Malonate-Induced Generation of Reactive Oxygen Species in Rat Strium Depends on Dopamine Release but Not on NMDA Receptor Activation

Intrastriatal injection of the reversible succinate dehydrogenase inhibitor malonate produces both energy depletion and striatal lesions similar to that seen in cerebral ischemia and Huntington's disease. The mechanisms of neuronal cell death involve secondary excitotoxicity and the generation of reactive oxygen species. Here, we investigated the effects of dopamine on malonate-induced generation of hydroxyl radicals and striatal lesion volumes. Using in vivo microdialysis, we found that malonate induced a 94-fold increase in extracellular striatal dopamine concentrations. This was paralleled by an increase in the generation of hydroxyl radicals. Prior unilateral lesioning of the nigrostriatal dopaminergic pathway by focal injection of 6-hydroxydopamine blocked the malonate-induced increase in dopamine concentrations and the generation of hydroxyl radicals and attenuated the lesion volume. In contrast, the NMDA receptor antagonist MK-801 attenuated malonate-induced lesion volumes but did not block the generation of hydroxyl radicals. Thus, the dopaminergic and glutamatergic pathways are essential in the pathogenesis of malonate-induced striatal lesions. Our results suggest that the malonate-induced release of dopamine but not NMDA receptor activation mediates hydroxyl radical formation.     

Extended therapeutic window for caspase inhibition and synergy with MK-801 in the treatment of cerebral histotoxic hypoxia

In rats, striatal histotoxic hypoxic lesions produced by the mitochondrial toxin malonate resemble those of focal cerebral ischemia. Intrastriatal injections of malonate induced cleavage of caspase-2 beginning at 6 h, and caspase-3-like activity as identified by DEVD biotin affinity-labeling within 12 h. DEVD affinity-labeling was prevented and lesion volume reduced in transgenic mice overexpressing BCL-2 in neuronal cells. Intrastriatal injection of the tripeptide, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD-fmk), a caspase inhibitor, at 3 h, 6 h, or 9 h after malonate injections reduced the lesion volume produced by malonate. A combination of pretreatment with the NMDA antagonist, dizocilpine (MK-801), and delayed treatment with zVAD-fmk provided synergistic protection compared with either treatment alone and extended the therapeutic window for caspase inhibition to 12 h. Treatment with cycloheximide and zVAD-fmk, but not with MK-801, blocked the malonate-induced cleavage of caspase-2. NMDA injections alone resulted in a weak caspase-2 cleavage. These results suggest that malonate toxicity induces neuronal death by more than one pathway. They strongly implicate early excitotoxicity and delayed caspase activation in neuronal loss after focal ischemic lesions and offer a new strategy for the treatment of stroke.     

Effects of CAG repeat length, HTT protein length and protein context on cerebral metabolism measured using magnetic resonance spectroscopy in transgenic mouse models of Huntington's disease

Huntington's disease is a neurodegenerative illness caused by expansion of CAG repeats at the N-terminal end of the protein huntingtin. We examined longitudinal changes in brain metabolite levels using in vivo magnetic resonance spectroscopy in five different mouse models. There was a large (>50%) exponential decrease in N-acetyl aspartate (NAA) with time in both striatum and cortex in mice with 150 CAG repeats (R6/2 strain). There was a linear decrease restricted to striatum in N171-82Q mice with 82 CAG repeats. Both the exponential and linear decreases of NAA were paralleled in time by decreases in neuronal area measured histologically. Yeast artificial chromosome transgenic mice with 72 CAG repeats, but low expression levels, had less striatal NAA loss than the N171-82Q mice (15% vs. 43%). We evaluated the effect of gene context in mice with an approximate 146 CAG repeat on the hypoxanthine phosphoribosyltransferase gene (HPRT). HPRT mice developed an obese phenotype in contrast to weight loss in the R6/2 and N171-82Q mice. These mice showed a small striatal NAA loss (21%), and a possible increase in brain lipids detectable by magnetic resonance (MR) spectroscopy and decreased brain water T1. Our results indicate profound metabolic defects that are strongly affected by CAG repeat length, as well as gene expression levels and protein context.     

Transgenic models of Huntington's disease.

