Progressive alterations in the hypothalamic-pituitary-adrenal axis in the R6/2 transgenic mouse model of Huntington's disease

Glucocorticoid receptor signaling in the brain and its involvement in cognitive function

The hypothalamic-pituitary-adrenal axis regulates the secretion of glucocorticoids in response to environmental challenges. In the brain, a nuclear receptor transcription factor, the glucocorticoid receptor, is an important component of the hypothalamic-pituitary-adrenal axis's negative feedback loop and plays a key role in regulating cognitive equilibrium and neuroplasticity. The glucocorticoid receptor influences cognitive processes, including glutamate neurotransmission, calcium signaling, and the activation of brain-derived neurotrophic factor-mediated pathways, through a combination of genomic and non-genomic mechanisms. Protein interactions within the central nervous system can alter the expression and activity of the glucocorticoid receptor, thereby affecting the hypothalamic-pituitary-adrenal axis and stress-related cognitive functions. An appropriate level of glucocorticoid receptor expression can improve cognitive function, while excessive glucocorticoid receptors or long-term exposure to glucocorticoids may lead to cognitive impairment. Patients with cognitive impairment-associated diseases, such as Alzheimer's disease, aging, depression, Parkinson's disease, Huntington's disease, stroke, and addiction, often present with dysregulation of the hypothalamic-pituitary-adrenal axis and glucocorticoid receptor expression. This review provides a comprehensive overview of the functions of the glucocorticoid receptor in the hypothalamic-pituitary-adrenal axis and cognitive activities. It emphasizes that appropriate glucocorticoid receptor signaling facilitates learning and memory, while its dysregulation can lead to cognitive impairment. This provides clues about how glucocorticoid receptor signaling can be targeted to overcome cognitive disability-related disorders.

Antiageing strategy for neurodegenerative diseases: from mechanisms to clinical advances

In the context of global ageing, the prevalence of neurodegenerative diseases and dementia, such as Alzheimer's disease (AD), is increasing. However, the current symptomatic and disease-modifying therapies have achieved limited benefits for neurodegenerative diseases in clinical settings. Halting the progress of neurodegeneration and cognitive decline or even improving impaired cognition and function are the clinically meaningful goals of treatments for neurodegenerative diseases. Ageing is the primary risk factor for neurodegenerative diseases and their associated comorbidities, such as vascular pathologies, in elderly individuals. Thus, we aim to elucidate the role of ageing in neurodegenerative diseases from the perspective of a complex system, in which the brain is the core and peripheral organs and tissues form a holistic network to support brain functions. During ageing, the progressive deterioration of the structure and function of the entire body hampers its active and adaptive responses to various stimuli, thereby rendering individuals more vulnerable to neurodegenerative diseases. Consequently, we propose that the prevention and treatment of neurodegenerative diseases should be grounded in holistic antiageing and rejuvenation means complemented by interventions targeting disease-specific pathogenic events. This integrated approach is a promising strategy to effectively prevent, pause or slow down the progression of neurodegenerative diseases.

Circadian Interventions in Preclinical Models of Huntington's Disease: A Narrative Review

Huntington's Disease (HD) is a neurodegenerative disorder caused by an autosomal-dominant mutation in the huntingtin gene, which manifests with a triad of motor, cognitive and psychiatric declines. Individuals with HD often present with disturbed sleep/wake cycles, but it is still debated whether altered circadian rhythms are intrinsic to its aetiopathology or a consequence. Conversely, it is well established that sleep/wake disturbances, perhaps acting in concert with other pathophysiological mechanisms, worsen the impact of the disease on cognitive and motor functions and are a burden to the patients and their caretakers. Currently, there is no cure to stop the progression of HD, however, preclinical research is providing cementing evidence that restoring the fluctuation of the circadian rhythms can assist in delaying the onset and slowing progression of HD. Here we highlight the application of circadian-based interventions in preclinical models and provide insights into their potential translation in clinical practice. Interventions aimed at improving sleep/wake cycles' synchronization have shown to improve motor and cognitive deficits in HD models. Therefore, a strong support for their suitability to ameliorate HD symptoms in humans emerges from the literature, albeit with gaps in our knowledge on the underlying mechanisms and possible risks associated with their implementation.

Potential molecular mechanism of exercise reversing insulin resistance and improving neurodegenerative diseases

Neurodegenerative diseases are debilitating nervous system disorders attributed to various conditions such as body aging, gene mutations, genetic factors, and immune system disorders. Prominent neurodegenerative diseases include Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and multiple sclerosis. Insulin resistance refers to the inability of the peripheral and central tissues of the body to respond to insulin and effectively regulate blood sugar levels. Insulin resistance has been observed in various neurodegenerative diseases and has been suggested to induce the occurrence, development, and exacerbation of neurodegenerative diseases. Furthermore, an increasing number of studies have suggested that reversing insulin resistance may be a critical intervention for the treatment of neurodegenerative diseases. Among the numerous measures available to improve insulin sensitivity, exercise is a widely accepted strategy due to its convenience, affordability, and significant impact on increasing insulin sensitivity. This review examines the association between neurodegenerative diseases and insulin resistance and highlights the molecular mechanisms by which exercise can reverse insulin resistance under these conditions. The focus was on regulating insulin resistance through exercise and providing practical ideas and suggestions for future research focused on exercise-induced insulin sensitivity in the context of neurodegenerative diseases.

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

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

Glucocorticoid receptor antagonist CORT113176 attenuates motor and neuropathological symptoms of Huntington's disease in R6/2 mice

Huntington's Disease (HD) is a progressive neurodegenerative disease caused by a mutation in the huntingtin gene. The mutation leads to a toxic gain of function of the mutant huntingtin (mHtt) protein resulting in cellular malfunction, aberrant huntingtin aggregation and eventually neuronal cell death. Patients with HD show impaired motor functions and cognitive decline. Elevated levels of glucocorticoids have been found in HD patients and in HD mouse models, and there is a positive correlation between increased glucocorticoid levels and the progression of HD. Therefore, antagonism of the glucocorticoid receptor (GR) may be an interesting strategy for the treatment of HD. In this study, we evaluated the efficacy of the selective GR antagonist CORT113176 in the commonly used R6/2 mouse model. In male mice, CORT113176 treatment significantly delayed the loss of grip strength, the development of hindlimb clasping, gait abnormalities, and the occurrence of epileptic seizures. CORT113176 treatment delayed loss of DARPP-32 immunoreactivity in the dorsolateral striatum. It also restored HD-related parameters including astrocyte markers in both the dorsolateral striatum and the hippocampus, and microglia markers in the hippocampus. This suggests that CORT113176 has both cell-type and brain region-specific effects. CORT113176 delayed the formation of mHtt aggregates in the striatum and the hippocampus. In female mice, we did not observe major effects of CORT113176 treatment on HD-related symptoms, with the exception of the anti-epileptic effects. We conclude that CORT113176 effectively delays several key symptoms related to the HD phenotype in male R6/2 mice and believe that GR antagonism may be a possible treatment option.

Deregulated Transcriptome as a Platform for Adrenal Huntington's Disease-Related Pathology

Huntington's disease (HD) is a neurodegenerative disorder that affects mainly the central nervous system (CNS) by inducing progressive deterioration in both its structure and function. In recent years, there has been growing interest in the impact of HD on peripheral tissue function. Herein, we used the R6/2 mouse model of HD to investigate the influence of the disease on adrenal gland functioning. A transcriptomic analysis conducted using a well-established quantitative method, an Affymetrix array, revealed changes in gene expression in the R6/2 model compared to genetic background controls. For the first time, we identified disruptions in cholesterol and sterol metabolism, blood coagulation, and xenobiotic metabolism in HD adrenal glands. This study showed that the disrupted expression of these genes may contribute to the underlying mechanisms of Huntington's disease. Our findings may contribute to developing a better understanding of Huntington's disease progression and aid in the development of novel diagnostic or therapeutic approaches.

