REST: a mammalian silencer protein that restricts sodium channel gene expression to neurons

Glucocorticoid control of glial gene expression

The glucocorticoid signaling pathway is responsive to a considerable number of internal and external signals and can therefore establish diverse patterns of gene expression. A glial-specific pattern, for example, is shown by the glucocorticoid-inducible gene glutamine synthetase. The enzyme is expressed at a particularly high level in glial cells, where it catalyzes the recycling of the neurotransmitter glutamate, and at a low level in most other cells, for housekeeping duties. Glial specificity of glutamine synthetase induction is achieved by the use of positive and negative regulatory elements, a glucocorticoid response element and a neural restrictive silencer element. Though not glial specific by themselves, these elements may establish a glial-specific pattern of expression through their mutual activity and their combined effect. The inductive activity of glucocorticoids is markedly repressed by the c-Jun protein, which is expressed at relatively high levels in proliferating glial cells. The signaling pathway of c-Jun is activated by the disruption of glia-neuron cell contacts, by transformation with v-src, and in proliferating retinal cells of early embryonic ages. The c-Jun protein inhibits the transcriptional activity of the glucocorticoid receptor and thus represses glutamine synthetase expression. This repressive mechanism might also affect the ability of glial cells to cope with glutamate neurotoxicity in injured tissues.

Neuron-specific splicing of zinc finger transcription factor REST/NRSF/XBR is frequent in neuroblastomas and conserved in human, mouse and rat

Neuron-restrictive silencer factor (NRSF), also known as repressor element RE1 binding transcription factor (REST) or repressor binding to the X2 box (XBR) (REST/NRSF/XBR), is a zinc finger transcription factor that during early embryogenesis is required to repress a subset of neuron-specific genes in non-neural tissues and undifferentiated neural precursors. We have previously shown that splicing within the coding region of rat REST/NRSF/XBR (rREST) generates several different transcripts all of which are expressed in the adult nervous system. rREST transcripts with short neuron-specific exons (exon N) have in-frame stop codons and encode truncated proteins which have an N-terminal repressor domain and weakened DNA binding activity. The aim of this study was to analyze the regulatory mechanisms underlying REST/NRSF/XBR activity in human and mouse as compared to rat. We show that the structure of REST/NRSF/XBR gene and its regulation by neuron-specific splicing is conserved in human, mouse and rat. Expression levels of REST/NRSF/XBR transcripts with the insertion of exon N are increased during the neuronal differentiation of mouse teratocarcinoma PCC7 and rat pheocromocytoma PC12 cells and are high in several human and mouse neuroblastoma cells as compared to the relatively low levels in the developing and adult nervous system. The exclusive expression of the neuronal forms of REST/NRSF/XBR mRNAs in mouse neuroblastoma Neuro-2A cells is not caused by rearrangement of the REST/NRSF/XBR gene nor by mutations in the sequence of the splice sites flanking exon N. These data suggest that changes in REST/NRSF/XBR splicing pattern may result from altered levels of splicing factors reflecting the formation and/or progression of neuroblastoma tumors.

Subfamily-specific posttranscriptional mechanism underlies K(+) channel expression in a developing neuronal blastomere

Na(+) and K(+) channels are the two key proteins that shape the action potentials in neurons. However, little is known about how the expression of these two channels is coordinated. To address this issue, we cloned a Shab-related K(+) channel gene from ascidian Halocynthia roretzi (TuKv2). In this animal, a blastomere of neuronal lineage isolated from the 8-cell embryo expresses single Na(+) channel and K(+) channel genes after neural induction. Expression of a dominant negative form of TuKv2 eliminated the native delayed rectifier K(+) currents, indicating that the entire delayed rectifier K(+) current of the neuronal blastomere is exclusively encoded by TuKv2. TuKv2 transcripts are expressed more broadly than Na(+) channel transcripts, which are restricted to the neuronal lineages. There is also a temporal mismatch in the expression of TuKv2 transcript and the K(+) current; TuKv2 transcripts are present throughout development, whereas delayed rectifier K(+) currents only appear after the tailbud stage, suggesting that the functional expression of the TuKv2 transcript is suppressed during the early embryonic stages. To test if this suppression occurs by a mechanism specific to the TuKv2 channel protein, an ascidian Shaker-related gene, TuKv1, was misexpressed in neural blastomeres. A TuKv1-encoded current was expressed earlier than the TuKv2 current. Furthermore, the introduction of the TuKv2-expressing plasmid into noninduced cells did not lead to the current expression. These results raise the possibility that the expression of TuKv2 is post-transcriptionally controlled through a mechanism that is dependent on neural induction.

Neuron-restrictive silencer elements mediate neuron specificity of adenoviral gene expression

Neuron-restrictive silencer elements (NRSEs) were used to target the gene expression of adenoviral vectors specifically to neuron cells in the central nervous system. By generating adenoviral constructs in which NRSE sequences were placed upstream from the ubiquitous phosphoglycerate kinase promoter, the specificity of expression of a luciferase reporter gene was tested in both cell lines and primary cultures. Whereas transgene expression was negligible in nonneuronal cells following infection with an adenovirus containing 12 NRSEs, neuronal cells strongly expressed luciferase when infected with the same adenovirus. The NRSEs restricted expression of the luciferase gene to neuronal cells in vivo when adenoviruses were injected both intramuscularly into mice and intracerebrally into rats. This NRSE strategy may avoid side effects resulting from the ectopic expression of therapeutic genes in the treatment of neurological diseases. In particular, it may allow the direct transfection of motor neurons without promoting transgene expression within inoculated muscles or the secretion of transgene products into the bloodstream.

E2F4 actively promotes the initiation and maintenance of nerve growth factor-induced cell differentiation

E2F transcription factors play a critical role in cell cycle progression through the regulation of genes required for G(1)/S transition. They are also thought to be important for growth arrest; however, their potential role in the cell differentiation process has not been previously examined. Here, we demonstrate that E2F4 is highly upregulated following the neuronal differentiation of PC12 cells with nerve growth factor (NGF), while E2F1, E2F3, and E2F5 are downregulated. Immunoprecipitation and subcellular fractionation studies demonstrated that both the nuclear localization of E2F4 and its association with the Rb family member p130 increased following neuronal differentiation. The forced expression of E2F4 markedly enhanced the rate of PC12 cell differentiation induced by NGF and also greatly lowered the rate at which cells lost their neuronal phenotype following NGF removal. Importantly, this effect occurred in the absence of any significant change in the growth regulation of PC12 cells by NGF. Further, the downregulation of E2F4 expression with antisense oligodeoxynucleotides inhibited NGF-induced neurite outgrowth, indicating an important role for this factor during PC12 cell differentiation. Finally, E2F4 expression was found to increase dramatically in the developing rat cerebral cortex and cerebellum, as neuroblasts became postmitotic and initiated terminal differentiation. These findings demonstrate that, in addition to its effects on cell proliferation, E2F4 actively promotes the neuronal differentiation of PC12 cells as well as the retention of this state. Further, this effect is independent of alterations in cell growth and may involve interactions between E2F4 and the neuronal differentiation program itself. E2F4 may be an important participant in the terminal differentiation of neuroblasts.

