Disruption of immune cell function by mutant huntingtin in Huntington's disease pathogenesis

Drug repurposing for neurodegenerative diseases

Neurodegenerative diseases (NDDs) are neuronal problems that include the brain and spinal cord and result in loss of sensory and motor dysfunction. Common NDDs include Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), Multiple Sclerosis (MS), and Amyotrophic Lateral Sclerosis (ALS) etc. The occurrence of these diseases increases with age and is one of the challenging problems among elderly people. Though, several scientific research has demonstrated the key pathologies associated with NDDs still the underlying mechanisms and molecular details are not well understood and need to be explored and this poses a lack of effective treatments for NDDs. Several lines of evidence have shown that NDDs have a high prevalence and affect more than a billion individuals globally but still, researchers need to work forward in identifying the best therapeutic target for NDDs. Thus, several researchers are working in the directions to find potential therapeutic targets to alter the disease pathology and treat the diseases. Several steps have been taken to identify the early detection of the disease and drug repurposing for effective treatment of NDDs. Moreover, it is logical that current medications are being evaluated for their efficacy in treating such disorders; therefore, drug repurposing would be an efficient, safe, and cost-effective way in finding out better medication. In the current manuscript we discussed the utilization of drugs that have been repurposed for the treatment of AD, PD, HD, MS, and ALS.

Mutant-Huntingtin Molecular Pathways Elucidate New Targets for Drug Repurposing

The spectrum of neurodegenerative diseases known today is quite extensive. The complexities of their research and treatment lie not only in their diversity. Even many years of struggle and narrowly focused research on common pathologies such as Alzheimer's, Parkinson's, and other brain diseases have not brought cures for these illnesses. What can be said about orphan diseases? In particular, Huntington's disease (HD), despite affecting a smaller part of the human population, still attracts many researchers. This disorder is known to result from a mutation in the HTT gene, but having this information still does not simplify the task of drug development and studying the mechanisms of disease progression. Nonetheless, the data accumulated over the years and their analysis provide a good basis for further research. Here, we review studies devoted to understanding the mechanisms of HD. We analyze genes and molecular pathways involved in HD pathogenesis to describe the action of repurposed drugs and try to find new therapeutic targets.

Pain in Huntington's disease and its potential mechanisms

Pain is common and frequent in many neurodegenerative diseases, although it has not received much attention. In Huntington's disease (HD), pain is often ignored and under-researched because attention is more focused on motor and cognitive decline than psychiatric symptoms. In HD progression, pain symptoms are complex and involved in multiple etiologies, particularly mental issues such as apathy, anxiety and irritability. Because of psychiatric issues, HD patients rarely complain of pain, although their bodies show severe pain symptoms, ultimately resulting in insufficient awareness and lack of research. In HD, few studies have focused on pain and pain-related features. A detailed and systemic pain history is crucial to assess and explore pain pathophysiology in HD. This review provides an overview concentrating on pain-related factors in HD, including neuropathology, frequency, features, affecting factors and mechanisms. More attention and studies are still needed in this interesting field in the future.

Amendment of Altered Immune Response by Curcumin in Drosophila Model of Huntington's Disease

BACKGROUND

Though primarily classified as a brain disorder, surplus studies direct Huntington's disease (HD) to be a multi-system disorder affecting various tissues and organs, thus affecting overall physiology of host. Recently, we have reported that neuronal expression of mutant huntingtin induces immune dysregulation in Drosophila and may pose chronic threat to challenged individuals. Therefore, we tested the polyphenolic compound curcumin to circumvent the impact of immune dysregulation in Drosophila model of HD.

OBJECTIVE

The present study examined the molecular basis underlying immune derangements and immunomodulatory potential of curcumin in HD.

METHODS

UAS-GAL4 system was used to imitate the HD symptoms in Drosophila, and the desired female progenies (elav > Httex1pQ25; control and elav > Httex1pQ93; diseased) were cultured on food mixed without and with 10 μM concentration of curcumin since early development. Effect of curcumin supplementation was investigated by monitoring the hemocytes' count and their functional abilities in diseased condition. Reactive oxygen species (ROS) level in cells was assessed by DHE staining and mitochondrial dysfunction was assessed by CMXros red dye. In addition, transcript levels of pro-inflammatory cytokines and anti-microbial peptides were monitored by qRT-PCR.

RESULTS

We found that curcumin supplementation commendably reduced higher crystal cell count and phenoloxidase activity in diseased flies. Interestingly, curcumin significantly managed altered plasmatocytes count, improved their phagocytic activity by upregulating the expression of key phagocytic receptors in HD condition. Moreover, substantial alleviation of ROS levels and mitochondria dysfunction was observed in plasmatocytes of diseased flies upon curcumin supplementation. Furthermore, curcumin administration effectively attenuated transcriptional expression of pro-inflammatory cytokines and AMPs in diseased flies.

