Ethyl-EPA treatment improves motor dysfunction, but not neurodegeneration in the YAC128 mouse model of Huntington disease

Influence of dietary patterns in the pathophysiology of Huntington's Disease: A literature review

Huntington's disease (HD), a rare autosomal dominant neurodegenerative disease, causes the gradual deterioration of neurons in the basal ganglia, specifically in the striatum. HD displays a wide range of symptoms, from motor disturbances such as chorea, dystonia, and bradykinesia to more debilitating symptoms such as cognitive decline, behavioral abnormalities, and psychiatric disturbances. Current research suggests the potential use of dietary interventions as viable strategies for slowing the progression of HD. Most notably, the Mediterranean, vegan, carnivore, paleo, and ketogenic diets have gained attention due to their hypothesized impact on neuroprotection and symptomatic modulation in various neurodegenerative disorders. Despite substantial nutritional differences among these diets, they share a fundamental premise-that dietary factors have an influential impact in modifying pertinent biological pathways linked to neurodegeneration. Understanding the intricate interactions between these dietary regimens and HD pathogenesis could open avenues for personalized interventions tailored to the individual's specific needs and genetic background. Ultimately, elucidating the multifaceted effects of these diets on HD offers a promising framework for developing comprehensive therapeutic approaches that integrate dietary strategies with conventional treatments.

From Pathogenesis to Therapeutics: A Review of 150 Years of Huntington's Disease Research

Huntington's disease (HD) is a debilitating neurodegenerative genetic disorder caused by an expanded polyglutamine-coding (CAG) trinucleotide repeat in the huntingtin (HTT) gene. HD behaves as a highly penetrant dominant disorder likely acting through a toxic gain of function by the mutant huntingtin protein. Widespread cellular degeneration of the medium spiny neurons of the caudate nucleus and putamen are responsible for the onset of symptomology that encompasses motor, cognitive, and behavioural abnormalities. Over the past 150 years of HD research since George Huntington published his description, a plethora of pathogenic mechanisms have been proposed with key themes including excitotoxicity, dopaminergic imbalance, mitochondrial dysfunction, metabolic defects, disruption of proteostasis, transcriptional dysregulation, and neuroinflammation. Despite the identification and characterisation of the causative gene and mutation and significant advances in our understanding of the cellular pathology in recent years, a disease-modifying intervention has not yet been clinically approved. This review includes an overview of Huntington's disease, from its genetic aetiology to clinical presentation and its pathogenic manifestation. An updated view of molecular mechanisms and the latest therapeutic developments will also be discussed.

The Role of Dietary Lipids in Cognitive Health: Implications for Neurodegenerative Disease

Neurodegenerative diseases are a group of disorders characterised by progressive loss of brain function. The most common of these is Alzheimer's disease, a form of dementia. Intake of macro- and micro-nutrients impacts brain function, including memory, learning, mood, and behaviour. Lipids, particularly phospholipids and sphingolipids, are crucial structural components of neural tissues and significantly affect cognitive function. The importance of functional foods in preventing cardiovascular disease is well-documented in the current literature. However, the significance of such foods for central nervous system health and neurodegenerative diseases is less recognized. Gut microbiome composition affects cognitive health and function, and dietary lipids are known to influence gut health. Thus, this review will discuss different sources of dietary lipids and their effect on cognitive functioning and their interaction with the gut microbiome in the context of neurodegenerative disease.

Human iPSC-derived neural precursor cells differentiate into multiple cell types to delay disease progression following transplantation into YAC128 Huntington's disease mouse model

OBJECTIVES

To investigate whether human HLA-homozygous induced pluripotent stem cell (iPSC)-derived neural precursor cells (iPSC-NPCs) can provide functional benefits in Huntington's disease (HD), we transplanted them into the YAC128 transgenic HD mouse model.

MATERIALS AND METHODS

CHAi001-A, an HLA-homozygous iPSC line (A*33:03-B*44:03-DRB1*13:02), was differentiated into neural precursor cells, and then, they were transplanted into 6 months-old YAC128 mice. Various behavioural and histological analyses were performed for five months after transplantation.

RESULTS

Motor and cognitive functions were significantly improved in transplanted animals. Cells transplanted in the striatum showed multipotential differentiation. Five months after transplantation, the donor cells had differentiated into neurons, oligodendrocytes and astrocytes. Transplantation restored DARPP-32 expression, synaptophysin density, myelin basic protein expression in the corpus callosum and astrocyte function.

CONCLUSION

Altogether, these results strongly suggest that iPSC-NPCs transplantation induces neuroprotection and functional recovery in a mouse model of HD and should be taken forward for clinical trials in HD patients.

Rich fatty acids diet of fish and olive oils modifies membrane properties in striatal rat synaptosomes

Background: Essential fatty acids (EFAs) and non-essential fatty acids (nEFAs) exert experimental and clinical neuroprotection in neurodegenerative diseases. The main EFAs, eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), nEFAs, and oleic acid (OA) contained in olive and fish oils are inserted into the cell membranes, but the exact mechanism through which they exert neuroprotection is still unknown. Objectives and Methods: In this study, we assessed the fatty acids content and membrane fluidity in striatal rat synaptosomes after fatty acid-rich diets (olive- or a fish-oil diet, 15% w/w). Then, we evaluated the effect of enriching striatum synaptosomes with fatty acids on the oxidative damage produced by the prooxidants ferrous sulfate (FeSO4) or quinolinic acid (QUIN). Results and Discussion: Lipid profile analysis in striatal synaptosomes showed that EPA content increased in the fish oil group in comparison with control and olive groups. Furthermore, we found that synaptosomes enriched with fatty acids and incubated with QUIN or FeSO4 showed a significant oxidative damage reduction. Results suggest that EFAs, particularly EPA, improve membrane fluidity and confer antioxidant effect.

Reactive Species in Huntington Disease: Are They Really the Radicals You Want to Catch?

