Striatal specificity of gene expression dysregulation in Huntington's disease

Microarray profiling of hypothalamic gene expression changes in Huntington's disease mouse models

Structural changes and neuropathology in the hypothalamus have been suggested to contribute to the non-motor manifestations of Huntington's disease (HD), a neurodegenerative disorder caused by an expanded cytosine-adenine-guanine (CAG) repeat in the huntingtin (HTT) gene. In this study, we investigated whether hypothalamic HTT expression causes transcriptional changes. Hypothalamic RNA was isolated from two different HD mouse models and their littermate controls; BACHD mice with ubiquitous expression of full-length mutant HTT (mHTT) and wild-type mice with targeted hypothalamic overexpression of either wild-type HTT (wtHTT) or mHTT fragments. The mHTT and wtHTT groups showed the highest number of differentially expressed genes compared to the BACHD mouse model. Gene Set Enrichment Analysis (GSEA) with leading-edge analysis showed that suppressed sterol- and cholesterol metabolism were shared between hypothalamic wtHTT and mHTT overexpression. Most distinctive for mHTT overexpression was the suppression of neuroendocrine networks, in which qRT-PCR validation confirmed significant downregulation of neuropeptides with roles in feeding behavior; hypocretin neuropeptide precursor (Hcrt), tachykinin receptor 3 (Tacr3), cocaine and amphetamine-regulated transcript (Cart) and catecholamine-related biological processes; dopa decarboxylase (Ddc), histidine decarboxylase (Hdc), tyrosine hydroxylase (Th), and vasoactive intestinal peptide (Vip). In BACHD mice, few hypothalamic genes were differentially expressed compared to age-matched WT controls. However, GSEA indicated an enrichment of inflammatory- and gonadotropin-related processes at 10 months. In conclusion, we show that both wtHTT and mHTT overexpression change hypothalamic transcriptome profile, specifically mHTT, altering neuroendocrine circuits. In contrast, the ubiquitous expression of full-length mHTT in the BACHD hypothalamus moderately affects the transcriptomic profile.

Cell-intrinsic glial pathology is conserved across human and murine models of Huntington's disease

Glial pathology is a causal contributor to the striatal neuronal dysfunction of Huntington's disease (HD). We investigate mutant HTT-associated changes in gene expression by mouse and human striatal astrocytes, as well as in mouse microglia, to identify commonalities in glial pathobiology across species and models. Mouse striatal astrocytes are fluorescence-activated cell sorted (FACS) from R6/2 and zQ175 mice, which respectively express exon1-only or full-length mHTT, and human astrocytes are generated either from human embryonic stem cells (hESCs) expressing full-length mHTT or from fetal striatal astrocytes transduced with exon1-only mHTT. Comparison of differential gene expression across these conditions, all with respect to normal HTT controls, reveals cell-type-specific changes in transcription common to both species, yet with differences that distinguish glia expressing truncated mHTT versus full-length mHTT. These data indicate that the differential gene expression of glia expressing truncated mHTT may differ from that of cells expressing full-length mHTT, while identifying a conserved set of dysregulated pathways in HD glia.

Integrated analysis on transcriptome and behaviors defines HTT repeat-dependent network modules in Huntington's disease

Huntington's disease (HD) is caused by a CAG repeat expansion in the huntingtin (HTT) gene. Knock-in mice carrying a CAG repeat-expanded Htt will develop HD phenotypes. Previous studies suggested dysregulated molecular networks in a CAG length genotype- and the age-dependent manner in brain tissues from knock-in mice carrying expanded Htt CAG repeats. Furthermore, a large-scale phenome analysis defined a behavioral signature for HD genotype in knock-in mice carrying expanded Htt CAG repeats. However, an integrated analysis correlating phenotype features with genotypes (CAG repeat expansions) was not conducted previously. In this study, we revealed the landscape of the behavioral features and gene expression correlations based on 445 mRNA samples and 445 microRNA samples, together with behavioral features (396 PhenoCube behaviors and 111 NeuroCube behaviors) in Htt CAG-knock-in mice. We identified 37 behavioral features that were significantly associated with CAG repeat length including the number of steps and hind limb stand duration. The behavioral features were associated with several gene coexpression groups involved in neuronal dysfunctions, which were also supported by the single-cell RNA sequencing data in the striatum and the spatial gene expression in the brain. We also identified 15 chemicals with significant responses for genes with enriched behavioral features, most of them are agonist or antagonist for dopamine receptors and serotonin receptors used for neurology/psychiatry. Our study provides further evidence that abnormal neuronal signal transduction in the striatum plays an important role in causing HD-related phenotypic behaviors and provided rich information for the further pharmacotherapeutic intervention possibility for HD.

Dysregulation of PGC-1α-Dependent Transcriptional Programs in Neurological and Developmental Disorders: Therapeutic Challenges and Opportunities

Substantial evidence indicates that mitochondrial impairment contributes to neuronal dysfunction and vulnerability in disease states, leading investigators to propose that the enhancement of mitochondrial function should be considered a strategy for neuroprotection. However, multiple attempts to improve mitochondrial function have failed to impact disease progression, suggesting that the biology underlying the normal regulation of mitochondrial pathways in neurons, and its dysfunction in disease, is more complex than initially thought. Here, we present the proteins and associated pathways involved in the transcriptional regulation of nuclear-encoded genes for mitochondrial function, with a focus on the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α). We highlight PGC-1α’s roles in neuronal and non-neuronal cell types and discuss evidence for the dysregulation of PGC-1α-dependent pathways in Huntington’s Disease, Parkinson’s Disease, and developmental disorders, emphasizing the relationship between disease-specific cellular vulnerability and cell-type-specific patterns of PGC-1α expression. Finally, we discuss the challenges inherent to therapeutic targeting of PGC-1α-related transcriptional programs, considering the roles for neuron-enriched transcriptional coactivators in co-regulating mitochondrial and synaptic genes. This information will provide novel insights into the unique aspects of transcriptional regulation of mitochondrial function in neurons and the opportunities for therapeutic targeting of transcriptional pathways for neuroprotection.

