Changes in GAD67 mRNA expression evidenced by in situ hybridization in the brain of R6/2 transgenic mice

A method to estimate the cellular composition of the mouse brain from heterogeneous datasets

The mouse brain contains a rich diversity of inhibitory neuron types that have been characterized by their patterns of gene expression. However, it is still unclear how these cell types are distributed across the mouse brain. We developed a computational method to estimate the densities of different inhibitory neuron types across the mouse brain. Our method allows the unbiased integration of diverse and disparate datasets into one framework to predict inhibitory neuron densities for uncharted brain regions. We constrained our estimates based on previously computed brain-wide neuron densities, gene expression data from in situ hybridization image stacks together with a wide range of values reported in the literature. Using constrained optimization, we derived coherent estimates of cell densities for the different inhibitory neuron types. We estimate that 20.3% of all neurons in the mouse brain are inhibitory. Among all inhibitory neurons, 18% predominantly express parvalbumin (PV), 16% express somatostatin (SST), 3% express vasoactive intestinal peptide (VIP), and the remainder 63% belong to the residual GABAergic population. We find that our density estimations improve as more literature values are integrated. Our pipeline is extensible, allowing new cell types or data to be integrated as they become available. The data, algorithms, software, and results of our pipeline are publicly available and update the Blue Brain Cell Atlas. This work therefore leverages the research community to collectively converge on the numbers of each cell type in each brain region.

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.

RNA-seq analysis reveals significant transcriptome changes in huntingtin-null human neuroblastoma cells

BACKGROUND

Huntingtin (Htt) protein is the product of the gene mutated in Huntington's disease (HD), a fatal, autosomal dominant, neurodegenerative disorder. Normal Htt is essential for early embryogenesis and the development of the central nervous system. However, the role of Htt in adult tissues is less defined. Following the recent promising clinical trial in which both normal and mutant Htt mRNA were knocked down in HD patients, there is an urgent need to fully understand the molecular consequences of knocking out/down Htt in adult tissues. Htt has been identified as an important transcriptional regulator. Unbiased investigations of transcriptome changes with RNA-sequencing (RNA-Seq) have been done in multiple cell types in HD, further confirming that transcriptional dysregulation is a central pathogenic mechanism in HD. However, there is lack of direct understanding of the transcriptional regulation by normal Htt.

METHODS

To investigate the transcriptional role of normal Htt, we first knocked out Htt in the human neuroblastoma SH-SY5Y cell line using the CRISPR (clustered regularly interspaced short palindromic repeats)-Cas9 (CRISPR-associated protein 9) gene editing approach. We then performed RNA-seq analysis on Htt-null and wild type SH-SY5Y cells to probe the global transcriptome changes induced by Htt deletion.

RESULTS

In general, Htt has a widespread effect on gene transcription. Functional analysis of the differentially expressed genes (DEGs) using various bioinformatic tools revealed irregularities in pathways related to cell communication and signaling, and more specifically those related to neuron development, neurotransmission and synaptic signaling. We further examined the transcription factors that may regulate these DEGs. Consistent with the disrupted pathways associated with cellular development, we showed that Htt-null cells exhibited slower cell proliferation than wild type cells. We finally validated some of the top DEGS with quantitative RT-PCR.

CONCLUSIONS

The widespread transcriptome changes in Htt-null cells could be directly caused by the loss of Htt-mediated transcriptional regulation or due to the secondary consequences of disruption in the gene regulatory network. Our study therefore provides valuable information about key genes associated with Htt-mediated transcription and improves our understanding of the molecular mechanisms underlying the cellular functions of normal and mutant Htt.