Huntington disease (HD) is a neurodegenerative condition and one of the so-called rare or minority diseases, due to its low prevalence (affecting 1–10 of every 100,000 people in western countries). The causative gene, HTT, encodes huntingtin, a protein with a yet unknown function. Mutant huntingtin causes a range of phenotypes, including oxidative stress and the activation of microglia and astrocytes, which leads to chronic inflammation of the brain. Although substantial efforts have been made to find a cure for HD, there is currently no medical intervention able to stop or even delay progression of the disease. Among the many targets of therapeutic intervention, oxidative stress and inflammation have been extensively studied and some clinical trials have been promoted to target them. In the present work, we review the basic research on oxidative stress in HD and the strategies used to fight it. Many of the strategies to reduce the phenotypes associated with oxidative stress have produced positive results, yet no substantial functional recovery has been observed in animal models or patients with the disease. We discuss possible explanations for this and suggest potential ways to overcome it.

Efficacy of ethyl-EPA as a treatment for Huntington disease: a systematic review and meta-analysis

OBJECTIVE

After MRI studies suggested the efficacy of ethyl-EPA in reducing the progressive brain atrophy in Huntington disease (HD), trials were conducted to test its efficacy as a treatment for HD. Trials that continued for 6 months did not find any significant improvement, urging discontinuation of the drug. However, trials that continued for 12 months indicated improvement of motor functions in these patients.

METHODS

We searched 12 electronic databases to find randomised clinical trials relevant to our inclusion criteria. After screening, only five papers were included. Continuous and binary variables were analysed to compute the pooled mean difference (MD) and risk ratio (RR), respectively. Quality effect model meta-analysis was used as a post hoc analysis for studies at 12 months.

FINDINGS

Meta-analysis indicated that ethyl-eicosapentaenoic acid (EPA) has no significant effect on any scale of HD at 6 months. At 12 months, two studies suggested significant improvements of the Total Motor Score and Total Motor Score-4 in both fixed and quality effect models [MD = -2.720, 95% CI (-4.76, -.68), p = 0.009; MD = -2.225, 95% CI (-3.842, -0.607), p = 0.007], respectively. Maximal chorea score showed significant results [MD = -1.013, 95% CI (-1.793, -0.233), p = 0.011] in only fixed-effect model, while no improvement was detected for Stroop colour naming test or symbol digit modality.

CONCLUSION

Meta-analysis indicated a significant improvement of motor scores only after 12 months. These results should be interpreted cautiously because only two studies had assessed the efficacy of ethyl-EPA after 12 months with one of them having a 6-month open-label phase.

Are immunotherapies for Huntington's disease a realistic option?

There is compelling evidence that the pathophysiology of many neurodegenerative diseases includes dysregulation of the immune system, with some elements that precede disease onset. However, if these alterations are prominent, why have clinical trials targeting this system failed to translate into long-lasting meaningful benefits for patients? This review focuses on Huntington's disease, a genetic disorder marked by notable cerebral and peripheral inflammation. We summarize ongoing and completed clinical trials that have involved pharmacological approaches to inhibit various components of the immune system and their pre-clinical correlates. We then discuss new putative treatment strategies using more targeted immunotherapies such as vaccination and intrabodies and how these may offer new hope in the treatment of Huntington's disease as well as other neurodegenerative diseases.

Altering cortical input unmasks synaptic phenotypes in the YAC128 cortico-striatal co-culture model of Huntington disease

BACKGROUND

Huntington disease (HD) is a fatal neurodegenerative disorder caused by a CAG expansion in the huntingtin (HTT) gene, leading to selective and progressive neuronal death predominantly in the striatum. Mutant HTT expression causes dysfunctional cortico-striatal (CS) transmission, loss of CS synapses, and striatal medium spiny neuron (MSN) dendritic spine instability prior to neuronal death. Co-culturing cortical and striatal neurons in vitro promotes the formation of functional CS synapses and is a widely used approach to elucidate pathogenic mechanisms of HD and to validate potential synapto-protective therapies. A number of relevant in vivo synaptic phenotypes from the YAC128 HD mouse model, which expresses full-length transgenic human mutant HTT, are recapitulated in CS co-culture by 21 days in vitro (DIV). However, striatal spine loss, which occurs in HD patients and in vivo animal models, has been observed in YAC128 CS co-culture in some studies but not in others, leading to difficulties in reproducing and interpreting results. Here, we investigated whether differences in the relative proportion of cortical and striatal neurons alter YAC128 synaptic phenotypes in this model.

RESULTS

YAC128 MSNs in 1:1 CS co-culture exhibited impaired dendritic length and complexity compared to wild-type, whereas reducing cortical input using a 1:3 CS ratio revealed a dramatic loss of YAC128 MSN dendritic spines. Chimeric experiments determined that this spine instability was primarily cell autonomous, depending largely on mutant HTT expression in striatal neurons. Moreover, we found that spontaneous electrophysiological MSN activity correlated closely with overall dendritic length, with no differences observed between genotypes in 1:3 co-cultures despite significant YAC128 spine loss. Finally, limiting cortical input with a 1:3 CS ratio impaired the basal survival of YAC128 neurons at DIV21, and this was partially selective for dopamine- and cAMP-regulated phosphoprotein 32-positive MSNs.

CONCLUSIONS

Our findings reconcile previous discordant reports of spine loss in this model, and improve the utility and reliability of the CS co-culture for the development of novel therapeutic strategies for HD.

New directions in therapeutics for Huntington disease

Huntington disease (HD) is an autosomal dominantly inherited neurodegenerative disease that affects motor, cognitive and psychiatric functions, and ultimately leads to death. The pathology of the disease is based on an expansion of CAG repeats in exon 1 of the huntingtin gene on chromosome 4, which produces a mutant huntingtin protein (mHtt). This protein is involved in neurotoxicity and brain atrophy, and can form β-sheets and abnormal mHtt aggregates. Currently, there are no approved effective treatments for HD, although tetrabenazine (Xenazine™) and deutetrabenazine (AUSTEDO™) have been approved for treatment of the motor symptom chorea in HD. This literature review aims to address the latest research on promising therapeutics based on influencing the hypothesized pathological mechanisms.