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by a CAG-polyglutamine repeat expansion. A mouse model of this disease has been generated by the introduction of exon 1 of the human HD gene carrying highly expanded CAG repeats into the mouse germ line (R6 lines). Transgenic mice develop a progressive neurological phenotype with a movement disorder and weight loss similar to that in HD. We have previously identified neuronal inclusions in the brains of these mice that have subsequently been established as the pathological hallmark of polyglutamine disease. Inclusions are present before symptoms, which in turn occur long before any selective neuronal cell death can be identified. We have extended the search for inclusions to skeletal muscle, which, like brain, contains terminally differentiated cells. We have conducted an investigation into the skeletal muscle atrophy that occurs in the R6 lines, (i) to provide possible insights into the muscle bulk loss observed in HD patients, and (ii) to conduct a parallel analysis into the consequence of inclusion formation to that being performed in brain. The identification of inclusions in skeletal muscle might be additionally useful in monitoring the ability of drugs to prevent inclusion formation in vivo.     

Malonate and 3-nitropropionic acid neurotoxicity are reduced in transgenic mice expressing a caspase-1 dominant-negative mutant

Increasing evidence implicates caspase-1-mediated cell death as a major mechanism of neuronal death in neurodegenerative diseases. In the present study we investigated the role of caspase-1 in neurotoxic experimental animal models of Huntington's disease (HD) by examining whether transgenic mice expressing a caspase-1 dominant-negative mutant are resistant to malonate and 3-nitropropionic acid (3-NP) neurotoxicity. Intrastriatal injection of malonate resulted in significantly smaller striatal lesions in mutant caspase-1 mice than those observed in littermate control mice. Caspase-1 was significantly activated following malonate intrastriatal administration in control mice but significantly attenuated in mutant caspase-1 mice. Systemic 3-NP treatment induced selective striatal lesions that were significantly smaller within mutant caspase-1 mice than in littermate control mice. These results provide further evidence of a functional role for caspase-1 in both malonate- and 3-NP-mediated neurotoxin models of HD.     

Reduction in Subventricular Zone-Derived Olfactory Bulb Neurogenesis in a Rat Model of Huntington’s Disease Is Accompanied by Striatal Invasion of Neuroblasts

Huntington’s disease (HD) is an inherited progressive neurodegenerative disorder caused by an expanded CAG repeat in exon 1 of the huntingtin gene (HTT). The primary neuropathology of HD has been attributed to the preferential degeneration of medium spiny neurons (MSN) in the striatum. Reports on striatal neurogenesis have been a subject of debate; nevertheless, it should be considered as an endogenous attempt to repair the brain. The subventricular zone (SVZ) might offer a close-by region to supply the degenerated striatum with new cells. Previously, we have demonstrated that R6/2 mice, a widely used preclinical model representing an early onset HD, showed reduced olfactory bulb (OB) neurogenesis but induced striatal migration of neuroblasts without affecting the proliferation of neural progenitor cell (NPCs) in the SVZ. The present study revisits these findings, using a clinically more relevant transgenic rat model of late onset HD (tgHD rats) carrying the human HTT gene with 51 CAG repeats and mimicking many of the neuropathological features of HD seen in patients. We demonstrate that cell proliferation is reduced in the SVZ and OB of tgHD rats compared to WT rats. In the OB of tgHD rats, although cell survival was reduced, the frequency of neuronal differentiation was not altered in the granule cell layer (GCL) compared to the WT rats. However, an increased frequency of dopamenergic neuronal differentiation was noticed in the glomerular layer (GLOM) of tgHD rats. Besides this, we observed a selective proliferation of neuroblasts in the adjacent striatum of tgHD rats. There was no evidence for neuronal maturation and survival of these striatal neuroblasts. Therefore, the functional role of these invading neuroblasts still needs to be determined, but they might offer an endogenous alternative for stem or neuronal cell transplantation strategies.     

Nuclear translocation of AMPK-��1 potentiates striatal neurodegeneration in Huntington��s disease

Adenosine monophosphate–activated protein kinase (AMPK) is a major energy sensor that maintains cellular energy homeostasis. Huntington’s disease (HD) is a neurodegenerative disorder caused by the expansion of CAG repeats in the huntingtin (Htt) gene. In this paper, we report that activation of the α1 isoform of AMPK (AMPK-α1) occurred in striatal neurons of humans and mice with HD. Overactivation of AMPK in the striatum caused brain atrophy, facilitated neuronal loss, and increased formation of Htt aggregates in a transgenic mouse model (R6/2) of HD. Such nuclear accumulation of AMPK-α1 was activity dependent. Prevention of nuclear translocation or inactivation of AMPK-α1 ameliorated cell death and down-regulation of Bcl2 caused by mutant Htt (mHtt). Conversely, enhanced expression of Bcl2 protected striatal cells from the toxicity evoked by mHtt and AMPK overactivation. These data demonstrate that aberrant activation of AMPK-α1 in the nuclei of striatal cells represents a new toxic pathway induced by mHtt.     

