Selective striatal neuronal loss in a YAC128 mouse model of Huntington disease

An expanded CAG repeat is the underlying genetic defect in Huntington disease, a disorder characterized by motor, psychiatric and cognitive deficits and striatal atrophy associated with neuronal loss. An accurate animal model of this disease is crucial for elucidation of the underlying natural history of the illness and also for testing experimental therapeutics. We established a new yeast artificial chromosome (YAC) mouse model of HD with the entire human HD gene containing 128 CAG repeats (YAC128) which develops motor abnormalities and age-dependent brain atrophy including cortical and striatal atrophy associated with striatal neuronal loss. YAC128 mice exhibit initial hyperactivity, followed by the onset of a motor deficit and finally hypokinesis. The motor deficit in the YAC128 mice is highly correlated with striatal neuronal loss, providing a structural correlate for the behavioral changes. The natural history of HD-related changes in the YAC128 mice has been defined, demonstrating the presence of huntingtin inclusions after the onset of behavior and neuropathological changes. The HD-related phenotypes of the YAC128 mice show phenotypic uniformity with low inter-animal variability present, which together with the age-dependent striatal neurodegeneration make it an ideal mouse model for the assessment of neuroprotective and other therapeutic interventions.     

Behavioral abnormalities precede neuropathological markers in rats transgenic for Huntington's disease

Huntington's disease (HD) is caused by an expanded CAG repeatleading to the synthesis of an aberrant protein and to the formationof polyglutamine (polyQ)-containing inclusions and aggregates.Limited information is available concerning the associationof neuropathological markers with the development of behavioralmarkers in HD. Using a previously generated transgenic rat modelof HD (tgHD rat), we performed association studies on the time-courseof behavioral symptoms (motor function, learning, anxiety) andthe appearance of striatal atrophy, 1C2 immunopositive aggregatesand polyQ recruitment sites, a precursor to these aggregates.At the age of 1 month, tgHD rats exhibited reduced anxiety andimproved motor performance, while at 6 months motor impairmentsand at 9 months cognitive decline occurred. In contrast, polyQrecruitment sites appeared at around 6–9 months of age,indicating that HD-like behavioral markers preceded the appearanceof currently detectable neuropathological markers. Interestingly,numerous punctate sites containing polyQ aggregates were alsoseen in areas receiving afferents from the densely recruitingregions suggesting either transport of recruitment-competentaggregates to terminal projections where initially 1C2 positiveaggregates were formed or different internal properties of neuronsin different regions. Furthermore, striatal atrophy was observedat the age of 12 months. Taken together, our findings supportthe hypothesis of a dynamic process leading to region- and age-specificpolyQ recruitment and aggregation. The dissociation of onsetbetween behavioral and neuropathological markers is suggestiveof as yet undetected processes, which contribute to the earlyphenotype of these HD transgenic rats.     

Significantly differential diffusion of neuropathological aggregates in the brain of transgenic mice carrying N-terminal mutant huntingtin fused with green fluorescent protein

Huntington’s disease (HD) is a genetically neurodegenerative disease, affecting the central nervous system and leading to mental and motor dysfunctions. To date, there is no cure for HD; as a result, HD patients gradually suffer devastating symptoms, such as chorea, weight loss, depression and mood swings, until death. According to previous studies, the exon 1 region of the huntingtin (HTT) gene with expanded CAG trinucleotide repeats plays a critical role in causing HD. In vitro studies using exon 1 of HTT fused with green fluorescent protein (GFP) gene have facilitated discovering several mechanisms of HD. However, whether this chimera construct exerts similar functions in vivo is still not clear. Here, we report the generation of transgenic mice carrying GFP fused with mutant HTT exon 1 containing 84 CAG trinucleotide repeats, and the evaluation of phenotypes via molecular, neuropathological and behavioral analyses. Results show that these transgenic mice not only displayed neuropathological characteristics, observed either by green fluorescent signals or by immunohistochemical staining, but also progressively developed pathological and behavioral symptoms of HD. Most interestingly, these transgenic mice showed significantly differential expression levels of nuclear aggregates between cortex and striatum regions, highly mimicking selective expression of mutant HTT in HD patients. To the best of our knowledge, this is the first report showing different nuclear diffusion profiling in mouse models with transgenic mice carrying the exon 1 region of mutant HTT. Our model will be beneficial for tracing the expression of mutant HTT and accelerating the understanding of selective pathological progression in HD.     