Dysregulation of PGC-1α-Dependent Transcriptional Programs in Neurological and Developmental Disorders: Therapeutic Challenges and Opportunities

Substantial evidence indicates that mitochondrial impairment contributes to neuronal dysfunction and vulnerability in disease states, leading investigators to propose that the enhancement of mitochondrial function should be considered a strategy for neuroprotection. However, multiple attempts to improve mitochondrial function have failed to impact disease progression, suggesting that the biology underlying the normal regulation of mitochondrial pathways in neurons, and its dysfunction in disease, is more complex than initially thought. Here, we present the proteins and associated pathways involved in the transcriptional regulation of nuclear-encoded genes for mitochondrial function, with a focus on the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α). We highlight PGC-1α's roles in neuronal and non-neuronal cell types and discuss evidence for the dysregulation of PGC-1α-dependent pathways in Huntington's Disease, Parkinson's Disease, and developmental disorders, emphasizing the relationship between disease-specific cellular vulnerability and cell-type-specific patterns of PGC-1α expression. Finally, we discuss the challenges inherent to therapeutic targeting of PGC-1α-related transcriptional programs, considering the roles for neuron-enriched transcriptional coactivators in co-regulating mitochondrial and synaptic genes. This information will provide novel insights into the unique aspects of transcriptional regulation of mitochondrial function in neurons and the opportunities for therapeutic targeting of transcriptional pathways for neuroprotection.

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.

Targeting Gpr52 lowers mutant HTT levels and rescues Huntington's disease-associated phenotypes

See Huang and Gitler (doi:10.1093/brain/awy112) for a scientific commentary on this article.

Lowering the levels of disease-causing proteins is an attractive treatment strategy for neurodegenerative disorders, among which Huntington’s disease is an appealing disease for testing this strategy because of its monogenetic nature. Huntington’s disease is mainly caused by cytotoxicity of the mutant HTT protein with an expanded polyglutamine repeat tract. Lowering the soluble mutant HTT may reduce its downstream toxicity and provide potential treatment for Huntington’s disease. This is hard to achieve by small-molecule compound drugs because of a lack of effective targets. Here we demonstrate Gpr52, an orphan G protein-coupled receptor, as a potential Huntington’s disease drug target. Knocking-out Gpr52 significantly reduces mutant HTT levels in the striatum and rescues Huntington’s disease-associated behavioural phenotypes in a knock-in Huntington’s disease mouse model expressing endogenous mutant Htt. Importantly, a novel Gpr52 antagonist E7 reduces mutant HTT levels and rescues Huntington’s disease-associated phenotypes in cellular and mouse models. Our study provides an entry point for Huntington’s disease drug discovery by targeting Gpr52.

Huntington's disease pattern of transcriptional dysregulation in the absence of mutant huntingtin is produced by knockout of neuronal GLT-1

GLT-1 is the major glutamate transporter in the brain, and is expressed in astrocytes and in axon terminals in the hippocampus, cortex, and striatum. Neuronal GLT-1 accounts for only 5-10% of total brain GLT-1 protein, and its function is uncertain. In HD, synaptic dysfunction of the corticostriate synapse is well-established. Transcriptional dysregulation is a key feature of HD. We hypothesized that deletion of neuronal GLT-1, because it is expressed in axon terminals in the striatum, might produce a synaptopathy similar to that present in HD. If true, then some of the gene expression changes observed in HD might also be observed in the neuronal GLT-1 knockout. In situ hybridization using ³³P labeled oligonucleotide probes was carried out to assess localization and expression of a panel of genes known to be altered in expression in HD. We found changes in the expression of cannabinoid receptors 1 and 2, preproenkaphalin, and PDE10A in the striatum of mice in which the GLT-1 gene was inactivated in neurons by expression of synapsin-Cre, compared to wild-type littermates. These changes in expression were observed at 12 weeks of age but not at 6 weeks of age. No changes in DARPP-32, PDE1B, NGFIA, or β-actin expression were observed. In addition, we found widespread alteration in expression of the dynamin 1 gene. The changes in expression in the neuronal GLT-1 knockout of genes thought to exemplify HD transcriptional dysregulation suggest an overlap in the synaptopathy caused by neuronal GLT-1 deletion and HD. These data further suggest that specific changes in expression of cannabinoid receptors, preproenkephalin, and PDE10A, considered to be the hallmark of HD transcriptional dysregulation, may be produced by an abnormality of glutamate homeostasis under the regulation of neuronal GLT-1, or a synaptic disturbance caused by that abnormality, independently of mutation in huntingtin.

Claire J, Earls JC, Kovalenko M, Gillis T, Mysore J, Gusella JF, Lee JM, Kwak S, Howland D, Lee MY, Baxter D, Scherler K, Wang K, Geman D, Carroll JB, MacDonald ME, Carlson G, Wheeler VC, Price ND, Hood LE. High resolution time-course mapping of early transcriptomic, molecular and cellular phenotypes in Huntington's disease CAG knock-in mice across multiple genetic backgrounds

Huntington's disease is a dominantly inherited neurodegenerative disease caused by the expansion of a CAG repeat in the HTT gene. In addition to the length of the CAG expansion, factors such as genetic background have been shown to contribute to the age at onset of neurological symptoms. A central challenge in understanding the disease progression that leads from the HD mutation to massive cell death in the striatum is the ability to characterize the subtle and early functional consequences of the CAG expansion longitudinally. We used dense time course sampling between 4 and 20 postnatal weeks to characterize early transcriptomic, molecular and cellular phenotypes in the striatum of six distinct knock-in mouse models of the HD mutation. We studied the effects of the HttQ111 allele on the C57BL/6J, CD-1, FVB/NCr1, and 129S2/SvPasCrl genetic backgrounds, and of two additional alleles, HttQ92 and HttQ50, on the C57BL/6J background. We describe the emergence of a transcriptomic signature in HttQ111/+  mice involving hundreds of differentially expressed genes and changes in diverse molecular pathways. We also show that this time course spanned the onset of mutant huntingtin nuclear localization phenotypes and somatic CAG-length instability in the striatum. Genetic background strongly influenced the magnitude and age at onset of these effects. This work provides a foundation for understanding the earliest transcriptional and molecular changes contributing to HD pathogenesis.

