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Volume 17, Number 22,
Issue of November 15, 1997
Cloning and Characterization of Murine Glial Cell-Derived
Neurotrophic Factor Inducible Transcription Factor (MGIF)
Shunsuke Yajima1,
Claas-Hinrich Lammers1,
Sang-Hyeon Lee1,
Yoshinobu Hara2,
Keiko Mizuno3, and
M. Maral Mouradian1
1 Genetic Pharmacology Unit, Experimental Therapeutics
Branch, National Institute of Neurological Diseases and Stroke,
2 Laboratory of Biochemical Genetics, National Heart, Lung,
and Blood Institute, and 3 Laboratory of Developmental
Neurobiology, National Institute of Child Health and Human Development,
Bethesda, Maryland 20892
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
FOOTNOTES
REFERENCES
ABSTRACT
The potent neurotrophic factor glial cell-derived neurotrophic
factor (GDNF) is a distant member of the transforming growth factor-
(TGF- ) superfamily of proteins. We report a transcription factor
that is the first nuclear protein known to be induced by GDNF, thus
designated murine GDNF inducible factor (mGIF). The cDNA was cloned in
the course of investigating transcription factors that bind to Sp1
consensus sequences, using the in situ filter detection
method, and it was found to encode a protein having the same
C2-H2 zinc finger motif as Sp1. Sequence
analysis indicated that mGIF is homologous to the human TGF-
inducible early gene (TIEG) and human early growth response gene-
(EGR- ). mGIF is widely distributed in the adult mouse with high mRNA
levels in kidney, lung, brain, liver, heart, and testis. In the adult
brain, mGIF is abundantly expressed in hippocampus, cerebral cortex, cerebellum, and amygdala with lower amounts in striatum, nucleus accumbens, olfactory tubercle, thalamus, and substantia nigra. During
development, mGIF mRNA also has a wide distribution, including in
cerebral cortex, cerebellar primordium, kidney, intestine, liver, and
lung. GDNF induces the expression of mGIF rapidly and transiently both
in a neuroblastoma cell line and in primary cultures of rat embryonic
cortical neurons. Co-transfection of the Drosophila SL2
cells using mGIF expression plasmid and reporter constructs having Sp1
binding sites indicated that mGIF represses transcription from a
TATA-containing as well as from a TATA-less promoter. These observations suggest that the zinc finger transcription factor mGIF
could be important in mediating some of the biological effects of
GDNF.
Key words:
glial cell-derived neurotrophic factor (GDNF);
zinc
finger;
Sp1;
transcription;
cloning;
transforming growth factor-
(TGF- )
INTRODUCTION
Glial cell-derived neurotrophic
factor (GDNF) is a distant member of the transforming growth factor-
(TGF- ) superfamily, which comprises an expanding list of
multifunctional proteins serving as regulators of cell proliferation
and differentiation (Kingsley, 1994 ). GDNF has trophic effects on
midbrain dopaminergic, spinal motor, cranial sensory, sympathetic, and
hindbrain noradrenergic neurons (Lin et al., 1993 ; Henderson et al.,
1994 ). It also protects and restores dopamine neurons from injuries
in vitro as well as in vivo (Beck et al., 1995 ;
Oppenheim et al., 1995 ; Tomac et al., 1995a ; Yan et al., 1995 ; Gash et
al., 1996 ). Thus, it is considered a potential therapeutic agent for
neurodegenerative diseases such as Parkinson's disease, although its
molecular signaling mechanisms remain to be elucidated. Although the
receptors mediating the physiological actions of GDNF are being
identified (Durbec et al., 1996 ; Jing et al., 1996 ; Trupp et al., 1996 ;
Baloh et al., 1997 ), the transcriptional regulatory mechanisms that
ensue activation of these receptors remain to be identified.
Transcription of genes in eukaryotes is controlled by a complex set of
regulatory factors that bind to distinct DNA elements. Although many
short DNA sequences are known to be specific transcription factor
binding sites, three main motifs in the DNA binding domain of
transcription factors have been identified including the zinc finger,
helix-turn-helix, and leucine zipper motifs (Harrison, 1991 ). Among
these, the zinc finger motif is composed of 25-30 amino acids
containing cysteine and histidine residues that are involved in the
tetrahedral coordination of a zinc atom. Considerable variations of
this motif are known to occur depending on the number of fingers and
the different combinations of cysteines or histidines in a finger.
A prototypical zinc finger protein is Sp1, which has a
three-Cys2-His2 zinc finger motif. It was
originally cloned as a transcription factor that binds to the simian
virus 40 early promoter (Kadonaga et al., 1987 ) and is now known to
activate a wide range of vertebrate genes. Several additional
transcription factors having the same zinc finger motif as Sp1 have
been cloned and found to bind to the GC box sequence. These include Sp2
(Kingsley and Winoto, 1992 ), Sp3 (Hagen et al., 1992 ; Kingsley and
Winoto, 1992 ), Sp4 (Hagen et al., 1992 ), basic transcription
element-binding protein (BTEB; Imataka et al., 1992 ), the
Drosophila buttonhead gene product (btd; Wimmer et al.,
1993 ), TGF- inducible early gene (TIEG; Subramaniam et al., 1995 ),
and early growth response gene- (EGR- ; Blok et al., 1995 ).