Polyglutamine disease in peripheral tissues

This year is a milestone anniversary of the discovery that Huntington's disease is caused by the presence of expanded polyglutamine repeats in the huntingtin gene leading to the formation of huntingtin aggregates. 30 years have elapsed and there is still no cure and the only FDA-approved treatment to alleviate the debilitating locomotor impairments presents several adverse effects. It has long been neglected that the huntingtin gene is almost ubiquitously expressed in many tissues outside of the nervous system. Growing evidence indicates that these peripheral tissues can contribute to the symptoms of the disease. New findings in Drosophila have shown that the selective expression of mutant huntingtin in muscle or fat is sufficient to cause detrimental effects in the absence of any neurodegeneration. In addition, it was discovered that a completely different tissue distribution of Htt aggregates in Drosophila muscles is responsible for a drastic aggravation of the detrimental effects. This review examines the peripheral tissues that express huntingtin with an added focus on the nature and distribution of the aggregates, if any.

The Urine Metabolome of R6/2 and zQ175DN Huntington's Disease Mouse Models

Huntington's disease (HD) is caused by the expansion of a polyglutamine (polyQ)-encoding tract in exon 1 of the huntingtin gene to greater than 35 CAG repeats. It typically has a disease course lasting 15-20 years, and there are currently no disease-modifying therapies available. Thus, there is a need for faithful mouse models of HD to use in preclinical studies of disease mechanisms, target validation, and therapeutic compound testing. A large variety of mouse models of HD were generated, none of which fully recapitulate human disease, complicating the selection of appropriate models for preclinical studies. Here, we present the urinary liquid chromatography-high-resolution mass spectrometry analysis employed to identify metabolic alterations in transgenic R6/2 and zQ175DN knock-in mice. In R6/2 mice, the perturbation of the corticosterone metabolism and the accumulation of pyrraline, indicative of the development of insulin resistance and the impairment of pheromone excretion, were observed. Differently from R6/2, zQ175DN mice showed the accumulation of oxidative stress metabolites. Both genotypes showed alterations in the tryptophan metabolism. This approach aims to improve our understanding of the molecular mechanisms involved in HD neuropathology, facilitating the selection of appropriate mouse models for preclinical studies. It also aims to identify potential biomarkers specific to HD.

AAV5-miHTT-mediated huntingtin lowering improves brain health in a Huntington's disease mouse model

Huntingtin (HTT)-lowering therapies show great promise in treating Huntington's disease. We have developed a microRNA targeting human HTT that is delivered in an adeno-associated serotype 5 viral vector (AAV5-miHTT), and here use animal behaviour, MRI, non-invasive proton magnetic resonance spectroscopy and striatal RNA sequencing as outcome measures in preclinical mouse studies of AAV5-miHTT. The effects of AAV5-miHTT treatment were evaluated in homozygous Q175FDN mice, a mouse model of Huntington's disease with severe neuropathological and behavioural phenotypes. Homozygous mice were used instead of the more commonly used heterozygous strain, which exhibit milder phenotypes. Three-month-old homozygous Q175FDN mice, which had developed acute phenotypes by the time of treatment, were injected bilaterally into the striatum with either formulation buffer (phosphate-buffered saline + 5% sucrose), low dose (5.2 × 109 genome copies/mouse) or high dose (1.3 × 1011 genome copies/mouse) AAV5-miHTT. Wild-type mice injected with formulation buffer served as controls. Behavioural assessments of cognition, T1-weighted structural MRI and striatal proton magnetic resonance spectroscopy were performed 3 months after injection, and shortly afterwards the animals were sacrificed to collect brain tissue for protein and RNA analysis. Motor coordination was assessed at 1-month intervals beginning at 2 months of age until sacrifice. Dose-dependent changes in AAV5 vector DNA level, miHTT expression and mutant HTT were observed in striatum and cortex of AAV5-miHTT-treated Huntington's disease model mice. This pattern of microRNA expression and mutant HTT lowering rescued weight loss in homozygous Q175FDN mice but did not affect motor or cognitive phenotypes. MRI volumetric analysis detected atrophy in four brain regions in homozygous Q175FDN mice, and treatment with high dose AAV5-miHTT rescued this effect in the hippocampus. Like previous magnetic resonance spectroscopy studies in Huntington's disease patients, decreased total N-acetyl aspartate and increased myo-inositol levels were found in the striatum of homozygous Q175FDN mice. These neurochemical findings were partially reversed with AAV5-miHTT treatment. Striatal transcriptional analysis using RNA sequencing revealed mutant HTT-induced changes that were partially reversed by HTT lowering with AAV5-miHTT. Striatal proton magnetic resonance spectroscopy analysis suggests a restoration of neuronal function, and striatal RNA sequencing analysis shows a reversal of transcriptional dysregulation following AAV5-miHTT in a homozygous Huntington's disease mouse model with severe pathology. The results of this study support the use of magnetic resonance spectroscopy in HTT-lowering clinical trials and strengthen the therapeutic potential of AAV5-miHTT in reversing severe striatal dysfunction in Huntington's disease.

A comprehensive perspective of Huntington's disease and mitochondrial dysfunction

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease. It is caused by the expansion of the CAG trinucleotide repeat sequence in the HTT gene. HD mainly manifests as involuntary dance-like movements and severe mental disorders. As it progresses, patients lose the ability to speak, think, and even swallow. Although the pathogenesis is unclear, studies have found that mitochondrial dysfunctions occupy an important position in the pathogenesis of HD. Based on the latest research advances, this review sorts out and discusses the role of mitochondrial dysfunction on HD in terms of bioenergetics, abnormal autophagy, and abnormal mitochondrial membranes. This review provides researchers with a more complete perspective on the mechanisms underlying the relationship between mitochondrial dysregulation and HD.

Altered Adipocyte Cell Size Distribution Prior to Weight Loss in the R6/2 Model of Huntington's Disease

BACKGROUND

Metabolic alterations contribute to disease onset and prognosis of Huntington's disease (HD). Weight loss in the R6/2 mouse model of HD is a consistent feature, with onset in mid-to-late stage of disease.

OBJECTIVE

In the present study, we aimed to investigate molecular and functional changes in white adipose tissue (WAT) that occur at weight loss in R6/2 mice. We further elaborated on the effect of leptin-deficiency and early obesity in R6/2 mice.

METHODS

We performed analyses at 12 weeks of age; a time point that coincides with the start of weight loss in our R6/2 mouse colony. Gonadal (visceral) and inguinal (subcutaneous) WAT depot weights were monitored, as well as adipocyte size distribution. Response to isoprenaline-stimulated glycerol release and insulin-stimulated glucose uptake in adipocytes from gonadal WAT was assessed.

RESULTS

In R6/2 mice, WAT depot weights were comparable to wildtype (WT) mice, and the response to insulin and isoprenaline in gonadal adipocytes was unaltered. Leptin-deficient R6/2 mice exhibited distinct changes compared to leptin-deficient WT mice. At 12 weeks, female leptin-deficient R6/2 mice had reduced body weight accompanied by an increased proportion of smaller adipocytes, while in contrast; male mice displayed a shift towards larger adipocyte sizes without a significant body weight reduction at this timepoint.

