(Received for publication, September 10, 1996, and in revised form, February 13, 1997)
From the Cell Biology Section, Laboratory of
Pulmonary Pathobiology, Gamete Biology Section and the
¶ Laboratory of Reproductive and Developmental Toxicology, NIEHS,
National Institutes of Health, Research Triangle Park, North Carolina
27709 and the § Department of Medicine III, Osaka University
Medical School, Yamadaoka, Suita, Osaka 565, Japan
Recently, we have reported the cloning of the
germ cell-specific, nuclear orphan receptor germ cell nuclear factor
(GCNF)/RTR. In this study, we characterize the RTR response elements by
an electrophoretic mobility shift assay/polymerase chain
reaction-based, DNA binding site selection strategy. RTR binds with the
greatest affinity to response elements containing
TCA(AG(G/T)TCA)2 (consensus RTR response element;
conRTRE), to which it binds as a homodimer. RTR is also able to bind as
a monomer to a single core motif TCAAG(G/T)TCA, albeit with a lower
affinity. Mutation analysis supports the specific requirements of the
5-flanking sequence and the core motif of the RTRE for optimal binding
of RTR. An RTR-specific antiserum (RTR-Ab2) was raised that causes
supershift of the RTR-conRTRE complex in EMSA. Based on the sequence of
the conRTRE, we located a putative RTRE, referred to as P2-RE, in the
5
promoter-flanking region of the mouse protamine 2 gene, which is
induced during the same stage of spermatogenesis as RTR. The ability of
RTR-Ab2 to cause a supershift of an RTR-RTRE complex with nuclear
extracts from different tissues correlated with the tissue- and
development-specific expression of RTR. Transfection of RTR in CV-1
cells was unable to cause RTRE-dependent transactivation of
a CAT reporter gene; however, an RTR-VP16 fusion protein could induce
transactivation through several RTREs, including P2-RE.
The nuclear receptor superfamily constitutes a class of ligand-dependent transcription factors that regulate gene expression during many biological processes, including development, cellular proliferation, and differentiation (1-5). This superfamily includes receptors for steroid hormones, retinoids, and vitamin D, and an increasing number of orphan receptors for which the ligand has not yet been identified (6-9). Members of this superfamily share a common modular structure consisting of four major domains (6, 10-13). The DNA-binding domain is the most conserved among the nuclear receptors and is composed of two "zinc finger" motifs that play a role in DNA recognition and protein-protein interactions (10, 14). The ligand-binding domain at the C terminus is involved in several functions including ligand binding, receptor dimerization, nuclear translocation, and transcriptional activation (11, 13-16). The functions of the N-terminal region and the hinge domain that separates the DNA-binding and ligand-binding domains are still poorly understood. Recent studies have indicated that in certain instances the amino-terminal domain contains a transactivation function (17) and that the hinge domain contain sites that interact with co-repressors (18).
Nuclear receptors control the transcription of target genes by binding to DNA sequences referred to as hormone response elements (14, 16). Most members of this superfamily bind, as homodimers or heterodimers, to cis-acting DNA sequences that contain two half-site core motifs of RGGTCA configured in a direct repeat, a palindrome, or an inverted palindrome separated by spacers of different lengths (2, 7, 14, 16, 19, 20). A number of orphan receptors, including members of the ROR/RZR family (21-23), NGF1-B/Nur77 (24), and FTZ-F1/SF-1 (24-26) bind as monomers to hormone response elements consisting of a single core motif, RGGTCA, preceded by an AT-rich sequence.
Recently, we reported the cloning of the novel nuclear orphan receptor RTR from a mouse testis cDNA library (27). This receptor has also been cloned from rat and has been named germ cell nuclear factor (GCNF1; Ref. 28). This orphan receptor is highly expressed in the testis and in particular in round spermatids (27). These observations suggest that GCNF/RTR functions as an important transcriptional factor in the regulation of gene expression during a very specific stage of spermatogenesis.
In this study, we have analyzed the characteristics of the RTR response
element (RTRE) more precisely by a DNA binding site selection strategy
that is based on a combination of polymerase chain reaction (PCR) and
electrophoretic mobility shift assay (EMSA) and by mutation analysis.
