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Molecular and Cellular Biology, March 2000, p. 2147-2157, Vol. 20, No. 6
Wellcome Laboratory for Molecular
Pharmacology, University College London, London WC1E
6BT,1 and School of Biochemistry and
Molecular Biology, University of Leeds, Leeds LS2
9JT,2 United Kingdom, and Division
of Biology, California Institute of Technology, Pasadena, California
911253
Received 25 October 1999/Accepted 6 December 1999
A large number of neuron-specific genes characterized to date are
under the control of negative transcriptional regulation. Many promoter
regions of neuron-specific genes possess the repressor element
repressor element 1/neuron-restrictive silencing element (RE1/NRSE).
Its cognate binding protein, REST/NRSF, is an essential transcription
factor; its null mutations result in embryonic lethality, and its
dominant negative mutants produce aberrant expression of
neuron-specific genes. REST/NRSF acts as a regulator of neuron-specific gene expression in both nonneuronal tissue and developing neurons. Here, we shown that heterologous expression of REST/NRSF in
Saccharomyces cerevisiae is able to repress transcription
from yeast promoters engineered to contain RE1/NRSEs. Moreover, we have
taken advantage of this observation to show that this repression
requires both yeast Sin3p and Rpd3p and that REST/NRSF physically
interacts with the product of the yeast SIN3 gene in vivo.
Furthermore, we show that REST/NRSF binds mammalian SIN3A and HDAC-2
and requires histone deacetylase activity to repress neuronal gene
transcription in both nonneuronal and neuronal cell lines. We show that
REST/NRSF binding to RE1/NRSE is accompanied by a decrease in the
acetylation of histones around RE1/NRSE and that this decrease requires
the N-terminal Sin3p binding domain of REST/NRSF. Taken together, these
data suggest that REST/NRSF represses neuronal gene transcription by
recruiting the SIN3/HDAC complex.
Neuronal differentiation is
accompanied by a massive change in cellular phenotype that
requires expression of both panneuronal and neuronal cell type-specific
genes. However, the transcriptional mechanisms that underlie
these changes in gene expression are only beginning to be understood.
Numerous experiments have implicated the role of both positive and
negative transcriptional regulation in determining the complement
of expressed genes in a cell both during the acquisition of neuronal
identity and also in its maintenance (3). It is becoming
increasingly apparent that negative transcriptional regulation plays a
major role in defining neuron-specific gene expression (51,
64). A great many 5' regulatory regions of neuron-specific genes
have been shown to possess negative-acting cis
elements. One such element is the RE1/NRSE (repressor element 1/neuron-restrictive silencer element), a 21-bp negative element that
has been found in numerous genes (52). Functional
characterization of this element has been demonstrated for many
promoters, including those of the sodium type II channel gene
(33), the SCG 10 gene (35), the M4
gene (34, 63), synapsin I gene (17, 32, 49), and
the GluR2 gene (38). In all these cases, the RE1/NRSE silences expression of the reporter gene in nonneuronal cells. However,
transgenic analysis of the promoters of the The RE1/NRSE is bound by the zinc finger transcription factor REST/NRSF
(12, 50), which is expressed in most embryonic and adult
nonneuronal tissue. In the nervous system, expression is largely
restricted to undifferentiated neuroepithelium, although differentiated neurons express low levels of REST/NRSF splice variants
(40). REST/NRSF knockout mice show up-regulation and ectopic
expression of the REST/NRSF has been shown to possess at least two independent repression
domains, one encompassed by the 83 N-terminal residues and the other by
the C-terminal zinc finger, both of which can function when fused to a
heterologous DNA binding domain (57). The C-terminal
repression domain has been shown to interact with a novel protein,
CoREST (2). CoREST contains two SANT
(SWI13/ADA2/N-CoR/TFIIIB) domains (1) separated by 191 amino
acids and is a motif found in N-CoR/SMRTe (16, 41).
The N-terminal repression domain of REST/NRSF does not show sequence
homology to any other known repression domains. Furthermore, nothing is
known about the mechanism by which it represses transcription.
Many repressors interact with components of the TFIID/RNApolII
holocomplex or activators to repress transcription (for a review, see
reference 21 and references therein). However, it
has recently become clear that the transparency of chromatin itself to
transcription factors and components of the general transcriptional
machinery can be modulated by repressors and activators in order to
allow gene-specific expression (29, 39, 46, 60). In
mammalian cells, a number of DNA binding repressors such as Mad:Max,
Mxi:Max, unliganded nuclear receptors, and Pit-1 recruit the SIN3
complex and associated histone deacetylase activity to mediate the
transcriptional repression of genes governing a variety of cellular
processes (reviewed in reference 43). In
Saccharomyces cerevisiae, Ume6 binds to the URS1 element and
recruits the Sin3-Rpd3 complex (23) to repress transcription
of a large variety of yeast promoters (55). Indeed it has
been shown that the histone deacetylase activity per se of rpd3p is
important for Sin3-Rpd3-mediated repression (23).
However, not all repressors require Sin3 to recruit histone
deacetylases. Transcriptional repressors can directly interact with
histone deacetylases, e.g., YY1 (65) or Rb (9),
or recruit histone deacetylases complexed with other proteins (such as
Mi2), e.g., the human papillomavirus E7 zinc finger protein
(10).
