The metabolic profile of early Huntington's disease- a combined human and transgenic mouse study

Maintenance of basal levels of autophagy in Huntington's disease mouse models displaying metabolic dysfunction

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by an expanded polyglutamine repeat in the huntingtin protein. Neuropathology in the basal ganglia and in the cerebral cortex has been linked to the motor and cognitive symptoms whereas recent work has suggested that the hypothalamus might be involved in the metabolic dysfunction. Several mouse models of HD that display metabolic dysfunction have hypothalamic pathology, and expression of mutant huntingtin in the hypothalamus has been causally linked to the development of metabolic dysfunction in mice. Although the pathogenic mechanisms by which mutant huntingtin exerts its toxic functions in the HD brain are not fully known, several studies have implicated a role for the lysososomal degradation pathway of autophagy. Interestingly, changes in autophagy in the hypothalamus have been associated with the development of metabolic dysfunction in wild-type mice. We hypothesized that expression of mutant huntingtin might lead to changes in the autophagy pathway in the hypothalamus in mice with metabolic dysfunction. We therefore investigated whether there were changes in basal levels of autophagy in a mouse model expressing a fragment of 853 amino acids of mutant huntingtin selectively in the hypothalamus using a recombinant adeno-associate viral vector approach as well as in the transgenic BACHD mice. We performed qRT-PCR and Western blot to investigate the mRNA and protein expression levels of selected autophagy markers. Our results show that basal levels of autophagy are maintained in the hypothalamus despite the presence of metabolic dysfunction in both mouse models. Furthermore, although there were no major changes in autophagy in the striatum and cortex of BACHD mice, we detected modest, but significant differences in levels of some markers in mice at 12 months of age. Taken together, our results indicate that overexpression of mutant huntingtin in mice do not significantly perturb basal levels of autophagy.

IGF-1 intranasal administration rescues Huntington's disease phenotypes in YAC128 mice

Huntington's disease (HD) is an autosomal dominant disease caused by an expansion of CAG repeats in the gene encoding for huntingtin. Brain metabolic dysfunction and altered Akt signaling pathways have been associated with disease progression. Nevertheless, conflicting results persist regarding the role of insulin-like growth factor-1 (IGF-1)/Akt pathway in HD. While high plasma levels of IGF-1 correlated with cognitive decline in HD patients, other data showed protective effects of IGF-1 in HD striatal neurons and R6/2 mice. Thus, in the present study, we investigated motor phenotype, peripheral and central metabolic profile, and striatal and cortical signaling pathways in YAC128 mice subjected to intranasal administration of recombinant human IGF-1 (rhIGF-1) for 2 weeks, in order to promote IGF-1 delivery to the brain. We show that IGF-1 supplementation enhances IGF-1 cortical levels and improves motor activity and both peripheral and central metabolic abnormalities in YAC128 mice. Moreover, decreased Akt activation in HD mice brain was ameliorated following IGF-1 administration. Upregulation of Akt following rhIGF-1 treatment occurred concomitantly with increased phosphorylation of mutant huntingtin on Ser421. These data suggest that intranasal administration of rhIGF-1 ameliorates HD-associated glucose metabolic brain abnormalities and mice phenotype.

Thyrotropin secretion patterns in health and disease

Thyroid hormones are extremely important for metabolism, development, and growth during the lifetime. The hypothalamo-pituitary-thyroid axis is precisely regulated for these purposes. Much of our knowledge of this hormonal axis is derived from experiments in animals and mutations in man. This review examines the hypothalamo-pituitary-thyroid axis particularly in relation to the regulated 24-hour serum TSH concentration profiles in physiological and pathophysiological conditions, including obesity, primary hypothyroidism, pituitary diseases, psychiatric disorders, and selected neurological diseases. Diurnal TSH rhythms can be analyzed with novel and precise techniques, eg, operator-independent deconvolution and approximate entropy. These approaches provide indirect insight in the regulatory components in pathophysiological conditions.