In the present investigation, we cloned and characterized a murine
Sp1-related zinc finger transcription factor cDNA, which is expressed
in the adult and developing brain. We found that GDNF induces the mRNA
expression of this nuclear protein, and, therefore, we named it murine
GDNF inducible factor (mGIF).
MATERIALS AND METHODS
Expression cloning and rapid amplification of cDNA
5 ends. Total RNA was prepared from the murine neuroblastoma cell
line NB41A3 [American Type Culture Collection (ATCC), Rockville, MD] by the guanidinium/cesium chloride method (Sambrook et al., 1989 ), and
poly(A)+ RNA was obtained using a Quick poly(A) column
(Stratagene, La Jolla, CA). A gt11 cDNA library was constructed
using oligo-dT primer and the SuperScript Choice System for cDNA
Synthesis (Life Technologies, Gaithersburg, MD) and packaged with
Gigapack II gold (Stratagene). The probe used was the Sp1(A) region in
the rat D2 dopamine receptor gene, which is one of two Sp1
binding sites within the NegB region of this gene (Minowa et al.,
1994 ). Synthetic oligodeoxynucleotides
5 -CTAGCAAG GGTTC-3 and 5 -CTAGGAACC CTTG-3 were end-labeled with
32P, annealed, and ligated to generate a concatenated
probe. The Sp1 consensus sequence is underlined, and the bases added to
the Sp1(A) sequence to generate the concatenated probe are in italic. Screening of the expression library followed published procedures (Vinson et al., 1988 ) with some modifications (Mizuno et al., 1990 ).
One of the positive clones was subsequently characterized and
designated mGIF.
To obtain the 5 portion of mGIF cDNA, the rapid amplification of
cDNA 5 ends (5 -RACE) system (Life Technologies) was used with 1 µg
of poly(A)+ RNA from NB41A3 cells, 2 pmol of mGIF
gene-specific primer (5 -CACCTGAGTGCCCATGAACAAC-3 , corresponding to
nucleotides +933 to +912 relative to the translation initiation site),
and a second mGIF gene-specific primer
(5 -GACGAAGGAGCTGGCTGAGAC-3 , +743 to +723). cDNA fragments of
positive clones were subcloned in pUC19 and sequenced.
Plasmid constructions. Full-length mGIF cDNA was constructed
by combining the original gt11 clone and the 5 -RACE clone using the
unique HaeII site within mGIF.
For in vitro transcription to generate an in situ
hybridization probe, the 578 bp HindIII-PstI
fragment of mGIF (nucleotides +31 to +609 relative to the first ATG)
was subcloned into the same sites of pGEM3Zf( ) (Promega, Madison,
WI).
For functional analysis, an expression plasmid of mGIF in the
Drosophila Schneider's cell line SL2 was constructed in
pRmHa3 (a kind gift from Carl Wu, National Cancer Institute, Bethesda, MD) (Rabindran et al., 1993 ), which has a multiple cloning site between
the Drosophila metalothionein promoter and the 3 -end sequence of the Drosophila alcohol dehydrogenase gene in
pUC18. The 2.6 kb HindIII-SacI fragment of mGIF
was inserted into the SacI site of pRmHa3 to generate
pRmGIF. This construct lacks nine amino acids from the N terminus of
mGIF. As reporter plasmids, BCAT-2 (a kind gift from Robert Tjian,
University of California, Berkeley, CA) (Pascal and Tjian, 1991 ) and
pCATD2-116 (Minowa et al., 1994 ) were used.
Southern blot analysis. Mouse genomic DNA was digested with
EcoRI or BamHI, electrophoresed in a 0.7%
agarose gel (7 µg/lane), and transferred to nitrocellulose filters.
Hybridization was performed using the random-primed,
32P-labeled, 578 bp HindIII-PstI
fragment of mGIF cDNA as probe in 4× SSC, 40% formamide, 0.1% SDS,
1× Denhardt's solution and 25 µg/ml sheared salmon DNA at 42°C
overnight. Filters were washed three times in 1× SSC and 0.1% SDS at
25°C and four times in 0.1× SSC and 0.1% SDS at 40°C as described
previously (Hara et al., 1992 ). Filters were then exposed to a phosphor
screen and analyzed with a PhosphorImager (Molecular Dynamics,
Sunnyvale, CA).