Although the N-terminal tails of histones H2A, H2B, H3, and H4 are
targets for acetylation and deacetylation (66), a number of
other proteins have been shown to be regulated by this kind of
modification, including p53 (15), TFIIE Many aspects of gene regulation are highly conserved between yeast and
higher eukaryotes to the point that when introduced into yeast, a
number of transcription factors from Drosophila melanogaster
and mammals, are able to modulate transcription at yeast promoters
(6, 26, 38, 45, 47, 62). Here, we report that upon
expression in yeast, the HZ4 fragment of REST/NRSF is able to repress
transcription from an RE1/NRSE-bearing GAL1 promoter. This repression
has a genetic requirement for the yeast SIN3 and
RPD3 genes and REST/NRSF associates with yeast Sin3p in
vivo. We also show that these interactions are conserved in mammalian
cells and that the N-terminal repression domain interacts with
mammalian SIN3A and HDAC-2. Also, the N-terminal repression domain
requires deacetylase activity to repress transcription of neuronal
genes. Moreover, we show by chromatin immunoprecipitation experiments
that REST/NRSF binding coincides with a reduction in the amount of
acetylated histone H3 binding around the RE1/NRSE. Our results
therefore indicate that the SIN3/HDAC complex is important for
REST/NRSF-mediated silencing and provide evidence that this corepressor
complex plays a direct role in regulating differentiated neuronal
phenotype via chromatin modification.
Yeast strains.
All yeast experiments were performed in
S. cerevisiae strain FM242 (designated YPH925 in reference
54) and derivatives thereof. Details of cloning
procedures, oligonucleotide sequence, etc., are available on request.
All transformations were performed according to Schiestl and Gietz
(48). The SIN3 disruption was generated by
cloning a yeast SIN3 fragment into pRS306(URA3 integrative) (53), linearizing it, transforming it into FM242, and
selecting for uracil autotrophy. SIN3 disruption was
confirmed by PCR. Strain yA2, expressing hemagglutination (HA)-tagged
ySin3p, was generated by cloning an in-frame HA tag-bearing PCR product
into pRS306, linearizing it, transforming it into FM242, and
selecting for uracil autotrophy. Integrants were screened for by
PCR. The rpd3 knockout strain yA3 was generated by single-step
gene disruption with a PCR fragment bearing the HIS3 gene
and RPD3 5' and 3' flanking sequences.
Plasmids.
Yeast reporter and expression plasmids are
summarized in Table 1. Details of cloning
procedures are available on request. The plasmid
pBM2389(RE1)3UAS was generated by cloning an RE1/NRSE double-stranded oligonucleotide derived from the M4
promoter (top strand,
5'-GATCCGAGCTGTCCGAGGTGCTGAATCTGGGAGCTGTCCGAGGTGCTGAATCTGCCTA-3') and Gal4p consensus binding site double-stranded oligonucleotide (top strand, 5'-GATCGCGGACTGTCCTCCGG-3') upstream of
GAL1TATA in pBM2389 (32a). The
plasmid pAR1000 was generated by replacing the HIS3 fragment
of pBM2389 with the lacZ fragment of p
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Transcriptional Repression by Neuron-Restrictive Silencer Factor
Is Mediated via the Sin3-Histone Deacetylase Complex
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ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
![]()
INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
2 nicotinic acetylcholine receptor subunit (7) and L1 cell adhesion
molecule gene (24) has demonstrated that the RE1/NRSE, as
well as mediating repression in both neuronal and nonneuronal cells,
can mediate activation in subsets of neuronal cells.
III tubulin gene, morphological perturbations in
the head mesenchyme and somites, and, ultimately, lethality around
embryonic day 9.5 to 10. Ectopic expression of a
dominant-negative REST/NRSF mutant in developing chick embryos results
in up-regulation of a number of RE1/NRSE-containing genes,
e.g., SCG10 and Ng-CAM, within the developing nervous system
(11). These observations underscore the importance of
REST/NRSF in normal development and in regulating the expression of
differentiated neuronal genes.
(19),
HMGI(Y) (36), and GATA-1 (8).
![]()
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
GAL (Clontech) and
recombining the (RE1)3UAS sequence from
pBM2389(RE1)3UAS into pBMGAL by gap rescue. The
expression plasmid pLEONOV(HX) was generated by cloning a
multiple-cloning site oligonucleotide (top strand, 5'-AGCTTGAATTCGCGGCCGCGGATCCCTCGAGT-3') into
HindIII-cut pGAD10. pLEONOV.NRSF(1-600)
was generated by cloning a myc epitope-tagged HZ4 sequence
(50) (corresponding to NRSF(1-600) from pMT.HZ4 into
pLEONOV. pLEONOV.NRSF(142-445) was generated by cloning a myc-tagged DNA binding domain of mREST (11) into pLEONOV.