CoREST: a functional corepressor required for regulation of neural-specific gene expression

Several genes encoding proteins critical to the neuronal phenotype, such as the brain type II sodium channel gene, are expressed to high levels only in neurons. This cell specificity is due, in part, to long-term repression in nonneural cells mediated by the repressor protein REST/NRSF (RE1 silencing transcription factor/neural-restrictive silencing factor). We show here that CoREST, a newly identified human protein, functions as a corepressor for REST. A single zinc finger motif in REST is required for CoREST interaction. Mutations of the motif that disrupt binding also abrogate repression. When fused to a Gal4 DNA-binding domain, CoREST functions as a repressor. CoREST is present in cell lines that express REST, and the proteins are found in the same immunocomplex. CoREST contains two SANT (SW13/ADA2/NCoR/TFIIIB B) domains, a structural feature of the nuclear receptor and silencing mediator for retinoid and thyroid human receptors (SMRT)-extended corepressors that mediate inducible repression by steroid hormone receptors. Together, REST and CoREST mediate repression of the type II sodium channel promoter in nonneural cells, and the REST/CoREST complex may mediate long-term repression essential to maintenance of cell identity.

Silencer-mediated repression and non-mediated activation of BDNF and c-fos gene promoters in primary glial or neuronal cells

Although the neuron-restrictive silencer element (NRSE/Regard) has been shown to function as a negative-acting DNA regulatory element to prevent the expression of neuron-specific genes in non-neuronal cells, little is known about its silencing effect on transcription in primary glial cells nor its effect on transcriptional activation in primary neurons. By DNA transfection in primary cultures of rat cortical neuronal or glial cells, we investigated the effect of NRSE on transcription mediated by the BDNF promoter I or c-fos promoter to which NRSE sequences derived from the SCG10 gene were linked. Transfection of plasmid DNAs to NIH3T3 fibroblasts resulted in a marked repressive effect of NRSE on BDNF promoter I- or c-fos promoter-mediated transcription. In primary neuronal cells, however, NRSE did not repress the basal promoter activities of BDNF and c-fos genes and allowed the transcriptional activation of these genes induced by membrane depolarization although NRSE slightly reduced the magnitude of BDNF promoter I activation. In contrast to neuronal cells, a marked repression of basal promoter activities of both genes was detected in primary glial culture and a two base pair-mutation of NRSE partially recovered the repression. These results indicate that NRSE negatively acts on its linked promoters in primary glial cells and does not interfere an activation of linked promoters in neuronal cells.

Diverse stabilities of expression in the rat brain from different cellular promoters in a helper virus-free herpes simplex virus type 1 vector system

Many neural gene transfer studies require both long-term and cell type-specific expression. We have reported a helper virus-free HSV-1 plasmid vector system (Fraefel et al., 1996), and this system supports at least some long-term expression from herpesvirus immediate-early promoters. In this study, we constructed vectors that placed the lacZ reporter gene under the regulation of five different cellular promoters. Vector stocks were microinjected into the midbrain, striatum, or hippocampus; the rats were sacrificed at 4 days to 2 months after gene transfer; and the numbers of X-Gal-positive cells were determined. A 6.8-kb fragment of the rat tyrosine hydroxylase (TH) promoter supported relatively stable expression for up to 2 months and targeted expression to TH-immunoreactive neurons in the substantia nigra pars compacta. The other promoters that were examined were chosen with the goal of obtaining long-term, neuronal-specific expression. At 4 days after gene transfer, a 766-bp fragment of the TH promoter supported expression in cells with neuronal morphology in the midbrain and striatum, consistent with results in transgenic mice. However, expression was absent by 2 weeks. Similarly, at 4 days after gene transfer, a mouse neurofilament heavy subunit promoter supported expression in cells with neuronal morphology in the midbrain, striatum, and hippocampus, but expression was absent by 2 weeks. A rat neuron-specific enolase promoter supported only a low level of expression in cultured neuronal cells, and expression was not detected in the brain. A rat voltage-gated sodium channel promoter supported only a low level of expression in PC12 cells and expression could not be detected in cultured cortical cells. These results demonstrate that different promoters support a wide range of levels and stabilities of expression in this vector system, and the results suggest approaches to improving the stability of long-term expression.

A PC12 variant lacking regulated secretory organelles: aberrant protein targeting and evidence for a factor inhibiting neuroendocrine gene expression

A variant of the PC12 pheochromocytoma cell line (termed A35C) has been isolated that lacks regulated secretory organelles and several constituent proteins. Northern and Southern blot analyses suggested a block at the transcriptional level. The proprotein-converting enzyme carboxypeptidase H was synthesised in the A35C cell line but was secreted by the constitutive pathway. Transient transfection of A35C cells with cDNAs encoding the regulated secretory proteins dopamine beta-hydroxylase and synaptotagmin I resulted in distinct patterns of mistargeting of these proteins. It is surprising that hybrid cells created by fusing normal PC12 cells with A35C cells exhibited the variant phenotype, suggesting that A35C cells express an inhibitory factor that represses neuroendocrine-specific gene expression.

TATA-driven transcriptional initiation and regulation of the rat serotonin 5-HT1A receptor gene

The transcriptional initiation and regulation of the rat serotonin 5-HT1A receptor gene were characterized. By three types of analyses, a single brain-specific site of transcriptional initiation was localized to -967 bp upstream of the translation initiation codon that is utilized both in hippocampus and in the rat raphe RN46A cell line. This major site of transcriptional initiation was located 58 bp downstream from a consensus TATA element, suggesting TATA-driven transcription of the rat 5-HT1A receptor. To identify the promoter activity of the receptor gene, progressive 5' deletions of the -2,719/-117-bp fragment of the 5-HT1A promoter linked to luciferase gene were transfected into 5-HT1A-negative (pituitary GH4C1, L6 myoblast, and C6 glioma) and 5-HT1A-positive (septal SN-48 and raphe RN46A) cell lines. Enhancer regions were identified within a fragment between nucleotides -426 and -117 that selectively enhanced transcription in 5-HT1A-positive cells. A nonselective enhancer/promoter that mediated expression in all cell lines was located upstream between -1,519 and -426 bp in a DNA segment containing consensus TATA, CCAAT, SP-1, and AP-1 elements as well as a poly-GT26 dinucleotide repeat. Strong repression of transcription in all cell lines was conferred by the region upstream of -1,519 bp that contains a 152-bp DNA segment with >80% identity to RANTES, tumor necrosis factor-beta, and other immune system genes. Our results indicate that TATA-driven expression of the 5-HT1A receptor is regulated by a novel proximal tissue-specific enhancer region, a nonselective promoter, and an upstream repressor region that is distinct from previously identified neuron-specific repressors.

