Elsevier

Life Sciences

Volume 74, Issue 18, 19 March 2004, Pages 2213-2225
Life Sciences

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Regulation of the cholinergic gene locus by the repressor element-1 silencing transcription factor/neuron restrictive silencer factor (REST/NRSF)

https://doi.org/10.1016/j.lfs.2003.08.045Get rights and content

Abstract

The cholinergic gene locus is comprised of two genes, the choline acetyltransferase gene and the vesicular acetylcholine transporter gene. The vesicular acetylcholine transporter gene is located within the first intron of the choline acetyltransferase gene. This arrangement permits coordinate regulation of the locus. Protein kinase A regulates expression of the cholinergic gene locus in PC12 cells. This regulation was found to be dependent on the presence of a 21-bp DNA sequence known as the repressor element-1 (RE-1)/neuron-restrictive silencer element (NRSE). Repressor element-1 silencing transcription factor (REST)/ neuron-restrictive silencer factor (NRSF), which binds to the RE-1/NRSE, is a zinc finger containing transcriptional repressor that blocks the expression of many neuronal RE-1/NRSE containing genes in nonneuronal cells. However, REST/NRSF expression has also been observed in neurons as well as the PC12 cell line used in these studies. REST/NRSF truncated isoforms were expressed in neuronal cells, suggesting they also function in regulating neuronal gene expression. A study of REST4, one of the REST/NRSF isoforms, suggests that it regulates transcription of the cholinergic gene locus by blocking the repressor activity of REST/NRSF. Protein kinase A regulation of the cholinergic gene locus in PC12 cells can thus be attributed, at least in part, to increased synthesis of REST4, which in turn derepresses the repressor activity of REST/NRSF.

Introduction

The ability of a cell to regulate its phenotype is dependent upon its ability to carefully and precisely control which genes are expressed and which are not. In the case of neurons there are many genes whose expression is necessary to produce a neuronal phenotype, but whose expression outside the central nervous system is not needed and likely to be detrimental. Recent evidence suggests that one mechanism utilized to prevent expression of neuronal genes outside the central nervous system is that of gene repression. This review focuses on the regulation of one neuron specific gene locus, the cholinergic gene locus, by transcriptional repression. In addition to describing the regulation of the cholinergic gene locus, the complexity of this system, its application to other neuronal genes, and the many questions yet to be answered are noted.

Cholinergic neurotransmission in the central nervous system is a key process dependent on the coexpression of proteins involved in the synthesis, storage, and release of the neurotransmitter acetylcholine (ACh). ACh plays an important role in fundamental brain processes such as memory, learning, and sleep Karczmar, 1976, Bartus et al., 1982, Aigner and Mishkin, 1986. ACh is used in a cyclic process involving the uptake of choline by a high-affinity uptake system (Jope, 1979), the formation of ACh through acetylation of choline catalyzed by the enzyme choline acetyltransferase (ChAT) (EC 2.3.1.6) and the transport of ACh into synaptic vesicles by the vesicular ACh transporter (VAChT). Upon stimulation, synaptic vesicles fuse with the plasma membrane and release ACh, which can then be hydrolyzed by acetylcholinesterase to produce free choline, thereby completing the cycle.

The cholinergic gene locus is comprised of both the choline acetyltransferase gene as well as the vesicular acetylcholine transporter gene, the latter being located within the first intron of the ChAT gene. ChAT is primarily if not exclusively a cytosolic protein whereas VAChT has 12 transmembrane domains and is integrated in the membrane of synaptic vesicles. In the CNS, ChAT and VAChT are both required for cholinergic neurotransmission. In some neurodegenerative disorders, i.e. Alzheimer's disease, amyotrophic lateral sclerosis, and schizophrenia, there is a dysfunction of central cholinergic neurons involving a loss of, or abnormality in, basal forebrain ChAT activity Davies and Maloney, 1976, Coyle et al., 1983, Price, 1986, Wu and Hersh, 1994.

Section snippets

Genomic structure and transcription of the cholinergic gene locus

The genes for mouse Misawa et al., 1992, Pu et al., 1993, rat Brice et al., 1989, Ishii et al., 1990, and human ChAT Kong et al., 1989, Hersh et al., 1993 have been isolated and shown to contain three 5′-noncoding exons; exon 1, referred to as the R exon; exon 2, referred to as the N exon; and exon 3, referred to as the M exon Misawa et al., 1992, Kengaku et al., 1993, Fig. 1. The alternative splicing of these three exons results in multiple 5′-mRNA species, which are transcribed from three

Coordinate regulation of ChAT and VAChT gene expression

The organization of the cholinergic gene locus suggested the possibility of coordinate regulation of the ChAT and VAChT genes at the transcriptional level Berrard et al., 1995, Berse and Blusztajn, 1995, Erickson et al., 1994, Shimojo et al., 1998. Prior to the discovery of the gene structure within the cholinergic gene locus, several studies reported that ChAT activity was regulated by growth factors Hefti et al., 1986, Gould and Butcher, 1989, Saadat et al., 1989, Knusel et al., 1991 and

Transcriptional regulation of cholinergic gene locus by REST/NRSF

Studies from a number of laboratories suggest that the transcriptional regulation of the ChAT and VAChT genes involves multiple cell specific regulatory elements. The region of the cholinergic gene locus upstream of the R-type promoter contains a consensus repressor element-1 (RE-1)/neuron-restrictive silencer element (NRSE) sequence and a cholinergic specific enhancer that presumably operate in a cooperative manner to control cholinergic gene expression in the CNS, Fig. 1. The RE-1/NRSE is a

Regulation of REST/NRSF function by its REST4 isoform

The finding that REST4 can block REST/NRSF repressor activity, coupled with REST4 expression in neuronal cells, can account for the presence of REST/NRSF in neurons without neuronal gene expression being repressed. Palm et al. (1998) reported the presence of REST/NRSF in adult neurons, albeit at rather low levels relative to its expression in non-neuronal cells. It was suggested that the inability of REST/NRSF to repress gene expression in neuronal cells could be explained by its low

Role of zinc finger domains of REST/NRSF

REST/NRSF contains nine Cys2-His2 type zinc finger domains, the first of these is near the N-terminus and is followed by a cluster of seven zinc finger domains, with the last zinc finger domain being near the C-terminus of the molecule, Fig. 2. REST4, being a truncated form of REST/NRSF contains only zinc finger domains 1–5, Fig. 2. It binds weakly to the RE-1/NRSE with the strength of binding dependent on the number of zinc finger domains present (Lee et al., 2000). These findings suggest that

REST/NRSF Repressor Domains

REST/NRSF is a modular protein that represses neuronal gene expression via two distinct repressor domains. Repression through an amino terminal domain is mediated by a Sin3-histone deacetylase (HDAC) complex, which affects chromosomal structure by promoting histone deacetylation Tapia-Ramirez et al., 1997, Grimes et al., 2000, Huang et al., 1999, Roopra et al., 2001, Naruse et al., 1999. The other repressor domain is found in the carboxyl-terminal domain and binds a novel protein, CoREST and

The future for REST

It is evident that the regulation of neuronal gene expression by REST/NRSF and its truncated isoforms is complex and far from understood. As noted above REST mediated gene repression involves two distinct repressor domains, one binding Sin3A and the other binding CoREST. Why two distinct repressor domains are present in REST/NRSF is puzzling. It is unclear whether both domains are utilized simultaneously or represent different modes of gene regulation that are utilized differentially. The need

Acknowledgements

This work was supported in part by grants AG46734 and AG05893 from the National Institutes of Health.

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