Mitochondrial matters of the brain: the role in Huntington’s disease

Targeted to neuronal organelles for CNS drug development

Significant evidences indicate that sub-cellular organelle dynamics is critical for both physiological and pathological events and therefore may be attractive drug targets displaying great therapeutic potential. Although the basic biological mechanism underlying the dynamics of intracellular organelles has been extensively studied, relative drug development is still limited. In the present review, we show that due to the development of technical advanced imaging tools, especially live cell imaging methods, intracellular organelle dynamics (including mitochondrial dynamics and membrane contact sites) can be dissected at the molecular level. Based on these identified molecular targets, we review and discuss the potential of drug development to target organelle dynamics, especially mitochondria dynamics and ER-organelle membrane contact dynamics, in the central nervous system for treating human diseases, including neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis.

HSF1 and Its Role in Huntington's Disease Pathology

PURPOSE OF REVIEW

Heat shock factor 1 (HSF1) is the master transcriptional regulator of the heat shock response (HSR) in mammalian cells and is a critical element in maintaining protein homeostasis. HSF1 functions at the center of many physiological processes like embryogenesis, metabolism, immune response, aging, cancer, and neurodegeneration. However, the mechanisms that allow HSF1 to control these different biological and pathophysiological processes are not fully understood. This review focuses on Huntington's disease (HD), a neurodegenerative disease characterized by severe protein aggregation of the huntingtin (HTT) protein. The aggregation of HTT, in turn, leads to a halt in the function of HSF1. Understanding the pathways that regulate HSF1 in different contexts like HD may hold the key to understanding the pathomechanisms underlying other proteinopathies. We provide the most current information on HSF1 structure, function, and regulation, emphasizing HD, and discussing its potential as a biological target for therapy.

DATA SOURCES

We performed PubMed search to find established and recent reports in HSF1, heat shock proteins (Hsp), HD, Hsp inhibitors, HSF1 activators, and HSF1 in aging, inflammation, cancer, brain development, mitochondria, synaptic plasticity, polyglutamine (polyQ) diseases, and HD.

STUDY SELECTIONS

Research and review articles that described the mechanisms of action of HSF1 were selected based on terms used in PubMed search.

RESULTS

HSF1 plays a crucial role in the progression of HD and other protein-misfolding related neurodegenerative diseases. Different animal models of HD, as well as postmortem brains of patients with HD, reveal a connection between the levels of HSF1 and HSF1 dysfunction to mutant HTT (mHTT)-induced toxicity and protein aggregation, dysregulation of the ubiquitin-proteasome system (UPS), oxidative stress, mitochondrial dysfunction, and disruption of the structural and functional integrity of synaptic connections, which eventually leads to neuronal loss. These features are shared with other neurodegenerative diseases (NDs). Currently, several inhibitors against negative regulators of HSF1, as well as HSF1 activators, are developed and hold promise to prevent neurodegeneration in HD and other NDs.

CONCLUSION

Understanding the role of HSF1 during protein aggregation and neurodegeneration in HD may help to develop therapeutic strategies that could be effective across different NDs.

Ceramide and Sphingosine-1-Phosphate in Neurodegenerative Disorders and Their Potential Involvement in Therapy

Neurodegenerative disorders (ND) are progressive diseases of the nervous system, often without resolutive therapy. They are characterized by a progressive impairment and loss of specific brain regions and neuronal populations. Cellular and animal model studies have identified several molecular mechanisms that play an important role in the pathogenesis of ND. Among them are alterations of lipids, in particular sphingolipids, that play a crucial role in neurodegeneration. Overall, during ND, ceramide-dependent pro-apoptotic signalling is promoted, whereas levels of the neuroprotective spingosine-1-phosphate are reduced. Moreover, ND are characterized by alterations of the metabolism of complex sphingolipids. The finding that altered sphingolipid metabolism has a role in ND suggests that its modulation might provide a useful strategy to identify targets for possible therapies. In this review, based on the current literature, we will discuss how bioactive sphingolipids (spingosine-1-phosphate and ceramide) are involved in some ND (Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis) and their possible involvement in therapies.

Neurodegenerative Disorders: Spotlight on Sphingolipids

Neurodegenerative diseases are incurable diseases of the nervous system that lead to a progressive loss of brain areas and neuronal subtypes, which is associated with an increase in symptoms that can be linked to the affected brain areas. The key findings that appear in many neurodegenerative diseases are deposits of proteins and the damage of mitochondria, which mainly affect energy production and mitophagy. Several causative gene mutations have been identified in various neurodegenerative diseases; however, a large proportion are considered sporadic. In the last decade, studies linking lipids, and in particular sphingolipids, to neurodegenerative diseases have shown the importance of these sphingolipids in the underlying pathogenesis. Sphingolipids are bioactive lipids consisting of a sphingoid base linked to a fatty acid and a hydrophilic head group. They are involved in various cellular processes, such as cell growth, apoptosis, and autophagy, and are an essential component of the brain. In this review, we will cover key findings that demonstrate the relevance of sphingolipids in neurodegenerative diseases and will focus on neurodegeneration with brain iron accumulation and Parkinson’s disease.

