Huntingtin interacts with REST/NRSF to modulate the transcription of NRSE-controlled neuronal genes

Scalable production in human cells and biochemical characterization of full-length normal and mutant huntingtin

Huntingtin (Htt) is a 350 kD intracellular protein, ubiquitously expressed and mainly localized in the cytoplasm. Huntington’s disease (HD) is caused by a CAG triplet amplification in exon 1 of the corresponding gene resulting in a polyglutamine (polyQ) expansion at the N-terminus of Htt. Production of full-length Htt has been difficult in the past and so far a scalable system or process has not been established for recombinant production of Htt in human cells. The ability to produce Htt in milligram quantities would be a prerequisite for many biochemical and biophysical studies aiming in a better understanding of Htt function under physiological conditions and in case of mutation and disease. For scalable production of full-length normal (17Q) and mutant (46Q and 128Q) Htt we have established two different systems, the first based on doxycycline-inducible Htt expression in stable cell lines, the second on “gutless” adenovirus mediated gene transfer. Purified material has then been used for biochemical characterization of full-length Htt. Posttranslational modifications (PTMs) were determined and several new phosphorylation sites were identified. Nearly all PTMs in full-length Htt localized to areas outside of predicted alpha-solenoid protein regions. In all detected N-terminal peptides methionine as the first amino acid was missing and the second, alanine, was found to be acetylated. Differences in secondary structure between normal and mutant Htt, a helix-rich protein, were not observed in our study. Purified Htt tends to form dimers and higher order oligomers, thus resembling the situation observed with N-terminal fragments, although the mechanism of oligomer formation may be different.

Neuron-restrictive silencer factor in periaqueductal gray contributes to remifentanil-induced postoperative hyperalgesia via repression of the mu-opioid receptor

BACKGROUND

The ultra-short-acting mu-opioid receptor (MOR) agonist remifentanil induces postoperative hyperalgesia both in preclinical and clinical research studies. However, the precise mechanisms remain unclear, although changes in opioid receptor expression might be a correlative feature. Neuron-restrictive silencer factor (NRSF) functions as a crucial regulator of MOR expression in specific neuronal cells. Using a mouse model of incisional postoperative pain, we assessed the expression of MOR and NRSF and investigated whether disruption of NRSF expression could prevent the postoperative nociceptive sensitization induced by surgical incision and subcutaneous infusion of remifentanil.

METHODS

Paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) were independently used to assess mechanical allodynia and thermal hyperalgesia after surgery and cerebral ventricle injection of NRSF antisense oligonucleotide. Western blotting analyses were preformed to assess the expression levels of MOR and NRSF.

RESULTS

NRSF expression levels were enhanced after intraoperative infusion of remifentanil, resulting in repression of MOR expression in the periaqueductal gray (PAG). NRSF blockade with an NRSF antisense oligonucleotide significantly enhanced the expression levels of MOR and alleviated mechanical allodynia and thermal hyperalgesia induced by intraoperative infusion of remifentanil.

CONCLUSION

NRSF functions as a negative regulator of MOR in PAG and contributes to remifentanil-induced postoperative hyperalgesia. NRSF in PAG may be a potential target for this pain therapy.

Rest represses maturation within migrating facial branchiomotor neurons

The vertebrate brain arises from the complex organization of millions of neurons. Neurogenesis encompasses not only cell fate specification from neural stem cells, but also the terminal molecular and morphological maturation of neurons at correct positions within the brain. RE1-silencing transcription factor (Rest) is expressed in non-neural tissues and neuronal progenitors where it inhibits the terminal maturation of neurons by repressing hundreds of neuron-specific genes. Here we show that Rest repression of maturation is intimately linked with the migratory capability of zebrafish facial branchiomotor neurons (FBMNs), which undergo a characteristic tangential migration from hindbrain rhombomere (r) 4 to r6/r7 during development. We establish that FBMN migration is increasingly disrupted as Rest is depleted in zebrafish rest mutant embryos, such that around two-thirds of FBMNs fail to complete migration in mutants depleted of both maternal and zygotic Rest. Although Rest is broadly expressed, we show that de-repression or activation of Rest target genes only within FBMNs is sufficient to disrupt their migration. We demonstrate that this migration defect is due to precocious maturation of FBMNs, based on both morphological and molecular criteria. We further show that the Rest target gene and alternative splicing factor srrm4 is a key downstream regulator of maturation; Srrm4 knockdown partially restores the ability of FBMNs to migrate in rest mutants while preventing their precocious morphological maturation. Rest must localize to the nucleus to repress its targets, and its subcellular localization is highly regulated: we show that targeting Rest specifically to FBMN nuclei rescues FBMN migration in Rest-deficient embryos. We conclude that Rest functions in FBMN nuclei to inhibit maturation until the neurons complete their migration.

Vaccinia-Related Kinase 2 Controls the Stability of the Eukaryotic Chaperonin TRiC/CCT by Inhibiting the Deubiquitinating Enzyme USP25

Molecular chaperones monitor the proper folding of misfolded proteins and function as the first line of defense against mutant protein aggregation in neurodegenerative diseases. The eukaryotic chaperonin TRiC is a potent suppressor of mutant protein aggregation and toxicity in early stages of disease progression. Elucidation of TRiC functional regulation will enable us to better understand the pathological mechanisms of neurodegeneration. We have previously shown that vaccinia-related kinase 2 (VRK2) downregulates TRiC protein levels through the ubiquitin-proteasome system by recruiting the E3 ligase COP1. However, although VRK2 activity was necessary in TRiC downregulation, the phosphorylated substrate was not determined. Here, we report that USP25 is a novel TRiC interacting protein that is also phosphorylated by VRK2. USP25 catalyzed deubiquitination of the TRiC protein and stabilized the chaperonin, thereby reducing accumulation of misfolded polyglutamine protein aggregates. Notably, USP25 deubiquitinating activity was suppressed when VRK2 phosphorylated the Thr(680), Thr(727), and Ser(745) residues. Impaired USP25 deubiquitinating activity after VRK2-mediated phosphorylation may be a critical pathway in TRiC protein destabilization.

Rest mutant zebrafish swim erratically and display atypical spatial preferences

The Rest/Nrsf transcriptional repressor modulates expression of a large set of neural specific genes. Many of these target genes have well characterized roles in nervous system processes including development, plasticity and synaptogenesis. However, the impact of Rest-mediated transcriptional regulation on behavior has been understudied due in part to the embryonic lethality of the mouse knockout. To investigate the requirement for Rest in behavior, we employed the zebrafish rest mutant to explore a range of behaviors in adults and larva. Adult rest mutants of both sexes showed abnormal behaviors in a novel environment including increased vertical swimming, erratic swimming patterns and a proclivity for the tank walls. Adult males also had diminished reproductive success. At 6 days post fertilization (dpf), rest mutant larva were hypoactive, but displayed normal evoked responses to light and sound stimuli. Overall, these results provide evidence that rest dysfunction produces atypical swimming patterns and preferences in adults, and reduced locomotor activity in larvae. This study provides the first behavioral analysis of rest mutants and reveals specific behaviors that are modulated by Rest.