CONCLUSIONS

Our results indicate that curcumin efficiently attenuates immune derangements in HD flies and may prove beneficial in alleviating complexities associated with HD.

Leukocyte Telomere Length as Potential Biomarker of HD Progression: A Follow-Up Study

The identification of biomarkers for neurodegenerative disorders such as Huntington’s disease (HD) is crucial for monitoring disease progression and therapeutic trial outcomes, especially in the pre-manifest disease stage (pre-HD). In a previous study, we observed that leukocyte telomere length (LTL) was strongly correlated with the estimated time to clinical onset in pre-HD subjects. To validate this hypothesis, we designed a follow-up study in which we analyzed LTL in 45 pre-HD stage subjects at baseline (T0) and then again after clinical onset at follow-up (T1); the follow-up interval was about 3 years, and the CAG range was 39–51 repeats; 90 peripheral blood mononuclear cell samples (PBMCs) were obtained from the Enroll-HD biorepository. In pre-HD subjects at T0, LTL was significantly reduced by 22% compared to the controls and by 14% from T0 at T1. No relationship was observed between the LTL and CAG numbers in subjects carrying different CAG repeats at T0 and at T1, suggesting that LTL reduction occurs independently of CAG number in pre-HD subjects. ROC curve analysis was used to test the validity of LTL as a potential biomarker of HD progression and showed that LTL measurement is extremely accurate in discriminating pre-HD subjects from the controls and even pre-HD from manifest HD, thus yielding a robust prognostic value in pre-HD subjects.

Behavioral- and blood-based biomarkers for Huntington's disease: Studies on the R6/1 mouse model with prospects for early diagnosis and monitoring of the disease

Background

Objective

Methods

Results

Conclusion

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Gender is one of the factors that determine the rate of progression of Huntington's disease symptoms.

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A set of non-invasive biomarkers that are useful in the diagnosis and monitoring of Huntington's disease progression.

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Hormonal profile may be a factor in the efficacy of potential therapy for Huntington's disease.

Pan-neuronal expression of human mutant huntingtin protein in Drosophila impairs immune response of hemocytes

Huntington's disease (HD) is a late-onset; progressive, dominantly inherited neurological disorder marked by an abnormal expansion of polyglutamine (poly Q) repeats in Huntingtin (HTT) protein. The pathological effects of mutant Huntingtin (mHTT) are not restricted to the nervous system but systemic abnormalities including immune dysregulation have been evidenced in clinical and experimental settings of HD. Indeed, mHTT is ubiquitously expressed and could induce cellular toxicity by directly acting on immune cells. However, it is still unclear if selective expression of mHTT exon1 in neurons could induce immune responses and hemocytes' function. In the present study, we intended to monitor perturbations in the hemocytes' population and their physiological functions in Drosophila, caused by pan-neuronal expression of mHTT protein. A measure of hemocyte count and their physiological activities caused by pan-neuronal expression of mHTT protein highlighted the extent of immune dysregulation occurring with disease progression. We found that pan-neuronal expression of mHTT significantly alters crystal cells and plasmatocyte count in larvae and adults with disease progression. Interestingly, plasmatocytes isolated from diseased conditions exhibit a gradual decline in phagocytic activity ex vivo at progressive stages of the disease as compared to age-matched control groups. In addition, diseased flies displayed elevated reactive oxygen species (ROS) in circulating plasmatocytes at the larval stage and in sessile plasmatocytes of hematopoietic pockets at terminal stages of disease. These findings strongly implicate that neuronal expression of mHTT alone is sufficient to induce non-cell-autonomous immune dysregulation in vivo.

Ginsenoside Rg1 exerts neuroprotective effects in 3-nitropronpionic acid-induced mouse model of Huntington's disease via suppressing MAPKs and NF-κB pathways in the striatum

Huntington's disease (HD) is one of main neurodegenerative diseases, characterized by striatal atrophy, involuntary movements, and motor incoordination. Ginsenoside Rg1 (Rg1), an active ingredient in ginseng, possesses a variety of neuroprotective effects with low toxicity and side effects. In this study, we investigated the potential therapeutic effects of Rg1 in a mouse model of HD and explored the underlying mechanisms. HD was induced in mice by injection of 3-nitropropionic acid (3-NP, i.p.) for 4 days. From the first day of 3-NP injection, the mice were administered Rg1 (10, 20, 40 mg·kg^-1, p.o.) for 5 days. We showed that oral pretreatment with Rg1 alleviated 3-NP-induced body weight loss and behavioral defects. Furthermore, pretreatment with Rg1 ameliorated 3-NP-induced neuronal loss and ultrastructural morphological damage in the striatum. Moreover, pretreatment with Rg1 reduced 3-NP-induced apoptosis and inhibited the activation of microglia, inflammatory mediators in the striatum. We revealed that Rg1 exerted neuroprotective effects by suppressing 3-NP-induced activation of the MAPKs and NF-κΒ signaling pathways in the striatum. Thus, our results suggest that Rg1 exerts therapeutic effects on 3-NP-induced HD mouse model via suppressing MAPKs and NF-κΒ signaling pathways. Rg1 may be served as a novel therapeutic option for HD.