Oxidative stress is increased in C. elegans models of Huntington's disease but does not contribute to polyglutamine toxicity phenotypes

Huntington's disease (HD) is an adult onset neurodegenerative disorder for which there is currently no cure. While HD patients and animal models of the disease exhibit increased oxidative damage, it is currently uncertain to what extent oxidative stress contributes to disease pathogenesis. In this work, we use a genetic approach to define the role of oxidative stress in HD. We find that a C. elegans model of HD expressing a disease-length polyglutamine tract in the body wall muscle is hypersensitive to oxidative stress and shows an upregulation of antioxidant defense genes, indicating that the HD worm model has increased levels of oxidative stress. To determine whether this increase in oxidative stress contributes to the development of polyglutamine-toxicity phenotypes in this HD model, we examined the effect of deleting individual superoxide dismutase (sod) genes in the HD worm model. As predicted, we found that deletion of sod genes in the HD worm model resulted in a clear increase in sensitivity to oxidative stress. However, we found that increasing oxidative stress in the HD worm model did not exacerbate deficits caused by polyglutamine toxicity. We confirmed these observations in two worm models expressing disease-length polyglutamine tracts in neurons. Furthermore, we found that treatment with antioxidants failed to rescue movement deficits or decrease aggregation in HD worm models. Combined, this suggests that the increase in oxidative stress in worm models of HD does not contribute to the phenotypic deficits observed in these worms, and provides a possible explanation for the failure of antioxidants in HD clinical trials.

Time-Course Analysis of Protein and Lipid Oxidation in the Brains of Yac128 Huntington's Disease Transgenic Mice

Huntington's disease (HD) is caused by an expansion of cytosine-adenine-guanine (CAG) repeats within the coding region of the HD gene, which expresses the protein huntingtin and is characterized by selective degeneration of specific neuronal populations, mainly in the striatum and the cortex. The mechanisms that account for this selective neuronal death are multifaceted, but oxidative stress might play an important role in this process. To determine whether changes in the intracellular redox state will result in oxidative damage to cellular macromolecules with disease progression, we analyzed levels of lipid peroxidation (with the thiobarbituric acid reactive substances [TBARS] assay) and protein carbonyl formation (using the 2,4-dinitrophenylhydrazine reaction) in the cerebellum, cerebral cortex, prefrontal cortex, striatum, and hippocampus of the YAC128 HD mouse model at 3, 6, and 12 months of age. With the exception of a transient increase in protein carbonyl levels in the YAC128 prefrontal cortex at 6 months of age, levels of lipid peroxidation and protein oxidation were not significantly different between YAC128 mice and their age-matched wild-type counterparts in any of the brain regions analyzed up to 12 months of age. However, age-related increases in oxidative stress were observed in various brain regions. These results suggest that lipid and protein oxidative damage is not a major contributor to neurodegeneration in the YAC128 brain up to 12 months of age.

Animal models of Huntington's disease for translation to the clinic: best practices

Mouse models of Huntington's disease (HD) recapitulate many aspects of the human disease. These genetically modified mice are powerful tools that are used not only to examine the pathogenesis of the disease, but also to assess the efficacy of potential new treatments. Disappointingly, in the past few years we have seen the success of potential therapies in animal studies, subsequently followed by failure in clinical trials. We discuss here a number of factors that influence the translatability of findings from the preclinical to the clinical realm. In particular, we discuss issues related to sample size, reporting of information regarding experimental protocols and mouse models, assignment to experimental groups, incorporation of cognitive measures for early phases of the disease, environmental enrichment, surrogate measures for survival, and the use of more than one HD mouse model. Although it is reasonable to question the appropriateness of the animal models used, we argue that it is more parsimonious to assume that improvements in experimental design and publication of negative results will lead to improved translatability to the clinic and insights about HD pathophysiology.

Huntington's disease: an update of therapeutic strategies

Huntington's disease (HD) is an autosomal dominant triplet repeat genetic disease, which results in progressive neuronal degeneration in the neostriatum and neocortex, and associated functional impairments in motor, cognitive, and psychiatric domains. Although the genetic mutation caused by abnormal CAG expansion within the htt gene on chromosome 4p16.3 is identified, the mechanism by which this leads to neuronal cell death and the question of why striatal neurones are targeted both remain unknown. Patients manifest a typical phenotype of sporadic, rapid, involuntary control of limb movement, stiffness of limbs, impaired cognition and severe psychiatric disturbances. There have been a number of therapeutic advances in the treatment of HD, such as fetal neural transplantation, RNA interference (RNAi) and transglutaminase inhibitors (Tgasei). Although there is intensive research into HD and recent findings seem promising, effective therapeutic strategies may not be developed until the next few decades.

Differential Changes in Postsynaptic Density Proteins in Postmortem Huntington's Disease and Parkinson's Disease Human Brains

NMDA and AMPA-type glutamate receptors and their bound membrane-associated guanylate kinases (MAGUKs) are critical for synapse development and plasticity. We hypothesised that these proteins may play a role in the changes in synapse function that occur in Huntington's disease (HD) and Parkinson's disease (PD). We performed immunohistochemical analysis of human postmortem brain tissue to examine changes in the expression of SAP97, PSD-95, GluA2 and GluN1 in human control, and HD- and PD-affected hippocampus and striatum. Significant increases in SAP97 and PSD-95 were observed in the HD and PD hippocampus, and PSD95 was downregulated in HD striatum. We observed a significant increase in GluN1 in the HD hippocampus and a decrease in GluA2 in HD and PD striatum. Parallel immunohistochemistry experiments in the YAC128 mouse model of HD showed no change in the expression levels of these synaptic proteins. Our human data show that major but different changes occur in glutamatergic proteins in HD versus PD human brains. Moreover, the changes in human HD brains differ from those occurring in the YAC128 HD mouse model, suggesting that unique changes occur at a subcellular level in the HD human hippocampus.

