. Even if Htt is ubiquitously expressed in tissues, the changes in the protein finally result in the clinical manifestation of motor and cognitive impairments observed in HD patients. The molecular ethiology of the disease is obscure: a number of cellular and animal models are used as essential tools in experimental approaches aimed at understanding it. Biochemical changes have been described that correlate with the malfunction of HD neurons (primarily in the striatum): consensus is gradually emerging that the dyshomeostasis of Ca(2+) and/or mitochondria stress are important factors in the linkage of the Htt mutation to the onset and progression of the disease. Here, we present a succint overview of the changes of Htt, of its possible effect on the transcription of critical genes and of its causative role in the disturbance of the neuronal Ca(2+) homeostasis. Particular emphasis will be placed on the role of mitochondria as key player in the molecular pathogenesis of the disease.

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

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

Licofelone attenuates quinolinic acid induced Huntington like symptoms: possible behavioral, biochemical and cellular alterations

Cyclo-oxygenase and lipoxygenase enzymes are involved in arachidonic acid metabolism. Emerging evidence indicates that cyclo-oxygenase and lipoxygenase inhibitors prevent neurodegenerative processes and related complications. Therefore, the present study has been designed to explore the neuroprotective potential of licofelone (dual COX-2/5-LOX inhibitor) against quinolinic acid induced Huntington like symptom in rats. Intrastriatal administration of quinolinic acid significantly caused reduction in body weight and motor function (locomotor activity, rotarod performance and beam walk test), oxidative defense (as evidenced by increased lipid peroxidation, nitrite concentration and decreased endogenous antioxidant enzymes), alteration in mitochondrial enzyme complex (I, II and IV) activities, raised TNF-α level and striatal lesion volume as compared to sham treated animals. Licofelone (2.5, 5 and 10 mg/kg) treatment significantly improved body weight, locomotor activity, rotarod performance, balance beam walk performance, oxidative defense, mitochondrial enzyme complex activities and attenuated TNF-α level and striatal lesion as compared to control (quinolinic acid). The present study highlights that licofelone attenuates behavioral, biochemical and cellular alterations against quinolinic acid induced neurotoxicity and this could be an important therapeutic avenue to ameliorate the Huntington like symptoms.

Energy deficit in Huntington disease: why it matters

Huntington disease (HD) is an autosomal dominant neurodegenerative disease with complete penetrance. Although the understanding of the cellular mechanisms that drive neurodegeneration in HD and account for the characteristic pattern of neuronal vulnerability is incomplete, defects in energy metabolism, particularly mitochondrial function, represent a common thread in studies of HD pathogenesis in humans and animal models. Here we review the clinical, biochemical, and molecular evidence of an energy deficit in HD and discuss the mechanisms underlying mitochondrial and related alterations.

Novel high-throughput assay to assess cellular manganese levels in a striatal cell line model of Huntington's disease confirms a deficit in manganese accumulation

In spite of the essentiality of manganese (Mn) as a trace element necessary for a variety of physiological processes, Mn in excess accumulates in the brain and has been associated with dysfunction and degeneration of the basal ganglia. Despite the high sensitivity, limited chemical interference, and multi-elemental advantages of traditional methods for measuring Mn levels, they lack the feasibility to assess Mn transport dynamics in a high-throughput manner. Our lab has previously reported decreased net Mn accumulation in a mutant striatal cell line model of Huntington's disease (STHdh(Q111/Q111)) relative to wild-type following Mn exposure. To evaluate Mn transport dynamics in these striatal cell lines, we have developed a high-throughput fluorescence-quenching extraction assay (Cellular Fura-2 Manganese Extraction Assay - CFMEA). CFMEA utilizes changes in fura-2 fluorescence upon excitation at 360 nm (Ca(2+) isosbestic point) and emission at 535 nm, as an indirect measurement of total cellular Mn content. Here, we report the establishment, development, and application of CFMEA. Specifically, we evaluate critical extraction and assay conditions (e.g. extraction buffer, temperature, and fura-2 concentration) required for efficient extraction and quantitative detection of cellular Mn from cultured cells. Mn concentrations can be derived from quenching of fura-2 fluorescence with standard curves based on saturation one-site specific binding kinetics. Importantly, we show that extracted calcium and magnesium concentrations below 10 μM have negligible influence on measurements of Mn by fura-2. CFMEA is able to accurately measure extracted Mn levels from cultured striatal cells over a range of at least 0.1-10 μM. We have used two independent Mn supplementation approaches to validate the quantitative accuracy of CFMEA over a 0-200 μM cellular Mn-exposure range. Finally, we have utilized CFMEA to experimentally confirm a deficit in net Mn accumulation in the mutant HD striatal cell line versus wild-type cells. To conclude, we have developed and applied a novel assay to assess Mn transport dynamics in cultured striatal cell lines. CFMEA provides a rapid means of evaluating Mn transport kinetics in cellular toxicity and disease models.