Free carnitine and branched chain amino acids are not good biomarkers in Huntington's disease

BACKGROUND

Huntington's disease (HD), caused by an expanded CAG repeat at HTT, has no treatment, and biomarkers are needed for future clinical trials.

OBJECTIVE

The objective of this study was to verify if free carnitine and branched chain amino acids levels behave as potential biomarkers in HD.

METHODS

Symptomatic and asymptomatic HD carriers and controls were recruited. Age, sex, body mass index (BMI), age of onset, disease duration, UHDRS scores, and expanded CAG tract were obtained; valine, leucine, isoleucine, and free carnitine were measured. Baseline and longitudinal analysis were performed.

RESULTS

Seventy-four symptomatic carriers, 20 asymptomatic carriers, and 22 non-carriers were included. At baseline, valine levels were reduced in symptomatic and asymptomatic HD carriers when compared to non-carriers. No difference in free carnitine or isoleucine+leucine levels were observed between groups. BMI of symptomatic individuals was lower than those of non-carriers. Valine levels correlated with BMI. Follow-up evaluation was performed in 43 symptomatic individuals. UHDRS total motor score increased 4.8 points/year on average. No significant reductions in BMI or valine were observed, whereas free carnitine and isoleucine+leucine levels increased.

CONCLUSIONS

Although valine levels were lower in HD carriers and were related to BMI losses observed in pre-symptomatic individuals, none of these metabolites seem to be biomarkers for HD.

Mitochondria, Microglia, and the Immune System-How Are They Linked in Affective Disorders?

Major depressive disorder (MDD) is a severe mood disorder and frequently associated with alterations of the immune system characterized by enhanced levels of circulating pro-inflammatory cytokines and microglia activation in the brain. Increasing evidence suggests that dysfunction of mitochondria may play a key role in the pathogenesis of MDD. Mitochondria are regulators of numerous cellular functions including energy metabolism, maintenance of redox and calcium homeostasis, and cell death and therefore modulate many facets of the innate immune response. In depression-like behavior of rodents, mitochondrial perturbation and release of mitochondrial components have been shown to boost cytokine production and neuroinflammation. On the other hand, pro-inflammatory cytokines may influence mitochondrial functions such as oxidative phosphorylation, production of adenosine triphosphate, and reactive oxygen species, thereby aggravating inflammation. There is strong interest in a better understanding of immunometabolic pathways in MDD that may serve as diagnostic markers and therapeutic targets. Here, we review the interaction between mitochondrial metabolism and innate immunity in the pathophysiology of MDD. We specifically focus on immunometabolic processes that govern microglial and peripheral myeloid cell functions, both cellular components involved in neuroinflammation in depression-like behavior. We finally discuss microglial polarization and associated metabolic states in depression-associated behavior and in MDD.

Fast-to-Slow Transition of Skeletal Muscle Contractile Function and Corresponding Changes in Myosin Heavy and Light Chain Formation in the R6/2 Mouse Model of Huntington's Disease

Huntington´s disease (HD) is a hereditary neurodegenerative disease resulting from an expanded polyglutamine sequence (poly-Q) in the protein huntingtin (HTT). Various studies report atrophy and metabolic pathology of skeletal muscle in HD and suggest as part of the process a fast-to-slow fiber type transition that may be caused by the pathological changes in central motor control or/and by mutant HTT in the muscle tissue itself. To investigate muscle pathology in HD, we used R6/2 mice, a common animal model for a rapidly progressing variant of the disease expressing exon 1 of the mutant human gene. We investigated alterations in the extensor digitorum longus (EDL), a typical fast-twitch muscle, and the soleus (SOL), a slow-twitch muscle. We focussed on mechanographic measurements of excised muscles using single and repetitive electrical stimulation and on the expression of the various myosin isoforms (heavy and light chains) using dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) of whole muscle and single fiber preparations. In EDL of R6/2, the functional tests showed a left shift of the force-frequency relation and decrease in specific force. Moreover, the estimated relative contribution of the fastest myosin isoform MyHC IIb decreased, whereas the contribution of the slower MyHC IIx isoform increased. An additional change occurred in the alkali MyLC forms showing a decrease in 3f and an increase in 1f level. In SOL, a shift from fast MyHC IIa to the slow isoform I was detectable in male R6/2 mice only, and there was no evidence of isoform interconversion in the MyLC pattern. These alterations point to a partial remodeling of the contractile apparatus of R6/2 mice towards a slower contractile phenotype, predominantly in fast glycolytic fibers.

The rate of training response to aerobic exercise affects brain function of rats

There is an increasing volume of data connecting capacity to respond to exercise training with quality of life and aging. In this study, we used a rat model in which animals were selectively bred for low and high gain in running distance to test t whether genetic segregation for trainability is associated with brain function and signaling processes in the hippocampus. Rats selected for low response (LRT) and high response training (HRT) were randomly divided into control or exercise group that trained five times a week for 30 min per day for three months at 70% VO2max. All four groups had similar running distance before training. With training, HRT rats showed significantly greater increases in VO2max and running distance than LRT rats (p < 0.05). On the reverse Morris Maze test HRT-trained rats outperformed HRT control ones. Significant difference was noted between LRT and HRT groups in redox milieu as assessed by levels of reactive oxygen species (ROS), carbonylation of proteins, nNOS and S-nitroso-cysteine. Moreover the silent information regulator 1 (SIRT1), brain-derived neurotrophic factor (BDNF), ratio of phospho and total cAMP-response element binding protein (CREB), and apoptotic index, also showed significant differences between LRT and HRT groups. These findings suggest that aerobic training responses are not localized to skeletal muscle, but differently involve signaling processes in the brain of LRT and HRT rats.