Inhibition of DNA Methyltransferases Blocks Mutant Huntingtin-Induced Neurotoxicity

Although epigenetic abnormalities have been described in Huntington's disease (HD), the causal epigenetic mechanisms driving neurodegeneration in HD cortex and striatum remain undefined. Using an epigenetic pathway-targeted drug screen, we report that inhibitors of DNA methyltransferases (DNMTs), decitabine and FdCyd, block mutant huntingtin (Htt)-induced toxicity in primary cortical and striatal neurons. In addition, knockdown of DNMT3A or DNMT1 protected neurons against mutant Htt-induced toxicity, together demonstrating a requirement for DNMTs in mutant Htt-triggered neuronal death and suggesting a neurodegenerative mechanism based on DNA methylation-mediated transcriptional repression. Inhibition of DNMTs in HD model primary cortical or striatal neurons restored the expression of several key genes, including Bdnf, an important neurotrophic factor implicated in HD. Accordingly, the Bdnf promoter exhibited aberrant cytosine methylation in mutant Htt-expressing cortical neurons. In vivo, pharmacological inhibition of DNMTs in HD mouse brains restored the mRNA levels of key striatal genes known to be downregulated in HD. Thus, disturbances in DNA methylation play a critical role in mutant Htt-induced neuronal dysfunction and death, raising the possibility that epigenetic strategies targeting abnormal DNA methylation may have therapeutic utility in HD.

DNA methylation in Huntington's disease: Implications for transgenerational effects

Huntington's disease (HD) is a devastating, neurodegenerative disorder caused by a CAG repeat mutation in the HTT gene. A growing body of evidence suggests that epigenetic modifications play a key role in HD pathogenesis. Expression of the disease protein, huntingtin, leads to extensive transcriptional dysregulation due to disruption of histone-modifying complexes and altered interactions with chromatin-related factors. Such epigenetic mechanisms also readily respond to environmental factors, which are now thought to influence the risk, onset and progression of neurodegenerative disorders, including HD. DNA methylation is an epigenetic modification that has been studied intensively, however, its role in HD is just emerging. In this review, DNA methylation differences associated with HD will be summarized, as well as the role of environmental factors to alter DNA methylation in a manner that could alter disease phenotypes. Further, transgenerational epigenetic inheritance will be discussed in the context of relevant environmental factors and their potential links to HD. The study of epigenetic states in HD presents an opportunity to gain new insights into risk factors and pathogenic mechanisms associated with HD, as well as to inform about treatment options.

The epigenetics of aging and neurodegeneration

Epigenetics is a quickly growing field encompassing mechanisms regulating gene expression that do not involve changes in the genotype. Epigenetics is of increasing relevance to neuroscience, with epigenetic mechanisms being implicated in brain development and neuronal differentiation, as well as in more dynamic processes related to cognition. Epigenetic regulation covers multiple levels of gene expression; from direct modifications of the DNA and histone tails, regulating the level of transcription, to interactions with messenger RNAs, regulating the level of translation. Importantly, epigenetic dysregulation currently garners much attention as a pivotal player in aging and age-related neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, and Huntington's disease, where it may mediate interactions between genetic and environmental risk factors, or directly interact with disease-specific pathological factors. We review current knowledge about the major epigenetic mechanisms, including DNA methylation and DNA demethylation, chromatin remodeling and non-coding RNAs, as well as the involvement of these mechanisms in normal aging and in the pathophysiology of the most common neurodegenerative diseases. Additionally, we examine the current state of epigenetics-based therapeutic strategies for these diseases, which either aim to restore the epigenetic homeostasis or skew it to a favorable direction to counter disease pathology. Finally, methodological challenges of epigenetic investigations and future perspectives are discussed.

Genetic analysis of genes causing hypertension and stroke in spontaneously hypertensive rats: Gene expression profiles in the kidneys

Spontaneously hypertensive rats (SHRs) and stroke-prone SHRs (SHRSP) are frequently used as models not only of essential hypertension and stroke, but also of attention-deficit hyperactivity disorder (ADHD). Normotensive Wistar-Kyoto (WKY) rats are normally used as controls in these studies. In the present study, we aimed to identify the genes causing hypertension and stroke, as well as the genes involved in ADHD using these rats. We previously analyzed gene expression profiles in the adrenal glands and brain. Since the kidneys can directly influence the functions of the cardiovascular, endocrine and sympathetic nervous systems, gene expression profiles in the kidneys of the 3 rat strains were examined using genome-wide microarray technology when the rats were 3 and 6 weeks old, a period in which rats are considered to be in a pre-hypertensive state. Gene expression profiles were compared between the SHRs and WKY rats and also between the SHRSP and SHRs. A total of 232 unique genes showing more than a 4-fold increase or less than a 4-fold decrease in expression were isolated as SHR- and SHRSP-specific genes. Candidate genes were then selected using two different web tools: the 1st tool was the Database for Annotation, Visualization and Integrated Discovery (DAVID), which was used to search for significantly enriched genes and categorized them using Gene Ontology (GO) terms, and the 2nd was Ingenuity Pathway Analysis (IPA), which was used to search for interactions among SHR- and also SHRSP‑specific genes. The analyses of SHR-specific genes using IPA revealed that B-cell CLL/lymphoma 6 (Bcl6) and SRY (sex determining region Y)-box 2 (Sox2) were possible candidate genes responsible for causing hypertension in SHRs. Similar analyses of SHRSP-specific genes revealed that angiotensinogen (Agt), angiotensin II receptor-associated protein (Agtrap) and apolipoprotein H (Apoh) were possible candidate genes responsible for triggering strokes. Since our results revealed that SHRSP-specific genes isolated from the kidneys of rats at 6 weeks of age, included 6 genes related to Huntington's disease, we discussed the genetic association between ADHD and Huntington's disease.

Striatal long noncoding RNA Abhd11os is neuroprotective against an N-terminal fragment of mutant huntingtin in vivo

A large number of gene products that are enriched in the striatum have ill-defined functions, although they may have key roles in age-dependent neurodegenerative diseases affecting the striatum, especially Huntington disease (HD). In the present study, we focused on Abhd11os, (called ABHD11-AS1 in human) which is a putative long noncoding RNA (lncRNA) whose expression is enriched in the mouse striatum. We confirm that despite the presence of 2 small open reading frames (ORFs) in its sequence, Abhd11os is not translated into a detectable peptide in living cells. We demonstrate that Abhd11os levels are markedly reduced in different mouse models of HD. We performed in vivo experiments in mice using lentiviral vectors encoding either Abhd11os or a small hairpin RNA targeting Abhd11os. Results show that Abhd11os overexpression produces neuroprotection against an N-terminal fragment of mutant huntingtin, whereas Abhd11os knockdown is protoxic. These novel results indicate that the loss lncRNA Abhd11os likely contribute to striatal vulnerability in HD. Our study emphasizes that lncRNA may play crucial roles in neurodegenerative diseases.