Longitudinal analysis of the electroencephalogram and sleep phenotype in the R6/2 mouse model of Huntington's disease

Deficits in sleep and circadian organization have been identified as common early features in patients with Huntington's disease that correlate with symptom severity and may be instrumental in disease progression. Studies in Huntington's disease gene carriers suggest that alterations in the electroencephalogram may reflect underlying neuronal dysfunction that is present in the premanifest stage. We conducted a longitudinal characterization of sleep/wake and electroencephalographic activity in the R6/2 mouse model of Huntington's disease to determine whether analogous electroencephalographic 'signatures' could be identified early in disease progression. R6/2 and wild-type mice were implanted for electroencephalographic recordings along with telemetry for the continuous recording of activity and body temperature. Diurnal patterns of activity and core body temperature were progressively disrupted in R6/2 mice, with a large reduction in the amplitude of these rhythms apparent by 13 weeks of age. The diurnal variation in sleep/wake states was gradually attenuated as sleep became more fragmented and total sleep time was reduced relative to wild-type mice. These genotypic differences were augmented at 17 weeks and evident across the entire 24-h period. Quantitative electroencephalogram analysis revealed anomalous increases in high beta and gamma activity (25-60 Hz) in all sleep/wake states in R6/2 mice, along with increases in theta activity during both non-rapid eye movement and rapid eye movement sleep and a reduction of delta power in non-rapid eye movement sleep. These dramatic alterations in quantitative electroencephalographic measures were apparent from our earliest recording (9 weeks), before any major differences in diurnal physiology or sleep/wake behaviour occurred. In addition, the homeostatic response to sleep deprivation was greatly attenuated with disease progression. These findings demonstrate the sensitivity of quantitative electroencephalographic analysis to identify early pathophysiological alterations in the R6/2 model of Huntington's disease and suggest longitudinal studies in other preclinical Huntington's disease models are needed to determine the generality of these observations as a potential adjunct in therapeutic development.

Characteristics, pathophysiology and clinical management of weight loss in Huntington’s disease

SUMMARY Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder caused by a CAG repeat expansion in the HTT gene. Clinically, the disease is characterized by motor impairment, cognitive deterioration and behavioral disturbances. Unintended weight loss is also a hallmark of the disease and frequently leads to general weakening and a decline in the quality of life of HD patients. Moreover, a higher BMI has been associated with a slower rate of disease progression. In this review, the authors first delineate the characteristics of weight loss in both HD patients and genetic models of the disease. Subsequently, they discuss the pathophysiological processes underlying weight loss in HD and highlight the implications for management and care of HD patients with, or at risk of, unintended weight loss.

Automated home cage assessment shows behavioral changes in a transgenic mouse model of spinocerebellar ataxia type 17

Spinocerebellar Ataxia type 17 (SCA17) is an autosomal dominantly inherited, neurodegenerative disease characterized by ataxia, involuntary movements, and dementia. A novel SCA17 mouse model having a 71 polyglutamine repeat expansion in the TATA-binding protein (TBP) has shown age related motor deficit using a classic motor test, yet concomitant weight increase might be a confounding factor for this measurement. In this study we used an automated home cage system to test several motor readouts for this same model to confirm pathological behavior results and evaluate benefits of automated home cage in behavior phenotyping. Our results confirm motor deficits in the Tbp/Q71 mice and present previously unrecognized behavioral characteristics obtained from the automated home cage, indicating its use for high-throughput screening and testing, e.g. of therapeutic compounds.

Mitochondria targeted therapeutic approaches in Parkinson's and Huntington's diseases

Substantial evidence from both genetic and toxin induced animal and cellular models and postmortem human brain tissue indicates that mitochondrial dysfunction plays a central role in pathophysiology of the neurodegenerative disorders including Parkinson's disease (PD), and Huntington's disease (HD). This review discusses the emerging understanding of the role of mitochondrial dysfunction including bioenergetics defects, mitochondrial DNA mutations, familial nuclear DNA mutations, altered mitochondrial fusion/fission and morphology, mitochondrial transport/trafficking, altered transcription and increased interaction of pathogenic proteins with mitochondria in the pathogenesis of PD and HD. This review recapitulates some of the key therapeutic strategies applied to surmount mitochondrial dysfunction in these debilitating disorders. We discuss the therapeutic role of mitochondrial bioenergetic agents such as creatine, Coenzyme-Q10, mitochondrial targeted antioxidants and peptides, the SIRT1 activator resveratrol, and the pan-PPAR agonist bezafibrate in toxin and genetic cellular and animal models of PD and HD. We also summarize the phase II-III clinical trials conducted using some of these agents. Lastly, we discuss PGC-1α, TORC and Sirtuins as potential therapeutic targets for mitochondrial dysfunction in neurodegenerative disorders. This article is part of a Special Issue entitled 'Mitochondrial function and dysfunction in neurodegeneration'.