Altered Ca(2+) signaling in skeletal muscle fibers of the R6/2 mouse, a model of Huntington's disease

Huntington's disease (HD) is caused by an expanded CAG trinucleotide repeat within the gene encoding the protein huntingtin. The resulting elongated glutamine (poly-Q) sequence of mutant huntingtin (mhtt) affects both central neurons and skeletal muscle. Recent reports suggest that ryanodine receptor-based Ca(2+) signaling, which is crucial for skeletal muscle excitation-contraction coupling (ECC), is changed by mhtt in HD neurons. Consequently, we searched for alterations of ECC in muscle fibers of the R6/2 mouse, a mouse model of HD. We performed fluorometric recordings of action potentials (APs) and cellular Ca(2+) transients on intact isolated toe muscle fibers (musculi interossei), and measured L-type Ca(2+) inward currents on internally dialyzed fibers under voltage-clamp conditions. Both APs and AP-triggered Ca(2+) transients showed slower kinetics in R6/2 fibers than in fibers from wild-type mice. Ca(2+) removal from the myoplasm and Ca(2+) release flux from the sarcoplasmic reticulum were characterized using a Ca(2+) binding and transport model, which indicated a significant reduction in slow Ca(2+) removal activity and Ca(2+) release flux both after APs and under voltage-clamp conditions. In addition, the voltage-clamp experiments showed a highly significant decrease in L-type Ca(2+) channel conductance. These results indicate profound changes of Ca(2+) turnover in skeletal muscle of R6/2 mice and suggest that these changes may be associated with muscle pathology in HD.

Enhanced neuronal glucose transporter expression reveals metabolic choice in a HD Drosophila model

Huntington’s disease is a neurodegenerative disorder caused by toxic insertions of polyglutamine residues in the Huntingtin protein and characterized by progressive deterioration of cognitive and motor functions. Altered brain glucose metabolism has long been suggested and a possible link has been proposed in HD. However, the precise function of glucose transporters was not yet determined. Here, we report the effects of the specifically-neuronal human glucose transporter expression in neurons of a Drosophila model carrying the exon 1 of the human huntingtin gene with 93 glutamine repeats (HQ93). We demonstrated that overexpression of the human glucose transporter in neurons ameliorated significantly the status of HD flies by increasing their lifespan, reducing their locomotor deficits and rescuing eye neurodegeneration. Then, we investigated whether increasing the major pathways of glucose catabolism, glycolysis and pentose-phosphate pathway (PPP) impacts HD. To mimic increased glycolytic flux, we overexpressed phosphofructokinase (PFK) which catalyzes an irreversible step in glycolysis. Overexpression of PFK did not affect HQ93 fly survival, but protected from photoreceptor loss. Overexpression of glucose-6-phosphate dehydrogenase (G6PD), the key enzyme of the PPP, extended significantly the lifespan of HD flies and rescued eye neurodegeneration. Since G6PD is able to synthesize NADPH involved in cell survival by maintenance of the redox state, we showed that tolerance to experimental oxidative stress was enhanced in flies co-expressing HQ93 and G6PD. Additionally overexpressions of hGluT3, G6PD or PFK were able to circumvent mitochondrial deficits induced by specific silencing of genes necessary for mitochondrial homeostasis. Our study confirms the involvement of bioenergetic deficits in HD course; they can be rescued by specific expression of a glucose transporter in neurons. Finally, the PPP and, to a lesser extent, the glycolysis seem to mediate the hGluT3 protective effects, whereas, in addition, the PPP provides increased protection to oxidative stress.

Mass spectrometry and redox proteomics: applications in disease

Proteomics techniques are continuously being developed to further understanding of biology and disease. Many of the pathways that are relevant to disease mechanisms rely on the identification of post-translational modifications (PTMs) such as phosphorylation, acetylation, and glycosylation. Much attention has also been focused on oxidative PTMs which include protein carbonyls, protein nitration, and the incorporation of fatty acids and advanced glycation products to amino acid side chains, amongst others. The introduction of these PTMs in the cell can occur due to the attack of reactive oxygen and nitrogen species (ROS and RNS, respectively) on proteins. ROS and RNS can be present as a result of normal metabolic processes as well as external factors such as UV radiation, disease, and environmental toxins. The imbalance of ROS and RNS with antioxidant cellular defenses leads to a state of oxidative stress, which has been implicated in many diseases. Redox proteomics techniques have been used to characterize oxidative PTMs that result as a part of normal cell signaling processes as well as oxidative stress conditions. This review highlights many of the redox proteomics techniques which are currently available for several oxidative PTMs and brings to the reader's attention the application of redox proteomics for understanding disease pathogenesis in neurodegenerative disorders and others such as cancer, kidney, and heart diseases.

