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      Is Huntingtin Dispensable in the Adult Brain?

      review-article
      , *
      Journal of Huntington's Disease
      IOS Press
      Huntingtin, HTT, Htt, Huntington’s disease, HD, cell stress, ROS, DNA damage

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          Abstract

          Huntingtin (HTT) is an essential protein during early embryogenesis and the development of the central nervous system (CNS). Conditional knock-out of mouse Huntingtin ( Htt) expression in the CNS beginning during neural development, as well as reducing Htt expression only during embryonic and early postnatal stages, results in neurodegeneration in the adult brain. These findings suggest that HTT is important for the development and/or maintenance of the CNS, but they do not address the question of whether HTT is required specifically in the adult CNS for its normal functions and/or homeostasis. Recently, it was reported that although removing Htt expression in young adult mice causes lethality due to acute pancreatitis, loss of Htt expression in the adult brain is well tolerated and does not result in either motor deficits or neurodegeneration for up to 7 months after Htt inactivation. However, recent studies have also demonstrated that HTT participates in several cellular functions that are important for neuronal homeostasis and survival including sensing reactive oxygen species (ROS), DNA damage repair, and stress responses, in addition to its role in selective macroautophagy. In this review, HTT’s functions in development and in the adult CNS will be discussed in the context of these recent discoveries, together with a discussion of their potential impact on the design of therapeutic strategies for Huntington’s disease (HD) aimed at lowering total HTT expression.

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          Most cited references108

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          Disruption of the glucocorticoid receptor gene in the nervous system results in reduced anxiety.

          The glucocorticoid receptor (Gr, encoded by the gene Grl1) controls transcription of target genes both directly by interaction with DNA regulatory elements and indirectly by cross-talk with other transcription factors. In response to various stimuli, including stress, glucocorticoids coordinate metabolic, endocrine, immune and nervous system responses and ensure an adequate profile of transcription. In the brain, Gr has been proposed to modulate emotional behaviour, cognitive functions and addictive states. Previously, these aspects were not studied in the absence of functional Gr because inactivation of Grl1 in mice causes lethality at birth (F.T., C.K. and G.S., unpublished data). Therefore, we generated tissue-specific mutations of this gene using the Cre/loxP -recombination system. This allowed us to generate viable adult mice with loss of Gr function in selected tissues. Loss of Gr function in the nervous system impairs hypothalamus-pituitary-adrenal (HPA)-axis regulation, resulting in increased glucocorticoid (GC) levels that lead to symptoms reminiscent of those observed in Cushing syndrome. Conditional mutagenesis of Gr in the nervous system provides genetic evidence for the importance of Gr signalling in emotional behaviour because mutant animals show an impaired behavioural response to stress and display reduced anxiety.
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            Huntingtin interacts with REST/NRSF to modulate the transcription of NRSE-controlled neuronal genes.

            Huntingtin protein is mutated in Huntington disease. We previously reported that wild-type but not mutant huntingtin stimulates transcription of the gene encoding brain-derived neurotrophic factor (BDNF; ref. 2). Here we show that the neuron restrictive silencer element (NRSE) is the target of wild-type huntingtin activity on BDNF promoter II. Wild-type huntingtin inhibits the silencing activity of NRSE, increasing transcription of BDNF. We show that this effect occurs through cytoplasmic sequestering of repressor element-1 transcription factor/neuron restrictive silencer factor (REST/NRSF), the transcription factor that binds to NRSE. In contrast, aberrant accumulation of REST/NRSF in the nucleus is present in Huntington disease. We show that wild-type huntingtin coimmunoprecipitates with REST/NRSF and that less immunoprecipitated material is found in brain tissue with Huntington disease. We also report that wild-type huntingtin acts as a positive transcriptional regulator for other NRSE-containing genes involved in the maintenance of the neuronal phenotype. Consistently, loss of expression of NRSE-controlled neuronal genes is shown in cells, mice and human brain with Huntington disease. We conclude that wild-type huntingtin acts in the cytoplasm of neurons to regulate the availability of REST/NRSF to its nuclear NRSE-binding site and that this control is lost in the pathology of Huntington disease. These data identify a new mechanism by which mutation of huntingtin causes loss of transcription of neuronal genes.
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              Cortical excitatory neurons and glia, but not GABAergic neurons, are produced in the Emx1-expressing lineage.

              By homologous recombination of an internal ribosome entry site and Cre recombinase coding region into the 3'-untranslated region of the mouse Emx1 gene, we have generated a strain of mice, Emx1(IRES)cre, that expresses the Cre recombinase in a spatial and temporal pattern like that observed for Emx1. When mated to reporter strains, these mice are a sensitive means to fate-map the Emx1-expressing cells of the developing forebrain. Our results demonstrate that radial glia, Cajal-Retzius cells, glutamatergic neurons, astrocytes, and oligodendrocytes of most pallial structures originate from an Emx1-expressing lineage. On the other hand, most of the pallial GABAergic neurons arise outside the Emx1-expressing lineage. Structures that are located near the basal ganglia (e.g., the amygdala and endopiriform nuclei) are not uniformly derived from Emx1-expressing cells.
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                Author and article information

                Journal
                J Huntingtons Dis
                J Huntingtons Dis
                JHD
                Journal of Huntington's Disease
                IOS Press (Nieuwe Hemweg 6B, 1013 BG Amsterdam, The Netherlands )
                1879-6397
                1879-6400
                21 March 2017
                28 March 2017
                2017
                : 6
                : 1
                : 1-17
                Affiliations
                [1]Department of Neuroscience, University of Virginia School of Medicine , Charlottesville, VA, USA
                Author notes
                [* ]Correspondence to: Scott O. Zeitlin, Ph.D., Department of Neuroscience, University of Virginia School of Medicine, 409 Lane Rd., Box 801392, MR4-5022, Charlottesville, VA 22908, USA. Tel.: +1 434 924 5011; Fax: +1 434 982 4380; E-mail: soz4n@ 123456eservices.virginia.edu .
                Article
                JHD170235
                10.3233/JHD-170235
                5389021
                28339401
                77921daa-134c-4732-80ae-3d0b270b677f
                IOS Press and the authors. All rights reserved

                This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial (CC BY-NC 4.0) License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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                Review

                huntingtin,htt,huntington’s disease,hd,cell stress,ros,dna damage
                huntingtin, htt, huntington’s disease, hd, cell stress, ros, dna damage

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