Effect of Immune Activation during Early Gestation or Late Gestation on Inhibitory Markers in Adult Male Rats

People with schizophrenia exhibit deficits in inhibitory neurons and cognition. The timing of maternal immune activation (MIA) may present distinct schizophrenia-like phenotypes in progeny. We investigated whether early gestation [gestational day (GD) 10] or late gestation (GD19) MIA, via viral mimetic polyI:C, produces deficits in inhibitory neuron indices (GAD1, PVALB, SST, SSTR2 mRNAs) within cortical, striatal, and hippocampal subregions of male adult rat offspring. In situ hybridisation revealed that polyI:C offspring had: (1) SST mRNA reductions in the cingulate cortex and nucleus accumbens shell, regardless of MIA timing; (2) SSTR2 mRNA reductions in the cortex and striatum of GD19, but not GD10, MIA; (3) no alterations in cortical or striatal GAD1 mRNA of polyI:C offspring, but an expected reduction of PVALB mRNA in the infralimbic cortex, and; (4) no alterations in inhibitory markers in hippocampus. Maternal IL-6 response negatively correlated with adult offspring SST mRNA in cortex and striatum, but not hippocampus. These results show lasting inhibitory-related deficits in cortex and striatum in adult offspring from MIA. SST downregulation in specific cortical and striatal subregions, with additional deficits in somatostatin-related signalling through SSTR2, may contribute to some of the adult behavioural changes resulting from MIA and its timing.

EGF Treatment Improves Motor Behavior and Cortical GABAergic Function in the R6/2 Mouse Model of Huntington's Disease

Recent evidence indicates that disruption of epidermal growth factor (EGF) signaling by mutant huntingtin (polyQ-htt) may contribute to the onset of behavioral deficits observed in Huntington's disease (HD) through a variety of mechanisms, including cerebrovascular dysfunction. Yet, whether EGF signaling modulates the development of HD pathology and the associated behavioral impairments remain unclear. To gain insight on this issue, we used the R6/2 mouse model of HD to assess the impact of chronic EGF treatment on behavior, and cerebrovascular and cortical neuronal functions. We found that bi-weekly treatment with a low dose of EGF (300 µg/kg, i.p.) for 6 weeks was sufficient to effectively improve motor behavior in R6/2 mice and diminish mortality, compared to vehicle-treated littermates. These beneficial effects of EGF treatment were dissociated from changes in cerebrovascular leakiness, a result that was surprising given that EGF ameliorates this deficit in other neurodegenerative diseases. Rather, the beneficial effect of EGF on R6/2 mice behavior was concomitant with a marked amelioration of cortical GABAergic function. As GABAergic transmission in cortical circuits is disrupted in HD, these novel data suggest a potential mechanistic link between deficits in EGF signaling and GABAergic dysfunction in the progression of HD.

Insights into GABAAergic system alteration in Huntington's disease

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disease that is characterized by a triad of motor, psychiatric and cognitive impairments. There is still no effective therapy to delay or halt the disease progress. The striatum and cortex are two particularly affected brain regions that exhibit dense reciprocal excitatory glutamate and inhibitory gamma-amino butyric acid (GABA) connections. Imbalance between excitatory and inhibitory signalling is known to greatly affect motor and cognitive processes. Emerging evidence supports the hypothesis that disrupted GABAergic circuits underlie HD pathogenesis. In the present review, we focused on the multiple defects recently found in the GABAergic inhibitory system, including altered GABA level and synthesis, abnormal subunit composition and distribution of GABA_A receptors and aberrant GABA_A receptor-mediated signalling. In particular, the important role of cation–chloride cotransporters (i.e. NKCC1 and KCC2) is discussed. Recent studies also suggest that neuroinflammation contributes significantly to the abnormal GABAergic inhibition in HD. Thus, GABA_A receptors and cation–chloride cotransporters are potential therapeutic targets for HD. Given the limited availability of therapeutic treatments for HD, a better understanding of GABAergic dysfunction in HD could provide novel therapeutic opportunities.