The omega-3 fatty acid eicosapentaenoic acid accelerates disease progression in a model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive fatal neurodegenerative disease characterised by loss of motor neurons that currently has no cure. Omega-3 polyunsaturated fatty acids, such as eicosapentaenoic acid (EPA), have many health benefits including neuroprotective and myoprotective potential. We tested the hypothesis that a high level of dietary EPA could exert beneficial effects in ALS. The dietary exposure to EPA (300 mg/kg/day) in a well-established mouse model of ALS expressing the G93A superoxide dismutase 1 (SOD1) mutation was initiated at a pre-symptomatic or symptomatic stage, and the disease progression was monitored until the end stage. Daily dietary EPA exposure initiated at the disease onset did not significantly alter disease presentation and progression. In contrast, EPA treatment initiated at the pre-symptomatic stage induced a significantly shorter lifespan. In a separate group of animals sacrificed before the end stage, the tissue analysis showed that the vacuolisation detected in G93A-SOD1 mice was significantly increased by exposure to EPA. Although EPA did not alter motor neurone loss, EPA reversed the significant increase in activated microglia and the astrocytic activation seen in G93A-SOD1 mice. The microglia in the spinal cord of G93A-SOD1 mice treated with EPA showed a significant increase in 4-hydroxy-2-hexenal, a highly toxic aldehydic oxidation product of omega-3 fatty acids. These data show that dietary EPA supplementation in ALS has the potential to worsen the condition and accelerate the disease progression. This suggests that great caution should be exerted when considering dietary omega-3 fatty acid supplements in ALS patients.

Hip14l-deficient mice develop neuropathological and behavioural features of Huntington disease

Palmitoylation, the dynamic post-translational addition of the lipid, palmitate, to proteins by Asp-His-His-Cys-containing palmitoyl acyltransferase (PAT) enzymes, modulates protein function and localization and plays a key role in the nervous system. Huntingtin-interacting protein 14 (HIP14), a well-characterized neuronal PAT, has been implicated in the pathogenesis of Huntington disease (HD), a fatal neurodegenerative disease associated with motor, psychiatric and cognitive symptoms, caused by a CAG expansion in the huntingtin gene (HTT). Mice deficient for Hip14 expression develop neuropathological and behavioural features similar to HD, and the catalytic activity of HIP14 is impaired in HD mice, most likely due to the reduced interaction of HIP14 with HTT. Huntingtin-interacting protein 14-like (HIP14L) is a paralog of HIP14, with identical domain structure. Together, HIP14 and HIP14L are the major PATs for HTT. Here, we report the characterization of a Hip14l-deficient mouse model, which develops adult-onset, widespread and progressive neuropathology accompanied by early motor deficits in climbing, impaired motor learning and reduced palmitoylation of a novel HIP14L substrate: SNAP25. Although the phenotype resembles that of the Hip14(-/-) mice, a more progressive phenotype, similar to that of the YAC128 transgenic mouse model of HD, is observed. In addition, HIP14L interacts less with mutant HTT than the wild-type protein, suggesting that reduced HIP14L-dependent palmitoylation of neuronal substrates may contribute to the pathogenesis of HD. Thus, both HIP14 and HIP14L may be dysfunctional in the disease.

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.

Therapeutic Strategies for Huntington's Disease

Huntington's disease (HD) is a devastating autosomal dominant neurodegenerative disease, caused by expansion of the CAG repeat in the huntingtin (HTT) gene and characterized pathologically by the loss of pyramidal neurons in several cortical areas, of striatal medium spiny neurons, and of hypothalamic neurons. Clinically, a distinguishing feature of the disease is uncontrolled involuntary movements (chorea, dyskensias) accompanied by progressive cognitive, motor, and psychiatric impairment. This review focuses on the current state of therapeutic development for the treatment of HD, including the preclinical and clinical development of small molecules and molecular therapies.

Mouse models of polyglutamine diseases in therapeutic approaches: review and data table. Part II

Mouse models of human diseases are created both to understand the pathogenesis of the disorders and to find successful therapies for them. This work is the second part in a series of reviews of mouse models of polyglutamine (polyQ) hereditary disorders and focuses on in vivo experimental therapeutic approaches. Like part I of the polyQ mouse model review, this work is supplemented with a table that contains data from experimental studies of therapeutic approaches in polyQ mouse models. The aim of this review was to characterize the benefits and outcomes of various therapeutic strategies in mouse models. We examine whether the therapeutic strategies are specific to a single disease or are applicable to more than one polyQ disorder in mouse models. In addition, we discuss the suitability of mouse models in therapeutic approaches. Although the majority of therapeutic studies were performed in mouse models of Huntington disease, similar strategies were also used in other disease models.

Screening of therapeutic strategies for Huntington's disease in YAC128 transgenic mice

Huntington’s disease (HD) is a hereditary neurodegenerative disorder caused by an unstable expansion of cytosine-adenine-guanine (CAG) repeats in the HD gene. The symptoms include cognitive dysfunction and severe motor impairment with loss of voluntary movement coordination that is later replaced by bradykinesia and rigidity. The neuropathology is characterized by neuronal loss mainly in the striatum and cortex, and the appearance of neuronal intranuclear inclusions of mutant huntingtin. The mechanisms responsible for neurodegeneration are still not fully understood although excitotoxicity and a consequent increase in intracellular calcium concentration as well as the activation of caspases and calapins are known to play a key role. There is currently no satisfactory treatment or cure for this disease. The YAC128 transgenic mice express the full-length human HD gene with 128 CAG repeats and constitute a unique model for the study of HD as they replicate the slow and biphasic progression of behavioral deficits characteristic of the human condition and show striatal neuronal loss. As such, these transgenic mice have been an invaluable model not only for the elucidation of the neurodegenerative pathways in HD, but also for the screening and development of new therapeutic approaches. Here, I will review the unique characteristics of this transgenic HD model and will provide a summary of the therapies that have been tested in these mice, namely: potentiation of the protective roles of wild-type huntingtin and mutant huntingtin aggregation, transglutaminase inhibition, inhibition of glutamate- and dopamine-induced toxicity, apoptosis inhibition, use of essential fatty acids, and the novel approach of intrabody gene therapy. The insights obtained from these and future studies will help identify potential candidates for clinical trials and will ultimately contribute to the discovery of a successful treatment for this devastating neurodegenerative disorder.

