Phenotypic abnormalities in the YAC128 mouse model of Huntington disease are penetrant on multiple genetic backgrounds and modulated by strain

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.

Onset and progression of behavioral and molecular phenotypes in a novel congenic R6/2 line exhibiting intergenerational CAG repeat stability

In the present study we report on the use of speed congenics to generate a C57BL/6J congenic line of HD-model R6/2 mice carrying 110 CAG repeats, which uniquely exhibits minimal intergenerational instability. We also report the first identification of the R6/2 transgene insertion site. The relatively stable line of 110 CAG R6/2 mice was characterized for the onset of behavioral impairments in motor, cognitive and psychiatric-related phenotypes as well as the progression of disease-related impairments from 4 to 10 weeks of age. 110Q mice exhibited many of the phenotypes commonly associated with the R6/2 model including reduced activity and impairments in rotarod performance. The onset of many of the phenotypes occurred around 6 weeks and was progressive across age. In addition, some phenotypes were observed in mice as early as 4 weeks of age. The present study also reports the onset and progression of changes in several molecular phenotypes in the novel R6/2 mice and the association of these changes with behavioral symptom onset and progression. Data from TR-FRET suggest an association of mutant protein state changes (soluble versus aggregated) in disease onset and progression.

Characterisation of a C1qtnf5 Ser163Arg knock-in mouse model of late-onset retinal macular degeneration

A single founder mutation resulting in a Ser163Arg substitution in the C1QTNF5 gene product causes autosomal dominant late-onset retinal macular degeneration (L-ORMD) in humans, which has clinical and pathological features resembling age-related macular degeneration. We generated and characterised a mouse “knock-in” model carrying the Ser163Arg mutation in the orthologous murine C1qtnf5 gene by site-directed mutagenesis and homologous recombination into mouse embryonic stem cells. Biochemical, immunological, electron microscopic, fundus autofluorescence, electroretinography and laser photocoagulation analyses were used to characterise the mouse model. Heterozygous and homozygous knock-in mice showed no significant abnormality in any of the above measures at time points up to 2 years. This result contrasts with another C1qtnf5 Ser163Arg knock-in mouse which showed most of the features of L-ORMD but differed in genetic background and targeting construct.

Mutant huntingtin, abnormal mitochondrial dynamics, defective axonal transport of mitochondria, and selective synaptic degeneration in Huntington's disease

Huntington's disease (HD) is a progressive, fatal neurodegenerative disease caused by expanded polyglutamine repeats in the HD gene. HD is characterized by chorea, seizures, involuntary movements, dystonia, cognitive decline, intellectual impairment and emotional disturbances. Research into mutant huntingtin (Htt) and mitochondria has found that mutant Htt interacts with the mitochondrial protein dynamin-related protein 1 (Drp1), enhances GTPase Drp1 enzymatic activity, and causes excessive mitochondrial fragmentation and abnormal distribution, leading to defective axonal transport of mitochondria and selective synaptic degeneration. This article summarizes latest developments in HD research and focuses on the role of abnormal mitochondrial dynamics and defective axonal transport in HD neurons. This article also discusses the therapeutic strategies that decrease mitochondrial fragmentation and neuronal damage in HD.

Combined treatment with the mood stabilizers lithium and valproate produces multiple beneficial effects in transgenic mouse models of Huntington's disease

Emerging evidence suggests that the mood stabilizers lithium and valproate (VPA) have broad neuroprotective and neurotrophic properties, and that these occur via inhibition of glycogen synthase kinase 3 (GSK-3) and histone deacetylases (HDACs), respectively. Huntington's disease (HD) is an inherited neurodegenerative disorder characterized by impaired movement, cognitive and psychiatric disturbances, and premature death. We treated N171-82Q and YAC128 mice, two mouse models of HD varying in genetic backgrounds and pathological progressions, with a diet containing therapeutic doses of lithium, VPA, or both. Untreated, these transgenic mice displayed a decrease in levels of GSK-3β serine 9 phosphorylation and histone H3 acetylation in the striatum and cerebral cortex around the onset of behavioral deficits, indicating a hyperactivity of GSK-3β and HDACs. Using multiple well-validated behavioral tests, we found that co-treatment with lithium and VPA more effectively alleviated spontaneous locomotor deficits and depressive-like behaviors in both models of HD mice. Furthermore, compared with monotherapy with either drug alone, co-treatment more successfully improved motor skill learning and coordination in N171-82Q mice, and suppressed anxiety-like behaviors in YAC128 mice. This combined treatment consistently inhibited GSK-3β and HDACs, and caused a sustained elevation in striatal as well as cortical brain-derived neurotrophic factor and heat shock protein 70. Importantly, co-treatment markedly prolonged median survival of N171-82Q mice from 31.6 to 41.6 weeks. Given that there is presently no proven treatment for HD, our results suggest that combined treatment with lithium and VPA, two mood stabilizers with a long history of safe use in humans, may have important therapeutic potential for HD patients.