Manipulation of Membrane Potential Modulates Malonate-Induced Striatal Excitotoxicity In Vivo

Abstract: Malonate is a reversible inhibitor of succinate dehydrogenase (SDH) that produces neurotoxicity by an N -methyl- d -aspartate (NMDA) receptor-dependent mechanism. We have examined the influence of pharmacological manipulation of membrane potential on striatal malonate toxicity in rats in vivo by analysis of lesion volume. Depolarization caused by coinjection of the Na⁺,K⁺-ATPase inhibitor ouabain or a high concentration of potassium greatly exacerbated malonate toxicity; this combined toxicity was blocked by the noncompetitive NMDA antagonist MK-801. The toxicity of NMDA was also exacerbated by ouabain. The overt toxicity of a high dose of ouabain (1 nmol) was largely prevented by MK-801. Coinjection of the K⁺ channel activator minoxidil (4 nmol) to reduce depolarization attenuated the toxicity of 1 µmol of malonate by ∼60% without affecting malonate-induced ATP depletion. These results indicate that membrane depolarization exacerbates malonate neurotoxicity and that membrane hyperpolarization protects against malonate-induced neuronal damage. We hypothesize that the effects of membrane potential on malonate toxicity are mediated through the NMDA receptor as a result of its combined agonist- and voltage-dependent properties.     

Striatal Damage and Oxidative Stress Induced by the Mitochondrial Toxin Malonate Are Reduced in Clorgyline-Treated Rats and MAO-A Deficient Mice

Intrastriatal administration of the succinate dehydrogenase (SDH) inhibitor malonate produces neuronal injury by a "secondary excitotoxic" mechanism involving the generation of reactive oxygen species (ROS). Recent evidence indicates dopamine may contribute to malonate-induced striatal neurodegeneration; infusion of malonate causes a pronounced increase in extracellular dopamine and dopamine deafferentation attenuates malonate toxicity. Inhibition of the catabolic enzyme monoamine oxidase (MAO) also attenuates striatal lesions induced by malonate. In addition to forming 3,4-dihydroxyphenylacetic acid, metabolism of dopamine by MAO generates H2O2, suggesting that dopamine metabolism may be a source of ROS in malonate toxicity. There are two isoforms of MAO, MAO-A and MAO-B. In this study, we have investigated the role of each isozyme in malonate-induced striatal injury using both pharmacological and genetic approaches. In rats treated with either of the specific MAO-A or -B inhibitors, clorgyline or deprenyl, respectively, malonate lesion volumes were reduced by 30% compared to controls. In knock-out mice lacking the MAO-A isoform, malonate-induced lesions were reduced by 50% and protein carbonyls, an index ROS formation, were reduced by 11%, compared to wild-type animals. In contrast, mice deficient in MAO-B showed highly variable susceptibility to malonate toxicity precluding us from determining the precise role of MAO-B in this form of brain damage. These findings indicate that normal levels of MAO-A participate in expression of malonate toxicity by a mechanism involving oxidative stress.     

Quantification of age-dependent somatic CAG repeat instability in Hdh CAG knock-in mice reveals different expansion dynamics in striatum and liver

Background

Age at onset of Huntington''s disease (HD) is largely determined by the CAG trinucleotide repeat length in the HTT gene. Importantly, the CAG repeat undergoes tissue-specific somatic instability, prevalent in brain regions that are disease targets, suggesting a potential role for somatic CAG repeat instability in modifying HD pathogenesis. Thus, understanding underlying mechanisms of somatic CAG repeat instability may lead to discoveries of novel therapeutics for HD. Investigation of the dynamics of the CAG repeat size changes over time may provide insights into the mechanisms underlying CAG repeat instability.