Mitochondrial membrane fluidity is consistently increased in different models of Huntington disease: restorative effects of olesoxime

Huntington disease (HD) is a fatal neurodegenerative disorder caused by a CAG repeat expansion in exon 1 of the huntingtin gene (HTT). One prominent target of the mutant huntingtin protein (mhtt) is the mitochondrion, affecting its morphology, distribution, and function. Thus, mitochondria have been suggested as potential therapeutic targets for the treatment of HD. Olesoxime, a cholesterol-like compound, promotes motor neuron survival and neurite outgrowth in vitro, and its effects are presumed to occur via a direct interaction with mitochondrial membranes (MMs). We examined the properties of MMs isolated from cell and animal models of HD as well as the effects of olesoxime on MM fluidity and cholesterol levels. MMs isolated from brains of aged Hdh Q111/Q111 knock-in mice showed a significant decrease in 1,6-diphenyl-hexatriene (DPH) anisotropy, which is inversely correlated with membrane fluidity. Similar increases in MM fluidity were observed in striatal STHdh Q111/Q111 cells as well as in MMs isolated from brains of BACHD transgenic rats. Treatment of STHdh cells with olesoxime decreased the fluidity of isolated MMs. Decreased membrane fluidity was also measured in olesoxime-treated MMs isolated from brains of HD knock-in mice. In both models, treatment with olesoxime restored HD-specific changes in MMs. Accordingly, olesoxime significantly counteracted the mhtt-induced increase in MM fluidity of MMs isolated from brains of BACHD rats after 12 months of treatment in vivo, possibly by enhancing MM cholesterol levels. Thus, olesoxime may represent a novel pharmacological tool to treat mitochondrial dysfunction in HD.

The anti-apoptotic and anti-inflammatory properties of puerarin attenuate 3-nitropropionic-acid induced neurotoxicity in rats

Puerarin (Pur), an isoflavonoid extracted from the dried roots of Pueraria lobata, has been reported to be useful in the treatment of various diseases. This study was designed to evaluate the anti-apoptotic and anti-inflammatory activities of Pur against 3-nitropropionic acid (3-NP) induced neurotoxicity. For 5 consecutive days, male Wistar rats were given Pur (200 mg/kg body mass) 30 min before treatment with 20 mg/kg body mass of 3-NP. The striata, hippocampi, and cortices of the 3-NP treated group showed apoptotic damage, inflammation, and energy deficit as well as histopathological lesions. The 3-NP-induced alteration in apoptotic biomarkers (caspase-3 activity/level, cytosolic cytochrome c, Bax/Bcl-2 levels) were significantly ameliorated by Pur treatment. Moreover, Pur pretreatment blocked 3-NP-induced inflammatory biomarkers (NF-κB, TNF-α, and iNOS) and prevented the energy deficit (ATP reduction). Nissl staining further confirmed Pur's neuroprotective effect. These results indicate that Pur may be a useful preventive approach to various neurodegenerative diseases with underlying apoptosis and neuroinflammation.

Mitochondrial diseases of the brain

Neurodegenerative disorders are debilitating diseases of the brain, characterized by behavioral, motor and cognitive impairments. Ample evidence underpins mitochondrial dysfunction as a central causal factor in the pathogenesis of neurodegenerative disorders including Parkinson's disease, Huntington's disease, Alzheimer's disease, Amyotrophic lateral sclerosis, Friedreich's ataxia and Charcot-Marie-Tooth disease. In this review, we discuss the role of mitochondrial dysfunction such as bioenergetics defects, mitochondrial DNA mutations, gene mutations, altered mitochondrial dynamics (mitochondrial fusion/fission, morphology, size, transport/trafficking, and movement), impaired transcription and the association of mutated proteins with mitochondria in these diseases. We highlight the therapeutic role of mitochondrial bioenergetic agents in toxin and in cellular and genetic animal models of neurodegenerative disorders. We also discuss clinical trials of bioenergetics agents in neurodegenerative disorders. Lastly, we shed light on PGC-1α, TORC-1, AMP kinase, Nrf2-ARE, and Sirtuins as novel therapeutic targets for neurodegenerative disorders.

SUMO-2 and PIAS1 modulate insoluble mutant huntingtin protein accumulation

A key feature in Huntington disease (HD) is the accumulation of mutant Huntingtin (HTT) protein, which may be regulated by posttranslational modifications. Here, we define the primary sites of SUMO modification in the amino-terminal domain of HTT, show modification downstream of this domain, and demonstrate that HTT is modified by the stress-inducible SUMO-2. A systematic study of E3 SUMO ligases demonstrates that PIAS1 is an E3 SUMO ligase for both HTT SUMO-1 and SUMO-2 modification and that reduction of dPIAS in a mutant HTT Drosophila model is protective. SUMO-2 modification regulates accumulation of insoluble HTT in HeLa cells in a manner that mimics proteasome inhibition and can be modulated by overexpression and acute knockdown of PIAS1. Finally, the accumulation of SUMO-2-modified proteins in the insoluble fraction of HD postmortem striata implicates SUMO-2 modification in the age-related pathogenic accumulation of mutant HTT and other cellular proteins that occurs during HD progression.