CONCLUSIONS

We here show that there are early sex-specific changes in adipocyte cell size distribution in WAT of R6/2 mice and leptin-deficient R6/2 mice.

Progressive transcriptional changes in metabolic genes and altered fatbody homeostasis in Drosophila model of Huntington's disease

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder marked by progressive neuronal atrophy, particularly in striatum and cerebral cortex. Although predominant manifestations of the disease include loss in the triad of motor, cognitive and behavioral capabilities, metabolic dysfunction in patients and HD models are being increasingly recognized. Patients display progressive body weight loss, which aggravates the disease and leads to cachexia in the terminal stages. Using the Drosophila model of HD, we have earlier reported that diseased flies exhibit an atypical pattern of lipid gain and loss with progression along with exhibiting extensive mitochondrial dysfunction, impaired calcium homeostasis and heightened apoptosis in the fatbody. Here, we first monitored the structural changes that abdominal fatbody undergoes with disease progression. Further, we checked the transcriptional changes of key metabolic genes in whole fly as well as genes regulating mitochondrial function, apoptosis, autophagy and calcium homeostasis in the abdominal fatbody. We found extensive alterations in whole-body and fatbody-specific transcriptional profile of the diseased flies, which was in consort with their stage-specific physiological state. Additionally, we also assessed lysosome-mediated autophagy in the fatbody of diseased flies in order to ascertain the mechanisms contributing to fatbody atrophy at the terminal stage. Interestingly, we found elevated autophagy in fatbody of flies throughout disease progression. This study provides new insights into the effect on peripheral metabolism due to degeneration of neurons in the neurodegenerative disease, thereby discerns novel mechanisms leading to cachexia in diseased flies and advocates for the need of managing metabolic dysfunctions in HD.

Hypothalamic expression of huntingtin causes distinct metabolic changes in Huntington's disease mice

OBJECTIVE

In Huntington's disease (HD), the disease-causing huntingtin (HTT) protein is ubiquitously expressed and causes both central and peripheral pathology. In clinical HD, a higher body mass index has been associated with slower disease progression, indicating the role of metabolic changes in disease pathogenesis. Underlying mechanisms of metabolic changes in HD remain poorly understood, but recent studies suggest the involvement of hypothalamic dysfunction. The present study aimed to investigate whether modulation of hypothalamic HTT levels would affect metabolic phenotype and disease features in HD using mouse models.

METHODS

We used the R6/2 and BACHD mouse models that express different lengths of mutant HTT to develop lean- and obese phenotypes, respectively. We utilized adeno-associated viral vectors to overexpress either mutant or wild-type HTT in the hypothalamus of R6/2, BACHD, and their wild-type littermates. The metabolic phenotype was assessed by body weight measurements over time and body composition analysis using dual-energy x-ray absorptiometry at the endpoint. R6/2 mice were further characterized using behavioral analyses, including rotarod, nesting-, and hindlimb clasping tests during early- and late-time points of disease progression. Finally, gene expression analysis was performed in R6/2 mice and wild-type littermates in order to assess transcriptional changes in the hypothalamus and adipose tissue.

RESULTS

Hypothalamic overexpression of mutant HTT induced significant gender-affected body weight gain in all models, including wild-type mice. In R6/2 females, early weight gain shifted to weight loss during the corresponding late stage of disease despite increased fat accumulation. Body weight changes were accompanied by behavioral alterations. During the period of early weight gain, R6/2 mice displayed a comparable locomotor capacity to wild-type mice. When assessing behavior just prior to weight loss onset in R6/2 mice, decreased locomotor performance was observed in R6/2 females with hypothalamic overexpression of mutant HTT. Transcriptional downregulation of beta-3 adrenergic receptor (B3AR), adipose triglyceride lipase (ATGL), and peroxisome proliferator-activated receptor-gamma (PPARγ) in gonadal white adipose tissue was accompanied by distinct alterations in hypothalamic gene expression profiles in R6/2 females after mutant HTT overexpression. No significant effect on metabolic phenotype in R6/2 was seen in response to wild-type HTT overexpression.

CONCLUSIONS

Taken together, our findings provide further support for the role of HTT in metabolic control via hypothalamic neurocircuits. Understanding the specific central neurocircuits and their peripheral link underlying metabolic imbalance in HD may open up avenues for novel therapeutic interventions.

Neurodegenerative Disorders of Alzheimer, Parkinsonism, Amyotrophic Lateral Sclerosis and Multiple Sclerosis: An Early Diagnostic Approach for Precision Treatment

Neurodegenerative diseases (NDs) are characterised by progressive dysfunction of synapses, neurons, glial cells and their networks. Neurodegenerative diseases can be classified according to primary clinical features (e.g., dementia, parkinsonism, or motor neuron disease), anatomic distribution of neurodegeneration (e.g., frontotemporal degenerations, extrapyramidal disorders, or spinocerebellar degenerations), or principal molecular abnormalities. The most common neurodegenerative disorders are amyloidosis, tauopathies, a-synucleinopathy, and TAR DNA-binding protein 43 (TDP-43) proteopathy. The protein abnormalities in these disorders have abnormal conformational properties along with altered cellular mechanisms, and they exhibit motor deficit, mitochondrial malfunction, dysfunctions in autophagic-lysosomal pathways, synaptic toxicity, and more emerging mechanisms such as the roles of stress granule pathways and liquid-phase transitions. Finally, for each ND, microglial cells have been reported to be implicated in neurodegeneration, in particular, because the microglial responses can shift from neuroprotective to a deleterious role. Growing experimental evidence suggests that abnormal protein conformers act as seed material for oligomerization, spreading from cell to cell through anatomically connected neuronal pathways, which may in part explain the specific anatomical patterns observed in brain autopsy sample. In this review, we mention the human pathology of select neurodegenerative disorders, focusing on how neurodegenerative disorders (i.e., Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis) represent a great healthcare problem worldwide and are becoming prevalent because of the increasing aged population. Despite many studies have focused on their etiopathology, the exact cause of these diseases is still largely unknown and until now with the only available option of symptomatic treatments. In this review, we aim to report the systematic and clinically correlated potential biomarker candidates. Although future studies are necessary for their use in early detection and progression in humans affected by NDs, the promising results obtained by several groups leads us to this idea that biomarkers could be used to design a potential therapeutic approach and preclinical clinical trials for the treatments of NDs.

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

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

Gastrointestinal disorders in hyperkinetic movement disorders and ataxia

BACKGROUND

Gastrointestinal (GI) disorders have been thoroughly investigated in hypokinetic disorders such as Parkinson's disease, but much less is known about GI disorders in hyperkinetic movement disorders and ataxia. The aim of this review is to draw attention to the GI disorders that are associated with these movement disorders.

METHODS

References for this systematic review were identified by searches of PubMed through May 2020. Only publications in English were reviewed.

RESULTS

Data from 249 articles were critically reviewed, compared, and integrated. The most frequently reported GI symptoms overall in hyperkinetic movement disorders and ataxia are dysphagia, sialorrhea, weight changes, esophago-gastritis, gastroparesis, constipation, diarrhea, and malabsorption. We report in detail on the frequency, characteristics, pathophysiology, and management of GI symptoms in essential tremor, restless legs syndrome, chorea, and spinocerebellar ataxias. The limited available data on GI disorders in dystonias, paroxysmal movement disorders, tardive dyskinesias, myoclonus, and non-SCA ataxias are also summarized.