Transcriptional repression by the zinc finger protein REST is mediated by titratable nuclear factors

The zinc finger protein REST (RE-1 silencing transcription factor) is a transcriptional repressor that inhibits neuronal gene transcription in non-neuronal tissues. REST may represent a master regulator of neuronal gene expression. REST contains two repressor domains located at the N- and C-termini of the molecule. To investigate the molecular mechanism of transcriptional repression by REST, in vivo competition experiments were performed. Both repression domains were expressed in the nucleus as fusion proteins with S. japonicum glutathione S-transferase (GST). The ability of these fusion proteins to block transcriptional repression mediated by the repressor domains of REST was tested. The results show that transcriptional repression by the N-terminal repression domain of REST could be overcome by expression of a GST fusion protein encoding the N-terminal, but not C-terminal repression domain, and vice versa, suggesting that both repression domains have to interact with distinct nuclear factors to exhibit biological activity. The GST-REST fusion proteins had no effect upon transcriptional repression mediated by the KRAB (Krüppel-associated box) domain, a strong mammalian repressor domain, or the repressor domain derived from the thyroid hormone receptors alpha. We conclude that REST has to interact with at least two distinct nuclear factors to inhibit transcription. These factors are distinct from the mammalian corepressor proteins KAP-1/KRIP-1 and N-CoR that mediate repression by the KRAB domain or the thyroid hormone receptor alpha. Thus, mammalian transcriptional repressors utilize different mechanisms to inhibit transcription by using different kinds of protein-protein interactions.

Genetic regulation of glutamate receptor ion channels

Transcriptional and translational regulation of glutamate receptor expression determines one of the key phenotypic features of neurons in the brain--the properties of their excitatory synaptic receptors. Up- and down-regulation of various glutamate receptor subunits occur throughout development, following ischemia, seizures, repetitive activation of afferents, or chronic administration of a variety of drugs. The promoters of the genes that encode the NR1, NR2B, NR2C, GluR1, GluR2, and KA2 subunits share several characteristics that include multiple transcriptional start sites within a CpG island, lack of TATA and CAAT boxes, and neuronal-selective expression. In most cases, the promoter regions include overlapping Sp1 and GSG motifs near the major initiation sites, and a silencer element, to guide expression in neurons. Manipulating the levels of glutamate receptors in vivo by generating transgenic and knockout mice has enhanced understanding of the role of specific glutamate receptor subunits in long-term potentiation and depression, learning, seizures, neural pattern formation, and survival. Neuron-specific glutamate receptor promoter fragments may be employed in the design of novel gene-targeting constructs to deliver future experimental transgene and therapeutic agents to selected neurons in the brain.

Knockout of REST/NRSF shows that the protein is a potent repressor of neuronally expressed genes in non-neural tissues

The protein repressor element 1 silencing transcription factor/neuron restrictive silencer factor (REST/NRSF) is a negative regulator of neuronal genes that contain a particular DNA sequence, the neuron restrictive silencer element (NRSE). REST is expressed ubiquitously in non-neural tissues but is down-regulated in neural precursors and turned off in postmitotic neurons, suggesting that it can act both to prevent extraneural expression of certain genes and to delay the differentiation of neuronal subtypes. In a recent paper, Chen et al.(1) describe the production of a null mutant for REST in mice and the mosaic inactivation of REST function in chicken embryos. Knockout of REST led to malformations in several non-neural tissues, as well as apoptosis and embryonic lethality in mice. In addition, the expression of several REST target genes was derepressed in non-neural tissues and in neural progenitors in both mouse and chicken embryos. These studies clearly demonstrate that active repression of tissue-specific genes is required for proper tissue differentiation during embryonic development.

Neuron-specific and developmental regulation of the synapsin II gene expression in transgenic mice

Synapsin II, a major phosphoprotein of synaptic vesicles, is believed to function in neurotransmitter release as well as in synapse formation. The expression of the synapsin II gene is neuron-specific, and correlates temporally with synaptogenesis. To understand the mechanisms by which the expression of the synapsin II gene is regulated in vivo, we generated transgenic mice carrying a 5.1-kb 5'-flanking sequence of the murine synapsin II gene fused to the firefly luciferase reporter gene. The synapsin II-luciferase transgene is specifically expressed in neural tissues, such as brain and spinal cord, but not in non-neural tissues. Throughout the brain, the expression of the transgene is widely distributed, and restricted only to neuronal cells. Moreover, the expression of the transgene is developmentally regulated, with a temporal profile similar to that of endogenous synapsin II expression. These results indicate that the 5.1-kb flanking sequence of the murine synapsin II gene contains cis-regulatory elements that are required for directing neuron-specific and synaptogenesis-regulated expression in vivo.

A silencer element in the regulatory region of glutamine synthetase controls cell type-specific repression of gene induction by glucocorticoids

Glutamine synthetase is a key enzyme in the recycling of the neurotransmitter glutamate. Expression of this enzyme is regulated by glucocorticoids, which induce a high level of glutamine synthetase in neural but not in various non-neural tissues. This is despite the fact that non-neural cells express functional glucocorticoid receptor molecules capable of inducing other target genes. Sequencing and functional analysis of the upstream region of the glutamine synthetase gene identified, 5' to the glucocorticoid response element (GRE), a 21-base pair glutamine synthetase silencer element (GSSE), which showed considerable homology with the neural restrictive silencer element NRSE. The GSSE was able to markedly repress the induction of gene transcription by glucocorticoids in non-neural cells and in embryonic neural retina. The repressive activity of the GSSE could be conferred on a heterologous GRE promoter and was orientation- and position-independent with respect to the transcriptional start site, but appeared to depend on a location proximal to the GRE. Gel-shift assays revealed that non-neural cells and cells of early embryonic retina contain a high level of GSSE binding activity and that this level declines progressively with age. Our results suggest that the GSSE might be involved in the restriction of glutamine synthetase induction by glucocorticoids to differentiated neural tissues.

Transcriptional regulation of GABAA receptor gamma2 subunit gene

We have cloned the promoter regions of the genes for the mouse and human gamma2 subunits of the type A receptors for gamma-aminobutyric acid (GABA). For the mouse, the two major transcription start sites were at +1 (by definition) and +43, as established by rapid amplification of cDNA ends (RACE) and primer extension. This numbering places the start methionine at +297. There was no TATA or CCAAT box. Both mouse and human sequences have a candidate neuron-restrictive silencer element (NRSE) site in the first intron (+956 in mouse). We made assorted mouse-based promoter/reporter (luciferase) constructs starting from a core extending from -331 to +136, varying sizes at both ends, and including and excluding the putative NRSE and more proximal sequences. These were tested by transient transfection in several neuron-like and non-neuronal cell lines. Both proximal and distal downstream elements appeared to help direct expression to neuron-like cells, the NRSE in the intron, by repression in non-neurons, and a 24-bp portion of the 5' untranslated region starting at +113 (named GPE1) by preferentially promoting expression in neuron-like cells. Cotransfected human NRSF (transcription factor for NRSE) reduced reporter expression in neuron-like cells for constructs containing the NRSE in two locations. In gel mobility shift assays, the mouse gamma2 NRSE and a consensus NRSE both bound in vitro translated NRSF very similarly, and the NRSF gave the same major shifted band with the mouse gamma2 NRSE as was observed with nuclear extracts.