Marked differences in neurochemistry and aggregates despite similar behavioural and neuropathological features of Huntington disease in the full-length BACHD and YAC128 mice

The development of animal models of Huntington disease (HD) has enabled studies that help define the molecular aberrations underlying the disease. The BACHD and YAC128 transgenic mouse models of HD harbor a full-length mutant huntingtin (mHTT) and recapitulate many of the behavioural and neuropathological features of the human condition. Here, we demonstrate that while BACHD and YAC128 animals exhibit similar deficits in motor learning and coordination, depressive-like symptoms, striatal volume loss and forebrain weight loss, they show obvious differences in key features characteristic of HD. While YAC128 mice exhibit significant and widespread accumulation of mHTT striatal aggregates, these mHTT aggregates are absent in BACHD mice. Furthermore, the levels of several striatally enriched mRNA for genes, such as DARPP-32, enkephalin, dopamine receptors D1 and D2 and cannabinoid receptor 1, are significantly decreased in YAC128 but not BACHD mice. These findings may reflect sequence differences in the human mHTT transgenes harboured by the BACHD and YAC128 mice, including both single nucleotide polymorphisms as well as differences in the nature of CAA interruptions of the CAG tract. Our findings highlight a similar profile of HD-like behavioural and neuropathological deficits and illuminate differences that inform the use of distinct endpoints in trials of therapeutic agents in the YAC128 and BACHD mice.

Comparative analysis of pathology and behavioural phenotypes in mouse models of Huntington's disease

The longitudinal characterisation of Huntington's disease (HD) mouse lines is essential for the understanding of the differential developmental time course, nature and severity of phenotype progression over time. This overview outlines detailed behavioural, neuropathological and gene expression studies in four HD mouse lines: R6/1, YAC128, HdhQ92 and HdhQ150 and outlines their relevance to human HD. The review describes the similarities and differences between the models at the behavioural, anatomical and genetic levels of pathology and how these phenotypes interact in the development of disease in the lines. The HdhQ150 mouse demonstrates the most similarities to the functional deficits observed in human HD. The neuropathological profile with early cortical development of intense aggregate/inclusion pathology in the YAC128 mouse suggests that this line most resembles the development of inclusion pathology in the human disease. The gene expression analyses of the mouse lines find significant similarities between each of the lines and human HD, which converge as the mice age. In the YAC128 and HdhQ92 mouse lines some severe functional deficits are progressive whilst others are not, despite the concomitant ongoing development of neuropathological and gene expression changes. We suggest that the YAC128 and R6/1 lines may be more representative of the juvenile form of HD. The suitability of the different mouse models studied here for different types of pre-clinical therapeutic trials is discussed.

Endogenous neuroprotection in chronic neurodegenerative disorders: with particular regard to the kynurenines

Parkinson's disease (PD) and Huntington's disease (HD) are progressive chronic neurodegenerative disorders that are accompanied by a considerable impairment of the motor functions. PD may develop for familial or sporadic reasons, whereas HD is based on a definite genetic mutation. Nevertheless, the pathological processes involve oxidative stress and glutamate excitotoxicity in both cases. A number of metabolic routes are affected in these disorders. The decrease in antioxidant capacity and alterations in the kynurenine pathway, the main pathway of the tryptophan metabolism, are features that deserve particular interest, because the changes in levels of neuroactive kynurenine pathway compounds appear to be strongly related to the oxidative stress and glutamate excitotoxicity involved in the disease pathogenesis. Increase of the antioxidant capacity and pharmacological manipulation of the kynurenine pathway are therefore promising therapeutic targets in these devastating disorders.

Mice lacking caspase-2 are protected from behavioral changes, but not pathology, in the YAC128 model of Huntington disease

BACKGROUND

Huntington Disease (HD) is a neurodegenerative disorder in which caspase activation and cleavage of substrates, including the huntingtin protein, has been invoked as a pathological mechanism. Specific changes in caspase-2 (casp2) activity have been suggested to contribute to the pathogenesis of HD, however unique casp2 cleavage substrates have remained elusive. We thus utilized mice completely lacking casp2 (casp2-/-) to examine the role played by casp2 in the progression of HD. This 'substrate agnostic' approach allows us to query the effect of casp2 on HD progression without pre-defining proteolytic substrates of interest.

RESULTS

YAC128 HD model mice lacking casp2 show protection from well-validated motor and cognitive features of HD, including performance on rotarod, swimming T-maze, pre-pulse inhibition, spontaneous alternation and locomotor tasks. However, the specific pathological features of the YAC128 mice including striatal volume loss and testicular degeneration are unaltered in mice lacking casp2. The application of high-resolution magnetic resonance imaging (MRI) techniques validates specific neuropathology in the YAC128 mice that is not altered by ablation of casp2.

CONCLUSIONS

The rescue of behavioral phenotypes in the absence of pathological improvement suggests that different pathways may be operative in the dysfunction of neural circuitry in HD leading to behavioral changes compared to the processes leading to cell death and volume loss. Inhibition of caspase-2 activity may be associated with symptomatic improvement in HD.

Cystamine and ethyl-eicosapentaenoic acid treatment fail to prevent malonate-induced striatal toxicity in mice

Cystamine has demonstrated neuroprotective activity in a variety of studies, and is currently being evaluated in a human clinical trial in Huntington's disease (HD). Cystamine treatment of various genetic models of HD demonstrated protection against neurodegeneration and/or improvement in behavior. Given the need for a rapid screening tool for HD therapeutics, we assessed the potential therapeutic benefits of cystamine in a short-term acute toxicity murine model of striatal cell death. Cystamine did not provide neuroprotection against bilateral intrastriatal malonate injections in mice as measured by lesion size, loss of striatal volume, or decreased striatal neuronal counts. Similar results were obtained for treatment with another potential therapeutic agent that was protective in genetic mouse models of HD, the essential fatty acid ethyl-eicosapentaenoic acid. Our findings suggest that this toxic model is not reflective or predictive of findings in genetic mouse models, and may not be useful as a preclinical screen for HD therapeutics.