Based on the consensus RTRE sequence obtained, we located a putative
RTR response element (P2-RE) in the 5 promoter-flanking region of the
mouse protamine 2 gene that is induced at the same stage of
spermatogenesis as GCNF/RTR (29, 30) and demonstrated that GCNF/RTR
specifically interacts with this DNA sequence. We provide evidence
showing that GCNF/RTR can mediate transactivation of a reporter gene
through either the consensus RTRE or P2-RE. These results suggest that
the protamine 2 gene may belong to a subset of genes that are regulated
by GCNF/RTR during the postmeiotic phase of spermatogenesis.
The plasmid pBSK-RTR containing the full-length
mouse RTR cDNA in pBluescript II SK(/+) (Stratagene) has been
described (27). The expression construct pZeoSV-RTR was made by
inserting the XbaI-KpnI fragment into the
SpeI-KpnI site of the pZeoSV vector (Invitrogen).
The plasmid pGEM3Z-RTR, which was used for in vitro translation of RTR, was created by inserting the
XbaI-KpnI fragment of pBSK-RTR containing the
full-length RTR into the expression vector pGEM3Z (Promega). The CAT
reporter gene constructs were created by inserting three copies of the
consensus RTRE (conRTRE) (TAA)3,
the half-site RTRE (GGAATCT)3, P2-RE
(TTTC)2TT
or a single copy of conRTRE into the HindIII and
BamHI sites of pBLCAT5 (31) to create the reporter plasmids
of (conRTRE)3-CAT, (1/2-RTRE)3-CAT, (P2RE)3-CAT, and (conRTRE)1-CAT. The
pSG5-VP16-RTR chimeric expression plasmid that encodes a fusion protein
consisting of the VP16 activation domain and the full-length coding
region of the RTR was created as follows. pGEM3Z-RTR was cut first with
EclXI and XhoI enzymes to obtain the full-length
RTR fragment, which was then inserted into the Bsp120 I and
XhoI sites of pBluescript II SK to create pBSK-RTR-2.
pSG5-VP16-RTR was obtained by inserting the
KpnI-XhoI fragment of pBSK-RTR-2 into the same
sites of pSG5-VP16. The pSG5-VP16 plasmid was obtained from Dr. J. Lehmann (Glaxo-Wellcome) and described previously (32). Expression of
fusion proteins VP16-RTR was verified by in vitro
translation and its subsequent characterization by polyacrylamide gel
electrophoresis and EMSA. pGEM3Z-RTR406, which encodes an
RTR truncated at amino acid 406, was created by inserting stop codons
at the NsiI site.
A peptide DSDHSSPGNRASESNQPSC (RTR-pep2) encoding amino acids 189-206 of RTR was synthesized and cross-linked via maleimide to KLH using the Imject kit from Pierce. The antigen was mixed 1:1 with Freund's adjuvant and injected subcutaneously into New Zealand White rabbits as described (33). Immunization boosts were given every 4 weeks. Antisera were tested by immunoprecipitation using protein A-agarose as described (33) and in EMSA using in vitro synthesized RTR. The obtained antiserum RTR-Ab2 was used at a 200-fold dilution.
Nuclear Extract PreparationNuclear extracts from brain, kidney, testis, and liver of 3-month-old CD-1 mice were prepared as described (34, 35). Nuclear extracts from juvenile mice at day 17 after birth were also prepared.
EMSAThe SP6 RNA-coupled reticulocyte lysate system
(Promega) was used to synthesize RTR protein. Oligonucleotide probes
for EMSA were end-labeled with [-32P]ATP by T4
polynucleotide kinase (Promega). Approximately 0.2-0.5 ng (50,000 cpm)
of the probe was used in a binding reaction with 2 µl of RTR
programmed reticulocyte lysate or 1-2 µl of nuclear extract (10-20
µg of protein) in a buffer containing 20 mM HEPES (pH
7.9), 50 mM KCl, 2.5 mM MgCl2, 1 mM DTT, and 10% glycerol. To prevent nonspecific binding,
1 µg of poly(dI-dC), 1 µg of salmon sperm DNA, and 0.5 µg of
nonspecific oligonucleotide were included in the reaction buffer. The
programmed lysates/nuclear extracts were first incubated with reaction
buffer for 10 min at room temperature and then in the presence of the
radiolabeled probe with or without competitor for 30 min. As a control,
probes were incubated with the same amount of unprogrammed lysate. For
supershift assays, 1 µl of RTR-Ab2 (1:200 final dilution) was
preincubated with the in vitro-translated RTR for 30 min at
room temperature prior to the addition of radiolabeled probe. The
RTR-nucleotide complexes were separated on 5% nondenaturing
polyacrylamide gels (National Diagnostics) containing 0.5 × TBE.