Mouse models of polyglutamine diseases: review and data table. Part I

Polyglutamine (polyQ) disorders share many similarities, such as a common mutation type in unrelated human causative genes, neurological character, and certain aspects of pathogenesis, including morphological and physiological neuronal alterations. The similarities in pathogenesis have been confirmed by findings that some experimental in vivo therapy approaches are effective in multiple models of polyQ disorders. Additionally, mouse models of polyQ diseases are often highly similar between diseases with respect to behavior and the features of the disease. The common features shared by polyQ mouse models may facilitate the investigation of polyQ disorders and may help researchers explore the mechanisms of these diseases in a broader context. To provide this context and to promote the understanding of polyQ disorders, we have collected and analyzed research data about the characterization and treatment of mouse models of polyQ diseases and organized them into two complementary Excel data tables. The data table that is presented in this review (Part I) covers the behavioral, molecular, cellular, and anatomic characteristics of polyQ mice and contains the most current knowledge about polyQ mouse models. The structure of this data table is designed in such a way that it can be filtered to allow for the immediate retrieval of the data corresponding to a single mouse model or to compare the shared and unique aspects of many polyQ models. The second data table, which is presented in another publication (Part II), covers therapeutic research in mouse models by summarizing all of the therapeutic strategies employed in the treatment of polyQ disorders, phenotypes that are used to examine the effects of the therapy, and therapeutic outcomes.

Measures of growth in children at risk for Huntington disease

OBJECTIVE

The effect of mHTT on human development was examined by evaluating measures of growth in children at risk for Huntington disease (HD).

METHODS

Children at risk for HD with no manifest symptoms (no juvenile HD included) were enrolled and tested for gene expansion for research purposes only. Measurements of growth (height, weight, body mass index [BMI], and head circumference) in children tested as gene-expanded (n = 20, 7-18 years of age, CAG repeats ≥39) were compared to those of a large database of healthy children (n = 152, 7-18 years of age).

RESULTS

Gene-expanded children had significantly lower measures of head circumference, weight, and BMI. Head circumference was abnormally low even after correcting for height, suggesting a specific deficit in brain growth, rather than a global growth abnormality.

CONCLUSIONS

These results indicate that, compared to a control population, children who were estimated to be decades from HD diagnosis have significant differences in growth. Further, they suggest that mHTT may play a role in atypical somatic, and in particular, brain development.

The Current Status of Neural Grafting in the Treatment of Huntington's Disease. A Review

Huntington’s disease (HD) is a devastating, fatal, autosomal dominant condition in which the abnormal gene codes for a mutant form of huntingtin that causes widespread neuronal dysfunction and death. This leads to a clinical presentation, typically in midlife, with a combination of motor, psychiatric, cognitive, metabolic, and sleep abnormalities, for which there are some effective symptomatic therapies that can produce some transient benefits. The disease, though, runs a progressive course over a 20-year period ultimately leading to death, and there are currently no proven disease modifying therapies. However whilst the neuronal dysfunction and loss affects much of the central nervous system, the striatum is affected early on in the disease and is one of the areas most affected by the pathogenic process. As a result the prospect of treating HD using neural transplants of striatal tissue has been explored and to date the clinical data is inconclusive. In this review we discuss the rationale for treating HD using this approach, before discussing the clinical trial data and what we have learnt to date using this therapeutic strategy.

No evidence of impaired gastric emptying in early Huntington's Disease

Background: Several factors, such as dysphagia, an increased motor activity, increased metabolic rate and a hypermetabolic state have been discussed as contributing to weight loss even at the early stages of Huntington’s Disease (HD). Aim of this pilot study was to investigate gastric emptying as a possible reason for weight loss in HD.

Methods: 11 HD participants at early stages of the disease and matched controls were investigated by using the well-established and non-invasive 13C-octanoate breath test. The “Gastroparesis Cardinal Symptom Index” and the “Short-Form Leeds Dyspepsia Questionnaire” were used for clinical evaluation of gastroparesis or dyspepsia.

Results: When compared to standard values ​​given in literature and controls all HD patients had normal breath test results. There was no evidence of gastroparesis or dyspepsia. There was a correlation of breath test results with the cognitive and functional performance of HD participants.

Conclusion: According to our data, there is no evidence of impaired gastric emptying in early HD. We can not exclude that gastric emptying contributes to weight loss at more advanced stages of the disease.