Methodology/Principal Findings

To understand how the HTT CAG repeat length changes over time, we quantified somatic instability of the CAG repeat in Huntington''s disease CAG knock-in mice from 2–16 months of age in liver, striatum, spleen and tail. The HTT CAG repeat in spleen and tail was very stable, but that in liver and striatum expanded over time at an average rate of one CAG per month. Interestingly, the patterns of repeat instability were different between liver and striatum. Unstable CAG repeats in liver repeatedly gained similar sizes of additional CAG repeats (approximately two CAGs per month), maintaining a distinct population of unstable repeats. In contrast, unstable CAG repeats in striatum gained additional repeats with different sizes resulting in broadly distributed unstable CAG repeats. Expanded CAG repeats in the liver were highly enriched in polyploid hepatocytes, suggesting that the pattern of liver instability may reflect the restriction of the unstable repeats to a unique cell type.

Conclusions/Significance

Our results are consistent with repeat expansion occurring as a consequence of recurrent small repeat insertions that differ in different tissues. Investigation of the specific mechanisms that underlie liver and striatal instability will contribute to our understanding of the relationship between instability and disease and the means to intervene in this process.     

Transgenic mice expressing mutated full-length HD cDNA: a paradigm for locomotor changes and selective neuronal loss in Huntington's disease.

Huntington's disease (HD) is a progressive neurodegenerative disorder characterized clinically by motor and psychiatric disturbances and pathologically by neuronal loss and gliosis (reactive astrocytosis) particularly in the striatum and cerebral cortex. We have recently created HD full-length cDNA transgenic mouse models that may serve as a paradigm for HD. A more detailed characterization of these models is presented here. The transgene encoding normal huntingtin consists of 9417 bp of the huntingtin coding sequences including 16 tandem CAGs coding for polyglutamines as part of exon 1. The transgene is driven by a heterologous cytomegalovirus promoter. Five independent transgenic mouse lines were obtained using this construct. An additional six transgenic lines were obtained using full-length HD constructs that have been modified to include either 48 or 89 CAG repeat expansions. Southern blot and densitometric analyses indicated unique integration sites for the transgene in each of the lines with a copy number ranging from two to 22 copies. Widespread expression of the transgene in brain, heart, spleen, kidney, lung, liver and gonads from each line was determined by Western blot analyses. In the brain, transgene expression was found in cerebral cortex, striatum, hippocampus and cerebellum. Expression of the transgene was as much as five times the endogenous mouse huntingtin level. Phenotypically, only mice expressing 48 or 89 CAG repeats manifested progressive behavioural and motor dysfunction. Early behavioural abnormalities were characterized by trunk curling and clasping of both fore- and hindlimbs when the animals were suspended by their tails. Subsequently, these mice exhibited hyperkinetic movements, including heightened exploratory activities, unidirectional rotational behaviour, backflipping and excessive grooming that lasted for several weeks. Eventually, the animals progressed to a hypokinetic phase consisting of slowed movements and lack of response to sensory stimuli. Urine retention or incontinence was also a prominent feature of the hypokinetic phase. At the end stage of the disease process, HD48(B,D) and HD89(A-C) mice became akinetic just prior to death. Neuropathological examination of mice at various stages indicated that it was only during the hypokinetic phase and thereafter when selective neuronal loss was most apparent. Regions of neurodegeneration and loss included the striatum, cerebral cortex, thalamus and hippocampus. TUNEL staining indicated an apoptotic mode of cell death in these brain regions. Comparative neuronal counts after Nissl staining showed as much as 20% loss of small and medium neurons in the striatum in mice at the hypokinetic and akinetic stages. Reactive astrocytosis accompanied the areas of neurodegeneration and loss. Polyglutamine aggregates in the form of neuronal intranuclear inclusions and diffuse nuclear and perinuclear aggregations were found in a small percentage of neurons, including those in brain regions that are typically spared in HD. This observation suggests that polyglutamine aggregates may not be sufficient to cause neuronal loss in HD. In both behavioural and neuropathological analyses, wild-type and transgenic animals with 16 CAG repeats were indistinguishable from each other and do not exhibit the changes observed for mice carrying the 48 and 89 CAG repeat mutations. Thus, animals expressing the CAG repeat expansions appear to represent clinically analogous models for HD pathogenesis, and may also provide insights into the underlying pathophysiological mechanisms of other triplet repeat disorders.     