Cell culture and transfection. For mGIF mRNA induction
experiments in the neuroblastoma cell line NB41A3, cells were cultured in 100 mm dishes containing DMEM (Mediatech, Herndon, VA) supplemented with 10% fetal bovine serum at 37°C in a humidified atmosphere containing 10% CO2 in the presence of 10% (for GDNF) or
0.2% (for TGF- ) serum for 36 hr. Subsequently, 800 ng of human
recombinant GDNF (R&D Systems, Minneapolis, MN) or 50 ng of human
platelet TGF- (Calbiochem, San Diego, CA) was added per dish. Cells
were harvested at different time points, and total RNA was prepared using RNAsolB (Tel-Test, Friendswood, TX).
For mGIF induction in primary neuronal cultures, whole neocortices of
embryonic day 18 (E18) rats were dissociated mechanically and plated
onto poly-D-lysine- and laminin-coated dishes at a density
of 1 × 103 cells/mm2. After
growing the cells in neurobasal medium (Life Technologies), 0.5 mM glutamine, 2% fetal bovine serum, and nutrient mixture N2 (Life Technologies) for 80 hr, the medium was replaced with fresh
medium lacking serum with or without 80 ng/ml GDNF or 5 ng/ml TGF- .
Total RNA was extracted at the indicated time points by an acid-phenol
method (Sambrook et al., 1989 ).
To study the transcriptional activity of mGIF, SL2 cells (ATCC) were
maintained in Schneider's Drosophila medium (Life
Technologies) supplemented with 10% fetal bovine serum (Intergen,
Purchase, NY) in ambient air. Transfections were performed using
Lipofectin (Life Technologies) with HyQ-CCM 3 serum-free medium
(HyClone, Logan, UT) in 60 mm dishes. To study plasmid dosage effect,
various amounts of BCAT-2 or pCATD2-116 were initially used
individually to co-transfect SL2 cells. Subsequently, 2 µg of BCAT-2
or 1 µg of pCATD2-116 and varying amounts of pRmGIF were used to
co-transfect SL2 cells. The vector pRmHa3 was added to keep the total
plasmid content equal for all dishes. After a 16 hr incubation, mGIF
expression was induced with 0.7 mM
CuSO4, and the cells were harvested 24 hr after
induction and lysed by repeated cycles of freezing and thawing.
Chloramphenicol acetyltransferase (CAT) activity was measured using a
CAT ELISA kit (Boehringer Mannheim, Indianapolis, IN).
Northern blot analysis. To study the distribution of mGIF, a
mouse multiple tissue northern (MTN) blot membrane (Clontech, Palo
Alto, CA) was hybridized with a 36 mer mGIF probe (5 -
GCAAGCTGTCATCCTCGGAGGTATCAGACACTGGTG-3 , bases +229 to +194) using
the manufacturer's protocol.
Induction of mGIF in NB41A3 cells was studied by electrophoresing 20 µg of total RNA in a 1.8% agarose-formaldehyde gel in 3-[N-morpholino]propane-sulfonic acid buffer, transferred
to a Zeta membrane (Bio-Rad Laboratories, Hercules, CA), and
cross-linked by UV light. The 578 bp
HindIII-PstI fragment of mGIF was
32P-labeled with the random-priming method and used as
probe in ExpressHyb (Clontech) at 68°C for 1 hr. A
glyceraldehyde-3-phosphate dehydrogenase (GAPDH) probe (Minowa et al.,
1996 ) was used as a control under the same hybridization conditions.
Band intensities were measured by a PhosphorImager.
To study mGIF induction in primary neuronal cultures, 40 µg of total
RNA was electrophoresed and subjected to Northern hybridization as
described above using the 578 bp mGIF probe and the control GAPDH probe
as described above. Filters were then exposed to a phosphor screen.
In situ hybridization. An adult mouse brain was
sectioned at 12 µm thickness, sections were placed on silanized
slides, and in situ hybridization was performed using a
previously described procedure (Bradley et al., 1992 ; Young et al.,
1993 ). The 578 bp mGIF riboprobe was labeled with
35S-uridine triphosphate (UTP) by in vitro
transcription using MAXIscript (Ambion, Austin, TX), and 1 × 106 cpm were applied to each slide. Finally,
hybridized slides were dipped in NTB3 emulsion (Eastman Kodak,
Rochester, NY). Sense riboprobe was used as a control.
To study the expression pattern of mGIF during development, E14 and E16
embryos of NIH Swiss mice were fixed in buffered 4% paraformaldehyde
for 24 hr at 4°C, embedded in paraffin, and sectioned at 7 µm
thickness followed by mounting onto Vectabond (Vector Laboratories,
Burlingame, CA)-treated slides. Sections were cleared of paraffin with
three 5 min washes in xylene and two washes in 100% ethanol. In
situ hybridization was then performed as described above. Sections
were counterstained with hematoxylin and eosin.
