Huntingtin regulates RE1-silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF) nuclear trafficking indirectly through a complex with REST/NRSF-interacting LIM domain protein (RILP) and dynactin p150 Glued

Understanding neurological disease mechanisms in the era of epigenetics

The burgeoning field of epigenetics is making a significant impact on our understanding of brain evolution, development, and function. In fact, it is now clear that epigenetic mechanisms promote seminal neurobiological processes, ranging from neural stem cell maintenance and differentiation to learning and memory. At the molecular level, epigenetic mechanisms regulate the structure and activity of the genome in response to intracellular and environmental cues, including the deployment of cell type-specific gene networks and those underlying synaptic plasticity. Pharmacological and genetic manipulation of epigenetic factors can, in turn, induce remarkable changes in neural cell identity and cognitive and behavioral phenotypes. Not surprisingly, it is also becoming apparent that epigenetics is intimately involved in neurological disease pathogenesis. Herein, we highlight emerging paradigms for linking epigenetic machinery and processes with neurological disease states, including how (1) mutations in genes encoding epigenetic factors cause disease, (2) genetic variation in genes encoding epigenetic factors modify disease risk, (3) abnormalities in epigenetic factor expression, localization, or function are involved in disease pathophysiology, (4) epigenetic mechanisms regulate disease-associated genomic loci, gene products, and cellular pathways, and (5) differential epigenetic profiles are present in patient-derived central and peripheral tissues.

Binding of the repressor complex REST-mSIN3b by small molecules restores neuronal gene transcription in Huntington's disease models

Transcriptional dysregulation is a hallmark of Huntington's disease (HD) and one cause of this dysregulation is enhanced activity of the REST-mSIN3a-mSIN3b-CoREST-HDAC repressor complex, which silences transcription through REST binding to the RE1/NRSE silencer. Normally, huntingtin (HTT) prevents this binding, allowing expressing of REST target genes. Here, we aimed to identify HTT mimetics that disrupt REST complex formation in HD. From a structure-based virtual screening of 7 million molecules, we selected 94 compounds predicted to interfere with REST complex formation by targeting the PAH1 domain of mSIN3b. Primary screening using DiaNRSELuc8 cells revealed two classes of compounds causing a greater than two-fold increase in luciferase. In particular, quinolone-like compound 91 (C91) at a non-toxic nanomolar concentration reduced mSIN3b nuclear entry and occupancy at the RE1/NRSE within the Bdnf locus, and restored brain-derived neurotrophic factor (BDNF) protein levels in HD cells. The mRNA levels of other RE1/NRSE-regulated genes were similarly increased while non-REST-regulated genes were unaffected. C91 stimulated REST-regulated gene expression in HTT-knockdown Zebrafish and increased BDNF mRNA in the presence of mutant HTT. Thus, a combination of virtual screening and biological approaches can lead to compounds reducing REST complex formation, which may be useful in HD and in other pathological conditions.

Roles of long noncoding RNAs in brain development, functional diversification and neurodegenerative diseases

Long noncoding RNAs (lncRNAs) have been attracting immense research interest, while only a handful of lncRNAs have been characterized thoroughly. Their involvement in the fundamental cellular processes including regulate gene expression at epigenetics, transcription, and post-transcription highlighted a central role in cell homeostasis. However, lncRNAs studies are still at a relatively early stage, their definition, conservation, functions, and action mechanisms remain fairly complicated. Here, we give a systematic and comprehensive summary of the existing knowledge of lncRNAs in order to provide a better understanding of this new studying field. lncRNAs play important roles in brain development, neuron function and maintenance, and neurodegenerative diseases are becoming increasingly evident. In this review, we also highlighted recent studies related lncRNAs in central nervous system (CNS) development and neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS), and elucidated some specific lncRNAs which may be important for understanding the pathophysiology of neurodegenerative diseases, also have the potential as therapeutic targets.

