Abnormal mitochondrial dynamics, mitochondrial loss and mutant huntingtin oligomers in Huntington's disease: implications for selective neuronal damage

SUMO-Modified FADD Recruits Cytosolic Drp1 and Caspase-10 to Mitochondria for Regulated Necrosis

Fas-associated protein with death domain (FADD) plays a key role in extrinsic apoptosis. Here, we show that FADD is SUMOylated as an essential step during intrinsic necrosis. FADD was modified at multiple lysine residues (K120/125/149) by small ubiquitin-related modifier 2 (SUMO2) during necrosis caused by calcium ionophore A23187 and by ischemic damage. SUMOylated FADD bound to dynamin-related protein 1 (Drp1) in cells both in vitro and in ischemic tissue damage cores, thus promoting Drp1 recruitment by mitochondrial fission factor (Mff) to accomplish mitochondrial fragmentation. Mitochondrial-fragmentation-associated necrosis was blocked by FADD or Drp1 deficiency and SUMO-defective FADD expression. Interestingly, caspase-10, but not caspase-8, formed a ternary protein complex with SUMO-FADD/Drp1 on the mitochondria upon exposure to A23187 and potentiated Drp1 oligomerization for necrosis. Moreover, the caspase-10 L285F and A414V mutants, found in autoimmune lymphoproliferative syndrome and non-Hodgkin lymphoma, respectively, regulated this necrosis. Our study reveals an essential role of SUMOylated FADD in Drp1- and caspase-10-dependent necrosis, providing insights into the mechanism of regulated necrosis by calcium overload and ischemic injury.

Ischemic brain injury decreases dynamin-like protein 1 expression in a middle cerebral artery occlusion animal model and glutamate-exposed HT22 cells

Dynamin-like protein I (DLP-1) is an important mitochondrial fission and fusion protein that is associated with apoptotic cell death in neurodegenerative diseases. In this study, we investigated DLP-1 expression in a focal cerebral ischemia animal model and glutamate-exposed hippocampal-derived cell line. Middle cerebral artery occlusion (MCAO) was surgically induced in adult male rats to induce focal cerebral ischemic injury. Brain tissues were collected 24 hours after the onset of MCAO. MCAO induces an increase in infarct volume and histopathological changes in the cerebral cortex. We identified a decrease in DLP-1 in the cerebral cortices of MCAO-injured animals using a proteomic approach and Western blot analysis. Moreover, glutamate treatment significantly decreased DLP-1 expression in a hippocampal-derived cell line. The decrease in DLP-1 indicates mitochondrial dysfunction. Thus, these results suggest that neuronal cell injury induces a decrease in DLP-1 levels and consequently leads to neuronal cell death.

The Role of Mitochondrial Aldehyde Dehydrogenase 2 (ALDH2) in Neuropathology and Neurodegeneration

Aldehydes-induced toxicity has been implicated in many neurodegenerative diseases. Exposure to reactive aldehydes from (1) alcohol and food metabolism; (2) environmental pollutants, including car, factory exhausts, smog, pesticides, herbicides; (3) metabolism of neurotransmitters, amino acids and (4) lipid peroxidation of biological membrane from excessive ROS, all contribute to 'aldehydic load' that has been linked to the pathology of neurodegenerative diseases. In particular, the α, β-unsaturated aldehydes derived from lipid peroxidation, 4-hydroxynonenal (4-HNE), DOPAL (MAO product of dopamine), malondialdehyde, acrolein and acetaldehyde, all readily form chemical adductions with proteins, DNA and lipids, thus causing neurotoxicity. Mitochondrial aldehyde dehydrogenase 2 (ALDH 2) is a major aldehyde metabolizing enzyme that protects against deleterious aldehyde buildup in brain, a tissue that has a particularly high mitochondrial content. In this review, we highlight the deleterious effects of increased aldehydic load in the neuropathology of ischemic stroke, Alzheimer's disease and Parkinson's disease. We also discuss evidence for the association between ALDH2 deficiency, a common East Asianspecific mutation, and these neuropathologies. A novel class of small molecule aldehyde dehydrogenase activators (Aldas), represented by Alda-1, reduces neuronal cell death in models of ischemic stroke, Alzheimer's disease and Parkinson's disease. Together, these data suggest that reducing aldeydic load by enhancing the activity of aldehyde dehydrogenases, such as ALDH2, represents as a therapeutic strategy for neurodegenerative diseases.

Mitochondrial Biology and Neurological Diseases

Mitochondria are extremely active organelles that perform a variety of roles in the cell including energy production, regulation of calcium homeostasis, apoptosis, and population maintenance through fission and fusion. Mitochondrial dysfunction in the form of oxidative stress and mutations can contribute to the pathogenesis of various neurodegenerative diseases such as Parkinson’s (PD), Alzheimer’s (AD), and Huntington’s diseases (HD). Abnormalities of Complex I function in the electron transport chain have been implicated in some neurodegenerative diseases, inhibiting ATP production and generating reactive oxygen species that can cause major damage to mitochondria

Mutations in both nuclear and mitochondrial DNA can contribute to neurodegenerative disease, although the pathogenesis of these conditions tends to focus on nuclear mutations. In PD, nuclear genome mutations in the PINK1 and parkin genes have been implicated in neurodegeneration [1], while mutations in APP, PSEN1 and PSEN2 have been implicated in a variety of clinical symptoms of AD [5]. Mutant htt protein is known to cause HD [2]. Much progress has been made to determine some causes of these neurodegenerative diseases, though permanent treatments have yet to be developed. In this review, we discuss the roles of mitochondrial dysfunction in the pathogenesis of these diseases.

Dynamin Functions and Ligands: Classical Mechanisms Behind

Dynamin is a GTPase that plays a vital role in clathrin-dependent endocytosis and other vesicular trafficking processes by acting as a pair of molecular scissors for newly formed vesicles originating from the plasma membrane. Dynamins and related proteins are important components for the cleavage of clathrin-coated vesicles, phagosomes, and mitochondria. These proteins help in organelle division, viral resistance, and mitochondrial fusion/fission. Dysfunction and mutations in dynamin have been implicated in the pathophysiology of various disorders, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, Charcot-Marie-Tooth disease, heart failure, schizophrenia, epilepsy, cancer, dominant optic atrophy, osteoporosis, and Down's syndrome. This review is an attempt to illustrate the dynamin-related mechanisms involved in the above-mentioned disorders and to help medicinal chemists to design novel dynamin ligands, which could be useful in the treatment of dynamin-related disorders.

