Characterization of synaptic dysfunction in an in vitro corticostriatal model system of Huntington’s disease

Huntington’s disease (HD) is an autosomal-dominant inherited neurodegenerative disease resulting from expanded amino acid (CAG) repeat in the gene that encodes protein huntingtin (Htt). HD remains incurable for now. A lot of evidence implicates aberrant synaptic connection between cortical and striatal neurons, a key component of HD pathophysilogy, which also leads to cognitive decline and motor disorders. In the present work synaptic activity between cortical and striatal neurons was studied on the corticostriatal co-culture model system of HD. Culture was prepared from HD mouse model YAC128. It was shown that first impairment appears on day 14 in vitro. Interestingly, these alterations occur in cortical neurons. Their activity in YAC128 cultures was higher than in cultures of wild-type neurons. At the same time, there were no differences in morphology of spines in striatal neurons. However, using novel optogenetic approach, we demonstrated that synaptic connections are already dysfunctional in YAC128 cultures. On day 19 in vitro the activity of cortical neurons in YAC128 cultures was reduced, which led to alterations on the post-synaptic side. Dendric spines of medium spiny neurons transformed and disappeared, which is possibly the main reason of neurodegenerative mechanisms during the HD development.     

Baclofen,a GABAB receptor agonist,enhances ubiquitin-proteasome system functioning and neuronal survival in Huntington’s disease model mice

Huntington’s disease (HD) is an autosomal neurodegenerative disease. Its manifestations is selective degeneration of medium-sized spiny neurons (MSN) in the striatum. The specificity of the vulnerability of these GABAergic MSNs can be explained by abnormal protein accumulation, excitotoxicity, mitochondrial dysfunction, and failure of trophic control, among other dysfunctions. In this study, we used in vitro and in vivo models of HD to study the effects of GABAergic neuron stimulation on the cellular protein degradation machinery. We administered the GABA_B receptor agonist, baclofen, to wild-type or mutant huntingtin-expressing striatal cells (HD19 or HD43). Chymotrypsin-like proteasome activity and cell viability were significantly increased in the mutant huntingtin-expressing striatal cells (HD43) after GABA_B receptor agonist treatment. In addition, we systemically administered baclofen to a HD model containing the entire human huntingtin gene with 128 CAG repeats (YAC128). Chymotrypsin-like proteasome activity was significantly increased in YAC128 transgenic mice after baclofen administration. Baclofen-injected mutant YAC128 mice also showed significantly reduced numbers of ubiquitin-positive neuronal intranuclear inclusions (NIIs) in the striatum. Baclofen markedly improved behavioral abnormalities in mutant YAC128 mice as determined by the rotarod performance test. These data indicate that stimulation of GABAergic neurons with the GABA_B receptor agonist, baclofen, enhances ubiquitin-proteasome system (UPS) function and cell survival in in vitro and in vivo models of HD.     

Brain-Derived Neurotrophic Factor Prevents Depressive-Like Behaviors in Early-Symptomatic YAC128 Huntington’s Disease Mice

Huntington disease (HD) is a neurodegenerative disorder caused by an expanded CAG repeat in the Huntington disease gene. The symptomatic stage of the disease is defined by the onset of motor symptoms. However, psychiatric disturbances, including depression, are common features of HD and can occur a decade before the manifestation of motor symptoms. We used the YAC128 transgenic mice (which develop motor deficits at a later stage, allowing more time to study depressive behaviors without the confounding effects of motor impairment) to test the effects of intranasal brain-derived neurotrophic factor (BDNF) treatment for 15 days in the occurrence of depressive-like behaviors. Using multiple well-validated behavioral tests, we found that BDNF treatment alleviated anhedonic and depressive-like behaviors in the YAC128 HD mice. Furthermore, we also investigated whether the antidepressant-like effects of BDNF were associated with an increase in adult hippocampal neurogenesis. However, BDNF treatment only increased cell proliferation and neuronal differentiation in the hippocampal dentate gyrus (DG) of wild-type (WT) mice, without altering these parameters in their YAC128 counterparts. Moreover, BDNF treatment did not cause an increase in the number of dendritic branches in the hippocampal DG when compared with animals treated with vehicle. In conclusion, our results suggest that non-invasive administration of BDNF via the intranasal route may have important therapeutic potential for treating mood disturbances in early-symptomatic HD patients.     