The role of the CoREST/REST repressor complex in herpes simplex virus 1 productive infection and in latency

REST is a key component of the HDAC1 or 2, CoREST, LSD1, REST (HCLR) repressor complex. The primary function of the HCLR complex is to silence neuronal genes in non-neuronal cells. HCLR plays a role in regulating the expression of viral genes in productive infections as a donor of LDS1 for expression of α genes and as a repressor of genes expressed later in infection. In sensory neurons the HCLR complex is involved in the silencing of viral genome in the course of establishment of latency. The thesis of this article is that (a) sensory neurons evolved a mechanism to respond to the presence and suppress the transmission of infectious agents from the periphery to the CNS and (b) HSV evolved subservience to the HCLR with at least two objectives: to maintain a level of replication consistent with maximal person-to-person spread and to enable it to take advantage of neuronal innate immune responses to survive and be available for reactivation shielded from adaptive immune responses of the host.

HSV carrying WT REST establishes latency but reactivates only if the synthesis of REST is suppressed

HSVs transit from vigorous replication at the portal of entry into the body to a latent state in sensory neurons in which only noncoding (e.g., latency-associated transcript) and micro-RNAs are expressed. In productive infection, viral genes must be sequentially derepressed at two checkpoints. A leading role in the repression of viral genes is carried out by histone deacetylase (HDAC)/corepressor element-1 silencing transcription factor (CoREST)/lysinespecific demethylase1(LSD1)/RE1-silencing transcription factor (REST) repressor complex (HCLR). Previously, we reported that to define the role of the components of the HCLR complex in the establishment of latency, we constructed recombinant virus (R112) carrying a dominant-negative REST that bound response elements in DNA but could not recruit repressive proteins. This recombinant virus was unable to establish latency. In the current studies, we constructed a virus (R111) carrying WT REST with a WT genome. We report the following findings: (a) R111 readily established latent infection in trigeminal ganglia; however, although the amounts of viral DNAs in latently infected neurons were similar to those of WT virus, the levels of latency-associated transcript and micro-RNAs were 50- to 100-fold lower; (b) R111 did not spontaneously reactivate in ganglionic organ cultures; however, viral genes were expressed if the synthesis of REST was blocked by cycloheximide; and (c) histone deacetylase inhibitors reactivated the WT parent but not the R111 recombinant virus. The results suggest that REST plays a transient role in the establishment of latency but not in reactivation and suggest the existence of at least two phases at both establishment and reactivation.

Multiple binding partners

GAPDH interacts with a plethora of diverse cellular proteins. The network of interacting partners, or interactome, is presented for GAPDH with the interacting molecules grouped into specific functional and structural categories. By organizing the binding partners in this way, certain common structural features are beginning to surface, such as acidic dipeptide sequences that are found in several of these binding proteins. Additionally, the consensus sequences for target polynucleotides are being brought to light. The categories, which are presented according to function, offer an opportunity for research into the corresponding structural correlates to these interactions. Recent discoveries of interacting proteins have revealed novel relationships that are generating emerging mechanisms. Proteins that are associated with age-related neurodegenerative diseases appear to be particularly prone to binding GAPDH, suggesting that GAPDH may be playing a role in these diseases. Neurodegenerative diseases that are discussed are the conformational diseases of aging, suggesting that GAPDH may be a global sensor for cellular conformational stress. In addition to GAPDH's oxidoreductase activity, several other enzymatic functions have been discovered, including peroxidase, nitrosylase, mono-ADP-ribosylase and kinase activities.

Epigenetic mechanisms governing the process of neurodegeneration

Studies elucidating how and why neurodegeneration unfolds suggest that a complex interplay between genetic and environmental factors is responsible for disease pathogenesis. Recent breakthroughs in the field of epigenetics promise to advance our understanding of these mechanisms and to promote the development of useful and effective pre-clinical risk stratification strategies, molecular diagnostic and prognostic methods, and disease-modifying treatments.

Advances in epigenetics and epigenomics for neurodegenerative diseases

In the post-genomic era, epigenetic factors-literally those that are "over" or "above" genetic ones and responsible for controlling the expression and function of genes-have emerged as important mediators of development and aging; gene-gene and gene-environmental interactions; and the pathophysiology of complex disease states. Here, we provide a brief overview of the major epigenetic mechanisms (ie, DNA methylation, histone modifications and chromatin remodeling, and non-coding RNA regulation). We highlight the nearly ubiquitous profiles of epigenetic dysregulation that have been found in Alzheimer's and other neurodegenerative diseases. We also review innovative methods and technologies that enable the characterization of individual epigenetic modifications and more widespread epigenomic states at high resolution. We conclude that, together with complementary genetic, genomic, and related approaches, interrogating epigenetic and epigenomic profiles in neurodegenerative diseases represent important and increasingly practical strategies for advancing our understanding of and the diagnosis and treatment of these disorders.