pLEONOV.NRSF(139-600) was derived from pLEONOV.NRSF(1-600) by
removing the coding region corresponding to residues 1 to 138 of
NRSF. The glutathione S-transferase (GST) fusions of
REST/NRSF and mMAD1 fragments were generated in the plasmid
pGEX4T3 (Pharmacia). The gal4-NRSF(1-73) expression plasmid was
generated by first cloning the gal4 DNA binding domain coding sequence in frame with myc-tagged HZ4, yielding pMT.G4.HZ4. The
gal4-NRSF(1-73) coding region was then amplified and cloned into pTarget (Promega). pMT.Gal4 was derived from pMT.G4.HZ4 by removing the NRSF(1-600) coding region. M4 reporter
constructs have been described elsewhere (63). The NaII
reporter plasmids were generated by cloning a
HindIII/PstI fragment of pSDK7 into pBluescript, excising a HindIII fragment, and cloning
into HindIII-cut pGL3 (Promega) to generate
pGL3.-1051/+177. pGL3.-134/+177 was generated by digesting
pGL3.-1051/+177 with BglII and religating. pGL3-5UAS-7TetO.Inr was generated as follows: a double-stranded oligonucleotide encompassing the adenovirus major late
promoter-initiator element (44) was cloned into pGL3 to form
pGL3.Inr. The XmaI fragment encompassing 5XUAS-TATA from
T7G5TATA (44), a gift from L. Lania, IIGB, Naples, Italy,
was cloned into pTRE-Luc (Clontech) to generate pTRE-UAS-TATA. The
7XTetO-5XUAS region was cloned into pGL3.Inr.
TABLE 1.
Stains and plasmids used in this study
Yeast reporter assays.
His3 growth assays were performed as
follows: single yeast colonies were streaked onto appropriate synthetic
dropout plates supplemented with either 2% glucose or 2%
galactose-2% raffinose and stated concentrations of
3-amino-1,2,4-triazole (3-AT) (Sigma), incubated at 30°C for 3 days, and assessed for growth.
-Galactosidase assays were performed
with Lumigal reagent (Clontech). Briefly, 100 µl of saturated
cultures in synthetic dropout medium with 2% glucose was pelleted,
resuspended in 1 ml of YP medium with an appropriate carbon source,
grown to an optical density at 600 nm of 0.5 to 1.0, harvested,
freeze-thawed into Z buffer (Clontech) with liquid nitrogen, analyzed
on a Turner TD-20e luminometer as per the manufacturer's instructions,
and normalized to cell density.
Chromatin immunoprecipitation (ChIP) assays. ChIP assays in yeast were performed essentially as previously described (13). Yeast strain yA2 (250 ml) expressing either NRSF(1-600) or NRSF(142-445) was grown to an optical density at 600 nm of 0.5, cross-linked with 1% formaldehyde, and treated according to Dedon et al. (13). Supernatant was subjected to PCR with primers GAL1s (5'-GCCACCTGACGTTTAAGAAACC-3') and GAL1.789a (5'-CTCCTTGACGTTAAAGTATAGAGG-3') to amplify (RE1)3UAS sequences from the reporter plasmid.
ChIP assays in mammalian cells were performed essentially as described in the Upstate Biotechnology home page (http://www.upstatebiotech.com). Briefly, a 10-cm2 dish of 25 to 50% confluent Neuro-2A cells was transfected with 5 µg of reporter plasmid and 5 µg of pMTGal4 or pMTGal4.NRSF(1-138) plasmid with 21 µl of Tfx-50 (Promega) in 2 ml of OPTImem and fed 4 h posttransfection. Forty-eight hours after feeding, cells were cross-linked and treated as per the protocol. One microgram of each indicated antibody was used (anti-mSIN3A, anti-HDAC2, and anti-HA antibody from Santa Cruz Biotechnology; anti-acetyl histone-H3 from Upstate Biotechnology). Precipitated DNA was resuspended in 50 µl of water. To detect the precipitated plasmid sequence, 1 µl of DNA was used in a 10-µl, 25-cycle PCR with primers RV3 (5'-CTAGCAAAATAGGCTGTCCC-3') (Promega) and M4 RE1a (5'-GTACAGGCAGATTCAGCACCTCGGACAGCTCC-3').GST pull-down assays. GST or GST fusion proteins were expressed in Escherichia coli and purified using glutathione-Sepharose beads (Pharmacia) equilibrated with NETN (20 mM Tris [pH 8.0], 100 mM NaCl, 1 mM EDTA, 0.5% NP-40) with 5 mM dithiothreitol and 1 mM PMSF (phenylmethylsulfonyl fluoride). Beads loaded with 10 µg of GST fusion protein were rocked at 4°C with 200 µg of HEK293 whole-cell extract, prepared by freeze-thaw of cells in whole-cell extraction buffer (20 mM HEPES [pH 7.9], 450 mM NaCl, 0.4 mM EDTA, 25% glycerol) and diluted 10-fold in NETN. After overnight incubation, the beads were washed three times with NETN and boiled, and bound protein was subjected to Western blot analysis. Bound mSIN3A was detected by immunoblotting with anti-mSIN3A antibody (Santa Cruz Biotechnology) and ECL detection (Amersham). For the SIN3 domain-mapping experiment, 5 µg of plasmid expressing the indicated fragments of mSIN3A (myc tagged) was transfected into 25 to 50% confluent Neuro-2A cells in 10-cm2 dishes. Protein was harvested 48 h later into 1 ml of NETN Complete Protease Block (Boehringer) and sonicated, and debris was cleared by centrifugation. A total of 5 µg of the indicated antibody and 25 µl of Sepharose G beads were added to 200 µg of protein and rocked overnight at 4°C. Beads were washed five times in NETN supplemented with 10% glycerol and 0.1% sodium dodecyl sulfate (SDS), boiled, subjected to SDS-polyacrylamide gel electrophoresis (PAGE), transferred, and probed with anti-myc antibody.