AAV-Dominant Negative Tumor Necrosis Factor (DN-TNF) Gene Transfer to the Striatum Does Not Rescue Medium Spiny Neurons in the YAC128 Mouse Model of Huntington's Disease

CNS inflammation is a hallmark of neurodegenerative disease, and recent studies suggest that the inflammatory response may contribute to neuronal demise. In particular, increased tumor necrosis factor (TNF) signaling is implicated in the pathology of both Parkinson''s disease (PD) and Alzheimer''s disease (AD). We have previously shown that localized gene delivery of dominant negative TNF to the degenerating brain region can limit pathology in animal models of PD and AD. TNF is upregulated in Huntington''s disease (HD), like in PD and AD, but it is unknown whether TNF signaling contributes to neuronal degeneration in HD. We used in vivo gene delivery to test whether selective reduction of soluble TNF signaling could attenuate medium spiny neuron (MSN) degeneration in the YAC128 transgenic (TG) mouse model of Huntington''s disease (HD). AAV vectors encoding cDNA for dominant-negative tumor necrosis factor (DN-TNF) or GFP (control) were injected into the striatum of young adult wild type WT and YAC128 TG mice and achieved 30–50% target coverage. Expression of dominant negative TNF protein was confirmed immunohistologically and biochemically and was maintained as mice aged to one year, but declined significantly over time. However, the extent of striatal DN-TNF gene transfer achieved in our studies was not sufficient to achieve robust effects on neuroinflammation, rescue degenerating MSNs or improve motor function in treated mice. Our findings suggest that alternative drug delivery strategies should be explored to determine whether greater target coverage by DN-TNF protein might afford some level of neuroprotection against HD-like pathology and/or that soluble TNF signaling may not be the primary driver of striatal neuroinflammation and MSN loss in YAC128 TG mice.     

Relationship between BDNF expression in major striatal afferents,striatum morphology and motor behavior in the R6/2 mouse model of Huntington's disease

Patients with Huntington's disease (HD) and transgenic mouse models of HD show neuronal loss in the striatum as a major feature, which contributes to cognitive and motor manifestations. Reduced expression of the neurotrophin brain‐derived neurotrophic factor (BDNF) in striatal afferents may play a role in neuronal loss. How progressive loss of BDNF expression in different cortical or subcortical afferents contributes to striatal atrophy and behavioral dysfunction in HD is not known, and may best be determined in animal models. We compared age‐dependent alterations of BDNF mRNA expression in major striatal afferents from the cerebral cortex, thalamus and midbrain in the R6/2 transgenic mouse model of HD. Corresponding changes in striatal morphology were quantified using unbiased stereology. Changes in motor behavior were measured using an open field, grip strength monitor, limb clasping and a rotarod apparatus. BDNF expression in cortical limbic and midbrain striatal afferents is reduced by age 4 weeks, prior to onset of motor abnormalities. BDNF expression in motor cortex and thalamic afferents is reduced by 6 weeks, coinciding with early motor dysfunction and reduced striatum volume. BDNF loss in afferents progresses until death at 13–15 weeks, correlating with progressive striatal neuronal loss and motor abnormalities. Mutant huntingtin protein expression in R6/2 mice results in progressive loss of BDNF in both cortical and subcortical striatal afferents. BDNF loss in limbic and dopaminergic striatal inputs may contribute to cognitive/psychiatric dysfunction in HD. Subsequent BDNF loss in cortical motor and thalamic afferents may accelerate striatal degeneration, resulting in progressive involuntary movements.     

Cystamine treatment is neuroprotective in the YAC128 mouse model of Huntington disease

Huntington disease (HD) is an adult onset neurodegenerative disorder characterized by selective atrophy and cell loss within the striatum. There is currently no treatment that can prevent the striatal neuropathology. Transglutaminase (TG) activity is increased in HD patients, is associated with cell death, and has been suggested to contribute to striatal neuronal loss in HD. This work assesses the therapeutic potential of cystamine, an inhibitor of TG activity with additional potentially beneficial effects. Specifically, we examine the effect of cystamine on striatal neuronal loss in the YAC128 mouse model of HD. We demonstrate here for the first time that YAC128 mice show a forebrain-specific increase in TG activity compared with wild-type (WT) littermates which is decreased by oral delivery of cystamine. Treatment of symptomatic YAC128 mice with cystamine starting at 7 months prevented striatal neuronal loss. Cystamine treatment also ameliorated the striatal volume loss and striatal neuronal atrophy observed in these animals, but was unable to prevent motor dysfunction or the down-regulation of dopamine and cyclic adenosine monophsophate-regulated phosphoprotein (DARPP-32) expression in the striatum. While the exact mechanism responsible for the beneficial effects of cystamine in YAC128 mice is uncertain, our findings suggest that cystamine is neuroprotective and may be beneficial in the treatment of HD.     