Glutamate-induced alterations in Ca2+ signaling are modulated by mitochondrial Ca2+ handling capacity in brain slices of R6/1 transgenic mice

Huntington's disease is a neurodegenerative disorder caused by an expansion of CAGs repeats and characterized by alterations in mitochondrial functions. Although changes in Ca(2+) handling have been suggested, the mechanisms involved are not completely understood. The aim of this study was to investigate the possible alterations in Ca(2+) handling capacity and the relationship with mitochondrial dysfunction evaluated by NAD(P)H fluorescence, reactive oxygen species levels, mitochondrial membrane potential (DeltaPsi(m)) measurements and respiration in whole brain slices from R6/1 mice of different ages, evaluated in situ by real-time real-space microscopy. We show that the cortex and striatum of the 9-month-old R6/1 transgenic mice present a significant sustained increase in cytosolic Ca(2+) induced by glutamate (Glu). This difference in Glu response was partially reduced in R6/1 when in the absence of extracellular Ca(2+), indicating that N-methyl-D-aspartate receptors participation in this response is more important in transgenic mice. In addition, Glu also lead to a decrease in NAD(P)H fluorescence, a loss in DeltaPsi(m) and a further increase in respiration, which may have evoked a decrease in mitochondrial Ca(2+) Ca(2+)(m) uptake capacity. Taken together, these results show that alterations in Ca(2+) homeostasis in transgenic mice are associated with a decrease in Ca(2+)(m) uptake mechanism with a diminished Ca(2+) handling ability that ultimately causes dysfunctions and worsening of the neurodegenerative and the disease processes.

Huntington's disease and Group I metabotropic glutamate receptors

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by involuntary body movement, cognitive impairment and psychiatric disturbance. A polyglutamine expansion in the amino-terminal region of the huntingtin (htt) protein is the genetic cause of HD. Htt protein interacts with a wide variety of proteins, and htt mutation causes cell signaling alterations in various neurotransmitter systems, including dopaminergic, glutamatergic, and cannabinoid systems, as well as trophic factor systems. This review will overview recent findings concerning htt-promoted alterations in cell signaling that involve different neurotransmitters and trophic factor systems, especially involving mGluR1/5, as glutamate plays a crucial role in neuronal cell death. The neuronal cell death that takes place in the striatum and cortex of HD patients is the most important factor underlying HD progression. Metabotropic glutamate receptors (mGluR1 and mGluR5) have a very controversial role in neuronal cell death and it is not clear whether mGluR1/5 activation either protects or exacerbates neuronal death. Thus, understanding how mutant htt protein affects glutamatergic receptor signaling will be essential to further establish a role for glutamate receptors in HD and develop therapeutic strategies to treat HD.