Possible involvement of self-defense mechanisms in the preferential vulnerability of the striatum in Huntington's disease

HD is caused by a mutation in the huntingtin gene that consists in a CAG repeat expansion translated into an abnormal poly-glutamine (polyQ) tract in the huntingtin (Htt) protein. The most striking neuropathological finding in HD is the atrophy of the striatum. The regional expression of mutant Htt (mHtt) is ubiquitous in the brain and cannot explain by itself the preferential vulnerability of the striatum in HD. mHtt has been shown to produce an early defect in transcription, through direct alteration of the function of key regulators of transcription and in addition, more indirectly, as a result of compensatory responses to cellular stress. In this review, we focus on gene products that are preferentially expressed in the striatum and have down- or up-regulated expression in HD and, as such, may play a crucial role in the susceptibility of the striatum to mHtt. Many of these striatal gene products are for a vast majority down-regulated and more rarely increased in HD. Recent research shows that some of these striatal markers have a pro-survival/neuroprotective role in neurons (e.g., MSK1, A2A, and CB1 receptors) whereas others enhance the susceptibility of striatal neurons to mHtt (e.g., Rhes, RGS2, D2 receptors). The down-regulation of these latter proteins may be considered as a potential self-defense mechanism to slow degeneration. For a majority of the striatal gene products that have been identified so far, their function in the striatum is unknown and their modifying effects on mHtt toxicity remain to be experimentally addressed. Focusing on these striatal markers may contribute to a better understanding of HD pathogenesis, and possibly the identification of novel therapeutic targets.

Proteostasis in striatal cells and selective neurodegeneration in Huntington's disease

Selective neuronal loss is a hallmark of neurodegenerative diseases, including Huntington’s disease (HD). Although mutant huntingtin, the protein responsible for HD, is expressed ubiquitously, a subpopulation of neurons in the striatum is the first to succumb. In this review, we examine evidence that protein quality control pathways, including the ubiquitin proteasome system, autophagy, and chaperones, are significantly altered in striatal neurons. These alterations may increase the susceptibility of striatal neurons to mutant huntingtin-mediated toxicity. This novel view of HD pathogenesis has profound therapeutic implications: protein homeostasis pathways in the striatum may be valuable targets for treating HD and other misfolded protein disorders.

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.

Striatal development involves a switch in gene expression networks, followed by a myelination event: implications for neuropsychiatric disease

Because abnormal development of striatal neurons is thought to be the part of pathology underlying major psychiatric illnesses, we studied the expression pattern of genes involved in striatal development and of genes comprising key striatal-specific pathways, during an active striatal maturation period, the first two postnatal weeks in rat. This period parallels human striatal development during the second trimester, when prenatal stress is though to lead to increased risk for neuropsychiatric disorders. To identify genes involved in this developmental process, we used subtractive hybridization, followed by quantitative real-time PCR, which allowed us to characterize the developmental expression of over 60 genes, many not previously known to play a role in neuromaturation. Of these 12 were novel transcripts, which did not match known genes, but which showed strict developmental expression and may play a role in striatal neurodevelopment. An additional 89 genes were identified as strong candidates for involvement in this neurodevelopmental process. We show that during the first two postnatal weeks in rat, an early gene expression network, still lacking key striatal-specific signaling pathways, is downregulated and replaced by a mature gene expression network, containing key striatal-specific genes including the dopamine D1 and D2 receptors, conferring to these neurons their functional identity. Therefore, before this developmental switch, striatal neurons lack many of their key phenotypic characteristics. This maturation process is followed by a striking rise in expression of myelination genes, indicating a striatal-specific myelination event. Such strictly controlled developmental program has the potential to be a point of susceptibility to disruption by external factors. Indeed, this period is known to be a susceptibility period in both humans and rats.

Selective histone deacetylase (HDAC) inhibition imparts beneficial effects in Huntington's disease mice: implications for the ubiquitin-proteasomal and autophagy systems

We previously demonstrated that the histone deacetylase (HDAC) inhibitor, 4b, which preferentially targets HDAC1 and HDAC3, ameliorates Huntington's disease (HD)-related phenotypes in different HD model systems. In the current study, we investigated extensive behavioral and biological effects of 4b in N171-82Q transgenic mice and further explored potential molecular mechanisms of 4b action. We found that 4b significantly prevented body weight loss, improved several parameters of motor function and ameliorated Huntingtin (Htt)-elicited cognitive decline in N171-82Q transgenic mice. Pathways analysis of microarray data from the mouse brain revealed gene networks involving post-translational modification, including protein phosphorylation and ubiquitination pathways, associated with 4b drug treatment. Using real-time qPCR analysis, we validated differential regulation of several genes in these pathways by 4b, including Ube2K, Ubqln, Ube2e3, Usp28 and Sumo2, as well as several other related genes. Additionally, 4b elicited increases in the expression of genes encoding components of the inhibitor of kappaB kinase (IKK) complex. IKK activation has been linked to phosphorylation, acetylation and clearance of the Htt protein by the proteasome and the lysosome, and accordingly, we found elevated levels of phosphorylated endogenous wild-type (wt) Htt protein at serine 16 and threonine 3, and increased AcK9/pS13/pS16 immunoreactivity in cortical samples from 4b-treated mice. We further show that HDAC inhibitors prevent the formation of nuclear Htt aggregates in the brains of N171-82Q mice. Our findings suggest that one mechanism of 4b action is associated with the modulation of the ubiquitin-proteasomal and autophagy pathways, which could affect accumulation, stability and/or clearance of important disease-related proteins, such as Htt.