Human Huntington's disease pluripotent stem cell-derived microglia develop normally but are abnormally hyper-reactive and release elevated levels of reactive oxygen species

Background

Neuroinflammation may contribute to the pathogenesis of Huntington’s disease, given evidence of activated microglia and elevated levels of inflammatory molecules in disease gene carriers, even those many years from symptom onset. We have shown previously that monocytes from Huntington’s disease patients are hyper-reactive to stimulation in a manner dependent on their autonomous expression of the disease-causing mutant HTT protein. To date, however, whether human microglia are similarly hyper-responsive in a cell-autonomous manner has not been determined.

Methods

Microglial-like cells were derived from human pluripotent stem cells (PSCs) expressing mutant HTT containing varying polyglutamine lengths. These included lines that are otherwise isogenic, such that any observed differences can be attributed with certainty to the disease mutation itself. Analyses by quantitative PCR and immunofluorescence microscopy respectively of key genes and protein markers were undertaken to determine whether Huntington’s disease PSCs differentiated normally to a microglial fate. The resultant cultures and their supernatants were then assessed by various biochemical assays and multiplex ELISAs for viability and responses to stimulation, including the release of pro-inflammatory cytokines and reactive oxygen species. Conditioned media were applied to PSC-derived striatal neurons, and vice versa, to determine the effects that the secretomes of each cell type might have on the other.

Results

Human PSCs generated microglia successfully irrespective of the expression of mutant HTT. These cells, however, were hyper-reactive to stimulation in the production of pro-inflammatory cytokines such as IL-6 and TNFα. They also released elevated levels of reactive oxygen species that have neurotoxic potential. Accompanying such phenotypes, human Huntington’s disease PSC-derived microglia showed increased levels of apoptosis and were more susceptible to exogenous stress. Such stress appeared to be induced by supernatants from human PSC-derived striatal neurons expressing mutant HTT with a long polyglutamine tract.

Conclusions

These studies show, for the first time, that human Huntington’s disease PSC-derived microglia are hyper-reactive due to their autonomous expression of mutant HTT. This provides a cellular basis for the contribution that neuroinflammation might make to Huntington’s disease pathogenesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02147-6.

Advances and Perspectives in Dental Pulp Stem Cell Based Neuroregeneration Therapies

Human dental pulp stem cells (hDPSCs) are some of the most promising stem cell types for regenerative therapies given their ability to grow in the absence of serum and their realistic possibility to be used in autologous grafts. In this review, we describe the particular advantages of hDPSCs for neuroregenerative cell therapies. We thoroughly discuss the knowledge about their embryonic origin and characteristics of their postnatal niche, as well as the current status of cell culture protocols to maximize their multilineage differentiation potential, highlighting some common issues when assessing neuronal differentiation fates of hDPSCs. We also review the recent progress on neuroprotective and immunomodulatory capacity of hDPSCs and their secreted extracellular vesicles, as well as their combination with scaffold materials to improve their functional integration on the injured central nervous system (CNS) and peripheral nervous system (PNS). Finally, we offer some perspectives on the current and possible future applications of hDPSCs in neuroregenerative cell therapies.

Wild-type huntingtin regulates human macrophage function

The huntingtin (HTT) protein in its mutant form is the cause of the inherited neurodegenerative disorder, Huntington's disease. Beyond its effects in the central nervous system, disease-associated mutant HTT causes aberrant phenotypes in myeloid-lineage innate immune system cells, namely monocytes and macrophages. Whether the wild-type form of the protein, however, has a role in normal human macrophage function has not been determined. Here, the effects of lowering the expression of wild-type (wt)HTT on the function of primary monocyte-derived macrophages from healthy, non-disease human subjects were examined. This demonstrated a previously undescribed role for wtHTT in maintaining normal macrophage health and function. Lowered wtHTT expression was associated, for instance, with a diminished release of induced cytokines, elevated phagocytosis and increased vulnerability to cellular stress. These may well occur by mechanisms different to that associated with the mutant form of the protein, given an absence of any effect on the intracellular signalling pathway predominantly associated with macrophage dysfunction in Huntington's disease.