Baseline striatal and nigral interneuronal protein levels in two distinct mice strains differ in accordance with their MPTP susceptibility

Epidemiological studies reveal an ethnicity-based bias in prevalence of Parkinson's disease (PD), deriving from the differences that exist between Caucasians and African or Asian populations. Experimental mice models provide a scope to analyse the cellular mechanisms of differential susceptibility to PD. C57BL/6J mice, for instance, are more susceptible to MPTP-induced Parkinsonism whereas CD-1 mice are resistant. In PD-pathogenesis, interneuronal contribution is also likely, although they comprise only 5-10% of the striatal cells. The interneurons harbour calcium binding proteins, like calretinin (Cal-R) and parvalbumin (PV), which are crucial in Ca²⁺ homeostasis for preventing calcium-induced excitotoxicity. GAD-67-immunoreactive interneurons are the other prominent set of GABAergic interneurons. In PD, dopamine loss up-regulates GAD-67 expression in striatal projection neurons and other basal ganglia circuit. We studied the possible contribution of interneurons in determining variable susceptibility by assessing the expression of calretinin, PV and GAD-67 in both striatum and substantia nigra pars compacta (SNpc) in two distinct mice strains, i.e. C57BL/6J and CD-1 under normal conditions, using unbiased stereology for quantification of immunoreactive cells and immunoblotting. The vulnerable C57BL/6J had lesser basal parvalbumin expression in both nigra and striatum whereas the calretinin levels were low only in the striatum. GAD-67 expression showed no perceptible differences in the striatum or SNpc of either of the strains. Differential expression of calcium buffering/binding proteins under normal physiological condition proffers a role for interneurons in the differential susceptibility to PD. Thus, even the baseline susceptibility indices i.e. without using the neurotoxin; can provide vital mechanistic insights into PD pathogenesis.

Developmental alterations in Huntington's disease neural cells and pharmacological rescue in cells and mice

Neural cultures derived from Huntington's disease (HD) patient-derived induced pluripotent stem cells were used for 'omics' analyses to identify mechanisms underlying neurodegeneration. RNA-seq analysis identified genes in glutamate and GABA signaling, axonal guidance and calcium influx whose expression was decreased in HD cultures. One-third of gene changes were in pathways regulating neuronal development and maturation. When mapped to stages of mouse striatal development, the profiles aligned with earlier embryonic stages of neuronal differentiation. We observed a strong correlation between HD-related histone marks, gene expression and unique peak profiles associated with dysregulated genes, suggesting a coordinated epigenetic program. Treatment with isoxazole-9, which targets key dysregulated pathways, led to amelioration of expanded polyglutamine repeat-associated phenotypes in neural cells and of cognitive impairment and synaptic pathology in HD model R6/2 mice. These data suggest that mutant huntingtin impairs neurodevelopmental pathways that could disrupt synaptic homeostasis and increase vulnerability to the pathologic consequence of expanded polyglutamine repeats over time.

A single dose of hypnotic corrects sleep and EEG abnormalities in symptomatic Huntington's disease mice

Sleep and electroencephalogram abnormalities are prominent early features of Huntington's disease (HD) that typically appear before the onset of characteristic motor symptoms. The changes in sleep and electroencephalogram seen in HD patients are largely recapitulated in mouse models of HD such as transgenic R6/2 lines. To test whether or not drugs with hypnotic properties can correct the sleep and electroencephalogram abnormalities seen in HD mice, we treated male wild-type (WT; N = 7) and R6/2 mice (N = 9) acutely with intraperitoneal injections of vehicle, zolpidem (5, 10 or 20 mg/kg) or amitriptyline (5, 10 or 20 mg/kg), and then monitored their sleep-wake behavior. In R6/2 mice, both zolpidem and amitriptyline suppressed the abnormally high REM sleep amount and electroencephalographic gamma (30-46 Hz) oscillations in a dose-dependent manner. Amitriptyline's effect on sleep was similar in both genotypes, whereas zolpidem showed significant genotype differences. Zolpidem exerted a strong hypnotic effect in WT mice by increasing electroencephalographic delta power, doubling the mean bout duration and the total amount of non-rapid eye movement sleep. However, no such effect was seen in R6/2 mice. Our study demonstrates that the pathophysiological changes seen in sleep and electroencephalogram are not 'hard-wired' in HD brain and can be reversed even at late stages of the disease. The diminished hypnotic effect of zolpidem suggests that the GABAergic control of sleep-wake states is impaired in HD mice. A better understanding of the neurochemical basis underlying these abnormalities should lead to more effective and rational therapies for HD.