Plasmids containing NRSE/RE1 sites enhance neurite outgrowth of retinal ganglion cells via sequestration of REST independent of NRSE dsRNA expression

Repressor element-1 silencing transcription factor (REST) is a transcriptional repressor of neuron-specific genes that binds to a conserved DNA element, the neuron restrictive silencer element (NRSE/RE1). Interestingly, increased REST activity is found in several neurological diseases like Huntington's disease and cerebral ischemia. Recently, it was shown that NRSE dsRNA, a double-stranded non-coding RNA can bind to REST during a defined period of neuronal differentiation, and thereby changes REST from a transcriptional repressor to an activator of neuron-specific genes. Here, we analyzed the effects of NRSE dsRNA expression in primary retinal ganglion cells. We found that NRSE dsRNA expression vectors significantly enhance neurite outgrowth even when axonal degeneration is induced by neurotrophin deprivation. Transfection of HEK cells with NRSE dsRNA-expressing vectors altered their morphology leading to the formation of thin processes and induced the expression of neurofilament-68. Surprisingly, control vectors containing REST-binding sites, but not expressing NRSE dsRNA, resulted in the same effects, also in the retinal ganglion cell model. Reporter assays and retention of REST in the cytoplasm with a labeled NRSE/RE1-containing plasmid incapable of entering the nucleus suggest that sequestration of REST in the cytoplasm is the reason for the observed effects. No evidence for a biological function of NRSE dsRNA could be found in these models. We conclude that sequestration of REST leads to enhanced neurite outgrowth in retinal ganglion cells and that an increased activity of REST, as it is found in several neurodegenerative diseases, can be effectively modulated by sequestration of REST with plasmids containing NRSE/RE1 sites.

Regulation of RE1 protein silencing transcription factor (REST) expression by HIP1 protein interactor (HIPPI)

Earlier we have shown that the proapoptotic protein HIPPI (huntingtin interacting protein 1 (HIP1) protein interactor) along with its molecular partner HIP1 could regulate transcription of the caspase-1 gene. Here we report that RE1-silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF) is a new transcriptional target of HIPPI. HIPPI could bind to the promoter of REST and increased its expression in neuronal as well as non-neuronal cells. Such activation of REST down-regulated expression of REST target genes, such as brain-derived neurotrophic factor (BDNF) or proenkephalin (PENK). The ability of HIPPI to activate REST gene transcription was dependent on HIP1, the nuclear transporter of HIPPI. Using a Huntington disease cell model, we have demonstrated that feeble interaction of HIP1 with mutant huntingtin protein resulted in increased nuclear accumulation of HIPPI and HIP1, leading to higher occupancy of HIPPI at the REST promoter, triggering its transcriptional activation and consequent repression of REST target genes. This novel transcription regulatory mechanism of REST by HIPPI may contribute to the deregulation of transcription observed in the cell model of Huntington disease.

REST/NRSF governs the expression of dense-core vesicle gliosecretion in astrocytes

The REST/NRSF transcriptional repressor prevents cultured astrocytes from forming DCVs, and its variable expression in human brain cortex astrocytes may account for their functional heterogeneity.

Astrocytes are the brain nonnerve cells that are competent for gliosecretion, i.e., for expression and regulated exocytosis of clear and dense-core vesicles (DCVs). We investigated whether expression of astrocyte DCVs is governed by RE-1–silencing transcription factor (REST)/neuron-restrictive silencer factor (NRSF), the transcription repressor that orchestrates nerve cell differentiation. Rat astrocyte cultures exhibited high levels of REST and expressed neither DCVs nor their markers (granins, peptides, and membrane proteins). Transfection of a dominant-negative construct of REST induced the appearance of DCVs filled with secretogranin 2 and neuropeptide Y (NPY) and distinct from other organelles. Total internal reflection fluorescence analysis revealed NPY–monomeric red fluorescent protein–labeled DCVs to undergo Ca²⁺-dependent exocytosis, which was largely prevented by botulinum toxin B. In the I–II layers of the human temporal brain cortex, all neurons and microglia exhibited the expected inappreciable and high levels of REST, respectively. In contrast, astrocyte REST was variable, going from inappreciable to high, and accompanied by a variable expression of DCVs. In conclusion, astrocyte DCV expression and gliosecretion are governed by REST. The variable in situ REST levels may contribute to the well-known structural/functional heterogeneity of astrocytes.