Specification of neurotransmitter identity by Phox2 proteins in neural crest stem cells

We have investigated the specification of noradrenergic neurotransmitter identity in neural crest stem cells (NCSCs). Retroviral expression of both wild-type and dominant-negative forms of the paired homeodomain transcription factor Phox2a indicates a crucial and direct role for this protein (and/or the closely related Phox2b) in the regulation of endogenous tyrosine hydroxylase (TH) and dopamine-beta hydroxylase (DBH) gene expression in these cells. In collaboration with cAMP, Phox2a can induce expression of TH but not of DBH or of panneuronal genes. Phox2 proteins are, moreover, necessary for the induction of both TH and DBH by bone morphogenetic protein 2 (BMP2) (which induces Phox2a/b) and forskolin. They are also necessary for neuronal differentiation. These data suggest that Phox2a/b coordinates the specification of neurotransmitter identity and neuronal fate by cooperating environmental signals in sympathetic neuroblasts.

Control elements of muscarinic receptor gene expression

Studies describing the structures of the M1, M2 and M4 muscarinic acetylcholine receptors (mAChR) genes and the genetic elements that control their expression are reviewed. In particular, we focus on the role of the neuron-restrictive silencer element/restriction element-1 (NRSE/RE-1) in the regulation of the M4 mAChR gene. The NRSE/RE-1 was first identified as a genetic control element that prevents the expression of the SCG-10 and type II sodium channel (NaII) genes in non-neuronal cells in culture. The NRSE/RE-1 inhibits gene expression by binding the repressor/silencer protein NRSF/REST, which is present in many non-neuronal cell lines and tissues. Our studies show that although the expression of the M4 mAChR gene is inhibited by NRSF/REST, this inhibition is not always complete. Rather, the efficiency of silencing by NRSF/REST is different in different cells. A plausible explanation for this differential silencing is that the NRSF/RE-1 interacts with distinct sets of promoter binding proteins in different types of cells. We hypothesize that modulation of NRSF/REST silencing activity by these proteins contributes to the cell-specific pattern of expression of the M4 mAChR in neuronal and non-neuronal cells. Recent studies that suggest a more complex role for the NRSE/RE-1 in regulating gene expression are also discussed.

Repression and activation of muscarinic receptor genes

The specific cellular response to muscarinic receptor activation is dependent upon appropriate expression of each of the five muscarinic receptor genes by individual cells. Here we summarise recent work describing some of the genomic regulatory elements and transcriptional mechanisms that control expression of the M1 and M4 genes.

beta43': An enhancer displaying neural-restricted activity is located in the 3'-untranslated exon of the rat nicotinic acetylcholine receptor beta4 gene

Members of a neuronal nicotinic acetylcholine receptor subunit gene cluster ordered beta4, alpha3, alpha5 in the vertebrate genome are expressed in highly restricted patterns in the PNS and CNS. Nothing is known, however, about the regulatory elements that control transcription of these genes in selected neuronal cell populations. We report here a novel enhancer, designated beta43', that is positioned in the beta4 3'-untranslated exon. It is composed of two nearly identical 37 bp direct repeats that are separated by 6 bp. Multimerization of the enhancer upstream of the alpha3 minimal promoter results in synergistic activation. Analysis in different cell types, including three neural lines and primary keratinocytes, shows that beta43' is preferentially active in the neural line PC12, which expresses all members of the cluster. Mobility shift assays reveal a cell-type-specific complex, which forms with the first repeat of the enhancer and PC12 extracts. Complexes co-migrating with the PC12 cell complex are not detected with extracts from other lines, which suggests that PC12 cells contain a differentially expressed factor that may be important for the restricted activity of beta43'. The cell-type-specific activity of the beta43' enhancer suggests that it is important for regulating restricted expression patterns of one or more clustered neuronal acetylcholine receptor genes. Its location within the beta4 gene may be a selective pressure for maintaining tight linkage of clustered neuronal nAchR genes.

Cloning from insulinoma cells of synapsin I associated with insulin secretory granules

Synapsin I is a synaptic vesicle-associated protein involved in neurotransmitter release. The functions of this protein are apparently regulated by Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). We reported evidence for CaM kinase II and a synapsin I-like protein present in mouse insulinoma MIN6 cells (Matsumoto, K., Fukunaga, K., Miyazaki, J., Shichiri, M., and Miyamoto, E. (1995) Endocrinology 136, 3784-3793). Phosphorylation of the synapsin I-like protein in these cells correlated with the activation of CaM kinase II and insulin secretion. In the present study, we screened the MIN6 cDNA library with the full-length cDNA probe of rat brain synapsin Ia and obtained seven positive clones; the largest one was then sequenced. The largest open reading frame deduced from the cDNA sequence of 3695 base pairs encoded a polypeptide of 670 amino acids, which exhibited significant sequence similarity to rat synapsin Ib. The cDNA contained the same sequence as the first exon of the mouse synapsin I gene. These results indicate that synapsin Ib is present in MIN6 cells. Synapsin I was expressed in normal rat islets, as determined by reverse transcriptase-polymerase chain reaction analysis. Immunoblot analysis after subcellular fractionation of MIN6 cells demonstrated that synapsin Ib and delta subunit of CaM kinase II co-localized with insulin secretory granules. By analogy concerning regulation of neurotransmitter release, our results suggest that phosphorylation of synapsin I by CaM kinase II may induce the release of insulin from islet cells.

Isolation and expression analysis of a novel human homologue of the Drosophila glial cells missing (gcm) gene

A novel human homologue (GCMB) of the Drosophila glial cells missing gene (dGCM) was isolated using RACE. GCMB contained a gcm motif sequence and a nuclear targeting sequence similar to that of dGCM and mouse GCMb. Homology searches indicated that GCMB was located within chromosome 6p24.2. Transcripts of GCMB were detected by means of RT-PCR in fetal brain, normal adult kidney, 3/3 medulloblastomas, 1/3 gliomas and 4/8 non-neuroepithelial tumor cell lines. Our data suggest that humans have two homologues of gcm like mice and that human gcm genes form a novel family which may function not only during fetal development but also in the postnatal or pathological stage.

Cis-regulatory elements controlling basal and inducible VIP gene transcription

The cis-acting elements of the VIP gene important for basal and stimulated transcription have been studied by transfection of VIP-reporter gene constructs into distinct human neuroblastoma cell lines in which VIP transcription is constitutively high, or can be induced to high levels by protein kinase stimulation. The 5.2 kb flanking sequence of the VIP gene conferring correct basal and inducible VIP gene expression onto a reporter gene in these cell lines was systematically deleted to define its minimal components. A 425-bp fragment (-4656 to -4231) fused to the proximal 1.55 kb of the VIP promoter-enhancer was absolutely required for cell-specific basal and inducible transcription. Four additional components of the VIP gene were required for full cell-specific expression driven by the 425 bp TSE (region A). Sequences from -1.55 to -1.37 (region B), -1.37 to -1.28 (region C), -1.28 to -.094 (region D), and the CRE-containing proximal 94 bp (region E) were deleted in various combinations to demonstrate the specific contributions of each region to correct basal and inducible VIP gene expression. Deletion of region B, or mutational inactivation of the CRE in region E, resulted in constructs with low transcriptional activity in VIP-expressing cell lines. Deletion of regions B and C together resulted in a gain of transcriptional activity, but without cell specificity. All five domains of the VIP gene were also required for cell-specific induction of VIP gene expression with phorbol ester. Gelshift analysis of putative regulatory sequences in regions A-D suggests that both ubiquitous and neuron-specific trans-acting proteins participate in VIP gene regulation.