Cholesterol biosynthesis pathway is disturbed in YAC128 mice and is modulated by huntingtin mutation

Our recent analyses of the cholesterol biosynthetic pathway in Huntington's disease (HD) cells, in the R6/2 huntingtin-fragment mouse model of HD as well as in human tissues have provided the first evidence of altered activity of this pathway in genetically identifiable HD samples. Here we report that these changes also occur in the full-length-huntingtin YAC128 (yeast artificial chromosome) mouse model, which shows a consistent reduction in the activity or levels of multiple components of the cholesterogenic pathway. We also show that this phenotype is progressive and is specific for the brain region most affected in HD. Mice over-expressing the wild-type protein with 18 CAG (YAC18 mice) show the opposite phenotype with higher activity of the cholesterol biosynthetic pathway compared with littermate mice. Finally, we report that plasma levels of cholesterol, its precursors and its brain-derived catabolite 24-S-hydroxycholesterol in YAC mice mirror brain biosynthetic levels supporting further investigation of their potential as peripheral biomarkers in HD.     

Striatal and nigral pathology in a lentiviral rat model of Machado-Joseph disease

Machado-Joseph disease (MJD) is a fatal, dominant neurodegenerative disorder. MJD results from polyglutamine repeat expansion in the MJD-1 gene, conferring a toxic gain of function to the ataxin-3 protein. In this study, we aimed at overexpressing ataxin-3 in the rat brain using lentiviral vectors (LV), to generate an in vivo MJD genetic model and, to study the disorder in defined brain regions: substantia nigra, an area affected in MJD, cortex and striatum, regions not previously reported to be affected in MJD. LV encoding mutant or wild-type human ataxin-3 was injected in the brain of adult rats and the animals were tested for behavioral deficits and neuropathological abnormalities. Striatal pathology was confirmed in transgenic mice and human tissue. In substantia nigra, unilateral overexpression of mutant ataxin-3 led to: apomorphine-induced turning behavior; formation of ubiquitinated ataxin-3 aggregates; alpha-synuclein immunoreactivity; and loss of dopaminergic markers (TH and VMAT2). No neuropathological changes were observed upon wild-type ataxin-3 overexpression. Mutant ataxin-3 expression in striatum and cortex, resulted in accumulation of misfolded ataxin-3, and within striatum, loss of neuronal markers. Striatal pathology was confirmed by observation in MJD transgenic mice of ataxin-3 aggregates and substantial reduction of DARPP-32 immunoreactivity and, in human striata, by ataxin-3 inclusions, immunoreactive for ubiquitin and alpha-synuclein. This study demonstrates the use of LV encoding mutant ataxin-3 to produce a model of MJD and brings evidence of striatal pathology, suggesting that this region may contribute to dystonia and chorea observed in some MJD patients and may represent a target for therapies.     

