Huntington's disease mouse models: unraveling the pathology caused by CAG repeat expansion

Huntington's disease (HD) is a neurodegenerative disease that results in motor and cognitive dysfunction, leading to early death. HD is caused by an expansion of CAG repeats in the huntingtin gene (HTT). Here, we review the mouse models of HD. They have been used extensively to better understand the molecular and cellular basis of disease pathogenesis as well as to provide non-human subjects to test the efficacy of potential therapeutics. The first and best-studied in vivo rodent model of HD is the R6/2 mouse, in which a transgene containing the promoter and exon 1 fragment of human HTT with 150 CAG repeats was inserted into the mouse genome. R6/2 mice express rapid, robust behavioral pathologies and display a number of degenerative abnormalities in neuronal populations most vulnerable in HD. The first conditional full-length mutant huntingtin (mHTT) mouse model of HD was the bacterial artificial chromosome (BAC) transgenic mouse model of HD (BACHD), which expresses human full-length mHTT with a mixture of 97 CAG-CAA repeats under the control of endogenous HTT regulatory machinery. It has been useful in identifying the role of mHTT in specific neuronal populations in degenerative processes. In the knock-in (KI) model of HD, the expanded human CAG repeats and human exon 1 are inserted into the mouse Htt locus, so a chimera of the full-length mouse protein with the N-terminal human portion is expressed. Many of aspects of the pathology and behavioral deficits in the KI model better mimic disease characteristics found in HD patients than other models. Accordingly, some have proposed that these mice may be preferable models of the disease over others. Indeed, as our understanding of HD advances, so will the design of animal models to test and develop HD therapies.

FACS-array-based cell purification yields a specific transcriptome of striatal medium spiny neurons in a murine Huntington disease model

Huntington disease (HD) is a neurodegenerative disorder caused by expanded CAG repeats in the Huntingtin gene. Results from previous studies have suggested that transcriptional dysregulation is one of the key mechanisms underlying striatal medium spiny neuron (MSN) degeneration in HD. However, some of the critical genes involved in HD etiology or pathology could be masked in a common expression profiling assay because of contamination with non-MSN cells. To gain insight into the MSN-specific gene expression changes in presymptomatic R6/2 mice, a common HD mouse model, here we used a transgenic fluorescent protein marker of MSNs for purification via FACS before profiling gene expression with gene microarrays and compared the results of this "FACS-array" with those obtained with homogenized striatal samples (STR-array). We identified hundreds of differentially expressed genes (DEGs) and enhanced detection of MSN-specific DEGs by comparing the results of the FACS-array with those of the STR-array. The gene sets obtained included genes ubiquitously expressed in both MSNs and non-MSN cells of the brain and associated with transcriptional regulation and DNA damage responses. We proposed that the comparative gene expression approach using the FACS-array may be useful for uncovering the gene cascades affected in MSNs during HD pathogenesis.

Ghrelin-mediated improvements in the metabolic phenotype in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a heritable neurodegenerative disorder, characterised by metabolic disturbances, along with cognitive and psychiatric impairments. Targeting metabolic HD dysfunction via the maintenance of body weight and fat mass and restoration of peripheral energy metabolism can improve the progression of neurological symptoms. In this respect, we focused on the therapeutic potential of the orexigenic peptide hormone ghrelin, which plays an important role in promoting a positive energy balance. In the present study, we found a significant disruption of circadian metabolic regulation in a R6/2 mouse HD model in the late stage of disease. Daily circadian rhythms of activity, energy expenditure, respiratory exchange ratio and feeding were strongly attenuated in R6/2 mice. During the rest phase, R6/2 mice had a higher total activity, elevated energy expenditure and excessive water consumption compared to control mice. We also found that, in the late stage of disease, R6/2 mice had ghrelin axis deficiency as a result of low circulating ghrelin levels, in addition to down-regulation of the ghrelin receptor and several key signalling molecules in the hypothalamus, as well as a reduced responsiveness to exogenous peripheral ghrelin. We demonstrated that, in pre-symptomatic mice, responsiveness to ghrelin is preserved. Chronic ghrelin treatment efficiently increased lean body mass and decreased the energy expenditure and fat utilisation of R6/2 mice in the early stage of disease. In addition, ghrelin treatment was also effective in the normalisation of drinking behaviour and the rest activity of these mice. Ghrelin treatment could provide a novel therapeutic possibility for delaying disease progression; however, deficiency in ghrelin receptor expression could limit its therapeutic potential in the late stage of disease.

Enhanced GABAergic Inputs Contribute to Functional Alterations of Cholinergic Interneurons in the R6/2 Mouse Model of Huntington's Disease

Although large cholinergic interneurons (LCIs) in striatum are spared in Huntington's disease (HD), deficits in cholinergic function have been described. Here we demonstrate in a mouse model of HD that the firing patterns of LCIs are disrupted and this is due to aberrant GABAergic neurotransmission. This explains cholinergic deficits in HD.