Reduced dynamin-related protein 1 protects against phosphorylated Tau-induced mitochondrial dysfunction and synaptic damage in Alzheimer's disease

The purpose of our study was to understand the protective effects of a partial reduction of dynamin-related protein 1 (Drp1) in Alzheimer’s disease (AD) progression and pathogenesis. Increasing evidence suggests that phosphorylated Tau and mitochondrial abnormalities are involved in the loss of synapses, defective axonal transport and cognitive decline, in patients with AD. In the current study, we investigated whether a partial reduction of Drp1 protect neurons from phosphorylated Tau-induced mitochondrial and synaptic toxicities in AD progression. We crossed Drp1+/− mice with Tau transgenic mice (P301L line) and created double mutant (TauXDrp1+/−) mice. Using real-time RT-PCR, immunoblotting and immunostaining analyses, we measured mRNA expressions and protein levels of genes related to the mitochondrial dynamics—Drp1 and Fis1 (fission), Mfn1, Mfn2 and Opa1 (fusion), CypD (matrix), mitochondrial biogenesis—Nrf1, Nrf2, PGC1α and TFAM and synaptic—synaptophysin, PSD95, synapsin 1, synaptobrevin 1, neurogranin, GAP43 and synaptopodin in brain tissues from 6-month-old Drp1+/−, Tau, TauXDrp1+/− and wild-type mice. Using biochemical and immunoblotting methods, mitochondrial function and phosphorylated Tau were measured. Decreased mRNA and protein levels of fission and matrix and increased levels of fusion, mitochondrial biogenesis, and synaptic genes were found in 6-month-old TauXDrp1+/− mice relative to Tau mice. Mitochondrial dysfunction was reduced in TauXDrp1+/− mice relative to Tau mice. Phosphorylated Tau found to be reduced in TauXDrp1+/− mice relative to Tau mice. These findings suggest that a partial reduction of Drp1 decreases the production of phosphorylated Tau, reduces mitochondrial dysfunction, and maintains mitochondrial dynamics, enhances mitochondrial biogenesis and synaptic activity in Tau mice. Findings of this study may have implications for the development of Drp1 based therapeutics for patients with AD and other tauopathies.

Novel Metabolic Abnormalities in the Tricarboxylic Acid Cycle in Peripheral Cells From Huntington's Disease Patients

Metabolic dysfunction is well-documented in Huntington's disease (HD). However, the link between the mutant huntingtin (mHTT) gene and the pathology is unknown. The tricarboxylic acid (TCA) cycle is the main metabolic pathway for the production of NADH for conversion to ATP via the electron transport chain (ETC). The objective of this study was to test for differences in enzyme activities, mRNAs and protein levels related to the TCA cycle between lymphoblasts from healthy subjects and from patients with HD. The experiments utilize the advantages of lymphoblasts to reveal new insights about HD. The large quantity of homogeneous cell populations permits multiple dynamic measures to be made on exactly comparable tissues. The activities of nine enzymes related to the TCA cycle and the expression of twenty-nine mRNAs encoding for these enzymes and enzyme complexes were measured. Cells were studied under baseline conditions and during metabolic stress. The results support our recent findings that the activities of the pyruvate dehydrogenase complex (PDHC) and succinate dehydrogenase (SDH) are elevated in HD. The data also show a large unexpected depression in MDH activities. Furthermore, message levels for isocitrate dehydrogenase 1 (IDH1) were markedly increased in in HD lymphoblasts and were responsive to treatments. The use of lymphoblasts allowed us to clarify that the reported decrease in aconitase activity in HD autopsy brains is likely due to secondary hypoxic effects. These results demonstrate the mRNA and enzymes of the TCA cycle are critical therapeutic targets that have been understudied in HD.

Alterations of mitochondrial dynamics allow retrograde propagation of locally initiated axonal insults

In chronic neurodegenerative syndromes, neurons progressively die through a generalized retraction pattern triggering retrograde axonal degeneration toward the cell bodies, which molecular mechanisms remain elusive. Recent observations suggest that direct activation of pro-apoptotic signaling in axons triggers local degenerative events associated with early alteration of axonal mitochondrial dynamics. This raises the question of the role of mitochondrial dynamics on both axonal vulnerability stress and their implication in the spreading of damages toward unchallenged parts of the neuron. Here, using microfluidic chambers, we assessed the consequences of interfering with OPA1 and DRP1 proteins on axonal degeneration induced by local application of rotenone. We found that pharmacological inhibition of mitochondrial fission prevented axonal damage induced by rotenone, in low glucose conditions. While alteration of mitochondrial dynamics per se did not lead to spontaneous axonal degeneration, it dramatically enhanced axonal vulnerability to rotenone, which had no effect in normal glucose conditions, and promoted retrograde spreading of axonal degeneration toward the cell body. Altogether, our results suggest a mitochondrial priming effect in axons as a key process of axonal degeneration. In the context of neurodegenerative diseases, like Parkinson's and Alzheimer's, mitochondria fragmentation could hasten neuronal death and initiate spatial dispersion of locally induced degenerative events.

Quantitative Proteomic Analysis Reveals Similarities between Huntington's Disease (HD) and Huntington's Disease-Like 2 (HDL2) Human Brains

The pathogenesis of HD and HDL2, similar progressive neurodegenerative disorders caused by expansion mutations, remains incompletely understood. No systematic quantitative proteomics studies, assessing global changes in HD or HDL2 human brain, were reported. To address this deficit, we used a stable isotope labeling-based approach to quantify the changes in protein abundances in the cortex of 12 HD and 12 control cases and, separately, of 6 HDL2 and 6 control cases. The quality of the tissues was assessed to minimize variability due to post mortem autolysis. We applied a robust median sweep algorithm to quantify protein abundance and performed statistical inference using moderated test statistics. 1211 proteins showed statistically significant fold changes between HD and control tissues; the differences in selected proteins were verified by Western blotting. Differentially abundant proteins were enriched in cellular pathways previously implicated in HD, including Rho-mediated, actin cytoskeleton and integrin signaling, mitochondrial dysfunction, endocytosis, axonal guidance, DNA/RNA processing, and protein transport. The abundance of 717 proteins significantly differed between control and HDL2 brain. Comparative analysis of the disease-associated changes in the HD and HDL2 proteomes revealed that similar pathways were altered, suggesting the commonality of pathogenesis between the two disorders.

Huntington's Disease-Induced Cardiac Disorders Affect Multiple Cellular Pathways

Huntington's disease (HD) is a rare, inherited, progressive, and fatal neurological disorder resulting from expanded polyglutamine repeats in the huntingtin protein. While HD is predominately characterized as a disease of the central nervous system, mortality surveys and epidemiological studies reveal heart disease as one of the leading causes of death in HD patients. Emerging evidence supports a link between HD and cardiovascular disease, such as cardiac amyloidosis (accumulation of aggregates in the heart). Experimental animal and clinical studies have attempted to explain the mechanisms of HD-induced cardiac pathology in the association of protein misfolding, autophagic defects, oxidative stress, mitochondrial dysfunction, and cell death. HD is increasingly understood as a complex disease with peripheral components of cardiac and skeletal muscle pathophysiology. While the discovery of these linkages and apparent pathological markers is promising, the mechanism of HD-induced cardiac pathology and the nature of its cell autonomy remain elusive. Further study of the wide-ranging cardiac function in HD patients is needed. This review highlights published literature on the pathological factors associated with HD-induced cardiac amyloidosis and other cardiovascular diseases, and addresses gaps in this expanding area of study. Through comprehensive experimental and clinical studies, potential drugs can be tested to attenuate and/or ameliorate HD-induced cardiac pathology and mortality.