Identification of novel radiation-induced p53-dependent transcripts extensively regulated during mouse brain development

Ionizing radiation is a potent activator of the tumor suppressor gene p53, which itself regulates the transcription of genes involved in canonical pathways such as the cell cycle, DNA repair and apoptosis as well as other biological processes like metabolism, autophagy, differentiation and development. In this study, we performed a meta-analysis on gene expression data from different in vivo and in vitro experiments to identify a signature of early radiation-responsive genes which were predicted to be predominantly regulated by p53. Moreover, we found that several genes expressed different transcript isoforms after irradiation in a p53-dependent manner. Among this gene signature, we identified novel p53 targets, some of which have not yet been functionally characterized. Surprisingly, in contrast to genes from the canonical p53-regulated pathways, our gene signature was found to be highly enriched during embryonic and post-natal brain development and during in vitro neuronal differentiation. Furthermore, we could show that for a number of genes, radiation-responsive transcript variants were upregulated during development and differentiation, while radiation non-responsive variants were not. This suggests that radiation exposure of the developing brain and immature cortical neurons results in the p53-mediated activation of a neuronal differentiation program. Overall, our results further increase the knowledge of the radiation-induced p53 network of the embryonic brain and provide more evidence concerning the importance of p53 and its transcriptional targets during mouse brain development.

Invited review: decoding the pathophysiological mechanisms that underlie RNA dysregulation in neurodegenerative disorders: a review of the current state of the art

Altered RNA metabolism is a key pathophysiological component causing several neurodegenerative diseases. Genetic mutations causing neurodegeneration occur in coding and noncoding regions of seemingly unrelated genes whose products do not always contribute to the gene expression process. Several pathogenic mechanisms may coexist within a single neuronal cell, including RNA/protein toxic gain-of-function and/or protein loss-of-function. Genetic mutations that cause neurodegenerative disorders disrupt healthy gene expression at diverse levels, from chromatin remodelling, transcription, splicing, through to axonal transport and repeat-associated non-ATG (RAN) translation. We address neurodegeneration in repeat expansion disorders [Huntington's disease, spinocerebellar ataxias, C9ORF72-related amyotrophic lateral sclerosis (ALS)] and in diseases caused by deletions or point mutations (spinal muscular atrophy, most subtypes of familial ALS). Some neurodegenerative disorders exhibit broad dysregulation of gene expression with the synthesis of hundreds to thousands of abnormal messenger RNA (mRNA) molecules. However, the number and identity of aberrant mRNAs that are translated into proteins - and how these lead to neurodegeneration - remain unknown. The field of RNA biology research faces the challenge of identifying pathophysiological events of dysregulated gene expression. In conclusion, we discuss current research limitations and future directions to improve our characterization of pathological mechanisms that trigger disease onset and progression.

Molecular mechanisms regulating the defects in fragile X syndrome neurons derived from human pluripotent stem cells

Fragile X syndrome (FXS) is caused by the absence of the fragile X mental retardation protein (FMRP). We have previously generated FXS-induced pluripotent stem cells (iPSCs) from patients' fibroblasts. In this study, we aimed at unraveling the molecular phenotype of the disease. Our data revealed aberrant regulation of neural differentiation and axon guidance genes in FXS-derived neurons, which are regulated by the RE-1 silencing transcription factor (REST). Moreover, we found REST to be elevated in FXS-derived neurons. As FMRP is involved in the microRNA (miRNA) pathway, we employed miRNA-array analyses and uncovered several miRNAs dysregulated in FXS-derived neurons. We found hsa-mir-382 to be downregulated in FXS-derived neurons, and introduction of mimic-mir-382 into these neurons was sufficient to repress REST and upregulate its axon guidance target genes. Our data link FMRP and REST through the miRNA pathway and show a new aspect in the development of FXS.

The Neuropathology of Huntington's Disease

The basal ganglia are a highly interconnected set of subcortical nuclei and major atrophy in one or more regions may have major effects on other regions of the brain. Therefore, the striatum which is preferentially degenerated and receives projections from the entire cortex also affects the regions to which it targets, especially the globus pallidus and substantia nigra pars reticulata. Additionally, the cerebral cortex is itself severely affected as are many other regions of the brain, especially in more advanced cases. The cell loss in the basal ganglia and the cerebral cortex is extensive. The most important new findings in Huntington's disease pathology is the highly variable nature of the degeneration in the brain. Most interestingly, this variable pattern of pathology appears to reflect the highly variable symptomatology of cases with Huntington's disease even among cases possessing the same number of CAG repeats.

microRNAs and Neurodegenerative Diseases

microRNAs (miRNAs) are small, noncoding RNA molecules that through imperfect base-pairing with complementary sequences of target mRNA molecules, typically cleave target mRNA, causing subsequent degradation or translation inhibition. Although an increasing number of studies have identified misregulated miRNAs in the neurodegenerative diseases (NDDs) Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis, which suggests that alterations in the miRNA regulatory pathway could contribute to disease pathogenesis, the molecular mechanisms underlying the pathological implications of misregulated miRNA expression and the regulation of the key genes involved in NDDs remain largely unknown. In this chapter, we provide evidence of the function and regulation of miRNAs and their association with the neurological events in NDDs. This will help improve our understanding of how miRNAs govern the biological functions of key pathogenic genes in these diseases, which potentially regulate several pathways involved in the progression of neurodegeneration. Additionally, given the growing interest in the therapeutic potential of miRNAs, we discuss current clinical challenges to developing miRNA-based therapeutics for NDDs.

Canadian Association of Neurosciences Review: Polyglutamine Expansion Neurodegenerative Diseases

Since the early 1990s, DNA triplet repeat expansions have been found to be the cause in an ever increasing number of genetic neurologic diseases. A subset of this large family of genetic diseases has the expansion of a CAG DNA triplet in the open reading frame of a coding exon. The result of this DNA expansion is the expression of expanded glutamine amino acid repeat tracts in the affected proteins, leading to the term, Polyglutamine Diseases, which is applied to this sub-family of diseases. To date, nine distinct genes are known to be linked to polyglutamine diseases, including Huntington's disease, Machado-Joseph Disease and spinobulbar muscular atrophy or Kennedy's disease. Most of the polyglutamine diseases are characterized clinically as spinocerebellar ataxias. Here we discuss recent successes and advancements in polyglutamine disease research, comparing these different diseases with a common genetic flaw at the level of molecular biology and early drug design for a family of diseases where many new research tools for these genetic disorders have been developed. Polyglutamine disease research has successfully used interdisciplinary collaborative efforts, informative multiple mouse genetic models and advanced tools of pharmaceutical industry research to potentially serve as the prototype model of therapeutic research and development for rare neurodegenerative diseases.

The oncogenic STP axis promotes triple-negative breast cancer via degradation of the REST tumor suppressor

Defining the molecular networks that drive breast cancer has led to therapeutic interventions and improved patient survival. However, the aggressive triple-negative breast cancer subtype (TNBC) remains recalcitrant to targeted therapies because its molecular etiology is poorly defined. In this study, we used a forward genetic screen to discover an oncogenic network driving human TNBC. SCYL1, TEX14, and PLK1 ("STP axis") cooperatively trigger degradation of the REST tumor suppressor protein, a frequent event in human TNBC. The STP axis induces REST degradation by phosphorylating a conserved REST phospho-degron and bridging REST interaction with the ubiquitin-ligase βTRCP. Inhibition of the STP axis leads to increased REST protein levels and impairs TNBC transformation, tumor progression, and metastasis. Expression of the STP axis correlates with low REST protein levels in human TNBCs and poor clinical outcome for TNBC patients. Our findings demonstrate that the STP-REST axis is a molecular driver of human TNBC.