Forkhead box protein p1 is a transcriptional repressor of immune signaling in the CNS: implications for transcriptional dysregulation in Huntington disease

Forkhead box protein p1 (Foxp1), a transcription factor showing highly enriched expression in the striatum, has been implicated in central nervous system (CNS) development, but its role in the mature brain is unknown. In order to ascertain functional roles for Foxp1 in the CNS, we have identified gene targets for Foxp1 both in vitro and in vivo using genome-wide expression microarrays and chromatin-immunoprecipitation followed by high-throughput sequencing (ChIP-seq) assays. We found that mouse Foxp1 overexpression in striatal cells elicited expression changes of genes related to immune signaling, transcriptional regulation and a manually curated Huntington's disease (HD)-signaling pathway. Similar results were found when the gene expression data set was integrated with Foxp1-binding data determined from ChIP-seq analysis. In vivo lentiviral-mediated overexpression of human FOXP1 in the context of mutant huntingtin (Htt) protein resulted in a robust downregulation of glial cell-associated, immune genes, including those encoding a variety of cytokines and chemokines. Furthermore, Foxp1-induced expression changes were significantly negatively correlated with those changes elicited by mutant Htt protein in several different HD mouse models, and most significantly in post-mortem caudate from human HD subjects. We finally show that Foxp1 interacts with mutant Htt protein in mouse brain and is present in nuclear Htt aggregates in the striatum of R6/1 transgenic mice. These findings implicate Foxp1 as a key repressor of immune signaling in the CNS and suggest that the loss of Foxp1-mediated gene regulation in HD contributes to the immune dysfunction in this disease. We further suggest that Foxp1-regulated pathways might be important mediators of neuronal-glial cell communication.

Suppression of aggregate formation of mutant huntingtin potentiates CREB-binding protein sequestration and apoptotic cell death

Although aggregates of mutant huntingtin are a pathological hallmark of Huntington's disease (HD), the role of inclusions in the pathogenesis remains inconclusive. Sequestration of CBP into mutant huntingtin has been reported to play a significant role in the pathogenesis of HD. However, whether aggregate formation of mutant huntingtin is necessary for the sequestration of CBP is not fully elucidated. In the present study, YFP was linked into either N- or C-terminus of exon 1 huntingtin to modulate the aggregation propensity of huntingtin. Efficient aggregation was observed with C-terminally YFP-tagged huntingtin (MT-YFP) whereas N-terminally YFP-tagged mutant huntingtin (YFP-MT) exhibited significantly attenuated aggregation frequency. The sequestration of CBP and apoptosis were significantly increased with YFP-MT. Microarray study showed transcriptional changes favoring apoptosis. Furthermore, expression of PGC1-α was significantly decreased with YFP-MT. The data strongly demonstrate that microscopically non-aggregate form of mutant huntingtin might exert essential pathogenic role of mutant huntingtin in HD.

Aging causes distinct characteristics of polyglutamine amyloids in vivo

Polyglutamine diseases, including Machado-Joseph disease and Huntington's disease, typically appear in midlife and are characterized by amyloid accumulations of abnormally expanded polyglutamine proteins. Although there is growing evidence that aging has an important role in the occurrence of such diseases, the role of aging in the late onset of these diseases is not well understood. Recent studies showed that differences in amyloid conformation from different brain regions lead to differing toxicity. We hypothesized that higher amyloid toxicity at later ages might cause the late onset of polyglutamine diseases. Using a method for temporal and regional gene expression targeting (TARGET) in Drosophila, we showed that transient polyglutamine expression caused more severe neurodegeneration in older flies than in younger flies. Moreover, the polyglutamine amyloids themselves showed distinct characteristics in relation to age; those from older flies were less resistant to SDS and more effective at seeding polymerization than those from younger flies, suggesting that the polyglutamine amyloids in aged individuals may have higher toxicity. These findings show that age-related changes in amyloid characteristics may be a trigger for late-onset polyglutamine diseases.

In vivo cell-autonomous transcriptional abnormalities revealed in mice expressing mutant huntingtin in striatal but not cortical neurons

Huntington's disease (HD), caused by a CAG repeat expansion in the huntingtin (HTT) gene, is characterized by abnormal protein aggregates and motor and cognitive dysfunction. Htt protein is ubiquitously expressed, but the striatal medium spiny neuron (MSN) is most susceptible to dysfunction and death. Abnormal gene expression represents a core pathogenic feature of HD, but the relative roles of cell-autonomous and non-cell-autonomous effects on transcription remain unclear. To determine the extent of cell-autonomous dysregulation in the striatum in vivo, we examined genome-wide RNA expression in symptomatic D9-N171-98Q (a.k.a. DE5) transgenic mice in which the forebrain expression of the first 171 amino acids of human Htt with a 98Q repeat expansion is limited to MSNs. Microarray data generated from these mice were compared with those generated on the identical array platform from a pan-neuronal HD mouse model, R6/2, carrying two different CAG repeat lengths, and a relatively high degree of overlap of changes in gene expression was revealed. We further focused on known canonical pathways associated with excitotoxicity, oxidative stress, mitochondrial dysfunction, dopamine signaling and trophic support. While genes related to excitotoxicity, dopamine signaling and trophic support were altered in both DE5 and R6/2 mice, which may be either cell autonomous or non-cell autonomous, genes related to mitochondrial dysfunction, oxidative stress and the peroxisome proliferator-activated receptor are primarily affected in DE5 transgenic mice, indicating cell-autonomous mechanisms. Overall, HD-induced dysregulation of the striatal transcriptome can be largely attributed to intrinsic effects of mutant Htt, in the absence of expression in cortical neurons.

Transcriptional activation of REST by Sp1 in Huntington's disease models

In Huntington's disease (HD), mutant huntingtin (mHtt) disrupts the normal transcriptional program of disease neurons by altering the function of several gene expression regulators such as Sp1. REST (Repressor Element-1 Silencing Transcription Factor), a key regulator of neuronal differentiation, is also aberrantly activated in HD by a mechanism that remains unclear. Here, we show that the level of REST mRNA is increased in HD mice and in NG108 cells differentiated into neuronal-like cells and expressing a toxic mHtt fragment. Using luciferase reporter gene assay, we delimited the REST promoter regions essential for mHtt-mediated REST upregulation and found that they contain Sp factor binding sites. We provide evidence that Sp1 and Sp3 bind REST promoter and interplay to fine-tune REST transcription. In undifferentiated NG108 cells, Sp1 and Sp3 have antagonistic effect, Sp1 acting as an activator and Sp3 as a repressor. Upon neuronal differentiation, we show that the amount and ratio of Sp1/Sp3 proteins decline, as does REST expression, and that the transcriptional role of Sp3 shifts toward a weak activator. Therefore, our results provide new molecular information to the transcriptional regulation of REST during neuronal differentiation. Importantly, specific knockdown of Sp1 abolishes REST upregulation in NG108 neuronal-like cells expressing mHtt. Our data together with earlier reports suggest that mHtt triggers a pathogenic cascade involving Sp1 activation, which leads to REST upregulation and repression of neuronal genes.