Targeting sirtuin-1 in Huntington's disease: rationale and current status

Huntington's disease (HD) is an autosomal dominant hereditary disease caused by a trinucleotide repeat mutation in the huntingtin gene that results in an increased number of glutamine residues in the N terminus of huntingtin protein. Mutant huntingtin leads to progressive impairment of motor function, cognitive dysfunction, and neuropsychiatric disturbance. There are no disease-modifying treatments available. During the past decade, sirtuin-1 (SIRT1) has been the focus of intense investigation and discussion because it regulates longevity in multiple organisms and has shown beneficial effects in a variety of models of neurodegenerative disorders. Studies in different animal models provide convincing evidence that SIRT1 protects neurons in mouse models of HD as well as in Caenorhabditis elegans, although controversial results were reported in a fly model. Indeed, many connections exist between the deacetylation function of SIRT1 and its role in neuroprotection. As a result, pharmacological interventions targeting SIRT1 might become promising strategies to combat HD. This review summarizes recent progress in SIRT1 research, with a focus on the specificity of this protein as a potential therapeutic target for HD, as well as existing challenges for developing SIRT1 modulators for clinical use.

Impaired mitochondrial oxidative phosphorylation in the peroxisomal disease X-linked adrenoleukodystrophy

X-linked adrenoleukodystrophy (X-ALD) is an inherited metabolic disorder of the nervous system characterized by axonopathy in spinal cords and/or cerebral demyelination, adrenal insufficiency and accumulation of very long-chain fatty acids (VLCFAs) in plasma and tissues. The disease is caused by malfunction of the ABCD1 gene, which encodes a peroxisomal transporter of VLCFAs or VLCFA-CoA. In the mouse, Abcd1 loss causes late onset axonal degeneration in the spinal cord, associated with locomotor disability resembling the most common phenotype in patients, adrenomyeloneuropathy. We have formerly shown that an excess of the VLCFA C26:0 induces oxidative damage, which underlies the axonal degeneration exhibited by the Abcd1(-) mice. In the present study, we sought to investigate the noxious effects of C26:0 on mitochondria function. Our data indicate that in X-ALD patients' fibroblasts, excess of C26:0 generates mtDNA oxidation and specifically impairs oxidative phosphorylation (OXPHOS) triggering mitochondrial ROS production from electron transport chain complexes. This correlates with impaired complex V phosphorylative activity, as visualized by high-resolution respirometry on spinal cord slices of Abcd1(-) mice. Further, we identified a marked oxidation of key OXPHOS system subunits in Abcd1(-) mouse spinal cords at presymptomatic stages. Altogether, our results illustrate some of the mechanistic intricacies by which the excess of a fatty acid targeted to peroxisomes activates a deleterious process of oxidative damage to mitochondria, leading to a multifaceted dysfunction of this organelle. These findings may be of relevance for patient management while unveiling novel therapeutic targets for X-ALD.

Neuroprotective effects of PPAR-γ agonist rosiglitazone in N171-82Q mouse model of Huntington's disease

Huntington's disease (HD) is a devastating genetic neurodegenerative disease caused by CAG trinucleotide expansion in the exon-1 region of the huntingtin gene. Currently, no cure is available. It is becoming increasingly apparent that mutant Huntingtin (HTT) impairs metabolic homeostasis and causes transcriptional dysregulation. The peroxisome proliferator-activated receptor gamma (PPAR-γ) is a transcriptional factor that plays a key role in regulating genes involved in energy metabolism; recent studies demonstrated that PPAR-γ activation prevented mitochondrial depolarization in cells expressing mutant HTT and attenuated neurodegeneration in various models of neurodegenerative diseases. PPAR-γ-coactivator 1α (PGC-1 α) transcription activity is also impaired by mutant HTT. We now report that the PPAR-γ agonist, rosiglitazone (RSG), significantly attenuated mutant HTT-induced toxicity in striatal cells and that the protective effect of RSG is mediated by activation of PPAR-γ. Moreover, chronic administration of RSG (10 mg/kg/day, i.p) significantly improved motor function and attenuated hyperglycemia in N171-82Q HD mice. RSG administration rescued brain derived neurotrophic factor(BDNF) deficiency in the cerebral cortex, and prevented loss of orexin-A-immunopositive neurons in the hypothalamus of N171-82Q HD mice. RSG also prevented PGC-1α reduction and increased Sirt6 protein levels in HD mouse brain. Our results suggest that modifying the PPAR-γ pathway plays a beneficial role in rescuing motor function as well as glucose metabolic abnormalities in HD.