Differential loss of thalamostriatal and corticostriatal input to striatal projection neuron types prior to overt motor symptoms in the Q140 knock-in mouse model of Huntington's disease

Motor slowing and forebrain white matter loss have been reported in premanifest Huntington's disease (HD) prior to substantial striatal neuron loss. These findings raise the possibility that early motor defects in HD may be related to loss of excitatory input to striatum. In a prior study, we showed that in the heterozygous Q140 knock-in mouse model of HD that loss of thalamostriatal axospinous terminals is evident by 4 months, and loss of corticostriatal axospinous terminals is evident at 12 months, before striatal projection neuron pathology. In the present study, we specifically characterized the loss of thalamostriatal and corticostriatal terminals on direct (dSPN) and indirect (iSPN) pathway striatal projection neurons, using immunolabeling to identify thalamostriatal (VGLUT2+) and corticostriatal (VGLUT1+) axospinous terminals, and D1 receptor immunolabeling to distinguish dSPN (D1+) and iSPN (D1-) synaptic targets. We found that the loss of corticostriatal terminals at 12 months of age was preferential for D1+ spines, and especially involved smaller terminals, presumptively of the intratelencephalically projecting (IT) type. By contrast, indirect pathway D1- spines showed little loss of axospinous terminals at the same age. Thalamostriatal terminal loss was comparable for D1+ and D1- spines at both 4 and 12 months. Regression analysis showed that the loss of VGLUT1+ terminals on D1+ spines was correlated with a slight decline in open field motor parameters at 12 months. Our overall results raise the possibility that differential thalamic and cortical input loss to SPNs is an early event in human HD, with cortical loss to dSPNs in particular contributing to premanifest motor slowing.

The role for alterations in neuronal activity in the pathogenesis of polyglutamine repeat disorders

Polyglutamine diseases are a class of neurodegenerative diseases that share an expansion of a glutamine-encoding CAG tract in the respective disease genes as a central hallmark. In all of these diseases there is progressive degeneration in a select subset of neurons, and the mechanisms behind this degeneration remain unclear. Emerging evidence from animal models of disease has identified abnormalities in synaptic signaling and intrinsic excitability in affected neurons, which coincide with the onset of symptoms and precede apparent neuropathology. The appearance of these early changes suggests that altered neuronal activity might be an important component of network dysfunction and that these alterations in network physiology could contribute to symptoms of disease. Here we review abnormalities in neuronal function that have been identified in both animal models and patients, and highlight ways in which these changes in neuronal activity may contribute to disease symptoms. We then review the literature supporting an emerging role for abnormalities in neuronal activity as a driver of neurodegeneration. Finally, we identify common themes that emerge from studies of neuronal dysfunction in polyglutamine disease.

Forkhead transcription factor FOXO3a levels are increased in Huntington disease because of overactivated positive autofeedback loop

Huntington disease (HD) is a fatal autosomal dominant neurodegenerative disorder caused by an increased number of CAG repeats in the HTT gene coding for huntingtin. Decreased neurotrophic support and increased mitochondrial and excitotoxic stress have been reported in HD striatal and cortical neurons. The members of the class O forkhead (FOXO) transcription factor family, including FOXO3a, act as sensor proteins that are activated upon decreased survival signals and/or increased cellular stress. Using immunocytochemical screening in mouse striatal Hdh^7/7 (wild type), Hdh^7/109 (heterozygous for HD mutation), and Hdh^109/109 (homozygous for HD mutation) cells, we identified FOXO3a as a differentially regulated transcription factor in HD. We report increased nuclear FOXO3a levels in mutant Hdh cells. Additionally, we show that treatment with mitochondrial toxin 3-nitropropionic acid results in enhanced nuclear localization of FOXO3a in wild type Hdh^7/7 cells and in rat primary cortical neurons. Furthermore, mRNA levels of Foxo3a are increased in mutant Hdh cells compared with wild type cells and in 3-nitropropionic acid-treated primary neurons compared with untreated neurons. A similar increase was observed in the cortex of R6/2 mice and HD patient post-mortem caudate tissue compared with controls. Using chromatin immunoprecipitation and reporter assays, we demonstrate that FOXO3a regulates its own transcription by binding to the conserved response element in Foxo3a promoter. Altogether, the findings of this study suggest that FOXO3a levels are increased in HD cells as a result of overactive positive feedback loop.

Biomarker development for C9orf72 repeat expansion in ALS

The expanded GGGGCC hexanucleotide repeat in the non-coding region of the C9orf72 gene on chromosome 9p21 has been discovered as the cause of approximately 20-50% of familial and up to 5-20% of sporadic amyotrophic lateral sclerosis (ALS) cases, making this the most common known genetic mutation of ALS to date. At the same time, it represents the most common genetic mutation in frontotemporal dementia (FTD; 10-30%). Because of the high prevalence of mutant C9orf72, pre-clinical efforts in identifying therapeutic targets and developing novel therapeutics for this mutation are highly pursued in the hope of providing a desperately needed disease-modifying treatment for ALS patients, as well as other patient populations affected by the C9orf72 mutation. The current lack of effective treatments for ALS is partially due to the lack of appropriate biomarkers that aide in assessing drug efficacy during clinical trials independent of clinical outcome measures, such as increased survival. In this review we will summarize the opportunities for biomarker development specifically targeted to the newly discovered C9orf72 repeat expansion. While drugs are being developed for this mutation, it will be crucial to provide a reliable biomarker to accompany the clinical development of these novel therapeutic interventions to maximize the chances of a successful clinical trial. This article is part of a Special Issue entitled ALS complex pathogenesis.

Mutant huntingtin affects cortical progenitor cell division and development of the mouse neocortex

A polyglutamine expansion in huntingtin (HTT) causes the specific death of adult neurons in Huntington's disease (HD). Most studies have thus focused on mutant HTT (mHTT) toxicity in adulthood, and its developmental effects have been largely overlooked. We found that mHTT caused mitotic spindle misorientation in cultured cells by altering the localization of dynein, NuMA, and the p150(Glued) subunit of dynactin to the spindle pole and cell cortex and of CLIP170 and p150(Glued) to microtubule plus-ends. mHTT also affected spindle orientation in dividing mouse cortical progenitors, altering the thickness of the developing cortex. The serine/threonine kinase Akt, which regulates HTT function, rescued the spindle misorientation caused by the mHTT, by serine 421 (S421) phosphorylation, in cultured cells and in mice. Thus, cortical development is affected in HD, and this early defect can be rescued by HTT phosphorylation at S421.

Preclinical and clinical investigations of mood stabilizers for Huntington's disease: what have we learned?

Huntington's disease (HD) is a lethal, autosomal dominant neurodegenerative disorder caused by CAG repeat expansions at exon 1 of the huntingtin (Htt) gene, which encodes for a mutant huntingtin protein (mHtt). Prominent symptoms of HD include motor dysfunction, characterized by chorea; psychiatric disturbances such as mood and personality changes; and cognitive decline that may lead to dementia. Pathologically multiple complex processes and pathways are involved in the development of HD, including selective loss of neurons in the striatum and cortex, dysregulation of cellular autophagy, mitochondrial dysfunction, decreased neurotrophic and growth factor levels, and aberrant regulation of gene expression and epigenetic patterns. No cure for HD presently exists, nor are there drugs that can halt the progression of this devastating disease. Therefore, the need to discover neuroprotective modalities to combat HD is critical. In basic and preclinical studies using cellular and animal HD models, the mood stabilizers lithium and valproic acid (VPA) have shown multiple beneficial effects, including behavioral and motor improvement, enhanced neuroprotection, and lifespan extension. Recent studies in transgenic HD mice support the notion that combined lithium/VPA treatment is more effective than treatment with either drug alone. In humans, several clinical studies of HD patients found that lithium treatment improved mood, and that VPA treatment both stabilized mood and moderately reduced chorea. In contrast, other studies observed that the hallmark features of HD were unaffected by treatment with either lithium or VPA. The current review discusses preclinical and clinical investigations of the beneficial effects of lithium and VPA on HD pathophysiology.

