Cardiac dysfunction in the R6/2 mouse model of Huntington's disease

Brain-Periphery Interactions in Huntington's Disease: Mediators and Lifestyle Interventions

Prominent pathological features of Huntington's disease (HD) are aggregations of mutated Huntingtin protein (mHtt) in the brain and neurodegeneration, which causes characteristic motor (such as chorea and dystonia) and non-motor symptoms. However, the numerous systemic and peripheral deficits in HD have gained increasing attention recently, since those factors likely modulate disease progression, including brain pathology. While whole-body metabolic abnormalities and organ-specific pathologies in HD have been relatively well described, the potential mediators of compromised inter-organ communication in HD have been insufficiently characterized. Therefore, we applied an exploratory literature search to identify such mediators. Unsurprisingly, dysregulation of inflammatory factors, circulating mHtt, and many other messenger molecules (hormones, lipids, RNAs) were found that suggest impaired inter-organ communication, including of the gut-brain and muscle-brain axis. Based on these findings, we aimed to assess the risks and potentials of lifestyle interventions that are thought to improve communication across these axes: dietary strategies and exercise. We conclude that appropriate lifestyle interventions have great potential to reduce symptoms and potentially modify disease progression (possibly via improving inter-organ signaling) in HD. However, impaired systemic metabolism and peripheral symptoms warrant particular care in the design of dietary and exercise programs for people with HD.

Intravenous MSC-Treatment Improves Impaired Brain Functions in the R6/2 Mouse Model of Huntington's Disease via Recovered Hepatic Pathological Changes

Huntington's disease (HD), a congenital neurodegenerative disorder, extends its pathological damages beyond the nervous system. The systematic manifestation of HD has been extensively described in numerous studies, including dysfunction in peripheral organs and peripheral inflammation. Gut dysbiosis and the gut-liver-brain axis have garnered greater emphasis in neurodegenerative research, and increased plasma levels of pro-inflammatory cytokines have been identified in HD patients and various in vivo models, correlating with disease progression. In the present study, we investigated hepatic pathological markers in the liver of R6/2 mice which convey exon 1 of the human mutant huntingtin gene. Furthermore, we evaluated the impact of intravenously administered Mesenchymal Stromal Cells (MSCs) on the liver enzymes, changes in hepatic inflammatory markers, as well as brain pathology and behavioral deficits in R6/2 mice. Our results revealed altered enzyme expression and increased levels of inflammatory mediators in the liver of R6/2 mice, which were significantly attenuated in the MSC-treated R6/2 mice. Remarkably, neuronal pathology and altered motor activities in the MSC-treated R6/2 mice were significantly ameliorated, despite the absence of MSCs in the postmortem brain. Our data highlight the importance of hepatic pathological changes in HD, providing a potential therapeutic approach. Moreover, the data open new perspectives for the search in blood biomarkers correlating with liver pathology in HD.

Rapamycin reduces neuronal mutant huntingtin aggregation and ameliorates locomotor performance in Drosophila

Huntington's disease (HD) is a neurodegenerative disease characterized by movement and cognitive dysfunction. HD is caused by a CAG expansion in exon 1 of the HTT gene that leads to a polyglutamine (PQ) repeat in the huntingtin protein, which aggregates in the brain and periphery. Previously, we used Drosophila models to determine that Htt-PQ aggregation in the heart causes shortened lifespan and cardiac dysfunction that is ameliorated by promoting chaperonin function or reducing oxidative stress. Here, we further study the role of neuronal mutant huntingtin and how it affects peripheral function. We overexpressed normal (Htt-PQ25) or expanded mutant (Htt-PQ72) exon 1 of huntingtin in Drosophila neurons and found that mutant huntingtin caused age-dependent Htt-PQ aggregation in the brain and could cause a loss of synapsin. To determine if this neuronal dysfunction led to peripheral dysfunction, we performed a negative geotaxis assay to measure locomotor performance and found that neuronal mutant huntingtin caused an age-dependent decrease in locomotor performance. Next, we found that rapamycin reduced Htt-PQ aggregation in the brain. These results demonstrate the role of neuronal Htt-PQ in dysfunction in models of HD, suggest that brain-periphery crosstalk could be important to the pathogenesis of HD, and show that rapamycin reduces mutant huntingtin aggregation in the brain.

Huntington’s disease phenotypes are improved via mTORC1 modulation by small molecule therapy

Huntington’s Disease (HD) is a dominantly inherited neurodegenerative disease for which the major causes of mortality are neurodegeneration-associated aspiration pneumonia followed by cardiac failure. mTORC1 pathway perturbations are present in HD models and human tissues. Amelioration of mTORC1 deficits by genetic modulation improves disease phenotypes in HD models, is not a viable therapeutic strategy. Here, we assessed a novel small molecule mTORC1 pathway activator, NV-5297, for its improvement of the disease phenotypes in the N171-82Q HD mouse model. Oral dosing of NV-5297 over 6 weeks activated mTORC1, increased striatal volume, improved motor learning and heart contractility. Further, the heart contractility, heart fibrosis, and survival were improved in response to the cardiac stressor isoprenaline when compared to vehicle-treated mice. Cummulatively, these data support mTORC1 activation as a therapeutic target in HD and consolidates NV-5297 as a promising drug candidate for treating central and peripheral HD phenotypes and, more generally, mTORC1-deficit related diseases.

Rosiglitazone Ameliorates Cardiac and Skeletal Muscle Dysfunction by Correction of Energetics in Huntington’s Disease

Huntington’s disease (HD) is a rare neurodegenerative disease that is accompanied by skeletal muscle atrophy and cardiomyopathy. Tissues affected by HD (central nervous system [CNS], skeletal muscle, and heart) are known to suffer from deteriorated cellular energy metabolism that manifests already at presymptomatic stages. This work aimed to test the effects of peroxisome proliferator-activated receptor (PPAR)-γ agonist—rosiglitazone on grip strength and heart function in an experimental HD model—on R6/1 mice and to address the mechanisms. We noted that rosiglitazone treatment lead to improvement of R6/1 mice grip strength and cardiac mechanical function. It was accompanied by an enhancement of the total adenine nucleotides pool, increased glucose oxidation, changes in mitochondrial number (indicated as increased citric synthase activity), and reduction in mitochondrial complex I activity. These metabolic changes were supported by increased total antioxidant status in HD mice injected with rosiglitazone. Correction of energy deficits with rosiglitazone was further indicated by decreased accumulation of nucleotide catabolites in HD mice serum. Thus, rosiglitazone treatment may not only delay neurodegeneration but also may ameliorate cardio- and myopathy linked to HD by improvement of cellular energetics.

Mitochondrial and redox modifications in early stages of Huntington's disease

Deficits in mitochondrial function and redox deregulation have been attributed to Huntington's disease (HD), a genetic neurodegenerative disorder largely affecting the striatum. However, whether these changes occur in early stages of the disease and can be detected in vivo is still unclear. In the present study, we analysed changes in mitochondrial function and production of reactive oxygen species (ROS) at early stages and with disease progression. Studies were performed in vivo in human brain by PET using [⁶⁴Cu]-ATSM and ex vivo in human skin fibroblasts of premanifest and prodromal (Pre-M) and manifest HD carriers. In vivo brain [⁶⁴Cu]-ATSM PET in YAC128 transgenic mouse and striatal and cortical isolated mitochondria were assessed at presymptomatic (3 month-old, mo) and symptomatic (6–12 mo) stages. Pre-M HD carriers exhibited enhanced whole-brain (with exception of caudate) [⁶⁴Cu]-ATSM labelling, correlating with CAG repeat number. Fibroblasts from Pre-M showed enhanced basal and maximal respiration, proton leak and increased hydrogen peroxide (H₂O₂) levels, later progressing in manifest HD. Mitochondria from fibroblasts of Pre-M HD carriers also showed reduced circularity, while higher number of mitochondrial DNA copies correlated with maximal respiratory capacity. In vivo animal PET analysis showed increased accumulation of [⁶⁴Cu]-ATSM in YAC128 mouse striatum. YAC128 mouse (at 3 months) striatal isolated mitochondria exhibited a rise in basal and maximal mitochondrial respiration and in ATP production, and increased complex II and III activities. YAC128 mouse striatal mitochondria also showed enhanced mitochondrial H₂O₂ levels and circularity, revealed by brain ultrastructure analysis, and defects in Ca²⁺ handling, supporting increased striatal susceptibility. Data demonstrate both human and mouse mitochondrial overactivity and altered morphology at early HD stages, facilitating redox unbalance, the latter progressing with manifest disease.