Dysregulated Brain Cholesterol Metabolism Is Linked to Neuroinflammation in Huntington's Disease

Huntington's disease is an autosomal-dominant, neurodegenerative disorder caused by a CAG repeat expansion in exon-1 of the huntingtin gene. Alterations in cholesterol metabolism and distribution have been reported in Huntington's disease, including abnormal interactions between mutant huntingtin and sterol regulatory element-binding proteins, decreased levels of apolipoprotein E/cholesterol/low-density lipoprotein receptor complexes, and alterations in the synthesis of ATP-binding cassette transporter A1. Plasma levels of 24S-hydroxycholestrol, a key intermediary in cholesterol metabolism and a possible marker in neurodegenerative diseases, decreased proportionally to the degree of caudate nucleus atrophy. The interaction of mutant huntingtin with sterol regulatory element-binding proteins is of particular interest given that sterol regulatory element-binding proteins play a dual role: They take part in lipid and cholesterol metabolism, but also in the inflammatory response that induces immune cell migration as well as toxic effects, particularly in astrocytes. This work summarizes current evidence on the metabolic and immune implications of sterol regulatory element-binding protein dysregulation in Huntington's disease, highlighting the potential use of drugs that modulate these alterations. © 2020 International Parkinson and Movement Disorder Society.

IDO and Kynurenine Metabolites in Peripheral and CNS Disorders

The importance of the kynurenine pathway in normal immune system function has led to an appreciation of its possible contribution to autoimmune disorders such as rheumatoid arthritis. Indoleamine-2,3-dioxygenase (IDO) activity exerts a protective function, limiting the severity of experimental arthritis, whereas deletion or inhibition exacerbates the symptoms. Other chronic disorder with an inflammatory component, such as atherosclerosis, are also suppressed by IDO activity. It is suggested that this overall anti-inflammatory activity is mediated by a change in the relative production or activity of Th17 and regulatory T cell populations. Kynurenines may play an anti-inflammatory role also in CNS disorders such as Huntington's disease, Alzheimer's disease and multiple sclerosis, in which signs of inflammation and neurodegeneration are involved. The possibility is discussed that in Huntington's disease kynurenines interact with other anti-inflammatory molecules such as Human Lymphocyte Antigen-G which may be relevant in other disorders. Kynurenine involvement may account for the protection afforded to animals with cerebral malaria and trypanosomiasis when they are treated with an inhibitor of kynurenine-3-monoxygenase (KMO). There is some evidence that changes in IL-10 may contribute to this protection and the relationship between kynurenines and IL-10 in arthritis and other inflammatory conditions should be explored. In addition, metabolites of kynurenine downstream of KMO, such as anthranilic acid and 3-hydroxy-anthranilic acid can influence inflammation, and the ratio of these compounds is a valuable biomarker of inflammatory status although the underlying molecular mechanisms of the changes require clarification. Hence it is essential that more effort be expended to identify their sites of action as potential targets for drug development. Finally, we discuss increasing awareness of the epigenetic regulation of IDO, for example by DNA methylation, a phenomenon which may explain differences between individuals in their susceptibility to arthritis and other inflammatory disorders.

Mutant huntingtin expression in microglia is neither required nor sufficient to cause the Huntington's disease-like phenotype in BACHD mice

Huntington's disease (HD) is caused by a CAG repeat expansion in the HTT gene and is characterized by early and selective striatal neurodegeneration. The huntingtin (HTT) protein is ubiquitously expressed in many tissues and the cellular pathogenesis of the disease is not fully understood. Immune cell dysfunction due to mutant HTT (mHTT) expression and aberrant immune system activation in HD patients suggests that inflammatory processes may contribute to HD pathogenesis. Here we used the BACHD mouse model of HD, which carries a conditional transgene expressing full-length human mHTT, to selectively deplete mHTT expression in myeloid lineage cells, including microglia, and evaluated the effects on HD-related behavior and neuropathology. In the converse experiment, we depleted mHTT expression in the majority of cells in the brain but specifically excluding microglia and again evaluated behavior and neuropathology. In mice with myeloid-specific mHTT-depletion, we observed no significant rescue of any behavioral or neuropathological outcome measures, while neural-specific knockout mice showed significant rescue of body weight, rotarod performance and striatal volume. We conclude that mHTT expression in microglia, though clearly affecting specific aspects of microglia function, does not alter disease pathogenesis in the BACHD mouse model. This may have implications for current or future therapeutic trials testing immune-modulating drugs in HD patients.