Treatment of Huntington's disease

Huntington's disease (HD) is a dominantly inherited progressive neurological disease characterized by chorea, an involuntary brief movement that tends to flow between body regions. HD is typically diagnosed based on clinical findings in the setting of a family history and may be confirmed with genetic testing. Predictive testing is available to family members at risk, but only experienced clinicians should perform the counseling and testing. Multiple areas of the brain degenerate, mainly involving the neurotransmitters dopamine, glutamate, and γ-aminobutyric acid. Although pharmacotherapies theoretically target these neurotransmitters, few well-conducted trials for symptomatic interventions have yielded positive results and current treatments have focused on the motor aspects of HD. Tetrabenazine is a dopamine-depleting agent that may be one of the more effective agents for reducing chorea, although it has a risk of potentially serious adverse effects. Some newer neuroleptic agents, such as olanzapine and aripiprazole, may have adequate efficacy with a more favorable adverse effect profile than older neuroleptic agents for treating chorea and psychosis. There are no current treatments to change the course of HD, but education and symptomatic therapies can be effective tools for clinicians to use with patients and families affected by HD.

DBZ (DISC1-binding zinc finger protein)-deficient mice display abnormalities in basket cells in the somatosensory cortices

Disrupted-in-schizophrenia 1 (DISC1)-binding zinc finger protein (DBZ) is a DISC1-interacting molecule and the interaction between DBZ and DISC1 is involved in neurite outgrowth in vitro. DBZ is highly expressed in brain, especially in the cortex. However, the physiological roles of DBZ in vivo have not been clarified. Here, we show that development of basket cells, a morphologically defined class of parvalbumin (PV)-containing interneurons, is disturbed in DBZ knockout (KO) mice. DBZ mRNA was highly expressed in the ventral area of the subventricular zone of the medial ganglionic eminence, where PV-containing cortical interneurons were generated, at embryonic 14.5 days (E14.5). Although the expression level for PV and the number of PV-containing interneurons were not altered in the cortices of DBZ KO mice, basket cells were less branched and had shorter processes in the somatosensory cortices of DBZ KO mice compared with those in the cortices of WT mice. Furthermore, in the somatosensory cortices of DBZ KO mice, the level of mRNAs for the gamma-aminobutyric acid-synthesizing enzymes GAD67 was decreased. These findings show that DBZ is involved in the morphogenesis of basket cells.

Modeling Huntington's disease with induced pluripotent stem cells

Huntington's disease (HD) causes severe motor dysfunction, behavioral abnormalities, cognitive impairment and death. Investigations into its molecular pathology have primarily relied on murine tissues; however, the recent discovery of induced pluripotent stem cells (iPSCs) has opened new possibilities to model neurodegenerative disease using cells derived directly from patients, and therefore may provide a human-cell-based platform for unique insights into the pathogenesis of HD. Here, we will examine the practical implementation of iPSCs to study HD, such as approaches to differentiate embryonic stem cells (ESCs) or iPSCs into medium spiny neurons, the cell type most susceptible in HD. We will explore the HD-related phenotypes identified in iPSCs and ESCs and review how brain development and neurogenesis may actually be altered early, before the onset of HD symptoms, which could inform the search for drugs that delay disease onset. Finally, we will speculate on the exciting possibility that ESCs or iPSCs might be used as therapeutics to restore or replace dying neurons in HD brains.

