Role of brain-derived neurotrophic factor in Huntington's disease

Neurotrophic factors are essential contributors to the survival of peripheral and central nervous system (CNS) neurons, and demonstration of their reduced availability in diseased brains indicates that they play a role in various neurological disorders. This paper will concentrate on the role of brain-derived neurotrophic factor (BDNF) in the survival and activity of the neurons that die in Huntington's disease (HD) by reviewing the evidence indicating that it involves profound changes in BDNF levels and that attempts to restore these levels are therapeutically interesting.

BDNF is a small dimeric protein that is widely expressed in adult mammalian brain and has been shown to promote the survival of all major neuronal types affected in Alzheimer's disease (AD) and Parkinson's disease (PD). Furthermore, cortical BDNF production is required for the correct activity of the corticostriatal synapse and the survival of the GABA-ergic medium-sized spiny striatal neurons that die in HD. We will highlight the available data concerning changes in BDNF levels in HD cells, mice and human postmortem samples, describe the molecular evidence underlying this alteration, and review the data concerning the impact of the experimental manipulation of BDNF levels on HD progression. Such studies have revealed a major loss of BDNF protein in the striatum of HD patients which may contribute to the clinical manifestations of the disease. They have also opened up a molecular window into the underlying pathogenic mechanism and new therapeutic perspectives by raising the possibility that one of the mechanisms triggering the reduction in BDNF in HD may also affect the activity of many other neuronal proteins.     

Optineurin in Huntington's disease intranuclear inclusions

Optineurin mutations cause adult-onset primary open-angle glaucoma and have been associated with some familial forms of amyotrophic lateral sclerosis (ALS). Optineurin is involved in many cellular processes and interacts with a variety of proteins, among them huntingtin (htt). Here we report that in Huntington's disease (HD) cortex, optineurin frequently occurs in neuronal intranuclear inclusions, and to a lesser extent, in inclusions in the neuropil and in perikarya. Most intranuclear optineurin-positive inclusions were co-labeled for ubiquitin, but they were only occasionally and more weakly co-labeled for htt. Optineurin-labeled neuropil and perikaryal inclusions were commonly co-labeled for ubiquitin and htt. Although these inclusions were common in cortex, they were rare in striatum. Our results show that in HD optineurin is present in intranuclear, neuropil and perikaryal inclusions. It is not clear whether this indicates a primary involvement in the disease process. In HD, the known interaction of htt and optineurin may suggest that a different process takes place as compared to other neurodegenerative disorders.     

Striatal ganglioside levels in the rat following kainic acid lesions: Comparison with Huntington's disease

Summary Ganglioside and DNA levels were estimated in the striatum of rats 10 days and 6 weeks after lesioning by intrastriatal injection of kainic acid. There was a moderate, 21–24% decrease of the ganglioside concentration per unit protein on the side of the lesion, which can be ascribed to the loss of the intrinsic striatal neurons following the injection of kainic acid. On the other hand, there was a 131 and 60% increase of DNA per unit protein in the kainate injected side 10 days and 6 weeks after the lesions, respectively; these changes apparently reflected the gliotic reaction brought about by the neurotoxin. Qualitatively similar findings — a decrease of ganglioside and an increase of DNA levels per unit protein — were also found in the brain of patients with Huntington's disease; however, as compared with the corresponding control material, the decrease of the ganglioside concentration was more pronounced in the striatum of Huntington's disease (by 38% in the caudate nucleus and by 46% in the putamen) than in the kainate lesioned rat striatum. This difference could be due to the different proportions of the intrinsic and extrinsic neuronal plasma membranes in the striatum of the two species; however, the possibility of a more generalized affection of neuronal plasma membranes in Huntington's disease may also be envisaged.The work was supported by Austrian Science Research Fund, Project No S-25     