Adenosine receptors and brain diseases: neuroprotection and neurodegeneration

Adenosine acts in parallel as a neuromodulator and as a homeostatic modulator in the central nervous system. Its neuromodulatory role relies on a balanced activation of inhibitory A(1) receptors (A1R) and facilitatory A(2A) receptors (A2AR), mostly controlling excitatory glutamatergic synapses: A1R impose a tonic brake on excitatory transmission, whereas A2AR are selectively engaged to promote synaptic plasticity phenomena. This neuromodulatory role of adenosine is strikingly similar to the role of adenosine in the control of brain disorders; thus, A1R mostly act as a hurdle that needs to be overcame to begin neurodegeneration and, accordingly, A1R only effectively control neurodegeneration if activated in the temporal vicinity of brain insults; in contrast, the blockade of A2AR alleviates the long-term burden of brain disorders in different neurodegenerative conditions such as ischemia, epilepsy, Parkinson's or Alzheimer's disease and also seem to afford benefits in some psychiatric conditions. In spite of this qualitative agreement between neuromodulation and neuroprotection by A1R and A2AR, it is still unclear if the role of A1R and A2AR in the control of neuroprotection is mostly due to the control of glutamatergic transmission, or if it is instead due to the different homeostatic roles of these receptors related with the control of metabolism, of neuron-glia communication, of neuroinflammation, of neurogenesis or of the control of action of growth factors. In spite of this current mechanistic uncertainty, it seems evident that targeting adenosine receptors might indeed constitute a novel strategy to control the demise of different neurological and psychiatric disorders.

The interrelationship between mitochondrial dysfunction and transcriptional dysregulation in Huntington disease

Huntington disease (HD) is an inherited neurodegenerative disease caused by an abnormal expansion of the CAG repeat region in the huntingtin (Htt) gene. Although the pathogenic mechanisms by which mutant Htt (mHtt) causes HD have not been fully elucidated, it is becoming increasingly apparent that mHtt can impair mitochondrial function directly, as well as indirectly by dysregulation of transcriptional processes. mHtt causes increased sensitivity to Ca(2+)-induced decreases in state 3 respiration and mitochondrial permeability transition pore (mPTP) opening concurrent with a reduction in mitochondrial Ca(2+) uptake capacity. Treatment of striatal cells expressing mHtt with thapsigargin results in a decrease in mitochondrial Ca(2+) uptake and membrane potential and an increase in reactive oxygen species (ROS) production. Transcriptional processes regulated by peroxisome proliferator-activated receptor gamma (PPAR gamma) coactivator-1 alpha (PGC-1 alpha), which are critical for mitochondrial biogenesis, have been shown to be impaired in HD. In addition, the PPAR gamma signaling pathway is impaired by mHtt and the activation of this pathway ameliorates many of the mitochondrial deficits, suggesting that PPAR gamma agonists may represent an important treatment strategy for HD.

Comparative neuroprotective profile of statins in quinolinic acid induced neurotoxicity in rats

A possible neuroprotective role has been recently suggested for 3H3MGCoA reductase inhibitors (statins). Here, we sought to determine neuroprotective effect of statins in quinolinic acid induced neurotoxicity in rats. Rats were surgically administered quinolinic acid and treated with Atorvastatin (10, 20 mg/kg), simvastatin (15, 30 mg/kg) and fluvastatin (5, 10 mg/kg) once daily up to 3 weeks. Atorvastatin (10, 20 mg/kg), simvastatin (30 mg/kg) and fluvastatin (10 mg/kg) treatment significantly attenuated the quinolinic acid induced behavioral (locomotor activity, rotarod performance and beam walk test), biochemical (lipid peroxidation, nitrite concentration, SOD and catalase), mitochondrial enzyme complex alterations in rats suggesting their free radical scavenging potential. Additionally, atorvastatin (10, 20 mg/kg), simvastatin (30 mg/kg) and fluvastatin (10 mg/kg) significantly decrease the TNF-α level and striatal lesion volume in quinolinic acid treated animals indicating their anti-inflammatory effects. In comparing the protective effect of different statins, atorvastatin is effective at both the doses while simvastatin and fluvastatins at respective lower doses were not able to produce the protective effect in quinolinic acid treated animals. These modulations can account, at least partly, for the beneficial effect of statins in our rodent model of striatal degeneration. Our findings show that statins could be explored as possible neuroprotective agents for neurodegenerative disorders such as HD.