CONCLUSION

The purpose of our systematic review is to draw attention that, although primarily motor disorders, hyperkinetic movement disorders and ataxia can involve the GI system. Raising awareness about the GI symptom burden in hyperkinetic movement disorders and ataxia could contribute to a new research interest in that field, as well as improved patient care.

Effects of excitotoxicity in the hypothalamus in transgenic mouse models of Huntington disease

Huntington disease (HD) is a fatal neurodegenerative movement disorder caused by an expanded CAG repeat in the huntingtin gene (HTT). The mutant huntingtin protein is ubiquitously expressed, but only certain brain regions are affected. The hypothalamus has emerged as an important area of pathology with selective loss of neurons expressing the neuropeptides orexin (hypocretin), oxytocin and vasopressin in human postmortem HD tissue. Hypothalamic changes in HD may have implications for early disease manifestations affecting the regulation of sleep, emotions and metabolism. The underlying mechanisms of selective vulnerability of certain neurons in HD are not fully understood, but excitotoxicity has been proposed to play a role. Further understanding of mechanisms rendering neurons sensitive to mutant huntingtin may reveal novel targets for therapeutic interventions. In the present study, we wanted to examine whether transgenic HD mice display altered sensitivity to excitotoxicity in the hypothalamus. We first assessed effects of hypothalamic injections of the excitotoxin quinolinic acid (QA) into wild-type (WT) mice. We show that neuronal populations expressing melanin-concentrating hormone (MCH) and cocaine and amphetamine-regulated transcript (CART) display a dose-dependent sensitivity to QA. In contrast, neuronal populations expressing orexin, oxytocin, vasopressin as well as tyrosine hydroxylase in the A13 area are resistant to QA-induced toxicity. We demonstrate that the R6/2 transgenic mouse model expressing a short fragment of mutant HTT displays hypothalamic neuropathology with discrete loss of the neuronal populations expressing orexin, MCH, CART, and orexin at 12 weeks of age. The BACHD mouse model expressing full-length mutant HTT does not display any hypothalamic neuropathology at 2 months of age. There was no effect of hypothalamic injections of QA on the neuronal populations expressing orexin, MCH, CART or oxytocin in neither HD mouse model. In conclusion, we find no support for a role of excitotoxicity in the loss of hypothalamic neuronal populations in HD.

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

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

Hypothalamic pathology in Huntington disease

Huntington's disease (HD), an autosomal dominant hereditary disorder associated with the accumulation of mutant huntingtin, is classically associated with cognitive decline and motor symptoms, notably chorea. However, growing evidence suggests that nonmotor symptoms are equally prevalent and debilitating. Some of these symptoms may be linked to hypothalamic pathology, demonstrated by findings in HD animal models and HD patients showing specific changes in hypothalamic neuropeptidergic populations and their associated functions. At least some of these alterations are likely due to local mutant huntingtin expression and toxicity, while others are likely caused by disturbed hypothalamic circuitry. Common problems include circadian rhythm disorders, including desynchronization of daily hormone excretion patterns, which could be targeted by novel therapeutic interventions, such as timed circadian interventions with light therapy or melatonin. However, translation of these findings from bench-to-bedside is hampered by differences in murine HD models and HD patients, including mutant huntingtin trinucleotide repeat length, which is highly heterogeneous across the various models. In this chapter, we summarize the current knowledge regarding hypothalamic alterations in HD patients and animal models, and the potential for these findings to be translated into clinical practice and management.

The Role of Hypothalamic Pathology for Non-Motor Features of Huntington's Disease

 Huntington’s disease (HD) is a fatal genetic neurodegenerative disorder. It has mainly been considered a movement disorder with cognitive symptoms and these features have been associated with pathology of the striatum and cerebral cortex. Importantly, individuals with the mutant huntingtin gene suffer from a spectrum of non-motor features often decades before the motor disorder manifests. These symptoms and signs include a range of psychiatric symptoms, sleep problems and metabolic changes with weight loss particularly in later stages. A higher body mass index at diagnosis is associated with slower disease progression. The common psychiatric symptom of apathy progresses with the disease. The fact that non-motor features are present early in the disease and that they show an association to disease progression suggest that unravelling the underlying neurobiological mechanisms may uncover novel targets for early disease intervention and better symptomatic treatment. The hypothalamus and the limbic system are important brain regions that regulate emotion, social cognition, sleep and metabolism. A number of studies using neuroimaging, postmortem human tissue and genetic manipulation in animal models of the disease has collectively shown that the hypothalamus and the limbic system are affected in HD. These findings include the loss of neuropeptide-expressing neurons such as orexin (hypocretin), oxytocin, vasopressin, somatostatin and VIP, and increased levels of SIRT1 in distinct nuclei of the hypothalamus. This review provides a summary of the results obtained so far and highlights the potential importance of these changes for the understanding of non-motor features in HD.

Altered cerebrovascular response to acute exercise in patients with Huntington's disease

The objective of this study was to determine whether a single session of exercise was sufficient to induce cerebral adaptations in individuals with Huntington’s disease and to explore the time dynamics of any acute cerebrovascular response. In this case–control study, we employed arterial-spin labelling MRI in 19 Huntington’s disease gene-positive participants (32–65 years, 13 males) and 19 controls (29–63 years, 10 males) matched for age, gender, body mass index and self-reported activity levels, to measure global and regional perfusion in response to 20 min of moderate-intensity cycling. Cerebral perfusion was measured at baseline and 15, 40 and 60 min after exercise cessation. Relative to baseline, we found that cerebral perfusion increased in patients with Huntington’s disease yet was unchanged in control participants in the precentral gyrus (P = 0.016), middle frontal gyrus (P = 0.046) and hippocampus (P = 0.048) 40 min after exercise cessation (+15 to +32.5% change in Huntington’s disease participants, −7.7 to 0.8% change in controls). The length of the disease‐causing trinucleotide repeat expansion in the huntingtin gene predicted the change in the precentral gyrus (P = 0.03) and the intensity of the exercise intervention predicted hippocampal perfusion change in Huntington’s disease participants (P < 0.001). In both groups, exercise increased hippocampal blood flow 60 min after exercise cessation (P = 0.039). These findings demonstrate the utility of acute exercise as a clinically sensitive experimental paradigm to modulate the cerebrovasculature. Twenty minutes of aerobic exercise induced transient cerebrovascular adaptations in the hippocampus and cortex selectively in Huntington’s disease participants and likely represents latent neuropathology not evident at rest.

Why Woody got the blues: The neurobiology of depression in Huntington's disease

Huntington's disease (HD) is an extraordinary disorder that usually strikes when individuals are in the prime of their lives, as was the case for the influential 20th century musician Woody Guthrie. HD demonstrates the exceptionally fine line between life and death in such 'genetic diseases', as the only difference between those who suffer horribly and die slowly of this disease is often just a handful of extra tandem repeats (beyond the normal polymorphic range) in a genome that constitutes over 3 billion paired nucleotides of DNA. Furthermore, HD presents as a complex and heterogenous combination of psychiatric, cognitive and motor symptoms, so can appear as an unholy trinity of 'three disorders in one'. The autosomal dominant nature of the disorder is also extremely challenging for affected families, as a 'flip of a coin' dictates which children inherit the mutation from their affected parent, and the gene-negative family members bear the burden of caring for the other half of the family that is affected. In this review, we will focus on one of the earliest, and most devastating, symptoms associated with HD, depression, which has been reported to affect approximately half of gene-positive HD family members. We will discuss the pathogenesis of HD, and depressive symptoms in particular, including molecular and cellular mechanisms, and potential genetic and environmental modifiers. This expanding understanding of HD pathogenesis may not only lead to novel therapeutic options for HD families, but may also provide insights into depression in the wider population, which has the greatest burden of disease of any disorder and an enormous unmet need for new therapies.