Altered cholesterol and fatty acid metabolism in Huntington disease

Huntington disease is an autosomal dominant neurodegenerative disorder characterized by behavioral abnormalities, cognitive decline, and involuntary movements that lead to a progressive decline in functional capacity, independence, and ultimately death. The pathophysiology of Huntington disease is linked to an expanded trinucleotide repeat of cytosine-adenine-guanine (CAG) in the IT-15 gene on chromosome 4. There is no disease-modifying treatment for Huntington disease, and novel pathophysiological insights and therapeutic strategies are needed. Lipids are vital to the health of the central nervous system, and research in animals and humans has revealed that cholesterol metabolism is disrupted in Huntington disease. This lipid dysregulation has been linked to specific actions of the mutant huntingtin on sterol regulatory element binding proteins. This results in lower cholesterol levels in affected areas of the brain with evidence that this depletion is pathologic. Huntington disease is also associated with a pattern of insulin resistance characterized by a catabolic state resulting in weight loss and a lower body mass index than individuals without Huntington disease. Insulin resistance appears to act as a metabolic stressor attending disease progression. The fish-derived omega-3 fatty acids, eicosapentaenoic acid and docosahexaenoic acid, have been examined in clinical trials of Huntington disease patients. Drugs that combat the dysregulated lipid milieu in Huntington disease may help treat this perplexing and catastrophic genetic disease.

Huntington's disease: can mice lead the way to treatment?

Mouse models for Huntington's Disease (HD) and HD patients demonstrate motor and behavioral dysfunctions, such as progressive loss of coordination and memory, and share similar transcriptional profiles and striatal neuron atrophy. Clear differences between the mouse and human diseases include almost complete striatal degeneration and rarity of intranuclear inclusions in HD, and the fact that mice expressing full-length mutant huntingtin do not demonstrate a shortened life span characteristic of HD. While no clinical interventions tested in mouse models to date have delayed disease progression, the mouse models provide an invaluable tool for both investigating the underlying pathogenic processes and developing new effective therapies. Inherent differences between humans and mice must be considered in the search for efficacious treatments for HD, but the striking similarities between human HD and mouse models support the view that these models are a biologically relevant system to support the identification and testing of potential clinical therapies.

Modulation of energy deficiency in Huntington's disease via activation of the peroxisome proliferator-activated receptor gamma

Huntington's disease (HD) is a neurodegenerative disease caused by the expansion of a CAG trinucleotide repeat in exon 1 of the huntingtin (HTT) gene. Here, we report that the transcript of the peroxisome proliferator-activated receptor-γ (PPARγ), a transcription factor that is critical for energy homeostasis, was markedly downregulated in multiple tissues of a mouse model (R6/2) of HD and in lymphocytes of HD patients. Therefore, downregulation of PPARγ seems to be a pathomechanism of HD. Chronic treatment of R6/2 mice with an agonist of PPARγ (thiazolidinedione, TZD) rescued progressive weight loss, motor deterioration, formation of mutant Htt aggregates, jeopardized global ubiquitination profiles, reduced expression of two neuroprotective proteins (brain-derived neurotrophic factor and Bcl-2) and shortened life span exhibited by these mice. By reducing HTT aggregates and, thus, ameliorating the recruitment of PPARγ into HTT aggregates, chronic TZD treatment also elevated the availability of the PPARγ protein and subsequently normalized the expression of two of its downstream genes (the glucose transporter type 4 and PPARγ coactivator-1 alpha genes). The protective effects described above appear to have been exerted, at least partially, via direct activation of PPARγ in the brain, as TZD was detected in the brains of mice treated with TZD and because a PPARγ agonist (rosiglitazone) protected striatal cells from mHTT-evoked energy deficiency and toxicity. We demonstrated that the systematic downregulation of PPARγ seems to play a critical role in the dysregulation of energy homeostasis observed in HD, and that PPARγ is a potential therapeutic target for this disease.

Molecular Mechanisms and Potential Therapeutical Targets in Huntington's Disease

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

Mouse models of Huntington disease: variations on a theme

An accepted prerequisite for clinical trials of a compound in humans is the successful alleviation of the disease in animal models. For some diseases, however, successful translation of drug effects from mouse models to the bedside has been limited. One question is whether the current models accurately reproduce the human disease. Here, we examine the mouse models that are available for therapeutic testing in Huntington disease (HD), a late-onset neurodegenerative disorder for which there is no effective treatment. The current mouse models show different degrees of similarity to the human condition. Significant phenotypic differences are seen in mouse models that express either truncated or full-length human, or full-length mouse, mutant huntingtin (mHTT). These differences in phenotypic expression may be attributable to the influences of protein context, mouse strain and a difference in regulatory sequences between the mouse Htt and human HTT genes.

Emerging drug therapies in Huntington's disease

Huntington's disease (HD) is a relentless neurodegenerative disease that results in profound disability through a triad of motor, cognitive and neuropsychiatric symptoms. At present, there are very few therapeutic interventions available with the exception of a limited number of drugs that offer mild symptomatic relief. Although the genetic basis of the disease has been identified, the mechanisms behind the cellular pathogenesis are still not clear and as a result no candidate drugs with the potential for disease modification have been found clinically until now. One of the major limitations in assessing the usefulness of drug treatments in HD is the lack of well-designed, double-blind, placebo-controlled clinical trials. Most studies have been open-label, using a small number of patients and tend to concentrate on the motor features of the disease, primarily the chorea. This review discusses the treatments now used for HD before evaluating the newer drugs at present being explored in both the clinic and in the laboratory in mouse models of the disease.

Effects of eicosapentaenoic acid on synaptic plasticity, fatty acid profile and phosphoinositide 3-kinase signaling in rat hippocampus and differentiated PC12 cells

Placebo-controlled clinical studies suggest that intake of n-3 polyunsaturated fatty acids improves neurological disorders such as Alzheimer's disease, Huntington's disease and schizophrenia. To evaluate the impact of eicosapentaenoic acid (EPA), we orally administered highly purified ethyl EPA (EPA-E) to rats at a dose of 1.0 mg/g per day and measured long-term potentiation of the CA1 hippocampal region, a physiological correlate of synaptic plasticity that is thought to underlie learning and memory. The mean field excitatory postsynaptic potential slope of the EPA-E group was significantly greater than that of the control group in the CA1 region. Gene expression of hippocampal p85alpha, one of the regulatory subunits of phosphatidylinositol 3-kinase (PI3-kinase), was increased with EPA-E administration. Investigation of fatty acid profiles of neuronal and glia-enriched fractions demonstrated that a single administration of EPA-E significantly increased neuronal and glial EPA content and glial docosahexaenoic acid content, clearly suggesting that EPA was indeed taken up by both neurons and glial cells. In addition, we investigated the direct effects of EPA on the PI3-kinase/Akt pathway in differentiated PC12 cells. Phosphorylated-Akt expression was significantly increased in EPA-treated cells, and nerve growth factor withdrawal-induced increases in cell death and caspase-3 activity were suppressed by EPA treatment. These findings suggest that EPA protects against neurodegeneration by modulating synaptic plasticity and activating the PI3-kinase/Akt pathway, possibly by its own functional effects in neurons and glial cells and by its capacity to increase brain docosahexaenoic acid.