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.

Altered Balance of Activity in the Striatal Direct and Indirect Pathways in Mouse Models of Huntington's Disease

Imbalance in the activity of striatal direct and indirect pathway neurons contributes to motor disturbances in several neurodegenerative diseases. In Huntington's disease (HD), indirect pathway [dopamine (DA) D2 receptor-expressing] medium-sized spiny neurons (MSNs) are believed to show earlier vulnerability than direct pathway MSNs. We examined synaptic activity and DA modulation in MSNs forming the direct and indirect pathways in YAC128 and BACHD mouse models of HD. To visualize the two types of MSNs, we used mice expressing enhanced green fluorescent protein under the control of the promoter for the DA D1 or D2 receptor. Experiments were performed in early symptomatic (1.5 months) and symptomatic (12 months) mice. Behaviorally, early symptomatic mice showed increased stereotypies while symptomatic mice showed decreased motor activity. Electrophysiologically, at the early stage, excitatory and inhibitory transmission onto D1-YAC128 and D1-BACHD MSNs were increased, while there was no change in D2 MSNs. DA modulation of spontaneous excitatory postsynaptic currents (sEPSCs) in slices was absent in YAC128 cells at the early stage, but was restored by treating the slices with the DA depleter tetrabenazine (TBZ). In BACHD mice TBZ restored paired-pulse ratios and a D1 receptor antagonist induced a larger decrease of sEPSCs than in D1-WT cells, suggesting increased DA tone. Finally, TBZ decreased stereotypies in BACHD mice. These results indicate that by reducing DA or antagonizing D1 receptors, increases in inhibitory and excitatory transmission in early phenotypic direct pathway neurons can be normalized. In symptomatic YAC128 mice, excitatory synaptic transmission onto D1 MSNs was decreased, while inhibitory transmission was increased in D2 MSNs. These studies provide evidence for differential and complex imbalances in glutamate and GABA transmission, as well as in DA modulation, in direct and indirect pathway MSNs during HD progression.

Premature death and neurologic abnormalities in transgenic mice expressing a mutant huntingtin exon-2 fragment

Huntington's disease (HD) is a fatal neurodegenerative disease characterized pathologically by aggregates composed of N-terminal fragments of the mutant form of the protein huntingtin (htt). The role of these N-terminal fragments in disease pathogenesis has been questioned based in part on studies in transgenic mice. In one important example, mice that express an N-terminal fragment of mutant htt terminating at the C-terminus of exon 2 (termed the Shortstop mouse) were reported to develop robust inclusion pathology without developing phenotypic abnormalities seen in the R6/2 or N171-82Q models of HD, which are also based on expression of mutant N-terminal htt fragments. To further explore the capacity of mutant exon-2 htt fragments to produce neurologic abnormalities (N-terminal 118 amino acids; N118), we generated transgenic mice expressing cDNA that encodes htt N118-82Q with the mouse prion promoter vector. In mice generated in this manner, we demonstrate robust inclusion pathology accompanied by early death and failure to gain weight. These phenotypes are the most robust abnormalities identified in the R6/2 and N171-82Q models. We conclude that the lack of an overt phenotype in the initial Shortstop mice cannot be completely explained by the properties of mutant htt N118 fragments.

