Full-Length Human Mutant Huntingtin with a Stable Polyglutamine Repeat Can Elicit Progressive and Selective Neuropathogenesis in BACHD Mice

Center for Neurobehavioral Genetics, Semel Institute for Neuroscience and Human Behavior, University of California, Los Angeles, Los Angeles, California 90095, USA.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 07/2008; 28(24):6182-95. DOI: 10.1523/JNEUROSCI.0857-08.2008
Source: PubMed


To elucidate the pathogenic mechanisms in Huntington's disease (HD) elicited by expression of full-length human mutant huntingtin (fl-mhtt), a bacterial artificial chromosome (BAC)-mediated transgenic mouse model (BACHD) was developed expressing fl-mhtt with 97 glutamine repeats under the control of endogenous htt regulatory machinery on the BAC. BACHD mice exhibit progressive motor deficits, neuronal synaptic dysfunction, and late-onset selective neuropathology, which includes significant cortical and striatal atrophy and striatal dark neuron degeneration. Power analyses reveal the robustness of the behavioral and neuropathological phenotypes, suggesting BACHD as a suitable fl-mhtt mouse model for preclinical studies. Additional analyses of BACHD mice provide novel insights into how mhtt may elicit neuropathogenesis. First, unlike previous fl-mhtt mouse models, BACHD mice reveal that the slowly progressive and selective pathogenic process in HD mouse brains can occur without early and diffuse nuclear accumulation of aggregated mhtt (i.e., as detected by immunostaining with the EM48 antibody). Instead, a relatively steady-state level of predominantly full-length mhtt and a small amount of mhtt N-terminal fragments are sufficient to elicit the disease process. Second, the polyglutamine repeat within fl-mhtt in BACHD mice is encoded by a mixed CAA-CAG repeat, which is stable in both the germline and somatic tissues including the cortex and striatum at the onset of neuropathology. Therefore, our results suggest that somatic repeat instability does not play a necessary role in selective neuropathogenesis in BACHD mice. In summary, the BACHD model constitutes a novel and robust in vivo paradigm for the investigation of HD pathogenesis and treatment.