Mitochondrial loss, dysfunction and altered dynamics in Huntington's disease

Although a direct causative pathway from the gene mutation to the selective neostriatal neurodegeneration remains unclear in Huntington's disease (HD), one putative pathological mechanism reported to play a prominent role in the pathogenesis of this neurological disorder is mitochondrial dysfunction. We examined mitochondria in preferentially vulnerable striatal calbindin-positive neurons in moderate-to-severe grade HD patients, using antisera against mitochondrial markers of COX2, SOD2 and cytochrome c. Combined calbindin and mitochondrial marker immunofluorescence showed a significant and progressive grade-dependent reduction in the number of mitochondria in spiny striatal neurons, with marked alteration in size. Consistent with mitochondrial loss, there was a reduction in COX2 protein levels using western analysis that corresponded with disease severity. In addition, both mitochondrial transcription factor A, a regulator of mtDNA, and peroxisome proliferator-activated receptor-co-activator gamma-1 alpha, a key transcriptional regulator of energy metabolism and mitochondrial biogenesis, were also significantly reduced with increasing disease severity. Abnormalities in mitochondrial dynamics were observed, showing a significant increase in the fission protein Drp1 and a reduction in the expression of the fusion protein mitofusin 1. Lastly, mitochondrial PCR array profiling in HD caudate nucleus specimens showed increased mRNA expression of proteins involved in mitochondrial localization, membrane translocation and polarization and transport that paralleled mitochondrial derangement. These findings reveal that there are both mitochondrial loss and altered mitochondrial morphogenesis with increased mitochondrial fission and reduced fusion in HD. These findings provide further evidence that mitochondrial dysfunction plays a critical role in the pathogenesis of HD.

Differential vulnerability of neurons in Huntington's disease: the role of cell type-specific features

Abnormal expansion of a polyglutamine tract in huntingtin (Htt) protein results in Huntington's disease (HD), an autosomal dominant neurodegenerative disorder involving progressive loss of motor and cognitive function. Contrasting with the ubiquitous tissue expression of polyglutamine-expanded Htt, HD pathology is characterized by the increased vulnerability of specific neuronal populations within the striatum and the cerebral cortex. Morphological, biochemical, and functional characteristics of neurons affected in HD that might render these cells more vulnerable to the toxic effects of polyglutamine-Htt are covered in this review. The differential vulnerability of neurons observed in HD is discussed in the context of various major pathogenic mechanisms proposed to date, and in line with evidence showing a 'dying-back' pattern of degeneration in affected neuronal populations.

Transcriptional changes in Huntington disease identified using genome-wide expression profiling and cross-platform analysis

Evaluation of transcriptional changes in the striatum may be an effective approach to understanding the natural history of changes in expression contributing to the pathogenesis of Huntington disease (HD). We have performed genome-wide expression profiling of the YAC128 transgenic mouse model of HD at 12 and 24 months of age using two platforms in parallel: Affymetrix and Illumina. The data from these two powerful platforms were integrated to create a combined rank list, thereby revealing the identity of additional genes that proved to be differentially expressed between YAC128 and control mice. Using this approach, we identified 13 genes to be differentially expressed between YAC128 and controls which were validated by quantitative real-time PCR in independent cohorts of animals. In addition, we analyzed additional time points relevant to disease pathology: 3, 6 and 9 months of age. Here we present data showing the evolution of changes in the expression of selected genes: Wt1, Pcdh20 and Actn2 RNA levels change as early as 3 months of age, whereas Gsg1l, Sfmbt2, Acy3, Polr2a and Ppp1r9a RNA expression levels are affected later, at 12 and 24 months of age. We also analyzed the expression of these 13 genes in human HD and control brain, thereby revealing changes in SLC45A3, PCDH20, ACTN2, DDAH1 and PPP1R9A RNA expression. Further study of these genes may unravel novel pathways contributing to HD pathogenesis. DDBJ/EMBL/GenBank accession no: GSE19677.

Neurocognitive disorders: cluster 1 of the proposed meta-structure for DSM-V and ICD-11

BACKGROUND

In an effort to group mental disorders on the basis of aetiology, five clusters have been proposed. In this paper, we consider the validity of the first cluster, neurocognitive disorders, within this proposal. These disorders are categorized as 'Dementia, Delirium, and Amnestic and Other Cognitive Disorders' in DSM-IV and 'Organic, including Symptomatic Mental Disorders' in ICD-10.

METHOD

We reviewed the literature in relation to 11 validating criteria proposed by a Study Group of the DSM-V Task Force as applied to the cluster of neurocognitive disorders.

RESULTS

'Neurocognitive' replaces the previous terms 'cognitive' and 'organic' used in DSM-IV and ICD-10 respectively as the descriptor for disorders in this cluster. Although cognitive/organic problems are present in other disorders, this cluster distinguishes itself by the demonstrable neural substrate abnormalities and the salience of cognitive symptoms and deficits. Shared biomarkers, co-morbidity and course offer less persuasive evidence for a valid cluster of neurocognitive disorders. The occurrence of these disorders subsequent to normal brain development sets this cluster apart from neurodevelopmental disorders. The aetiology of the disorders is varied, but the neurobiological underpinnings are better understood than for mental disorders in any other cluster.

CONCLUSIONS

Neurocognitive disorders meet some of the salient criteria proposed by the Study Group of the DSM-V Task Force to suggest a classification cluster. Further developments in the aetiopathogenesis of these disorders will enhance the clinical utility of this cluster.

Expression analysis of novel striatal-enriched genes in Huntington disease

Selective degeneration of striatal neurons is a pathologic hallmark of Huntington disease (HD). The exact mechanism(s) behind this specific neurodegeneration is still unknown. Expression studies of diseased human post-mortem brain, as well as different mouse models exhibiting striatal degeneration, have demonstrated changes in the expression of many important genes with a large proportion of changes being observed in the striatal-enriched genes. These investigations have raised questions about how enrichment of particular transcripts in the striatum can lead to its selective vulnerability to neurodegeneration. Monitoring the expression changes of striatal-enriched genes during the course of the disease may be informative about their potential involvement in selective degeneration. In this study, we analyzed a Serial Analysis of Gene Expression (SAGE) database (www.mouseatlas.org) and compared the mouse striatum to 18 other brain regions to generate a novel list of striatal-enriched transcripts. These novel striatal-enriched transcripts were subsequently evaluated for expression changes in the YAC128 mouse model of HD, and differentially expressed transcripts were further examined in human post-mortem caudate samples. We identified transcripts with altered expression in YAC128 mice, which also showed consistent expression changes in human post-mortem tissue. The identification of novel striatal-enriched genes with altered expression in HD offers new avenues of study, leading towards a better understanding of specific pathways involved in the selective degeneration of striatal neurons in HD.