Corticostriatal synaptic function in mouse models of Huntington's disease: early effects of huntingtin repeat length and protein load

Huntington's disease (HD) is an autosomal dominant, late onset, neurodegenerative disease characterized by motor deficits and dementia that is caused by expansion of a CAG repeat in the HD gene. Clinical manifestations result from selective neuronal degeneration of predominantly GABAergic striatal medium-sized spiny neurons (MSNs). A growing number of studies demonstrate that personality, mood and cognitive disturbances are some of the earliest signs of HD and may reflect synaptic dysfunction prior to neuronal loss. Previous studies in striatal MSNs demonstrated early alterations in NMDA-type glutamate receptor currents in several HD mouse models, as well as evidence for presynaptic dysfunction prior to disease manifestations in the R6/2 HD fragment mouse model. We have compared corticostriatal synaptic function in full-length, human HD gene-carrying YAC transgenic mice expressing a non-pathogenic CAG repeat (YAC18; control) with three increasingly severe variants of pathogenic HD gene-expressing mice (YAC72 and two different lines of YAC128), at ages that precede any detectable disease phenotype. We report presynaptic dysfunction and a propensity towards synaptic depression in YAC72 and YAC128 compared to YAC18 mice, and, in the most severe model, we also observed altered AMPA receptor function. When normalized to evoked AMPAR currents, postsynaptic NMDAR currents are augmented in all three pathogenic HD YAC variants. These findings demonstrate multiple perturbations to corticostriatal synaptic function in HD mice, furthering our understanding of the early effects of the HD mutation that may contribute to cognitive dysfunction, mood disorders and later development of more serious dysfunction. Furthermore, this study provides a set of neurophysiological sequelae against which to test and compare other mouse models and potential therapies in HD.     

In vivo dopamine release and uptake impairments in rats treated with 3-nitropropionic acid

Recent evidence has suggested that mitochondrial dysfunction may lead to impaired neurotransmitter exocytosis in transgenic Huntington's disease (HD) model mice. To gain insight into the impact of mitochondrial impairment on striatal dopamine release in vivo, we used fast-scan cyclic voltammetry (FSCV) at carbon fiber microelectrodes to measure dopamine release and uptake kinetics in anesthetized Lewis rats continuously treated for 5 days with 3-nitropropionic acid (3NP). Our results indicate that, even though striatal dopamine content was unchanged, remotely stimulated dopamine release evoked per electrical stimulus pulse ([DA]_p) is decreased in 3NP-treated rats (33% of that observed in sham control rats) and that this decrease is uniform throughout all stereotaxic depths tested. Nevertheless, unlike data collected previously from transgenic HD model rodents, the maximum rate of dopamine uptake (V_max) in 3NP-treated rats is diminished (30% of controls) while K_m is unchanged. Treatment with 3NP also resulted in a corresponding decrease in locomotor activity, presumably due in part to the impaired dopamine release. These results indicate that dopamine release is degraded in this HD model, as is observed in transgenic HD model rodents; however, the results also imply that there are fundamental differences in dopamine uptake between 3NP-treated animals and transgenic animals.     

Compensatory changes in the ubiquitin-proteasome system, brain-derived neurotrophic factor and mitochondrial complex II/III in YAC72 and R6/2 transgenic mice partially model Huntington's disease patients

Intraneuronal protein aggregates of the mutated huntingtin in Huntington's disease (HD) brains suggest an overload and/or dysfunction of the ubiquitin-proteasome system (UPS). There is a general inhibition of the UPS in many brain regions (cerebellum, cortex, substantia nigra and caudate-putamen) and skin fibroblasts from HD patients. In the current experiment, the widely used mutant huntingtin-exon 1 CAG repeat HD transgenic mice model (R6/2) (with 144 CAG repeat and exon 1) during late-stage pathology, had increases in proteasome activity in the striatum. However, this discrepancy with HD patient tissue was not apparent in the mutant CAG repeat huntingtin full-length HD (YAC72) transgenic mouse model during post-symptomatic and late-stage pathology, which then also showed UPS inhibition similar to HD patients' brains. In both types of HD model mice, we determined biochemical changes, including expression of brain-derived neurotrophic factor (BDNF) and mitochondrial complex II/III (MCII/III) activities related to HD pathology. We found increases of both BDNF expression, and MCII/III activities in YAC72 transgenic mice, and no change of BDNF expression in R6/2 mice. Our data show that extreme CAG repeat lengths in R6/2 mice is paradoxically associated with increased proteasome activity, probably as a cellular compensatory biochemical change in response to the underlying mutation. Changes in HD patients for UPS function, BDNF expression and MCII/III activity are only partially modeled in R6/2 and YAC72 mice, with the latter at 16 months of age being most congruent with the human disease.     