Huntington's disease and the striatal medium spiny neuron: cell-autonomous and non-cell-autonomous mechanisms of disease

Huntington's disease is an autosomal dominant disorder caused by a mutation in the gene encoding the protein huntingtin on chromosome 4. The mutation is an expanded CAG repeat in the first exon, encoding a polyglutamine tract. If the polyglutamine tract is > 40, penetrance is 100% and death is inevitable. Despite the widespread expression of huntingtin, HD has long been considered primarily as a disease of the striatum. It is characterized by selective vulnerability with dysfunction followed by death of the medium size spiny neuron. Considerable effort is being expended to determine whether striatal damage is cell-autonomous, non-cell-autonomous, requiring cell-cell and region to region communication, or both. We review data supporting both mechanisms. We also attempt to organize the data into common mechanisms that may arise outside the medium, spiny neuron, but ultimately have their greatest impact in the striatum.

Reduced striatal acetylcholine efflux in the R6/2 mouse model of Huntington's disease: an examination of the role of altered inhibitory and excitatory mechanisms

Huntington's disease (HD) is a genetic neurodegenerative disorder that is characterized by the progressive onset of cognitive, psychiatric, and motor symptoms. In parallel, the neuropathology of HD is characterized by progressive loss of projection neurons in cortex and striatum; striatal cholinergic interneurons are relatively spared. Nonetheless, there is evidence that striatal acetylcholine (ACh) function is altered in HD. The present study is the first to examine striatal ACh function in awake, behaving animals, using the R6/2 mouse model of HD, which is transgenic for exon 1 of the mutant huntingtin gene. Physiological levels of extracellular striatal ACh were monitored in R6/2 mice and wild type controls using in vivo microdialysis. Results indicate that spontaneous ACh release is reduced in R6/2 mice relative to controls. Intrastriatal application of the GABA(A) antagonist bicuculline methiodide (10.0 μM) significantly elevated ACh levels in both R6/2 mice and wild type controls, while overall ACh levels were reduced in the R6/2 mice compared to the wild type group. In contrast, systemic administration of the D(1) dopamine receptor partial agonist, SKF-38393 (10.0mg/kg, IP), elevated ACh levels in control animals, but not R6/2 mice. Taken together, the present results suggest that GABA-mediated inhibition of striatal ACh release is intact in R6/2 mice, further demonstrating that cholinergic interneurons are capable of increased ACh release, whereas D(1) receptor-dependent activation of excitatory inputs to striatal cholinergic interneurons is dysfunctional in R6/2 mice. Reduced levels of extracellular striatal ACh in HD may reflect abnormalities in the excitatory innervation of cholinergic interneurons, which may have implications ACh-dependent processes that are altered in HD, including corticostriatal plasticity.

A2A receptor knockout worsens survival and motor behaviour in a transgenic mouse model of Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative genetic disorder that leads to motor, cognitive, and psychiatric disturbances. The primary neuropathological hallmark is atrophy of the striatum. HD preferentially affects efferent striato-pallidal neurons that express enkephalin as well as dopamine D2 and A(2A) adenosine receptors (A(2A)Rs). Expression and function of A(2A)Rs are altered in HD but, despite being an important modulator of the striato-pallidal function, the subsequent pathophysiological consequence of such changes remains unclear. Whether blockade of A(2A)Rs is of therapeutic interest in HD remains ill-defined. In the present work, we aimed to determine the pathophysiological consequences of genetic deletion of A(2A)Rs in HD by crossing A(2A)R knockout mice with the N171-82Q HD transgenic model. Our data demonstrate that knockout of A(2A)Rs moderately but significantly worsens motor performances and survival of N171-82Q mice and leads to a decrease in striatal enkephalin expression. These results support that early and chronic blockade of A(2A)Rs might not be beneficial in HD.