Zebrafish Prickle1b mediates facial branchiomotor neuron migration via a farnesylation-dependent nuclear activity

The facial branchiomotor neurons (FBMNs) undergo a characteristic tangential migration in the vertebrate hindbrain. We previously used a morpholino knockdown approach to reveal that zebrafish prickle1b (pk1b) is required for this migration. Here we report that FBMN migration is also blocked in a pk1b mutant with a disruption in the consensus farnesylation motif. We confirmed that this lipid modification is required during FBMN migration by disrupting the function of farnesyl biosynthetic enzymes. Furthermore, farnesylation of a tagged Pk1b is required for its nuclear localization. Using a unique rescue approach, we have demonstrated that Pk1b nuclear localization and farnesylation are required during FBMN migration. Our data suggest that Pk1b acts at least partially independently of core planar cell polarity molecules at the plasma membrane, and might instead be acting at the nucleus. We also found that the neuronal transcriptional silencer REST is necessary for FBMN migration, and we provide evidence that interaction between Pk1b and REST is required during this process. Finally, we demonstrate that REST protein, which is normally localized in the nuclei of migrating FBMNs, is depleted from the nuclei of Pk1b-deficient neurons. We conclude that farnesylation-dependent nuclear localization of Pk1b is required to regulate REST localization and thus FBMN migration.

Huntington's disease, calcium, and mitochondria

Huntington's disease (HD) is caused by a mutation that increases the number of CAG repeats in the gene encoding for the protein Huntingtin (Htt). The mutation results in the pathological expansion of the polyQ stretch that is normally present within the N-terminal region of Htt. Even if Htt is ubiquitously expressed in tissues, the changes in the protein finally result in the clinical manifestation of motor and cognitive impairments observed in HD patients. The molecular ethiology of the disease is obscure: a number of cellular and animal models are used as essential tools in experimental approaches aimed at understanding it. Biochemical changes have been described that correlate with the malfunction of HD neurons (primarily in the striatum): consensus is gradually emerging that the dyshomeostasis of Ca(2+) and/or mitochondria stress are important factors in the linkage of the Htt mutation to the onset and progression of the disease. Here, we present a succint overview of the changes of Htt, of its possible effect on the transcription of critical genes and of its causative role in the disturbance of the neuronal Ca(2+) homeostasis. Particular emphasis will be placed on the role of mitochondria as key player in the molecular pathogenesis of the disease.

Oligonucleotide therapeutic approaches for Huntington disease

Huntington disease is an autosomal dominant neurodegenerative disorder caused by a toxic expansion in the CAG repeat region of the huntingtin gene. Oligonucleotide approaches based on RNAi and antisense oligonucleotides provide promising new therapeutic strategies for direct intervention through reduced production of the causative mutant protein. Allele-specific and simultaneous mutant and wild-type allele-lowering strategies are being pursued with local delivery to the brain, each with relative merits. Delivery remains a key challenge for translational success, especially with chronic therapy. The potential of disease-modifying oligonucleotide approaches for Huntington disease will be revealed as they progress into clinical trials.

HuB/C/D, nPTB, REST4, and miR-124 regulators of neuronal cell identity are also utilized in the lens

PURPOSE

An interlocking network of transcription factors, RNA binding proteins, and miRNAs globally regulates gene expression and alternative splicing throughout development, and ensures the coordinated mutually exclusive expression of non-neural and neuronal forms of these factors during neurogenesis. Striking similarities between lens fiber cell and neuron cell morphology led us to determine if these factors are also used in the lens. HuR and polypyrimidine tract binding protein (PTB) have been described as 'global regulators' of RNA alternative splicing, stability, and translation in non-neuronal (including ectodermal) tissues examined to date in diverse species, and REST/NRSF (RE-1 Silencing Transcription Factor/Neuron Restrictive Silencing Factor) represses>2,000 neuronal genes in all non-neuronal tissues examined to date, but has not included the lens. During neurogenesis these factors are replaced by what has been considered neuron-specific HuB/C/D, nPTB, and alternatively spliced REST (REST4), which work with miR-124 to activate this battery of genes, comprehensively reprogram neuronal alternative splicing, and maintain their exclusive expression in post-mitotic neurons.