Genetic modifiers of neurological disease

Genetic modifiers make an important contribution to neurological disease phenotypes. Significant progress has been made by studying genetic modifiers in model organisms. The ability to study complex genetic interactions in model systems contributes to our understanding of the genetic factors that influence neurological disease. This will lead to the development of novel therapeutic strategies and personalized treatment based on genetic risk.

Molecular biology of Huntington's disease

It has been more than 17 years since the causative mutation for Huntington's disease was discovered as the expansion of the triplet repeat in the N-terminal portion of the Huntingtin (HTT) gene. In the intervening time, researchers have discovered a great deal about Huntingtin's involvement in a number of cellular processes. However, the role of Huntingtin in the key pathogenic mechanism leading to neurodegeneration in the disease process has yet to be discovered. Here, we review the body of knowledge that has been uncovered since gene discovery and include discussions of the HTT gene, CAG triplet repeat expansion, HTT expression, protein features, posttranslational modifications, and many of its known protein functions and interactions. We also highlight potential pathogenic mechanisms that have come to light in recent years.

Transgenic Animal Models of Huntington's Disease

Huntington's disease (HD) is a devastating neurodegenerative disorder that currently has no cure. In order to develop effective treatment, an understanding of HD pathogenesis and the evaluation of therapeutic efficacy of novel medications with the aid of animal models are critical steps. Transgenic animals sharing similar genetic defects that lead to HD have provided important discoveries in HD mechanisms that cell models are not able to replicate, which include psychiatric impairment, cognitive behavioral impact, and motor functions. Although transgenic HD rodent models have been widely used in HD research, it is clear that an animal model with comparable physiology to man, similar genetic defects that lead to HD, and the ability to develop similar cognitive and behavioral impairments is critical for explaining HD pathogenesis and the development of cures. Compared to HD rodents, HD transgenic nonhuman primates have not only developed comparable neuropathology but also present HD clinical features such as rigidity, seizure, dystonia, bradykinesia, and chorea that no other animal model has been able to replicate. Distinctive degenerating neurons and the accumulation of neuropil aggregates observed in HD monkey brain strongly support the hypothesis that the unique neuropathogenic events seen in HD monkey brain recapitulate HD in man. The latest development of transgenic HD primates has opened a new era of animal modeling that better represents human genetic disorders such as HD, which will accelerate the development of diagnostic tools and identifying novel biomarkers through longitudinal studies including gene expression and metabolite profiling, and noninvasive imaging. Furthermore, novel treatments with predictable efficacy in human patients can be developed using HD monkeys because of comparable neuropathology and clinical features.

BDNF overexpression in the forebrain rescues Huntington's disease phenotypes in YAC128 mice

Huntington's disease (HD) is caused by an expansion of the polyglutamine tract at the N terminus of huntingtin. This mutation reduces levels of BDNF in the striatum, likely by inhibiting cortical Bdnf gene expression and anterograde transport of BDNF from the cerebral cortex to the striatum. Substantial evidence suggests that this reduction of striatal BDNF plays a crucial role in HD pathogenesis. Here we report that overexpression of BDNF in the forebrain rescues many disease phenotypes in YAC128 mice that express a full-length human huntingtin mutant with a 128-glutamine tract. The Bdnf transgene, under the control of the promoter for α subunit of Ca(2+)/calmodulin-dependent protein kinase II, greatly increased BDNF levels in the cerebral cortex and striatum. BDNF overexpression in YAC128 mice prevented loss and atrophy of striatal neurons and motor dysfunction, normalized expression of the striatal dopamine receptor D2 and enkephalin, and improved procedural learning. Furthermore, quantitative analyses of Golgi-impregnated neurons revealed a decreased spine density and abnormal spine morphology in striatal neurons of YAC128 mice, which was also reversed by increasing BDNF levels in the striatum. These results demonstrate that reduced striatal BDNF plays a crucial role in the HD pathogenesis and suggest that attempts to restore striatal BDNF level may have therapeutic effects to the disease.