Microcirculation response to local cooling in patients with Huntington's disease

Altered autonomic nervous system (ANS) functioning in early stages of Huntington's disease (HD) has been suggested, presumably due to distorted high-order autonomic control. ANS functioning in the early stages of HD was further investigated. Laser-Doppler (LD) flux in the skin of the fingertips, heart rate (HR), HR variability, systolic and diastolic blood pressure were measured during rest and during a 6 min cooling of one hand at 15°C. Data of 15 presymptomatic gene mutation carriers (PHD), 15 early symptomatic HD patients (EHD), and two groups of 15 age- and sex-matched controls were compared. The area under the low frequency (LF) and high frequency (HF) bands of the HR variability spectrum were calculated. An augmented reduction of cutaneous LD flux was found in response to the direct cooling in the PHD group (37.5 ± 8.5% of resting value) compared to the PHD controls (67.27 ± 8.4%) (p < 0.05). In addition, the PHD group had higher (LF/(LF + HF) index of primary sympathetic modulation of the HR at rest (53.6 ± 3.3) compared to the EHD patients (39.7 ± 4.2) (p < 0.05). In the EHD group, a significantly smaller change of HR during cooling (100.26 ± 1.2%) was found compared to the EHD controls (95.9 ± 1.0%) (p < 0.05). The results are in line with the hypothesis that ANS dysfunction occurs even in PHD subjects. Further, they support the hypothesis that dysfunction of the high-order autonomic centres are involved in HD.

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

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

IRS2 increases mitochondrial dysfunction and oxidative stress in a mouse model of Huntington disease

Aging is a major risk factor for the progression of neurodegenerative diseases, including Huntington disease (HD). Reduced neuronal IGF1 or Irs2 signaling have been shown to extend life span in mice. To determine whether Irs2 signaling modulates neurodegeneration in HD, we genetically modulated Irs2 concentrations in the R6/2 mouse model of HD. Increasing Irs2 levels in the brains of R6/2 mice significantly reduced life span and increased neuronal oxidative stress and mitochondrial dysfunction. In contrast, reducing Irs2 levels throughout the body (except in β cells, where Irs2 expression is needed to prevent diabetes onset; R6/2•Irs2+/-•Irs2βtg mice) improved motor performance and extended life span. The slower progression of HD-like symptoms was associated with increased nuclear localization of the transcription factor FoxO1 and increased expression of FoxO1-dependent genes that promote autophagy, mitochondrial function, and resistance to oxidative stress. Mitochondrial function improved and the number of autophagosomes increased in R6/2•Irs2+/-•Irs2βtg mice, whereas aggregate formation and oxidative stress decreased. Thus, our study suggests that Irs2 signaling can modulate HD progression. Since we found the expression of Irs2 to be normal in grade II HD patients, our results suggest that decreasing IRS2 signaling could be part of a therapeutic approach to slow the progression of HD.

Molecular characterization of skeletal muscle atrophy in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD), a neurodegenerative disorder caused by mutant huntingtin, is characterized by a catabolic phenotype. To determine the mechanisms underlying muscle wasting, we examined key signal transduction pathways governing muscle protein metabolism, apoptosis, and autophagy in R6/2 mice, a well-characterized transgenic model of HD. R6/2 mice exhibited increased adiposity, elevated energy expenditure, and decreased body weight and lean mass without altered food intake. Severe skeletal muscle wasting accounted for a majority of the weight loss. Protein synthesis was unexpectedly increased 19% in gastrocnemius muscle, which was associated with overactivation of basal and refeeding-stimulated mammalian target of rapamycin (mTOR) signaling, elevated Akt expression and Ser(473) phosphorylation, and decreased AMPK Thr(172) phosphorylation. Moreover, mRNA abundance of atrogenes muscle ring finger-1 and atrophy F-box, was markedly attenuated during fasting and refeeding, and the urinary excretion of 3-methylhistidine was decreased, arguing against a role for the ubiquitin proteasome-mediated proteolysis in the atrophy. In contrast, mRNA expression of several caspase genes and genes involved in the extrinsic or intrinsic apoptotic pathway, caspase-3/7, -8, and -9 activity, protein abundance of caspase-3 and -9, Fas, and Fadd, and cytochrome c release were elevated. Protein expressions of LC3B-I and -II, beclin-I, and atg5 and -7 in muscle were upregulated. Thus, mutant huntingtin in skeletal muscle results in increased protein synthesis and mTOR signaling, which is countered by activation of the apoptotic and autophagic pathways, contributing to an overall catabolic phenotype and the severe muscle wasting.

Circadian Abnormalities in Motor Activity in a BAC Transgenic Mouse Model of Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disease marked by psychiatric and motor problems. Recently, these findings have been extended to deficits in sleep and circadian function that can be observed in HD patients and in HD mouse models, with abnormal sleep patterns correlating with symptom severity in patients. Here, we studied the behavior of the BAC HD mouse model using an 24/7 automated system; the results indicate significant lengthening of the circadian period in the mutant mice. These results reinforce previous findings in HD models and symptomatic HD patients, indicating that circadian dysfunction is a core feature of HD.