Neurological benefits of omega-3 fatty acids

The central nervous system is highly enriched in long-chain polyunsaturated fatty acid (PUFA) of the omega-6 and omega-3 series. The presence of these fatty acids as structural components of neuronal membranes influences cellular function both directly, through effects on membrane properties, and also by acting as a precursor pool for lipid-derived messengers. An adequate intake of omega-3 PUFA is essential for optimal visual function and neural development. Furthermore, there is increasing evidence that increased intake of the long-chain omega-3 PUFA, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), may confer benefits in a variety of psychiatric and neurological disorders, and in particular neurodegenerative conditions. However, the mechanisms underlying these beneficial effects are still poorly understood. Recent evidence also indicates that in addition to the positive effects seen in chronic neurodegenerative conditions, omega-3 PUFA may also have significant neuroprotective potential in acute neurological injury. Thus, these compounds offer an intriguing prospect as potentially new therapeutic approaches in both chronic and acute conditions. The purpose of this article is to review the current evidence of the neurological benefits of omega-3 PUFA, looking specifically at neurodegenerative conditions and acute neurological injury.

Hypothalamic dysfunction and neuroendocrine and metabolic alterations in Huntington's disease: clinical consequences and therapeutic implications

Huntington's disease (HD) is a hereditary neurodegenerative disorder characterized by cognitive, psychiatric, behavioural and motor disturbances. Although the course of HD is also frequently complicated by unintended weight loss, sleep disturbances and autonomic nervous system dysfunction, the aetiology of these signs and symptoms remains largely unknown. In recent years, many novel findings from both animal and human studies have emerged that indicate considerable hypothalamic, endocrine and metabolic alterations in HD. However, a comprehensive overview of these findings is lacking and their precise clinical significance is far from clear. Therefore, in this review we attempt to put these recent developments in the field into perspective by integrating them with previous findings in a comprehensible manner, and by discussing their clinical relevance, with a special focus on body weight, sleep and autonomic functions in HD, which will also allow for the identification of future lines of research in this area.

Comparison of biochemical effects of statins and fish oil in brain: the battle of the titans

Neural membranes are composed of glycerophospholipids, sphingolipids, cholesterol and proteins. The distribution of these lipids within the neural membrane is not random but organized. Neural membranes contain lipid rafts or microdomains that are enriched in sphingolipids and cholesterol. These rafts act as platforms for the generation of glycerophospholipid-, sphingolipid-, and cholesterol-derived second messengers, lipid mediators that are necessary for normal cellular function. Glycerophospholipid-derived lipid mediators include eicosanoids, docosanoids, lipoxins, and platelet-activating factor. Sphingolipid-derived lipid mediators include ceramides, ceramide 1-phosphates, and sphingosine 1-phosphate. Cholesterol-derived lipid mediators include 24-hydroxycholesterol, 25-hydroxycholesterol, and 7-ketocholesterol. Abnormal signal transduction processes and enhanced production of lipid mediators cause oxidative stress and inflammation. These processes are closely associated with the pathogenesis of acute neural trauma (stroke, spinal cord injury, and head injury) and neurodegenerative diseases such as Alzheimer disease. Statins, the HMG-CoA reductase inhibitors, are effective lipid lowering agents that significantly reduce risk for cardiovascular and cerebrovascular diseases. Beneficial effects of statins in neurological diseases are due to their anti-excitotoxic, antioxidant, and anti-inflammatory properties. Fish oil omega-3 fatty acids, eicosapentaenoic acid and docosahexaenoic acid, have similar anti-excitotoxic, antioxidant and anti-inflammatory effects in brain tissue. Thus the lipid mediators, resolvins, protectins, and neuroprotectins, derived from eicosapentaenoic acid and docosahexaenoic acid retard neuroinflammation, oxidative stress, and apoptotic cell death in brain tissue. Like statins, ingredients of fish oil inhibit generation of beta-amyloid and provide protection from oxidative stress and inflammatory processes. Collective evidence suggests that antioxidant, anti-inflammatory, and anti-apoptotic properties of statins and fish oil contribute to the clinical efficacy of treating neurological disorders with statins and fish oil. We speculate that there is an overlap between neurochemical events associated with neural cell injury in stroke and neurodegenerative diseases. This commentary compares the neurochemical effects of statins with those of fish oil.

Phenotypic abnormalities in the YAC128 mouse model of Huntington disease are penetrant on multiple genetic backgrounds and modulated by strain

The YAC128 mouse model of Huntington disease (HD) exhibits motor abnormalities, cognitive dysfunction and selective neuropathology which are similar to the human disease. Backcrossing YAC128 mice from the FVB/N strain onto the C57BL/6 strain and the 129 strain revealed that striatal volume loss and motor dysfunction are penetrant on all three genetic backgrounds. The severity of HD-like phenotypes in these mice is modulated by strain and this variation is not accounted for by differences in mutant huntingtin expression. In contrast, nuclear localization of mutant htt is modulated by strain and is correlated with the severity of neuropathology. Differences in phenotypic severity between the strains provide the opportunity to identify modifier genes which could impact the pathogenesis of HD. Importantly, the demonstration of penetrance across all three strains permits examining the effect of specific genes on the phenotypic severity in YAC128 mice without necessarily backcrossing onto the FVB/N strain background.