Sensitivity to neurotoxic stress is not increased in progranulin-deficient mice

Loss-of-function mutations in the progranulin (GRN) gene are a common cause of autosomal dominant frontotemporal lobar degeneration, a fatal and progressive neurodegenerative disorder common in people less than 65 years of age. In the brain, progranulin is expressed in multiple regions at varying levels, and has been hypothesized to play a neuroprotective or neurotrophic role. Four neurotoxic agents were injected in vivo into constitutive progranulin knockout (Grn^−/−) mice and their wild-type (Grn^+/+) counterparts to assess neuronal sensitivity to toxic stress. Administration of 3-nitropropionic acid, quinolinic acid, kainic acid, and pilocarpine induced robust and measurable neuronal cell death in affected brain regions, but no differential cell death was observed between Grn^+/+ and Grn^−/− mice. Thus, constitutive progranulin knockout mice do not have increased sensitivity to neuronal cell death induced by the acute chemical models of neuronal injury used in this study.     

Galantamine reduces striatal degeneration in 3-nitropropionic acid model of Huntington's disease

The acetylcholinesterase inhibitor (AChEI) galantamine is currently used to treat mild to moderate Alzheimer's disease (AD), and it has been suggested to have several neuroprotective effects. To investigate the potential application of this drug to the treatment of Huntington's disease, we examined whether galantamine can reduce the striatal degeneration induced by the mitochondrial toxin, 3-nitropropionic acid (3NP). 3NP (63 mg/kg/day) was delivered to Lewis rats by osmotic pumps for 5 consecutive days, and the rats received intraperitoneal administration of either different concentrations of galantamine (1mg/kg/day or 10 mg/kg/day, twice daily) or vehicle (saline) throughout the experiment. Galantamine attenuated the 3NP-induced neurologic deficits on days 2-5. Galantamine-treated rats showed smaller striatal lesion volumes measured by Nissl staining and lower numbers of TUNEL(+) apoptotic cells when compared to the vehicle-treated rats. Galantamine failed to reduce the striatal lesion volume when co-administered with mecamylamine, a nicotinic acetylcholine receptor antagonist. Our data indicate that galantamine can attenuate neurodegeneration in a Huntington's disease model by modulating nAChR.     

Huntingtin fragmentation and increased caspase 3, 8 and 9 activities in lymphoblasts with heterozygous and homozygous Huntington's disease mutation

Huntington's disease (HD) is caused by mutated huntingtin (htt), a toxic protein ubiquitously expressed in nervous and non-nervous system tissues. Fragmentation of htt by caspases and further accumulation in cells of protein aggregates contribute to cell dysfunction and death. In the attempt to elucidate whether this mechanism depends on patients' genotype, we analysed the pattern of htt fragmentation, the caspase 3, 8 and 9 activities and their variation in lymphoblasts with heterozygous and homozygous CAG mutation and in controls. Cells homozygous for expanded mutation showed greater amount of mutated fragments than heterozygotes and controls, caspase 3, 8 and 9 activities greater in mutated than control cell lines, after cyanide treatment, the caspase 3 and 8 particularly increased in homozygotes. This data offers a biological explanation to the clinical in-patients evidence of mutation homozygosity associated with more severe phenotype.     

Aspects of PET imaging relevant to the assessment of striatal transplantation in Huntington's disease

Proper assessment of outcome in clinical trials of neural transplantation requires both biochemical and imaging indices of graft survival, and behavioural and physiological indices of graft function. For transplantation in Huntington's disease, a variety of ligands that are selective for striatal degeneration and graft-derived replacement are available, notably ligands of dopaminergic receptors on striatal neurons. However, the validity of such ligands is potentially compromised by adjunctive drug therapies (e.g. neuroleptics) given to patients in the course of normal clinical care. We review the present state of experimental and clinical understanding of the selectivity of available ligands for striatal imaging, their interaction with other drug treatments, and strategies for refining valid assessment protocols in patients.     