Phosphorylation of huntingtin at Ser421 in YAC128 neurons is associated with protection of YAC128 neurons from NMDA-mediated excitotoxicity and is modulated by PP1 and PP2A

YAC transgenic mice expressing poly(Q)-expanded full-length huntingtin (mhtt) recapitulate many behavioral and neuropathological features of Huntington disease (HD). We have previously observed a reduction in phosphorylation of mhtt at S421 in the presence of the mutation for HD. In addition, phosphorylation of normal S421-htt is reduced after excitotoxic stimulation of NMDA receptors (NMDARs). To test whether NMDAR stimulation contributes to reduced pS421-htt levels in HD, we determined phosphorylation of htt at Ser421 after NMDA-induced excitotoxicity in neurons from YAC128 mice. Here, we report that the total level of pS421-htt is reduced in YAC128 primary neurons after excitotoxic NMDAR stimulation. Similarly, the total level of pS421-htt is reduced in YAC128 transgenic mice after quinolinic acid injection into the striatum. In contrast, loss of phosphorylation of pS421-htt is prevented in YAC mice that never develop clinical or neuropathological features of HD [the caspase 6-resistant YAC128 transgene (C6R)]. To gain insight into the mechanisms underlying these findings, we determined that the Ser/Thr protein phosphatases PP1 and PP2A dephosphorylate pS421-htt in situ and after excitotoxic stimulation of NMDARs in neurons. Furthermore, increasing the phosphorylation of htt at S421 by blocking PP1 and PP2A activity protects YAC128 striatal neurons from NMDA-induced cell death. These results, together with the observed modulation of pS421-htt levels by dopamine, the reduced expression of PP1 inhibitor Darpp-32 in the striatum of YAC128 mice, and the reduced phosphorylation of PP1 substrate CreB, point to altered regulation of phosphatase activity in HD and highlight enhancing phosphorylation of htt at S421 as a therapeutic target.

Palmitoylation and function of glial glutamate transporter-1 is reduced in the YAC128 mouse model of Huntington disease

Excitotoxicity plays a key role in the selective vulnerability of striatal neurons in Huntington disease (HD). Decreased glutamate uptake by glial cells could account for the excess glutamate at the synapse in patients as well as animal models of HD. The major molecule responsible for clearing glutamate at the synapses is glial glutamate transporter GLT-1. In this study, we show that GLT-1 is palmitoylated at cysteine38 (C38) and further, that this palmitoylation is drastically reduced in HD models both in vitro and in vivo. Palmitoylation is required for normal GLT-1 function. Blocking palmitoylation either with the general palmitoylation inhibitor, 2-bromopalmitate, or with a GLT-1 C38S mutation, severely impairs glutamate uptake activity. In addition, GLT-1-mediated glutamate uptake is indeed impaired in the YAC128 HD mouse brain, with the defect in the striatum evident as early as 3 months prior to obvious neuropathological findings, and in both striatum and cortex at 12 months. These phenotypes are not a result of changes in GLT1 protein expression, suggesting a crucial role of palmitoylation in GLT-1 function. Thus, it appears that impaired GLT-1 palmitoylation is present early in the pathogenesis of HD, and may influence decreased glutamate uptake, excitotoxicity, and ultimately, neuronal cell death in HD.

Longitudinal analysis of the behavioural phenotype in Hdh(CAG)150 Huntington's disease knock-in mice

In people with Huntington's disease, an expanded CAG repeat sequence on the HTT gene confers a toxic gain function resulting in a progressive and fatal neurodegeneration. The Hdh((CAG)Q150) Huntington's disease mouse line is a knock-in model of the disease that carries ∼150 CAG repeats on the normal mouse Htt locus. To determine that these mice are a useful model of the disease, they were assessed longitudinally for motor and cognitive deficits relevant to the human disease state. Each test was conducted bi-monthly across the lifespan of the animal. The results indicate that the Hdh(Q150/Q150) mice were impaired on each of the measures used, with deficits appearing on a 3-stage water maze test at 4 months of age and on prepulse inhibition at 6 months of age, both of which were prior to the manifestation of motor abnormalities. Grip strength, as measured by the inverted cage lid test, was reduced in the Hdh(Q150/Q150) mice from 10 months of age, when the male mice also exhibited weight loss relative to their wildtype littermates. On the accelerating rotarod, deficits in the carrier mice did not appear until they were 21 months old. Our results demonstrate that the Hdh((CAG)150) is a valid model of HD that displays early and progressive cognitive deficits that precede the onset of motor abnormalities.