Gel shift assays.
The two primers, NRSF 1s
(5'-GAGACCATGGCCACCCAGGTGATGGG-3') and NRSF 1040a
(5'-GAGATCTAGACTTCATGCTGATTAGAGGCCAC-3'), were used to
amplify the coding region of
NRSF with cDNA derived from Neuro-2A
cells. The resulting PCR product was cloned into pTARGET (Promega) and
used as a template for in vitro transcription with Sp6 RNA polymerase.
NRSF RNA was then translated in vitro with wheat germ extracts
(Promega) according to the manufacturer's instructions. Aliquots of
the in vitro-translated protein were used in gel mobility
shift assays which were performed as previously described
(63). The DNA probe used was a
HindIII/BglII fragment from the sodium
type II promoter containing the RE1/NRSE (27). Monoclonal
anti-REST/NRSF antibody 12C11 was used in supershift experiments.
Cell line maintenance and transfection analysis. HEK293 and Neuro-2A cells were maintained in Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum, 2 mM glutamine, streptomycin (10 g/liter), and penicillin (10 g/liter). All transfections were performed in 24-well plates, with wells of 1-cm diameter each. For experiments using gal4/NRSF fusions, 0.1 ng of CMV-Renilla (Promega) and 8.3 ng (each) of pUHC13-3 (expressing Tet-VP16), the indicated gal4 fusion plasmid, and a reporter plasmid were combined with 0.056 µl of Tfx-50 (Promega) in 20 µl of OPTImem for 10 min at room temperature, made up to a total volume of 200 µl of OPTImem, and added to cells. Cells were fed after 4 h. For experiments using M4 or NaII reporter plasmids, 27 ng of reporter plasmid and 0.1 ng of CMV-Renilla were combined with 0.056 µl of Tfx-50 and treated as above. Where required, Trichostatin-A (TSA) (Wako Chemical) was added at 100 nM 24 h posttransfection and harvested 24 h later. Firefly luciferase and Renilla luciferase assays were performed in a Turner TD-20e luminometer with the Dual Luciferase kit (Promega) as per the manufacturer's instructions.
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RESULTS |
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In order to perform genetic screens in yeast to decipher
mechanisms by which REST/NRSF regulates gene transcription in
mammalian cells, we wished to determine whether REST/NRSF could repress the GAL1 promoter in yeast. A single consensus
gal4 binding site, UASgal1, and three
RE1/NRSEs from the M4 promoter were placed upstream of
GAL1-HIS3 and GAL1-lacZ to generate the
centromeric reporter plasmids pBM2389(RE1)3UAS and
pAR1000 respectively (Fig. 1a). These
were introduced into yeast strain FM242 along with plasmids driving
expression of various fragments of REST/NRSF or the empty
expression vector (Fig. 1a) and grown on galactose to activate the
GAL1 promoter. NRSF(1-600) was able to repress Gal4p-activated GAL1 expression as judged by the inability
of the yeast to grow on plates lacking histidine (Fig. 1b) and a >10-fold reduction in lacZ expression than cells not
expressing NRSF(1-600) (Fig. 2
compare lanes 1 and 3). Yeast expressing just the DNA binding domain of
REST/NRSF [NRSF(142-445)] failed to repress lacZ
expression (Fig. 2 compare lanes 1 and 7).
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The 83 N-terminal residues encompass one of two repression domains identified by Tapia-Ramirez et al. (57). To assess whether the repression mechanisms employed by the REST/NRSF fragment in yeast were the same as those used in mammalian cells, we took advantage of the observation that only the N-terminal repression domain was contained within NRSF(1-600). Removal of residues 1 to 138 resulted in significant, though incomplete, loss of repression as judged by the inability of the reporter strain to grow in the presence of 10 mM 3-AT (Fig. 1b). Also, lacZ assays showed that NRSF(139-600) only repressed GAL1 expression to 38% of maximum compared to 7.6% with NRSF(1-600) (Fig. 2, compare lanes 1 and 5). This is consistent with the removal of a repressor domain. However, NRSF(139-600) still retained repressor ability, which could be abolished by deleting residues 446 to 600, the lysine-rich domain (LRD) of REST/NRSF, to give NRSF(142-445). The ability of the LRD to repress transcription in mammalian cells and its physiological relevence to REST/NRSF function are currently under investigation.