Body composition in premanifest Huntington's disease reveals lower bone density compared to controls

Huntington’s disease (HD) is a debilitating autosomal dominant, neurodegenerative disease with a fatal prognosis. Classical features include motor disturbances, dementia and psychiatric problems but are not restricted to this triad as patients often experience other abnormalities such as unintended weight loss, the exact cause of which is unknown.

We studied the body composition of 25 premanifest HD and compared it to 25 control subjects using a dual energy x-ray absorptiometer (DEXA) scan. Like the R6/2 transgenic mouse model, we identified significantly lower bone mineral density z-scores in premanifest individuals, that was not related to any difference in testosterone, cortisol, leptin or Vitamin D levels.

These results identify an early gene-related change that occurs in HD which not only could lead to a potential biomarker for the disease, but given it is also seen in other manifest neurodegenerative diseases, could also reveal a common disease related process.

Energy deficit in Huntington disease: why it matters

Huntington disease (HD) is an autosomal dominant neurodegenerative disease with complete penetrance. Although the understanding of the cellular mechanisms that drive neurodegeneration in HD and account for the characteristic pattern of neuronal vulnerability is incomplete, defects in energy metabolism, particularly mitochondrial function, represent a common thread in studies of HD pathogenesis in humans and animal models. Here we review the clinical, biochemical, and molecular evidence of an energy deficit in HD and discuss the mechanisms underlying mitochondrial and related alterations.

Changes in key hypothalamic neuropeptide populations in Huntington disease revealed by neuropathological analyses

Huntington disease (HD) is a fatal neurodegenerative disorder caused by expansion of a CAG repeat in the HD gene. Degeneration concentrating in the basal ganglia has been thought to account for the characteristic psychiatric symptoms, cognitive decline and motor dysfunction. However, the homeostatic control of emotions and metabolism are disturbed early in HD, and focused studies have identified a loss of orexin (hypocretin) neurons in the lateral hypothalamus in HD patients. There has been limited assessment of other hypothalamic cell populations that may be involved. In this study, we quantified the neuropeptide-expressing hypothalamic neurons known to regulate metabolism and emotion in patients with HD compared to healthy controls using unbiased stereological methods. We confirmed the loss of orexin-expressing neurons in HD and revealed substantial differences in the peptide expression of other neuronal populations in the same patients. Both oxytocin- and vasopressin-expressing neurons were decreased by 45 and 24%, respectively, while the number of cocaine- and amphetamine-regulated transcript (CART)-expressing neurons was increased by 30%. The increased expression of CART in the hypothalamus is consistent with a previous study showing increased CART levels in cerebrospinal fluid from HD patients. There was no difference in the numbers of neuropeptide Y-expressing neurons. These results show significant and specific alterations in the peptide expression of hypothalamic neurons known to regulate metabolism and emotion. They may be important in the development of psychiatric symptoms and metabolic disturbances in HD, and may provide potential targets for therapeutic interventions.

Leptin secretion rate increases with higher CAG repeat number in Huntington's disease patients

BACKGROUND

Huntington's disease (HD) is a hereditary neurodegenerative disorder caused by an increased number of CAG repeats in the huntingtin gene. A hallmark of HD is unintended weight loss, the cause of which is unknown.

OBJECTIVE

To perform a detailed analysis of adipose tissue function in HD patients as abnormal fat tissue function could contribute to the weight loss.

DESIGN, SETTING AND PARTICIPANTS

In a clinical research laboratory, 24-h plasma concentrations of leptin, adiponectin and resistin were studied in nine early-stage, medication-free HD patients and nine age-, gender- and body mass index (BMI)-matched controls.

MEASUREMENTS

Leptin was measured every 20 min whereas adiponectin and resistin were measured hourly. Autodeconvolution and cosinor regression were applied to quantify secretion characteristics of leptin and diurnal variations in leptin, adiponectin and resistin levels.

RESULTS

Plasma levels and diurnal rhythmicity of leptin, adiponectin and resistin were not significantly different between HD patients and controls. However, although leptin production increased with higher BMI and fat mass in controls, no such relation was present in HD patients. Moreover, when corrected for fat mass, mean plasma leptin concentration as well as basal, pulsatile and total secretion rates increased with the size of the CAG repeat mutation (r = +0.72 to r = +0.80; all P < 0.05). Both higher pulsatile leptin secretion and higher mean adiponectin levels were associated with a greater degree of motor and functional impairment in HD patients.

CONCLUSIONS

CAG-repeat size-dependent interference of the HD mutation with adipose tissue function may contribute to weight loss in HD patients.