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    • "One of the first transgenic mouse models of HD, the R6/2 fragment model, carries exon 1 of the mutant gene and produces an aggressive phenotype more akin to the juvenile form of HD (Mangiarini et al. 1996). The most widely studied full-length mouse models express human Htt in yeast or bacterial artificial chromosomes (YAC and BAC, respectively) (Gray et al. 2008; Hodgson et al. 1999; Slow et al. 2003). These models tend to display a more protracted and less severe phenotype. "
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    ABSTRACT: The Q175 knock-in mouse model of Huntington's disease (HD) carries a CAG trinucleotide expansion of the human mutant huntingtin allele in its native mouse genomic context and recapitulates the genotype more closely than transgenic models. In this study we examined the progression of changes in intrinsic membrane properties and excitatory and inhibitory synaptic transmission using whole-cell patch clamp recordings of medium-sized spiny neurons (MSNs) in the dorsolateral striatum and cortical pyramidal neurons (CPNs) in layers 2/3 of the primary motor cortex in brain slices from heterozygous (Q175(+/-)) and homozygous (Q175(+/+)) mice. Input resistance in MSNs from Q175(+/+) and Q175(+/-) mice was significantly increased compared to wildtype (WT) littermates beginning at 2 months. Furthermore, the frequency of spontaneous and miniature excitatory postsynaptic currents (EPSCs) was significantly reduced in MSNs from Q175(+/+) and Q175(+/-) mice compared to WTs beginning at 7 months. In contrast, the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) and IPSC/EPSC ratios were increased in MSNs from Q175(+/+) mice beginning at 2 months. Morphologically, significant decreases in spine density of MSNs from Q175(+/-) and Q175(+/+) mice occurred at 7 and 12 months. In CPNs, sIPSC frequencies and IPSC/EPSC ratios were significantly increased in Q175(+/-) mice compared to WTs at 12 months. There were no changes in intrinsic membrane properties or morphology. In summary, we show a number of alterations in electrophysiological and morphological properties of MSNs in Q175 mice that are similar to other HD mouse models. However, unlike other models, CPN inhibitory activity is increased in Q175(+/-) mice, indicating reduced cortical excitability. Copyright © 2014, Journal of Neurophysiology.
    Journal of Neurophysiology 02/2015; 113(7):jn.01056.2014. DOI:10.1152/jn.01056.2014 · 2.89 Impact Factor
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    • "Expression of mutant HTT by the use of stereotactic injections of recombinant adeno-associated viral (rAAV) vectors in the hypothalamus of wild-type (WT) mice led to increased body weight as well as insulin and leptin resistance. Another experiment used the BACHD mouse model of HD expresses, which human full-length mutant HTT with a floxed exon1 with 97 CAG, allowing selective deletion of mutant HTT in the presence of Cre-recombinase (Cre; [16]). The mouse model displays both depressive and anxiety-like behaviors as well as a robust metabolic phenotype with leptin and insulin resistance at an early age [14], [17]. "
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    ABSTRACT: Psychiatric and metabolic features appear several years before motor disturbances in the neurodegenerative Huntington's disease (HD), caused by an expanded CAG repeat in the huntingtin (HTT) gene. Although the mechanisms leading to these aspects are unknown, dysfunction in the hypothalamus, a brain region controlling emotion and metabolism, has been suggested. A direct link between the expression of the disease causing protein, huntingtin (HTT), in the hypothalamus and the development of metabolic and psychiatric-like features have been shown in the BACHD mouse model of HD. However, precisely which circuitry in the hypothalamus is critical for these features is not known. We hypothesized that expression of mutant HTT in the ventromedial hypothalamus, an area involved in the regulation of metabolism and emotion would be important for the development of these non-motor aspects. Therefore, we inactivated mutant HTT in a specific neuronal population of the ventromedial hypothalamus expressing the transcription factor steroidogenic factor 1 (SF1) in the BACHD mouse using cross-breeding based on a Cre-loxP system. Effects on anxiety-like behavior were assessed using the elevated plus maze and novelty-induced suppressed feeding test. Depressive-like behavior was assessed using the Porsolt forced swim test. Effects on the metabolic phenotype were analyzed using measurements of body weight and body fat, as well as serum insulin and leptin levels. Interestingly, the inactivation of mutant HTT in SF1-expressing neurons exerted a partial positive effect on the depressive-like behavior in female BACHD mice at 4 months of age. In this cohort of mice, no anxiety-like behavior was detected. The deletion of mutant HTT in SF1 neurons did not have any effect on the development of metabolic features in BACHD mice. Taken together, our results indicate that mutant HTT regulates metabolic networks by affecting hypothalamic circuitries that do not involve the SF1 neurons of the ventromedial hypothalamus.
    PLoS ONE 10/2014; 9(10):e107691. DOI:10.1371/journal.pone.0107691 · 3.23 Impact Factor
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    • "Many transgenic animal models of HD show an altered body weight compared to their WT littermates. Animals that express a fragment of the disease-causing gene typically have a reduced body weight [25], [26], [27], while the ones that express the full-length gene typically have an increased body weight [10], [11]. We show here, that although BACHD rats did not differ from WT rats in terms of body weight, they displayed several changes in body composition. "
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    ABSTRACT: Huntington disease (HD) is an inherited neurodegenerative disease characterized by motor, cognitive, psychiatric and metabolic symptoms. Animal models of HD show phenotypes that can be divided into similar categories, with the metabolic phenotype of certain models being characterized by obesity. Although interesting in terms of modeling metabolic symptoms of HD, the obesity phenotype can be problematic as it might confound the results of certain behavioral tests. This concerns the assessment of cognitive function in particular, as tests for such phenotypes are often based on food depriving the animals and having them perform tasks for food rewards. The BACHD rat is a recently established animal model of HD, and in order to ensure that behavioral characterization of these rats is done in a reliable way, a basic understanding of their physiology is needed. Here, we show that BACHD rats are obese and suffer from discrete developmental deficits. When assessing the motivation to lever push for a food reward, BACHD rats were found to be less motivated than wild type rats, although this phenotype was dependent on the food deprivation strategy. Specifically, the phenotype was present when rats of both genotypes were deprived to 85% of their respective free-feeding body weight, but not when deprivation levels were adjusted in order to match the rats' apparent hunger levels. The study emphasizes the importance of considering metabolic abnormalities as a confounding factor when performing behavioral characterization of HD animal models.
    PLoS ONE 08/2014; 9(8):e105662. DOI:10.1371/journal.pone.0105662 · 3.23 Impact Factor
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