Many repressors of transcription work by recruiting corepressor
complexes. The Sin3p molecule has been implicated in the repression mechanism of many repressors and is conserved between yeast and mammals. To assess the role of Sin3p in REST/NRSF-mediated repression, we generated a sin3
yeast strain, yA1, in
which we performed repression assays. Figures 1c and 2 show that loss
of SIN3 results in the abolition of REST/NRSF's ability to
repress Gal4p-activated transcription of the GAL1 promoter, as judged by the ability to grow on plates lacking histidine and by
maximal activity of lacZ expression (Fig. 2, lanes 9 to 12). Sin3p has been shown to be associated with the product of the RPD3 gene, the deacetylase activity of which is important in
mediating repression (22). To determine whether Rpd3p was
required for REST/NRSF repression in yeast, we generated the strain
yA3, which is deleted for the RPD3 gene in which we
performed lacZ assays. As is seen in Fig. 2, abolition of
the RPD3 gene results in loss of ability of NRSF(1-600)
to repress activated GAL1 expression in yeast (Fig. 2,
compare lanes 13 and 15). Thus, REST/NRSF is able to repress
transcription in yeast in a manner that requires the products of the
yeast SIN3 and RPD3 genes.
As expected, glucose repression of the GAL1 promoter was still intact in strain yA1 (data not shown), indicating that SIN3 disruption had not rendered the GAL1 promoter in pBM2389(RE1)3UAS immune to repressor signals in general. Further, REST/NRSF-mediated repression of GAL1 transcription in a strain disrupted for the SSN6 gene, whose product is part of the Ssn6p/Tup1p corepressor complex involved in glucose repression (59), was not affected (data not shown), suggesting that REST/NRSF has a specific requirement for the Sin3p corepressor complex.
To see if the genetic interaction between REST/NRSF and Sin3p/Rpd3p was
due to a physical interaction between the two molecules in vivo, we
performed a ChIP assay (Fig. 3). A strain
expressing HA epitope-tagged Sin3p was generated (yA2) bearing the
reporter pBM2389(RE1)3UAS, which expressed either
myc-tagged NRSF(1-600) or myc-tagged NRSF(142-445).
Formaldehyde cross-linking and immunoprecipitation with anti-HA
antibody precipitated the reporter sequence in the strain expressing
NRSF(1-600) but not in the strain expressing NRSF(142-445)
(Fig. 3, compare lanes 1 and 2). Anti-myc antibody precipitated the
reporter sequence in both strains (Fig. 3, compare lanes 3 and 4).
Also, anti-HA antibody precipitated promoter DNA from the
IME2 gene, which is known to recruit Sin3p (22),
but not SUC2 promoter DNA, which is not regulated by Sin3p
(data not shown). The HZ4 fragment of REST/NRSF [NRSF(1-600)] is
therefore able to interact with the Sin3p complex and recruit Sin3p to
the GAL1 promoter via RE1/NRSE sequences in vivo.
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Having established that REST/NRSF is able to repress transcription in
yeast by recruiting the Sin3p complex, we sought to determine if
REST/NRSF was able to interact with mammalian SIN3. The 138 N-terminal
residues of REST/NRSF encompassing the previously characterized
N-terminal repression domain (57) were fused to GST and
purified from E. coli. The fusion protein was incubated with
cell extracts from HEK293 cells, spun down, and washed, and the pellet
was assessed for the presence of mSIN3A by Western blot analysis.
GST-NRSF(1-138) was able to precipitate mSIN3A under conditions
which failed to allow mSIN3A to bind GST alone (Fig.
4a, compare lanes 1 and 3). As expected,
GST fused to the SIN3 interaction domain of MAD1 (residues 1 to 45)
(4) precipitated mSIN3A (Fig. 4a, compare lanes 2 and 3).
This demonstrates that NRSF(1-138), a fragment of REST/NRSF
required for full repression in yeast and known to repress autonomously
in mammalian cells, interacts with mammalian SIN3A protein in vitro.
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To address whether full-length REST/NRSF could interact with mammalian SIN3A, a plasmid driving myc-tagged full-length REST/NRSF expression was transfected into Neuro-2A cells, and the cell extract was subjected to immunoprecipitation with anti-myc antibody. The presence of mSIN3A was assessed by Western analysis of the immunocomplex with anti-mSIN3A antibody. Figure 4b shows that mSIN3A was precipitated with anti-myc antibody (lane 3) but failed to precipitate when anti-HA antibody was used (lane 2), demonstrating that full-length REST/NRSF can interact with mSIN3A in mammalian cells. Anti-REST/NRSF antibody (12c11) was then used in immunoprecipitation assays to see whether endogenous REST/NRSF was bound to endogenous mSIN3A as an in vivo complex. Figure 4c shows that mSIN3A is precipitated with anti-REST/NRSF antibody (lane 3) but not with anti-HA antibody (lane 2) from Neuro-2A cell extracts. REST/NRSF therefore forms a complex with mSIN3A in vivo.
In order to define more closely the domain of mSIN3A that REST/NRSF binds, we generated a series of myc-tagged mSIN3A deletion mutants: mSIN3A(1-680), mSIN3A(1-479), and mSIN3A(1-272). The fragments were expressed in HEK cells and subjected to GST pull-down assays (Fig. 4d). GST-NRSF(1-138) was able to bind all three mutants (lane 3) under conditions that prevented GST alone from binding (lane 2). This result suggests that mSIN3A residues 1 to 272 are sufficient for REST/NRSF binding.