Mutant huntingtin causes metabolic imbalance by disruption of hypothalamic neurocircuits

In Huntington's disease (HD), the mutant huntingtin protein is ubiquitously expressed. The disease was considered to be limited to the basal ganglia, but recent studies have suggested a more widespread pathology involving hypothalamic dysfunction. Here we tested the hypothesis that expression of mutant huntingtin in the hypothalamus causes metabolic abnormalities. First, we showed that bacterial artificial chromosome-mediated transgenic HD (BACHD) mice developed impaired glucose metabolism and pronounced insulin and leptin resistance. Selective hypothalamic expression of a short fragment of mutant huntingtin using adeno-associated viral vectors was sufficient to recapitulate these metabolic disturbances. Finally, selective hypothalamic inactivation of the mutant gene prevented the development of the metabolic phenotype in BACHD mice. Our findings establish a causal link between mutant huntingtin expression in the hypothalamus and metabolic dysfunction, and indicate that metabolic parameters are powerful readouts to assess therapies aimed at correcting dysfunction in HD by silencing huntingtin expression in the brain.     

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.     

Mitochondrial dysfunction in Huntington's disease: the bioenergetics of isolated and in situ mitochondria from transgenic mice

Mitochondrial dysfunction is believed to participate in Huntington's disease (HD) pathogenesis. Here we compare the bioenergetic behavior of forebrain mitochondria isolated from different transgenic HD mice (R6/2, YAC128 and Hdh150 knock-in) and wild-type littermates with the first determination of in situ respiratory parameters in intact HD striatal neurons. We assess the Ca2+-loading capacity of isolated mitochondria by steady Ca2+-infusion. Mitochondria from R6/2 mice (12-13 weeks) and 12 months YAC128, but not homozygous or heterozygous Hdh150 knock-in mice (15-17 weeks), exhibit increased Ca2+-loading capacity when compared with respective wild-type littermates. In situ mitochondria in intact striatal neurons show high respiratory control. Moreover, moderate expression of full-length mutant huntingtin (in Hdh150 knock-in heterozygotes) does not significantly impair mitochondrial respiration in unstimulated neurons. However, when challenged with energy-demanding stimuli (NMDA-receptor activation in pyruvate-based media to accentuate the mitochondria role in Ca2+-handling), Hdh150 neurons are more vulnerable to Ca2+-deregulation than neurons from their wild-type littermates. These results stress the importance of assessing HD mitochondrial function in the cellular context.     

Increased Levels of Rictor Prevent Mutant Huntingtin-Induced Neuronal Degeneration

Rictor associates with mTOR to form the mTORC2 complex, which activity regulates neuronal function and survival. Neurodegenerative diseases are characterized by the presence of neuronal dysfunction and cell death in specific brain regions such as for example Huntington’s disease (HD), which is characterized by the loss of striatal projection neurons leading to motor dysfunction. Although HD is caused by the expression of mutant huntingtin, cell death occurs gradually suggesting that neurons have the capability to activate compensatory mechanisms to deal with neuronal dysfunction and later cell death. Here, we analyzed whether mTORC2 activity could be altered by the presence of mutant huntingtin. We observed that Rictor levels are specifically increased in the striatum of HD mouse models and in the putamen of HD patients. Rictor-mTOR interaction and the phosphorylation levels of Akt, one of the targets of the mTORC2 complex, were increased in the striatum of the R6/1 mouse model of HD suggesting increased mTORC2 signaling. Interestingly, acute downregulation of Rictor in striatal cells in vitro reduced mTORC2 activity, as shown by reduced levels of phospho-Akt, and increased mutant huntingtin-induced cell death. Accordingly, overexpression of Rictor increased mTORC2 activity counteracting cell death. Furthermore, normalization of endogenous Rictor levels in the striatum of R6/1 mouse worsened motor symptoms suggesting an induction of neuronal dysfunction. In conclusion, our results suggest that increased Rictor striatal levels could counteract neuronal dysfunction induced by mutant huntingtin.     