Metformin therapy in a transgenic mouse model of Huntington's disease

Huntington's disease (HD) is a hereditary neurodegenerative disease that leads to striatal degeneration and a severe movement disorder. We used a transgenic mouse model of HD (the R6/2 line with approximately 150 glutamine repeats) to test a new therapy for this disease. We treated HD mice with metformin, a widely used anti-diabetes drug, in the drinking water (0, 2 or 5mg/ml) starting at 5 weeks of age. Metformin treatment significantly prolonged the survival time of male HD mice at the 2mg/ml dose (20.1% increase in lifespan) without affecting fasting blood glucose levels. This dose of metformin also decreased hind limb clasping time in 11-week-old mice. The higher dose did not prolong survival, and neither dose of metformin was effective in female HD mice. Collectively, our results suggest that metformin may be worth further investigation in additional HD models.     

Effects of simvastatin on neuroprotection and modulation of Bcl-2 and BAX in the rat quinolinic acid model of Huntington's disease

A possible neuroprotective role has been recently suggested for 3H3MGCoA reductase inhibitors. Here, we sought to determine whether simvastatin exerts a neuroprotective effect in our rat model of HD. Rats were surgically administered quinolinic acid and treated with simvastatin 1mg/kg intraperitoneally (i.p.) once daily up to 2 or 8 weeks. Two more groups of animals received a pretreatment with 1mg/kg simvastatin i.p. for 2 weeks before the QA lesion and then were treated with simvastatin for the following 2 weeks or 8 weeks, respectively. In the simvastatin treated groups (both pretreated and non-pretreated), striatal lesion size was about 36% smaller while neuronal counts where higher than in the vehicle treated ones at 2 weeks. The neuroprotective effects of simvastatin was still evident at 8 weeks post lesion, where the non-pretreated group had a 8% smaller lesion size than the saline group, and the pretreated group had an 11% smaller lesion size than the saline group. Simvastatin also induced immunoreactivity for Bcl-2, an anti-apoptotic factor, on one hand, and down-regulated immunoreactivity for Bax, a proapoptotic factor. Bcl-2/Bax modulation can account, at least partly, for the beneficial effect of simvastatin in our rodent model of striatal degeneration. Our findings show that statins could be explored as possible neuroprotective agents for neurodegenerative disorders such as HD.     

IGF-1 exacerbates the neurotoxicity of the mitochondrial inhibitor 3NP in rats

Insulin-like Growth Factor 1 (IGF-1) has broad-range neuroprotective effects and is a therapeutic candidate for Huntington's disease (HD). IGF-1 protects striatal neurons from the toxicity of mutated huntingtin in vitro and improves neuronal survival in vivo in a phenotypic model of HD involving excitotoxic cell death. Because HD is a multifactorial disease, it is important to evaluate the neuroprotective role of IGF-1 in other pathological situations involved in HD progression. We have evaluated the neuroprotective effects of IGF-1 in vivo, using the 3-nitropropionic acid (3NP) rat model which replicates the mitochondrial dysfunction observed in HD. Continuous intracerebroventricular infusion of recombinant IGF-1 at a low dose (0.025 microg/h for 5 days) did not alleviate motor impairment and weight loss induced by 3NP treatment. In addition, histological evaluation and quantification of DNA fragmentation evidenced no improvement in neuronal survival. Of interest, we found that a higher concentration of IGF-1 (0.25 microg/h) resulted in an exacerbation of 3NP toxicity on striatal neurons. These results suggest that intracerebral delivery of IGF-1 may not provide a fully effective therapeutic strategy for HD or other disorders involving mitochondrial impairment.     

Analysis of the (CAG)n repeat causing Huntington's disease in a Mexican population

We investigated the allele distribution of the polymorphic (CAG)n repeat in the IT15 gene in 96 normal subjects from the Mexican population and 83 unrelated patients with Huntington's disease. Our results show that the size distributions of normal and affected alleles do not overlap. Normal alleles range from 13 to 32 triplets, with 18 being the most frequent allele, while HD alleles contain 37 to 76 repeats with 42 being the most frequent. One allele in the range of intermediate alleles was found (32 repeats) in a normal subject. The juvenile onset cases in this study are associated with an expansion greater than 49 repeats. In the available parent-offspring pairs, paternal alleles show instability with an expansion of 28 repeats in one case.     