Defective mitochondrial disulfide relay system, altered mitochondrial morphology and function in Huntington's disease

A number of studies have been conducted that link mitochondrial dysfunction (MD) to Huntington's disease (HD); however, contradicting results had resulted in a lack of a clear mechanism that links expression of mutant Huntingtin protein and MD. Mouse homozygous (HM) and heterozygous (HT) mutant striatal cells with two or one allele encoding for a mutant huntingtin protein with 111 polyGln repeats showed a significant impairment of the mitochondrial disulfide relay system (MDRS). This system (consisting of two proteins, Gfer and Mia40) is involved in the mitochondrial import of Cys-rich proteins. The Gfer-to-Mia40 ratio was significantly altered in HM cells compared with controls, along with the expression of mitochondrial proteins considered substrates of the MDRS. In progenitors and differentiated neuron-like HM cells, impairment of MDRS were accompanied by deficient oxidative phosphorylation, Complex I, IV and V activities, decreased mtDNA copy number and transcripts, accumulation of mtDNA deletions and changes in mitochondrial morphology, consistent with other MDRS-deficient biological models, thus providing a framework for the energy deficits observed in this HD model. The majority (>90%) of the mitochondrial outcomes exhibited a gene-dose dependency with the expression of mutant Htt. Finally, decreases in the mtDNA copy number, along with the accumulation of mtDNA deletions, provide a mechanism for the progressive neurodegeneration observed in HD patients.

Transcriptional analysis of rat photoreceptor cells reveals daily regulation of genes important for visual signaling and light damage susceptibility

Photoreceptor cells face the challenge of adjusting their function and, possibly, their susceptibility to light damage to the marked daily changes in ambient light intensity. To achieve a better understanding of photoreceptor adaptation at the transcriptional level, this study aimed to identify genes which are under daily regulation in photoreceptor cells using microarray analysis and quantitative PCR. Included in the gene set obtained were a number of genes which up until now have not been shown to be expressed in photoreceptor cells, such as Atf3 (activating transcription factor 3) and Pde8a (phosphodiesterase 8A), and others with a known impact on phototransduction and/or photoreceptor survival, such as Grk1 (G protein-coupled receptor kinase 1) and Pgc-1α (peroxisome proliferator-activated receptor γ, coactivator 1alpha). According to their daily dynamics, the genes identified could be clustered in two groups: those with peak expression during the second part of the day which are uniformly promoted to cycle by light/dark transitions and those with peak expression during the second part of the night which are predominantly driven by a clock. Since Grk1 and Pgc-1α belong in the first group, the present results support a concept in which transcriptional regulation of genes by ambient light contributes to the functional adjustment of photoreceptor cells over the 24-h period.

Omega-3 fatty acids in neurodegenerative diseases: focus on mitochondria

Mitochondrial dysfunction represents a common early pathological event in brain aging and in neurodegenerative diseases, e.g., in Alzheimer's (AD), Parkinson's (PD), and Huntington's disease (HD), as well as in ischemic stroke. In vivo and ex vivo experiments using animal models of aging and AD, PD, and HD mainly showed improvement of mitochondrial function after treatment with polyunsaturated fatty acids (PUFA) such as docosahexaenoic acid (DHA). Thereby, PUFA are particular beneficial in animals treated with mitochondria targeting toxins. However, DHA showed adverse effects in a transgenic PD mouse model and it is not clear if a diet high or low in PUFA might provide neuroprotective effects in PD. Post-treatment with PUFA revealed conflicting results in ischemic animal models, but intravenous administered DHA provided neuroprotective efficacy after acute occlusion of the middle cerebral artery. In summary, the majority of preclinical data indicate beneficial effects of n-3 PUFA in neurodegenerative diseases, whereas most controlled clinical trials did not meet the expectations. Because of the high half-life of DHA in the human brain clinical studies may have to be initiated much earlier and have to last much longer to be more efficacious.

Primary cultures of astrocytes: their value in understanding astrocytes in health and disease

During the past few decades of astrocyte research it has become increasingly clear that astrocytes have taken a central position in all central nervous system activities. Much of our new understanding of astrocytes has been derived from studies conducted with primary cultures of astrocytes. Such cultures have been an invaluable tool for studying roles of astrocytes in physiological and pathological states. Many central astrocytic functions in metabolism, amino acid neurotransmission and calcium signaling were discovered using this tissue culture preparation and most of these observations were subsequently found in vivo. Nevertheless, primary cultures of astrocytes are an in vitro model that does not fully mimic the complex events occurring in vivo. Here we present an overview of the numerous contributions generated by the use of primary astrocyte cultures to uncover the diverse functions of astrocytes. Many of these discoveries would not have been possible to achieve without the use of astrocyte cultures. Additionally, we address and discuss the concerns that have been raised regarding the use of primary cultures of astrocytes as an experimental model system.

N-acetylcysteine normalizes neurochemical changes in the glutathione-deficient schizophrenia mouse model during development

BACKGROUND

Glutathione (GSH) is the major cellular redox-regulator and antioxidant. Redox-imbalance due to genetically impaired GSH synthesis is among the risk factors for schizophrenia. Here we used a mouse model with chronic GSH deficit induced by knockout (KO) of the key GSH-synthesizing enzyme, glutamate-cysteine ligase modulatory subunit (GCLM).

METHODS

With high-resolution magnetic resonance spectroscopy at 14.1 T, we determined the neurochemical profile of GCLM-KO, heterozygous, and wild-type mice in anterior cortex throughout development in a longitudinal study design.

RESULTS

Chronic GSH deficit was accompanied by an elevation of glutamine (Gln), glutamate (Glu), Gln/Glu, N-acetylaspartate, myo-Inositol, lactate, and alanine. Changes were predominantly present at prepubertal ages (postnatal days 20 and 30). Treatment with N-acetylcysteine from gestation on normalized most neurochemical alterations to wild-type level.