Nanoscale studies link amyloid maturity with polyglutamine diseases onset

The presence of expanded poly-glutamine (polyQ) repeats in proteins is directly linked to the pathogenesis of several neurodegenerative diseases, including Huntington’s disease. However, the molecular and structural basis underlying the increased toxicity of aggregates formed by proteins containing expanded polyQ repeats remain poorly understood, in part due to the size and morphological heterogeneity of the aggregates they form in vitro. To address this knowledge gap and technical limitations, we investigated the structural, mechanical and morphological properties of fibrillar aggregates at the single molecule and nanometer scale using the first exon of the Huntingtin protein as a model system (Exon1). Our findings demonstrate a direct correlation of the morphological and mechanical properties of Exon1 aggregates with their structural organization at the single aggregate and nanometric scale and provide novel insights into the molecular and structural basis of Huntingtin Exon1 aggregation and toxicity.

Mitochondrial Ca2+ handling in Huntington's and Alzheimer's diseases - Role of ER-mitochondria crosstalk

Mitochondria play a relevant role in Ca²⁺ buffering, governing energy metabolism and neuronal function. Huntington's disease (HD) and Alzheimer's disease (AD) are two neurodegenerative disorders that, although clinically distinct, share pathological features linked to selective brain damage. These include mitochondrial dysfunction, intracellular Ca²⁺ deregulation and mitochondrial Ca²⁺ handling deficits. Both diseases are associated with misfolding and aggregation of specific proteins that physically interact with mitochondria and interfere with endoplasmic reticulum (ER)/mitochondria-contact sites. Cumulating evidences indicate that impairment of mitochondrial Ca²⁺ homeostasis underlies the susceptibility to selective neuronal death observed in HD and AD; however data obtained with different models and experimental approaches are not always consistent. In this review, we explore the recent literature on deregulation of mitochondrial Ca²⁺ handling underlying the interplay between mitochondria and ER in HD and AD-associated neurodegeneration.

Impaired GAPDH-induced mitophagy contributes to the pathology of Huntington's disease

Mitochondrial dysfunction is implicated in multiple neurodegenerative diseases. In order to maintain a healthy population of functional mitochondria in cells, defective mitochondria must be properly eliminated by lysosomal machinery in a process referred to as mitophagy. Here, we uncover a new molecular mechanism underlying mitophagy driven by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) under the pathological condition of Huntington’s disease (HD) caused by expansion of polyglutamine repeats. Expression of expanded polyglutamine tracts catalytically inactivates GAPDH (iGAPDH), which triggers its selective association with damaged mitochondria in several cell culture models of HD. Through this mechanism, iGAPDH serves as a signaling molecule to induce direct engulfment of damaged mitochondria into lysosomes (micro-mitophagy). However, abnormal interaction of mitochondrial GAPDH with long polyglutamine tracts stalled GAPDH-mediated mitophagy, leading to accumulation of damaged mitochondria, and increased cell death. We further demonstrated that overexpression of inactive GAPDH rescues this blunted process and enhances mitochondrial function and cell survival, indicating a role for GAPDH-driven mitophagy in the pathology of HD.

Mutant Huntingtin and Elusive Defects in Oxidative Metabolism and Mitochondrial Calcium Handling

Elongation of a polyglutamine (polyQ) stretch in huntingtin protein (Htt) is linked to Huntington's disease (HD) pathogenesis. The mutation in Htt correlates with neuronal dysfunction in the striatum and cerebral cortex and eventually leads to neuronal cell death. The exact mechanisms of the injurious effect of mutant Htt (mHtt) on neurons are not completely understood but might include aberrant gene transcription, defective autophagy, abnormal mitochondrial biogenesis, anomalous mitochondrial dynamics, and trafficking. In addition, deficiency in oxidative metabolism and defects in mitochondrial Ca(2+) handling are considered essential contributing factors to neuronal dysfunction in HD and, consequently, in HD pathogenesis. Since the discovery of the mutation in Htt, the questions whether mHtt affects oxidative metabolism and mitochondrial Ca(2+) handling and, if it does, what mechanisms could be involved were in focus of numerous investigations. However, despite significant research efforts, the detrimental effect of mHtt and the mechanisms by which mHtt might impair oxidative metabolism and mitochondrial Ca(2+) handling remain elusive. In this paper, I will briefly review studies aimed at clarifying the consequences of mHtt interaction with mitochondria and discuss experimental results supporting or arguing against the mHtt effects on oxidative metabolism and mitochondrial Ca(2+) handling.

Impaired PLP-dependent metabolism in brain samples from Huntington disease patients and transgenic R6/1 mice

Oxidative stress has been described as important to Huntington disease (HD) progression. In a previous HD study, we identified several carbonylated proteins, including pyridoxal kinase and antiquitin, both of which are involved in the metabolism of pyridoxal 5´-phosphate (PLP), the active form of vitamin B6. In the present study, pyridoxal kinase levels were quantified and showed to be decreased both in HD patients and a R6/1 mouse model, compared to control samples. A metabolomic analysis was used to analyze metabolites in brain samples of HD patients and R6/1 mice, compared to control samples using mass spectrometry. This technique allowed detection of increased concentrations of pyridoxal, the substrate of pyridoxal kinase. In addition, PLP, the product of the reaction, was decreased in striatum from R6/1 mice. Furthermore, glutamate and cystathionine, both substrates of PLP-dependent enzymes were increased in HD. This reinforces the hypothesis that PLP synthesis is impaired, and could explain some alterations observed in the disease. Together, these results identify PLP as a potential therapeutic agent.

The entangled ER-mitochondrial axis as a potential therapeutic strategy in neurodegeneration: A tangled duo unchained

Endoplasmic reticulum (ER) and mitochondrial function have both been shown to be critical events in neurodegenerative diseases. The ER mediates protein folding, maturation, sorting as well acts as calcium storage. The unfolded protein response (UPR) is a stress response of the ER that is activated by the accumulation of misfolded proteins within the ER lumen. Although the molecular mechanisms underlying ER stress-induced apoptosis are not completely understood, increasing evidence suggests that ER and mitochondria cooperate to signal cell death. Similarly, calcium-mediated mitochondrial function and dynamics not only contribute to ATP generation and calcium buffering but are also a linchpin in mediating cell fate. Mitochondria and ER form structural and functional networks (mitochondria-associated ER membranes [MAMs]) essential to maintaining cellular homeostasis and determining cell fate under various pathophysiological conditions. Regulated Ca(2+) transfer from the ER to the mitochondria is important in maintaining control of pro-survival/pro-death pathways. In this review, we summarize the latest therapeutic strategies that target these essential organelles in the context of neurodegenerative diseases.