Impairment of PGC-1alpha expression, neuropathology and hepatic steatosis in a transgenic mouse model of Huntington's disease following chronic energy deprivation

We investigated the ability of AMP-activated protein kinase (AMPK) to activate PPARgamma coactivator-1alpha (PGC-1alpha) in the brain, liver and brown adipose tissue (BAT) of the NLS-N171-82Q transgenic mouse model of Huntington's disease (HD). In the striatum of the HD mice, the baseline levels of PGC-1alpha, NRF1, NRF2, Tfam, COX-II, PPARdelta, CREB and ERRalpha mRNA and mitochondrial DNA (mtDNA), were significantly reduced. Administration of the creatine analog beta guanidinopropionic acid (GPA) reduced ATP and PCr levels and increased AMPK mRNA in both the cerebral cortex and striatum. Treatment with GPA significantly increased expression of PGC-1alpha, NRF1, Tfam and downstream genes in the striatum and cerebral cortex of wild-type (WT) mice, but there was no effect on these genes in the HD mice. The striatum of the untreated HD mice showed microvacuolation in the neuropil, as well as gliosis and huntingtin aggregates, which were exacerbated by treatment with GPA. GPA treatment produced a significant increase in mtDNA in the cerebral cortex and striatum of WT mice, but not in HD mice. The HD mice treated with GPA had impaired activation of liver PGC-1alpha and developed hepatic steatosis with accumulation of lipids, degeneration of hepatocytes and impaired activation of gluconeogenesis. The BAT in the HD mice showed vacuolation due to accumulation of neutral lipids, and age-dependent impairment of UCP-1 activation and temperature regulation. Impaired activation of PGC-1alpha, therefore, plays an important role in the behavioral phenotype, metabolic disturbances and pathology of HD, which suggests the possibility that agents that enhance PGC-1alpha function will exert therapeutic benefits in HD patients.

Dietary anaplerotic therapy improves peripheral tissue energy metabolism in patients with Huntington's disease

We previously identified a systemic metabolic defect associated with early weight loss in patients with Huntington's disease (HD), suggesting a lack of substrates for the Krebs cycle. Dietary anaplerotic therapy with triheptanoin is used in clinical trials to promote energy production in patients with peripheral and brain Krebs cycle deficit, as its metabolites - C5 ketone bodies - cross the blood-brain barrier. We conducted a short-term clinical trial in six HD patients (UHDRS (Unified Huntington Disease Rating Scale)=33+/-13, 15-49) to monitor the tolerability of triheptanoin. We also assessed peripheral markers of short-term efficacy that were shown to be altered in the early stages of HD, that is, low serum IGF1 and (31)P-NMR spectroscopy (NMRS) in muscle. At baseline, (31)P-NMRS displayed two patients with end-exercise muscle acidosis despite a low work output. On day 2, the introduction of triheptanoin was well tolerated in all patients, and in particular, there was no evidence of mitochondrial overload from triheptanoin-derived metabolites. After 4 days of triheptanoin-enriched diet, muscle pH regulation was normalized in the two patients with pretreatment metabolic abnormalities. A significant increase in serum IGF1 was also observed in all patients (205+/-60 ng/ml versus 246+/-68 ng/ml, P=0.010). This study provides a rationale for extending our anaplerotic approach with triheptanoin in HD.

Formation of polyglutamine inclusions in a wide range of non-CNS tissues in the HdhQ150 knock-in mouse model of Huntington's disease

Background

Huntington's disease (HD) is an inherited progressive neurodegenerative disorder caused by a CAG repeat expansion in the ubiquitously expressed HD gene resulting in an abnormally long polyglutamine repeat in the huntingtin protein. Polyglutamine inclusions are a hallmark of the neuropathology of HD. We have previously shown that inclusion pathology is also present in the peripheral tissues of the R6/2 mouse model of HD which expresses a small N-terminal fragment of mutant huntingtin. To determine whether this peripheral pathology is a consequence of the aberrant expression of this N-terminal fragment, we extend this analysis to the genetically precise knock-in mouse model of HD, HdhQ150, which expresses mutant mouse huntingtin.

Methodology/Principal Findings

We have previously standardized the CAG repeat size and strain background of the R6/2 and HdhQ150 knock-in mouse models and found that they develop a comparable and widespread neuropathology. To determine whether HdhQ150 knock-in mice also develop peripheral inclusion pathology, homozygous Hdh^Q150/Q150 mice were perfusion fixed at 22 months of age, and tissues were processed for histology and immunohistochemistry with the anti-huntingtin antibody S830. The peripheral inclusion pathology was almost identical to that found in R6/2 mice at 12 weeks of age with minor differences in inclusion abundance.