CONCLUSIONS

Changes observed in GCLM-KO anterior cortex, notably the increase in Gln, Glu, and Gln/Glu, were similar to those reported in early schizophrenia, emphasizing the link between redox imbalance and the disease and validating the model. The data also highlight the prepubertal period as a sensitive time for redox-related neurochemical changes and demonstrate beneficial effects of early N-acetylcysteine treatment. Moreover, the data demonstrate the translational value of magnetic resonance spectroscopy to study brain disease in preclinical models.

Impaired mitochondrial function in psychiatric disorders

Major psychiatric illnesses such as mood disorders and schizophrenia are chronic, recurrent mental illnesses that affect the lives of millions of individuals. Although these disorders have traditionally been viewed as 'neurochemical diseases', it is now clear that they are associated with impairments of synaptic plasticity and cellular resilience. Although most patients with these disorders do not have classic mitochondrial disorders, there is a growing body of evidence to suggest that impaired mitochondrial function may affect key cellular processes, thereby altering synaptic functioning and contributing to the atrophic changes that underlie the deteriorating long-term course of these illnesses. Enhancing mitochondrial function could represent an important avenue for the development of novel therapeutics and also presents an opportunity for a potentially more efficient drug-development process.

Oxidative stress underlying axonal degeneration in adrenoleukodystrophy: a paradigm for multifactorial neurodegenerative diseases?

X-linked adrenoleukodystrophy (X-ALD) is an inherited neurodegenerative disorder expressed as four disease variants characterized by adrenal insufficiency and graded damage in the nervous system. X-ALD is caused by a loss of function of the peroxisomal ABCD1 fatty-acid transporter, resulting in the accumulation of very long chain fatty acids (VLCFA) in the organs and plasma, which have potentially toxic effects in CNS and adrenal glands. We have recently shown that treatment with a combination of antioxidants containing α-tocopherol, N-acetyl-cysteine and α-lipoic acid reversed oxidative damage and energetic failure, together with the axonal degeneration and locomotor impairment displayed by Abcd1 null mice, the animal model of X-ALD. This is the first direct demonstration that oxidative stress, which is a hallmark not only of X-ALD, but also of other neurodegenerative processes, such as Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD), contributes to axonal damage. The purpose of this review is, first, to discuss the molecular and cellular underpinnings of VLCFA-induced oxidative stress, and how it interacts with energy metabolism and/or inflammation to generate a complex syndrome wherein multiple factors are contributing. Particular attention will be paid to the dysregulation of redox homeostasis by the interplay between peroxisomes and mitochondria. Second, we will extend this analysis to the aforementioned neurodegenerative diseases with the aim of defining differences as well as the existence of a core pathogenic mechanism that would justify the exchange of therapeutic opportunities among these pathologies.

Neuroprotective role of Sirt1 in mammalian models of Huntington's disease through activation of multiple Sirt1 targets

Huntington's disease is a fatal neurodegenerative disorder caused by an expanded polyglutamine repeat in huntingtin (HTT) protein. We previously showed that calorie restriction ameliorated Huntington's disease pathogenesis and slowed disease progression in mice that model Huntington's disease (Huntington's disease mice). We now report that overexpression of sirtuin 1 (Sirt1), a mediator of the beneficial metabolic effects of calorie restriction, protects neurons against mutant HTT toxicity, whereas reduction of Sirt1 exacerbates mutant HTT toxicity. Overexpression of Sirt1 improves motor function, reduces brain atrophy and attenuates mutant-HTT-mediated metabolic abnormalities in Huntington's disease mice. Further mechanistic studies suggested that Sirt1 prevents the mutant-HTT-induced decline in brain-derived neurotrophic factor (BDNF) concentrations and the signaling of its receptor, TrkB, and restores dopamine- and cAMP-regulated phosphoprotein, 32 kDa (DARPP32) concentrations in the striatum. Sirt1 deacetylase activity is required for Sirt1-mediated neuroprotection in Huntington's disease cell models. Notably, we show that mutant HTT interacts with Sirt1 and inhibits Sirt1 deacetylase activity, which results in hyperacetylation of Sirt1 substrates such as forkhead box O3A (Foxo3a), thereby inhibiting its pro-survival function. Overexpression of Sirt1 counteracts the mutant-HTT-induced deacetylase deficit, enhances the deacetylation of Foxo3a and facilitates cell survival. These findings show a neuroprotective role for Sirt1 in mammalian Huntington's disease models and open new avenues for the development of neuroprotective strategies in Huntington's disease.