A huntingtin-mediated fast stress response halting endosomal trafficking is defective in Huntington's disease

Cellular stress is a normal part of the aging process and is especially relevant in neurodegenerative disease. Canonical stress responses, such as the heat shock response, activate following exposure to stress and restore proteostasis through the action of isomerases and chaperones within the cytosol. Through live-cell imaging, we demonstrate involvement of the Huntington's disease (HD) protein, huntingtin, in a rapid cell stress response that lies temporally upstream of canonical stress responses. This response is characterized by the formation of distinct cytosolic puncta and reversible localization of huntingtin to early endosomes. The formation of these puncta, which we have termed huntingtin stress bodies (HSBs), is associated with arrest of early-to-recycling and early-to-late endosomal trafficking. The critical domains for this response have been mapped to two regions of huntingtin flanking the polyglutamine tract, and we observe polyglutamine-expanded huntingtin-expressing cells to be defective in their ability to recover from this stress response. We propose that HSB formation rapidly diverts high ATP use from vesicular trafficking during stress, thus mobilizing canonical stress responses without relying on increased energy metabolism, and that restoration from this response is defective in HD.

BDNF and Huntingtin protein modifications by manganese: implications for striatal medium spiny neuron pathology in manganese neurotoxicity

High levels of manganese (Mn) exposure decrease striatal medium spiny neuron (MSN) dendritic length and spine density, but the mechanism(s) are not known. The Huntingtin (HTT) gene has been functionally linked to cortical brain-derived neurotrophic factor (BDNF) support of striatal MSNs via phosphorylation at serine 421. In Huntington's disease, pathogenic CAG repeat expansions of HTT decrease synthesis and disrupt transport of cortical-striatal BDNF, which may contribute to disease, and Mn is a putative environmental modifier of Huntington's disease pathology. Thus, we tested the hypothesis that changes in MSN dendritic morphology Mn due to exposure are associated with decreased BDNF levels and alterations in Htt protein. We report that BDNF levels are decreased in the striatum of Mn-exposed non-human primates and in the cerebral cortex and striatum of mice exposed to Mn. Furthermore, proBDNF and mature BDNF concentrations in primary cortical and hippocampal neuron cultures were decreased by exposure to Mn confirming the in vivo findings. Mn exposure decreased serine 421 phosphorylation of Htt in cortical and hippocampal neurons and increased total Htt levels. These data strongly support the hypothesis that Mn-exposure-related MSN pathology is associated with decreased BDNF trophic support via alterations in Htt.

C-terminal domain small phosphatase 1 and MAP kinase reciprocally control REST stability and neuronal differentiation

The repressor element 1 (RE1) silencing transcription factor (REST) in stem cells represses hundreds of genes essential to neuronal function. During neurogenesis, REST is degraded in neural progenitors to promote subsequent elaboration of a mature neuronal phenotype. Prior studies indicate that part of the degradation mechanism involves phosphorylation of two sites in the C terminus of REST that require activity of beta-transducin repeat containing E3 ubiquitin protein ligase, βTrCP. We identify a proline-directed phosphorylation motif, at serines 861/864 upstream of these sites, which is a substrate for the peptidylprolyl cis/trans isomerase, Pin1, as well as the ERK1/2 kinases. Mutation at S861/864 stabilizes REST, as does inhibition of Pin1 activity. Interestingly, we find that C-terminal domain small phosphatase 1 (CTDSP1), which is recruited by REST to neuronal genes, is present in REST immunocomplexes, dephosphorylates S861/864, and stabilizes REST. Expression of a REST peptide containing S861/864 in neural progenitors inhibits terminal neuronal differentiation. Together with previous work indicating that both REST and CTDSP1 are expressed to high levels in stem cells and down-regulated during neurogenesis, our results suggest that CTDSP1 activity stabilizes REST in stem cells and that ERK-dependent phosphorylation combined with Pin1 activity promotes REST degradation in neural progenitors.

The Drosophila Huntington's disease gene ortholog dhtt influences chromatin regulation during development

Huntington's disease is an autosomal dominant neurodegenerative disorder caused by a CAG expansion mutation in HTT, the gene encoding huntingtin. Evidence from both human genotype-phenotype relationships and mouse model systems suggests that the mutation acts by dysregulating some normal activity of huntingtin. Recent work in the mouse has revealed a role for huntingtin in epigenetic regulation during development. Here, we examine the role of the Drosophila huntingtin ortholog (dhtt) in chromatin regulation in the development of the fly. Although null dhtt mutants display no overt phenotype, we found that dhtt acts as a suppressor of position-effect variegation (PEV), suggesting that it influences chromatin organization. We demonstrate that dhtt affects heterochromatin spreading in a PEV model by modulating histone H3K9 methylation levels at the heterochromatin-euchromatin boundary. To gain mechanistic insights into how dhtt influences chromatin function, we conducted a candidate genetic screen using RNAi lines targeting known PEV modifier genes. We found that dhtt modifies phenotypes caused by knockdown of a number of key epigenetic regulators, including chromatin-associated proteins, histone demethylases (HDMs) and methyltransferases. Notably, dhtt strongly modifies phenotypes resulting from loss of the HDM dLsd1, in both the ovary and wing, and we demonstrate that dhtt appears to act as a facilitator of dLsd1 function in regulating global histone H3K4 methylation levels. These findings suggest that a fundamental aspect of huntingtin function in heterochromatin/euchromatin organization is evolutionarily conserved across phyla.

BDNF signaling and survival of striatal neurons

The striatum, a major component of the basal ganglia, performs multiple functions including control of movement, reward, and addiction. Dysfunction and death of striatal neurons are the main causes for the motor disorders associated with Huntington’s disease (HD). Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, is among factors that promote survival and proper function of this neuronal population. Here, we review recent studies showing that BDNF determines the size of the striatum by supporting survival of the immature striatal neurons at their origin, promotes maturation of striatal neurons, and facilitates establishment of striatal connections during brain development. We also examine the role of BDNF in maintaining proper function of the striatum during adulthood, summarize the mechanisms that lead to a deficiency in BDNF signaling and subsequently striatal degeneration in HD, and highlight a potential role of BDNF as a therapeutic target for HD treatment.

Metabotropic glutamate receptor 5 as a potential therapeutic target in Huntington's disease

INTRODUCTION

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a polyglutamine expansion in the amino-terminal region of the huntingtin (htt) protein, which underlies the loss of striatal and cortical neurons. Glutamate has been implicated in a number of neurodegenerative diseases, and several studies suggest that the metabotropic glutamate receptor 5 (mGluR5) may represent a target for the treatment of HD.

AREAS COVERED

The main goal of this review is to discuss the current data in the literature regarding the role of mGluR5 in HD and evaluate the potential of mGluR5 as a therapeutic target for the treatment of HD. mGluR5 is highly expressed in the brain regions affected in HD and is involved in movement control. Moreover, mGluR5 interacts with htt and mutated htt profoundly affects mGluR5 signaling. However, mGluR5 stimulation can activate both neuroprotective and neurotoxic signaling pathways, depending on the context of activation.

EXPERT OPINION

Although the data published so far strongly indicate that mGluR5 plays a major role in HD-associated neurodegeneration, htt aggregation and motor symptoms, it is not clear whether mGluR5 stimulation can diminish or intensify neuronal cell loss and HD progression. Thus, future experiments will be necessary to further investigate the outcome of drugs acting on mGluR5 for the treatment of neurodegenerative diseases.