RESULTS
Cloning the cDNA for the zinc finger protein mGIF
In our efforts to clone transcription factors that bind to Sp1
consensus sequences, we used the NegB sequence of the rat
D2 dopamine receptor gene (Minowa et al., 1994 ) as a probe
with an NB41A3 cell expression library in the in situ filter
detection method. The NegB region consists of three TGGG repeats and
one Sp1 consensus sequence [GGGCGG; Sp1(A)], both of which bind to the same proteins (Minowa et al., 1994 ). Initially, we used a concatenated, double-stranded oligonucleotide probe containing Sp1(A)
sequence for library screening. After establishing positive clones, we
confirmed their DNA-binding profile using a probe having the TGGG
repeat sequence. Among ~500,000 plaques screened, six positive clones
were identified.
DNA sequencing revealed that one of the clones, mGIF, codes for a
three-Cys2-His2 zinc finger motif similar to
Sp1. Because this clone did not include the entire open reading frame,
we performed the 5 -RACE procedure using a gene-specific primer located
in the 5 end of the available sequence. A single band, about 900 bp
long, was obtained and PCR products were subcloned in pUC19. Two
independent clones were sequenced for confirmation and found to have
identical sequence. Subsequently, the 5 fragment from the 5 -RACE
clone and the 3 fragment obtained from the original library screening
were ligated using the unique HaeII site to yield the
full-length expression clone pRmGIF. Both strands of the 3036 bp insert
were sequenced. The deduced amino acid sequence and other structural
features are shown in Figure 1. Sequence comparison with the GenBank database showed that mGIF had 85% identity
in its amino acid sequence to human TIEG (Subramaniam et al., 1995 ) and
human EGR- (Blok et al., 1995 ).
Fig. 1.
Sequence alignment, homology analysis, and primary
structure of mGIF. A, Amino acid sequence alignment
between mGIF and human TIEG (Subramaniam et al., 1995 ). Only amino
acids that are different from mGIF are shown in the TIEG sequence.
Human EGR has the same amino acid sequence as TIEG except for 12 residues absent from its N terminus (Blok et al., 1995 ). C-terminal
boxed residues indicate the zinc finger motif.
Underlined sequences are putative SH-3 binding sites
(Pro-X-X-Pro). Filled,
open, and shaded circles show putative
phosphorylation sites by MAP kinase, GSK, and CDK, respectively.
B, Diagram of the primary structure of mGIF protein. Vertical lines represent proline residues, and
T or S shows Thr or Ser followed by
proline. Putative phosphorylated residues are indicated by the name of
kinases on the T or S. Shadowed
boxes show the zinc finger regions. Dashes
indicate the positions of putative SH-3 binding sites.
[View Larger Version of this Image (41K GIF file)]
Southern blot analysis with mGIF probe
To assess the possible presence of other genes homolgous to mGIF
in the mouse genome, we performed Southern blot analysis followed by
washing at low-stringency conditions (Fig.
2). This experiment revealed a single
intense band as well as at least two additional bands, suggesting the
presence of other genes having various degrees of sequence homology to
mGIF.
Fig. 2.
Genomic DNA Blot Analysis with mGIF probe.
Southern hybridization was performed on mouse genomic DNA followed by
washing at low-stringency conditions. Each lane contains 7 µg of
genomic DNA. The probe used is the 578 bp
HindIII-PstI mGIF fragment labeled with
32P by random priming. E,
EcoRI; B, BamHI.
[View Larger Version of this Image (65K GIF file)]
Tissue distribution of mGIF
To establish the tissue distribution of mGIF mRNA in the adult
mouse, we performed Northern blot analysis using a mouse MTN blot
membrane (Fig. 3). A 3.6-kb-long mRNA was
detected in various tissues at different abundance levels. Highest
amounts of mGIF mRNA were found in kidney and lung, followed by brain,
liver, and heart. Minimal levels were seen in testis, and none was seen in skeletal muscle or spleen. Thus, mGIF has a relatively wide tissue
distribution.
Fig. 3.
Multiple-tissue Northern blot analysis of mGIF.
Each lane contains 2 µg of poly(A)+ RNA from mouse
tissues. The probe used is a 5 end-labeled 36 mer mGIF
oligonucleotide.
[View Larger Version of this Image (62K GIF file)]
Brain regional expression of mGIF
To determine the brain regional distribution of mGIF, in
situ hybridization was performed using adult mouse brain sections and a probe generated by in vitro transcription with
35S-UTP (Fig. 4). Specific
hybridization was observed throughout the cerebral cortex, including
neocortex, archicortex, and paleocortex. In the neocortex, mGIF was
expressed most prominently in layer 6 and to a lesser extent in other
cortical layers. In the hippocampus, strong expression was present in
pyramidal cells of CA1, CA2, and CA3, as well as in granule cells of
the dentate gyrus and in tenia tecta. Intense signal was also detected
in pyriform cortex. Caudate-putamen, accumbens, and olfactory tubercle
had significant but weaker signals. In the amygdala, the most intense
hybridization was detected in the basolateral and lateral nuclei with
weaker signals in other areas. In the thalamus, the parafascicular
nucleus had a detectable signal. In the substantia nigra, weak but
significant signal was detected in pars compacta with scattered
positive cells in pars reticulata. Prominent hybridization was also
seen in the granule cell layer of cerebellum. Specific hybridization
was also present in the trigeminal motor nucleus of the brainstem.