Insights into the Therapeutic Potential of Glucocorticoid Receptor Modulators for Neurodegenerative Diseases

Glucocorticoids are crucial for stress-coping, resilience, and adaptation. However, if the stress hormones become dysregulated, the vulnerability to stress-related diseases is enhanced. In this brief review, we discuss the role of glucocorticoids in the pathogenesis of neurodegenerative disorders in both human and animal models, and focus in particular on amyotrophic lateral sclerosis (ALS). For this purpose, we used the Wobbler animal model, which mimics much of the pathology of ALS including a dysfunctional hypothalamic–pituitary–adrenal axis. We discuss recent studies that demonstrated that the pathological cascade characteristic for motoneuron degeneration of ALS is mimicked in the genetically selected Wobbler mouse and can be attenuated by treatment with the selective glucocorticoid receptor antagonist (GRA) CORT113176. In long-term treatment (3 weeks) GRA attenuated progression of the behavioral, inflammatory, excitatory, and cell-death-signaling pathways while increasing the survival signal of serine–threonine kinase (pAkt). The action mechanism of the GRA may be either by interfering with GR deactivation or by restoring the balance between pro- and anti-inflammatory signaling pathways driven by the complementary mineralocorticoid receptor (MR)- and GR-mediated actions of corticosterone. Accordingly, GR antagonism may have clinical relevance for the treatment of neurodegenerative diseases.

Neuroendocrine Disturbances in Neurodegenerative Disorders: A Scoping Review

BACKGROUND

Neurodegenerative diseases cause progressive irreversible neuronal loss that has broad downstream effects. The neuroendocrine system regulates homeostasis of circuits that control critical functions such as the stress response, metabolism, reproduction, fluid balance, and glucose control. These systems are frequently disrupted in neurodegenerative disorders yet often overlooked in clinical practice.

OBJECTIVE

This review aims to gather the available data regarding these disturbances in Alzheimer's disease, Parkinson's disease, frontotemporal dementia, amyotrophic lateral sclerosis, and Huntington's disease and also to demonstrate the volume of literature in these individual arenas.

METHODS

Using the scoping review framework, a literature search was performed in PubMed to identify relevant articles published within the past 30 years (January 1988 to November 2018). The search criteria produced a total of 2022 articles, 328 of which were identified as relevant to this review.

RESULTS

Several major themes emerged from this review. These neuroendocrine disturbances may be a precursor to the illness, a part of the primary pathophysiology, or a direct consequence of the disease or independent of it. They have the potential to further understanding of the disease, exacerbate the underlying pathology, or provide therapeutic benefit.

CONCLUSIONS

By synthesizing the data from a systems' perspective, we aim to broaden how clinicians think about these illnesses and provide care.

Deterioration of neuroregenerative plasticity in association with testicular atrophy and dysregulation of the hypothalamic-pituitary-gonadal (HPG) axis in Huntington's disease: A putative role of the huntingtin gene in steroidogenesis

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder mainly affecting the structure and functions of the striatum, cerebral cortex and hippocampus leading to movement disorders, cognitive dysfunctions and emotional disturbances. The onset of HD has been linked to a pathogenic CAG repeat expansion in the huntingtin (HTT) gene that encodes for the polyglutamine (polyQ) stretches in the huntingtin (Htt) protein. Notably, the neuropathogenic events of the mutant HTT gene appear to be primed during adulthood and magnified along the ageing process. While the normal Htt protein is vital for the neuronal differentiation and neuroprotection, experimental HD models and postmortem human HD brains have been characterized by neurodegeneration and defects in neuroregenerative plasticity in the basal ganglia and limbic system including the hippocampus. Besides gonadal dysfunctions, reduced androgen levels and abnormal hypothalamic-pituitary-gonadal (HPG) axis have increasingly been evident in HD. Recently, ageing-related changes in levels of steroid sex hormones have been proposed to play a detrimental effect on the regulation of hippocampal neurogenesis in the adult brain. Considering its adult-onset nature, a potential relationship between dysregulation in the synthesis of sex steroid hormones and the pathogenesis of the mutant HTT gene appears to be an important clinical issue in HD. While the hippocampus and testis are the major sites of steroidogenesis, the presence of Htt in both areas is conclusively evident. Hence, the expression of the normal HTT gene may take part in the steroidogenic events in aforementioned organs in the physiological state, whereas the mutant HTT gene may cause defects in steroidogenesis in HD. Therefore, this review article comprehends the potential relationship between the gonadal dysfunctions and abnormal hippocampal plasticity in HD and represents a hypothesis for the putative role of the HTT gene in the regulation of steroidogenesis in gonads and in the brain.

A Critical Evaluation of Wet Biomarkers for Huntington's Disease: Current Status and Ways Forward

There is an unmet clinical need for objective biomarkers to monitor disease progression and treatment response in Huntington's disease (HD). The aim of this review is, therefore, to provide practical advice for biomarker discovery and to summarise studies on biofluid markers for HD. A PubMed search was performed to review literature with regard to candidate saliva, urine, blood and cerebrospinal fluid biomarkers for HD. Information has been organised into tables to allow a pragmatic approach to the discussion of the evidence and generation of practical recommendations for future studies. Many of the markers published converge on metabolic and inflammatory pathways, although changes in other analytes representing antioxidant and growth factor pathways have also been found. The most promising markers reflect neuronal and glial degeneration, particularly neurofilament light chain. International collaboration to standardise assays and study protocols, as well as to recruit sufficiently large cohorts, will facilitate future biomarker discovery and development.

Exercise, diet and stress as modulators of gut microbiota: Implications for neurodegenerative diseases

The last decade has witnessed an exponentially growing interest in gut microbiota and the gut-brain axis in health and disease. Accumulating evidence from preclinical and clinical research indicate that gut microbiota, and their associated microbiomes, may influence pathogenic processes and thus the onset and progression of various diseases, including neurological and psychiatric disorders. In fact, gut dysbiosis (microbiota dysregulation) has been associated with a range of neurodegenerative diseases, including Alzheimer's, Parkinson's, Huntington's and motor neuron disease, as well as multiple sclerosis. The gut microbiota constitutes a dynamic microbial system constantly challenged by many biological variables, including environmental factors. Since the gut microbiota constitute a changeable and experience-dependent ecosystem, they provide potential therapeutic targets that can be modulated as new interventions for dysbiosis-related disorders, including neurodegenerative diseases. This article reviews the evidence for environmental modulation of gut microbiota and its relevance to brain disorders, exploring in particular the implications for neurodegenerative diseases. We will focus on three major environmental factors that are known to influence the onset and progression of those diseases, namely exercise, diet and stress. Further exploration of environmental modulation, acting via both peripheral (e.g. gut microbiota and associated metabolic dysfunction or 'metabolopathy') and central (e.g. direct effects on CNS neurons and glia) mechanisms, may lead to the development of novel therapeutic approaches, such as enviromimetics, for a wide range of neurological and psychiatric disorders.