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Pre-manifest HD carriers and presymptomatic YAC128 mice show increased brain [⁶⁴Cu]-ATSM labelling.

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Increased [⁶⁴Cu]-ATSM brain retention correlates with raised ROS levels in human and mouse samples.

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Increased [⁶⁴Cu]-ATSM correlates with enhanced mitochondrial activity and mtDNA copy number.

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Presymptomatic YAC128 mouse striatal mitochondria show altered morphology and Ca²⁺ handling.

Metabolism in Huntington's disease: a major contributor to pathology

Huntington's disease (HD) is a progressively debilitating neurodegenerative disease exhibiting autosomal-dominant inheritance. It is caused by an unstable expansion in the CAG repeat tract of HD gene, which transforms the disease-specific Huntingtin protein (HTT) to a mutant form (mHTT). The profound neuronal death in cortico-striatal circuits led to its identification and characterisation as a neurodegenerative disease. However, equally disturbing are the concomitant whole-body manifestations affecting nearly every organ of the diseased individuals, at varying extents. Altered central and peripheral metabolism of energy, proteins, nucleic acids, lipids and carbohydrates encompass the gross pathology of the disease. Intense fluctuation of body weight, glucose homeostasis and organ-specific subcellular abnormalities are being increasingly recognised in HD. Many of these metabolic abnormalities exist years before the neuropathological manifestations such as chorea, cognitive decline and behavioural abnormalities develop, and prove to be reliable predictors of the disease progression. In this review, we provide a consolidated overview of the central and peripheral metabolic abnormalities associated with HD, as evidenced from clinical and experimental studies. Additionally, we have discussed the potential of metabolic biomolecules to translate into efficient biomarkers for the disease onset as well as progression. Finally, we provide a brief outlook on the efficacy of existing therapies targeting metabolic remediation. While it is clear that components of altered metabolic pathways can mark many aspects of the disease, it is only conceivable that combinatorial therapies aiming for neuronal protection in consort with metabolic upliftment will prove to be more efficient than the existing symptomatic treatment options.

Autonomic disorders and myocardial 123I-metaiodobenzylguanidine scintigraphy in Huntington's disease

BACKGROUND

Huntington's disease (HD) patients often present with abnormal modulation of blood pressure and heart rate. We investigated whether cardiac autonomic innervation assessed by 123I-metaiodobenzylguanidine (MIBG) imaging is impaired in HD patients, in comparison with controls (Ctrl).

METHODS

Fifteen patients (6 F and 9 M) were assessed by the motor section of the Unified HD Rating Scale, the Total Function Capacity, and the scale for outcomes in Parkinson's disease-autonomic (SCOPA-AUT) questionnaire. All patients and 10 Ctrl (5 F and 5 M) underwent 123I-MIBG imaging. From planar images, the early and late heart-to-mediastinum (H/M) ratios and myocardial washout rates (WR) were calculated.

RESULTS

We did not find significant differences in early and late H/M ratios and WR between the two groups. At individual level, three patients showed reduced early and/or late H/M ratios. The most common autonomic complaints were gastrointestinal and genitourinary disorders. SCOPA-AUT questionnaire score results positively correlated with the disease duration and WR.

CONCLUSIONS

Our study indicates that myocardial postganglionic sympathetic innervation is essentially preserved or only minimally involved in HD. These findings suggest that the cardiovascular dysfunction might be mainly due to the impairment of brain areas associated with the regulation and modulation of the heart function.

Targeted Genetic Reduction of Mutant Huntingtin Lessens Cardiac Pathology in the BACHD Mouse Model of Huntington's Disease

Individuals affected by Huntington's disease (HD) present with progressive degeneration that results in a wide range of symptoms, including cardiovascular (CV) dysfunction. The huntingtin gene (HTT) and its product are ubiquitously expressed, hence, the cardiomyopathy could also be driven by defects caused by its mutated form (mHTT) in the cardiomyocytes themselves. In the present study, we sought to determine the contribution of the mHTT expressed in the cardiomyocytes to CV symptoms. We utilized the BACHD mouse model, which exhibits many of the HD core symptoms, including CV dysfunction. This model allows the targeted genetic reduction of mHTT expression in the cardiomyocytes while maintaining the expression of the mHTT in the rest of the body. The BACHD line was crossed with a line of mice in which the expression of Cre recombinase is driven by the cardiac-specific alpha myosin-heavy chain (Myh6) promoter. The offspring of this cross (BMYO mice) exhibited a dramatic reduction in mHTT in the heart but not in the striatum. The BMYO mice were evaluated at 6 months old, as at this age, the BACHD line displays a strong CV phenotype. Echocardiogram measurements found improvement in the ejection fraction in the BMYO line compared to the BACHD, while hypertrophy was observed in both mutant lines. Next, we examined the expression of genes known to be upregulated during pathological cardiac hypertrophy. As measured by qPCR, the BMYO hearts exhibited significantly less expression of collagen1a as well as Gata4, and brain natriuretic peptide compared to the BACHD. Fibrosis in the hearts assessed by Masson's trichrome stain and the protein levels of fibronectin were reduced in the BMYO hearts compared to BACHD. Finally, we examined the performance of the mice on CV-sensitive motor tasks. Both the overall activity levels and grip strength were improved in the BMYO mice. Therefore, we conclude that the reduction of mHtt expression in the heart benefits CV function in the BACHD model, and suggest that cardiomyopathy should be considered in the treatment strategies for HD.

Hydrogen Sulfide Ameliorates Angiotensin II-Induced Atrial Fibrosis Progression to Atrial Fibrillation Through Inhibition of the Warburg Effect and Endoplasmic Reticulum Stress

Atrial fibrosis is the basis for the occurrence and development of atrial fibrillation (AF) and is closely related to the Warburg effect, endoplasmic reticulum stress (ERS) and mitochondrion dysfunctions-induced cardiomyocyte apoptosis. Hydrogen sulfide (H₂S) is a gaseous signalling molecule with cardioprotective, anti-myocardial fibrosis and improved energy metabolism effects. Nevertheless, the specific mechanism by which H₂S improves the progression of atrial fibrosis to AF remains unclear. A case-control study of patients with and without AF was designed to assess changes in H₂S, the Warburg effect, and ERS in AF. The results showed that AF can significantly reduce cystathionine-γ-lyase (CSE) and 3-mercaptopyruvate thiotransferase (3-MST) expression and the H₂S level, induce cystathionine-β-synthase (CBS) expression; increase the Warburg effect, ERS and atrial fibrosis; and promote left atrial dysfunction. In addition, AngII-treated SD rats had an increased Warburg effect and ERS levels and enhanced atrial fibrosis progression to AF compared to wild-type SD rats, and these conditions were reversed by sodium hydrosulfide (NaHS), dichloroacetic acid (DCA) or 4-phenylbutyric acid (4-PBA) supplementation. Finally, low CSE levels in AngII-induced HL-1 cells were concentration- and time-dependent and associated with mitochondrial dysfunction, apoptosis, the Warburg effect and ERS, and these effects were reversed by NaHS, DCA or 4-PBA supplementation. Our research indicates that H₂S can regulate the AngII-induced Warburg effect and ERS and might be a potential therapeutic drug to inhibit atrial fibrosis progression to AF.

Management of altered metabolic activity in Drosophila model of Huntington's disease by curcumin

Huntington's disease (HD) is a devastating polyglutamine disorder characterized by extensive neurodegeneration and metabolic abnormalities at systemic, cellular and intracellular levels. Metabolic alterations in HD manifest as abnormal body weight, dysregulated biomolecule levels, impaired adipocyte functions, and defective energy state which exacerbate disease progression and pose acute threat to the health of challenged individuals in form of insulin resistance, cardiovascular disease, and energy crisis. To colossally mitigate disease symptoms, we tested the efficacy of curcumin in Drosophila model of HD. Curcumin is the bioactive component of turmeric (Curcuma longa Linn), well-known for its ability to modulate metabolic activities. We found that curcumin effectively managed abnormal body weight, dysregulated lipid content, and carbohydrate level in HD flies. In addition, curcumin administration lowered elevated reactive-oxygen-species levels in adult adipose tissue of diseased flies, and improved survival and locomotor function in HD flies at advanced disease stage. Altogether, these findings clearly suggest that curcumin efficiently attenuates metabolic derangements in HD flies and can prove beneficial in alleviating the complexities associated with HD.