Tetrabenazine: the first approved drug for the treatment of chorea in US patients with Huntington disease

Huntington disease (HD) is a dominantly inherited progressive neurological disease characterized by chorea, an involuntary brief movement that tends to flow between body regions. HD is typically diagnosed based on clinical findings in the setting of a family history and may be confirmed with genetic testing. Predictive testing is available to those at risk, but only experienced clinicians should perform the counseling and testing. Multiple areas of the brain degenerate mainly involving the neurotransmitters dopamine, glutamate, and γ-aminobutyric acid. Although pharmacotherapies theoretically target these neurotransmitters, few well-conducted trials for symptomatic or neuroprotective interventions yielded positive results. Tetrabenazine (TBZ) is a dopamine-depleting agent that may be one of the more effective agents for reducing chorea, although it has a risk of potentially serious adverse effects. Some newer antipsychotic agents, such as olanzapine and aripiprazole, may have adequate efficacy with a more favorable adverse-effect profile than older antipsychotic agents for treating chorea and psychosis. This review will address the epidemiology and diagnosis of HD as background for understanding potential pharmacological treatment options. Because TBZ is the only US Food and Drug Administration-approved medication in the United States for HD, the focus of this review will be on its pharmacology, efficacy, safety, and practical uses. There are no current treatments to change the course of HD, but education and symptomatic therapies can be effective tools for clinicians to use with patients and families affected by HD.

A novel mechanism for GABA synthesis and packaging into synaptic vesicles

This review focuses on the recent advances that were made in understanding the fundamental mechanisms of the regulation of l-glutamic acid decarboxylase (GAD; E.C. 4.1.1.15), the enzyme responsible for the synthesis of the major inhibitory neurotransmitter gamma-amino butyric acid (GABA). In the brain, there are two isoforms of GAD- GAD67 and GAD65, where 67 and 65 refer to their respective molecular weights in kDa. A number of neurodegenerative diseases are known to occur as a result of insufficient inhibition due to failure of GABA neurotransmission. Since the rate-limiting step in GABA biosynthesis is the decarboxylation of glutamate by GAD, it is important to understand how GAD is regulated. So far, we know that GAD is regulated at the transcriptional level by alternate splicing and at the post-translational level by protein phosphorylation, palmitoylation and activity-dependent cleavage. Here, we present new evidence of the presence of GAD65 associated with mitochondria in the axon terminal and project a model in which ATP generated by mitochondrial GAD65 may serve an important function in providing energy for GAD65 mediated GABA biosynthesis and packaging into synaptic vesicles by vesicular GABA transporter (VGAT).

Transcriptional signatures in Huntington's disease

While selective neuronal death has been an influential theme in Huntington's disease (HD), there is now a preponderance of evidence that significant neuronal dysfunction precedes frank neuronal death. The best evidence for neuronal dysfunction is the observation that gene expression is altered in HD brain, suggesting that transcriptional dysregulation is a central mechanism. Studies of altered gene expression began with careful observations of postmortem human HD brain and subsequently were accelerated by the development of transgenic mouse models. The application of DNA microarray technology has spurred tremendous progress with respect to the altered transcriptional processes that occur in HD, through gene expression studies of both transgenic mouse models as well as cellular models of HD. Gene expression profiles are remarkably comparable across these models, bolstering the idea that transcriptional signatures reflect an essential feature of disease pathogenesis. Finally, gene expression studies have been applied to human HD, thus not only validating the approach of using model systems, but also solidifying the idea that altered transcription is a key mechanism in HD pathogenesis. In the future, gene expression profiling will be used as a readout in clinical trials aimed at correcting transcriptional dysregulation in Huntington's disease.

Abnormal sensitivity to cannabinoid receptor stimulation might contribute to altered gamma-aminobutyric acid transmission in the striatum of R6/2 Huntington's disease mice

BACKGROUND

One of the earliest neurochemical alterations observed in both Huntington's disease (HD) patients and HD animal models is the dysregulation of the endocannabinoid system, an alteration that precedes the development of identifiable striatal neuropathology. How this alteration impacts striatal synaptic transmission is unknown.