Kid-1 expression is high in differentiated renal proximal tubule cells and suppressed in cyst epithelia

The cDNA coding for the transcriptional repressor protein Kid-1 was cloned in a screen for zinc finger proteins, which are regulated during renal development and after renal ischemia. Kid-1 mRNA levels increase in the course of postnatal renal development and decrease after acute renal injury caused by ischemia or administration of folic acid. We have raised a monoclonal anti-Kid-1 antibody and demonstrate that the Kid-1 protein is strongly expressed in the proximal tubule of the adult rat kidney. During nephron development, the Kid-1 protein appears after the S-shaped body stage concomitantly with the brush-border enzyme alkaline phosphatase. In two animal models of polycystic kidney disease, the expression of Kid-1 is downregulated. The loss of expression of Kid-1 in cyst wall cells correlates with the loss of alkaline phosphatase histochemical staining. Kid-1 mRNA levels are also reduced in rodent renal cell carcinomas, another condition characterized by epithelial cell dedifferentiation and increased proliferation. We propose that Kid-1 plays an important role during the differentiation of the proximal tubule.

Protein kinase A-dependent derepression of the human prodynorphin gene via differential binding to an intragenic silencer element

Induction of the prodynorphin gene has been implicated in medium and long-term adaptation during memory acquisition and pain. By 5' deletion mapping and site-directed mutagenesis of the human prodynorphin promoter, we demonstrate that both basal transcription and protein kinase A (PKA)-induced transcription in NB69 and SK-N-MC human neuroblastoma cells are regulated by the GAGTCAAGG sequence centered at position +40 in the 5' untranslated region of the gene (named the DRE, for downstream regulatory element). The DRE repressed basal transcription in an orientation-independent and cell-specific manner when placed downstream from the heterologous thymidine kinase promoter. Southwestern blotting and UV cross-linking experiments with nuclear extracts from human neuroblastoma cells or human brain revealed a protein complex of approximately 110 kDa that specifically bound to the DRE. Forskolin treatment reduced binding to the DRE, and the time course paralleled that for an increase in prodynorphin gene expression. Our results suggest that under basal conditions, expression of the prodynorphin gene is repressed by occupancy of the DRE site. Upon PKA stimulation, binding to the DRE is reduced and transcription increases. We propose a model for human prodynorphin activation through PKA-dependent derepression at the DRE site.

MEBA derepresses the proximal myelin basic protein promoter in oligodendrocytes

The central nervous system expression of myelin basic protein (MBP) is restricted to oligodendrocytes and is developmentally regulated; these regulatory features are transcriptionally mediated. We have previously shown that the proximal 149 nucleotides of the MBP promoter were both necessary and sufficient to activate the transcription of MBP in cultured oligodendrocytes, but not in other cell types. Sequences within the distal portion of this promoter, which contains a nuclear factor 1 (NF1) binding site, repressed activation of the MBP promoter in Cos-7 cells, but not in oligodendrocytes. We now describe a sequence upstream of and partially overlapping the NF1 site that activates the MBP promoter in oligodendrocytes, but not in Cos-7 cells. A protein complex binds to this site, designated MEBA (myelinating glia-enriched DNA binding activity), and is enriched in nuclear extracts prepared from the brain, oligodendrocytes, and Schwann cells. The amount of MEBA parallels MBP expression and myelinogenesis in the developing brain and parallels new MBP expression as purified oligodendrocytes differentiate. Mutational analyses of binding and function distinguish MEBA, an activator, from NF1, a repressor of MBP transcription, and suggest that MEBA consists of at least two proteins. Because the binding sites of MEBA and NF1 overlap, we suggest that MEBA may either compete with or modify NF1 binding, thereby activating the MBP promoter in oligodendrocytes.

Biological activity and modular structure of RE-1-silencing transcription factor (REST), a repressor of neuronal genes

The zinc finger protein RE-1-silencing transcription factor (REST)1 is a transcriptional repressor that represses neuronal genes in nonneuronal tissues. Transfection experiments of neuroblastoma cells using a REST expression vector revealed that synapsin I promoter activity is controlled by REST. The biological activity of REST was further investigated using a battery of model promoters containing strong promoters/enhancers and REST binding sites. REST functioned as a transcriptional repressor when REST binding motifs derived from the genes encoding synapsin I, SCG10, alpha1-glycine receptor, the beta2-subunit of the neuronal nicotinic acetylcholine receptor, and the m4-subunit of the muscarinic acetylcholine receptor were present in the promoter region. No differences in the biological activity of these REST binding motifs tested were detected. Moreover, we found that REST functioned very effectively as a transcriptional repressor at a distance. Thus, REST represents a general transcriptional repressor that blocks transcription regardless of the location or orientation of its binding site relative to the enhancer and promoter. This biological activity could also be attributed to isolated domains of REST. Both repressor domains identified at the N and C termini of REST were transferable to a heterologous DNA binding domain and functioned from proximal and distal positions, similar to the REST protein.

Synergistic activation of the N-methyl-D-aspartate receptor subunit 1 promoter by myocyte enhancer factor 2C and Sp1

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor plays important roles in neuronal development, plasticity, and cell death. NMDA receptor subunit 1 (NR1) is an essential subunit of the NMDA receptor and is developmentally expressed in postnatal neurons of the central nervous system. Here we identify on the NR1 promoter a binding site for myocyte enhancer factor 2C (MEF2C), a developmentally expressed neuron/muscle transcription factor found in cerebrocortical neurons, and study its regulation of the NR1 gene. Co-expression of MEF2C and Sp1 cDNAs in primary neurons or cell lines synergistically activates the NR1 promoter. Disruption of the MEF2 site or the MEF2C DNA binding domain moderately reduces this synergism. Mutation of the Sp1 sites or the activation domains of Sp1 protein strongly reduces the synergism. Results of yeast two-hybrid and co-immunoprecipitation experiments reveal a physical interaction between MEF2C and Sp1 proteins. The MEF2C DNA binding domain is sufficient for this interaction. Dominant-negative MEF2C interferes with expression of NR1 mRNA in neuronally differentiated P19 cells. Growth factors, including epidermal growth factor and basic fibroblast growth factor, can up-regulate NR1 promoter activity in stably transfected PC12 cells, even in the absence of the MEF2 site, but the Sp1 sites are necessary for this growth factor regulation, suggesting that Sp1 sites may mediate these effects.