Distinct conformations of in vitro and in vivo amyloids of huntingtin-exon1 show different cytotoxicity

A hallmark of polyglutamine diseases, including Huntington disease (HD), is the formation of beta-sheet-rich aggregates, called amyloid, of causative proteins with expanded polyglutamines. However, it has remained unclear whether the polyglutamine amyloid is a direct cause or simply a secondary manifestation of the pathology. Here we show that huntingtin-exon1 (thtt) with expanded polyglutamines remarkably misfolds into distinct amyloid conformations under different temperatures, such as 4 degrees C and 37 degrees C. The 4 degrees C amyloid has loop/turn structures together with mostly beta-sheets, including exposed polyglutamines, whereas the 37 degrees C amyloid has more extended and buried beta-sheets. By developing a method to efficiently introduce amyloid into mammalian cells, we found that the formation of the 4 degrees C amyloid led to substantial toxicity, whereas the toxic effects of the 37 degrees C amyloid were very small. Importantly, thtt amyloids in different brain regions of HD mice also had distinct conformations. The thermolabile thtt amyloid with loop/turn structures in the striatum showed higher toxicity, whereas the rigid thtt amyloid with more extended beta-sheets in the hippocampus and cerebellum had only mild toxic effects. These studies show that the thtt protein with expanded polyglutamines can misfold into distinct amyloid conformations and, depending on the conformations, the amyloids can be either toxic or nontoxic. Thus, the amyloid conformation of thtt may be a critical determinant of cytotoxicity in HD.

Mutant huntingtin and mitochondrial dysfunction

Huntington's disease (HD) is a fatal, inherited neurodegenerative disorder that gradually robs affected individuals of memory, cognitive skills and normal movements. Although research has identified a single faulty gene, the huntingtin gene, as the cause of the disease, a cure remains elusive. Strong evidence indicates that mitochondrial impairment plays a key part in HD pathogenesis. Here, we highlight how mutant huntingtin (mtHtt) might cause mitochondrial dysfunction by either perturbing transcription of nuclear-encoded mitochondrial proteins or by direct interaction with the organelle and modulation of respiration, mitochondrial membrane potential and Ca(2+) buffering. In addition, we propose that mtHtt might convey its neurotoxicity by evoking defects in mitochondrial dynamics, organelle trafficking and fission and fusion, which, in turn, might result in bioenergetic failure and HD-linked neuronal dysfunction and cell death. Finally, we speculate how mitochondria might dictate selective vulnerability of long projection neurons, such as medium spiny neurons, which are particularly affected in HD.

The HDAC inhibitor 4b ameliorates the disease phenotype and transcriptional abnormalities in Huntington's disease transgenic mice

Transcriptional dysregulation has emerged as a core pathologic feature of Huntington's disease (HD), one of several triplet-repeat disorders characterized by movement deficits and cognitive dysfunction. Although the mechanisms contributing to the gene expression deficits remain unknown, therapeutic strategies have aimed to improve transcriptional output via modulation of chromatin structure. Recent studies have demonstrated therapeutic effects of commercially available histone deacetylase (HDAC) inhibitors in several HD models; however, the therapeutic value of these compounds is limited by their toxic effects. Here, beneficial effects of a novel pimelic diphenylamide HDAC inhibitor, HDACi 4b, in an HD mouse model are reported. Chronic oral administration of HDACi 4b, beginning after the onset of motor deficits, significantly improved motor performance, overall appearance, and body weight of symptomatic R6/2(300Q) transgenic mice. These effects were associated with significant attenuation of gross brain-size decline and striatal atrophy. Microarray studies revealed that HDACi 4b treatment ameliorated, in part, alterations in gene expression caused by the presence of mutant huntingtin protein in the striatum, cortex, and cerebellum of R6/2(300Q) transgenic mice. For selected genes, HDACi 4b treatment reversed histone H3 hypoacetylation observed in the presence of mutant huntingtin, in association with correction of mRNA expression levels. These findings suggest that HDACi 4b, and possibly related HDAC inhibitors, may offer clinical benefit for HD patients and provide a novel set of potential biomarkers for clinical assessment.

Functional roles for the striatal-enriched transcription factor, Bcl11b, in the control of striatal gene expression and transcriptional dysregulation in Huntington's disease

Transcriptional dysregulation has emerged as a central pathogenic mechanism in Huntington's disease (HD), which is associated with neuropathological changes predominantly in the striatum. Here we demonstrate that expression of Bcl11b (a.k.a. CTIP2), a transcription factor exhibiting highly-enriched localization in adult striatum, is significantly decreased in HD cells, mouse models and human subjects and that overexpression of Bcl11b attenuates toxic effects of mutant huntingtin in cultured striatal neurons. We show that Bcl11b directly activates the proximal promoter regions of striatal-enriched genes and can increase mRNA levels of striatal-expressing genes. We further demonstrate an interaction between Bcl11b and huntingtin protein in cultured cells and brain homogenates from HD R6/1 and YAC72 transgenic mice. We propose that sequestration and/or decreased expression of Bcl11b in HD is responsible, at least in part, for the dysregulation of striatal gene expression observed in HD and may contribute to the specificity of pathology observed in this disease.