The gene silencing transcription factor REST represses miR-132 expression in hippocampal neurons destined to die

The gene silencing transcription factor REST [repressor element 1 silencing transcription factor]/NRSF (neuron-restrictive silencer factor) actively represses a large array of coding and noncoding neuron-specific genes important to synaptic plasticity including miR-132. miR-132 is a neuron-specific microRNA and plays a pivotal role in synaptogenesis, synaptic plasticity and structural remodeling. However, a role for miR-132 in neuronal death is not, as yet, well-delineated. Here we show that ischemic insults promote REST binding and epigenetic remodeling at the miR-132 promoter and silencing of miR-132 expression in selectively vulnerable hippocampal CA1 neurons. REST occupancy was not altered at the miR-9 or miR-124a promoters despite the presence of repressor element 1 sites, indicating REST target specificity. Ischemia induced a substantial decrease in two marks of active gene transcription, dimethylation of lysine 4 on core histone 3 (H3K4me2) and acetylation of lysine 9 on H3 (H3K9ac) at the miR-132 promoter. RNAi-mediated depletion of REST in vivo blocked ischemia-induced loss of miR-132 in insulted hippocampal neurons, consistent with a causal relation between activation of REST and silencing of miR-132. Overexpression of miR-132 in primary cultures of hippocampal neurons or delivered directly into the CA1 of living rats by means of the lentiviral expression system prior to induction of ischemia afforded robust protection against ischemia-induced neuronal death. These findings document a previously unappreciated role for REST-dependent repression of miR-132 in the neuronal death associated with global ischemia and identify a novel therapeutic target for amelioration of the neurodegeneration and cognitive deficits associated with ischemic stroke.

Extracellular signal-related kinase 2/specificity protein 1/specificity protein 3/repressor element-1 silencing transcription factor pathway is involved in Aroclor 1254-induced toxicity in SH-SY5Y neuronal cells

Polychlorinated biphenyls (PCBs) cause a wide spectrum of toxic effects in the brain through undefined mechanisms. Exposure to the PCB mixture Aroclor-1254 (A1254) increases the repressor element-1 silencing transcription factor (REST) expression, leading to neuronal death. This study sought to understand the sequence of some molecular mechanisms to determine whether A1254 could increase REST expression and the cytoprotective effect of the phorbol ester tetradecanoylphorbol acetate (TPA) on A1254-induced toxicity in SH-SY5Y cells. As shown by Western blot analysis, A1254 (10 µg/ml) downregulates extracellular signal-related kinase 2 (ERK2) phosphorylation in a time-dependent manner, thereby triggering the binding of specificity protein 1 (Sp1) and Sp3 to the REST gene promoter as revealed by chromatin immunoprecipitation analysis. This chain of events results in an increase in REST mRNA and cell death, as assessed by quantitative real-time polymerase chain reaction and dimethylthiazolyl-2-5-diphenyltetrazolium-bromide assay, respectively. Accordingly, TPA prevented both the A1254-induced decrease in ERK2 phosphorylation and the A1254-induced increase in Sp1, Sp3, and REST protein expression. After 48 hr, TPA prevented A1254-induced cell death. ERK2 overexpression counteracted the A1254-induced increase in Sp1 and Sp3 protein expression and prevented A1254-induced Sp1 and Sp3 binding to the REST gene promoter, thus counteracting the increase in REST mRNA expression induced by the toxicant. In neuroblastoma SH-SY5Y cells, ERK2/Sp1/SP3/REST is a new pathway underlying the neurotoxic effect of PCB. The ERK2/Sp1/Sp3/REST pathway, which underlies A1254-induced neuronal death, might represent a new drug signaling cascade in PCB-induced neuronal toxicity.

Identification of REST-regulated genes and pathways using a REST-targeted antisense approach

The repressor element-1 silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF) is one of the first negative-acting transcriptional regulators implicated in vertebrate development thought to regulate hundreds of neuron-specific genes. However, its function in the adult system remains elusive. Here we employ second-generation antisense oligonucleotides (ASOs) to study the impact of rest-mediated suppression on gene expression. We demonstrate specific reductions in REST levels in vitro, and in vivo in mouse liver following treatment with ASOs, and we show that ASO mediated-REST suppression results in the elevation in expression of many neuronal genes including brain-derived neurotrophic factor, Synapsin1 (syn1) and β3-tubulin in BALB/c liver. Furthermore, we show the elevation of the affected proteins in plasma following ASO treatment. Finally, microarray analysis was applied to identify a broad range of genes modulated by REST suppression in mouse liver. Our findings suggest that REST may be an important target for neurodegenerative diseases like Huntington's disease, is also involved in the regulation of a broad range of additional cellular pathways, and that the antisense approach is a viable strategy for selectively modulating REST activity in vivo.

Huntingtin is critical both pre- and postsynaptically for long-term learning-related synaptic plasticity in Aplysia

Patients with Huntington’s disease exhibit memory and cognitive deficits many years before manifesting motor disturbances. Similarly, several studies have shown that deficits in long-term synaptic plasticity, a cellular basis of memory formation and storage, occur well before motor disturbances in the hippocampus of the transgenic mouse models of Huntington’s disease. The autosomal dominant inheritance pattern of Huntington’s disease suggests the importance of the mutant protein, huntingtin, in pathogenesis of Huntington’s disease, but wild type huntingtin also has been shown to be important for neuronal functions such as axonal transport. Yet, the role of wild type huntingtin in long-term synaptic plasticity has not been investigated in detail. We identified a huntingtin homolog in the marine snail Aplysia, and find that similar to the expression pattern in mammalian brain, huntingtin is widely expressed in neurons and glial cells. Importantly the expression of mRNAs of huntingtin is upregulated by repeated applications of serotonin, a modulatory transmitter released during learning in Aplysia. Furthermore, we find that huntingtin expression levels are critical, not only in presynaptic sensory neurons, but also in the postsynaptic motor neurons for serotonin-induced long-term facilitation at the sensory-to-motor neuron synapse of the Aplysia gill-withdrawal reflex. These results suggest a key role for huntingtin in long-term memory storage.

Huntington disease: can a zebrafish trail leave more than a ripple?

Over the past decade, zebrafish has proved to be very useful in modeling neurodegenerative conditions. It poses a number of advantages and has been accepted as one of the best models for elucidating pathophysiological mechanisms of neurodegenerative diseases, including Huntington disease (HD). HD is a debilitating neurodegenerative genetic disorder that affects a person's ability to think, talk, and move. The pathophysiology of HD is not completely understood, which prevents the development of effective therapeutic approaches. Using zebrafish as a model organism, scientific advancements can be made in understanding the HD pathology/mechanisms with the hope of developing potential therapies in the near future.

Causes and Consequences of MicroRNA Dysregulation in Neurodegenerative Diseases

Neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS), originate from a loss of neurons in the central nervous system (CNS) and are severely debilitating. The incidence of neurodegenerative diseases increases with age, and they are expected to become more common due to extended life expectancy. Because of no clear mechanisms, these diseases have become a major challenge in neurobiology. It is well recognized that these disorders become the culmination of many different genetic and environmental influences. Prior studies have shown that microRNAs (miRNAs) are pathologically altered during the inexorable course of some neurodegenerative diseases, suggesting that miRNAs may be the contributing factor in neurodegeneration. Here, we review what is known about the involvement of miRNAs in the pathogenesis of neurodegenerative diseases. The biogenesis of miRNAs and various functions of miRNAs that act as the chief regulators will be discussed. We focus in particular on dysregulation of miRNAs which leads to several neurodegenerative diseases from three aspects: miRNA-generating disorders, miRNA-targeting genes and epigenetic alterations. Furthermore, recent evidences have shown that circulating miRNA expression levels are changed in patients with neurodegenerative diseases. Circulating miRNA expression levels are reported in patients in order to evaluate their application as biomarkers of these diseases. A discussion is included with a potential diagnostic biomarker and the possible future direction in exploring the nexus between miRNAs and various neurodegenerative diseases.