Transgenic rat model of Huntington's disease

Huntington's disease (HD) is a late manifesting neurodegenerative disorder in humans caused by an expansion of a CAG trinucleotide repeat of more than 39 units in a gene of unknown function. Several mouse models have been reported which show rapid progression of a phenotype leading to death within 3-5 months (transgenic models) resembling the rare juvenile course of HD (Westphal variant) or which do not present with any symptoms (knock-in mice). Owing to the small size of the brain, mice are not suitable for repetitive in vivo imaging studies. Also, rapid progression of the disease in the transgenic models limits their usefulness for neurotransplantation. We therefore generated a rat model transgenic of HD, which carries a truncated huntingtin cDNA fragment with 51 CAG repeats under control of the native rat huntingtin promoter. This is the first transgenic rat model of a neurodegenerative disorder of the brain. These rats exhibit adult-onset neurological phenotypes with reduced anxiety, cognitive impairments, and slowly progressive motor dysfunction as well as typical histopathological alterations in the form of neuronal nuclear inclusions in the brain. As in HD patients, in vivo imaging demonstrates striatal shrinkage in magnetic resonance images and a reduced brain glucose metabolism in high-resolution fluor-deoxy-glucose positron emission tomography studies. This model allows longitudinal in vivo imaging studies and is therefore ideally suited for the evaluation of novel therapeutic approaches such as neurotransplantation.     

Striatal potassium channel dysfunction in Huntington's disease transgenic mice

Huntington's disease (HD) is a neurodegenerative disorder that mainly affects the projection neurons of the striatum and cerebral cortex. Genetic mouse models of HD have shown that neurons susceptible to the mutation exhibit morphological and electrophysiological dysfunctions before and during development of the behavioral phenotype. We used HD transgenic mouse models to examine inwardly and outwardly rectifying K+ conductances, as well as expression of some related K+ channel subunits. Experiments were conducted in slices and dissociated cells from two mouse models, the R6/2 and TgCAG100, at the beginning and after full development of overt behavioral phenotypes. Striatal medium-sized spiny neurons (MSNs) from symptomatic transgenic mice had increased input resistances, depolarized resting membrane potentials, and reductions in both inwardly and outwardly rectifying K+ currents. These changes were more dramatic in the R6/2 model than in the TgCAG100. Parallel immunofluorescence studies detected decreases in the expression of K+ channel subunit proteins, Kir2.1, Kir2.3, and Kv2.1 in MSNs, which contribute to the formation of the channel ionophores for these currents. Attenuation in K+ conductances and channel subunit expression contribute to altered electrophysiological properties of MSNs and may partially account for selective cellular vulnerability in the striatum.     

Reactions during hemodialysis caused by allergy to ethylene oxide gas sterilization

In patients receiving long-term hemodialysis (HD), we have examined the presence of IgE-dependent sensitization to ethylene oxide (EO) gas, which is used for sterilization of disposable medical products including dialyzers. Serum was obtained from 25 patients who experienced acute allergic reactions during HD, five patients receiving HD with isolated eosinophilia, and 37 unselected patients receiving HD. Sera from 22 of 25 of the allergic reaction group and from five of 35 of the unselected group were demonstrated to contain IgE antibodies with specificity for EO. Corresponding IgG antibodies were also present. No such antibodies were detected in serum from normal controls or ragweed-allergic patients. The serum from one patient with isolated eosinophilia had a borderline elevated IgE antibody level. These results demonstrate a close relationship between the presence of IgE antibodies to EO and HD-related allergic reactions in this patient population.     