Brain-derived neurotrophic factor over-expression in the forebrain ameliorates Huntington's disease phenotypes in mice

Huntington's disease (HD), a dominantly inherited neurodegenerative disorder characterized by relatively selective degeneration of striatal neurons, is caused by an expanded polyglutamine tract of the huntingtin (htt) protein. The htt mutation reduces levels of brain-derived neurotrophic factor (BDNF) in the striatum, likely by inhibiting cortical BDNF gene expression and anterograde transport of BDNF from cortex to striatum. However, roles of the BDNF reduction in HD pathogenesis have not been established conclusively. We reasoned that increasing striatal BDNF through over-expression would slow progression of the disease if BDNF reduction plays a pivotal role in HD pathogenesis. We employed a Bdnf transgene driven by the promoter for the alpha subunit of Ca²⁺/calmodulin-dependent kinase II to over-express BDNF in the forebrain of R6/1 mice which express a fragment of mutant htt with a 116-glutamine tract. The Bdnf transgene increased BDNF levels and TrkB signaling activity in the striatum, ameliorated motor dysfunction, and reversed brain weight loss in R6/1 mice. Furthermore, it normalized DARPP-32 expression of the 32 kDa dopamine and cAMP-regulated phosphoprotein, increased the number of enkephalin-containing boutons, and reduced formation of neuronal intranuclear inclusions in the striatum of R6/1 mice. These results demonstrate crucial roles of reduced striatal BDNF in HD pathogenesis and suggest potential therapeutic values of BDNF to HD.     

Characterization of subventricular zone-derived progenitor cells from mild and late symptomatic YAC128 mouse model of Huntington's disease

Huntington's disease (HD) is caused by an expansion of CAG repeats in the HTT gene, leading to expression of mutant huntingtin (mHTT) and selective striatal neuronal loss, frequently associated with mitochondrial dysfunction and decreased support of brain-derived neurotrophic factor (BDNF). New neurons derived from the subventricular zone (SVZ) are apparently not able to rescue HD pathological features. Thus, we analyzed proliferation, migration and differentiation of adult SVZ-derived neural stem/progenitor cells (NSPC) from mild (6 month-old (mo)) and late (10 mo) symptomatic HD YAC128 mice expressing full-length (FL)-mHTT versus age-matched wild-type (WT) mice. SVZ cells derived from 6 mo YAC128 mice exhibited higher migratory capacity and a higher number of MAP2 + and synaptophysin + cells, compared to WT cells; MAP2 labeling was enhanced after exposure to BDNF. However, BDNF-evoked neuronal differentiation was not observed in 10 mo YAC128 SVZ-derived cells. Interestingly, 6 mo YAC128 SVZ-derived cells showed increased intracellular Ca²⁺ levels in response to KCl, which was potentiated by BDNF, evidencing the presence of differentiated neurons. In contrast, KCl depolarization-induced intracellular Ca²⁺ increase in 10 mo YAC128 SVZ-derived cells was shown to be increased only in BDNF-treated YAC128 SVZ-derived cells, suggestive of decreased differentiation capacity. In addition, BDNF-untreated NSPC from 10 mo YAC128 mice exhibited lower mitochondrial membrane potential and increased mitochondrial Ca²⁺ accumulation, in relation with NSPC from 6 mo YAC128 mice. Data evidence age-dependent reduced migration and decreased acquisition of a neuronal phenotype, accompanied by decreased mitochondrial membrane potential in SVZ-derived cells from YAC128 mice through HD symptomatic phases.     

Azadiradione Restores Protein Quality Control and Ameliorates the Disease Pathogenesis in a Mouse Model of Huntington’s Disease

Huntington’s disease (HD) is an autosomal dominantly inherited neurodegenerative disorder caused by expansion of CAG repeats in the coding area of huntingtin gene. In the HD brain, mutant huntingtin protein goes through proteolysis, and its amino-terminal portion consisting of polyglutamine repeats accumulate as inclusions that result in progressive impairment of cellular protein quality control system. Here, we demonstrate that partial rescue of the defective protein quality control in HD model mouse by azadiradione (a bioactive limonoids found in the seed of Azadirachta indica) could potentially improve the disease pathology. Prolonged treatment of azadiradione to HD mice significantly improved the progressive deterioration in body weight, motor functioning along with extension of lifespan. Azadiradione-treated HD mice brain also exhibited considerable decrease in mutant huntingtin aggregates load and improvement of striatal pathology in comparison with age-matched saline-treated HD controls. Biochemical analysis further revealed upregulation and activation of not only HSF1 (master regulator of protein folding) but also Ube3a (an ubiquitin ligase involved in the clearance of mutant huntingtin) in azadiradione-treated mice. Our results indicate that azadiradione-mediated enhanced folding and clearance of mutant huntingtin might underlie improved disease pathology in HD mice and suggests that it could be a potential therapeutic molecule to delay the progression of HD.     