Behavioral phenotyping of glutathione-deficient mice: relevance to schizophrenia and bipolar disorder

Redox-dysregulation represents a common pathogenic mechanism in schizophrenia (SZ) and bipolar disorder (BP). It may in part arise from a genetically compromised synthesis of glutathione (GSH), the major cellular antioxidant and redox-regulator. Allelic variants of the genes coding for the rate-limiting GSH synthesizing enzyme glutamate-cysteine-ligase modifier (GCLM) and/or catalytic (GCLC) subunit have been associated with SZ and BP. Using mice knockout (KO) for GCLM we have previously shown that impaired GSH synthesis is associated with morphological, functional and neurochemical anomalies similar to those in patients. Here we asked whether GSH deficit is also associated with SZ- and BP-relevant behavioral and cognitive anomalies. Accordingly, we subjected young adult GCLM-wildtype (WT), heterozygous and KO males to a battery of standard tests. Compared to WT, GCLM-KO mice displayed hyperlocomotion in the open field and forced swim test but normal activity in the home cage, suggesting that hyperlocomotion was selective to environmental novelty and mildly stressful situations. While spatial working memory and latent inhibition remained unaffected, KO mice showed a potentiated hyperlocomotor response to an acute amphetamine injection, impaired sensorymotor gating in the form of prepulse inhibition and altered social behavior compared to WT. These anomalies resemble important aspects of both SZ and the manic component of BP. As such our data support the notion that redox-dysregulation due to GSH deficit is implicated in both disorders. Moreover, our data propose the GCLM-KO mouse as a valuable model to study the behavioral and cognitive consequences of redox dysregulation in the context of psychiatric disease.

Lipid peroxidation and neurodegenerative disease

Lipid peroxidation is a complex process involving the interaction of oxygen-derived free radicals with polyunsaturated fatty acids, resulting in a variety of highly reactive electrophilic aldehydes. Since 1975, lipid peroxidation has been extensively studied in a variety of organisms. As neurodegenerative diseases became better understood, research establishing a link between this form of oxidative damage, neurodegeneration, and disease has provided a wealth of knowledge to the scientific community. With the advent of proteomics in 1995, the identification of biomarkers for neurodegenerative disorders became of paramount importance to better understand disease pathogenesis and develop potential therapeutic strategies. This review focuses on the relationship between lipid peroxidation and neurodegenerative diseases. It also demonstrates how findings in current research support the common themes of altered energy metabolism and mitochondrial dysfunction in neurodegenerative disorders.

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

The purpose of our study was to determine the relationship between mutant huntingtin (Htt) and mitochondrial dynamics in the progression of Huntington's disease (HD). We measured the mRNA levels of electron transport chain genes, and mitochondrial structural genes, Drp1 (dynamin-related protein 1), Fis1 (fission 1), Mfn1 (mitofusin 1), Mfn2 (mitofusin 2), Opa1 (optric atrophy 1), Tomm40 (translocase of outermembrane 40) and CypD (cyclophilin D) in grade III and grade IV HD patients and controls. The mutant Htt oligomers and the mitochondrial structural proteins were quantified in the striatum and frontal cortex of HD patients. Changes in expressions of the electron transport chain genes were found in HD patients and may represent a compensatory response to mitochondrial damage caused by mutant Htt. Increased expression of Drp1 and Fis1 and decreased expression of Mfn1, Mfn2, Opa1 and Tomm40 were found in HD patients relative to the controls. CypD was upregulated in HD patients, and this upregulation increased as HD progressed. Significantly increased immunoreactivity of 8-hydroxy-guanosine was found in the cortical specimens from stage III and IV HD patients relative to controls, suggesting increased oxidative DNA damage in HD patients. In contrast, significantly decreased immunoreactivities of cytochrome oxidase 1 and cytochrome b were found in HD patients relative to controls, indicating a loss of mitochondrial function in HD patients. Immunoblotting analysis revealed 15, 25 and 50 kDa mutant Htt oligomers in the brain specimens of HD patients. All oligomeric forms of mutant Htt were significantly increased in the cortical tissues of HD patients, and mutant Htt oligomers were found in the nucleus and in mitochondria. The increase in Drp1, Fis1 and CypD and the decrease in Mfn1 and Mfn2 may be responsible for abnormal mitochondrial dynamics that we found in the cortex of HD patients, and may contribute to neuronal damage in HD patients. The presence of mutant Htt oligomers in the nucleus of HD neurons and in mitochondria may disrupt neuronal functions. Based on these findings, we propose that mutant Htt in association with mitochondria imbalance and mitochondrial dynamics impairs axonal transport of mitochondria, decreases mitochondrial function and damages neurons in affected brain regions of HD patients.

Mitochondrial matters in Huntington disease

Huntington Disease (HD) is a relatively common inherited neuropathy with characteristic cognitive and behavioral features. HD usually has a late onset and often is not recognized until the third or fourth decades of life. Transmitted as an autosomal dominant trait, HD has become a prototype for understanding a group of neurogenetic disorders. As a class, HD and the others are manifestations of the expansion of a trinucleotide repeat within the gene coding or structural region. In HD expansion of the (CAG)(n) repeat in the first exon from an average of 18 (normal) to a median of 44 is the underlying molecular biologic change. In affected individuals, the mutant HD protein (Huntingtin, mHtt) thus contains an extended polyglutamine repeat. Clinical and neuropathic changes in the caudate and putamen nuclei occur relatively early with other brain regions being affected later. Mitochondrial structure, altered electron transport and increased brain lactate levels have implicated mitochondria in HD pathophysiology. There is also evidence that reduced transcription of the peroxisome proliferator-activated receptor-gamma coactivator (PGC-1 alpha) leads to altered downstream gene regulation. Further evidence for mitochondrial involvement is presented in the following reviews. Clarifying mitochondrial derangements has led to some possibilities for therapeutic intervention.