Tissue-specific enhancement and restriction of galanin gene expression in transgenic mice by 5' flanking sequences

Galanin (GAL) is a 29/30 amino acid residue neuropeptide that regulates a wide variety of neuroendocrine functions. Galanin is expressed in specific populations of neurons in the hypothalamus and other regions of the brain and in numerous peripheral sites. Previous studies in which galanin-reporter genes were transfected into neural crest-derived neuroblastoma and other tumor cells indicated that cell-specific galanin expression is controlled by gene elements on the 5' flanking sequence which enhance and restrict transcriptional activity. To determine how the gene sequences act in vivo, we first determined the distribution of endogenous galanin gene expression in normal mice. Galanin mRNA was detected in several parts of the central nervous system (CNS), and in several peripheral organs, including the pituitary, pancreas, small and large intestine, adrenal gland, lung, tongue, testes, ovary-fallopian tubes, and uterus, but not at detectable levels in the heart, liver, kidney, urinary bladder or skeletal muscle. We then created several lines of transgenic mice which contained either 5 or 0.131 kilobases (kb) of the bovine galanin gene 5' flanking sequence fused to the luciferase (luc) reporter gene (5GAL-luc vs. 0.1GAL-luc mice, respectively) and compared luciferase activity in these and other organs. In some regions of the CNS that expressed high amounts of galanin mRNA, such as the spinal cord, hypothalamus, thalamus, and medulla, transgene expression was significantly higher in 5GAL-luc vs. 0.1GAL-luc mice, whereas in certain other regions of the brain and in all peripheral organs, the ratio was strikingly reversed. It is concluded that 5 kb of flanking sequence contains elements that mediate basal transcriptional activity in certain parts of the CNS, but also contains sequences that restrict expression in many tissues. However, because the larger transgene was expressed at very low levels in some peripheral sites of high galanin expression such as the pituitary, pancreas, adrenal gland, and intestine, it is concluded that sequences on the 5 kb transgene are not sufficient to direct expression to these peripheral tissues in mice.

When, where, and how much? Expression of the Kv3.1 potassium channel in high-frequency firing neurons

The Kv3.1 potassium channel gene is expressed in neurons that fire action potentials at high frequencies. Neurons that express this gene, such as auditory brain stem neurons, have high-threshold voltage-dependent potassium currents that activate and deactivate unusually rapidly, and whose characteristics match those of the Kv3.1 subunit expressed heterologously. The level of Kv3.1 expression in neurons is regulated during development and by environmental stimuli. Pharmacological and computer modeling studies indicate that changes in the level of this channel alter the ability of a neuron to follow synaptic inputs at high frequencies. To understand the transcriptional mechanisms that control Kv3.1 expression, an initial characterization of the primary promoter for the Kv3.1 gene was carried out. This review summarizes current knowledge regarding Kv3.1 gene transcription and the roles of upstream regulatory elements in conferring cell-type specificity and long-term regulation by extrinsic factors.

NRSF/REST is required in vivo for repression of multiple neuronal target genes during embryogenesis

The neuron-restrictive silencer factor NRSF (also known as REST and XBR) can silence transcription from neuronal promoters in non-neuronal cell lines, but its function during normal development is unknown. In mice, a targeted mutation of Rest, the gene encoding NRSF, caused derepression of neuron-specific tubulin in a subset of non-neural tissues and embryonic lethality. Mosaic inhibition of NRSF in chicken embryos, using a dominant-negative form of NRSF, also caused derepression of neuronal tubulin, as well as of several other neuronal target genes, in both non-neural tissues and central nervous system neuronal progenitors. These results indicate that NRSF is required to repress neuronal gene expression in vivo, in both extra-neural and undifferentiated neural tissue.

Neuronal expression of the 5HT3 serotonin receptor gene requires nuclear factor 1 complexes

The 5HT3 receptor (5HT3R) is a serotonin-gated ion channel whose expression is restricted to a subset of cells within the central and peripheral nervous systems. In vitro analysis shows that a small proximal region of the TATA-less 5HT3R promoter is sufficient to direct neuronal-specific reporter gene expression. Three potential regulatory elements conserved between the mouse and human genes were identified within this proximal promoter, two of which are known sites for the ubiquitously expressed factors Sp1 and nuclear factor 1 (NF1). Surprisingly, mutation of the NF1 binding site abolished all reporter activity in cell transfection studies, suggesting that this element is essential for neuronal-specific transcriptional activity of the 5HT3R. Furthermore, a complex of neuronal proteins that includes a member(s) of the NF1 family binds to this site, as shown by gel mobility super shift and DNaseI footprinting analyses. Although NF1 has been proposed to mediate basal transcription of many ubiquitously expressed genes, our data suggest that a member of the NF1 transcription factor family participates in neuronal-specific gene expression by promoting interactions with other regulatory factors found in sensory ganglia.

Transcriptional regulation of the GluR2 gene: neural-specific expression, multiple promoters, and regulatory elements

To understand how neurons control the expression of the AMPA receptor subunit GluR2, we cloned the 5' proximal region of the rat gene and investigated GluR2 promoter activity by transient transfection. RNase protection and primer extension of rat brain mRNA revealed multiple transcription initiation sites from -340 to -481 bases upstream of the GluR2 AUG codon. The relative use of 5' start sites was different in cortex and cerebellum, indicating complexity of GluR2 transcript expression among different sets of neurons. When GluR2 promoter activity was investigated by plasmid transfection into cultured cortical neurons, cortical glia, and C6 glioma cells, the promoter construct with the strongest activity, per transfected cell, was 29.4-fold (+/- 3.7) more active in neurons than in non-neural cells. Immunostaining of cortical cultures showed that >97% of the luciferase-positive cells also expressed the neuronal marker MAP-2. Evaluation of internal deletion and substitution mutations identified a functional repressor element I RE1-like silencer and functional Sp1 and nuclear respiratory factor-1 (NRF-1) elements within a GC-rich proximal GluR2 promoter region. The GluR2 silencer reduced promoter activity in glia and non-neuronal cell lines by two- to threefold, was without effect in cortical neurons, and could bind the RE1-silencing transcription factor (REST) because cotransfection of REST into neurons reduced GluR2 promoter activity in a silencer-dependent manner. Substitution of the GluR2 silencer by the homologous NaII RE1 silencer further reduced GluR2 promoter activity in non-neuronal cells by 30-47%. Maximal positive GluR2 promoter activity required both Sp1 and NRF-1 cis elements and an interelement nucleotide bridge sequence. These results indicate that GluR2 transcription initiates from multiple sites, is highly neuronal selective, and is regulated by three regulatory elements in the 5' proximal promoter region.