Systemic manifestation and contribution of peripheral tissues to Huntington's disease pathogenesis

Huntington disease (HD) is an autosomal dominant neurodegenerative disease that is caused by expansion of cytosine/adenosine/guanine repeats in the huntingtin (HTT) gene, which leads to a toxic, aggregation-prone, mutant HTT-polyQ protein. Beyond the well-established mechanisms of HD progression in the central nervous system, growing evidence indicates that also peripheral tissues are affected in HD and that systemic signaling originating from peripheral tissues can influence the progression of HD in the brain. Herein, we review the systemic manifestation of HD in peripheral tissues, and the impact of systemic signaling on HD pathogenesis. Mutant HTT induces a body wasting syndrome (cachexia) primarily via its activity in skeletal muscle, bone, adipose tissue, and heart. Additional whole-organism effects induced by mutant HTT include decline in systemic metabolic homeostasis, which stems from derangement of pancreas, liver, gut, hypothalamic-pituitary-adrenal axis, and circadian functions. In addition to spreading via the bloodstream and a leaky blood brain barrier, HTT-polyQ may travel long distance via its uptake by neurons and its axonal transport from the peripheral to the central nervous system. Lastly, signaling factors that are produced and/or secreted in response to therapeutic interventions such as exercise or in response to mutant HTT activity in peripheral tissues may impact HD. In summary, these studies indicate that HD is a systemic disease that is influenced by intertissue signaling and by the action of pathogenic HTT in peripheral tissues. We propose that treatment strategies for HD should include the amelioration of HD symptoms in peripheral tissues. Moreover, harnessing signaling between peripheral tissues and the brain may provide a means for reducing HD progression in the central nervous system.

Purine Nucleotides Metabolism and Signaling in Huntington's Disease: Search for a Target for Novel Therapies

Huntington’s disease (HD) is a multi-system disorder that is caused by expanded CAG repeats within the exon-1 of the huntingtin (HTT) gene that translate to the polyglutamine stretch in the HTT protein. HTT interacts with the proteins involved in gene transcription, endocytosis, and metabolism. HTT may also directly or indirectly affect purine metabolism and signaling. We aimed to review existing data and discuss the modulation of the purinergic system as a new therapeutic target in HD. Impaired intracellular nucleotide metabolism in the HD affected system (CNS, skeletal muscle and heart) may lead to extracellular accumulation of purine metabolites, its unusual catabolism, and modulation of purinergic signaling. The mechanisms of observed changes might be different in affected systems. Based on collected findings, compounds leading to purine and ATP pool reconstruction as well as purinergic receptor activity modulators, i.e., P2X7 receptor antagonists, may be applied for HD treatment.

Progressive cardiac arrhythmias and ECG abnormalities in the Huntington's disease BACHD mouse model

Huntington's disease (HD) is a dominantly inherited neurodegenerative disease. There is accumulating evidence that HD patients have increased prevalence of conduction abnormalities and compromised sinoatrial node function which could lead to increased risk for arrhythmia. We used mutant Huntingtin (mHTT) expressing bacterial artificial chromosome Huntington's disease mice to determine if they exhibit electrocardiogram (ECG) abnormalities involving cardiac conduction that are known to increase risk of sudden arrhythmic death in humans. We obtained surface ECGs and analyzed arrhythmia susceptibility; we observed prolonged QRS duration, increases in PVCs as well as PACs. Abnormal histological and structural changes that could lead to cardiac conduction system dysfunction were seen. Finally, we observed decreases in desmosomal proteins, plakophilin-2 and desmoglein-2, which have been reported to cause cardiac arrhythmias and reduced conduction. Our study indicates that mHTT could cause progressive cardiac conduction system pathology that could increase the susceptibility to arrhythmias and sudden cardiac death in HD patients.

Deciphering the key mechanisms leading to alteration of lipid metabolism in Drosophila model of Huntington's disease

Huntington's disease (HD) is an inherited, progressively debilitating disorder marked by prominent degeneration in striatal and cortical brain regions. HD is caused by (CAG)_n repeat expansion in huntingtin (HTT) gene that translates into a mutant form of the ubiquitously present Huntingtin (HTT) protein. Extensive metabolic dysfunction coexisting with overt neuropathies has been evidenced in clinical and experimental settings of HD. Body weight loss despite normal to high caloric intake remains a critical determinant of the disease progression and a challenge for therapeutic interventions. In the present study, we intended to monitor the cellular and molecular perturbations in Drosophila, caused by pan-neuronal expression of mHTT (mutant Huntingtin) protein. We found aberrant transcription profile of key lipolytic and lipogenic genes in whole-body of the fly with disease progression. Interestingly, fatbody undergoes extensive alteration of vital cellular processes and eventually surrenders to increased apoptotic cell death in terminal stage of the disease. Extensive mitochondrial dysfunction from early disease stage along with calcium derangement at terminal stage were observed in fatbody, which contribute to its deteriorating integrity. All the mechanisms were monitored progressively, at different disease stages, and many alterations were documented in the early stage itself. Our study hence provides insight into the mechanisms through which neuronal expression of mHTT might be inflicting the profound systemic effects, specifically on lipid metabolism, and may open new therapeutic avenues for alleviation of the multidimensional disease.

Cardiac electrical remodeling and neurodegenerative diseases association

Cardiac impairment contributes significantly to the mortality associated with several neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD), primarily recognized as brain pathologies. These diseases may be caused by aggregation of a misfolded protein, most often, in the brain, although new evidence also reveals peripheral abnormalities. After characterization of the cardiac involvement in neurodegenerative diseases, several studies concentrated on elucidating the cause of the impaired cardiac function. However, most of the current knowledge is focused on the mechanical aspects of the heart rather than the electrical disturbances. The main objective of this review is to summarize the most recent advances in the elucidation of cardiac electrical remodeling in the neurodegenerative environment. We aimed to determine a crosstalk between the heart and the brain in three neurodegenerative conditions: AD, PD, and HD. We found that the most studies demonstrated important alterations in the electrocardiogram (ECG) of patients with neurodegeneration and in animal models of the conditions. We also showed that little is described when considering excitability disruptions in cardiomyocytes, for example, action potential impairments. It is a matter of contention whether central nervous system abnormalities or the peripheral ones increase the risk of heart diseases in patients with neurodegenerative conditions. To determine this notion, there is a need for new heart studies focusing specifically on the cardiac electrophysiology (e.g., ECG and cardiomyocyte excitability). This review could serve as an important guide in designing novel accurate approaches targeting the heart in neuronal conditions.

Influence of 3-nitropropionic acid on physiological and behavioral responses in zebrafish larvae and adults

Long-term treatment with 3-nitropropionic acid (3-NPA), a toxin derived from plants and fungi, may reproduce symptoms and biochemical characteristics of Huntington's disease (HD). Our study evaluated the effects of 3-NPA on the physiological and behavioral responses in zebrafish larvae and adults. Larvae exposed to 0.1, 0.2, or 0.5 mM 3-NPA exhibited an increase in heart rate at 2- and 5-days post-fertilization (dpf). There was a decrease in the ocular distance at 5 dpf with 0.05 mM 3-NPA treatment. However, 3-NPA did not alter larval locomotor parameters. Adult zebrafish received 3-NPA intraperitoneal injections (a total of seven injections at doses 10, 20, or 60 mg/kg every 96 h) and showed a decrease in body weight , locomotion and aggressive behavior. No changes were observed in anxiety-like behavior and social interaction between 3-NPA-exposed animals and control groups. However, 3-NPA-treated animals (at 60 mg/kg) demonstrated impaired long-term aversive memory. Overall, 3-NPA exposure induced morphological and heart rate alterations in zebrafish larvae. Additionally, our study showed behavioral changes in zebrafish that were submitted to long-term 3-NPA treatment, which could be related to HD symptoms.