Striatal expression of a calmodulin fragment improved motor function, weight loss, and neuropathology in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder, caused by a polyglutamine expansion in the huntingtin protein (htt). Increasing evidence suggests that transglutaminase (TGase) plays a critical role in the pathophysiology of HD possibly by stabilizing monomeric, polymeric and aggregated htt. We previously reported that in HEK293 and SH-SY5Y cells expression of a calmodulin (CaM)-fragment, consisting of amino acids 76-121 of CaM, decreased binding of CaM to mutant htt, TGase-modified htt and cytotoxicity associated with mutant htt and normalized intracellular calcium release. In this study, an adeno-associated virus (AAV) that expresses the CaM-fragment was injected into the striatum of HD transgenic R6/2 mice. The CaM-fragment significantly reduced body weight loss and improved motor function as indicated by improved rotarod performance, longer stride length, lower stride frequency, fewer low mobility bouts and longer travel distance than HD controls. A small but insignificant increase in survival was observed in R6/2 mice with CaM-fragment expression. Immunoprecipitation studies show that expression of the CaM-fragment reduced TGase-modified htt in the striatum of R6/2 mice. The percentage of htt-positive nuclei and the size of intranuclear htt aggregates were reduced by the CaM-fragment without striatal volume changes. The effects of CaM-fragment appear to be selective, as activity of another CaM-dependent enzyme, CaM-dependent kinase II, was not altered. Moreover, inhibition of TGase-modified htt was substrate-specific since overall TGase activity in the striatum was not altered by treatment with the CaM-fragment. Together, these results suggest that disrupting CaM-htt interaction may provide a new therapeutic strategy for HD.

Huntington's disease: the case for genetic modifiers

For almost three decades, Huntington's disease has been a prototype for the application of genetic strategies to human disease. HD, the Huntington's disease gene, was the first autosomal defect mapped using only DNA markers, a finding in 1983 that helped to spur similar studies in many other disorders and contributed to the concept of the human genome project. The search for the genetic defect itself pioneered many mapping and gene-finding technologies, and culminated in the identification of the HD gene, its mutation and its novel protein product in 1993. Since that time, extensive investigations into the pathogenic mechanism have utilized the knowledge of the disease gene and its defect but, with notable exceptions, have rarely relied for guidance on the genetic findings in human patients to interpret the relevance of findings in non-human model systems. However, the human patient still has much to teach us through a detailed analysis of genotype and phenotype. Such studies have implicated the existence of genetic modifiers - genes whose natural polymorphic variation contributes to altering the development of Huntington's disease symptoms. The search for these modifiers, much as the search for the HD gene did in the past, offers to open new entrées into the process of Huntington's disease pathogenesis by unlocking the biochemical changes that occur many years before diagnosis, and thereby providing validated target proteins and pathways for development of rational therapeutic interventions.

Systematic behavioral evaluation of Huntington's disease transgenic and knock-in mouse models

Huntington's disease (HD) is one of the few neurodegenerative diseases with a known genetic cause, knowledge that has enabled the creation of animal models using genetic manipulations that aim to recapitulate HD pathology. The study of behavioral and neuropathological phenotypes of these HD models, however, has been plagued by inconsistent results across laboratories stemming from the lack of standardized husbandry and testing conditions, in addition to the intrinsic differences between the models. We have compared different HD models using standardized conditions to identify the most robust phenotypic differences, best suited for preclinical therapeutic efficacy studies. With a battery of tests of sensory-motor function, such as the open field and prepulse inhibition tests, we replicate previous results showing a strong and progressive behavioral deficit in the R6/2 line with an average of 129 CAG repeats in a mixed CBA/J and C57BL/6J background. We present the first behavioral characterization of a new model, an R6/2 line with an average of 248 CAG repeats in a pure C57BL/6J background, which also showed a progressive and robust phenotype. The BACHD in a FVB/N background showed robust and progressive behavioral phenotype, while the YAC128 full-length model on either an FVB/N or a C57BL/6J background generally showed milder deficits. Finally, the Hdh(Q111) knock-in mouse on a CD1 background showed very mild deficits. This first extensive standardized cross-characterization of several HD animal models under standardized conditions highlights several behavioral outcomes, such as hypoactivity, amenable to standardized preclinical therapeutic drug screening.