BAC-based cellular model for screening regulators of BDNF gene transcription

Background

Brain derived neurotrophic factor (BDNF) belongs to a family of structurally related proteins called neurotrophins that have been shown to regulate survival and growth of neurons in the developing central and peripheral nervous system and also to take part in synaptic plasticity related processes in adulthood. Since BDNF is associated with several nervous system disorders it would be beneficial to have cellular reporter system for studying its expression regulation.

Methods

Using modified bacterial artificial chromosome (BAC), we generated several transgenic cell lines expressing humanised Renilla luciferase (hRluc)-EGFP fusion reporter gene under the control of rat BDNF gene regulatory sequences (rBDNF-hRluc-EGFP) in HeLa background. To see if the hRluc-EGFP reporter was regulated in response to known regulators of BDNF expression we treated cell lines with substances known to regulate BDNF and also overexpressed transcription factors known to regulate BDNF gene in established cell lines.

Results

rBDNF-hRluc-EGFP cell lines had high transgene copy numbers when assayed with qPCR and FISH analysis showed that transgene was maintained episomally in all cell lines. Luciferase activity in transgenic cell lines was induced in response to ionomycin-mediated rise of intracellular calcium levels, treatment with HDAC inhibitors and by over-expression of transcription factors known to increase BDNF expression, indicating that transcription of the transgenic reporter is regulated similarly to the endogenous BDNF gene.

Conclusions

Generated rBDNF-hRluc-EGFP BAC cell lines respond to known modulators of BDNF expression and could be used for screening of compounds/small molecules or transcription factors altering BDNF expression.

Comparison of REST cistromes across human cell types reveals common and context-specific functions

Recent studies have shown that the transcriptional functions of REST are much broader than repressing neuronal genes in non-neuronal systems. Whether REST occupies similar chromatin regions in different cell types and how it interacts with other transcriptional regulators to execute its functions in a context-dependent manner has not been adequately investigated. We have applied ChIP-seq analysis to identify the REST cistrome in human CD4+ T cells and compared it with published data from 15 other cell types. We found that REST cistromes were distinct among cell types, with REST binding to several tumor suppressors specifically in cancer cells, whereas 7% of the REST peaks in non-neuronal cells were ubiquitously called and <25% were identified for ≥ 5 cell types. Nevertheless, using a quantitative metric directly comparing raw ChIP-seq signals, we found the majority (∼80%) was shared by ≥ 2 cell types. Integration with RNA-seq data showed that REST binding was generally correlated with low gene expression. Close examination revealed that multiple contexts were correlated with reduced expression of REST targets, e.g., the presence of a cognate RE1 motif and cellular specificity of REST binding. These contexts were shown to play a role in differential corepressor recruitment. Furthermore, transcriptional outcome was highly influenced by REST cofactors, e.g., SIN3 and EZH2 co-occupancy marked higher and lower expression of REST targets, respectively. Unexpectedly, the REST cistrome in differentiated neurons exhibited unique features not observed in non-neuronal cells, e.g., the lack of RE1 motifs and an association with active gene expression. Finally, our analysis demonstrated how REST could differentially regulate a transcription network constituted of miRNAs, REST complex and neuronal factors. Overall, our findings of contexts playing critical roles in REST occupancy and regulatory outcome provide insights into the molecular interactions underlying REST's diverse functions, and point to novel roles of REST in differentiated neurons.

Casein kinase 1 suppresses activation of REST in insulted hippocampal neurons and halts ischemia-induced neuronal death

Repressor Element-1 (RE1) Silencing Transcription Factor/Neuron-Restrictive Silencer Factor (REST/NRSF) is a gene-silencing factor that is widely expressed during embryogenesis and plays a strategic role in neuronal differentiation. Recent studies indicate that REST can be activated in differentiated neurons during a critical window of time in postnatal development and in adult neurons in response to neuronal insults such as seizures and ischemia. However, the mechanism by which REST is regulated in neurons is as yet unknown. Here, we show that REST is controlled at the level of protein stability via β-TrCP-dependent, ubiquitin-based proteasomal degradation in differentiated neurons under physiological conditions and identify Casein Kinase 1 (CK1) as an upstream effector that bidirectionally regulates REST cellular abundance. CK1 associates with and phosphorylates REST at two neighboring, but distinct, motifs within the C terminus of REST critical for binding of β-TrCP and targeting of REST for proteasomal degradation. We further show that global ischemia in rats in vivo triggers a decrease in CK1 and an increase in REST in selectively vulnerable hippocampal CA1 neurons. Administration of the CK1 activator pyrvinium pamoate by in vivo injection immediately after ischemia restores CK1 activity, suppresses REST expression, and rescues neurons destined to die. Our results identify a novel and previously unappreciated role for CK1 as a brake on REST stability and abundance in adult neurons and reveal that loss of CK1 is causally related to ischemia-induced neuronal death. These findings point to CK1 as a potential therapeutic target for the amelioration of hippocampal injury and cognitive deficits associated with global ischemia.

Coordinated regulation of dendrite arborization by epigenetic factors CDYL and EZH2

Dendritic arborization is one of the key determinants of precise circuits for information processing in neurons. Unraveling the molecular mechanisms underlying dendrite morphogenesis is critical to understanding the establishment of neuronal connections. Here, using gain- and loss-of-function approaches, we defined the chromodomain protein and transcription corepressor chromodomain Y-like (CDYL) protein as a negative regulator of dendrite morphogenesis in rat/mouse hippocampal neurons both in vitro and in vivo. Overexpressing CDYL decreased, whereas knocking it down increased, the dendritic complexity of the primary cultured rat neurons. High-throughput DNA microarray screening identified a number of CDYL downstream target genes, including the brain-derived neurotrophic factor (BDNF). Knock-down of CDYL in neuronal cells led to increased expression of BDNF, which is primarily responsible for CDYL's effects on dendrite patterns. Mechanistically, CDYL interacts with EZH2, the catalytic subunit of Polycomb Repressive Complex 2 (PRC2), directly and recruits the H3K27 methyltransferase activity to the promoter region of the BDNF gene. In doing so, CDYL and EZH2 coordinately restrict dendrite morphogenesis in an interdependent manner. Finally, we found that neural activity increased dendritic complexity through degradation of CDYL protein to unleash its inhibition on BDNF. These results link, for the first time, the epigenetic regulators CDYL and EZH2 to dendrite morphogenesis and might shed new light on our understanding of the regulation of the neurodevelopment.

High throughput screening for inhibitors of REST in neural derivatives of human embryonic stem cells reveals a chemical compound that promotes expression of neuronal genes

Decreased expression of neuronal genes such as brain-derived neurotrophic factor (BDNF) is associated with several neurological disorders. One molecular mechanism associated with Huntington disease (HD) is a discrete increase in the nuclear activity of the transcriptional repressor REST/NRSF binding to repressor element-1 (RE1) sequences. High-throughput screening of a library of 6,984 compounds with luciferase-assay measuring REST activity in neural derivatives of human embryonic stem cells led to identify two benzoimidazole-5-carboxamide derivatives that inhibited REST silencing in a RE1-dependent manner. The most potent compound, X5050, targeted REST degradation, but neither REST expression, RNA splicing nor binding to RE1 sequence. Differential transcriptomic analysis revealed the upregulation of neuronal genes targeted by REST in wild-type neural cells treated with X5050. This activity was confirmed in neural cells produced from human induced pluripotent stem cells derived from a HD patient. Acute intraventricular delivery of X5050 increased the expressions of BDNF and several other REST-regulated genes in the prefrontal cortex of mice with quinolinate-induced striatal lesions. This study demonstrates that the use of pluripotent stem cell derivatives can represent a crucial step toward the identification of pharmacological compounds with therapeutic potential in neurological affections involving decreased expression of neuronal genes associated to increased REST activity, such as Huntington disease.