Conclusions/Significance

The highly comparable peripheral inclusion pathology that is present in both the R6/2 and HdhQ150 knock-in models of HD indicates that the presence of peripheral inclusions in R6/2 mice is not a consequence of the aberrant expression of an N-terminal huntingtin protein. It remains to be determined whether peripheral inclusions are a pathological feature of the human disease. Both mouse models carry CAG repeats that cause childhood disease in humans, and therefore, inclusion pathology may be a feature of the childhood rather than the adult forms of HD. It is important to establish the extent to which peripheral pathology causes the peripheral symptoms of HD from the perspective of a mechanistic understanding and future treatment options.

Biomarkers in Huntington's and Parkinson's Disease

Parkinson's (PD) and Huntington's disease (HD) are chronic neurodegenerative conditions of the brain with a variety of clinical presentations including a disorder of movement and a range of nonmotor deficits. HD is genetic in origin and the causative gene and protein known, namely mutant Huntingtin, which leads to widespread early neuronal dysfunction and death throughout the brain. In contrast, the etiology of sporadic PD is unknown, and the pathology targets the nigrostriatal dopaminergic neurons with the formation of alpha-synuclein positive Lewy bodies. In both diseases, the ability to accurately diagnose the disease in the early stages and monitor progression over time remains a major challenge given the majority of the pathology is sited deep within the CNS. This challenge has gained extra significance as the development of disease-modifying drugs starts to emerge into the clinic. To this end, there is a need to find biomarkers that will help in the accurate diagnosis of the disease and/or prediction of its clinical onset as well as biomarkers that are able to faithfully track disease progression independent of any symptomatic effects of any therapies. In addition, these same markers may also help stratify each of these heterogeneous disorders into specific subtypes that share particular clinical and pathological characteristics.

Emerging drug therapies in Huntington's disease

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

Impaired PGC-1alpha function in muscle in Huntington's disease

We investigated the role of PPAR gamma coactivator 1alpha (PGC-1alpha) in muscle dysfunction in Huntington's disease (HD). We observed reduced PGC-1alpha and target genes expression in muscle of HD transgenic mice. We produced chronic energy deprivation in HD mice by administering the catabolic stressor beta-guanidinopropionic acid (GPA), a creatine analogue that reduces ATP levels, activates AMP-activated protein kinase (AMPK), which in turn activates PGC-1alpha. Treatment with GPA resulted in increased expression of AMPK, PGC-1alpha target genes, genes for oxidative phosphorylation, electron transport chain and mitochondrial biogenesis, increased oxidative muscle fibers, numbers of mitochondria and motor performance in wild-type, but not in HD mice. In muscle biopsies from HD patients, there was decreased PGC-1alpha, PGC-1beta and oxidative fibers. Oxygen consumption, PGC-1alpha, NRF1 and response to GPA were significantly reduced in myoblasts from HD patients. Knockdown of mutant huntingtin resulted in increased PGC-1alpha expression in HD myoblast. Lastly, adenoviral-mediated delivery of PGC-1alpha resulted increased expression of PGC-1alpha and markers for oxidative muscle fibers and reversal of blunted response for GPA in HD mice. These findings show that impaired function of PGC-1alpha plays a critical role in muscle dysfunction in HD, and that treatment with agents to enhance PGC-1alpha function could exert therapeutic benefits. Furthermore, muscle may provide a readily accessible tissue in which to monitor therapeutic interventions.

Huntington's disease: the current state of research with peripheral tissues

Huntington's disease (HD) is a genetically dominant condition caused by expanded CAG repeats. These repeats code for a glutamine tract in the HD gene product huntingtin (htt), which is a protein expressed in almost all tissues. Although most HD symptoms reflect preferential neuronal death in specific brain regions, even before the HD gene was identified numerous reports had described additional abnormalities in the peripheral tissues of HD patients, including weight loss, altered glucose homeostasis, and sub-cellular abnormalities in fibroblasts, lymphocytes and erythrocytes. Several years have elapsed since the HD mutation was discovered, and analyses of peripheral tissues from HD patients have helped to understand the molecular pathogenesis of the disease and revealed that the molecular mechanisms through which mutated htt leads to cell dysfunction are widely shared between central nervous system (CNS) and peripheral tissues. These studies show that in peripheral tissues, mutated htt causes accumulation of intracellular protein aggregates, impairment of energetic metabolism, transcriptional deregulation and hyperactivation of programmed cell-death mechanisms. Here, we review the current knowledge of peripheral tissue alterations in HD patients and in animal models of HD and focus on how this information can be used to identify potential therapeutic possibilities and biomarkers for disease progression.