Differential regulation of basal and cyclic adenosine 3',5'-monophosphate-induced somatostatin gene transcription in neural cells by DNA control elements that bind homeodomain proteins

A number of genes encoding neuropeptides are expressed in the peripheral and central nervous systems, in different endocrine organs, and in specialized cells distributed along the gastrointestinal tract. Whether expression of the same neuropeptide gene in different tissues is regulated by similar transcriptional mechanisms or by mechanisms that differ in a cell-specific manner remains unclear. We report on promoter studies on the regulation of the somatostatin gene in immortalized neural precursor cells derived from developing rat forebrain. Expression of the somatostatin gene in these cells was determined by RT-PCR/Southern blot analysis, by immunocytochemistry, and by RIA. We show that in cerebrocortical and hippocampal cells, expression of the somatostatin gene is regulated by several negative and positive DNA cis-regulatory elements located throughout the promoter region. The somatostatin cAMP-response element appears to play a prominent role in neural somatostatin gene expression by acting as a strong enhancer even in the absence of cAMP stimulation. Site-directed mutagenesis followed by transient transfection assays indicated that SMS-TAAT1, SMS-TAAT2, and SMS-UE, three previously identified homeodomain protein-binding regulatory elements that enhance transcription in pancreatic cells, act as repressors of transcription in neural cells. Electrophoretic mobility shifts assays indicate that those elements bind protein complexes that differ between neural and pancreatic cells. Our results support the notion that expression of the somatostatin gene in neural cells occurs via transcriptional mechanisms that are different from those regulating expression of the same gene in pancreatic cells.

Multiple promoter elements interact to control the transcription of the potassium channel gene, KCNJ2

Potassium channels play important roles in shaping the electrical properties of excitable cells. Toward understanding the transcriptional regulation of a member of the inwardly rectifying potassium channel family, we have characterized the genomic structure and 5'-proximal promoter of the murine Kcnj2 gene (also referred to as IRK1 and Kir2.1). The Kcnj2 transcription unit is composed of two exons separated by a 5.5-kilobase pair intron. Deletion analysis of 5'-flanking sequences identified a promiscuously active 172-base pair minimal promoter, whereas expression from a construct containing additional upstream sequences was cell type-restricted. The minimal promoter contained an E box, a Y box, and three GC box consensus elements but lacked both TATA and CCAAT box elements. The activity of the minimal promoter was found to be controlled by a combination of the activities of the transcription factors Sp1, Sp3, and NF-Y. The interplay between Sp1, Sp3, and NF-Y within the architecture of the Kcnj2 promoter, the ubiquitous nature of these trans-acting factors, and the action of tissue-selective repressor element(s) may combine to enable a wide variety of cell types to differentially regulate Kcnj2 expression through transcriptional control.

GC- and E-box motifs as regulatory elements in the proximal promoter region of the neuronal nicotinic receptor alpha7 subunit gene

The alpha7 subunit is a component of alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors expressed in bovine adrenomedullary chromaffin cells. The proximal promoter of the gene coding for this subunit contains several GC-boxes and one E-box. Deletion analysis and transient transfections showed that a 120-base pair region (-77 to +43) including all of these elements gave rise to approximately 70 and 95% of the maximal transcriptional activity observed in chromaffin and SHSY-5Y neuroblastoma cells, respectively. Site-directed mutagenesis of the different elements indicated that both GC and E motifs contribute to the activity of the alpha7 gene in a very prominent way. Using electrophoretic mobility shift assays, the upstream stimulatory factor (USF) was shown to be a component of the complexes that interacted with the E-box when nuclear extracts from chromaffin and SHSY-5Y cells were used. Binding of the early growth response gene transcription factor (Egr-1) to three different GC-boxes was also demonstrated by shift assays and DNase I footprint analysis. Likewise, alpha7 promoter activity increased by up to 5-fold when alpha7 constructs and an Egr-1 expression vector were cotransfected into chromaffin cell cultures. Mutagenesis of individual GC-boxes had little effect on Egr-1 activation. By contrast, pairwise suppression of GC-boxes abolished activation, especially when the most promoter-proximal of the Egr-1 sites was removed. Taken together, these studies indicate that the alpha7 gene is likely to be a target for multiple signaling pathways, in which various regulatory elements are involved.

Molecular cloning of a novel transcriptional repressor protein of the rat type 1 vasoactive intestinal peptide receptor gene

This study demonstrates that the transcriptional repressor sequence of the rat vasoactive intestinal peptide receptor (VIPR) gene constitutes a 42-base pair core element that is the binding site for a nuclear protein. We showed that this element was able to confer transcriptional repression to a heterologous promoter and that deletion or point mutations within this element resulted in loss of transcriptional repression. Southwestern blot analysis indicated that the VIPR repressor element interacts specifically with a nuclear protein of about 72 kDa. By screening a rat lung expression library coupled with rapid amplification of cDNA ends polymerase chain reactions, we isolated a cDNA clone (designated as VIPR-RP) that contains an open reading frame of 656 amino acids. VIPR-RP is 78% identical to a previously characterized protein, differentiation-specific element-binding protein, which is a member of a family of proteins including components of the DNA replication factor C complex. However, VIPR-RP cDNA encodes for a much smaller protein than differentiation-specific element-binding protein because of a frameshift. VIPR-RP mRNA is expressed in multiple tissues, including lung, liver, brain, heart, kidney, spleen, and testis. VIPR-RP protein specifically interacts with the VIPR repressor element as demonstrated by gel shift assays. Transfection of VIP-RP expression vector into Cos cells resulted in transcriptional repression of a reporter construct containing multiple copies of the VIPR repressor element. These results indicate that VIPR-RP is a novel transcriptional repressor protein that regulates VIPR expression.

The activity of a highly promiscuous AP-1 element can be confined to neurons by a tissue-selective repressive element

Tissue-specific gene transcription can be determined by the use of either positive-acting or negative-acting DNA regulatory elements. We have analyzed a promoter from the growth-associated protein 43 (GAP-43) gene and found that it uses both of these mechanisms to achieve its high degree of neuron-specific activity. Two novel transcription factor binding sites, designated Cx1 and Cx2, drive promoter activity in neurons from developing cerebral cortex but not in several other cell types. The promoter also contains an activator protein 1 (AP-1) site that contributes to activity in neurons. The AP-1 site can drive promoter activity in a wide range of non-neuronal cells that express little or no endogenous GAP-43, but only in the absence of a tissue-specific repressive element located downstream of the GAP-43 TATA box. These findings suggest that the GAP-43 repressive element plays an important role in allowing AP-1 signaling pathways to modulate activity of the GAP-43 gene in neurons, without also causing inappropriate activation by AP-1 transcription factors in other cell types.

Functional properties of the neuronal nicotinic acetylcholine receptor beta3 promoter in the developing central nervous system

Within the chick central nervous system, expression of the beta3 nicotinic acetylcholine receptor gene is restricted to a subset of retinal neurons, the majority of which are ganglion cells. Transient transfection in retinal neurons and in neural and non-neural cells from other regions of the chick embryo allowed the identification of the cis-regulatory domain of the beta3 gene. Within this domain, a 75-base pair fragment located immediately upstream of the transcription start site suffices to reproduce the neuron-specific expression pattern of beta3. This fragment encompasses an E-box and a CAAT box, both of which are shown to be key positive regulatory elements of the beta3 promoter. Co-transfection experiments into retinal, telencephalic, and tectal neurons with plasmid reporters of beta3 promoter activity and a number of vectors expressing different neuronal (ASH-1, NeuroM, NeuroD, CTF-4) and non-neuronal (MyoD) basic helix-loop-helix transcription factors indicate that the cis-regulatory domain of beta3 has the remarkable property of discriminating accurately between related members of the basic helix-loop-helix protein family. The sequence located immediately 3' of the E-box participates in this selection, and the E-box acts in concert with the nearby CAAT box.