Fig. 4.
mGIF mRNA localization in the adult mouse brain.
Dark-field photomicrographs from liquid emulsion autoradiograms of
coronal sections (A-D; anterior to posterior) and x-ray
film autoradiograms of sagittal sections (E, F; medial
and lateral, respectively). Acb, Accumbens;
Amy, amygdala; Cbl, cerebellum;
CPu, caudate putamen; Cx, cerebral
cortex; OB, olfactory bulb; OTu,
olfactory tubercle; PF, parafascicular thalamic nucleus;
Pir, pyriform cortex; TT, tenia tecta.
Twelve-micrometer-thick sections were hybridized with the 578 bp
HindIII-PstI mGIF riboprobe.
[View Larger Version of this Image (188K GIF file)]
Expression of mGIF during development
Whole mouse embryos were also analyzed for mGIF mRNA expression by
in situ hybridization histochemistry. Using embryos at E14
and E16, we found that mGIF is widely expressed (Fig.
5). In the nervous system, it is most
prominent in cerebral cortex, cerebellar primordium, and the dorsal
edge of the spinal cord. Although the level of mGIF expression in the
embryonic brain is not as high as in the adult brain, clearly some
similarities are noted, especially in the cerebral cortex and
cerebellar primordium. mGIF is also expressed in the body surface, most
clearly in the vibrissa of the nose and to a lesser extent in other
areas of skin. Bones and differentiating mesenchyme surrounding the
nasal cavity, jaw, basisphenoid bone of the skull, as well as back
bones and ribs also have this transcript. In the respiratory and
digestive systems, significant levels of mGIF mRNA are present in lung, liver, and intestine, including myenteric ganglia. In the excretory system, mGIF is prominently expressed in the kidney at E16.
Fig. 5.
Expression pattern of mGIF mRNA during
embryogenesis. Expression of mGIF mRNA was analyzed by in
situ hybridization histochemistry in the whole mouse embryo on
embryonic E14 (first and second
rows) and E16 (third row). Photomicrographs of
sagittal sections counter-stained by hematoxylin and eosin
(HE) and x-ray film autoradiograms hybridized with
antisense (AS) and sense (S) mGIF
probes are shown in the first, second, and third
columns, respectively. BS, Basisphenoid bone;
BB, back bone; Cbl, cerebellum;
Cx, cerebral cortex; Int, intestine;
Li, liver; Lu, lung; Nas,
nasal cavity; Sp, spinal cord; Vib;
Vibrissa. Seven-micrometer-thick sections were hybridized with the 578 bp HindIII-PstI mGIF riboprobe.
[View Larger Version of this Image (92K GIF file)]
Induction of mGIF mRNA expression by GDNF
Because mGIF is homolgous to a human gene that was cloned based on
its inducibility by TGF- , we sought to determine whether GDNF, which
belongs to the TGF- superfamily, induces mGIF. Thus, GDNF was added
to NB41A3 cells, and the expression level of endogenous mGIF mRNA was
examined by Northern blot analysis. We consistently found at least a
twofold to threefold increase in mGIF mRNA levels attaining a peak at
1.5 hr after the addition of GDNF (Fig.
6A). This induction was
brief, with a rapid decline as early as 2 hr. Northern blot analysis
using the control probe GAPDH indicated uniformity of the amount of
mRNA loaded per lane as well as specificity of mGIF induction. Similar
to human TIEG, mGIF could also be induced at least 2- to 2.5-fold by
TGF- peaking at 1.5 hr and persisting for at least 4 hr (Fig.
6B).
Fig. 6.
mGIF mRNA induction by GDNF and TGF- . Northern
blot analysis of mGIF mRNA levels after GDNF (A)
or TGF- (B) induction in NB41A3 cells.
Indicated times show incubation periods. The probe was the 578 bp
HindIII-PstI mGIF fragment labeled with
32P by the random-primed method. C,
Induction of mGIF mRNA in primary rat embryonic neuronal cultures after
treatment with 80 ng/ml GDNF or 5 ng/ml TGF- . Northern analysis for
GAPDH mRNA was performed as a control.
[View Larger Version of this Image (57K GIF file)]
The ability of GDNF and TGF- to induce mGIF mRNA expression was also
evaluated in primary cultures of rat embryonic cortical neurons (Fig.
6C). RNA was extracted after different periods of treatment,
and Northern blot analysis was performed using the mouse mGIF
riboprobe. A rapid and transient induction of mGIF expression was found
peaking at 30 min after incubation with GDNF or TGF- (Fig.
6C).