Loss of Mitochondrial Function Impairs Lysosomes

Alterations in mitochondrial function, as observed in neurodegenerative diseases, lead to disrupted energy metabolism and production of damaging reactive oxygen species. Here, we demonstrate that mitochondrial dysfunction also disrupts the structure and function of lysosomes, the main degradation and recycling organelle. Specifically, inhibition of mitochondrial function, following deletion of the mitochondrial protein AIF, OPA1, or PINK1, as well as chemical inhibition of the electron transport chain, impaired lysosomal activity and caused the appearance of large lysosomal vacuoles. Importantly, our results show that lysosomal impairment is dependent on reactive oxygen species. Given that alterations in both mitochondrial function and lysosomal activity are key features of neurodegenerative diseases, this work provides important insights into the etiology of neurodegenerative diseases.

Oxidative metabolism and Ca2+ handling in isolated brain mitochondria and striatal neurons from R6/2 mice, a model of Huntington's disease

Alterations in oxidative metabolism and defects in mitochondrial Ca²⁺ handling have been implicated in the pathology of Huntington's disease (HD), but existing data are contradictory. We investigated the effect of human mHtt fragments on oxidative metabolism and Ca²⁺ handling in isolated brain mitochondria and cultured striatal neurons from the R6/2 mouse model of HD. Non-synaptic and synaptic mitochondria isolated from the brains of R6/2 mice had similar respiratory rates and Ca²⁺ uptake capacity compared with mitochondria from wild-type (WT) mice. Respiratory activity of cultured striatal neurons measured with Seahorse XF24 flux analyzer revealed unaltered cellular respiration in neurons derived from R6/2 mice compared with neurons from WT animals. Consistent with the lack of respiratory dysfunction, ATP content of cultured striatal neurons from R6/2 and WT mice was similar. Mitochondrial Ca²⁺ accumulation was also evaluated in cultured striatal neurons from R6/2 and WT animals. Our data obtained with striatal neurons derived from R6/2 and WT mice show that both glutamate-induced increases in cytosolic Ca²⁺ and subsequent carbonilcyanide p-triflouromethoxyphenylhydrazone-induced increases in cytosolic Ca²⁺ were similar between WT and R6/2, suggesting that mitochondria in neurons derived from both types of animals accumulated comparable amounts of Ca²⁺ Overall, our data argue against respiratory deficiency and impaired Ca²⁺ handling induced by human mHtt fragments in both isolated brain mitochondria and cultured striatal neurons from transgenic R6/2 mice.

GSK-3β-induced Tau pathology drives hippocampal neuronal cell death in Huntington's disease: involvement of astrocyte-neuron interactions

Glycogen synthase kinase-3β (GSK-3β) has emerged as a critical factor in several pathways involved in hippocampal neuronal maintenance and function. In Huntington's disease (HD), there are early hippocampal deficits both in patients and transgenic mouse models, which prompted us to investigate whether disease-specific changes in GSK-3β expression may underlie these abnormalities. Thirty-three postmortem hippocampal samples from HD patients (neuropathological grades 2-4) and age- and sex-matched normal control cases were analyzed using real-time quantitative reverse transcription PCRs (qPCRs) and immunohistochemistry. In vitro and in vivo studies looking at hippocampal pathology and GSK-3β were also undertaken in transgenic R6/2 and wild-type mice. We identified a disease and stage-dependent upregulation of GSK-3β mRNA and protein levels in the HD hippocampus, with the active isoform pGSK-3β-Tyr(216) being strongly expressed in dentate gyrus (DG) neurons and astrocytes at a time when phosphorylation of Tau at the AT8 epitope was also present in these same neurons. This upregulation of pGSK-3β-Tyr(216) was also found in the R6/2 hippocampus in vivo and linked to the increased vulnerability of primary hippocampal neurons in vitro. In addition, the increased expression of GSK-3β in the astrocytes of R6/2 mice appeared to be the main driver of Tau phosphorylation and caspase3 activation-induced neuronal death, at least in part via an exacerbated production of major proinflammatory mediators. This stage-dependent overactivation of GSK-3β in HD-affected hippocampal neurons and astrocytes therefore points to GSK-3β as being a critical factor in the pathological development of this condition. As such, therapeutic targeting of this pathway may help ameliorate neuronal dysfunction in HD.

Targeting mitochondrial biogenesis to overcome drug resistance to MAPK inhibitors

Targeting multiple components of the MAPK pathway can prolong the survival of patients with BRAFV600E melanoma. This approach is not curative, as some BRAF-mutated melanoma cells are intrinsically resistant to MAPK inhibitors (MAPKi). At the systemic level, our knowledge of how signaling pathways underlie drug resistance needs to be further expanded. Here, we have shown that intrinsically resistant BRAF-mutated melanoma cells with a low basal level of mitochondrial biogenesis depend on this process to survive MAPKi. Intrinsically resistant cells exploited an integrated stress response, exhibited an increase in mitochondrial DNA content, and required oxidative phosphorylation to meet their bioenergetic needs. We determined that intrinsically resistant cells rely on the genes encoding TFAM, which controls mitochondrial genome replication and transcription, and TRAP1, which regulates mitochondrial protein folding. Therefore, we targeted mitochondrial biogenesis with a mitochondrium-targeted, small-molecule HSP90 inhibitor (Gamitrinib), which eradicated intrinsically resistant cells and augmented the efficacy of MAPKi by inducing mitochondrial dysfunction and inhibiting tumor bioenergetics. A subset of tumor biopsies from patients with disease progression despite MAPKi treatment showed increased mitochondrial biogenesis and tumor bioenergetics. A subset of acquired drug-resistant melanoma cell lines was sensitive to Gamitrinib. Our study establishes mitochondrial biogenesis, coupled with aberrant tumor bioenergetics, as a potential therapy escape mechanism and paves the way for a rationale-based combinatorial strategy to improve the efficacy of MAPKi.

Enhanced mitochondrial biogenesis ameliorates disease phenotype in a full-length mouse model of Huntington's disease

Huntington's disease (HD) is a devastating illness and at present there is no disease modifying therapy or cure for it; and management of the disease is limited to a few treatment options for amelioration of symptoms. Recently, we showed that the administration of bezafibrate, a pan-PPAR agonist, increases the expression of PGC-1α and mitochondrial biogenesis, and improves phenotype and survival in R6/2 transgenic mouse model of HD. Since the R6/2 mice represent a 'truncated' huntingtin (Htt) mouse model of HD, we tested the efficacy of bezafibrate in a 'full-length' Htt mouse model, the BACHD mice. Bezafibrate treatment restored the impaired PPARγ, PPARδ, PGC-1α signaling pathway, enhanced mitochondrial biogenesis and improved antioxidant defense in the striatum of BACHD mice. Untreated BACHD mice show robust and progressive motor deficits, as well as late-onset and selective neuropathology in the striatum, which was markedly ameliorated in the BACHD mice treated with bezafibrate. Our data demonstrate the efficacy of bezafibrate in ameliorating both neuropathological features and disease phenotype in BACHD mice, and taken together with our previous studies with the R6/2 mice, highlight the strong therapeutic potential of bezafibrate for treatment of HD.