In Huntington’s disease (HD), a hereditary neurodegenerative disorder, striatal medium-sized spiny neurons undergo degenerative changes. In contrast, large cholinergic interneurons (LCIs) are relatively spared. However, their ability to release acetylcholine (ACh) is impaired. The present experiments examined morphological and electrophysiological properties of LCIs in the R6/2 mouse model of HD. R6/2 mice show a severe, rapidly progressing phenotype. Immunocytochemical analysis of choline acetyltransferase-positive striatal neurons showed that, although the total number of cells was not changed, somatic areas were significantly smaller in symptomatic R6/2 mice compared to wild-type (WT) littermates, For electrophysiology, brain slices were obtained from presymptomatic (3-4 weeks) and symptomatic (>8 weeks) R6/2 mice and their WT littermates. Striatal LCIs were identified by somatic size and spontaneous action potential firing in the cell-attached mode. Passive and active membrane properties of LCIs were similar in presymptomatic R6/2 and WT mice. In contrast, LCIs from symptomatic R6/2 animals displayed smaller membrane capacitance and higher input resistance, consistent with reduced somatic size. In addition, more LCIs from symptomatic mice displayed irregular firing patterns and bursts of action potentials. They also displayed a higher frequency of spontaneous GABAergic IPSCs and larger amplitude of electrically evoked IPSCs. Selective optogenetic stimulation of somatostatin- but not parvalbumin-containing interneurons also evoked larger amplitude IPSCs in LCIs from R6/2 mice. In contrast, glutamatergic spontaneous or evoked postsynaptic currents were not affected. Morphological and electrophysiological alterations, in conjunction with the presence of mutant huntingtin in LCIs, could explain impaired ACh release in HD mouse models.

Treatment with a herbal formula B401 enhances neuroprotection and angiogenesis in the R6/2 mouse model of Huntington's disease

Huntington’s disease (HD) is a neurodegenerative disease characterized by motor dysfunction and early death. Despite years of research, the mechanisms responsible for chronic neurodegeneration of HD remain elusive. Chinese traditional medicines might provide new insights or new therapy for HD. The Chinese herbal formula B401 is a well-known Taiwan–US patent formula and a health supplement for promoting blood circulation and enhancing brain function. This study aimed to elucidate the neuroprotective effects of the Chinese herbal formula B401 on the syndrome of HD. Then, we compared the life span and body weight of R6/2 HD mice with and without oral B401 treatment. The ameliorative effects of B401 on the symptom of HD mice were investigated through behavior tests. Expressions of proteins for neuroprotection, angiogenesis, and inflammation in the brain tissue of R6/2 HD mice were compared by using immunostaining and Western blotting techniques. Our study in vitro showed that viabilities of glutamate-treated SH-SY5Y cells were significantly increased under B401 treatment. Our results in vivo showed that duration of survival was increased, body weight loss was reduced, and motor ability was improved in R6/2 HD mice under oral B401 treatment. Subcutaneous microcirculation was enhanced in 3-month R6/2 HD mice under intraperitoneal B401 injections as observed by using moorFLPI laser Doppler imager. Atrophy of cerebrum, midbrain, and cerebellum in 3-month R6/2 HD mice under oral B401 treatment was alleviated as observed by utilizing magnetic resonance imaging. Evidence from immunostaining and Western blotting analysis showed that expressions of mutant huntingtin and tumor necrosis factor-alpha were reduced, while expressions of brain-derived neurotrophic factor and vascular endothelial growth factor were enhanced in the brain tissue of 2-month R6/2 HD mice under oral B401 treatment. We suggest that the herbal formula B401 can be developed as a medical supplement for ameliorating neurodegenerative diseases of HD via reducing mutant huntingtin aggregation and excitotoxicity, enhancing neuroprotection and angiogenesis, and alleviating inflammation in the brain.

Fused 3-Hydroxy-3-trifluoromethylpyrazoles Inhibit Mutant Huntingtin Toxicity

Here, we describe the selection and optimization of a chemical series active in both a full-length and a fragment-based Huntington's disease (HD) assay. Twenty-four thousand small molecules were screened in a phenotypic HD assay, identifying a series of compounds bearing a 3-hydroxy-3-trifluoromethylpyrazole moiety as able to revert the toxicity induced by full-length mutant Htt by up to 50%. A chemical exploration around the series led to the identification of compound 4f, which demonstrated to be active in a Htt171-82Q rat primary striatal neuron assay and a PC12-Exon-1 based assay. This compound was selected for testing in R6/2 mice, in which it was well-tolerated and showed a positive effect on body weight and a positive trend in preventing ventricular volume enlargment. These studies provide strong rationale for further testing the potential benefits of 3-hydroxy-3-trifluoromethylpyrazoles in treating HD.

Evidence from the R6/2 Mouse Model of Huntington's Disease for Using Abnormal Brain Metabolism as a Biomarker for Evaluating Therapeutic Approaches for Treatment

Huntington's disease (HD) is an autosomal dominant genetic disorder characterized by a progression of motor abnormalities as well as cognitive and psychiatric symptoms [1]. Presently, there is no cure for HD and no treatment to reverse its course or prevent its onset. HD has been characterized primarily by significant degeneration of the striatum. In addition, imaging studies have shown alterations in extra-striatal regions including the cortex [2, 3], hippocampus, and hypothalamus [4]. Although previous functional magnetic resonance imaging (fMRI) studies in patients have yielded complex and heterogeneous findings, identifying functional alterations may serve as a useful tool for tracking the progression of HD and assessing the effects of therapeutic interventions. In a recent article Cepeda-Prado et el. use novel and groundbreaking fMRI methods to elucidate functional, structural, and metabolic alterations in the R6/2 mouse model of HD. Based on changes in relative cerebral brain volume (rCBV), neuronal activity, and glucose utilization, the authors suggest that R6/2 mice have impaired neurometabolic coupling. They propose the use of rCBV as a biomarker of HD progression, providing a basis for future research examining functional alterations in animal models.