Meclizine is neuroprotective in models of Huntington's disease

Defects in cellular energy metabolism represent an early feature in a variety of human neurodegenerative diseases. Recent studies have shown that targeting energy metabolism can protect against neuronal cell death in such diseases. Here, we show that meclizine, a clinically used drug that we have recently shown to silence oxidative metabolism, suppresses apoptotic cell death in a murine cellular model of polyglutamine (polyQ) toxicity. We further show that this protective effect extends to neuronal dystrophy and cell death in Caenorhabditis elegans and Drosophila melanogaster models of polyQ toxicity. Meclizine's mechanism of action is not attributable to its anti-histaminergic or anti-muscarinic activity, but rather, strongly correlates with its ability to suppress mitochondrial respiration. Since meclizine is an approved drug that crosses the blood-brain barrier, it may hold therapeutic potential in the treatment of polyQ toxicity disorders, such as Huntington's disease.     

Homozygosity in Huntington's disease: new ethical dilemma caused by molecular diagnosis

Huntington's disease (HD) is a degenerative disorder of the central nervous system with autosomal dominant inheritance. Genetic counseling has always been difficult in this disorder with anguish, depression and denial being very common in both the patient and family members. The discovery of the causal gene has led to precise diagnostic procedures allowing homozygotes for the disease to be identified. Contrary to what occurs in some other autosomal dominant diseases, the course of the disease is not more severe in the homozygote than in the heterozygote. The present authors describe a family comparing two affected siblings: one is heterozygotic and the other homozygous for the HD mutation. They confirm that the age and symptoms of onset did not differ significantly between the subjects; however, the disease seemed to have a more severe progression in the heterozygote than in the homozygote. The authors discuss the ethical dilemma derived from the genetic counseling of a homozygotic patient, given the fact that all his offspring will be affected. Letting the offspring know about their 100% probability of inheriting the disorder is equivalent to delivering a non-requested predictive test, while not informing them constitutes withholding crucial information from the individual.     

The endocannabinoid pathway in Huntington's disease: a comparison with other neurodegenerative diseases

Endocannabinoids are endogenous agonists of cannabinoid receptors, and comprise amides, esters and ethers of long chain polyunsaturated fatty acids. Anandamide (N-arachidonoylethanolamine) and 2-arachidonoylglycerol are the best-studied members of this class of lipid mediators, and it is now widely accepted that their in vivo concentration and biological activity are largely dependent on a “metabolic control.” Therefore, the proteins that synthesize, transport and degrade endocannabinoids, and that together with the target receptors form the so-called “endocannabinoid system,” are the focus of intense research. This new system will be presented in this review, in order to put in a better perspective the impact of its modulation on Huntington's disease. In particular, the effect of agonists/antagonists of endocannabinoid receptors, or of inhibitors of endocannabinoid metabolism, will be discussed in the context of onset and progression of Huntington's disease, and will be compared with other neurodegenerative diseases like Parkinson's disease, Alzheimer's disease, and amyotropic lateral sclerosis. Also the plastic changes of endocannabinoids in multiple sclerosis will be reviewed, as a paradigm of their impact in neuroinflammatory disorders.     