Partial ablation of mu-opioid receptor rich striosomes produces deficits on a motor-skill learning task

Basal ganglia striosomes, or patches, are rich in mu opioid receptors (MOR) and form a three-dimensional labyrinth of cells that extend throughout the mid- and anterior striatum in mice. Though previous studies have suggested that striosomes could affect drug-induced motor output in rodents, the functional role of these compartmentalized MOR-rich striosomes is not well understood. To investigate any relationship between the striosomes and motor behavior we used the toxin dermorphin-saporin (DS) to selectively ablate MOR-rich striosomal cells. FVB mice were bilaterally infused with DS in the midstriatum alone or in the mid- and anterior striatum, and were tested on three motor tasks and in an open field. Two volume measurement procedures and stereological cell counts were used to confirm the induced pathology. Mice that received DS injections showed significantly smaller volumes (-26% to -44%) and fewer cells (-30% to -49%) in the striosome compartment compared to mice that received control injections of saline or saporin. Striosome pathology was greatest in the dorsolateral striatum. The extrastriosomal matrix was not significantly affected, resulting in an imbalance in the ratio of striosome-to-matrix cells. Behaviorally, toxin injections caused deficits on an accelerating rotarod task and the deficit was worse in mice that received mid and anterior injections than those that received midstriatal injections alone. However, DS-injected mice did not differ from control mice on other motor tasks. We conclude that the MOR-rich cells of the striosomes are necessary for optimal rotarod performance, including learning and/or improvement on the task.

Mitochondrial structural and functional dynamics in Huntington's disease

Huntington's disease (HD) is an autosomal, dominantly inherited neurodegenerative disorder, characterized by chorea, involuntary movements, and cognitive impairments. Tremendous progress has been made since the discovery of HD gene in 1993, in terms of developing animal models to study the disease process, unraveling the expression and function of wild-type and mutant huntingtin (Htt) proteins in the central and peripheral nervous systems, and understanding expanded CAG repeat containing mutant Htt protein interactions with CNS proteins in the disease process. HD progression has been found to involve several pathomechanisms, including expanded CAG repeat protein interaction with other CNS proteins, transcriptional dysregulation, calcium dyshomeostasis, abnormal vesicle trafficking, and defective mitochondrial bioenergetics. Recent studies have found that mutant Htt is associated with mitochondria and causes mitochondrial structural changes, decreases mitochondrial trafficking, and impairs mitochondrial dynamics in the neurons affected by HD. This article discusses recent developments in HD research, with a particular focus on intracellular and intramitochondrial calcium influx, mitochondrial DNA defects, and mitochondrial structural and functional abnormalities in HD development and progression. Further, this article outlines the current status of mitochondrial therapeutics with a special reference to Dimebon.

Force-plate quantification of progressive behavioral deficits in the R6/2 mouse model of Huntington's disease

The R6/2 mouse is a popular model of Huntington's disease (HD) because of its rapid progression and measurable behavioral phenotype. Yet current behavioral phenotyping methods are usually univariate (e.g., latency to fall from a rotarod) and labor intensive. We used a force-plate actometer and specialized computer algorithms to partition the data into topographically specific behavioral categories that were sensitive to HD-like abnormalities. Seven R6/2 male mice and 7 wild-type (WT) controls were placed in a 42 cm x 42 cm force-plate actometer for 20-min recording sessions at 6-7, 8-9, 10-11 and 12-13 weeks of age. Distance traveled, number of wall rears, and number of straight runs (traveling 175 mm or more in 1.5s) were reduced in R6/2 relative to WT mice at all ages tested. Low mobility bouts (each defined as remaining continuously in a virtual circle of 15 mm radius for 5s) were increased in R6/2 mice at 6-7 weeks and beyond. Independent of body weight, force off-load during wall rears was reduced in R6/2 mice except at 6-7 weeks. Power spectra of force variation during straight runs indicated an age-related progressive loss of rhythmicity in R6/2 compared to WT, suggesting gait dysrhythmia and dysmetria. Collectively, these data, which extend results obtained with other widely different behavioral phenotyping methods, document a multifaceted syndrome of motor abnormalities in R6/2 mice. We suggest, moreover, that the force-plate actometer offers a high-throughput tool for screening drugs that may affect symptom expression in R6/2 or other HD model mice.