Transcription, epigenetics and ameliorative strategies in Huntington's Disease: a genome-wide perspective

Transcriptional dysregulation in Huntington’s disease (HD) is an early event that shapes the brain transcriptome by both the depletion and ectopic activation of gene products that eventually affect survival and neuronal functions. Disruption in the activity of gene expression regulators, such as transcription factors, chromatin-remodeling proteins, and noncoding RNAs, accounts for the expression changes observed in multiple animal and cellular models of HD and in samples from patients. Here, I review the recent advances in the study of HD transcriptional dysregulation and its causes to finally discuss the possible implications in ameliorative strategies from a genome-wide perspective. To date, the use of genome-wide approaches, predominantly based on microarray platforms, has been successful in providing an extensive catalog of differentially regulated genes, including biomarkers aimed at monitoring the progress of the pathology. Although still incipient, the introduction of combined next-generation sequencing techniques is enhancing our comprehension of the mechanisms underlying altered transcriptional dysregulation in HD by providing the first genomic landscapes associated with epigenetics and the occupancy of transcription factors. In addition, the use of genome-wide approaches is becoming more and more necessary to evaluate the efficacy and safety of ameliorative strategies and to identify novel mechanisms of amelioration that may help in the improvement of current preclinical therapeutics. Finally, the major conclusions obtained from HD transcriptomics studies have the potential to be extrapolated to other neurodegenerative disorders.

Mood disorders in Huntington's disease: from behavior to cellular and molecular mechanisms

Huntington's disease (HD) is a neurodegenerative disorder that is best known for its effect on motor control. Mood disturbances such as depression, anxiety, and irritability also have a high prevalence in patients with HD, and often start before the onset of motor symptoms. Various rodent models of HD recapitulate the anxiety/depressive behavior seen in patients. HD is caused by an expanded polyglutamine stretch in the N-terminal part of a 350 kDa protein called huntingtin (HTT). HTT is ubiquitously expressed and is implicated in several cellular functions including control of transcription, vesicular trafficking, ciliogenesis, and mitosis. This review summarizes progress in efforts to understand the cellular and molecular mechanisms underlying behavioral disorders in patients with HD. Dysfunctional HTT affects cellular pathways that are involved in mood disorders or in the response to antidepressants, including BDNF/TrkB and serotonergic signaling. Moreover, HTT affects adult hippocampal neurogenesis, a physiological phenomenon that is implicated in some of the behavioral effects of antidepressants and is linked to the control of anxiety. These findings are consistent with the emerging role of wild-type HTT as a crucial component of neuronal development and physiology. Thus, the pathogenic polyQ expansion in HTT could lead to mood disorders not only by the gain of a new toxic function but also by the perturbation of its normal function.

Physical exercise-induced adult neurogenesis: a good strategy to prevent cognitive decline in neurodegenerative diseases?

Cumulative evidence has indicated that there is an important role for adult hippocampal neurogenesis in cognitive function. With the increasing prevalence of cognitive decline associated with neurodegenerative diseases among the ageing population, physical exercise, a potent enhancer of adult hippocampal neurogenesis, has emerged as a potential preventative strategy/treatment to reduce cognitive decline. Here we review the functional role of adult hippocampal neurogenesis in learning and memory, and how this form of structural plasticity is altered in neurodegenerative diseases known to involve cognitive impairment. We further discuss how physical exercise may contribute to cognitive improvement in the ageing brain by preserving adult neurogenesis, and review the recent approaches for measuring changes in neurogenesis in the live human brain.

Emerging bioinformatics approaches for analysis of NGS-derived coding and non-coding RNAs in neurodegenerative diseases

Neurodegenerative diseases in general and specifically late-onset Alzheimer’s disease (LOAD) involve a genetically complex and largely obscure ensemble of causative and risk factors accompanied by complex feedback responses. The advent of “high-throughput” transcriptome investigation technologies such as microarray and deep sequencing is increasingly being combined with sophisticated statistical and bioinformatics analysis methods complemented by knowledge-based approaches such as Bayesian Networks or network and graph analyses. Together, such “integrative” studies are beginning to identify co-regulated gene networks linked with biological pathways and potentially modulating disease predisposition, outcome, and progression. Specifically, bioinformatics analyses of integrated microarray and genotyping data in cases and controls reveal changes in gene expression of both protein-coding and small and long regulatory RNAs; highlight relevant quantitative transcriptional differences between LOAD and non-demented control brains and demonstrate reconfiguration of functionally meaningful molecular interaction structures in LOAD. These may be measured as changes in connectivity in “hub nodes” of relevant gene networks (Zhang etal., 2013). We illustrate here the open analytical questions in the transcriptome investigation of neurodegenerative disease studies, proposing “ad hoc” strategies for the evaluation of differential gene expression and hints for a simple analysis of the non-coding RNA (ncRNA) part of such datasets. We then survey the emerging role of long ncRNAs (lncRNAs) in the healthy and diseased brain transcriptome and describe the main current methods for computational modeling of gene networks. We propose accessible modular and pathway-oriented methods and guidelines for bioinformatics investigations of whole transcriptome next generation sequencing datasets. We finally present methods and databases for functional interpretations of lncRNAs and propose a simple heuristic approach to visualize and represent physical and functional interactions of the coding and non-coding components of the transcriptome. Integrating in a functional and integrated vision coding and ncRNA analyses is of utmost importance for current and future analyses of neurodegenerative transcriptomes.

Therapeutic Approaches for Inhibition of Protein Aggregation in Huntington's Disease

Huntington's disease (HD) is a late-onset and progressive neurodegenerative disorder that is caused by aggregation of mutant huntingtin protein which contains expanded-polyglutamine. The molecular chaperones modulate the aggregation in early stage and known for the most potent protector of neurodegeneration in animal models of HD. Over the past decades, a number of studies have demonstrated molecular chaperones alleviate the pathogenic symptoms by polyQ-mediated toxicity. Moreover, chaperone-inducible drugs and anti-aggregation drugs have beneficial effects on symptoms of disease. Here, we focus on the function of molecular chaperone in animal models of HD, and review the recent therapeutic approaches to modulate expression and turn-over of molecular chaperone and to develop anti-aggregation drugs.

Long noncoding RNAs: emerging stars in gene regulation, epigenetics and human disease

Noncoding RNAs (ncRNAs) are classes of transcripts that are encoded by the genome and transcribed but never get translated into proteins. Though not translated into proteins, ncRNAs play pivotal roles in a variety of cellular functions. Here, we review the functions of long noncoding RNAs (lncRNAs) and their implications in various human diseases. Increasing numbers of studies demonstrate that lncRNAs play critical roles in regulation of protein-coding genes, maintenance of genomic integrity, dosage compensation, genomic imprinting, mRNA processing, cell differentiation, and development. Misregulation of lncRNAs is associated with a variety of human diseases, including cancer, immune and neurological disorders. Different classes of lncRNAs, their functions, mechanisms of action, and associations with different human diseases are summarized in detail, highlighting their as yet untapped potential in therapy.