Because repression by HZ4 in yeast was dependant not only on Sin3p but also on Rpd3p, we wished to determine whether REST/NRSF recruited mammalian Rpd3p (HDAC2) as part of the mammalian SIN3A complex. Flag-tagged HDAC2 and myc-tagged REST/NRSF were expressed in Neuro-2A cells, and protein extracts were subjected to immunoprecipitation with anti-myc antibody. As seen in Fig. 4e, FLAG-HDAC2 is precipitated by anti-myc antibody (lane 3) but not by anti-HA antibody (lane 2). HDAC2 therefore is able to interact with REST/NRSF to form a REST/NRSF-SIN3-HDAC2 complex.
Based on the above genetic and biochemical data, we postulated that
REST/NRSF represses RE1/NRSE-containing genes by recruiting the
SIN3/HDAC complex to maintain neighboring nucleosomes in a hypoacetylated state. To test this, we performed a ChIP analysis on
Neuro-2A/1 cells (Neuro-2A/1 cells express full-length
REST/NRSF [data not shown]) transfected with the M4
reporter plasmid pGL3LC.-677/+80 (see below) which possesses a
functional RE1/NRSE (63) in combination with a plasmid
expressing either the yeast gal4 DNA binding domain or
gal4 fused to residues 1 to 138 of human NRSF [G4(1-138)]. We reasoned that overexpressing G4(1-138) would result in competition of the SIN3/HDAC complex away from endogenous REST/NRSF and so should
result in (i) a decrease in the occupancy of the RE1/NRSE by
mSIN3A and HDAC2, and (ii) an increase in the presence of
acetylated histones around REST/NRSF-occupied RE1/NRSEs. After
cross-linking with formaldehyde, cells were lysed, and the
extract was subjected to immunoprecipitation with antibodies to
acetylated histone H3, mSIN3A, HDAC2, or HA epitope (as a negative
control). Precipitated DNA was washed, liberated from cross-linked
protein, and subjected to PCR with primers around the M4
RE1/NRSE (Fig. 5). Antibodies to mSIN3A
and HDAC2 clearly precipitate the reporter sequence in cells expressing
just the gal4 DNA binding domain (Fig. 5, lanes 3 and 4 with
lane 5). However, in cells overexpressing G4(1-138), the same
antibodies fail to precipitate significant reporter DNA compare lane 8 with lanes 3 and 10 and lane 9 with lanes 4 and 10). In contrast,
immunoprecipitation with antibody to acetylated histone H3 yielded
greater quantities of reporter DNA from cells transfected with
G4(1-138) than the gal4 DNA binding domain alone (compare
lane 7 with lanes 2 and 10). The ChIP observations show that mSIN3A and
HDAC2 are bound to REST/NRSF at the M4 RE1/NRSE via the N
terminus of REST/NRSF and that their binding is associated with a
reduction of hyperacetylated nucleosomes at this site.
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We next wanted to see whether the nucleosomal hypoacetylation observed
when mSIN3A and HDAC2 were bound to the N terminus of REST/NRSF
resulted in the repressed transcription of a reporter gene. To test
this, we performed reporter assays on transfected cells treated with
the histone deacetylase inhibitor TSA. HEK293 cells were cotransfected
with reporter plasmid pGL3.UAS.TRE.Inr (five GAL1 upstream
activation sequence elements and seven tetO elements driving the
initiator from the adenovirus major late promoter), pTETOFF (expressing
tet-VP16), and fusions of the gal4 DNA binding domain with
fragments of REST/NRSF. Figure 6a shows that the 73 N-terminal residues, when fused to gal4, can repress VP16-activated Inr-derived transcription. Further,
gal4NRSF(1-73)-mediated repression is sensitive to TSA (Fig. 6b).
These data show that the N-terminal repression domain of REST/NRSF can
repress heterologous promoters via a mechanism that requires histone
deacetylase activity in mammalian cells.
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Functional characterization of RE1/NRSE has been demonstrated for a
number of promoters of neuronal specific genes (20, 32, 35,
42), including those of the sodium type II channel gene (NaII)
(33) and the type 4 muscarinic acetylcholine receptor gene
(M4) (34, 63). To determine whether REST/NRSF
repressed neuronal-specific genes via a histone
deacetylase-dependent mechanism, we performed transient
transfection assays in 3T3 fibroblasts with reporter constructs bearing
5' regions of the NaII and M4 genes with or without their
respective RE1/NRSEs. Figure 7 shows that
there is a 2.4-fold increase in reporter activity of the M4
promoter construct pGL3LC.-677/+80 (which possesses an RE1/NRSE) over
pGL3LC.-151/+80 (which lacks an RE1/NRSE) in the presence of 100 nM TSA. There is a 2.0-fold increase in reporter activity of the
NaII promoter construct pGL3.-1053/+177 (which possesses an RE1/NRSE)
over pGL3.-134/+177 (which lacks an RE1/NRSE) in the presence of 100 nM
TSA. These data are consistent with REST/NRSF repressing
neuronal-specific genes via a mechanism that requires histone
deacetylase activity.