Neuroprotective effects of inositol 1,4,5-trisphosphate receptor C-terminal fragment in a Huntington's disease mouse model

Huntington's disease (HD) is a dominantly inherited, progressive neurodegenerative disease caused by an expanded polyglutamine tract in huntingtin protein (Htt). Medium spiny striatal neurons (MSNs) are primarily affected in HD. Mutant huntingtin protein (Htt(exp)) specifically binds to and activates type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1), an intracellular Ca(2+) release channel. Htt(exp)-InsP(3)R1 association is mediated by a cytosolic C-terminal tail of InsP(3)R1 (a 122-aa-long IC10 fragment). To evaluate an importance of Htt(exp) association with InsP(3)R1 for HD pathology, we generated lentiviral and adeno-associated viruses expressing GFP-IC10 fusion protein and performed a series of experiments with YAC128 HD transgenic mouse. Infection with Lenti-GFP-IC10 virus stabilized Ca(2+) signaling in cultured YAC128 MSNs and protected YAC128 MSNs from glutamate-induced apoptosis. Intrastriatal injections of AAV1-GFP-IC10 significantly alleviated motor deficits and reduced MSN loss and shrinkage in YAC128 mice. Our results demonstrate an importance of InsP(3)R1-Htt(exp) association for HD pathogenesis and suggested that InsP(3)R1 is a potential therapeutic target for HD. Our data also support potential use of IC10 peptide as a novel HD therapeutic agent.

Striosome-matrix pathology and motor deficits in the YAC128 mouse model of Huntington's disease

Huntington's disease is characterized by striatal degeneration and progressive motor deficits. To examine striatal compartment-specific pathology and its relation to motor symptoms, we used immunohistochemistry to identify and measure the striosomes and matrix of 7-13-month-old YAC128 and wild type (WT) mice that were previously tested on motor tasks. Compared to WTs, 13-month-old YAC128s showed volume shrinkage in striosomes, and cell loss in both compartments. The percent cell loss was greater in striosomes than matrix. Striosome volume and cell loss was greatest in the dorsolateral striatum. YAC128 rotarod and balance beam deficits preceded volume and cell loss. At 13 months, YAC128 balance beam slips and striosome cell number were inversely correlated. The results show that pathology in older YAC128s manifests as an abnormal striosome to matrix ratio and suggest that this imbalance can contribute to some motor symptoms.

Extensive early motor and non-motor behavioral deficits are followed by striatal neuronal loss in knock-in Huntington's disease mice

Huntington's disease is a neurodegenerative disorder, caused by an elongation of CAG repeats in the huntingtin gene. Mice with an insertion of an expanded polyglutamine repeat in the mouse huntingtin gene (knock-in mice) most closely model the disease because the mutation is expressed in the proper genomic and protein context. However, few knock-in mouse lines have been extensively characterized and available data suggest marked differences in the extent and time course of their behavioral and pathological phenotype. We have previously described behavioral anomalies in the open field as early as 1 month of age, followed by the appearance at 2 months of progressive huntingtin neuropathology, in a mouse carrying a portion of human exon 1 with approximately 140 CAG repeats inserted into the mouse huntingtin gene. Here we extend these observations by showing that early behavioral anomalies exist in a wide range of motor (climbing, vertical pole, rotarod, and running wheel performance) and non-motor functions (fear conditioning and anxiety) starting at 1-4 months of age, and are followed by progressive gliosis and decrease in dopamine and cyclic AMP-regulated phosphoprotein with molecular weight 32 kDa (DARPP32) (12 months) and a loss of striatal neurons at 2 years. At this age, mice also present striking spontaneous behavioral deficits in their home cage. The data show that this line of knock-in mice reproduces canonical characteristics of Huntington's disease, preceded by deficits which may correspond to the protracted pre-manifest phase of the disease in humans. Accordingly, they provide a useful model to elucidate early mechanisms of pathophysiology and the progression to overt neurodegeneration.