Mitochondrial Dysfunctions: A Red Thread across Neurodegenerative Diseases

Mitochondria play a central role in a plethora of processes related to the maintenance of cellular homeostasis and genomic integrity. They contribute to preserving the optimal functioning of cells and protecting them from potential DNA damage which could result in mutations and disease. However, perturbations of the system due to senescence or environmental factors induce alterations of the physiological balance and lead to the impairment of mitochondrial functions. After the description of the crucial roles of mitochondria for cell survival and activity, the core of this review focuses on the "mitochondrial switch" which occurs at the onset of neuronal degeneration. We dissect the pathways related to mitochondrial dysfunctions which are shared among the most frequent or disabling neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's, Amyotrophic Lateral Sclerosis, and Spinal Muscular Atrophy. Can mitochondrial dysfunctions (affecting their morphology and activities) represent the early event eliciting the shift towards pathological neurobiological processes? Can mitochondria represent a common target against neurodegeneration? We also review here the drugs that target mitochondria in neurodegenerative diseases.

Protein and Mitochondria Quality Control Mechanisms and Cardiac Aging

Cardiovascular disease (CVD) is the number one cause of death in the United States. Advancing age is a primary risk factor for developing CVD. Estimates indicate that 20% of the US population will be ≥65 years old by 2030. Direct expenditures for treating CVD in the older population combined with indirect costs, secondary to lost wages, are predicted to reach $1.1 trillion by 2035. Therefore, there is an eminent need to discover novel therapeutic targets and identify new interventions to delay, lessen the severity, or prevent cardiovascular complications associated with advanced age. Protein and organelle quality control pathways including autophagy/lysosomal and the ubiquitin-proteasome systems, are emerging contributors of age-associated myocardial dysfunction. In general, two findings have sparked this interest. First, strong evidence indicates that cardiac protein degradation pathways are altered in the heart with aging. Second, it is well accepted that damaged and misfolded protein aggregates and dysfunctional mitochondria accumulate in the heart with age. In this review, we will: (i) define the different protein and mitochondria quality control mechanisms in the heart; (ii) provide evidence that each quality control pathway becomes dysfunctional during cardiac aging; and (iii) discuss current advances in targeting these pathways to maintain cardiac function with age.

Cardiac mTORC1 Dysregulation Impacts Stress Adaptation and Survival in Huntington's Disease

Huntington’s disease (HD) is a dominantly inherited neurological disorder caused by CAG-repeat expansion in exon 1 of Huntingtin (HTT). But in addition to the neurological disease, mutant HTT (mHTT), which is ubiquitously expressed, impairs other organ systems. Indeed, epidemiological and animal model studies suggest higher incidence of and mortality from heart disease in HD. Here, we show that the protein complex mTORC1 is dysregulated in two HD mouse models through a mechanism that requires intrinsic mHTT expression. Moreover, restoring cardiac mTORC1 activity with constitutively active Rheb prevents mortality and relieves the mHTT-induced block to hypertrophic adaptation to cardiac stress. Finally, we show that chronic mTORC1 dysregulation is due in part to mislocalization of endogenous Rheb. These data provide insight into the increased cardiac-related mortality of HD patients, with cardiac mHTT expression inhibiting mTORC1 activity, limiting heart growth, and decreasing the heart’s ability to compensate to chronic stress.

Circadian dysfunction in the Q175 model of Huntington's disease: Network analysis

Disturbances in sleep/wake cycle are a common complaint of individuals with Huntington's disease (HD) and are displayed by HD mouse models. The underlying mechanisms, including the possible role of the circadian timing system, have been the topic of a number of recent studies. The (z)Q175 mouse is a knock-in model in which the human exon 1 sequence of the huntingtin gene is inserted into the mouse DNA with approximately 190 CAG repeats. Among the numerous models available, the heterozygous Q175 offers strong construct validity with a single copy of the mutation, genetic precision of the insertion and control of mutation copy number. In this review, we will summarize the evidence that this model exhibits disrupted diurnal and circadian rhythms in locomotor activity. We found overwhelming evidence for autonomic dysfunction including blunted daily rhythms in heart rate and core body temperature (CBT), reduced heart rate variability, and almost a complete failure of the sympathetic arm of the autonomic nervous system to function during the baroreceptor reflex. Mechanistically, the Q175 mouse model exhibits deficits in the neural output of the central circadian clock, the suprachiasmatic nucleus along with an enhancement of at least one type of potassium current in these neurons. Finally, we report a novel network analysis examining the phase coherence between activity, CBT, and cardiovascular measures. Such analyses found that even young Q175 mutants (heterozygous or homozygous) show coherence degradation, and suggests that loss of phase coherence is a variable that should be considered as a possible biomarker for HD.

Do Disruptions in the Circadian Timing System Contribute to Autonomic Dysfunction in Huntington's Disease?

Huntington's disease (HD) patients suffer from a progressive neurodegenerative disorder that inflicts both motor and non-motor symptoms. HD is caused by a CAG repeat expansion within the first exon of the huntingtin (HTT) gene that produces a polyglutamine repeat that leads to protein misfolding, soluble aggregates, and inclusion bodies detected throughout the body. Both clinical and preclinical research indicate that cardiovascular dysfunction should be considered a core symptom in at least a subset of HD patients. There is strong evidence for dysautonomia (dysfunctional autonomic nervous system, ANS) in HD patients that can be detected early in the disease progression. The temporal patterning of ANS function is controlled by the circadian timing system based in the anterior hypothalamus. Patients with neurodegenerative diseases including HD exhibit disrupted sleep/wake cycle and, in preclinical models, there is compelling evidence that the circadian timing system is compromised early in the disease process. Here we review data from preclinical models of HD that explore the intersection between disruption of circadian rhythms and dysautonomia. This work will lead to new therapeutic strategies and standards of care for HD and other neurodegenerative diseases.

Transcriptomic Biomarkers for Huntington's Disease: Are Gene Expression Signatures in Whole Blood Reliable Biomarkers?

Huntington's disease (HD) is a severe neurodegenerative disorder manifesting as progressive impairment of motor function, cognitive decline, psychiatric symptoms, and immunological and endocrine dysfunction. We explored the consistency of blood transcriptomic biomarkers in HD based on a novel Slovene patient cohort and expert review of previous studies. HumanHT-12 v4 BeadChip microarrays were performed on the whole blood samples of a cohort of 23 HD mutation carriers and 23 controls to identify differentially expressed (DE) transcripts. In addition, we performed an expert review of DE transcripts identified in comparable HD studies from whole blood, to identify any consistent signature of HD. In the Slovene cohort, we identified 740 DE transcripts (p < 0.01 and a false discovery rate (FDR) of <0.1) of which 414 were downregulated and 326 were upregulated. Pathway analyses of DE transcripts showed enrichment for pathways involved in systemic, rather than neural processes in HD. With an expert review of comparable studies, we have further identified 15 DE transcripts shared by 3 studies. We suggest transcriptomic changes in blood reflect systemic changes in HD pathogenesis, rather than being a direct result of the neuropathological processes in the central nervous system during HD progression, and thus, have limited value as disease biomarkers.

Cellular Models: HD Patient-Derived Pluripotent Stem Cells

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by expanded polyglutamine (polyQ)-encoding repeats in the Huntingtin (HTT) gene. Traditionally, HD cellular models consisted of either patient cells not affected by disease or rodent neurons expressing expanded polyQ repeats in HTT. As these models can be limited in their disease manifestation or proper genetic context, respectively, human HD pluripotent stem cells (PSCs) are currently under investigation as a way to model disease in patient-derived neurons and other neural cell types. This chapter reviews embryonic stem cell (ESC) and induced pluripotent stem cell (iPSC) models of disease, including published differentiation paradigms for neurons and their associated phenotypes, as well as current challenges to the field such as validation of the PSCs and PSC-derived cells. Highlighted are potential future technical advances to HD PSC modeling, including transdifferentiation, complex in vitro multiorgan/system reconstruction, and personalized medicine. Using a human HD patient model of the central nervous system, hopefully one day researchers can tease out the consequences of mutant HTT (mHTT) expression on specific cell types within the brain in order to identify and test novel therapies for disease.