Expression of Human Mutant Huntingtin Protein in Drosophila Hemocytes Impairs Immune Responses

The pathogenic effect of mutant HTT (mHTT) which causes Huntington disease (HD) are not restricted to nervous system. Such phenotypes include aberrant immune responses observed in the HD models. However, it is still unclear how this immune dysregulation influences the innate immune response against pathogenic infection. In the present study, we used transgenic Drosophila melanogaster expressing mutant HTT protein (mHTT) with hemocyte-specific drivers and examined the immune responses and hemocyte function. We found that mHTT expression in the hemocytes did not affect fly viability, but the numbers of circulating hemocytes were significantly decreased. Consequently, we observed that the expression of mHTT in the hemocytes compromised the immune responses including clot formation and encapsulation which lead to the increased susceptibility to entomopathogenic nematode and parasitoid wasp infections. In addition, mHTT expression in Drosophila macrophage-like S2 cells in vitro reduced ATP levels, phagocytic activity and the induction of antimicrobial peptides. Further effects observed in mHTT-expressing cells included the altered production of cytokines and activation of JAK/STAT signaling. The present study shows that the expression of mHTT in Drosophila hemocytes causes deficient cellular and humoral immune responses against invading pathogens. Our findings provide the insight into the pathogenic effects of mHTT in the immune cells.

Impaired Remyelination in a Mouse Model of Huntington Disease

White matter (WM) abnormalities are a well-established feature of Huntington disease (HD), although their nature is not fully understood. Here, we asked whether remyelination as a measure of WM plasticity is impaired in a model of HD. Using the cuprizone assay, we examined demyelination and remyelination responses in YAC128 HD mice. Treatment with 0.2% cuprizone (CPZ) for 6 weeks resulted in significant reduction in mature (GSTπ-positive) oligodendrocyte counts and FluoroMyelin staining in the corpus callosum, leading to similar demyelination states in YAC128 and wild-type (WT) mice. Six weeks following cessation of CPZ, we observed robust remyelination in WT mice as indicated by an increase in mature oligodendrocyte counts and FluoroMyelin staining. In contrast, YAC128 mice exhibited an impaired remyelination response. The increase in mature oligodendrocyte counts in YAC128 HD mice following CPZ cessation was lower than that of WT. Furthermore, there was no increase in FluoroMyelin staining compared to the demyelinated state in YAC128 mice. We confirmed these findings using electron microscopy where the CPZ-induced reduction in myelinated axons was reversed following CPZ cessation in WT but not YAC128 mice. Our findings demonstrate that remyelination is impaired in YAC128 mice and suggest that WM plasticity may be compromised in HD.

Leukocyte telomere shortening in Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by an expanded CAG repeat. Though symptom onset commonly occurs at midlife and inversely correlates with the CAG repeat expansion, age at clinical onset and progression rate are variable. In the present study we investigated the relationship between leukocyte telomere length (LTL) and HD development. LTL was measured by real-time PCR in manifest HD patients (HD, n = 62), pre-manifest HD patients (pre-HD, n = 38), and age-matched controls (n = 76). Significant LTL differences were observed between the three groups (p < .0001), with LTL values in the order: HD < pre-HD < controls. The relationship between LTL and age was different in the three groups. An inverse relationship between mean LTL and CAG repeat number was found in the pre-HD (p = .03). The overall data seem to indicate that after age 30 years, LT begins to shorten markedly in pre-HD patients according to CAG number and increasing age, up to the values observed in HD. This very suggestive picture allowed us to hypothesize that in pre-manifest HD, LTL could be a measure of time to clinical HD onset. The possible use of LTL as a reliable biomarker to track HD development and progression was evaluated and discussed.

Platelet-derived extracellular vesicles in Huntington's disease

The production and release of extracellular vesicles (EV) is a property shared by all eukaryotic cells and a phenomenon frequently exacerbated in pathological conditions. The protein cargo of EV, their cell type signature and availability in bodily fluids make them particularly appealing as biomarkers. We recently demonstrated that platelets, among all types of blood cells, contain the highest concentrations of the mutant huntingtin protein (mHtt)-the genetic product of Huntington's disease (HD), a neurodegenerative disorder which manifests in adulthood with a complex combination of motor, cognitive and psychiatric deficits. Herein, we used a cohort of 59 HD patients at all stages of the disease, including individuals in pre-manifest stages, and 54 healthy age- and sex-matched controls, to evaluate the potential of EV derived from platelets as a biomarker. We found that platelets of pre-manifest and manifest HD patients do not release more EV even if they are activated. Importantly, mHtt was not found within EV derived from platelets, despite them containing high levels of this protein. Correlation analyses also failed to reveal an association between the number of platelet-derived EV and the age of the patients, the number of CAG repeats, the Unified Huntington Disease Rating Scale total motor score, the Total Functional Capacity score or the Burden of Disease score. Our data would, therefore, suggest that EV derived from platelets with HD is not a valuable biomarker in HD.