Early defect of transforming growth factor β1 formation in Huntington’s disease

A defective expression or activity of neurotrophic factors, such as brain‐ and glial‐derived neurotrophic factors, contributes to neuronal damage in Huntington’s disease (HD). Here, we focused on transforming growth factor‐β (TGF‐β₁), a pleiotropic cytokine with an established role in mechanisms of neuroprotection. Asymptomatic HD patients showed a reduction in TGF‐β₁ levels in the peripheral blood, which was related to trinucleotide mutation length and glucose hypometabolism in the caudate nucleus. Immunohistochemical analysis in post‐mortem brain tissues showed that TGF‐β₁ was reduced in cortical neurons of asymptomatic and symptomatic HD patients. Both YAC128 and R6/2 HD mutant mice showed a reduced expression of TGF‐β₁ in the cerebral cortex, localized in neurons, but not in astrocytes. We examined the pharmacological regulation of TGF‐β₁ formation in asymptomatic R6/2 mice, where blood TGF‐β₁ levels were also reduced. In these R6/2 mice, both the mGlu2/3 metabotropic glutamate receptor agonist, LY379268, and riluzole failed to increase TGF‐β₁ formation in the cerebral cortex and corpus striatum, suggesting that a defect in the regulation of TGF‐β₁ production is associated with HD. Accordingly, reduced TGF‐β₁ mRNA and protein levels were found in cultured astrocytes transfected with mutated exon 1 of the human huntingtin gene, and in striatal knock‐in cell lines expressing full‐length huntingtin with an expanded glutamine repeat. Taken together, our data suggest that serum TGF‐β₁ levels are potential biomarkers of HD development during the asymptomatic phase of the disease, and raise the possibility that strategies aimed at rescuing TGF‐β₁ levels in the brain may influence the progression of HD.     

Age-dependent changes in the calcium sensitivity of striatal mitochondria in mouse models of Huntington's Disease

Striatal and cortical mitochondria from knock-in and transgenic mutant huntingtin mice were examined for their sensitivity to calcium induction of the permeability transition, a cause of mitochondrial depolarization and ATP loss. The permeability transition has been suggested to contribute to cell death in Huntington's Disease. Mitochondria were examined from slowly progressing knock-in mouse models with different length polyglutarnine expansions (Q20, Q50, Q92, Q111) and from the rapidly progressing transgenic R6/2 mice overexpressing exon I of human huntingtin with more than 110 polyglutamines. As previously observed in rats, striatal mitochondria from background strain CD1 and C57BL/6 control mice were more sensitive to calcium than cortical mitochondria. Between 5 and 12 months in knock-in Q92 mice and between 8 and 12 weeks in knock-in Q111 mice, striatal mitochondria developed resistance, becoming equally sensitive to calcium as cortical mitochondria, while those from Q50 mice were unchanged. Cortical mitochondrial calcium sensitivity did not change. In R6/2 mice striatal and cortical mitochondria were equally resistant to Ca2+ while striatal mitochondria from littermate controls were more susceptible. No increases in calcium sensitivity were observed in the mitochondria from Huntington's Disease (HD) mice compared to controls. Neither motor abnormalities, nor expression of cyclophilin D corresponded to the changes in mitochondrial sensitivity. Polyglutamine expansions in huntingtin produced an early increased resistance to calcium in striatal mitochondria suggesting mitochondria undergo compensatory changes in calcium sensitivity in response to the many cellular changes wrought by polyglutamine expansion.     