Altered palmitoylation and neuropathological deficits in mice lacking HIP14

Huntingtin interacting protein 14 (HIP14, ZDHHC17) is a huntingtin (HTT) interacting protein with palmitoyl transferase activity. In order to interrogate the function of Hip14, we generated mice with disruption in their Hip14 gene. Hip14-/- mice displayed behavioral, biochemical and neuropathological defects that are reminiscent of Huntington disease (HD). Palmitoylation of other HIP14 substrates, but not Htt, was reduced in the Hip14-/- mice. Hip14 is dysfunctional in the presence of mutant htt in the YAC128 mouse model of HD, suggesting that altered palmitoylation mediated by HIP14 may contribute to HD.     

Magnetic resonance imaging and spectroscopy in assessing 3-nitropropionic acid-induced brain lesions: an animal model of Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disease, in which there is progressive motor and cognitive deterioration, and for which the pathogenesis of neuronal death remains controversial. Mitochondrial toxins like 3-nitropropionic acid (3-NP) and malonate, functioning as the inhibitors of the complex II of mitochondrial respiratory chain, have been found to effectively induce specific behavioral changes and selective striatal lesions in rats and non-human primates mimicking those in HD. Furthermore, several kinds of transgenic mouse models of HD have been recently developed, and used in the development and assessment of novel treatments for HD. In the past, most studies evaluating the animal models for HD were based on histological changes or in vitro neuronal cultures. With the emergence of advanced magnetic resonance technologies, non-invasive magnetic resonance imaging (MRI) and spectroscopy provide more detail of cerebral alterations, including the changes of cerebral structure, function and metabolites. These studies support the hypothesis that mitochondrial dysfunction with increased excitation of N-methyl-D-aspartate (NMDA) receptors can replicate the neurobehavioral changes, selective brain injury and neurochemical alterations in HD. The present review focuses on our work as well as that of others regarding 3-NP-induced neurotoxicity and other animal models of HD. Using both conventional and advanced MRI and spectroscopy, we summarize the pathogenesis and possible therapeutic strategies in chemical and transgenic models of HD. The results show magnetic resonance techniques to be powerful techniques in the evaluation of pathogenesis and therapeutic intervention for both chemical and transgenic models of HD.     

Huntington's Disease (HD): Neurodegeneration of Brodmann's Primary Visual Area 17 (BA17)

Huntington's disease (HD), an autosomal dominantly inherited polyglutamine or CAG repeat disease along with somatomotor, oculomotor, psychiatric and cognitive symptoms, presents clinically with impairments of elementary and complex visual functions as well as altered visual‐evoked potentials (VEPs). Previous volumetric and pathoanatomical post‐mortem investigations pointed to an involvement of Brodmann's primary visual area 17 (BA17) in HD. Because the involvement of BA17 could be interpreted as an early onset brain neurodegeneration, we further characterized this potential primary cortical site of HD‐related neurodegeneration neuropathologically and performed an unbiased estimation of the absolute nerve cell number in thick gallocyanin‐stained frontoparallel tissue sections through the striate area of seven control individuals and seven HD patients using Cavalieri's principle for volume and the optical disector for nerve and glial cell density estimations. This investigation showed a reduction of the estimated absolute nerve cell number of BA17 in the HD patients (71 044 037 ± 12 740 515 nerve cells) of 32% in comparison with the control individuals (104 075 067 ± 9 424 491 nerve cells) (Mann–Whitney U‐test; P < 0.001). Additional pathoanatomical studies showed that nerve cell loss was most prominent in the outer pyramidal layer III, the inner granular layers IVa and IVc as well as in the multiform layer VI of BA17 of the HD patients. Our neuropathological results in BA17 confirm and extend previous post‐mortem, biochemical and in vivo neuroradiological HD findings and offer suitable explanations for the elementary and complex visual dysfunctions, as well as for the altered VEP observed in HD patients.