Mortal engines: Mitochondrial bioenergetics and dysfunction in neurodegenerative diseases

Mitochondria are best known for their role in ATP generation. However, studies over the past two decades have shown that mitochondria do much more than that. Mitochondria regulate both necrotic and apoptotic cell death pathways, they store and therefore coordinate cellular Ca²⁺ signaling, they generate and metabolize important building blocks, by-products and signaling molecules, and they also generate and are targets of free radical species that modulate many aspects of cell physiology and pathology. Most estimates suggest that although the brain makes up only 2 percent of body weight, utilizes about 20 percent of the body's total ATP. Thus, mitochondrial dysfunction greatly impacts brain functions and is indeed associated with numerous neurodegenerative diseases. Furthermore, a number of abnormal disease-associated proteins have been shown to interact directly with mitochondria, leading to mitochondrial dysfunction and subsequent neuronal cell death. Here, we discuss the role of mitochondrial dynamics impairment in the pathological processes associated with neurodegeneration and suggest that a therapy targeting mitochondrialdysfunction holds a great promise.

TSPO-PET imaging using [18F]PBR06 is a potential translatable biomarker for treatment response in Huntington's disease: preclinical evidence with the p75NTR ligand LM11A-31

Huntington's disease (HD) is an inherited neurodegenerative disorder that has no cure. HD therapeutic development would benefit from a non-invasive translatable biomarker to track disease progression and treatment response. A potential biomarker is using positron emission tomography (PET) imaging with a translocator protein 18 kDa (TSPO) radiotracer to detect microglial activation, a key contributor to HD pathogenesis. The ability of TSPO-PET to identify microglial activation in HD mouse models, essential for a translatable biomarker, or therapeutic efficacy in HD patients or mice is unknown. Thus, this study assessed the feasibility of utilizing PET imaging with the TSPO tracer, [18F]PBR06, to detect activated microglia in two HD mouse models and to monitor response to treatment with LM11A-31, a p75NTR ligand known to reduce neuroinflammation in HD mice. [18F]PBR06-PET detected microglial activation in striatum, cortex and hippocampus of vehicle-treated R6/2 mice at a late disease stage and, notably, also in early and mid-stage symptomatic BACHD mice. After oral administration of LM11A-31 to R6/2 and BACHD mice, [18F]PBR06-PET discerned the reductive effects of LM11A-31 on neuroinflammation in both HD mouse models. [18F]PBR06-PET signal had a spatial distribution similar to ex vivo brain autoradiography and correlated with microglial activation markers: increased IBA-1 and TSPO immunostaining/blotting and striatal levels of cytokines IL-6 and TNFα. These results suggest that [18F]PBR06-PET is a useful surrogate marker of therapeutic efficacy in HD mice with high potential as a translatable biomarker for preclinical and clinical HD trials.

High-Throughput Functional Analysis Distinguishes Pathogenic, Nonpathogenic, and Compensatory Transcriptional Changes in Neurodegeneration

Discriminating transcriptional changes that drive disease pathogenesis from nonpathogenic and compensatory responses is a daunting challenge. This is particularly true for neurodegenerative diseases, which affect the expression of thousands of genes in different brain regions at different disease stages. Here we integrate functional testing and network approaches to analyze previously reported transcriptional alterations in the brains of Huntington disease (HD) patients. We selected 312 genes whose expression is dysregulated both in HD patients and in HD mice and then replicated and/or antagonized each alteration in a Drosophila HD model. High-throughput behavioral testing in this model and controls revealed that transcriptional changes in synaptic biology and calcium signaling are compensatory, whereas alterations involving the actin cytoskeleton and inflammation drive disease. Knockdown of disease-driving genes in HD patient-derived cells lowered mutant Huntingtin levels and activated macroautophagy, suggesting a mechanism for mitigating pathogenesis. Our multilayered approach can thus untangle the wealth of information generated by transcriptomics and identify early therapeutic intervention points.

Overlap between age-at-onset and disease-progression determinants in Huntington disease

OBJECTIVE

A fundamental but still unresolved issue regarding Huntington disease (HD) pathogenesis is whether the factors that determine age at onset are the same as those that govern disease progression. Because elucidation of this issue is crucial for the development as well as optimal timing of administration of novel disease-modifying therapies, we aimed to assess the extent of overlap between age-at-onset and disease-progression determinants in HD.

METHODS

Using observational data from Enroll-HD, the largest cohort of patients with HD worldwide, in this study we present, validate, and apply an intuitive method based on linear mixed-effect models to quantify the variability in the rate of disease progression in HD.