|
However, REST/NRSF is known to possess at least two repression domains
other than the 73 N-terminal residues, i.e., the C-terminal Zinc finger
(57) and the LRD (unpublished data). To be sure that the TSA
sensitivity observed in cells expressing full-length REST/NRSF is due
to the recruitment of a SIN3/HDAC complex by the N-terminal repression
domain, we took advantage of the following observations: (i) REST/NRSF
exists in multiple spliced isoforms (40); (ii) the cell line
Neuro-2A expresses only the short isoform (
NRSF) which encompasses
residues 1 to 327 of full-length REST/NRSF and thus contains only the
N-terminal repression domain and five of eight putative zinc fingers
(40); and (iii) Neuro-2A cells do not express the
M4 gene but do express the NaII gene (A. Roopra and I. C. Wood, unpublished observations). To show that
NRSF could bind DNA
despite a shortened DNA binding domain, we performed gel mobility shift
assays with in vitro-translated
NRSF (Fig. 8). In vitro-translated
NRSF produced
a shifted band when incubated with the RE1/NRSE from the NaII gene
(albeit less robust than the shift obtained with in vitro-translated
full-length REST/NRSF; data not shown) as seen in Fig. 8, lane 1, which was able to be competed with excess cold competitor (lane 2). The
shifted band could be supershifted with anti-REST/NRSF antibody (lane
3) but not the unrelated antibody, anti-myc (lane 4). Identical results were obtained with the M4 gene promoter (data not shown).
|
Having shown that
NRSF is able to bind the RE1/NRSE, we tested
whether
NRSF-mediated repression of neuronal genes was TSA sensitive. Neuro-2A cells were transfected with reporter plasmids bearing the 5' region of the M4 and NaII genes with or
without an RE1/NRSE. As is shown in Fig.
9a, promoter-luciferase reporter construct
677/+80 was repressed to 36% of -151/+80. However, -1053/+177 expressed the reporter gene to the same extent as -134/+177. We therefore conclude that
NRSF can repress M4 gene
expression in transient transfection assays but fails to repress NaII
expression. Figure 9b shows that while TSA has little effect on the
NaII promoter, TSA causes a 2.4-fold derepression of the M4
promoter in the presence of the M4 RE1/NRSE.
|
Therefore, we conclude that REST/NRSF represses neuronal promoters via their RE1/NRSEs by recruiting the mSIN3A/HDAC2 complex. Because the repression seen is sensitive to TSA, we conclude that the deacetylase activity of the mSIN3A/HDAC2 complex is important for REST/NRSF-mediated repression.
| |
DISCUSSION |
|---|
|
|
|---|
REST/NRSF is an essential transcription factor which regulates the expression of numerous genes associated with neuronal differentiation. Using a functional assay in yeast, we have identified the SIN3/HDAC complex as a corepressor of REST/NRSF. We show that REST/NRSF interacts with yeast and mammalian SIN3 in vivo. Repression of neuronal promoters by REST/NRSF requires deacetylase activity and occurs concommitantly with core histone deacetylation.
The ability of REST/NRSF to repress the yeast GAL1 promoter suggested that REST/NRSF was repressing via a mechanism conserved between yeast and mammals. The corepressor SIN3 is highly conserved across species from yeast to mammals and has been found to associate with many transcription factors regulating a variety of cellular processes (see references 14, 22, and 43 and references therein). The genetic requirement and physical interaction with yeast Sin3p and mammalian SIN3A suggests that REST/NRSF recruits the SIN3 complex to repress transcription at RE1/NRSE-containing promoters. The N-terminal repression domain of REST/NRSF is able to interact with the 213 N-terminal residues of mSIN3A. Whether this interaction is direct or not is not known. However mSIN3A, mSIN3B, and yeast SIN3 possess four paired amphipathic helices, motifs that mediate protein-protein interactions (5). Like MAD, which possesses an N-terminal amphipathic helix with which it interacts with PAH2 of SIN3 (5), REST/NRSF residues 1 to 73 model well to an ampipathic helix (data not shown) and so may interact with PAH1 (residues 140 to 187 in mSIN3A).
Yeast Sin3p and mSIN3A are associated with histone deacetylases (Rpd3p
and HDAC1/HDAC2/HDAC3, respectively) (22, 56) which are the
catalytic cores of the SIN3 complex. The observation that the
deacetylase activity of histone deacetylases is required for repression
(23) and that repression of genes is associated with histone
deacetylation around repression elements (reviewed in reference
14) suggests that REST/NRSF represses transcription by recruiting histone deacetylase activity to RE1/NRSE-containing genes. Although we show that REST/NRSF interacts with HDAC2, as judged
by immunoprecipitation (Fig. 4e) and ChIP (Fig. 5) assays, it is
unlikely that REST/NRSF binds directly to HDAC2; REST/NRSF repression
in yeast is lost in a sin3
RPD3+ strain, and so it is likely that REST/NRSF
requires Sin3p to mediate the interaction.