Is There an Association of Physical Activity with Brain Volume, Behavior, and Day-to-day Functioning? A Cross Sectional Design in Prodromal and Early Huntington Disease

BACKGROUND

Huntington disease (HD) is a genetic neurodegenerative disease leading to progressive motor, cognitive, and behavioral decline. Subtle changes in these domains are detectable up to 15 years before a definitive motor diagnosis is made. This period, called prodromal HD, provides an opportunity to examine lifestyle behaviors that may impact disease progression.

THEORETICAL FRAMEWORK

Physical activity relates to decreased rates of brain atrophy and improved cognitive and day-to-day functioning in Alzheimer disease and healthy aging populations. Previous research has yielded mixed results regarding the impact of physical activity on disease progression in HD and paid little attention to the prodromal phase.

METHODS

We conducted analyses of associations among current physical activity level, current and retrospective rate of change for hippocampus and striatum volume, and cognitive, motor, and day-to-day functioning variables. Participants were 48 gene-expanded cases with prodromal and early-diagnosed HD and 27 nongene-expanded control participants. Participants wore Fitbit Ultra activity monitors for three days and completed the self-reported International Physical Activity Questionnaire (IPAQ). Hippocampal and striatal white matter volumes were measured using magnetic resonance imaging. Cognitive tests included the Stroop Color and Word Test, and the Symbol Digit Modalities Test (SDMT). Motor function was assessed using the Unified Huntington's Disease Rating Scale total motor score (TMS). Day-to-day functioning was measured using the World Health Organization Disability Assessment Schedule (WHODAS) version 2.0.

RESULTS

Higher Fitbit activity scores were significantly related to better scores on the SDMT and WHODAS in case participants but not in controls. Fitbit activity scores tracked better with TMS scores in the group as a whole, though the association did not reach statistical significance in the case participants. Higher Fitbit activity scores related to less day-to-day functioning decline in retrospective slope analyses. Fitbit activity scores did not differ significantly between cases and controls.

CONCLUSIONS

This is the first known study examining the associations between activity level and imaging, motor, cognitive, and day-to-day functioning outcomes in prodromal and early HD. Preliminary results suggest physical activity positively correlates with improved cognitive and day-to-day functioning and possibly motor function in individuals in the prodromal and early phase of the condition.

A novel mitochondrial tRNA(Ala) gene variant causes chronic progressive external ophthalmoplegia in a patient with Huntington disease

Chronic progressive external ophthalmoplegia is a mitochondrial disorder usually caused by single or multiple mitochondrial DNA (mtDNA) deletions and, more rarely, by maternally inherited mtDNA point mutations, most frequently in tRNA genes (MTT). We report on a patient presenting with a progressive eyelid ptosis with bilateral ophthalmoparesis, dysphagia, dysphonia and mild proximal limb weakness associate with a mild movement disorder characterized by abnormal involuntary movements involving head and limbs, imbalance and gait instability. Muscle biopsy demonstrated the presence of ragged red fibers and several cytochrome-C-oxidase negative fibers. Molecular analysis showed the novel m.5613T > C heteroplasmic mutation in the mitochondrial tRNA(Ala) gene (MTTA) which disrupts a conserved site and fulfills the accepted criteria of pathogenicity. Moreover, a 38 CAG trinucleotide repeat expansion was found on the huntingtin gene, thus configuring a singular CPEO/"reduced penetrance" Huntington disease "double trouble". With this novel MTTA point mutation, we extend the spectrum of provisional pathogenic changes in this gene, which is a very rare site of pathogenic mutation, and confirm that clinical expression of these mutations is hardly ever heterogeneous, including myopathy and CPEO. Mitochondrial involvement is an emerging key determinant in the pathogenesis of Huntington disease and it is well known that mutant huntingtin influences the mitochondrial respiratory complexes II and III. A synergist effect of the HTT and MTTA mutations on respiratory chain function may be hypothesized in our patient and should be regarded as a spur for further studies on the mtDNA/HTT reciprocal interactions.

Mitochondria-targeted molecules MitoQ and SS31 reduce mutant huntingtin-induced mitochondrial toxicity and synaptic damage in Huntington's disease

The objective of this study was to determine the protective effects of the mitochondria-targeted molecules MitoQ and SS31 in striatal neurons that stably express mutant huntingtin (Htt) (STHDhQ111/Q111) in Huntington's disease (HD). We studied mitochondrial and synaptic activities by measuring mRNA and the protein levels of mitochondrial and synaptic genes, mitochondrial function, and ultra-structural changes in MitoQ- and SS31-treated mutant Htt neurons relative to untreated mutant Htt neurons. We used gene expression analysis, biochemical methods, transmission electron microscopy (TEM) and confocal microscopy methods. In the MitoQ- and SS31-treated mutant Htt neurons, fission genes Drp1 and Fis1 were down-regulated, and fusion genes Mfn1, Mfn2 and Opa1 were up-regulated relative to untreated neurons, suggesting that mitochondria-targeted molecules reduce fission activity. Interestingly, the mitochondrial biogenesis genes PGC1α, PGC1β, Nrf1, Nrf2 and TFAM were up-regulated in MitoQ- and SS31-treated mutant Htt neurons. The synaptic genes synaptophysin and PSD95 were up-regulated, and mitochondrial function was normal in the MitoQ- and SS31-treated mutant Htt neurons. Immunoblotting findings of mitochondrial and synaptic proteins agreed with the mRNA findings. TEM studies revealed decreased numbers of structurally intact mitochondria in MitoQ- and SS31-treated mutant Htt neurons. These findings suggest that mitochondria-targeted molecules MitoQ and SS31 are protective against mutant Htt-induced mitochondrial and synaptic damage in HD neurons, and these mitochondria-targeted molecules are potential therapeutic molecules for the treatment of HD neurons.

Mitochondrial Fission in Human Diseases

Mitochondria are an essential component of multicellular life - from primitive organisms, to highly complex entities like mammals. The importance of mitochondria is underlined by their plethora of well-characterized essential functions such as energy production through oxidative phosphorylation (OX-PHOS), calcium and reactive oxygen species (ROS) signaling, and regulation of apoptosis. In addition, novel roles and attributes of mitochondria are coming into focus through the recent years of mitochondrial research. In particular, over the past decade the study of mitochondrial shape and dynamics has achieved special significance, as they are found to impact mitochondrial function. Recent advances indicate that mitochondrial function and dynamics are inter-connected, and maintain the balance between health and disease at a cellular and an organismal level. For example, excessive mitochondrial division (fission) is associated with functional defects, and is implicated in multiple human diseases from neurodegenerative diseases to cancer. In this chapter we examine the recent literature on the mitochondrial dynamics-function relationship, and explore how it impacts on the development and progression of human diseases. We will also highlight the implications of therapeutic manipulation of mitochondrial dynamics in treating various human pathologies.