Characterization of the human Megalin/LRP-2 promoter in vitro and in primary parathyroid cells

The gp330/Megalin/LRP-2 protein belongs to the low-density lipoprotein receptor gene family and is believed to function as an endocytic receptor for the uptake of lipoproteins and many other ligands. Other functions of this protein may include a role in calcium sensing in the parathyroid glands and other tissues. In order to study the transcriptional regulation of the human LRP-2 gene, a clone containing the 5'-flanking region was isolated from a genomic DNA library, and a transient transfection protocol for primary bovine parathyroid cells was established. RNA mapping techniques located the transcriptional start site 136 bp upstream of the initiation codon. Transient expression in several cell types, including primary parathyroid cells, and in vitro transcription in HeLa cell nuclear extracts showed that sequences between -120 and -35 were important for activated transcription. This region contains consensus binding sites (GC boxes) for transcription factor Sp1. Mutation of the GC boxes abolished binding of Sp1 in vitro and resulted in reduced transcription in vitro and in transfected cells. Furthermore, Sp1 stimulated transcription when tethered to the LRP-2 core promoter through a heterologous DNA-binding domain. Through site-directed mutagenesis, we identified a novel atypical TATA element with the sequence TAGAAAA. Intriguingly, this sequence motif was shown previously not to mediate transcription in a systematic mutational analysis of the TATA motif. Possible roles of this novel TATA element in the regulation of transcription initiation are discussed. The isolation and characterization of the LRP-2 promoter and the 5'-flanking region and the establishment of a transient expression assay in primary parathyroid cells will facilitate studies on the regulatory mechanisms of the LRP-2 gene and of other genes expressed in the parathyroid glands.

Activation and repression in the nervous system

The mechanisms underlying transcriptional activation and repression have become much clearer. Recent evidence suggests that transcription factors that do not bind DNA directly, the co-activators and co-repressors, mediate a large number of cell signaling events. Their association with histone acetylases, to mediate activation, or deacetylases, to mediate repression, provide a model for explaining how gene expression is regulated.

The structures of the mouse and human urocortin genes (Ucn and UCN)

The mouse and human urocortin genes (Ucn and UCN, respectively) have been isolated, characterized, and found to have very similar structures. Each has two exons, and the entire coding region is located in the second exon, as is the case for the gene of the related peptide, corticotropin-releasing factor. Several putative transcription factor-binding sites were identified in each of the urocortin promoters, including a TATA box, a cyclic AMP response element (CRE), GATA-binding sites, and a C/EBP-binding site as well as a Brn-2-binding site(s). Sequence analyses of the mouse and human genes also revealed the presence of a previously identified gene, Mpv17, in the 5' region upstream of the urocortin gene. Functional studies following transient transfection of urocortin reporter plasmids in PC12 cells revealed that the urocortin promoter is controlled by both positive and negative elements; the CRE is important for basal activity as well as responsiveness to forskolin stimulation.

Gene regulation of cell adhesion: a key step in neural morphogenesis

A mounting body of evidence suggests that cell adhesion molecules (CAMs) play important roles in morphogenetic patterning of the nervous system. The combined factors that control the expression of CAMs during early neural development are, however, largely unknown. We have hypothesized that the coordinate expression of homeobox (Hox) and paired box (Pax) proteins in the neural axis leads to the differential expression of particular CAM genes. Following this hypothesis, we have characterized the promoters and identified cis-regulatory sequences that bind to and respond to Hox and Pax proteins in the genes for three neurally expressed CAMs - the neural cell adhesion molecule, N-CAM, the neuron-glia cell adhesion molecule, Ng-CAM, and L1. Experiments on transgenic mice carrying N-CAM promoter/lacZ reporter gene constructs indicated that mutation of either the HBS or the PBS disrupted patterning of N-CAM expression in the embryonic spinal cord. To examine the factors that restrict the expression of certain CAMs to the nervous system, we identified regulatory elements that block expression of the Ng-CAM and L1 genes in non-neural cells. We characterized a 310 base pair region of the first intron of the Ng-CAM gene containing five neural restrictive silencer elements (NRSEs) and a binding site for the Pax-3 protein. These elements silenced activity of the Ng-CAM promoter in NIH3T3 fibroblasts, but had no effect on its activity in N2A neuroblastoma cells line. Similar analyses of the L1 gene revealed a single NRSE within the second intron that was important for silencing in this cellular transfection system. To analyze the role of the NRSE in vivo, we prepared transgenic mice containing two L1 gene/lacZ constructs, one containing the NRSE and another in which the NRSE was deleted. The wild type L1lacZ transgene showed a neurally restricted pattern of expression, whereas the NRSE-mutated L1 construct showed extensive extraneural expression of the L1 gene. Thus, neural specificity of CAM expression is controlled by the NRSE. The general significance of these observations is that they connect the expression of important families of transcriptional regulators with gene products capable of direct cellular mechanochemistry.

Structure and function of voltage-gated sodium channels

1. Sodium channels mediate fast depolarization and conduct electrical impulses throughout nerve, muscle and heart. This paper reviews the links between sodium channel structure and function. 2. Sodium channels have a modular architecture, with distinct regions for the pore and the gates. The separation is far from absolute, however, with extensive interaction among the various parts of the channel. 3. At a molecular level, sodium channels are not static: they move extensively in the course of gating and ion translocation. 4. Sodium channels bind local anaesthetics and various toxins. In some cases, the relevant sites have been partially identified. 5. Sodium channels are subject to regulation at the levels of transcription, subunit interaction and post-translational modification (notably glycosylation and phosphorylation).

Two E-boxes are the focal point of muscle-specific skeletal muscle type 1 Na+ channel gene expression

We have characterized a group of cis-regulatory elements that control muscle-specific expression of the rat skeletal muscle type 1 sodium channel (SkM1) gene. These elements are located within a 3. 1-kilobase fragment that encompasses the 5'-flanking region, first exon, and part of the first intron of SkM1. We sequenced the region between -1062 and +311 and determined the start sites of transcription; multiple sites were identified between +1 and +30. The basal promoter (-65/+11) lacks cell-type specificity, while an upstream repressor (-174/-65) confers muscle-specific expression. A positive element (+49/+254) increases muscle-specific expression. Within these broad elements, two E boxes play a pivotal role. One E box at -31/-26 within the promoter, acting in part through its ability to bind the myogenic basic helix-loop-helix proteins, recruits additional factor(s) that bind elsewhere within the SkM1 sequence to control positive expression of the gene. A second E box at -90/-85 within the repressor controls negative regulation of the gene and acts through a different complex of proteins. Several of these cis-regulatory elements share both sequence and functional similarities with cis-regulatory elements of the acetylcholine receptor delta-subunit; the different arrangement of these elements may contribute to unique expression patterns for the two genes.