METHODS

Immunoprecipitation, western blot, immunofluorescence, and immunohistochemistry were used to determine the expression and distribution of proteins in mouse and rat lenses. Mobility shift assays were used to examine lenses for REST/NRSF DNA binding activity, and RT-PCR, DNA sequencing, and northern blots were used to identify RNA expression and alternative splicing events in lenses from mouse, rat, and goldfish (N. crassa).

RESULTS

We demonstrated that REST, HuR, and PTB proteins are expressed predominantly in epithelial cells in mouse and rat lenses, and showed these factors are also replaced by the predominant expression of REST4, HuB/C/D and nPTB in post-mitotic fiber cells, together with miR-124 expression in vertebrate lenses. REST-regulated gene products were found to be restricted to fiber cells where REST is decreased. These findings predicted nPTB- and HuB/C/D-dependent splicing reactions can also occur in lenses, and we showed Neuronal C-src and Type 1 Neurofibromatosis 1 splicing as well as calcitonin gene related peptide (CGRP) and neural cell adhesion molecule (NCAM-180) alternative transcripts in lenses. Transgenic mice with increased HuD in lens also showed increased growth associated protein 43 (GAP43) and Ca++/Calmodulin dependent kinase IIα (CamKIIα) HuD target gene expression in the lens, similar to brain.

CONCLUSIONS

The present study provides the first evidence this fundamental set of regulatory factors, previously considered to have a unique role in governing neurogenesis are also used in the lens, and raises questions about the origins of these developmental factors and mechanisms in lens and neuronal cells that also have a basic role in determining the neuronal phenotype.

Loss of striatal type 1 cannabinoid receptors is a key pathogenic factor in Huntington's disease

Endocannabinoids act as neuromodulatory and neuroprotective cues by engaging type 1 cannabinoid receptors. These receptors are highly abundant in the basal ganglia and play a pivotal role in the control of motor behaviour. An early downregulation of type 1 cannabinoid receptors has been documented in the basal ganglia of patients with Huntington's disease and animal models. However, the pathophysiological impact of this loss of receptors in Huntington's disease is as yet unknown. Here, we generated a double-mutant mouse model that expresses human mutant huntingtin exon 1 in a type 1 cannabinoid receptor-null background, and found that receptor deletion aggravates the symptoms, neuropathology and molecular pathology of the disease. Moreover, pharmacological administration of the cannabinoid Δ(9)-tetrahydrocannabinol to mice expressing human mutant huntingtin exon 1 exerted a therapeutic effect and ameliorated those parameters. Experiments conducted in striatal cells show that the mutant huntingtin-dependent downregulation of the receptors involves the control of the type 1 cannabinoid receptor gene promoter by repressor element 1 silencing transcription factor and sensitizes cells to excitotoxic damage. We also provide in vitro and in vivo evidence that supports type 1 cannabinoid receptor control of striatal brain-derived neurotrophic factor expression and the decrease in brain-derived neurotrophic factor levels concomitant with type 1 cannabinoid receptor loss, which may contribute significantly to striatal damage in Huntington's disease. Altogether, these results support the notion that downregulation of type 1 cannabinoid receptors is a key pathogenic event in Huntington's disease, and suggest that activation of these receptors in patients with Huntington's disease may attenuate disease progression.

The Huntington's disease mutation impairs Huntingtin's role in the transport of NF-κB from the synapse to the nucleus

Expansion of a polyglutamine (polyQ) tract in the Huntingtin (Htt) protein causes Huntington's disease (HD), a fatal inherited neurodegenerative disorder. Loss of the normal function of Htt is thought to be an important pathogenetic component of HD. However, the function of wild-type Htt is not well defined. Htt is thought to be a multifunctional protein that plays distinct roles in several biological processes, including synaptic transmission, intracellular transport and neuronal transcription. Here, we show with biochemical and live cell imaging studies that wild-type Htt stimulates the transport of nuclear factor κ light-chain-enhancer of activated B cells (NF-κB) out of dendritic spines (where NF-κB is activated by excitatory synaptic input) and supports a high level of active NF-κB in neuronal nuclei (where NF-κB stimulates the transcription of target genes). We show that this novel function of Htt is impaired by the polyQ expansion and thus may contribute to the etiology of HD.