Tissue transglutaminase contributes to disease progression in the R6/2 Huntington's disease mouse model via aggregate-independent mechanisms

Huntington's disease (HD) is caused by an expansion of CAG repeats within the huntingtin gene and is characterized by intraneuronal mutant huntingtin protein aggregates. In order to determine the role of tissue transglutaminase (tTG) in HD aggregate formation and disease progression, we cross-bred the R6/2 HD mouse model with a tTG knockout mouse line. R6/2 mice that were tTG heterozygous knockouts (R6/2 : tTG+/-) and tTG homozygous knockouts (R6/2 : tTG-/-) showed a very similar increase in aggregate number within the striatum compared with R6/2 mice that were wild-type with respect to tTG (R6/2 : tTG+/+). Interestingly, a significant delay in the onset of motor dysfunction and death occurred in R6/2 : tTG-/- mice compared with both R6/2 : tTG+/+ and R6/2 : tTG+/- mice. As aggregate number was similarly increased in the striatum of both R6/2 : tTG+/- and R6/2 : tTG-/- mice, whereas only R6/2 : tTG-/- mice showed delayed disease progression, these data suggest that the contribution of tTG towards motor dysfunction and death in the R6/2 mouse is independent of its ability to negatively regulate aggregate formation. Moreover, the combined results from this study suggest that the formation of striatal huntingtin aggregates does not directly influence motor dysfunction or death in this HD mouse model.     

Proteomic analysis of protein expression and oxidative modification in r6/2 transgenic mice: a model of Huntington disease

Huntington disease (HD) is a hereditary neurodegenerative disorder characterized by motor, psychiatric, and cognitive symptoms. The genetic defect responsible for the onset of the disease, expansion of CAG repeats in exon 1 of the gene that codes for huntingtin on chromosome 4, has been unambiguously identified. On the other hand, the mechanisms by which the mutation causes the disease are not completely understood yet. However, defects in energy metabolism of affected cells may cause oxidative damage, which has been proposed as one of the underlying molecular mechanisms that participate in the etiology of the disease. In our effort to investigate the extent of oxidative damage occurring at the protein level, we used a parallel proteomic approach to identify proteins potentially involved in processes upstream or downstream of the disease-causing huntingtin in a well established HD mouse model (R6/2 transgenic mice). We have demonstrated that the expression levels of dihydrolipoamide S-succinyltransferase and aspartate aminotransferase increase consistently over the course of disease (10-week-old mice). In contrast, pyruvate dehydrogenase expression levels were found to be decreased in 10-week-old HD transgenic mice compared with young (4-week-old) mice. Our experimental approach also led to the identification of oxidatively modified proteins. Six proteins were found to be significantly oxidized in old R6/2 transgenic mice compared with either young transgenic mice or non-transgenic mice. These proteins are alpha-enolase, gamma-enolase (neuron-specific enolase), aconitase, the voltage-dependent anion channel 1, heat shock protein 90, and creatine kinase. Because oxidative damage has proved to play an important role in the pathogenesis and the progression of Huntington disease, our results for the first time identify specific oxidatively modified proteins that potentially contribute to the pathogenesis of Huntington disease.     

Euploidy in somatic cells from R6/2 transgenic Huntington's disease mice

Background  

Huntington's disease (HD) is a hereditary neurodegenerative disorder caused by a CAG repeat expansion in the HD gene. The huntingtin protein expressed from HD has an unknown function but is suggested to interact with proteins involved in the cell division machinery. The R6/2 transgenic mouse is the most widely used model to study HD. In R6/2 fibroblast cultures, a reduced mitotic index and high frequencies of multiple centrosomes and aneuploid cells have recently been reported. Aneuploidy is normally a feature closely connected to neoplastic disease. To further explore this unexpected aspect of HD, we studied cultures derived from 6- and 12-week-old R6/2 fibroblasts, skeletal muscle cells, and liver cells.     

Alterations in striatal synaptic transmission are consistent across genetic mouse models of Huntington's disease

Since the identification of the gene responsible for HD (Huntington''s disease), many genetic mouse models have been generated. Each employs a unique approach for delivery of the mutated gene and has a different CAG repeat length and background strain. The resultant diversity in the genetic context and phenotypes of these models has led to extensive debate regarding the relevance of each model to the human disorder. Here, we compare and contrast the striatal synaptic phenotypes of two models of HD, namely the YAC128 mouse, which carries the full-length huntingtin gene on a yeast artificial chromosome, and the CAG140 KI (knock-in) mouse, which carries a human/mouse chimaeric gene that is expressed in the context of the mouse genome, with our previously published data obtained from the R6/2 mouse, which is transgenic for exon 1 mutant huntingtin. We show that striatal MSNs (medium-sized spiny neurons) in YAC128 and CAG140 KI mice have similar electrophysiological phenotypes to that of the R6/2 mouse. These include a progressive increase in membrane input resistance, a reduction in membrane capacitance, a lower frequency of spontaneous excitatory postsynaptic currents and a greater frequency of spontaneous inhibitory postsynaptic currents in a subpopulation of striatal neurons. Thus, despite differences in the context of the inserted gene between these three models of HD, the primary electrophysiological changes observed in striatal MSNs are consistent. The outcomes suggest that the changes are due to the expression of mutant huntingtin and such alterations can be extended to the human condition.     