Drp1/Fis1-mediated mitochondrial fragmentation leads to lysosomal dysfunction in cardiac models of Huntington's disease

Huntington's disease (HD) is a fatal hereditary neurodegenerative disorder, best known for its clinical triad of progressive motor impairment, cognitive deficits and psychiatric disturbances, is caused by CAG-repeat expansion in exon 1 of Huntingtin (HTT). However, in addition to the neurological disease, mutant HTT (mHTT), which is ubiquitously expressed in all tissues, impairs other organ systems. Not surprisingly, cardiovascular dysautonomia as well as the deterioration of circadian rhythms are among the earliest detectable pathophysiological changes in individuals with HD. Mitochondrial dysfunction in the brain and skeletal muscle in HD has been well documented, as the disease progresses. However, not much is known about mitochondrial abnormalities in the heart. In this study, we describe a role for Drp1/Fis1-mediated excessive mitochondrial fission and dysfunction, associated with lysosomal dysfunction in H9C2 expressing long polyglutamine repeat (Q73) and in human iPSC-derived cardiomyocytes transfected with Q77. Expression of long polyglutamine repeat led to reduced ATP production and mitochondrial fragmentation. We observed an increased accumulation of damaged mitochondria in the lysosome that was coupled with lysosomal dysfunction. Importantly, reducing Drp1/Fis1-mediated mitochondrial damage significantly improved mitochondrial function and cell survival. Finally, reducing Fis1-mediated Drp1 recruitment to the mitochondria, using the selective inhibitor of this interaction, P110, improved mitochondrial structure in the cardiac tissue of R6/2 mice. We suggest that drugs focusing on the central nervous system will not address mitochondrial function across all organs, and therefore will not be a sufficient strategy to treat or slow down HD disease progression.

Evidence of cardiac electrical remodeling in patients with Huntington disease

OBJECTIVE

Although Huntington's disease (HD) is a disease of the central nervous system, HD mortality surveys indicate heart disease as a major cause of death. Cardiac dysfunction in HD might be a primary consequence of peripherally expressed mutant huntingtin or secondary to either a general decline in health or the onset of neurological dysfunction. The aim of the study was to clarify the heart muscle involvement.

MATERIALS AND METHODS

We measured conventional and advanced resting ECG indices. Thirty-one subjects with a confirmed huntingtin gene mutation and 31 age- and gender-matched controls were included. The HD subjects were divided into four groups based on their Unified Huntington Disease Rating Scale (UHDRS) motor score.

RESULTS

We detected changes in advanced ECG variables connected with electrical ventricular remodeling (t test, p < 0.01). The increase in the unexplained part of both QT variability and the standard deviation of normal-to-normal QT intervals, presumably reflecting beat-to-beat changes in repolarization, was most pronounced. Further, both variables correlated with the product of the cytosine-adenine-guanine (CAG) triplets' repeat length and the subjects' age (CAP), the former R = 0.423 (p = 0.018) and the latter R = 0.499 (p = 0.004). There was no correlation between the CAP score and any of variables representing autonomic nervous system activity.

CONCLUSIONS

Both autonomic nervous system dysfunction and cardiac electrical remodeling are present in patients with HD. The changes in advanced ECG variables observed in the study evolve with HD progression. The increased values of QT unexplained variability may be a marker of temporal inhomogeneity in ventricular repolarization associated with malignant ventricular arrhythmias.

Increased nuclear DNA damage precedes mitochondrial dysfunction in peripheral blood mononuclear cells from Huntington's disease patients

Huntington’s disease (HD) is a progressive neurodegenerative disorder primarily affecting the basal ganglia and is caused by expanded CAG repeats in the huntingtin gene. Except for CAG sizing, mitochondrial and nuclear DNA (mtDNA and nDNA) parameters have not yet proven to be representative biomarkers for disease and future therapy. Here, we identified a general suppression of genes associated with aerobic metabolism in peripheral blood mononuclear cells (PBMCs) from HD patients compared to controls. In HD, the complex II subunit SDHB was lowered although not sufficiently to affect complex II activity. Nevertheless, we found decreased level of factors associated with mitochondrial biogenesis and an associated dampening of the mitochondrial DNA damage frequency in HD, implying an early defect in mitochondrial activity. In contrast to mtDNA, nDNA from HD patients was four-fold more modified than controls and demonstrated that nDNA integrity is severely reduced in HD. Interestingly, the level of nDNA damage correlated inversely with the total functional capacity (TFC) score; an established functional score of HD. Our data show that PBMCs are a promising source to monitor HD progression and highlights nDNA damage and diverging mitochondrial and nuclear genome responses representing early cellular impairments in HD.

Huntington's disease: novel therapeutic perspectives hanging in the balance

INTRODUCTION

Huntington's disease (HD), an autosomal dominant neurodegenerative disorder caused by an expansion of CAG repeats in the huntingtin gene, has long been characterized by the presence of motor symptoms due to the loss of striatal projection neurons. Cognitive dysfunction and neuropsychiatric symptoms are also present and they occur in the absence of cell death in most mouse models, pointing to neuronal dysfunction and abnormal synaptic plasticity as causative mechanisms. Areas covered: Here, we focus on those common mechanisms altered by the presence of mutant huntingtin affecting corticostriatal and hippocampal function as therapeutic targets that could prove beneficial to ameliorate both cognitive and motor function in HD. Specifically, we discuss the importance of reestablishing the balance in (1) synaptic/extrasynaptic N-methyl-D-aspartate receptor signaling, (2) mitochondrial dynamics/trafficking, (3) TrkB/p75^NTR signaling, and (4) transcriptional activity. Expert opinion: Mutant huntingtin has a broad impact on multiple cellular processes, which makes it very challenging to design a curative therapeutic strategy. As we point out here, novel therapeutic interventions should look for multi-purpose drugs targeting common and early affected processes leading to corticostriatal and hippocampal dysfunction that additionally operate in a feedforward vicious cycle downstream the activation of extrasynaptic N-methyl-D-aspartate receptor.

Neurocardiovascular deficits in the Q175 mouse model of Huntington's disease

Cardiovascular dysautonomia as well as the deterioration of circadian rhythms are among the earliest detectable pathophysiological changes in individuals with Huntington's disease (HD). Preclinical research requires mouse models that recapitulate disease symptoms and the Q175 knock-in model offers a number of advantages but potential autonomic dysfunction has not been explored. In this study, we sought to test the dual hypotheses that cardiovascular dysautonomia can be detected early in disease progression in the Q175 model and that this dysfunction varies with the daily cycle. Using radiotelemetry implants, we observed a significant reduction in the diurnal and circadian activity rhythms in the Q175 mutants at the youngest ages. By middle age, the autonomically driven rhythms in core body temperature were highly compromised, and the Q175 mutants exhibited striking episodes of hypothermia that increased in frequency with mutant huntingtin gene dosage. In addition, Q175 mutants showed higher resting heart rate (HR) during sleep and greatly reduced correlation between activity and HR HR variability was reduced in the mutants in both time and frequency domains, providing more evidence of autonomic dysfunction. Measurement of the baroreceptor reflex revealed that the Q175 mutant could not appropriately increase HR in response to a pharmacologically induced decrease in blood pressure. Echocardiograms showed reduced ventricular mass and ejection fraction in mutant hearts. Finally, cardiac histopathology revealed localized points of fibrosis resembling those caused by myocardial infarction. Thus, the Q175 mouse model of HD exhibits cardiovascular dysautonomia similar to that seen in HD patients with prominent sympathetic dysfunction during the resting phase of the activity rhythm.

Molecular mechanisms of heart failure: insights from Drosophila

Heart failure places an enormous burden on health and economic systems worldwide. It is a complex disease that is profoundly influenced by both genetic and environmental factors. Neither the molecular mechanisms underlying heart failure nor effective prevention strategies are fully understood. Fortunately, relevant aspects of human heart failure can be experimentally studied in tractable model animals, including the fruit fly, Drosophila, allowing the in vivo application of powerful and sophisticated molecular genetic and physiological approaches. Heart failure in Drosophila, as in humans, can be classified into dilated cardiomyopathies and hypertrophic cardiomyopathies. Critically, many genes and cellular pathways directing heart development and function are evolutionarily conserved from Drosophila to humans. Studies of molecular mechanisms linking aging with heart failure have revealed that genes involved in aging-associated energy homeostasis and oxidative stress resistance influence cardiac dysfunction through perturbation of IGF and TOR pathways. Importantly, ion channel proteins, cytoskeletal proteins, and integrins implicated in aging of the mammalian heart have been shown to play significant roles in heart failure. A number of genes previously described having roles in development of the Drosophila heart, such as genes involved in Wnt signaling pathways, have recently been shown to play important roles in the adult fly heart. Moreover, the fly model presents opportunities for innovative studies that cannot currently be pursued in the mammalian heart because of technical limitations. In this review, we discuss progress in our understanding of genes, proteins, and molecular mechanisms that affect the Drosophila adult heart and heart failure.