Wild-type huntingtin ameliorates striatal neuronal atrophy but does not prevent other abnormalities in the YAC128 mouse model of Huntington disease

BACKGROUND

Huntington disease (HD) is an adult onset neurodegenerative disorder caused by a polyglutamine expansion in the huntingtin (htt) protein. Htt function is essential for embryonic survival as well as normal function during the postnatal period. In addition to having roles in transcription and transport, recent evidence demonstrates that wild-type htt is neuroprotective in vivo. To determine whether treatment with wild-type htt would be beneficial in HD, we crossed the YAC128 mouse model of HD with mice that over-express wild-type htt (YAC18 mice) to generate YAC128 mice that over-express wild-type htt (YAC18/128 mice).

RESULTS

YAC18/128 mice were found to express mutant htt at the same level as YAC128 mice and wild-type htt at the same level as YAC18 mice. YAC18/128 mice show no significant behavioural improvement compared to YAC128 mice in the rotarod test of motor coordination or in an automated open field test. In the brain, YAC18/128 mice show no significant improvement in striatal volume, striatal neuronal numbers or striatal DARPP-32 expression compared to YAC128 mice. In contrast, striatal neuronal cross-sectional area showed significant improvement in YAC18/128 mice compared to YAC128 mice.

CONCLUSION

While the over-expression of wild-type htt results in a mild improvement in striatal neuropathology in YAC128 mice, our findings suggest that treatment with wild-type htt may not be sufficient to ameliorate the symptoms of HD in this model.

Selective degeneration in YAC mouse models of Huntington disease

Huntington disease (HD) is one of at least nine polyglutamine disorders caused by a CAG expansion in the coding region of a disease-causing gene. These disorders are characterized by selective degeneration of different regions of the brain, which is not explained by the expression pattern of the mutant protein. In HD, degeneration primarily occurs in the striatum and cortex. To examine the mechanisms responsible for the selective neuronal loss in HD, we have generated yeast artificial chromosome (YAC) transgenic models of HD that express full length mutant huntingtin (htt) from a YAC. These mice have appropriate tissue-specific and temporal expression of mutant htt and accordingly recapitulate the motor deficits, cognitive impairment and selective degeneration of HD. As in human patients, mutant htt expression is not increased in the affected regions of the brain. In contrast, detection of mutant htt in the nucleus is earliest and greatest in the striatum, the region most affected in HD, suggesting that selective nuclear localization of mutant htt may contribute to the region specific atrophy in these mice. Selective phosphorylation of mutant htt on serine 421 may also contribute, as phosphorylation of mutant htt reduces its toxicity and is decreased in the striatum compared to other regions of the brain. Finally, the fact that mutant htt expression increases the susceptibility of striatal neurons to excitotoxicity but not neurons from the cerebellum, suggests that altered sensitization to excitotoxic death may also contribute to selective degeneration in YAC mice. Overall, YAC mice recapitulate the region specific damage that occurs in HD and provide a suitable model for examining the mechanisms underlying of selective degeneration.

Ethyl-EPA in Huntington disease: potentially relevant mechanism of action

The pathomechanisms involved in the neuronal dysfunction in Huntington disease (HD) are still unresolved and may be heterogeneous. One potential mechanism might be related to the induction of mitochondrial dysfunction in the CNS. This might lead firstly to neuronal dysfunction and finally to the activation of apoptotic pathways. Several compounds, which should alleviate mitochondrial dysfunction, have been tested in preclinical models as well as in clinical trials of different scale. Recently we reported the efficacy of Ethyl-eicosapentaenoic acid (Ethyl-EPA) in patients with HD. Ethyl-EPA is a polyunsaturated fatty acid from the n-3 group, which is in clinical development for HD and melancholic depression. In our trial with Ethyl-EPA in HD responding patients could be characterized by either a lower CAG repeat number or a chorea-predominant clinical expression of the disease. Here we would like to describe some evidence on the potential mechanism of action of Ethyl-EPA in HD. We specifically focus on pathways, which are known to be influenced in HD and are modified by Ethyl-EPA and which points to an involvement of mitochondrial function as a common target. Some attention is given to the NF-kappa B pathway and the c-Jun amino-terminal kinases (JNK) pathway, which both may lead to an activation of the antiproliferative factor p53 and consequently mitochondrial dysfunction. Further the effects of EPA or Ethyl-EPA in preclinical models of HD are described. The evidence from these studies led to the design of phase III clinical trials, which are ongoing.

Cholesterol dysfunction in neurodegenerative diseases: Is Huntington's disease in the list?

Brain cholesterol is an essential component of cell membranes, and involved in a number of biological functions such as membrane trafficking, signal transduction, myelin formation and synaptogenesis. Given these widespread activities and the knowledge that all brain cholesterol derives from local synthesis, it is not surprising that dysfunctions in cholesterol synthesis, storage, transport and removal may lead to human brain diseases. Some of these diseases emerge as a consequence of genetic defects in the enzymes involved in cholesterol biosynthesis; in other cases, such as Alzheimer's disease, there is a link between cholesterol metabolism and the formation and deposition of amyloid-beta peptide. Emerging evidence indicates that changes in cholesterol synthesis may also occur in Huntington's disease, an inherited, autosomal dominant neurodegenerative disorder that primarily affects striatal neurons of the brain. We here provide an overview of the involvement of cholesterol in normal brain function and its impact on neurodegenerative diseases. In particular, we consider the available clinical, biological and molecular evidence indicating a potential dysregulation of cholesterol homeostasis in Huntington's disease.

Update on huntington’s disease

Huntington's disease (HD) is a devastating neurodegenerative disease causing progressive movement disorders, cognitive dysfunction, and behavioral changes. Since the causative mutation of an expanded polyglutamine repeat in the huntingtin gene was identified, significant progress has been achieved in elucidating pathogenic mechanisms. This review summarizes recent developments in evaluating the role of abnormal protein aggregation, transcriptional dysregulation, mitochondrial and bioenergetic dysfunction, excitotoxicity, and abnormal cellular trafficking in the pathogenesis of HD. In addition, although therapeutic options in HD have been limited, progress in developing targeted therapies continues, and these advancements and future directions are reviewed.