Huntington's disease: pathogenesis to animal models

Huntington's disease (HD) is an inherited genetic disorder, characterized by cognitive dysfunction and abnormal body movements called chorea. George Huntington, an Ohio physician, described the disease precisely in 1872. HD is a dominantly inherited disorder, characterized by progressive neurodegeneration of the striatum but also involves other regions, primarily the cerebral cortex. The mutation responsible for this fatal disease is an abnormally expanded and unstable CAG repeat within the coding region of the gene encoding the huntingtin protein. Various hypotheses have been put forward to explain the pathogenic mechanisms of mutant huntingtin-induced neuronal dysfunction and cell death. None of these hypotheses, however, offers a clear explanation; thus, it remains a topic of research interest. HD is considered to be an important disease, embodying many of the major themes in modern neuroscience, including molecular genetics, selective neuronal vulnerability, excitotoxicity, mitochondrial dysfunction, apoptosis and transcriptional dysregulation. A number of recent reports have concluded that oxidative stress plays a key role in HD pathogenesis. Although there is no specific treatment available to block disease progression, treatments are available to help in controlling the chorea symptoms. As animal models are the best tools to evaluate any therapeutic agent, there are also different animal models available, mimicking a few or a larger number of symptoms. Each model has its own advantages and limitations. The present review deals with the pathophysiology and various cascades contributing to HD pathogenesis and progression as well as drug targets, such as dopaminergic, gamma-amino butyric acid (GABA)ergic, glutamate adenosine receptor, peptidergic pathways, cannabinoid receptor, and adjuvant therapeutic drug targets such as oxidative stress and mitochondrial dysfunction that can be targeted for future experimental study. The present review also focuses on the animal models (behavioral and genetic) used to unravel pathogenetic mechanisms and the identification of novel drug targets.

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.

Calcium signalling-dependent mitochondrial dysfunction and bioenergetics regulation in respiratory chain Complex II deficiency

Despite advanced knowledge on the genetic basis of oxidative phosphorylation-related diseases, the molecular and/or cellular determinants for tissue-specific dysfunction are not completely understood. Here, we report the cellular events associated with mitochondrial respiratory Complex II deficiency occurring before cell death. Mutation or chronic inhibition of Complex II determined a large increase of basal and agonist-evoked Ca(2+) signals in the cytosol and the mitochondria, in parallel with mitochondrial dysfunction characterized by membrane potential (Δψ(mit)) loss, [ATP] reduction and increased reactive oxygen species production. Cytosolic and mitochondrial Ca(2+) overload are linked to increased endoplasmic reticulum (ER) Ca(2+) leakage, and to SERCA2b and PMCA proteasome-dependent degradation. Increased [Ca(2+)](mit) is also contributed by decreased mitochondrial motility and increased ER-mitochondria contact sites. Interestingly, increased intracellular [Ca(2+)] activated on the one hand a compensatory Ca(2+)-dependent glycolytic ATP production and determined on the second hand mitochondrial pathology. These results revealed the primary function for Ca(2+) signalling in the control of mitochondrial dysfunction and cellular bioenergetics outcomes linked to respiratory chain Complex II deficiency.

The fateful encounter of mitochondria with calcium: how did it happen?