Targeting ENT1 and adenosine tone for the treatment of Huntington's disease

Huntington's disease (HD) is caused by an abnormal CAG expansion in the exon 1 of huntingtin gene. The treatment of HD is an unmet medical need. Given the important role of adenosine in modulating brain activity, in this study, levels of adenosine and adenine nucleotides in the cerebral spinal fluid of patients with HD and in the brain of two mouse models of HD (R6/2 and Hdh150Q) were analysed. The expression and activity of ENT1 in the striatum of mice with HD were measured. Targeting adenosine tone for treating HD was examined in R6/2 mice by genetic removal of ENT1 and by giving an ENT1 inhibitor, respectively. The results showed that the adenosine homeostasis is dysregulated in the brain of patients and mice with HD. In patients, the ratio of adenosine/ATP in the cerebral spinal fluid was negatively correlated with the disease duration, and tended to have a positive correlation with independence scale and functional capacity. In comparison to controls, mRNA level of ENT1 was higher in the striatum of R6/2 and Hdh150Q mice. Intrastriatal administration of ENT1 inhibitors increased extracellular level of adenosine in the striatum of R6/2 mice to a much higher level than controls. Chronic inhibition of ENT1 or by genetic removal of ENT1 enhanced the survival of R6/2 mice. Collectively, adenosine homeostasis and ENT1 expression are altered in HD. The inhibition of ENT1 can enhance extracellular adenosine level and be a potential therapeutic approach for treating HD.

Proteostasis in cardiac health and disease

The incidence and prevalence of cardiac diseases, which are the main cause of death worldwide, are likely to increase because of population ageing. Prevailing theories about the mechanisms of ageing feature the gradual derailment of cellular protein homeostasis (proteostasis) and loss of protein quality control as central factors. In the heart, loss of protein patency, owing to flaws in genetically-determined design or because of environmentally-induced 'wear and tear', can overwhelm protein quality control, thereby triggering derailment of proteostasis and contributing to cardiac ageing. Failure of protein quality control involves impairment of chaperones, ubiquitin-proteosomal systems, autophagy, and loss of sarcomeric and cytoskeletal proteins, all of which relate to induction of cardiomyocyte senescence. Targeting protein quality control to maintain cardiac proteostasis offers a novel therapeutic strategy to promote cardiac health and combat cardiac disease. Currently marketed drugs are available to explore this concept in the clinical setting.

3-NP-induced Huntington's-like disease impairs Nrf2 activation without loss of cardiac function in aged rats

Cardiovascular diseases (CVDs) are one of the leading causes of death in patients over 60years with Huntington's disease (HD). Here, we investigated if age-related oxidative stress (OS) is a relevant factor to develop cardiac damage in an in vivo model of striatal neurodegeneration induced by 3-nitropropionic acid (3-NP). We also evaluated the potential effect of tert-butylhydroquinone (tBHQ) to increase the Nrf2-regulated antioxidant response in hearts from adult and aged rats intoxicated with 3-NP. Our results showed that 3-NP-treatment did not induce cardiac dysfunction, neither in adult nor in aged rats. However, at the cellular level, adult animals showed higher susceptibility to 3-NP-induced damage than aged rats, which suggest that chronic oxidative stress ongoing during aging might have induced an hormetic response that probably prevented from further 3-NP damage. We also found that the oxidative unbalance concurs with unresponsiveness of the Nrf2-mediated antioxidant response in old animals.

High-resolution respirometry of fine-needle muscle biopsies in pre-manifest Huntington's disease expansion mutation carriers shows normal mitochondrial respiratory function

Alterations in mitochondrial respiration are an important hallmark of Huntington's disease (HD), one of the most common monogenetic causes of neurodegeneration. The ubiquitous expression of the disease causing mutant huntingtin gene raises the prospect that mitochondrial respiratory deficits can be detected in skeletal muscle. While this tissue is readily accessible in humans, transgenic animal models offer the opportunity to cross-validate findings and allow for comparisons across organs, including the brain. The integrated respiratory chain function of the human vastus lateralis muscle was measured by high-resolution respirometry (HRR) in freshly taken fine-needle biopsies from seven pre-manifest HD expansion mutation carriers and nine controls. The respiratory parameters were unaffected. For comparison skeletal muscle isolated from HD knock-in mice (HdhQ111) as well as a broader spectrum of tissues including cortex, liver and heart muscle were examined by HRR. Significant changes of mitochondrial respiration in the HdhQ knock-in mouse model were restricted to the liver and the cortex. Mitochondrial mass as quantified by mitochondrial DNA copy number and citrate synthase activity was stable in murine HD-model tissue compared to control. mRNA levels of key enzymes were determined to characterize mitochondrial metabolic pathways in HdhQ mice. We demonstrated the feasibility to perform high-resolution respirometry measurements from small human HD muscle biopsies. Furthermore, we conclude that alterations in respiratory parameters of pre-manifest human muscle biopsies are rather limited and mirrored by a similar absence of marked alterations in HdhQ skeletal muscle. In contrast, the HdhQ111 murine cortex and liver did show respiratory alterations highlighting the tissue specific nature of mutant huntingtin effects on respiration.

Protein post-translational modifications: In silico prediction tools and molecular modeling

Post-translational modifications (PTMs) occur in almost all proteins and play an important role in numerous biological processes by significantly affecting proteins' structure and dynamics. Several computational approaches have been developed to study PTMs (e.g., phosphorylation, sumoylation or palmitoylation) showing the importance of these techniques in predicting modified sites that can be further investigated with experimental approaches. In this review, we summarize some of the available online platforms and their contribution in the study of PTMs. Moreover, we discuss the emerging capabilities of molecular modeling and simulation that are able to complement these bioinformatics methods, providing deeper molecular insights into the biological function of post-translational modified proteins.

Huntington's disease blood and brain show a common gene expression pattern and share an immune signature with Alzheimer's disease

There is widespread transcriptional dysregulation in Huntington's disease (HD) brain, but analysis is inevitably limited by advanced disease and postmortem changes. However, mutant HTT is ubiquitously expressed and acts systemically, meaning blood, which is readily available and contains cells that are dysfunctional in HD, could act as a surrogate for brain tissue. We conducted an RNA-Seq transcriptomic analysis using whole blood from two HD cohorts, and performed gene set enrichment analysis using public databases and weighted correlation network analysis modules from HD and control brain datasets. We identified dysregulated gene sets in blood that replicated in the independent cohorts, correlated with disease severity, corresponded to the most significantly dysregulated modules in the HD caudate, the most prominently affected brain region, and significantly overlapped with the transcriptional signature of HD myeloid cells. High-throughput sequencing technologies and use of gene sets likely surmounted the limitations of previously inconsistent HD blood expression studies. Our results suggest transcription is disrupted in peripheral cells in HD through mechanisms that parallel those in brain. Immune upregulation in HD overlapped with Alzheimer's disease, suggesting a common pathogenic mechanism involving macrophage phagocytosis and microglial synaptic pruning, and raises the potential for shared therapeutic approaches.