Molecular Mechanisms and Potential Therapeutical Targets in Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the gene encoding for huntingtin protein. A lot has been learned about this disease since its first description in 1872 and the identification of its causative gene and mutation in 1993. We now know that the disease is characterized by several molecular and cellular abnormalities whose precise timing and relative roles in pathogenesis have yet to be understood. HD is triggered by the mutant protein, and both gain-of-function (of the mutant protein) and loss-of-function (of the normal protein) mechanisms are involved. Here we review the data that describe the emergence of the ancient huntingtin gene and of the polyglutamine trait during the last 800 million years of evolution. We focus on the known functions of wild-type huntingtin that are fundamental for the survival and functioning of the brain neurons that predominantly degenerate in HD. We summarize data indicating how the loss of these beneficial activities reduces the ability of these neurons to survive. We also review the different mechanisms by which the mutation in huntingtin causes toxicity. This may arise both from cell-autonomous processes and dysfunction of neuronal circuitries. We then focus on novel therapeutical targets and pathways and on the attractive option to counteract HD at its primary source, i.e., by blocking the production of the mutant protein. Strategies and technologies used to screen for candidate HD biomarkers and their potential application are presented. Furthermore, we discuss the opportunities offered by intracerebral cell transplantation and the likely need for these multiple routes into therapies to converge at some point as, ideally, one would wish to stop the disease process and, at the same time, possibly replace the damaged neurons.

Transcriptional dysregulation of coding and non-coding genes in cellular models of Huntington's disease

HD (Huntington's disease) is a late onset heritable neurodegenerative disorder that is characterized by neuronal dysfunction and death, particularly in the cerebral cortex and medium spiny neurons of the striatum. This is followed by progressive chorea, dementia and emotional dysfunction, eventually resulting in death. HD is caused by an expanded CAG repeat in the first exon of the HD gene that results in an abnormally elongated polyQ (polyglutamine) tract in its protein product, Htt (Huntingtin). Wild-type Htt is largely cytoplasmic; however, in HD, proteolytic N-terminal fragments of Htt form insoluble deposits in both the cytoplasm and nucleus, provoking the idea that mutHtt (mutant Htt) causes transcriptional dysfunction. While a number of specific transcription factors and co-factors have been proposed as mediators of mutHtt toxicity, the causal relationship between these Htt/transcription factor interactions and HD pathology remains unknown. Previous work has highlighted REST [RE1 (repressor element 1)-silencing transcription factor] as one such transcription factor. REST is a master regulator of neuronal genes, repressing their expression. Many of its direct target genes are known or suspected to have a role in HD pathogenesis, including BDNF (brain-derived neurotrophic factor). Recent evidence has also shown that REST regulates transcription of regulatory miRNAs (microRNAs), many of which are known to regulate neuronal gene expression and are dysregulated in HD. Thus repression of miRNAs constitutes a second, indirect mechanism by which REST can alter the neuronal transcriptome in HD. We will describe the evidence that disruption to the REST regulon brought about by a loss of interaction between REST and mutHtt may be a key contributory factor in the widespread dysregulation of gene expression in HD.

Brain-derived neurotrophic factor in neurodegenerative diseases

Changes in the levels and activities of neurotrophic factors, such as brain-derived neurotrophic factor (BDNF), have been described in a number of neurodegenerative disorders, including Huntington disease, Alzheimer disease and Parkinson disease. It is only in Huntington disease, however, that gain-of-function and loss-of-function experiments have linked BDNF mechanistically with the underlying genetic defect. Altogether, these studies have led to the development of experimental strategies aimed at increasing BDNF levels in the brains of animals that have been genetically altered to mimic the aforementioned human diseases, with a view to ultimately influencing the clinical treatment of these conditions. In this article, we will review the current knowledge about the involvement of BDNF in a number of neurodegenerative diseases, with particular emphasis on Huntington disease, and will provide the rationale for and discuss the problems in proposing BDNF treatment as a beneficial and feasible therapeutic approach in the clinic.