METHODS

We measured the effects of cannabinoid receptor stimulation on gamma-aminobutyric acid (GABA)-ergic synaptic currents recorded from striatal neurons of R6/2 HD mice in the early phase of their disease.

RESULTS

The sensitivity of striatal GABA synapses to cannabinoid receptor stimulation is severely impaired in R6/2 HD mice. In particular, whereas in control animals activation of cannabinoid CB1 receptors results in a significant inhibition of both evoked and spontaneous GABA-mediated synaptic events by a presynaptic mechanism, in R6/2 mice this treatment fails to reduce GABA currents but causes, in contrast, a slight increase of spontaneous inhibitory postsynaptic currents (sIPSCs).

CONCLUSIONS

Experimental HD was also associated with enhanced frequency of sIPSCs, a result consistent with the conclusion that loss of cannabinoid-mediated control of GABA transmission might contribute to hyperactivity of GABA synapses in the striatum of HD mice. Accordingly, spontaneous excitatory postsynaptic currents, which were not upregulated in R6/2 mice, were still sensitive to cannabinoid receptor stimulation.

Pathological Cell-Cell Interactions Elicited by a Neuropathogenic Form of Mutant Huntingtin Contribute to Cortical Pathogenesis in HD Mice

Expanded polyglutamine (polyQ) proteins in Huntington's disease (HD) as well as other polyQ disorders are known to elicit a variety of intracellular toxicities, but it remains unclear whether polyQ proteins can elicit pathological cell-cell interactions which are critical to disease pathogenesis. To test this possibility, we have created conditional HD mice expressing a neuropathogenic form of mutant huntingtin (mhtt-exon1) in discrete neuronal populations. We show that mhtt aggregation is a cell-autonomous process. However, progressive motor deficits and cortical neuropathology are only observed when mhtt expression is in multiple neuronal types, including cortical interneurons, but not when mhtt expression is restricted to cortical pyramidal neurons. We further demonstrate an early deficit in cortical inhibition, suggesting that pathological interactions between interneurons and pyramidal neurons may contribute to the cortical manifestation of HD. Our study provides genetic evidence that pathological cell-cell interactions elicited by neuropathogenic forms of mhtt can critically contribute to cortical pathogenesis in a HD mouse model.

Increased GABAergic function in mouse models of Huntington's disease: Reversal by BDNF

Huntington's disease (HD) is characterized by loss of striatal gamma-aminobutyric acid (GABA)ergic medium-sized spiny projection neurons (MSSNs), whereas some classes of striatal interneurons are relatively spared. Striatal interneurons provide most of the inhibitory synaptic input to MSSNs and use GABA as their neurotransmitter. We reported previously alterations in glutamatergic synaptic activity in the R6/2 and R6/1 mouse models of HD. In the present study, we used whole-cell voltage clamp recordings to examine GABAergic synaptic currents in MSSNs from striatal slices in these two mouse models compared to those in age-matched control littermates. The frequency of spontaneous GABAergic synaptic currents was increased significantly in MSSNs from R6/2 transgenics starting around 5-7 weeks (when the overt behavioral phenotype begins) and continuing in 9-14-week-old mice. A similar increase was observed in 12-15-month-old R6/1 transgenics. Bath application of brain-derived neurotrophic factor, which is downregulated in HD, significantly reduced the frequency of spontaneous GABAergic synaptic currents in MSSNs from R6/2 but not control mice at 9-14 weeks. Increased GABA current densities also occurred in acutely isolated MSSNs from R6/2 animals. Immunofluorescence demonstrated increased expression of the ubiquitous alpha1 subunit of GABA(A) receptors in MSSNs from R6/2 animals. These results indicate that increases in spontaneous GABAergic synaptic currents and postsynaptic receptor function occur in parallel to progressive decreases in glutamatergic inputs to MSSNs. In conjunction, both changes will severely alter striatal outputs to target areas involved in the control of movement.