RESULTS

A total of 3,411 patients with HD met inclusion criteria. We found that (1) about two-thirds of the rate of functional, motor, and cognitive progression in HD is determined by the same factors that also determine age at onset, with CAG repeat-dependent mechanisms having by far the largest effect; (2) although expanded HTT CAG repeat size had a large influence on average body weight, the rate of weight loss was largely independent of factors that determine age at onset in HD; and (3) about one-third of the factors that determine the rate of functional, motor, and cognitive progression are different from those that govern age at onset and need further elucidation.

CONCLUSION

Our findings imply that targeting of CAG repeat-dependent mechanisms, for example through gene-silencing approaches, is likely to affect the rate of functional, motor, and cognitive impairment, but not weight loss, in manifest HD mutation carriers.

On the Role of Store-Operated Calcium Entry in Acute and Chronic Neurodegenerative Diseases

In both excitable and non-excitable cells, calcium (Ca²⁺) signals are maintained by a highly integrated process involving store-operated Ca²⁺ entry (SOCE), namely the opening of plasma membrane (PM) Ca²⁺ channels following the release of Ca²⁺ from intracellular stores. Upon depletion of Ca²⁺ store, the stromal interaction molecule (STIM) senses Ca²⁺ level reduction and migrates from endoplasmic reticulum (ER)-like sites to the PM where it activates the channel proteins Orai and/or the transient receptor potential channels (TRPC) prompting Ca²⁺ refilling. Accumulating evidence suggests that SOCE dysregulation may trigger perturbation of intracellular Ca²⁺ signaling in neurons, glia or hematopoietic cells, thus participating to the pathogenesis of diverse neurodegenerative diseases. Under acute conditions, such as ischemic stroke, neuronal SOCE can either re-establish Ca²⁺ homeostasis or mediate Ca²⁺ overload, thus providing a non-excitotoxic mechanism of ischemic neuronal death. The dualistic role of SOCE in brain ischemia is further underscored by the evidence that it also participates to endothelial restoration and to the stabilization of intravascular thrombi. In Parkinson's disease (PD) models, loss of SOCE triggers ER stress and dysfunction/degeneration of dopaminergic neurons. Disruption of neuronal SOCE also underlies Alzheimer's disease (AD) pathogenesis, since both in genetic mouse models and in human sporadic AD brain samples, reduced SOCE contributes to synaptic loss and cognitive decline. Unlike the AD setting, in the striatum from Huntington's disease (HD) transgenic mice, an increased STIM2 expression causes elevated synaptic SOCE that was suggested to underlie synaptic loss in medium spiny neurons. Thus, pharmacological inhibition of SOCE is beneficial to synapse maintenance in HD models, whereas the same approach may be anticipated to be detrimental to cortical and hippocampal pyramidal neurons. On the other hand, up-regulation of SOCE may be beneficial during AD. These intriguing findings highlight the importance of further mechanistic studies to dissect the molecular pathways, and their corresponding targets, involved in synaptic dysfunction and neuronal loss during aging and neurodegenerative diseases.

Brain Regions Showing White Matter Loss in Huntington's Disease Are Enriched for Synaptic and Metabolic Genes

BACKGROUND

The earliest white matter changes in Huntington's disease are seen before disease onset in the premanifest stage around the striatum, within the corpus callosum, and in posterior white matter tracts. While experimental evidence suggests that these changes may be related to abnormal gene transcription, we lack an understanding of the biological processes driving this regional vulnerability.

METHODS

Here, we investigate the relationship between regional transcription in the healthy brain, using the Allen Institute for Brain Science transcriptome atlas, and regional white matter connectivity loss at three time points over 24 months in subjects with premanifest Huntington's disease relative to control participants. The baseline cohort included 72 premanifest Huntington's disease participants and 85 healthy control participants.

RESULTS

We show that loss of corticostriatal, interhemispheric, and intrahemispheric white matter connections at baseline and over 24 months in premanifest Huntington's disease is associated with gene expression profiles enriched for synaptic genes and metabolic genes. Corticostriatal gene expression profiles are predominately associated with motor, parietal, and occipital regions, while interhemispheric expression profiles are associated with frontotemporal regions. We also show that genes with known abnormal transcription in human Huntington's disease and animal models are overrepresented in synaptic gene expression profiles, but not in metabolic gene expression profiles.

CONCLUSIONS

These findings suggest a dual mechanism of white matter vulnerability in Huntington's disease, in which abnormal transcription of synaptic genes and metabolic disturbance not related to transcription may drive white matter loss.

Assessing and Modulating Kynurenine Pathway Dynamics in Huntington's Disease: Focus on Kynurenine 3-Monooxygenase

The link between disturbances in kynurenine pathway (KP) metabolism and Huntington's disease (HD) pathogenesis has been explored for a number of years. Several novel genetic and pharmacological tools have recently been developed to modulate key regulatory steps in the KP such as the reaction catalyzed by the enzyme kynurenine 3-monooxygenase (KMO). This insight has offered new options for exploring the mechanistic link between this metabolic pathway and HD, and provided novel opportunities for the development of candidate drug-like compounds. Here, we present an overview of the field, focusing on some novel approaches for interrogating the pathway experimentally.