Regulation of non-coding RNA networks in the nervous system--what's the REST of the story?

Recent advances are now providing novel insights into the mechanisms that underlie how cellular complexity, diversity, and connectivity are encoded within the genome. The repressor element-1 silencing transcription factor/neuron-restrictive silencing factor (REST/NRSF) and non-coding RNAs (ncRNAs) are emerging as key regulators that seem to orchestrate almost every aspect of nervous system development, homeostasis, and plasticity. REST and its primary cofactor, CoREST, dynamically recruit highly malleable macromolecular complexes to widely distributed genomic regulatory sequences, including the repressor element-1/neuron restrictive silencer element (RE1/NRSE). Through epigenetic mechanisms, such as site-specific targeting and higher-order chromatin remodeling, REST and CoREST can mediate cell type- and developmental stage-specific gene repression, gene activation, and long-term gene silencing for protein-coding genes and for several classes of ncRNAs (e.g. microRNAs [miRNAs] and long ncRNAs). In turn, these ncRNAs have similarly been implicated in the regulation of chromatin architecture and dynamics, transcription, post-transcriptional processing, and RNA editing and trafficking. In addition, REST and CoREST expression and function are tightly regulated by context-specific transcriptional and post-transcriptional mechanisms including bidirectional feedback loops with various ncRNAs. Not surprisingly, deregulation of REST and ncRNAs are both implicated in the molecular pathophysiology underlying diverse disorders that range from brain cancer and stroke to neurodevelopmental and neurodegenerative diseases. This review summarizes emerging aspects of the complex mechanistic relationships between these intricately interlaced control systems for neural gene expression and function.

Gene dysregulation in Huntington's disease: REST, microRNAs and beyond

Huntington's disease (HD) is an incurable, fatal neurodegenerative disorder that is caused by a polyglutamine expansion in the huntingtin (Htt) protein. Neuronal death in the striatum-the most obvious manifestation of the disease-is likely to result from widespread dysregulation of gene expression in various brain regions. To date, several potential mechanisms for this have been discovered, including one involving REST (RE1-Silencing Transcription Factor), a master regulator of neuronal genes. Recently, independent studies have demonstrated that post-transcriptional gene regulation by microRNAs is also disrupted in HD. Expression of key neuronal microRNAs-including mir-9/9*, mir-124 and mir-132-is repressed in the brains of human HD patients and mouse models. These changes occur downstream of REST, and are likely to result in major disruption of mRNA regulation and neuronal function. In this study we will discuss these findings and their implications for our understanding of HD. Using updated bioinformatic analysis, we predict 21 new candidate microRNAs in HD. We propose future strategies for unifying large-scale transcriptional and microRNA datasets with the aim of explaining HD aetiology. By way of example, we show how available genomic datasets can be integrated to provide independent, analytical validation for dysregulation of REST and microRNA mir-124 in HD. As a consequence, gene ontology analysis indicates that HD is characterised by a broad-based depression of neural genes in the caudate and motor cortex. Thus, we propose that a combination of REST, microRNAs and possibly other non-coding RNAs profoundly affect the neuronal transcriptome in HD.

REST and the RESTless: in stem cells and beyond

Since its original discovery as a negative regulator of neuronal differentiation, the repressor element (RE)-1 silencing transcription factor (REST), also known as the neuron-restrictive silencer factor, has been implicated in novel processes such as maintenance of embryonic stem cell pluripotency and self-renewal and regulation of mitotic fidelity in non-neural cells. REST expression and activity is tightly controlled by transcriptional and post-transcriptional mechanisms in a cell and developmental stage-specific manner and perturbations in its levels or function are associated with various pathological states. REST differentially influences target-gene expression through interaction with a wide variety of cellular cofactors in a context-dependent manner. However, the influence of the microenvironment on REST-mediated regulation of gene expression is poorly understood. This review will present our current understanding of REST signaling with a greater focus on its emerging ties with noncoding RNAs and novel interacting partners, as well as its roles in embryonic stem cell self-renewal, cellular plasticity and oncogenesis/tumor suppression.