Functional characterization of mGIF
Based on the deduced amino acid sequence of mGIF, which has an
Sp1-like zinc finger motif, as well as its DNA-binding activity, we
expected it to function as a transcription factor. To examine this
possibility, we performed transient co-transfection assays in
Schneider's Drosophila cells using the mGIF expression
plasmid and two different CAT reporter plasmids. These cells were
chosen to allow analysis of mGIF function under controlled conditions, because they do not express Sp family proteins, which bind to the same
DNA sequences as mGIF (Santoro et al., 1988 ).
The BCAT-2 reporter plasmid has two Sp1 binding sites and a TATA
box followed by the CAT gene. SL2 cells were transfected with varying
amounts of BCAT-2 alone to establish the linear portion of plasmid
concentration-CAT level curve (Fig.
7A). Subsequently, when SL2
cells were co-transfected with a fixed "nonsaturated" amount of
BCAT-2 and the mGIF expression plasmid pRmGIF, CAT activity decreased
significantly in a dosage-dependent manner with increasing amounts of
pRmGIF (Fig. 7C). We also investigated the activity of mGIF
on the NegB sequence in the TATA-less D2 dopamine receptor promoter (Minowa et al., 1994 ), which was originally used as the screening probe. We used the reporter plasmid pCATD2-116, which has the
NegB sequence. After performing titration experiments with pCATD2-116
(Fig. 7B), the results of co-transfection with pRmGIF were
similar to those obtained with BCAT-2 (Fig. 7D). Thus, mGIF
could repress promoter activity in two different reporter constructs.
Fig. 7.
Functional analysis of mGIF by co-transfection CAT
assay. SL2 cells were transfected with increasing amounts of BCAT-2
(A) or pCATD2-116 (B)
reporter plasmids. SL2 cells were co-transfected with various amounts
of mGIF expression plasmid pRmGIF and either 2 µg of BCAT-2
(C) or 1 µg of pCATD2-116
(D) reporter plasmids. Means ± SE for
triplicates are shown as percentage of basal activity for each reporter
plasmid. *ANOVA, p < 0.01; **p < 0.05 compared with no pRmGIF. Similar experiments repeated two to
three times yielded reproducible results.
[View Larger Version of this Image (49K GIF file)]
DISCUSSION
We have cloned the cDNA encoding a murine GDNF inducible
transcription factor designated mGIF. It is homolgous to two human genes, TIEG (Subramaniam et al., 1995 ) and EGR- (Blok et al., 1995 ).
TIEG was cloned from fetal osteoblastic cells and found to be induced
by TGF- and by epidermal growth factor (EGF), whereas EGR- was
cloned from prostate carcinoma cells and found to be induced by EGF and
repressed by androgens. TIEG and EGR have identical amino acid
sequences except for 12 residues absent in the N terminus of EGR .
Thus, TIEG and EGR appear to be encoded by the same gene. Sequence
comparison between murine mGIF and these two human proteins indicates
85% amino acid identity. Comparison of their nucleotide sequences
revealed that although these cDNAs are homologous within their open
reading frame, more diversity exists in their 3 untranslated regions.
Both the human and murine proteins are rich in proline. mGIF has two
proline-rich regions; one contains 17 prolines of 90 residues, and the
other contains 31 prolines of 160 residues. Although mGIF does not show
any sequence homology to the TGF- -responsive domain located in the
proline-rich region of CTF/NF-1 (Alevizopoulos et al., 1995 ), it has 10 Pro-X-X-Pro sequences, which are putative Src
homology 3 (SH-3) binding motifs (Yu et al., 1994 ) and 16 Ser/Thr-Pro
sequences, which may be targets for proline-directed kinases. Among
these Ser/Thr-Pro sequences, six are putative target sites for
mitogen-activated protein (MAP) kinase (Pro-basic/neutral-Ser/Thr-Pro)
(Alvarez et al., 1991 ; Clark-Lewis et al., 1991 ; Mukhopadhyay et al.,
1992 ; Sanghera et al., 1992 ), two are for cyclin-dependent protein
kinase (CDK) (Ser/Thr-ProX-Lys/Arg) (Moodie et al.,
1993 ), and one is for glycogen synthase kinase (GSK)
(Ser/Thr-Pro-X-X-phospho-Ser/phospho-Thr) (Pelech, 1995 ) (Fig. 1A,B). These features
suggest that mGIF could be regulated post-translationally and could be
involved in signal transduction pathways modulating the expression of
various genes.
Although the similarity between mGIF and TIEG/EGR- is 85% at the
amino acid level and 81% at the nucleotide level, substantial differences exist in both their sequence and tissue distribution. mGIF
mRNA in the adult mouse is present in heart, lung, brain, liver,
kidney, and testis but not in spleen or skeletal muscle. On the other
hand, TIEG is strongly expressed in skeletal muscle but not in brain,
lung, or kidney (Subramaniam et al., 1995 ). Furthermore, Southern blot
analysis revealed the presence of at least three genes in the mouse
genome with various degrees of sequence homology to mGIF. Thus, these
observations suggest a new class of TGF- family inducible
transcription factors, which, to date, include mGIF and TIEG/EGR- .