Transcriptional Signature of an Altered Purine Metabolism in the Skeletal Muscle of a Huntington's Disease Mouse Model

Huntington's disease (HD) is a fatal neurodegenerative disorder, caused by a polyglutamine expansion in the huntingtin protein (HTT). HD has a peripheral component to its pathology: skeletal muscles are severely affected, leading to atrophy, and malfunction in both pre-clinical and clinical settings. We previously used two symptomatic HD mouse models to demonstrate the impairment of the contractile characteristics of the hind limb muscles, which was accompanied by a significant loss of function of motor units. The mice displayed a significant reduction in muscle force, likely because of deteriorations in energy metabolism, decreased oxidation, and altered purine metabolism. There is growing evidence suggesting that HD-related skeletal muscle malfunction might be partially or completely independent of CNS degeneration. The pathology might arise from mutant HTT within muscle (loss or gain of function). Hence, it is vital to identify novel peripheral biomarkers that will reflect HD skeletal muscle atrophy. These will be important for upcoming clinical trials that may target HD peripherally. In order to identify potential biomarkers that might reflect muscle metabolic changes, we used qPCR to validate key gene transcripts in different skeletal muscle types. Consequently, we report a number of transcript alterations that are linked to HD muscle pathology.

Motor phenotype is not associated with vascular dysfunction in symptomatic Huntington's disease transgenic R6/2 (160 CAG) mice

Whereas Huntington’s disease (HD) is unequivocally a neurological disorder, a critical mass of emerging studies highlights the occurrence of peripheral pathology like cardiovascular defects in both animal models and humans. The overt impairment in cardiac function is normally expected to be associated with peripheral vascular dysfunction, however whether this assumption is reasonable or not in HD is still unknown. In this study we functionally characterized the vascular system in R6/2 mouse model (line 160 CAG), which recapitulates several features of human pathology including cardiac disease. Vascular reactivity in different arterial districts was determined by wire myography in symptomatic R6/2 mice and age-matched wild type (WT) littermates. Disease stage was assessed by using well-validated behavioural tests like rotarod and horizontal ladder task. Surprisingly, no signs of vascular dysfunction were detectable in symptomatic mice and no link with motor phenotype was found.

Low cancer prevalence in polyglutamine expansion diseases

Objective:

Polyglutamine (PolyQ) diseases are dominantly transmitted neurologic disorders, caused by coding and expanded CAG trinucleotide repeats. Cancer was reported retrospectively to be rare in patients with PolyQ diseases and we aimed to investigate its prevalence in France.

Methods:

Consecutive patients with Huntington disease (HD) and spinocerebellar ataxia (SCA) were questioned about cancer, cardiovascular diseases, and related risk factors in 4 university hospitals in Paris, Toulouse, Strasbourg, and Montpellier. Standardized incidence ratios (SIR), based on age- and sex-adjusted rate of the French population, were assessed for different types of cancer.

Results:

We questioned 372 patients with HD and 134 patients with SCA. SIR showed significantly reduced risk of cancer in HD: 23 observed cases vs 111.05 expected ones (SIR 0.21, 95% confidence interval [CI] 0.13–0.31), as well as in SCA: 7 observed cases vs 34.73 expected (SIR 0.23, 95% CI 0.08–0.42). This was surprising since risk behavior for cancer was increased in these patients, with significantly greater tobacco and alcohol consumption in patients with HD vs patients with SCA (p < 0.0056). There was no association between CAG repeat size and cancer or cardiovascular disease. However, in patients with HD, skin cancers were more frequent than expected (5 vs 0.98, SIR 5.11, 95% CI 1.65–11.95).

Conclusions:

There was a decreased cancer rate in PolyQ diseases despite high incidence of risk factors. Intriguingly, skin cancer incidence was higher, suggesting a crosstalk between neurodegeneration and skin tumorigenesis.

Delayed Onset and Reduced Cognitive Deficits through Pre-Conditioning with 3-Nitropropionic Acid is Dependent on Sex and CAG Repeat Length in the R6/2 Mouse Model of Huntington's Disease

BACKGROUND

Impairments in energy metabolism are implicated in Huntington's disease (HD) pathogenesis. Reduced levels of the mitochondrial enzyme succinate dehydrogenase (SDH), the main element of complex II, are observed post mortem in the brains of HD patients, and energy metabolism defects have been identified in both presymptomatic and symptomatic HD patients.

OBJECTIVE

Chemical preconditioning with 3-nitropropionic acid (3-NP), an irreversible inhibitor of SDH, has been shown to increase tolerance against experimental hypoxia in both heart and brain. Here we studied the effect of chronic preconditioning in the R6/2 mouse model of HD using mice carrying CAG repeat lengths of either 250 or 400 repeats. Both are transgenic fragment models, with 250CAG mice having a more rapid disease progression than 400CAG mice.

METHODS

Low doses of 3-NP (24 mg/kg) were administered via the drinking water and the effect on phenotype progression and cognition function assessed.

RESULTS

After 3-NP treatment there were significant improvements in all aspects of the behavioural phenotype, apart from body weight, with timing and magnitude of improvements dependent on both CAG repeat length and sex. Specifically, a delay in the deterioration of general health (as shown by delayed onset of glycosuria and increased survival) was seen in both male and female 400CAG mice and in female 250CAG mice and was consistent with improved appearance of 3-NP treated R6/2 mice. Male 250CAG mice showed improvements but these were short term, and 3-NP treatment eventually had deleterious effects on their survival rate. When cognitive performance of 250CAG mice was assessed using a two-choice discrimination touchscreen task, we found that female mice showed significant improvements.

DISCUSSION

Together, our results support the idea that energy metabolism contributes to the pathogenesis of HD, and suggest that improving energy deficits might be a therapeutically useful target.

Cardiac Fas-Dependent and Mitochondria-Dependent Apoptotic Pathways in a Transgenic Mouse Model of Huntington's Disease

Huntington's disease is an autosomal dominant neurodegenerative disease caused by a CAG repeat expansion in the huntingtin gene. Heart disease is the second leading cause of death in patients with Huntington's disease. This study was to evaluate whether cardiac Fas-dependent and mitochondria-dependent apoptotic pathways are activated in transgenic mice with Huntington's disease. Sixteen Huntington's disease transgenic mice (HD) and sixteen wild-type (WT) littermates were studied at 10.5 weeks of age. The cardiac characteristics, myocardial architecture, and two major apoptotic pathways in the excised left ventricle from mice were measured by histopathological analysis, Western blotting, and TUNEL assays. The whole heart weight and the left ventricular weight decreased significantly in the HD group, as compared to the WT group. Abnormal myocardial architecture, enlarged interstitial spaces, and more cardiac TUNEL-positive cells were observed in the HD group. The key components of Fas-dependent apoptosis (TNF-alpha, TNFR1, Fas ligand, Fas death receptors, FADD, activated caspase-8, and activated caspase-3) and the key components of mitochondria-dependent apoptosis (Bax, Bax-to-Bcl-2 ratio, cytosolic cytochrome c, activated caspase-9, and activated caspase-3) increased significantly in the hearts of the HD group. Cardiac Fas-dependent and mitochondria-dependent apoptotic pathways were activated in transgenic mice with Huntington's disease, which might provide one of possible mechanisms to explain why patients with Huntington's disease will develop heart failure.

Discovery of Therapeutic Approaches for Polyglutamine Diseases: A Summary of Recent Efforts

Polyglutamine (PolyQ) diseases are a group of neurodegenerative disorders caused by the expansion of cytosine-adenine-guanine (CAG) trinucleotide repeats in the coding region of specific genes. This leads to the production of pathogenic proteins containing critically expanded tracts of glutamines. Although polyQ diseases are individually rare, the fact that these nine diseases are irreversibly progressive over 10 to 30 years, severely impairing and ultimately fatal, usually implicating the full-time patient support by a caregiver for long time periods, makes their economic and social impact quite significant. This has led several researchers worldwide to investigate the pathogenic mechanism(s) and therapeutic strategies for polyQ diseases. Although research in the field has grown notably in the last decades, we are still far from having an effective treatment to offer patients, and the decision of which compounds should be translated to the clinics may be very challenging. In this review, we provide a comprehensive and critical overview of the most recent drug discovery efforts in the field of polyQ diseases, including the most relevant findings emerging from two different types of approaches-hypothesis-based candidate molecule testing and hypothesis-free unbiased drug screenings. We hereby summarize and reflect on the preclinical studies as well as all the clinical trials performed to date, aiming to provide a useful framework for increasingly successful future drug discovery and development efforts.

Functional impairment of precerebral arteries in Huntington disease

BACKGROUND

Cardiovascular pathology of Huntington disease (HD) appears to be complex; while microvascular dysfunction seems to appear early, deaths from cardiomyopathy and stroke might occur in the late phase of HD.