Neocortical expression of mutant huntingtin is not required for alterations in striatal gene expression or motor dysfunction in a transgenic mouse

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disease caused by an expanded polyglutamine tract in the ubiquitously expressed huntingtin protein. Clinically, HD is characterized by motor, cognitive and psychiatric deficits. Striking degeneration of the striatum is observed in HD with the medium spiny neurons (MSNs) being the most severely affected neuronal subtype. Dysfunction of MSNs is marked by characteristic changes in gene expression which precede neuronal death. The ubiquitous expression of the huntingtin protein raises the question as to whether the selective vulnerability of the MSN is cell-autonomous, non-cell-autonomous, or a combination thereof. In particular, growing evidence suggests that abnormalities of the cortex and corticostriatal projections may be primary causes of striatal vulnerability. To examine this issue, we developed transgenic mice that, within the forebrain, selectively express a pathogenic huntingtin species in the MSNs, specifically excluding the neocortex. These mice develop a number of abnormalities characteristic of pan-cellular HD mouse models, including intranuclear inclusion bodies, motor impairment, and changes in striatal gene expression. As this phenotype develops in the presence of normal levels of brain-derived neurotrophic factor and its major striatal receptor, tropomyosin-related kinase B, these data represent the first demonstration of in vivo cell-autonomous transcriptional dysregulation in an HD mouse model. Furthermore, our findings suggest that therapies targeted directly to the striatum may be efficacious at reversing some of the molecular abnormalities present in HD.

Longitudinal evaluation of the Hdh(CAG)150 knock-in murine model of Huntington's disease

Several murine genetic models of Huntington's disease (HD) have been developed. Murine genetic models are crucial for identifying mechanisms of neurodegeneration in HD and for preclinical evaluation of possible therapies for HD. Longitudinal analysis of mutant phenotypes is necessary to validate models and to identify appropriate periods for analysis of early events in the pathogenesis of neurodegeneration. Here we report longitudinal characterization of the murine Hdh(CAG)150 knock-in model of HD. A series of behavioral tests at five different time points (20, 40, 50, 70, and 100 weeks) demonstrates an age-dependent, late-onset behavioral phenotype with significant motor abnormalities at 70 and 100 weeks of age. Pathological analysis demonstrated loss of striatal dopamine D1 and D2 receptor binding sites at 70 and 100 weeks of age, and stereological analysis showed significant loss of striatal neuron number at 100 weeks. Late-onset behavioral abnormalities, decrease in striatal dopamine receptors, and diminished striatal neuron number observed in this mouse model recapitulate key features of HD. The Hdh(CAG)150 knock-in mouse is a valid model to evaluate early events in the pathogenesis of neurodegeneration in HD.

Dopaminergic signaling and striatal neurodegeneration in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by polyglutamine (polyQ) expansion in Huntingtin protein (Htt). PolyQ expansion in Htt(exp) causes selective degeneration of striatal medium spiny neurons (MSNs) in HD patients. Striatal MSN neurons receive glutamatergic input from the cortex and dopaminergic input from the substantia nigra. In previous studies with the yeast artificial chromosome (YAC128) transgenic HD mouse model, we established a connection between glutamate receptor activation, disturbed calcium (Ca2+) signaling, and apoptosis of HD MSNs (Tang et al., 2005). Here, we used the same YAC128 mouse model to investigate the role of dopaminergic signaling in HD. We discovered that glutamate and dopamine signaling pathways act synergistically to induce elevated Ca2+ signals and to cause apoptosis of YAC128 MSNs in vitro. We demonstrated that potentiating effects of dopamine are mediated by D1-class dopamine receptors (DARs) and not by D2-class DARs. Consistent with in vitro findings, in whole-animal experiments we found that persistent elevation of striatal dopamine levels exacerbated the behavioral motor deficits and MSN neurodegeneration in YAC128 mice. We further discovered that the clinically relevant dopamine pathway inhibitor tetrabenazine alleviated the motor deficits and reduced striatal cell loss in YAC128 mice. Our results suggest that dopamine signaling pathway plays an important role in HD pathogenesis and that antagonists of dopamine pathway such as tetrabenazine or dopamine receptor blockers may have a therapeutic potential for treatment of HD beyond well established "symptomatic" benefit.