N-Methyl-d-aspartate (NMDA) receptor function and excitotoxicity in Huntington's disease

Many lines of evidence support a role for neuronal damage arising as a result of excessive activation of glutamate receptors by excitatory amino acids in the pathogenesis of Huntington disease. The N-methyl-d-aspartate subclass of ionotropic glutamate receptors (NMDARs) is more selective and effective than the other subclasses in mediating this damage. As well, neurons expressing high levels of NMDARs are lost early from the striatum of individuals affected with Huntington's disease (HD), and injection of NMDAR agonists into the striatum of rodents or non-human primates recapitulates the pattern of neuronal damage observed in HD. Altered NMDAR function has been reported in corticostriatal synapses in one mouse model of HD, and NMDAR-mediated current and/or toxicity have been found to be potentiated in striatal neurons from several HD mouse models as well as heterologous cells expressing the mutant huntingtin protein. Changes in NMDAR activity have been correlated with altered calcium homeostasis, mitochondrial membrane depolarization and caspase activation. NMDAR stimulation is also closely linked to mitochondrial activity, as treatment with mitochondrial toxins has been demonstrated to produce striatal damage that can be reversed by the addition of NMDAR antagonists. Recent efforts have focused on the elucidation of molecular pathways linking huntingtin to NMDARs, as well as the mechanisms which underlie the enhancement of NMDAR activity by mutant huntingtin. Here, we review the literature to date and recent findings concerning the role of NMDARs in HD pathogenesis.     

Predictive testing for Huntington's disease: I. Predictors of uptake in South Wales

In Wales, predictive testing for Huntington's disease (HD) has not been offered proactively to families and uptake of testing is low in comparison to other centres. Little is known of those not requesting testing, particularly those not in direct contact with the genetics service. This study examined differences between a cohort of 22 test applicants and a random group of 32 'non-requesters', drawn from the South Wales HD register. Respondents were interviewed by means of a semi-structured schedule in their own homes. The study groups differed significantly on a number of variables including: knowledge of the availability of testing; perceived attitudes of family members and significant others to testing; length of knowledge and perceived stressfulness of being at risk; and perceived ability to cope with an unfavourable result. Overall, knowledge of testing procedures was poor and at-risk individuals' understanding of genetic terminology was at odds with scientific distinctions. Discussion focuses on the organisational and psychological factors associated with lack of knowledge of the availability of testing and the interpretation of reported coping capacities.     

Exclusion mapping of the benign hereditary chorea gene from the Huntington's disease locus: report of a family

A Greek family is presented in which seven members suffered from benign hereditary chorea (a rare autosomal dominant non-progressive chorea without dementia). All patients and three informative healthy relatives were submitted to DNA analysis using probes from loci linked to Huntington's disease. The results confirm one previous suggestion that these two disorders are not allelic and that they should be considered as two distinct genetic entities.     

Neuroprotective effect of neural stem cell-conditioned media in in vitro model of Huntington's disease

Although neural stem cell (NSC) transplantation has been investigated as a promising tool for reconstituting damaged brains, recent evidences suggest that NSCs may rescue the brain via paracrine effects rather than by direct cell replacements. In this study, we attempted to determine the neuroprotective effect of NSC-conditioned media (NSC-CM) in in vitro model of Huntington's disease. Cerebral hybrid neurons (A1) were transfected with either wild-type huntingtin (18 CAG repeats) or mutant huntingtin (100 CAG repeats). At 24h after the transfection, immunocytochemical patterns of the huntingtin aggregations, as well as the level of N-terminal proteolytic cleavages of huntingtin were analyzed. Neuronal apoptosis was evaluated with flowcytometry after Annexin-V and propidium iodide (PI) staining. Cerebral hybrid neurons transfected with mutant huntingtin showed five aggregates patterns, including diffuse cytoplasmic, dispered vacuoles, perinuclear vacuoles, nuclear inclusions (NI), and cytoplasmic inclusions (CI). NSC-CM reduced the levels of nuclear and cytoplasmic inclusions. The transfection with mutant huntingtin increased the level of N-terminal cleavages, which was reduced by the NSC-CM treatment. In addition, NSC-CM reduced the Annexin-V(+)PI(+) and Annexin-V(+)PI(-) neurons which were induced by the mutant huntingtin transfection. In summary, NSC-CM was neuroprotective in in vitro model of Huntington's disease with modulating mutant huntingtin-induced cytotoxicity.