Adipose tissue dysfunction tracks disease progression in two Huntington's disease mouse models

In addition to the hallmark neurological manifestations of Huntington's disease (HD), weight loss with metabolic dysfunction is often observed in the later stages of disease progression and is associated with poor prognosis. The mechanism for weight loss in HD is unknown. Using two mouse models of HD, the R6/2 transgenic and CAG140 knock-in mouse strains, we demonstrate that adipose tissue dysfunction is detectable at early ages and becomes more pronounced as the disease progresses. Adipocytes acquire a 'de-differentiated' phenotype characterized by impaired expression of fat storage genes. In addition, HD mice exhibit reduced levels of leptin and adiponectin, adipose tissue-derived hormones that regulate food intake and glucose metabolism. Importantly, some of these changes occur prior to weight loss and development of some of the characteristic neurological symptoms. We demonstrate that impaired gene expression and lipid accumulation in adipocytes can be recapitulated by expression of an inducible mutant huntingtin transgene in an adipocyte cell line and that mutant huntingtin inhibits transcriptional activity of the PGC-1alpha co-activator in adipocytes, which may contribute to aberrant gene expression. Thus, our findings indicate that mutant huntingtin has direct detrimental effects in cell types other than neurons. The results also indicate that circulating adipose-tissue-derived hormones may be accessible markers for HD prognosis and progression and suggest that adipose tissue may be a useful therapeutic target to improve standard of life for HD patients.

Hypothalamic involvement in Huntington's disease: an in vivo PET study

Recent studies have shown alterations in metabolism, sleep and circadian rhythms as well as in several neuropeptides derived from the hypothalamic-pituitary axis in Huntington's disease patients; however, the pathology underlying these abnormalities is not known. Our aim was to assess in vivo D(2) receptor's loss/dysfunction and increases in microglial activation in the hypothalamus of symptomatic Huntington's disease patients and premanifest Huntington's disease gene carriers using PET with (11)C-raclopride (RAC), a specific D(2) receptor ligand and (11)C-(R)-PK11195 (PK), a marker of microglial activation. We have studied 9 symptomatic Huntington's disease patients (age = 46.8 +/- 4.7 years; mean +/- SD) and 10 premanifest Huntington's disease gene carriers (age = 41.9 +/- 8.2 years; mean +/- SD). RAC and PK findings for these subjects were compared with those of a group of normal controls (RAC, n = 9; PK, n = 10). In the symptomatic Huntington's disease group, we found a significant decrease (P = 0.0012) in mean hypothalamic RAC binding potential (BP) and a significant increase in mean hypothalamic PK BP (P = 0.0008). Similarly, a significant decrease (P = 0.0143) in mean hypothalamic RAC BP and a significant increase in mean hypothalamic PK BP (P = 0.0057) were observed in the premanifest Huntington's disease group. Hypothalamic RAC and PK BP values correlated with each other in combined Huntington's disease groups (r = -0.6180, P = 0.0048) but not with striatal RAC and PK BP values. Our data demonstrate, for the first time, significant D(2) receptor loss and microglia activation in the hypothalamus of Huntington's disease. These pathological changes occur very early in the course of the disease and may partly explain the development of commonly reported symptoms in Huntington's disease including progressive weight loss, alterations in sexual behaviour and disturbances in the wake-sleep cycle.

Voxel-based morphometry in the R6/2 transgenic mouse reveals differences between genotypes not seen with manual 2D morphometry

The R6/2 mouse is the most common mouse model used for Huntington's disease (HD), a fatal, inherited neurodegenerative CAG disorder characterized by marked brain atrophy. We scanned 47 R6/2 transgenic and 42 wildtype (WT) ex vivo mouse brains at 18 weeks of age using high resolution, three-dimensional magnetic resonance imaging (MRI) for automated voxel-based morphometry (VBM) analysis. We found differences between genotypes in specific brain structures. Many of these changes were bilateral and were found in regions known to be involved in the behavioral deficits present in both R6/2 mice and HD patients. In particular, changes were evident in the basal ganglia, hippocampus, cortex and hypothalamus. In the striatum, changes were heterogenous and reminiscent of striosomal distribution. Changes were also seen in the cerebellum, as might be expected in a mouse carrying a repeat length typical of juvenile onset HD. Many of these changes were not detected by manual 2D morphometry from the same MR images. These data indicate that VBM will be a valuable technique for in vivo measurement of developing pathology in HD transgenic mice, and may be particularly useful for correlating histologically undetectable changes with behavioral deficits.