The ChIP assays (Fig. 5) show that mSIN3A and HDAC2 form a complex with REST/NRSF bound at the RE1/NRSE. Overexpression of the N terminus of REST/NRSF results in quenching of the amount of SIN3/HDAC bound to the RE1/NRSE but also a large increase in the amount of acetylated histone H3 associated around the RE1/NRSE. Overexpression of the REST/NRSF N terminus also results in diminished repression of reporter gene expression due to quenching (31 and our unpublished observations). These observations are entirely consistent with REST/NRSF recruiting histone deacetylase activity to generate a region of hypoacetylated core histone around the RE1/NRSE to mediate repression of RE1/NRSE-containing genes.
Because deacetylation occurs in the vicinity of the repressor binding site (14), it is tempting to speculate that REST/NRSF-induced hypoacetylation around the RE1/NRSE causes reduced access to the M4 initiator element by the TFIID/RNAP holocomplex. Indeed, the RE1/NRSE at the M4 locus is only 540 bp upstream of the transcription start site (63), and so local changes to nucleosomal structure may have a direct effect on TFIID/RNAP holocomplex access to the promoter. However, RE1/NRSEs are found at various distances from the promoters of other neuron-specific genes, e.g., the L1 gene has a functional RE1/NRSE 18 kb downstream from the transcription start site. It is unclear how the effects of local nucleosomal hypoacetylation induced by REST/NRSF binding at such a distant RE1/NRSE will be transmitted to the promoter of such genes.
Although REST/NRSF binding is associated with hypoacetylation and reduced reporter gene expression, many other molecular targets of acetyltransferases have recently been identified which may also be legitimate substrates for the REST/NRSF-recruited SIN3 complex (8, 15, 19, 36), and so core histone deacetylation may not be the only mechanism by which REST/NRSF-recruited deacetylases mediate repression.
HEK293 cells express full-length REST/NRSF which, in addition to the
N-terminal repression domain, possesses a further two repression
domains: the C-terminal zinc finger (57) and the LRD
(unpublished data). We therefore turned to the neuroblastoma cell line
Neuro-2A which exclusively expresses a splice variant of REST/NRSF,
NRSF possessing only the N-terminal repression domain and a
truncated DNA binding domain (residues 1 to 327) (40).
Despite a truncated DNA binding domain,
NRSF can still bind
RE1/NRSEs from both the NaII and M4 genes, albeit less
efficiently than full-length REST/NRSF (Fig. 7 and data not shown). The
RE1/NRSE-containing M4 promoter was repressed in this cell
line, and importantly, this repression was sensitive to TSA, consistent
with the N terminus recruiting the SIN3 complex to repress
M4 transcription. The NaII promoter however was not
repressed via the RE1/NRSE. It is not clear at present whether this
lack of repression is due to a lower affinity of
NRSF with the NaII
RE1 or if the factors that drive NaII expression in Neuro-2A cells
render the NaII promoter immune to
NRSF-mediated repression. The
observation that
NRSF differentially regulates M4 and
NaII transcription in Neuro-2A cells opens up the possibility that
REST/NRSF splicing may allow differential gene regulation of
RE1/NRSE-containing genes within the nervous system. Indeed, both yeast
and mammalian SIN3 are associated with a number of other peptides, at
least one of which, SAP30, can confer transcription factor selectivity
on the complex (28). Also, N-CoR (originally isolated in a
screen to identify corepressors of unliganded nuclear receptors
[16]) can confer histone deacetylation-independent repressor activity upon the SIN3 complex by directly interacting with
the general transcription factors (37). Whether REST/NRSF can use these other factors and associated modes of repression is not
yet known.
The regulation of histone deacetylase binding to CBF1 by Notch has been implicated in the acquisition of neuronal cell fate via the Notch/Delta pathway (18, 25). Here, we provide evidence suggesting that REST/NRSF, a gene required to maintain appropriate expression of neuronal genes within the nervous system (7, 11, 24, 58) and silence expression outside the nervous system (11) utilizes the SIN3-HDAC complex to silence transcription. HDACs therefore seem to play a direct role in both acquisition of neuronal cell fate and neuronal differentiation.
| |
ACKNOWLEDGMENTS |
|---|
We are indebted to the lab of Mark Johnston, Washington University, St. Louis, Mo., for plasmid pBM2389 and invaluable guidance throughout the course of the yeast work. We thank Luigi Lania, IIGB, Naples, Italy, for the T7G5TATA reporter plasmid. Plasmid pSKD7 containing the NaII promoter region was a generous gift from Gail Mandel, Stony Brook, N.Y. We thank Mireia Garriga-Canut for construction of pGL3.Inr, Helene Marie for construction of pLEONOV.NRSF(139-600), Francesca Caccuci and Richard Rowe for their contributions, and Martin Raff, University College London, London, United Kingdom, for critical reading of the manuscript.
This work was supported by the Wellcome Trust and by NIH grant NS23476 to David Anderson, Howard Hughes Medical Institute, California Institute of Technology. A.R. is the recipient of a Wellcome Prize Fellowship, and A.J.P. is supported by an NIH predoctoral training grant.
| |
FOOTNOTES |
|---|
* Corresponding author. Mailing address: School of Biochemistry and Molecular Biology, University of Leeds, Leeds LS2 9JT, United Kingdom. Phone: 44 113 233 3015. Fax: 113 233 3019. E-mail: aroopra{at}hgmp.mrc.ac.uk.
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