Neurovascular glucocorticoid receptors and glucocorticoids: implications in health, neurological disorders and drug therapy

Glucocorticoid receptors (GRs) are ubiquitous transcription factors widely studied for their role in controlling events related to inflammation, stress and homeostasis. Recently, GRs have reemerged as crucial targets of investigation in neurological disorders, with a focus on pharmacological strategies to direct complex mechanistic GR regulation and improve therapy. In the brain, GRs control functions necessary for neurovascular integrity, including responses to stress, neurological changes mediated by the hypothalamic-pituitary-adrenal axis and brain-specific responses to corticosteroids. Therefore, this review will examine GR regulation at the neurovascular interface in normal and pathological conditions, pharmacological GR modulation and glucocorticoid insensitivity in neurological disorders.

Leptin deficiency reverses high metabolic state and weight loss without affecting central pathology in the R6/2 mouse model of Huntington's disease

Body weight has been shown to be a predictor of clinical progression in Huntington's disease (HD). Alongside widespread neuronal pathology, both HD patients and the R6/2 mouse model of HD exhibit weight loss and increased energy expenditure, providing a rationale for targeting whole-body energy metabolism in HD. Leptin-deficient mice display low energy expenditure and increased body weight. We therefore hypothesized that normalizing energy metabolism in R6/2 mice, utilizing leptin- deficiency, would lead to a slower disease progression in the R6/2 mouse. In this study, we show that R6/2 mice on a leptin-deficient genetic background display increased body weight and increased fat mass compared to R6/2 mice, as well as wild type littermates. The increased body weight was accompanied by low energy expenditure, illustrated by a reduction in respiratory exchange rate. Leptin-deficient R6/2 mice had large white adipocytes with white adipocyte gene expression characteristics, in contrast to white adipose tissue in R6/2 mice, where white adipose tissue showed signs of browning. Leptin-deficient R6/2 mice did not exhibit improved neuropathological measures. Our results indicate that lowering energy metabolism in HD, by increasing fat mass and reducing respiratory exchange rate, is not sufficient to affect neuropathology. Further studies targeting energy metabolism in HD are warranted.

Normalizing glucocorticoid levels attenuates metabolic and neuropathological symptoms in the R6/2 mouse model of huntington's disease

Huntington's disease (HD) is a fatal genetic neurological disorder caused by a mutation in the human Huntingtin (HTT) gene. This mutation confers a toxic gain of function of the encoded mutant huntingtin (mHTT) protein, leading to widespread neuropathology including the formation of mHTT-positive inclusion bodies, gene dysregulation, reduced levels of adult dentate gyrus neurogenesis and neuron loss throughout many regions of the brain. Additionally, because HTT is ubiquitously expressed, several peripheral tissues are also affected. HD patients suffer from progressive motor, cognitive, psychiatric, and metabolic symptoms, including weight loss and skeletal muscle wasting. HD patients also show neuroendocrine changes including a robust, significant elevation in circulating levels of the glucocorticoid, cortisol. Previously, we confirmed that the R6/2 mouse model of HD exhibits elevated corticosterone (the rodent homolog of cortisol) levels and demonstrated that experimentally elevated corticosterone exacerbates R6/2 HD symptomology, resulting in severe and rapid weight loss and a shorter latency to death. Given that efficacious therapeutics are lacking for HD, here we investigated whether normalizing glucocorticoid levels could serve as a viable therapeutic approach for this disease. We tested the hypothesis that normalizing glucocorticoids to wild-type levels would ameliorate HD symptomology. Wild-type (WT) and transgenic R6/2 mice were allocated to three treatment groups: 1) adrenalectomy with normalized, WT-level corticosterone replacement (10 μg/ml), 2) adrenalectomy with high HD-level corticosterone replacement (35 μg/ml), or 3) sham surgery with no corticosterone replacement. Normalizing corticosterone to WT levels led to an improvement in metabolic rate in male R6/2 mice, as indicated by indirect calorimetry, including a reduction in oxygen consumption and normalization of respiratory exchange ratio values (p < .05 for both). Normalizing corticosterone also ameliorated brain atrophy in female R6/2 mice and skeletal muscle wasting in both male and female R6/2 mice (p < .05 for all). Female R6/2 mice given WT-level corticosterone replacement also showed a reduction in HD neuropathological markers, including a reduction in mHTT inclusion burden in the striatum, cortex, and hippocampus (p < .05 for all). This data illustrates that ameliorating glucocorticoid dysregulation leads to a significant improvement in HD symptomology in the R6/2 mouse model and suggests that cortisol-reducing therapeutics may be of value in improving HD patient quality of life.

Ghrelin rescues skeletal muscle catabolic profile in the R6/2 mouse model of Huntington's disease

Accumulating evidence suggests altered energy metabolism as a key feature in Huntington’s disease (HD) pathology. Hyper-catabolism, including weight loss and muscle atrophy, is seen in HD patients and HD mouse models. Metabolic hormones are key players, not only in energy metabolism, but also in neurodegenerative processes. Ghrelin, a gut peptide-hormone, plays an important role in regulating energy metabolism, stimulating appetite, and affects brain function and increases neuronal survival. The R6/2 mouse model of HD has previously been shown to exhibit progressive weight loss, dysregulated glucose metabolism, skeletal muscle atrophy and altered body composition. In this study, we targeted energy metabolism in R6/2 mice using ghrelin administration, with the primary aim to delay weight loss and reduce muscle atrophy. We also evaluated glucose metabolism and behaviour. We here demonstrate that ghrelin administration (subcutaneous 150 μg/kg daily injections) for 4 weeks, reversed the catabolic gene expression profile (increased expression of Caspase 8, Traf-5 and Creb1) seen in R6/2 mouse skeletal muscle. Skeletal muscle morphology was also improved with ghrelin, and importantly, ghrelin administration normalized behavioural deficits in R6/2 mice. Taken together, our findings encourage further studies targeting metabolism in HD.

Prolonged day length exposure improves circadian deficits and survival in a transgenic mouse model of Huntington's disease

The circadian disruption seen in patients of Huntington's disease (HD) is recapitulated in the R6/2 mouse model. As the disease progresses, the activity of R6/2 mice increases dramatically during the rest (light) period and decreases during the active (dark) period, eventually leading to a complete disintegration of rest-activity rhythms by the age of ~16 weeks. The suprachiasmatic nucleus controls circadian rhythms by entraining the rest-activity rhythms to the environmental light-dark cycle. Since R6/2 mice can shift their rest-activity rhythms in response to a jet-lag paradigm and also respond positively to bright light therapy (1000 lx), we investigated whether or not a prolonged day length exposure could reduce their daytime activity and improve their behavioural circadian rhythms. We found that a long-day photoperiod (16 h light/8 h dark cycle; 100 lx) significantly improved the survival of R6/2 female mice by 2.4 weeks, compared to mice kept under standard conditions (12 h light/12 h dark cycle). Furthermore, a long-day photoperiod improved the nocturnality of R6/2 female mice. Mice kept under long-day photoperiod also maintained acrophase in activity rhythms (a parameter of rhythmicity strength) in phase with that of WT mice, even if they were symptomatic. By contrast, a short-day photoperiod (8 h light/16 h dark cycle) was deleterious to R6/2 female mice and further reduced the survival by ~1 week. Together, our results support the idea that light therapy may be beneficial for improving circadian dysfunction in HD patients.

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Chronic exposure to a long day (16:8 LD) is beneficial to R6/2 female mice.

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The 16:8 LD cycle slowed body weight loss and improved survival of R6/2 mice.

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Lifespan of R6/2 female mice was extended by ~2.4 weeks under 16:8 LD cycle.

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R6/2 female mice under 16:8 LD had stabilised acrophase in activity rhythms.

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Lifespan of R6/2 female mice was reduced by chronic exposure to a short day (8:16 LD).