A number of findings in the 1950s had offered indirect indications that mitochondria could accumulate Ca2+. In 1961, the phenomenon was directly demonstrated using isolated mitochondria: the uptake process was driven by respiratory chain activity or by the hydrolysis of added ATP. It could be accompanied by the simultaneous uptake of inorganic phosphate, in which case precipitates of hydroxyapatite were formed in the matrix, buffering its free Ca2+ concentration. The properties of the uptake process were established in the 1960s and 1970s: the uptake of Ca2+ occurred electrophoretically on a carrier that has not yet been molecularly identified, and was released from mitochondria via a Na+/Ca2+ antiporter. A H+/Ca2+ release exchanger was also found to operate in some mitochondrial types. The permeability transition pore was later also found to mediate the efflux of Ca2+ from mitochondria. In the mitochondrial matrix two TCA cycle dehydrogenases and pyruvate dehydrogenase phosphate phosphatase were found to be regulated in the matrix by the cycling of Ca2+ across the inner membrane. In conditions of cytoplasmic Ca2+ overload mitochondria could store for a time large amounts of precipitated Ca2+-phosphate, thus permitting cells to survive situations of Ca2+ emergency. The uptake process was found to have very low affinity for Ca2+: since the bulk concentration of Ca2+ in the cytoplasm is in the low to mid-nM range, it became increasingly difficult to postulate a role of mitochondria in the regulation of cytoplsmic Ca2+. A number of findings had nevertheless shown that energy linked Ca2+ transport occurred efficiently in mitochondria of various tissues in situ. The paradox was only solved in the 1990s, when it was found that the concentration of Ca2+ in the cytoplasm is not uniform: perimitochondrial micropools are created by the agonist-promoted discharge of Ca2+ from vicinal stores in which the concentration of Ca2+ is high enough to activate the low affinity mitochondrial uniporter. Mitochondria thus regained center stage as important regulators of cytoplasmic Ca2+ (not only of their own internal Ca2+). Their Ca2+ uptake systems was found to react very rapidly to cytoplasmic Ca2+ demands, even in the 150-200 msec time scale of processes like the contraction and relaxation of heart. An important recent development in the area of mitochondrial Ca2+ transport is its involvement in the disease process. Ca2+ signaling defects are now gaining increasing importance in the pathogenesis of diseases, e.g., neurodegenerative diseases. Since mitochondria have now regained a central role in the regulation of cytoplasmic Ca2+, dysfunctions of their Ca2+ controlling systems have expectedly been found to be involved in the pathogenesis of numerous disease processes.

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.

Basic mechanisms of neurodegeneration: a critical update

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

Signal transduction to the permeability transition pore

The permeability transition pore (PTP) is an inner mitochondrial membrane channel that has been thoroughly characterized functionally, yet remains an elusive molecular entity. The best characterized PTP-regulatory component, cyclophilin (CyP) D, is a matrix protein that favors pore opening. CyP inhibitors, CyP-D null animals, and in situ PTP readouts have established the role of PTP as an effector mechanism of cell death, and the growing definition of PTP signalling mechanisms. This review briefly covers the functional features of the PTP and the role played by its dysregulation in disease pathogenesis. Recent progress on PTP modulation by kinase/phosphatase signal transduction is discussed, with specific emphasis on hexokinase and on the Akt-ERK-GSK3 axis, which might modulate the PTP through CyP-D phosphorylation.

3-Nitropropionic acid as a tool to study the mechanisms involved in Huntington's disease: past, present and future

Huntington's disease (HD) is an inheritable autosomal-dominant disorder whose causal mechanisms remain unknown. Experimental models have begun to uncover these pathways, thus helping to understand the mechanisms implicated and allowing for the characterization of potential targets for new therapeutic strategies. 3-Nitropropionic acid is known to produce in animals behavioural, biochemical and morphologic changes similar to those occurring in HD. For this reason, this phenotypic model is gaining attention as a valuable tool to mimick this disorder and further developing new therapies. In this review, we will focus on the past and present research of this molecule, to finally bring a perspective on what will be next in this promising field of study.