Protein misfolding detected early in pathogenesis of transgenic mouse model of Huntington disease using amyloid seeding assay

Huntington disease (HD) is one of several fatal neurodegenerative disorders associated with misfolded proteins. Here, we report a novel method for the sensitive detection of misfolded huntingtin (HTT) isolated from the brains of transgenic (Tg) mouse models of HD and humans with HD using an amyloid seeding assay (ASA), which is based on the propensity of misfolded proteins to act as a seed and shorten the nucleation-associated lag phase in the kinetics of amyloid formation in vitro. Using synthetic polyglutamine peptides as the substrate for amyloid formation, we found that partially purified misfolded HTT obtained from end-stage brain tissue of two Tg HD mouse models and brain tissue of post-mortem human HD patients was capable of specifically accelerating polyglutamine amyloid formation compared with unseeded reactions and controls. Alzheimer and prion disease brain tissues did not do so, demonstrating the specificity of the ASA. It is unclear whether early intermediates or later conformational species in the protein misfolding process act as seeds in the ASA for HD. However, we were able to detect misfolded protein in the brains of YAC128 mice early in disease pathogenesis (11 weeks of age), whereas large inclusion bodies have not been observed in the brains of these mice by histology until 78 weeks of age, much later in the pathogenic process. The sensitive detection of misfolded HTT protein early in the disease pathogenesis in the YAC128 Tg mouse model strengthens the argument for a causative role of protein misfolding in HD.     

Disease-toxicant screen reveals a neuroprotective interaction between Huntington's disease and manganese exposure

Recognizing the similarities between Huntington's disease (HD) pathophysiology and the neurotoxicology of various metals, we hypothesized that they may exhibit disease-toxicant interactions revealing cellular pathways underlying neurodegeneration. Here, we utilize metals and the ST Hdh mouse striatal cell line model of HD to perform a gene–environment interaction screen. We report that striatal cells expressing mutant Huntingtin exhibit elevated sensitivity to cadmium toxicity and resistance to manganese toxicity. This neuroprotective gene–environment interaction with manganese is highly specific, as it does not occur with iron, copper, zinc, cobalt, cadmium, lead, or nickel ions. Analysis of the Akt cell stress signaling pathway showed diminished activation with manganese exposure and elevated activation after cadmium exposure in the mutant cells. Direct examination of intracellular manganese levels found that mutant cells have a significant impairment in manganese accumulation. Furthermore, YAC128Q mice, a HD model, showed decreased total striatal manganese levels following manganese exposure relative to wild-type mice. Thus, this disease-toxicant interaction screen has revealed that expression of mutant Huntingtin results in heightened sensitivity to cadmium neurotoxicity and a selective impairment of manganese accumulation.     

Specific caspase interactions and amplification are involved in selective neuronal vulnerability in Huntington's disease

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder resulting in selective neuronal loss and dysfunction in the striatum and cortex. The molecular pathways leading to the selectivity of neuronal cell death in HD are poorly understood. Proteolytic processing of full-length mutant huntingtin (Htt) and subsequent events may play an important role in the selective neuronal cell death found in this disease. Despite the identification of Htt as a substrate for caspases, it is not known which caspase(s) cleaves Htt in vivo or whether regional expression of caspases contribute to selective neuronal cells loss. Here, we evaluate whether specific caspases are involved in cell death induced by mutant Htt and if this correlates with our recent finding that Htt is cleaved in vivo at the caspase consensus site 552. We find that caspase-2 cleaves Htt selectively at amino acid 552. Further, Htt recruits caspase-2 into an apoptosome-like complex. Binding of caspase-2 to Htt is polyglutamine repeat-length dependent, and therefore may serve as a critical initiation step in HD cell death. This hypothesis is supported by the requirement of caspase-2 for the death of mouse primary striatal cells derived from HD transgenic mice expressing full-length Htt (YAC72). Expression of catalytically inactive (dominant-negative) forms of caspase-2, caspase-7, and to some extent caspase-6, reduced the cell death of YAC72 primary striatal cells, while the catalytically inactive forms of caspase-3, -8, and -9 did not. Histological analysis of post-mortem human brain tissue and YAC72 mice revealed activation of caspases and enhanced caspase-2 immunoreactivity in medium spiny neurons of the striatum and the cortical projection neurons when compared to controls. Further, upregulation of caspase-2 correlates directly with decreased levels of brain-derived neurotrophic factor in the cortex and striatum of 3-month YAC72 transgenic mice and therefore suggests that these changes are early events in HD pathogenesis. These data support the involvement of caspase-2 in the selective neuronal cell death associated with HD in the striatum and cortex.