The nucleotide sequence, DNA damage location, and protein stoichiometry influence the base excision repair outcome at CAG/CTG repeats

Expansion of CAG/CTG repeats is the underlying cause of >14 genetic disorders, including Huntington's disease (HD) and myotonic dystrophy. The mutational process is ongoing, with increases in repeat size enhancing the toxicity of the expansion in specific tissues. In many repeat diseases, the repeats exhibit high instability in the striatum, whereas instability is minimal in the cerebellum. We provide molecular insights into how base excision repair (BER) protein stoichiometry may contribute to the tissue-selective instability of CAG/CTG repeats by using specific repair assays. Oligonucleotide substrates with an abasic site were mixed with either reconstituted BER protein stoichiometries mimicking the levels present in HD mouse striatum or cerebellum, or with protein extracts prepared from HD mouse striatum or cerebellum. In both cases, the repair efficiency at CAG/CTG repeats and at control DNA sequences was markedly reduced under the striatal conditions, likely because of the lower level of APE1, FEN1, and LIG1. Damage located toward the 5' end of the repeat tract was poorly repaired, with the accumulation of incompletely processed intermediates as compared to an AP lesion in the center or at the 3' end of the repeats or within control sequences. Moreover, repair of lesions at the 5' end of CAG or CTG repeats involved multinucleotide synthesis, particularly at the cerebellar stoichiometry, suggesting that long-patch BER processes lesions at sequences susceptible to hairpin formation. Our results show that the BER stoichiometry, nucleotide sequence, and DNA damage position modulate repair outcome and suggest that a suboptimal long-patch BER activity promotes CAG/CTG repeat instability.     

Glycolysis Inhibition Decreases the Levels of Glutamate Transporters and Enhances Glutamate Neurotoxicity in the R6/2 Huntington′s Disease Mice

Excitotoxicity has been associated with the loss of medium spiny neurons (MSN) in Huntington’s disease (HD). We have previously observed that the content of the glial glutamate transporters, glutamate transporter 1 (GLT-1) and glutamate-aspartate transporter (GLAST), diminishes in R6/2 mice at 14 weeks of age but not at 10 weeks, and that this change correlates with an increased vulnerability of striatal neurons to glutamate toxicity. We have also reported that inhibition of the glycolytic pathway decreases glutamate uptake and enhances glutamate neurotoxicity in the rat brain. We now show that at 10-weeks of age, glutamate excitotoxicity is precipitated in R6/2 mice, after the treatment with iodoacetate (IOA), an inhibitor of the glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). IOA induces a larger inhibition of GAPDH in R6/2 mice, while it similarly reduces the levels of GLT-1 and GLAST in wild-type and transgenic animals. Results suggest that metabolic failure and altered glutamate uptake are involved in the vulnerability of striatal neurons to glutamate excitotoxicity in HD.     

Increased sensitivity to N-methyl-D-aspartate receptor-mediated excitotoxicity in a mouse model of Huntington's disease

Previous work suggests N-methyl-D-aspartate receptor (NMDAR) activation may be involved in degeneration of medium-sized spiny striatal neurons in Huntington's disease (HD). Here we show that these neurons are more vulnerable to NMDAR-mediated death in a YAC transgenic FVB/N mouse model of HD expressing full-length mutant huntingtin, compared with wild-type FVB/N mice. Excitotoxic death of these neurons was increased after intrastriatal injection of quinolinate in vivo, and after NMDA but not AMPA exposure in culture. NMDA-induced cell death was abolished by an NR2B subtype-specific antagonist. In contrast, NMDAR-mediated death of cerebellar granule neurons was not enhanced, consistent with cell-type and NMDAR subtype specificity. Moreover, increased NMDA-evoked current amplitude and caspase-3 activity were observed in transgenic striatal neurons. Our data support a role for NR2B-subtype NMDAR activation as a trigger for selective neuronal degeneration in HD.