Mutant huntingtin and glycogen synthase kinase 3-beta accumulate in neuronal lipid rafts of a presymptomatic knock-in mouse model of Huntington's disease

Patients with Huntington's disease have an expanded polyglutamine tract in huntingtin and suffer severe brain atrophy and neurodegeneration. Because membrane dysfunction can occur in Huntington's disease, we addressed whether mutant huntingtin in brain and primary neurons is present in lipid rafts, which are cholesterol-enriched membrane domains that mediate growth and survival signals. Biochemical analysis of detergent-resistant membranes from brains and primary neurons of wild-type and presymptomatic Huntington's disease knock-in mice showed that wild-type and mutant huntingtin were recovered in lipid raft-enriched detergent-resistant membranes. The association with lipid rafts was stronger for mutant huntingtin than wild-type huntingtin. Lipid rafts extracted from Huntington's disease mice had normal levels of lipid raft markers (G(alphaq), Ras, and flotillin) but significantly more glycogen synthase kinase 3-beta. Increases in glycogen synthase kinase 3-beta have been associated with apoptotic cell death. Treating Huntington's disease primary neurons with inhibitors of glycogen synthase kinase 3-beta reduced neuronal death. We speculate that accumulation of mutant huntingtin and glycogen synthase kinase 3-beta in lipid rafts of presymptomatic Huntington's disease mouse neurons contributes to neurodegeneration in Huntington's disease.

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.

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.

Transglutaminase activation in neurodegenerative diseases

The following review examines the role of calcium in promoting the in vitro and in vivo activation of transglutaminases in neurodegenerative disorders. Diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease exhibit increased transglutaminase activity and rises in intracellular calcium concentrations, which may be related. The aberrant activation of transglutaminase by calcium is thought to give rise to a variety of pathological moieties in these diseases, and the inhibition has been shown to have therapeutic benefit in animal and cellular models of neurodegeneration. Given the potential clinical relevance of transglutaminase inhibitors, we have also reviewed the recent development of such compounds.

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.

Mitochondrial calcium function and dysfunction in the central nervous system

The ability of isolated brain mitochondria to accumulate, store and release calcium has been extensively characterized. Extrapolation to the intact neuron led to predictions that the in situ mitochondria would reversibly accumulate Ca(2+) when the concentration of the cation in the vicinity of the mitochondria rose above the 'set-point' at which uptake and efflux were in balance, storing Ca(2+) as a complex with phosphate, and slowly releasing the cation when plasma membrane ion pumps lowered the cytoplasmic free Ca(2+). Excessive accumulation of the cation was predicted to lead to activation of the permeability transition, with catastrophic consequences for the neuron. Each of these predictions has been confirmed with intact neurons, and there is convincing evidence for the permeability transition in cellular Ca(2+) overload associated with glutamate excitotoxicity and stroke, while the neurodegenerative disease in which possible defects in mitochondrial Ca(2+) handling have been most intensively investigated is Huntington's Disease. In this brief review evidence that mitochondrial Ca(2+) transport is relevant to neuronal survival in these conditions will be discussed.

Mitochondria, calcium and cell death: a deadly triad in neurodegeneration

Mitochondrial Ca(2+) accumulation is a tightly controlled process, in turn regulating functions as diverse as aerobic metabolism and induction of cell death. The link between Ca(2+) (dys)regulation, mitochondria and cellular derangement is particularly evident in neurodegenerative disorders, in which genetic models and environmental factors allowed to identify common traits in the pathogenic routes. We will here summarize: i) the current view of mechanisms and functions of mitochondrial Ca(2+) homeostasis, ii) the basic principles of organelle Ca(2+) transport, iii) the role of Ca(2+) in neuronal cell death, and iv) the new information on the pathogenesis of Alzheimer's, Huntington's and Parkinson's diseases, highlighting the role of Ca(2+) and mitochondria.

Calcium signaling and neurodegenerative diseases

Neurodegenerative disorders, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD) and spinocerebellar ataxias (SCAs), present an enormous medical, social, financial and scientific problem. Recent evidence indicates that neuronal calcium (Ca2+) signaling is abnormal in many of these disorders. Similar, but less severe, changes in neuronal Ca2+ signaling occur as a result of the normal aging process. The role of aberrant neuronal Ca2+ signaling in the pathogenesis of neurodegenerative disorders is discussed here. The potential utility of Ca2+ blockers for treatment of these disorders is also highlighted. It is reasoned that Ca2+ blockers will be most beneficial clinically when used in combination with other disease-specific therapeutic approaches.