METHODS

Our study evaluated global risk factors for coronary heart disease (CHD), structure and function of precerebral arteries in 41 HD subjects and 41 matched controls. HD subjects were divided into groups by the United Huntington disease rating scale (presymptomatic-PHD, early-EHD, midstage-MHD and late-LHD). CHD risk factors assessment and Doppler examination of precerebral arteries were performed, including measurements of the carotid artery intima-media thickness (IMT), and parameters indicating local carotid artery distensibility (stiffness index β, pulse wave velocity, pressure strain elasticity module and carotid artery compliance).

RESULTS

In the HD and controls we identified a comparable number of non-obstructive plaques (<50% lumen narrowing). No obstructive plaques (>50% lumen narrowing) were found. There was significantly increased IMT in MHD. In PHD and EHD the parameters of arterial stiffness were significantly higher and the carotid artery compliance was significantly lower.

CONCLUSIONS

Our results reveal functional vascular pathology in PHD, EHD, and MHD. Precerebral arteries dysfunction in HD therefore appears to be mostly functional and in agreement with recently described autonomic nervous system changes in HD.

An impaired metabolism of nucleotides underpins a novel mechanism of cardiac remodeling leading to Huntington's disease related cardiomyopathy

Huntington's disease (HD) is mainly thought of as a neurological disease, but multiple epidemiological studies have demonstrated a number of cardiovascular events leading to heart failure in HD patients. Our recent studies showed an increased risk of heart contractile dysfunction and dilated cardiomyopathy in HD pre-clinical models. This could potentially involve metabolic remodeling, that is a typical feature of the failing heart, with reduced activities of high energy phosphate generating pathways. In this study, we sought to identify metabolic abnormalities leading to HD-related cardiomyopathy in pre-clinical and clinical settings. We found that HD mouse models developed a profound deterioration in cardiac energy equilibrium, despite AMP-activated protein kinase hyperphosphorylation. This was accompanied by a reduced glucose usage and a significant deregulation of genes involved in de novo purine biosynthesis, in conversion of adenine nucleotides, and in adenosine metabolism. Consequently, we observed increased levels of nucleotide catabolites such as inosine, hypoxanthine, xanthine and uric acid, in murine and human HD serum. These effects may be caused locally by mutant HTT, via gain or loss of function effects, or distally by a lack of trophic signals from central nerve stimulation. Either may lead to energy equilibrium imbalances in cardiac cells, with activation of nucleotide catabolism plus an inhibition of re-synthesis. Our study suggests that future therapies should target cardiac mitochondrial dysfunction to ameliorate energetic dysfunction. Importantly, we describe the first set of biomarkers related to heart and skeletal muscle dysfunction in both pre-clinical and clinical HD settings.

Altered lipid metabolism in Drosophila model of Huntington's disease

Huntington’s disease (HD) is late-onset, progressive neurodegenerative disorder caused by expansion of polyglutamine (polyQ) repeat within Huntingtin (Htt) protein. In HD patients, energy-related manifestations such as modulation of weight during entire course of disease with energy deficit at terminal stage have been reported, however, underlying reason remains elusive till date. Lipids, carbohydrate and protein constitute a predominant fraction of body’s energy reservoir and perturbation in their homeostasis may influence weight. To discern role of these energy molecules in weight alteration, we quantified them in an in vivo transgenic Drosophila model of HD. We document that diseased flies exhibit change in weight due to an altered lipid metabolism, as evident from considerably high lipid levels at the time of disease onset followed by a pathologic decline at end-stage. An alteration in intracellular lipid droplet size suggested altered cellular lipid turnover. Furthermore, diseased flies displayed substantial changes in carbohydrate and protein content. Interestingly, alteration in weight and lipid levels are independent of the feeding pattern in diseased condition and exhibit weak correlation with insulin-like peptide or adipokinetic hormone producing cells. We propose that therapeutic intervention aimed at restoring lipid levels and associated metabolic pathways may improve longevity and quality of patient’s life.

White Adipose Tissue Browning in the R6/2 Mouse Model of Huntington's Disease

Huntington’s disease (HD) is a fatal, autosomal dominantly inherited neurodegenerative disorder, characterised not only by progressive cognitive, motor and psychiatric impairments, but also of peripheral pathology. In both human HD and in mouse models of HD there is evidence of increased energy expenditure and weight loss, alongside altered body composition. Unlike white adipose tissue (WAT), brown adipose tissue (BAT), as well as brown-like cells within WAT, expresses the mitochondrial protein, uncoupling protein 1 (UCP1). UCP1 enables dissociation of cellular respiration from ATP utilization, resulting in the release of stored energy as heat. Hyperplasia of brown/beige cells in WAT has been suggested to enhance energy expenditure. In this study, we therefore investigated the gene expression profile, histological appearance, response to cold challenge and functional aspects of WAT in the R6/2 HD mouse model and selected WAT gene expression in the full-length Q175 mouse model of HD. WAT from R6/2 mice contained significantly more brown-like adipocyte regions and had a gene profile suggestive of the presence of brown-like adipocytes, such as higher Ucp1 expression. Cold exposure induced Ucp1 expression in R6/2 inguinal WAT to a markedly higher degree as compared to the thermogenic response in WT WAT. Alongside this, gene expression of transcription factors (Zfp516 and Pparα), important inducers of WAT browning, were increased in R6/2 inguinal WAT, and Creb1 was highlighted as a key transcription factor in HD. In addition to increased WAT Ucp1 expression, a trend towards increased mitochondrial oxygen consumption due to enhanced uncoupling activity was found in inguinal R6/2 WAT. Key gene expressional changes (increased expression of (Zfp516 and Pparα)) were replicated in inguinal WAT obtained from Q175 mice. In summary, for the first time, we here show that HD mouse WAT undergoes a process of browning, resulting in molecular and functional alterations that may contribute to the weight loss and altered metabolism observed with disease progression.

Methylene Blue Partially Rescues Heart Defects in a Drosophila Model of Huntington's Disease

BACKGROUND

Huntington's disease (HD) is a Polyglutamine disease caused by the presence of CAG repeats in the first exon of Huntingtin (Htt), a large protein with multiple functions. In addition to neurodegeneration of specific brain regions, notably the striatum, HD also shows alterations in peripheral tissues, such as the heart, skeletal muscles or peripheral endocrine glands. Mutant Huntingtin (mHtt)-driven mitochondrial impairment may underlie some of the CNS and peripheral tissues dysfunctions, especially in tissues with high energy demand such as the heart.

OBJECTIVE

The aim of this study is to characterize two new inducible Drosophila HD heart models and to assay the therapeutic potential of methylene blue in these HD models.

METHODS

We report the construction of inducible Drosophila HD heart models, expressing two Nter fragments of the protein encompassing either exon 1 or the first 171 amino acids and the characterization of heart phenotypes in vivo.

RESULTS

We show that both mHtt fragments are able to impair fly cardiac function with different characteristics. Additionally, expression of mHtt, which was limited to adulthood only, leads to mild heart impairment, as opposed to a strong and age-dependent phenotype observed when mHtt expression was driven during both developmental and adult stages. We report that treatment with methylene blue (MB), a protective compound in mitochondria-related diseases, partially protects the fly's heart against mHtt-induced toxicity, but does not rescue neuronal or glial phenotypes in other fly models of HD. This may be linked to its low penetration through the fly's blood-brain barrier.

CONCLUSIONS

Our data suggest that improvement of mitochondrial function by MB, or related compounds, could be an efficient therapeutic strategy to prevent cardiac failure in HD patients.

Developing stem cell therapies for juvenile and adult-onset Huntington's disease

Stem cell therapies have been explored as a new avenue for the treatment of neurologic disease and damage within the CNS in part due to their native ability to mimic repair mechanisms in the brain. Mesenchymal stem cells have been of particular clinical interest due to their ability to release beneficial neurotrophic factors and their ability to foster a neuroprotective microenviroment. While early stem cell transplantation therapies have been fraught with technical and political concerns as well as limited clinical benefits, mesenchymal stem cell therapies have been shown to be clinically beneficial and derivable from nonembryonic, adult sources. The focus of this review will be on emerging and extant stem cell therapies for juvenile and adult-onset Huntington’s disease.