Melatonin: A Potential Anti-Oxidant Therapeutic Agent for Mitochondrial Dysfunctions and Related Disorders

Mitochondria play a central role in cellular physiology. Besides their classic function of energy metabolism, mitochondria are involved in multiple cell functions, including energy distribution through the cell, energy/heat modulation, regulation of reactive oxygen species (ROS), calcium homeostasis, and control of apoptosis. Simultaneously, mitochondria are the main producer and target of ROS with the result that multiple mitochondrial diseases are related to ROS-induced mitochondrial injuries. Increased free radical generation, enhanced mitochondrial inducible nitric oxide synthase (iNOS) activity, enhanced nitric oxide (NO) production, decreased respiratory complex activity, impaired electron transport system, and opening of mitochondrial permeability transition pores have all been suggested as factors responsible for impaired mitochondrial function. Because of these, neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and aging, are caused by ROS-induced mitochondrial dysfunctions. Melatonin, the major hormone of the pineal gland, also acts as an anti-oxidant and as a regulator of mitochondrial bioenergetic function. Melatonin is selectively taken up by mitochondrial membranes, a function not shared by other anti-oxidants, and thus has emerged as a major potential therapeutic tool for treating neurodegenerative disorders. Multiple in vitro and in vivo experiments have shown the protective role of melatonin for preventing oxidative stress-induced mitochondrial dysfunction seen in experimental models of PD, AD, and HD. With these functions in mind, this article reviews the protective role of melatonin with mechanistic insights against mitochondrial diseases and suggests new avenues for safe and effective treatment modalities against these devastating neurodegenerative diseases. Future insights are also discussed.

Modelling the molecular mechanism of protein-protein interactions and their inhibition: CypD-p53 case study

Cyclophilin D (CypD) is an important regulatory protein involved in mitochondrial membrane permeability transition and cell death. Further, the mitochondrial CypD-p53 axis is an important contributor to necroptosis, a form of programmed necrosis, involved in various cardiovascular and neurological disorders. The CypD ligand, Cyclosporin A (CsA), was identified as an inhibitor of this interaction. In this study, using computational methods, we have attempted to model the CypD-p53 interaction in order to delineate their mode of binding and also to disclose the molecular mechanism, by means of which CsA interferes with this interaction. It was observed that p53 binds at the CsA-binding site of CypD. The knowledge obtained from this modelling was employed to identify novel CypD inhibitors through structure-based methods. Further, the identified compounds were tested by a similar strategy, adopted during the modelling process. This strategy could be applied to study the mechanism of protein-protein interaction (PPI) inhibition and to identify novel PPI inhibitors.

Heterogeneous nuclear ribonucleoprotein A1 post-transcriptionally regulates Drp1 expression in neuroblastoma cells

Excessive mitochondrial fission is associated with the pathogenesis of neurodegenerative diseases. Dynamin-related protein 1 (Drp1) possesses specific fission activity in the mitochondria and peroxisomes. Various post-translational modifications of Drp1 are known to modulate complex mitochondrial dynamics. However, the post-transcriptional regulation of Drp1 remains poorly understood. Here, we show that the heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) regulates Drp1 expression at the post-transcriptional level. hnRNP A1 directly interacts with Drp1 mRNA at its 3′UTR region, and enhances translation potential without affecting mRNA stability. Down-regulation of hnRNP A1 induces mitochondrial elongation by reducing Drp1 expression. Moreover, depletion of hnRNP A1 suppresses 3-NP-mediated mitochondrial fission and dysfunction. In contrast, over-expression of hnRNP A1 promotes mitochondrial fragmentation by increasing Drp1 expression. Additionally, hnRNP A1 significantly exacerbates 3-NP-induced mitochondrial dysfunction and cell death in neuroblastoma cells. Interestingly, treatment with 3-NP induces subcellular translocation of hnRNP A1 from the nucleus to the cytoplasm, which accelerates the increase in Drp1 expression in hnRNP A1 over-expressing cells. Collectively, our findings suggest that hnRNP A1 controls mitochondrial dynamics by post-transcriptional regulation of Drp1.

•

hnRNP A1 increases Drp1 expression through the interaction with 3′UTR of Drp1 mRNA.

•

Down-regulation of hnRNP A1 increases mitochondrial elongation by reducing drp1 expression.

•

Down-regulation of hnRNPA1 inhibits 3-NP-mediated mitochondrial dysfunction.

•

Over-expression of hnRNP A1 potentiates 3-NP-mediated mitochondrial dysfunction and cell death.

•

Treatment of 3-NP promotes translocation of hnRNP A1 to the cytoplasm and enhances Drp1 expression.

Mitochondrial division inhibitor 1 protects against mutant huntingtin-induced abnormal mitochondrial dynamics and neuronal damage in Huntington's disease

The objective of this study was to determine the protective effects of the mitochondrial division inhibitor 1 (Mdivi1) in striatal neurons that stably express mutant Htt (STHDhQ111/Q111) and wild-type (WT) Htt (STHDhQ7/Q7). Using gene expression analysis, biochemical methods, transmission electron microscopy (TEM) and confocal microscopy methods, we studied (i) mitochondrial and synaptic activities by measuring mRNA and the protein levels of mitochondrial and synaptic genes, (ii) mitochondrial function and (iii) ultra-structural changes in mutant Htt neurons relative to WT Htt neurons. We also studied these parameters in Mdivil-treated and untreated WT and mutant Htt neurons. Increased expressions of mitochondrial fission genes, decreased expression of fusion genes and synaptic genes were found in the mutant Htt neurons relative to the WT Htt neurons. Electron microscopy of the mutant Htt neurons revealed a significantly increased number of mitochondria, indicating that mutant Htt fragments mitochondria. Biochemical analysis revealed defective mitochondrial functioning. In the Mdivil-treated mutant Htt neurons, fission genes were down-regulated, and fusion genes were up-regulated, suggesting that Mdivil decreases fission activity. Synaptic genes were up-regulated, and mitochondrial function was normal in the Mdivi1-treated mutant Htt neurons. Immunoblotting findings of mitochondrial and synaptic proteins agreed with mRNA findings. The TEM studies revealed that increased numbers of structurally intact mitochondria were present in Mdivi1-treated mutant Htt neurons. Increased synaptic and mitochondrial fusion genes and decreased fission genes were found in the Mdivi1-treated WT Htt neurons, indicating that Mdivi1 beneficially affects healthy neurons. Taken together, these findings suggest that Mdivi1 is protective against mutant Htt-induced mitochondrial and synaptic damage in HD neurons and that Mdivi1 may be a promising molecule for the treatment of HD patients.