Increased expression of Bim contributes to the potentiation of serum deprivation-induced apoptotic cell death in Huntington's disease knock-in striatal cell line

Given that mutant huntingtin may cause dysregulation of gene expression in striatal neurons leading to the neuronal death, we examined the expression level of Bcl-2 interacting mediator of cell death (Bim) in immortalized wild type STHdh(Q7) and knock-in mutant STHdh(Q111) striatal cell lines to understand the underlying mechanism by which mutant huntingtin causes selective death of striatal neurons. Mutant STHdh(Q111) exhibited significantly increased expression level of Bim compared to STHdh(Q7). Serum deprivation resulted in potentiated apoptotic death in STHdh(Q111) compared to STHdh(Q7). However, the expression level of Bim was not changed with serum deprivation in both cell lines. Activation of pro-survival pathway with IGF-1 significantly attenuated serum deprivation-induced neuronal death in both cell lines and attenuated mutant huntingtin-mediated potentiated apoptotic death in STHdh(Q111). The level of active Akt was significantly elevated in STHdh(Q111) compared to STHdh(Q7) resulting in the phosphorylation of a FKHRL1, a forkhead transcription factor regulating Bim expression in neuronal cells. These data suggest that the presence of mutant huntingtin causes transcriptional dysregulation favoring apoptosis and that Akt pro-survival pathway in STHdh(Q111) is not compromised due to the presence of mutant huntingtin. Therefore, activation of this pathway may contribute to the protection of striatal neurons in Huntington's disease.

Polyglutamine gene function and dysfunction in the ageing brain

The coordinated regulation of gene expression and protein interactions determines how mammalian nervous systems develop and retain function and plasticity over extended periods of time such as a human life span. By studying mutations that occur in a group of genes associated with chronic neurodegeneration, the polyglutamine (polyQ) disorders, it has emerged that CAG/glutamine stretches play important roles in transcriptional regulation and protein-protein interactions. However, it is still unclear what the many structural and functional roles of CAG and other low-complexity sequences in eukaryotic genomes are, despite being the most commonly shared peptide fragments in such proteomes. In this review we examine the function of genes responsible for at least 10 polyglutamine disorders in relation to the nervous system and how expansion mutations lead to neuronal dysfunction, by particularly focusing on Huntington's disease (HD). We argue that the molecular and cellular pathways that turn out to be dysfunctional during such diseases, as a consequence of a CAG expansion, are also involved in the ageing of the central nervous system. These are pathways that control protein degradation systems (including molecular chaperones), axonal transport, redox-homeostasis and bioenergetics. CAG expansion mutations confer novel properties on proteins that lead to a slow-progressing neuronal pathology and cell death similar to that found in other age-related conditions such as Alzheimer's and Parkinson's diseases.

Tiagabine, a GABA uptake inhibitor, attenuates 3-nitropropionic acid-induced alterations in various behavioral and biochemical parameters in rats

Huntington's disease is an incurable, adult-onset, dominantly inherited neurodegenerative disease. The clinical symptoms of the disease are primarily related to the progressive death of medium spiny gamma-amino butyric acid (GABAergic) neurons in the striatum and the deep layers of the cortex. Further in the later stage of life, the degeneration extends to a variety of brain regions, including the hypothalamus and hippocampus. Various GABAergic agents are being attempted for the treatment of Huntington's disease. Tiagabine [(R)-N-(4, 4-di-(3-methylthien-2-yl) but-3-enyl) nipecotic acid], a GABA uptake inhibitor, widely used in the treatment of seizures, is suggested to have neuroprotective properties. However, none of the study has elucidated its effect in the treatment of Huntington's disease and related pathologies. We explored whether tiagabine may attenuate various behavioral and biochemical alterations induced by systemic administration of 3-nitropropionic acid (an inhibitor of complex II of the electron transport chain), an accepted experimental animal model of Huntington's disease phenotype. Intraperitoneal administration of 3-nitropropionic acid (20 mg/kg., i.p.) for 4 days produced hypolocomotion, muscle incoordination and memory deficit. Daily treatment with tiagabine (5 and 10 mg/kg., i.p.) 30 min prior to 3-nitropropionic acid administration for a total of 4 days, significantly improved the 3-nitropropionic acid-induced motor and cognitive impairment. Biochemical analysis of the whole brain revealed that systemic 3-nitropropionic acid administration significantly increased lipid peroxidation, nitrite levels, total RNA levels and decreased reduced glutathione and succinate dehydrogenase activity which was reversed by daily treatment with tiagabine. Further, there was a decrease in adrenal ascorbic acid levels following daily administration of 3-nitropropionic acid, which was reversed by administration of tiagabine. The results of the present study indicate that tiagabine (5 and 10 mg/kg., i.p.) significantly reversed 3-nitropropionic acid-induced alterations in various behavioral and biochemical parameters and it could be a therapeutic agent for the treatment of Huntington's disease.

Palmitoyl protein thioesterase 1 (Ppt1)-deficient mouse neurons show alterations in cholesterol metabolism and calcium homeostasis prior to synaptic dysfunction

Infantile neuronal ceroid lipofuscinosis (INCL) is a severe neurodegenerative disorder of children, characterized by selective death of neocortical neurons. To understand early disease mechanisms in INCL, we have studied Ppt1(Deltaex4) knock-out mouse neurons in culture and acute brain slices. Global transcript profiling showed deregulation of key neuronal functions in knock-out mice including cholesterol metabolism, neuronal maturation, and calcium homeostasis. Cholesterol metabolism showed major changes; sterol biosynthesis was enhanced and steady-state amounts of sterols were altered at the cellular level. Changes were also present in early maturation of Ppt1(Deltaex4) neurons indicated by increased proliferative capacity of neuronal stem cells. Knock-out neurons presented unaltered electrophysiological properties suggesting uncompromised synaptic function in young animals. However, knock-out neurons exhibited more efficient recovery from glutamate-induced calcium transients, possibly indicating neuroprotective activation. This study established that the neuronal deregulation in INCL is linked to neuronal maturation, lipid metabolism and calcium homeostasis.

Glycolipid and ganglioside metabolism imbalances in Huntington's disease

We have explored genome-wide expression of genes related to glycobiology in exon 1 transgenic Huntington's disease (HD) mice using a custom-designed GLYCOv2 chip and Affymetrix microarray analyses. We validated, using quantitative real-time PCR, abnormal expression levels of genes encoding glycosyltransferases in the striatum of R6/1 transgenic mice, as well as in postmortem caudate from human HD subjects. Many of these genes show differential regional expression within the CNS, as indicated by in situ hybridization analysis, suggesting region-specific regulation of this system in the brain. We further show disrupted patterns of glycolipids (acidic and neutral lipids) and/or ganglioside levels in both the forebrain of the R6/1 transgenic mice and caudate samples from human HD subjects. These findings reveal novel disruptions in glycolipid/ganglioside metabolic pathways in the pathology of HD and suggest that the development of new targets to restore glycosphingolipid balance may act to ameliorate some symptoms of HD.