Altered lipid metabolism in Drosophila model of Huntington's disease

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

Corticosterone dysregulation exacerbates disease progression in the R6/2 transgenic mouse model of Huntington's disease

Huntington's disease (HD) is a genetic neurological disorder that causes severe and progressive motor, cognitive, psychiatric, and metabolic symptoms. There is a robust, significant elevation in circulating levels of the stress hormone, cortisol, in HD patients; however, the causes and consequences of this elevation are largely uncharacterized. Here, we evaluated whether elevated levels of corticosterone, the rodent homolog of cortisol, contributed to the development of symptomology in transgenic HD mice. Wild-type (WT) and transgenic R6/2 mice were given either 1) adrenalectomy with WT-level corticosterone replacement (10ng/ml), 2) adrenalectomy with high HD-level corticosterone replacement (60ng/ml), or 3) sham surgery without replacement. R6/2 mice on HD-level replacement showed severe and rapid weight loss (p<0.05) and a shorter latency to death (p<0.01) relative to the HD mice on WT-level replacement. We further evaluated basal and stress-induced levels of circulating corticosterone in R6/2 mice throughout the course of their life. We found that R6/2 transgenic HD mice display a spontaneous elevation in circulating corticosterone levels that became significant at 10weeks of age. Furthermore, we identified significant dysregulation of circadian rhythmicity of corticosterone release measured over a 24h period compared to wild-type controls. Unexpectedly, we found that R6/2 transgenic mice show a blunted corticosterone response to restraint stress, compared to wild-type mice. Together, these data provide further evidence that HPA-axis activity is abnormal in R6/2 mice, and highlight the important role that cortisol plays in HD symptom development. Our findings suggest that cortisol-reducing therapeutics may be of value in improving HD patient quality of life.

Impaired Nitric Oxide Mediated Vasodilation In The Peripheral Circulation In The R6/2 Mouse Model Of Huntington's Disease

Recent evidence shows that the Huntington's disease (HD) extends beyond the nervous system to other sites, including the cardiovascular system. Further, the cardiovascular pathology pre-dates neurological decline, however the mechanisms involved remain unclear. We investigated in the R6/2 mouse model of HD nitric oxide (NO) dependent and independent endothelial mechanisms. Femoral artery reactivity was determined by wire myography in wild type (WT) and R6/2 mice at 12 and 16 weeks of adulthood. WT mice showed increased endothelial relaxation between 12 and 16 weeks (Rmax: 72 ± 7% vs. 97 ± 13%, P < 0.05). In contrast, R6/2 mice showed enhanced endothelial relaxation already by 12 weeks (Rmax at 12w: 72 ± 7% vs. 94 ± 5%, WT vs. R6/2, P < 0.05) that declined by 16 weeks compared with WT mice (Rmax at 16w: 97 ± 13% vs. 68 ± 7%, WT vs. R6/2, P < 0.05). In WT mice, the increase in femoral relaxation between 12 and 16 weeks was due to enhanced NO dependent mechanisms. By 16 weeks of adult age, the R6/2 mouse developed overt endothelial dysfunction due to an inability to increase NO dependent vasodilation. The data add to the growing literature of non-neural manifestations of HD and implicate NO depletion as a key mechanism underlying the HD pathophysiology in the peripheral vasculature.

A Metabolic Study of Huntington's Disease

Background

Huntington’s disease patients have a number of peripheral manifestations suggestive of metabolic and endocrine abnormalities. We, therefore, investigated a number of metabolic factors in a 24-hour study of Huntington’s disease gene carriers (premanifest and moderate stage II/III) and controls.

Methods

Control (n = 15), premanifest (n = 14) and stage II/III (n = 13) participants were studied with blood sampling over a 24-hour period. A battery of clinical tests including neurological rating and function scales were performed. Visceral and subcutaneous adipose distribution was measured using magnetic resonance imaging. We quantified fasting baseline concentrations of glucose, insulin, cholesterol, triglycerides, lipoprotein (a), fatty acids, amino acids, lactate and osteokines. Leptin and ghrelin were quantified in fasting samples and after a standardised meal. We assessed glucose, insulin, growth hormone and cortisol concentrations during a prolonged oral glucose tolerance test.

Results

We found no highly significant differences in carbohydrate, protein or lipid metabolism markers between healthy controls, premanifest and stage II/III Huntington’s disease subjects. For some markers (osteoprotegerin, tyrosine, lysine, phenylalanine and arginine) there is a suggestion (p values between 0.02 and 0.05) that levels are higher in patients with premanifest HD, but not moderate HD. However, given the large number of statistical tests performed interpretation of these findings must be cautious.

Conclusions

Contrary to previous studies that showed altered levels of metabolic markers in patients with Huntington’s disease, our study did not demonstrate convincing evidence of abnormalities in any of the markers examined. Our analyses were restricted to Huntington’s disease patients not taking neuroleptics, anti-depressants or other medication affecting metabolic pathways. Even with the modest sample sizes studied, the lack of highly significant results, despite many being tested, suggests that the majority of these markers do not differ markedly by disease status.

A 24-Hour Study of the Hypothalamo-Pituitary Axes in Huntington's Disease

Background

Huntington’s disease is an inherited neurodegenerative disorder characterised by motor, cognitive and psychiatric disturbances. Patients exhibit other symptoms including sleep and mood disturbances, muscle atrophy and weight loss which may be linked to hypothalamic pathology and dysfunction of hypothalamo-pituitary axes.

Methods

We studied neuroendocrine profiles of corticotropic, somatotropic and gonadotropic hypothalamo-pituitary axes hormones over a 24-hour period in controlled environment in 15 healthy controls, 14 premanifest and 13 stage II/III Huntington’s disease subjects. We also quantified fasting levels of vasopressin, oestradiol, testosterone, dehydroepiandrosterone sulphate, thyroid stimulating hormone, free triiodothyronine, free total thyroxine, prolactin, adrenaline and noradrenaline. Somatotropic axis hormones, growth hormone releasing hormone, insulin-like growth factor-1 and insulin-like factor binding protein-3 were quantified at 06:00 (fasting), 15:00 and 23:00. A battery of clinical tests, including neurological rating and function scales were performed.

Results

24-hour concentrations of adrenocorticotropic hormone, cortisol, luteinizing hormone and follicle-stimulating hormone did not differ significantly between the Huntington’s disease group and controls. Daytime growth hormone secretion was similar in control and Huntington’s disease subjects. Stage II/III Huntington’s disease subjects had lower concentration of post-sleep growth hormone pulse and higher insulin-like growth factor-1:growth hormone ratio which did not reach significance. In Huntington’s disease subjects, baseline levels of hypothalamo-pituitary axis hormones measured did not significantly differ from those of healthy controls.

Conclusions

The relatively small subject group means that the study may not detect subtle perturbations in hormone concentrations. A targeted study of the somatotropic axis in larger cohorts may be warranted. However, the lack of significant results despite many variables being tested does imply that the majority of them do not differ substantially between HD and controls.

Hypothalamic overexpression of mutant huntingtin causes dysregulation of brown adipose tissue

Expression of mutant huntingtin (htt) protein has been shown to cause metabolic imbalance in animal models of Huntington disease (HD). The pathways involved are not fully understood but dysfunction of both the hypothalamus and brown adipose tissue (BAT) has been implicated. Here we show that targeted expression of mutant HTT in the hypothalamus leads to loss of the A13 dopaminergic cell group located in the zona incerta and reduced mRNA expression of neuropeptide Y1 receptor in the hypothalamus. Furthermore, this is accompanied by downregulation of uncoupling protein 1 expression and PPARγ coactivator-1 alpha in BAT and a rapid body weight gain. Taken together, our data might provide a mechanistic link between expression of mutant HTT, reduced activity of a hypothalamic dopaminergic pathway and dysfunction of BAT and in part explain the development of an obese phenotype in HD mouse models.