Connectivity mapping uncovers small molecules that modulate neurodegeneration in Huntington's disease models

Huntington's disease (HD) is a genetic disease caused by a CAG trinucleotide repeat expansion encoding a polyglutamine tract in the huntingtin (HTT) protein, ultimately leading to neuronal loss and consequent cognitive decline and death. As no treatments for HD currently exist, several chemical screens have been performed using cell-based models of mutant HTT toxicity. These screens measured single disease-related endpoints, such as cell death, but had low 'hit rates' and limited dimensionality for therapeutic detection. Here, we have employed gene expression microarray analysis of HD samples--a snapshot of the expression of 25,000 genes--to define a gene expression signature for HD from publically available data. We used this information to mine a database for chemicals positively and negatively correlated to the HD gene expression signature using the Connectivity Map, a tool for comparing large sets of gene expression patterns. Chemicals with negatively correlated expression profiles were highly enriched for protective characteristics against mutant HTT fragment toxicity in in vitro and in vivo models. This study demonstrates the potential of using gene expression to mine chemical activity, guide chemical screening, and detect potential novel therapeutic compounds.

KEY MESSAGES

Single-endpoint chemical screens have low therapeutic discovery hit-rates. In the context of HD, we guided a chemical screen using gene expression data. The resulting chemicals were highly enriched for suppressors of mutant HTT fragment toxicity. This study provides a proof of concept for wider usage in all chemical screening.

Splitting up the powerhouse: structural insights into the mechanism of mitochondrial fission

Mitochondria are dynamic organelles whose shape is regulated by the opposing processes of fission and fusion, operating in conjunction with organelle distribution along the cytoskeleton. The importance of fission and fusion homeostasis has been highlighted by a number of disease states linked to mutations in proteins involved in regulating mitochondrial morphology, in addition to changes in mitochondrial dynamics in Alzheimer's, Huntington's and Parkinson's diseases. While a number of mitochondrial morphology proteins have been identified, how they co-ordinate to assemble the fission apparatus is not clear. In addition, while the master mediator of mitochondrial fission, dynamin-related protein 1, is conserved throughout evolution, the adaptor proteins involved in its mitochondrial recruitment are not. This review focuses on our current understanding of mitochondrial fission and the proteins that regulate this process in cell homeostasis, with a particular focus on the recent mechanistic insights based on protein structures.

Garrison Institute on Aging: A New Hope for Elderly Individuals and Patients with Alzheimer's Disease

The Garrison Institute on Aging (GIA) is an established institute within Texas Tech University Health Sciences Center, whose mission is to promote healthy aging through cutting-edge research on Alzheimer's disease (AD) and other diseases of aging through innovative educational opportunities for students, clinicians, researchers, health care professionals, and the public. The GIA has multiple programs, including both research and education on healthy aging and AD, community outreach, caregiving, the Retired Senior Volunteer Program, Healthy Lubbock, the GIA Brain Bank, healthy aging seminars, research seminars, and collaborations and scholarships. The GIA programs connect basic and clinical researchers and health care professionals, and provide a unique environment to help our growing elderly population and patients with AD and their families.

Mitochondrial dynamics and quality control in Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by polyglutamine expansion mutations in the huntingtin protein. Despite its ubiquitous distribution, expression of mutant huntingtin (mHtt) is particularly detrimental to medium spiny neurons within the striatum. Mitochondrial dysfunction has been associated with HD pathogenesis. Here we review the current evidence for mHtt-induced abnormalities in mitochondrial dynamics and quality control, with a particular focus on brain and neuronal data pertaining to striatal vulnerability. We address mHtt effects on mitochondrial biogenesis, protein import, complex assembly, fission and fusion, mitochondrial transport, and on the degradation of damaged mitochondria via autophagy (mitophagy). For an integrated perspective on potentially converging pathogenic mechanisms, we also address impaired autophagosomal transport and abnormal mHtt proteostasis in HD.

Optical coherence tomography findings in Huntington's disease: a potential biomarker of disease progression

Previous reports of ocular abnormalities in Huntington's disease (HD) have detailed eye movement disorders. The objective of this case-control study was to investigate optic nerve and macular morphology in HD using optical coherence tomography (OCT). A total of 26 HD patients and 29 controls underwent a thorough ophthalmic examination including spectral domain OCT scans of the macula and peripapillary retinal nerve fibre layer (RNFL). Genetic testing results, disease duration, HD disease burden scores and Unified HD Rating Scale motor scores were acquired for HD patients. Temporal RNFL thickness was significantly reduced in the HD group (62.3 vs. 69.8 μm, p = 0.005), and there was a significant negative correlation between temporal RNFL thickness and disease duration (R (2) = -0.51, p = 0.04). Average peripapillary RNFL thickness was not significantly different between the HD and control groups. There was a significant negative correlation between macular volume and disease duration (R (2) = -0.71, p = 0.002), and motor scores (R (2) = -0.56, p = 0.01). Colour vision was significantly poorer in the HD group. Temporal RNFL is preferentially thinned in HD patients, possibly implicating mitochondrial dysfunction as the temporal RNFL is reduced in the patients with some mitochondrial disorders, including Leber's hereditary optic neuropathy. The correlation between the decrease in macular volume and temporal RNFL, and increasing disease severity suggests that OCT may be a useful biomarker for disease progression in HD. Larger, longitudinal studies are required.

Huntington disease: pathogenesis and treatment

Huntington disease (HD) is an autosomal dominant inherited neurodegenerative disease characterized by progressive motor, behavioral, and cognitive decline, culminating in death. It is caused by an expanded CAG repeat in the huntingtin gene. Even years before symptoms become overt, mutation carriers show subtle but progressive striatal and cerebral white matter atrophy by volumetric MRI. Although there is currently no direct treatment of HD, management options are available for several symptoms. A better understanding of HD pathogenesis, and more sophisticated clinical trials using newer biomarkers, may lead to meaningful treatments. This article reviews the current knowledge of HD pathogenesis and treatment.

Cdk5-mediated mitochondrial fission: A key player in dopaminergic toxicity in Huntington's disease

The molecular mechanisms underlying striatal vulnerability in Huntington's disease (HD) are still unknown. However, growing evidence suggest that mitochondrial dysfunction could play a major role. In searching for a potential link between striatal neurodegeneration and mitochondrial defects we focused on cyclin-dependent kinase 5 (Cdk5). Here, we demonstrate that increased mitochondrial fission in mutant huntingtin striatal cells can be a consequence of Cdk5-mediated alterations in Drp1 subcellular distribution and activity since pharmacological or genetic inhibition of Cdk5 normalizes Drp1 function ameliorating mitochondrial fragmentation. Interestingly, mitochondrial defects in mutant huntingtin striatal cells can be worsened by D1 receptor activation a process also mediated by Cdk5 as down-regulation of Cdk5 activity abrogates the increase in mitochondrial fission, the translocation of Drp1 to the mitochondria and the raise of Drp1 activity induced by dopaminergic stimulation. In sum, we have demonstrated a new role for Cdk5 in HD pathology by mediating dopaminergic neurotoxicity through modulation of Drp1-induced mitochondrial fragmentation, which underscores the relevance for pharmacologic interference of Cdk5 signaling to prevent or ameliorate striatal neurodegeneration in HD.