Whether mGIF and human TIEG represent homologs of the same gene in two
different species or distinct members of a group of proteins remains to
be determined by isolating additional homologs.
The induction of mGIF by GDNF was hypothesized based on its homology to
TIEG and based on its high expression level in brain and other
GDNF-responsive tissues. mGIF expression in NB41A3 cells, from which it
was originally cloned and which express Ret receptor mRNA determined by
reverse transcription-PCR (data not shown), increased transiently after
GDNF treatment. In addition, mGIF mRNA levels increased briefly in
primary cultures of rat embryonic cortical neurons after GDNF
treatment. Thus, mGIF is the first recognized transcription factor to
be induced by GDNF. mGIF is also inducible by TGF- . However, in
contrast to the 10-fold induction of TIEG (Subramaniam et al., 1995 ),
mGIF mRNA increased to a much lesser extent after TGF-
treatment.
The distribution of mGIF mRNA was examined in the adult mouse brain by
in situ hybridization, because we found it to be inducible by GDNF. The results showed a specific expression pattern with strong
hybridization in several regions particularly in hippocampus, cerebral
cortex, amygdala, and cerebellum, with weaker signals in striatum,
substantia nigra, nucleus accumbens, olfactory tubercle, and thalamus,
supporting an important role for mGIF in various brain functions.
Comparison of the brain expression pattern of mGIF with that of
GDNF and its cell surface receptors revealed both similarities and
differences. The mRNA distribution of GDNF and its currently known
receptors is complex and only partially overlapping. In the developing
brain, GDNF mRNA is expressed mainly in the striatum and olfactory
tubercle but barely detectable in substantia nigra, where this factor
exerts its trophic actions after retrograde transport from the striatum
(Tomac et al., 1995b ). On the other hand, the basal expression of GDNF
in the adult brain is very low (Nosrat et al., 1996 ). Three GDNF
receptor molecules, Ret, GDNFR- (TrnR1), and TrnR2, have thus far
been identified (Durbec et al., 1996 ; Jing et al., 1996 ; Trupp et al.,
1996 ; Baloh et al., 1997 ). Ret is highly expressed in the adult brain
in substantia nigra, pons, medulla, thalamus, and cerebellum but not in
cerebral cortex, hippocampus, or striatum (Trupp et al., 1996 ). On the other hand, GDNFR- mRNA is clearly expressed in cerebral cortex and
hippocampus, and it partially overlaps with Ret mRNA in substantia nigra, pons, and thalamus. Furthermore, in several areas of known GDNF
action, including nigral and motor neurons, TrnR2 expression is low or
undetectable (Baloh et al., 1997 ). These variations in mRNA expression
pattern among GDNF and its receptors are considered indications that
such trophic circuits use multiple mechanisms for different neuronal
populations (Trupp et al., 1997 ). The expression pattern of mGIF
overlaps with that of GDNF receptors, including in thalamus,
cerebellum, and substantia nigra with Ret and in cerebral cortex,
hippocampus, cerebellum, and nigra with GDNFR- . Although GDNF mRNA
is barely expressed at basal conditions in the adult brain, treatment
with kainic acid or pilocarpine increases its expression in striatum,
hippocampus, and cerebral cortex (Humpel et al., 1994 ; Schmidt-Kastner
et al., 1994 ). Thus, mGIF could transduce the GDNF signal after
stimulation of one of its receptors in response to certain
physiological or pathological conditions. mGIF is also induced by
TGF- , which transduces its signals across the cell membrane via
serine and threonine kinase receptors (Massague, 1996 ). Although the
mRNA expression of Habrec-1, a recently identified TGF- type I-like
receptor, overlaps with that of mGIF (Lorentzon et al., 1996 ), such as
in striatum, olfactory tubercle, and cerebral cortex, the expression
patterns of mGIF and Habrec-1 are not identical. Nevertheless, mGIF
could transduce TGF- signals in certain cell populations. These
observations taken collectively could suggest that mGIF is involved in
signal transduction from multiple TGF- -like signals including
GDNF.
The distribution of mGIF mRNA in the mouse during embryogenesis also
supports its role in transducing the GDNF signal. mGIF is widely
expressed in the embryo but most prominently in kidney and
intestine, including the enteric nervous system, consistent with
the fact that GDNF-null mice lack kidneys and enteric neurons (Schuchardt et al., 1994 , 1996 ; Moore et al., 1996 ; Pichel et al.,
1996 ; Sanchez et al., 1996 ), and that GDNF mutations are associated
with the human intestinal disorder Hirschsprung disease (Angrist et
al., 1996 ; Ivanchuk et al., 1996 ). Additional overlap is noted in the
distribution of mGIF, GDNF, and its receptors in the embryo. It would,
thus, be interesting to compare the phenotype of mGIF-null mice with
strains lacking the gene for GDNF or its receptors in future
studies.