Mechanisms underlying neurodegeneration in Huntington disease: applications to novel disease-modifying therapies

The CAG repeat expansion mutation that causes Huntington Disease (HD) was discovered more than 20 years ago, yet no treatment has yet been developed to stop the relentless course of the disease. Nonetheless, substantial progress has been made in understanding HD pathogenesis. We review insights that have been gleaned from HD genetics, metabolism, and pathology; HD mouse and cell models; the structure, function and post-translational modification of normal and mutant huntingtin (htt) protein; gene expression profiles in HD cells and tissue; the neurotoxicy of mutant htt RNA; and the expression of an antisense transcript from the HD locus. We conclude that rationale therapeutics for HD is within sight, though many questions remain to be answered.

Oxidative Stress and Huntington's Disease: The Good, The Bad, and The Ugly

Redox homeostasis is crucial for proper cellular functions, including receptor tyrosine kinase signaling, protein folding, and xenobiotic detoxification. Under basal conditions, there is a balance between oxidants and antioxidants. This balance facilitates the ability of oxidants, such as reactive oxygen species, to play critical regulatory functions through a direct modification of a small number of amino acids (e.g. cysteine) on signaling proteins. These signaling functions leverage tight spatial, amplitude, and temporal control of oxidant concentrations. However, when oxidants overwhelm the antioxidant capacity, they lead to a harmful condition of oxidative stress. Oxidative stress has long been held to be one of the key players in disease progression for Huntington's disease (HD). In this review, we will critically review this evidence, drawing some intermediate conclusions, and ultimately provide a framework for thinking about the role of oxidative stress in the pathophysiology of HD.

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.

Laquinimod rescues striatal, cortical and white matter pathology and results in modest behavioural improvements in the YAC128 model of Huntington disease

Increasing evidence supports a role for abnormal immune activation and inflammatory responses in Huntington disease (HD). In this study, we evaluated the therapeutic potential of laquinimod (1 and 10 mg/kg), a novel immunomodulatory agent shown to be protective in a number of neuroinflammatory conditions, in the YAC128 mouse model of HD. Treatment with laquinimod for 6 months rescued atrophy in the striatum, in certain cortical regions, and in the corpus callosum of YAC128 HD mice. Diffusion tensor imaging showed that white matter microstructural abnormalities in the posterior corpus callosum were improved following treatment with low dose (1 mg/kg) laquinimod, and were paralleled by reduced levels of interleukin-6 in the periphery of YAC128 HD mice. Functionally, treatment with laquinimod (1 and 10 mg/kg) led to modest improvements in motor function and in depressive-like behaviour. Taken together, these results suggest that laquinimod may improve some features of pathology in HD, and provides support for the role of immune activation in the pathogenesis of HD.

Integration of targeted metabolomics and transcriptomics identifies deregulation of phosphatidylcholine metabolism in Huntington's disease peripheral blood samples

INTRODUCTION

Metabolic changes have been frequently associated with Huntington's disease (HD). At the same time peripheral blood represents a minimally invasive sampling avenue with little distress to Huntington's disease patients especially when brain or other tissue samples are difficult to collect.

OBJECTIVES

We investigated the levels of 163 metabolites in HD patient and control serum samples in order to identify disease related changes. Additionally, we integrated the metabolomics data with our previously published next generation sequencing-based gene expression data from the same patients in order to interconnect the metabolomics changes with transcriptional alterations.

METHODS

This analysis was performed using targeted metabolomics and flow injection electrospray ionization tandem mass spectrometry in 133 serum samples from 97 Huntington's disease patients (29 pre-symptomatic and 68 symptomatic) and 36 controls.

RESULTS

By comparing HD mutation carriers with controls we identified 3 metabolites significantly changed in HD (serine and threonine and one phosphatidylcholine-PC ae C36:0) and an additional 8 phosphatidylcholines (PC aa C38:6, PC aa C36:0, PC ae C38:0, PC aa C38:0, PC ae C38:6, PC ae C42:0, PC aa C36:5 and PC ae C36:0) that exhibited a significant association with disease severity. Using workflow based exploitation of pathway databases and by integrating our metabolomics data with our gene expression data from the same patients we identified 4 deregulated phosphatidylcholine metabolism related genes (ALDH1B1, MBOAT1, MTRR and PLB1) that showed significant association with the changes in metabolite concentrations.

CONCLUSION

Our results support the notion that phosphatidylcholine metabolism is deregulated in HD blood and that these metabolite alterations are associated with specific gene expression changes.