Implications of mitochondrial dynamics on neurodegeneration and on hypothalamic dysfunction

Mitochondrial dynamics is a term that encompasses the movement of mitochondria along the cytoskeleton, regulation of their architecture, and connectivity mediated by tethering and fusion/fission. The importance of these events in cell physiology and pathology has been partially unraveled with the identification of the genes responsible for the catalysis of mitochondrial fusion and fission. Mutations in two mitochondrial fusion genes (MFN2 and OPA1) cause neurodegenerative diseases, namely Charcot-Marie Tooth type 2A and autosomal dominant optic atrophy (ADOA). Alterations in mitochondrial dynamics may be involved in the pathophysiology of prevalent neurodegenerative conditions. Moreover, impairment of the activity of mitochondrial fusion proteins dysregulates the function of hypothalamic neurons, leading to alterations in food intake and in energy homeostasis. Here we review selected findings in the field of mitochondrial dynamics and their relevance for neurodegeneration and hypothalamic dysfunction.

Oxidative metabolism in YAC128 mouse model of Huntington's disease

Alterations in oxidative metabolism are considered to be one of the major contributors to Huntington's disease (HD) pathogenesis. However, existing data about oxidative metabolism in HD are contradictory. Here, we investigated the effect of mutant huntingtin (mHtt) on oxidative metabolism in YAC128 mice. Both mHtt and wild-type huntingtin (Htt) were associated with mitochondria and the amount of bound Htt was four-times higher than the amount of bound mHtt. Percoll gradient-purified brain synaptic and non-synaptic mitochondria as well as unpurified brain, liver and heart mitochondria, isolated from 2- and 10-month-old YAC128 mice and age-matched WT littermates had similar respiratory rates. There was no difference in mitochondrial membrane potential or ADP and ATP levels. Expression of selected nuclear-encoded mitochondrial proteins in 2- and 10-month-old YAC128 and WT mice was similar. Cultured striatal and cortical neurons from YAC128 and WT mice had similar respiratory and glycolytic activities as measured with Seahorse XF24 analyzer in medium containing 10 mm glucose and 15 mm pyruvate. In the medium with 2.5 mm glucose, YAC128 striatal neurons had similar respiration, but slightly lower glycolytic activity. Striatal neurons had lower maximal respiration compared with cortical neurons. In vivo experiments with YAC128 and WT mice showed similar O2 consumption, CO2 release, physical activity, food consumption and fasted blood glucose. However, YAC128 mice were heavier and had more body fat compared with WT mice. Overall, our data argue against respiratory deficiency in YAC128 mice and, consequently, suggest that mitochondrial respiratory dysfunction is not essential for HD pathogenesis.

The choreography of neuroinflammation in Huntington's disease

Currently, the concept of 'neuroinflammation' includes inflammation associated with neurodegenerative diseases, in which there is little or no infiltration of blood-derived immune cells into the brain. The roles of brain-resident and peripheral immune cells in these inflammatory settings are poorly understood, and it is unclear whether neuroinflammation results from immune reaction to neuronal dysfunction/degeneration, and/or represents cell-autonomous phenotypes of dysfunctional immune cells. Here, we review recent studies examining these questions in the context of Huntington's disease (HD), where mutant Huntingtin (HTT) is expressed in both neurons and glia. Insights into the cellular and molecular mechanisms underlying neuroinflammation in HD may provide a better understanding of inflammation in more complex neurodegenerative disorders, and of the contribution of the neuroinflammatory component to neurodegenerative disease pathogenesis.

An update on Huntington's disease: from the gene to the clinic

PURPOSE OF REVIEW

This review highlights the recent advances in Huntington's disease, with a particular focus on development of disease biomarkers for use in therapeutic trials in the premotor phase of the disease, as well as the growing literature regarding pathophysiological mechanisms and their relevance to potential therapeutic targets.

RECENT FINDINGS

There have been continued advances in the development of disease biomarkers, and promising neuroprotection trials are beginning to emerge in the premotor stage of Huntington's disease. Deeper understanding of the pathophysiological mechanisms is being translated into potential therapeutic strategies.

SUMMARY

The premotor stage of Huntington's disease provides an ideal time to trial disease-modifying therapy, but reliable biomarkers are required for monitoring disease progression, and this remains an area of intense research. Our understanding of the underlying pathophysiological mechanisms continues to expand, and a number of promising therapeutic strategies are emerging, including strategies to silence mutant huntingtin expression.

Mitochondrial iron homeostasis and its dysfunctions in neurodegenerative disorders

Synthesis of the iron-containing prosthetic groups-heme and iron-sulfur clusters-occurs in mitochondria. The mitochondrion is also an important producer of reactive oxygen species (ROS), which are derived from electrons leaking from the electron transport chain. The coexistence of both ROS and iron in the secluded space of the mitochondrion makes this organelle particularly prone to oxidative damage. Here, we review the elements that configure mitochondrial iron homeostasis and discuss the principles of iron-mediated ROS generation in mitochondria. We also review the evidence for mitochondrial dysfunction and iron accumulation in Alzheimer's disease, Huntington Disease, Friedreich's ataxia, and in particular Parkinson's disease. We postulate that a positive feedback loop of mitochondrial dysfunction, iron accumulation, and ROS production accounts for the process of cell death in various neurodegenerative diseases in which these features are present.

Inhibitors of mitochondrial fission as a therapeutic strategy for diseases with oxidative stress and mitochondrial dysfunction

Mitochondria are essential cytoplasmic organelles, critical for cell survival and death. Recent mitochondrial research revealed that mitochondrial dynamics-the balance of fission and fusion in normal mitochondrial dynamics--is an important cellular mechanism in eukaryotic cell and is involved in the maintenance of mitochondrial morphology, structure, number, distribution, and function. Research into mitochondria and cell function has revealed that mitochondrial dynamics is impaired in a large number of aging and neurodegenerative diseases, and in several inherited mitochondrial diseases, and that this impairment involves excessive mitochondrial fission, resulting in mitochondrial structural changes and dysfunction, and cell damage. Attempts have been made to develop molecules to reduce mitochondrial fission while maintaining normal mitochondrial fusion and function in those diseases that involve excessive mitochondrial fission. This review article discusses mechanisms of mitochondrial fission in normal and diseased states of mammalian cells and discusses research aimed at developing therapies, such as Mdivi, Dynasore and P110, to prevent or to inhibit excessive mitochondrial fission.