Silencing mutant huntingtin by adeno-associated virus-mediated RNA interference ameliorates disease manifestations in the YAC128 mouse model of Huntington's disease

Huntington's disease (HD) is a fatal autosomal dominant neurodegenerative disease caused by an increase in the number of polyglutamine residues in the huntingtin (Htt) protein. With the identification of the underlying basis of HD, therapies are being developed that reduce expression of the causative mutant Htt. RNA interference (RNAi) that seeks to selectively reduce the expression of such disease-causing agents is emerging as a potential therapeutic strategy for this and similar disorders. This study examines the merits of administering a recombinant adeno-associated viral (AAV) vector designed to deliver small interfering RNA (siRNA) that targets the degradation of the Htt transcript. The aim was to lower Htt levels and to correct the behavioral, biochemical, and neuropathological deficits shown to be associated with the YAC128 mouse model of HD. Our data demonstrate that AAV-mediated RNAi is effective at transducing greater than 80% of the cells in the striatum and partially reducing the levels (~40%) of both wild-type and mutant Htt in this region. Concomitant with these reductions are significant improvements in behavioral deficits, reduction of striatal Htt aggregates, and partial correction of the aberrant striatal transcriptional profile observed in YAC128 mice. Importantly, a partial reduction of both the mutant and wild-type Htt levels is not associated with any notable overt neurotoxicity. Collectively, these results support the continued development of AAV-mediated RNAi as a therapeutic strategy for HD.

Transcriptional dysregulation in Huntington's disease: a failure of adaptive transcriptional homeostasis

Huntington's disease (HD) is a signature polyglutamine disorder. An enduring theory of HD pathogenesis has involved dysregulation of transcription. Indeed, transcriptional regulatory proteins can be modulated to overcome cardinal features of HD-modeled mice, and efforts to move these into human studies are ongoing. Here, we discuss a unifying hypothesis emerging from these studies, which is that HD represents the pathological disruption of evolutionarily conserved adaptive gene programs to counteract oxidative stress, mitochondrial dysfunction and accumulation of misfolded proteins. Transcriptional dyshomeostasis of adaptive genes is further exacerbated by repression of genes involved in normal synaptic activity or growth factor signaling.

Adenosine receptors and Huntington's disease

Adenosine regulates important pathophysiological functions via four distinct adenosine receptor subtypes (A1, A2A, A2B, and A3). The A1 and A2A adenosine receptors (A1R and A2AR) are major targets of caffeine and have been extensively investigated. Huntington's disease (HD) is a dominant neurodegenerative disease caused by an abnormal CAG expansion in the Huntingtin gene. Since the first genetic HD model was created almost two decades ago, tremendous progress regarding the function of the adenosine receptors in HD has been made. Chronic intake of caffeine was recently shown to be positively associated with the disease onset of HD. Moreover, genetic polymorphism of A2AR is believed to impact the age of onset. Given the importance of adenosine receptors as drug targets for human diseases, this review highlights the recent findings that delineate the roles of adenosine receptors in HD and discusses their potential for serving as drug targets and/or biomarkers for HD. Adenosine is a purine nucleoside that regulates important physiological functions via four different adenosine receptors (A1, A2A, A2B, and A3). These adenosine receptors have seven transmembrane domains and belong to the G protein-coupled receptor family. The functions of the A1 adenosine receptor (A1R) and A2A adenosine receptor (A2AR) have been investigated relative to HD. In this review, we summarize the recent findings regarding the role of adenosine receptors in HD and discuss the potential application of adenosine receptors as drug targets and biomarkers for HD.

Adenosine receptor control of cognition in normal and disease

Adenosine and adenosine receptors (ARs) are increasingly recognized as important therapeutic targets for controlling cognition under normal and disease conditions for its dual roles of neuromodulation as well as of homeostatic function in the brain. This chapter first presents the unique ability of adenosine, by acting on the inhibitory A1 and facilitating A2A receptor, to integrate dopamine, glutamate, and BNDF signaling and to modulate synaptic plasticity (e.g., long-term potentiation and long-term depression) in brain regions relevant to learning and memory, providing the molecular and cellular bases for adenosine receptor (AR) control of cognition. This led to the demonstration of AR modulation of social recognition memory, working memory, reference memory, reversal learning, goal-directed behavior/habit formation, Pavlovian fear conditioning, and effort-related behavior. Furthermore, human and animal studies support that AR activity can also, through cognitive enhancement and neuroprotection, reverse cognitive impairments in animal models of Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease, and schizophrenia. Lastly, epidemiological evidence indicates that regular human consumption of caffeine, the most widely used psychoactive drug and nonselective AR antagonists, is associated with the reduced cognitive decline in aging and AD patients, and with the reduced risk in developing PD. Thus, there is a convergence of the molecular studies revealing AR as molecular targets for integrating neurotransmitter signaling and controlling synaptic plasticity, with animal studies demonstrating the strong procognitive impact upon AR antagonism in normal and disease brains and with epidemiological and clinical evidences in support of caffeine and AR drugs for therapeutic modulation of cognition. Since some of adenosine A2A receptor antagonists are already in phase III clinical trials for motor benefits in PD patients with remarkable safety profiles, additional animal and human studies to better understand the mechanism underlying the AR-mediated control of cognition under normal and disease conditions will provide the required rationale to stimulate the necessary clinical investigation to rapidly translate adenosine and AR drug as a novel strategy to control memory impairment in neuropsychiatric disorders.

Huntington's disease

Changes in the level and activity of brain-derived neurotrophic factor (BDNF) have been described in a number of neurodegenerative disorders since early 1990s. However, only in Huntington disease (HD) gain- and loss-of-function experiments have mechanistically linked these abnormalities with the genetic defect.In this chapter we will describe how huntingtin protein, whose mutation causes HD, is involved in the physiological control of BDNF synthesis and transport in neurons and how both processes are simultaneously disrupted in HD. We will describe the underlying molecular mechanisms and discuss pre-clinical data concerning the impact of the experimental manipulation of BDNF levels on HD progression. These studies have revealed that a major loss of BDNF protein in the brain of HD patients may contribute to the clinical manifestations of the disease. The experimental strategies under investigation to increase brain BDNF levels in animal models of HD will also be described, with a view to ultimately improving the clinical treatment of this condition.

Neurotrophins: transcription and translation

Neurotrophins are powerful molecules. Small quantities of these secreted proteins exert robust effects on neuronal survival, synapse stabilization, and synaptic function. Key functions of the neurotrophins rely on these proteins being expressed at the right time and in the right place. This is especially true for BDNF, stimulus-inducible expression of which serves as an essential step in the transduction of a broad variety of extracellular stimuli into neuronal plasticity of physiologically relevant brain regions. Here we review the transcriptional and translational mechanisms that control neurotrophin expression with a particular focus on the activity-dependent regulation of BDNF.

Neurobiology of Huntington's Disease

Of the neurodegenerative diseases presented in this book, Huntington's disease (HD) stands as the archetypal autosomal dominantly inherited neurodegenerative disorder. Its occurrence through generations of affected families was noted long before the basic genetic underpinnings of hereditary diseases was understood. The early classification of HD as a distinct hereditary neurodegenerative disorder allowed the study of this disease to lead the way in the development of our understanding of the mechanisms of human genetic disorders. Following its clinical and pathologic characterization, the causative genetic mutation in HD was subsequently identified as a trinucleotide (CAG) repeat expansion in the huntingtin (HTT) gene, and consequently, the HTT gene and huntingtin protein have been studied in great detail. Despite this concentrated effort, there is still much about the function of huntingtin that still remains unknown. Presented in this chapter is an overview of the current knowledge on the normal function of huntingtin and some of the potential neurobiologic mechanisms by which the mutant HTT gene may mediate neurodegeneration in HD.