What's wrong with my mouse cage? Methodological considerations for modeling lifestyle factors and gene-environment interactions in mice

The mechanistic understanding of lifestyle contributions to disease has been largely driven by work in laboratory rodent models using environmental interventions. These interventions show an array of methodologies and sometimes unclear collective conclusions, hampering clinical interpretations. Here we discuss environmental enrichment, exercise and stress interventions to illustrate how different protocols can affect the interpretations of environmental factors in disease. We use Huntington's disease (HD) as an example because its mouse models exhibit excellent validity and HD was the first genetic animal model in which environmental stimulation was found to be beneficial. We make a number of observations and recommendations. Firstly, environmental enrichment and voluntary exercise generally show benefits across laboratories and mouse models. However, the extent to which these environmental interventions have beneficial effects depends on parameters such as the structural complexity of the cage in the case of enrichment, the timing of the intervention and the nature of the control conditions. In particular, clinical interpretations should consider deprived control living conditions and the ethological relevance of the enrichment. Secondly, stress can have negative effects on the phenotype in mouse models of HD and other brain disorders. When modeling stress, the effects of more than one type of experimental stressor should be investigated due to the heterogeneity and complexity of stress responses. With stress in particular, but ideally in all studies, both sexes should be used and the randomized group sizes need to be sufficiently powered to detect any sex effects. Opportunities for clinical translation will be guided by the 'environmental construct validity' of the preclinical data, including the culmination of complementary protocols across multiple animal models. Environmental interventions in mouse models of HD provide illustrative examples of how valid preclinical studies can lead to conclusions relevant to clinical populations.

Hypothalamic-pituitary-adrenal axis functioning in Huntington's disease and its association with depressive symptoms and suicidality

Hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis has been reported in Huntington's disease (HD). In non-HD populations, alterations in HPA axis activity have been associated with depression and suicidality. The present study aims to compare HPA axis activity between HD mutation carriers and controls, and examine its association with depressive symptoms and suicidality. To this end, salivary cortisol concentrations at seven time points, as well as depressive symptoms and suicidality, were assessed in 49 pre-motor, 102 motor symptomatic mutation carriers and 55 controls, at baseline and follow-up combined. Differences in parameters of HPA axis activity between these three groups, and their associations with depressive symptoms and suicidality in HD mutation carriers, were analysed using multilevel regression analyses. There were no differences in parameters of HPA axis activity between mutation carriers and controls, whereas pre-motor symptomatic mutation carriers had a significantly higher area under the curve to the increase (AUCi ) compared to motor symptomatic mutation carriers. In the entire HD cohort, HPA axis activity was not associated with depressive symptoms or suicidality. After stratifying mutation carriers into pre-motor, early and advanced disease stages, β values differed between these groups. Remarkably, a higher AUCi was significantly associated with depressive symptoms in pre-motor and early disease stage mutation carriers, with a reverse nonsignificant association in advanced disease stage mutation carriers. The lower AUCi in motor symptomatic mutation carriers and the varying associations with depressive symptoms and suicidality in pre-motor, early and advanced disease stages could possibly be explained by exhaustion of the HPA axis after prolonged stress-induced HPA axis hyperactivity and deserves further longitudinal study.

Is Dysregulation of the HPA-Axis a Core Pathophysiology Mediating Co-Morbid Depression in Neurodegenerative Diseases?

There is increasing evidence of prodromal manifestation of neuropsychiatric symptoms in a variety of neurodegenerative diseases such as Parkinson's disease (PD) and Huntington's disease (HD). These affective symptoms may be observed many years before the core diagnostic symptoms of the neurological condition. It is becoming more apparent that depression is a significant modifying factor of the trajectory of disease progression and even treatment outcomes. It is therefore crucial that we understand the potential pathophysiologies related to the primary condition, which could contribute to the development of depression. The hypothalamic-pituitary-adrenal (HPA)-axis is a key neuroendocrine signaling system involved in physiological homeostasis and stress response. Disturbances of this system lead to severe hormonal imbalances, and the majority of such patients also present with behavioral deficits and/or mood disorders. Dysregulation of the HPA-axis is also strongly implicated in the pathology of major depressive disorder. Consistent with this, antidepressant drugs, such as the selective serotonin reuptake inhibitors have been shown to alter HPA-axis activity. In this review, we will summarize the current state of knowledge regarding HPA-axis pathology in Alzheimer's, PD and HD, differentiating between prodromal and later stages of disease progression when evidence is available. Both clinical and preclinical evidence will be examined, but we highlight animal model studies as being particularly useful for uncovering novel mechanisms of pathology related to co-morbid mood disorders. Finally, we purpose utilizing the preclinical evidence to better inform prospective, intervention studies.

Analysis of White Adipose Tissue Gene Expression Reveals CREB1 Pathway Altered in Huntington's Disease

BACKGROUND

In addition to classical neurological symptoms, Huntington's disease (HD) is complicated by peripheral pathology and both the mutant gene and the protein are found in cells and tissues throughout the body. Despite the adipose tissue gene expression alterations described in HD mouse models, adipose tissue and its gene expression signature have not been previously explored in human HD.

OBJECTIVE

We investigated gene expression signatures in subcutaneous adipose tissue obtained from control subjects, premanifest HD gene carriers and manifest HD subjects with the aim to identify gene expression changes and signalling pathway alterations in adipose tissue relevant to HD.

METHODS

Gene expression was assessed using Affymetrix GeneChip® Human Gene 1.0 ST Array. Target genes were technically validated using real-time quantitative PCR and the expression signature was validated in an independent subject cohort.

RESULTS

In subcutaneous adipose tissue, more than 500 genes were significantly different in premanifest HD subjects as compared to healthy controls. Pathway analysis suggests that the differentially expressed genes found here in HD adipose tissue are involved in fatty acid metabolism pathways, angiotensin signalling pathways and immune pathways. Transcription factor analysis highlights CREB1. Using RT-qPCR, we found that MAL2, AGTR2, COBL and the transcription factor CREB1 were significantly upregulated, with CREB1 and AGT also being significantly upregulated in a separate cohort.

CONCLUSIONS

Distinct gene expression profiles can be seen in HD subcutaneous adipose tissue, with CREB1 highlighted as a key transcription factor.

Increase of angiotensin II type 1 receptor auto-antibodies in Huntington's disease

Background

In the recent years, a role of the immune system in Huntington’s disease (HD) is increasingly recognized. Here we investigate the presence of T cell activating auto-antibodies against angiotensin II type 1 receptors (AT1R) in all stages of the disease as compared to healthy controls and patients suffering from multiple sclerosis (MS) as a prototype neurologic autoimmune disease.

Results

As compared to controls, MS patients show higher titers of anti-AT1R antibodies, especially in individuals with active disease. In HD, anti-AT1R antibodies are more frequent than in healthy controls or even MS and occur in 37.9% of patients with relevant titers ≥ 20 U/ml. In a correlation analysis with clinical parameters, the presence of AT1R antibodies in the sera of HD individuals inversely correlated with the age of onset and positively with the disease burden score as well as with smoking and infection.

Conclusions

These data suggest a dysfunction of the adaptive immune system in HD which may be triggered by different stimuli including autoimmune responses, infection and possibly also smoking.