The conserved Sp1-like zinc finger motif in mGIF as well as its ability
to bind to Sp1 consensus sequences suggested that it functions as a
transcription factor. Among the Sp family of proteins, Sp1 is generally
known to be a strong activator, whereas Sp3 can either activate or
repress transcription, depending on promoter context (Hagen et al.,
1994 ; Liang et al., 1996 ). Functional analysis of mGIF by
co-transfection CAT assays in SL2 cells, which lack Sp family proteins,
allowing interpretation of these reconstitution experiments, revealed
that mGIF represses promoter activity both from TATA-containing and
TATA-less promoters. Thus, it appears that GDNF-induced increase in
mGIF expression results in decreased transcription rate of its target
gene(s). It is conceivable, however, that mGIF could also act as an
activator in certain promoters.
Several growth factors have been demonstrated to influence the
expression of specific genes, mostly activating them. For examples, nerve growth factor activates the ornithine decarboxylase gene (Feinstein et al., 1985 ); brain-derived neurotrophic factor,
neurotrophin-3 (NT-3), and NT-4 induce c-fos (Ip et al., 1993 ); and
TGF- activates cyclin-dependent kinase inhibitor
p15INK4B (Li et al., 1995 ; Collazo et al., 1992 ) and
2(I) collagen gene (Inagaki et al., 1994 ) both through Sp1 consensus
sites in the respective promoters. On the other hand, nerve growth
factor suppresses the epidermal growth factor receptor gene (Huff et
al., 1981 ; Lazarovici et al., 1987 ), and basic fibroblast growth factor
represses the NMDA receptor gene (Mattson et al., 1993 ). Although the
physiological target gene(s) for mGIF remain(s) to be determined, our
results suggest that one of the functions of mGIF is negative
transcriptional regulation in the GDNF- and TGF- -initiated cascades
in vivo. At present, we have no evidence that mGIF is
involved in the in vivo regulation of the D2
dopamine receptor gene, although its consensus Sp1 binding sequences
were initially used to clone mGIF.
These observations taken together suggest that mGIF induction could be
a key step downstream from GDNF- and TGF- -related factors. The
overlap as well as the diversity in the expression of mGIF with that of
TGF- superfamily members and their respective receptors suggest that
the intracellular cascades after TGF- -related signals might converge
on mGIF. The latter could, thus, operate as one of the final common
pathways in these signal-transcription coupling events in certain
neuronal populations.
FOOTNOTES
Received March 19, 1997; revised Aug. 21, 1997; accepted Aug. 25, 1997.
We thank Dr. Shioko Kimura of the National Cancer Institute for
technical support with cDNA cloning and Dr. Eva Mezey of the National
Institute of Neurological Diseases and Stroke for assistance with
in situ hybridization.
Correspondence should be addressed to Dr. M. Maral Mouradian, Building
10, Room 5C116, 10 Center Drive, MSC 1406, Bethesda, MD 20892-1406.
Dr. Yajima's present address: Tokyo University of Agriculture,
Sakuragaoka 1-1-1, Setagaya-ku, Tokyo 156, Japan.
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T. Cook and R. Urrutia
TIEG proteins join the Smads as TGF-beta -regulated transcription factors that control pancreatic cell growth
Am J Physiol Gastrointest Liver Physiol,
April 1, 2000;
278(4):
G513 - G521.
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T. Cook, B. Gebelein, M. Belal, K. Mesa, and R. Urrutia
Three Conserved Transcriptional Repressor Domains Are a Defining Feature of the TIEG Subfamily of Sp1-like Zinc Finger Proteins
J. Biol. Chem.,
October 8, 1999;
274(41):
29500 - 29504.
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T. Cook, B. Gebelein, K. Mesa, A. Mladek, and R. Urrutia
Molecular Cloning and Characterization of TIEG2 Reveals a New Subfamily of Transforming Growth Factor-beta -inducible Sp1-like Zinc Finger-encoding Genes Involved in the Regulation of Cell Growth
J. Biol. Chem.,
October 2, 1998;
273(40):
25929 - 25936.
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T. E. Hefferan, G. G. Reinholz, D. J. Rickard, S. A. Johnsen, K. M. Waters, M. Subramaniam, and T. C. Spelsberg
Overexpression of a Nuclear Protein, TIEG, Mimics Transforming Growth Factor-beta Action in Human Osteoblast Cells
J. Biol. Chem.,
June 30, 2000;
275(27):
20255 - 20259.
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C. K. Hwang, U. M. D'Souza, A. J. Eisch, S. Yajima, C.-H. Lammers, Y. Yang, S.-H. Lee, Y.-M. Kim, E. J. Nestler, and M. M. Mouradian
Dopamine receptor regulating factor, DRRF: A zinc finger transcription factor
PNAS,
June 19, 2001;
98(13):
7558 - 7563.
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