To select for the DNA
binding sites of RTR, a similar procedure was used as reported
previously (21). A mixture of 70-bp DNA fragments was synthesized by
PCR using the random oligomer 5-CGCGGATCCTGCAGCTCGAGN30GTCGACAAGCTTCTAGAGCA-3
as a
template and 5
-CGCGGATCCTGCAGCTCGAG-3
(primer 1) and
5
-TGCTCTAGAAGCTTGTCGAC-3
(primer 2) as the forward and reverse
primers, respectively. In a separate selection, the oligomer
5
-CGCGGATCCTGCAGCTCGAGN7N7GTCGACAAGCTTCTAGAGCA-3
was used as template. The PCR amplification was carried out using 20 pmol of oligomer, 100 pmol of 32P-end-labeled forward
primer, and 100 pmol of reverse primer for three cycles under the
following conditions: 1 min at 94 °C, 1 min at 55 °C, and 1 min
at 72 °C for each cycle. The double-stranded mixed DNA fragments
generated were purified and incubated with in vitro
synthesized RTR, and complexes were analyzed by EMSA. A band
corresponding to the RTR-RTRE complex was excised, and the DNA was
eluted in TE buffer. Recovered DNA was amplified by PCR for 15 cycles
and used for EMSA analysis with RTR under the conditions described
above. This procedure was repeated four times. In the fifth round, PCR
products were cloned into TA vector (Invitrogen). The inserts from
individual white colonies were amplified and used as competitors in
EMSA. DNA that competed in EMSA effectively was subjected to sequence
analysis. The sequences of 37 independent clones were analyzed.
CV-1 cells were plated in 6-well
dishes containing minimal essential medium and 10% fetal bovine serum.
The next day, cells were transfected in Opti-MEM (Life Technologies,
Inc.) with the expression vector pSG5-VP16-RTR or pZeoSV-RTR and
pBL-tk-CAT reporter DNA under the control of various response elements
using lipofectamine (Life Technologies) as described (19, 23). The
plasmid -actin-LUC was used as an internal control to correct for
differences in transfection efficiency. The reporter constructs used
were (conRTRE)3CAT, (1/2-RTRE)3-CAT,
(P2-RE)3-CAT, and (conRTRE)1-CAT. Cells were collected 48 h after transfection, and CAT activity was determined with an enzyme-linked immunosorbent assay kit (Boehringer Mannheim) following the manufacturer's instructions.
The amino acid sequence in the C-terminal region of the first zinc finger of nuclear receptors is key in classifying their DNA binding specificity (11, 12, 14). In RTR, this sequence is CEGCKG, suggesting that RTR, like the retinoid, thyroid hormone, vitamin D, and peroxisome proliferator activating receptors, recognizes response elements containing the consensus core motif AGGTCA (2, 14, 16). On this basis, we screened a series of well characterized natural and synthetic response elements that contain the consensus core motif RGGTCA configured either as a direct repeat, palindrome, or inverted palindrome spaced by different numbers of nucleotides for RTR binding. These results showed that RTR was able to bind to a DR0 sequence (not shown).
To determine in an unbiased manner the consensus sequence of the
response element that binds RTR with high affinity (RTRE), we used a
DNA-binding site selection strategy that is based on a combination of
PCR and EMSA. A mixture of 70-bp oligonucleotides containing 30 random
nucleotides flanked by two 20-bp primers was used as template (see
"Experimental Procedures"; Fig. 1A). After four rounds of selection with in vitro synthesized RTR
protein, a strong radiolabeled band consisting of RTR-oligonucleotide
complexes was observed in EMSA. The PCR products generated after the
fourth selection were cloned into the TA vector, and the sequence of 37 independent clones was analyzed as described under "Experimental Procedures" (Fig. 1A). From these sequences the percentage
of A, G, T, and C at each position was calculated, and the RTRE
consensus sequence was derived (Fig. 1B). These data
demonstrated that RTR bound most effectively to a DR0 consisting of a
direct repeat of the core motif AG(G/T)TCA. In addition, these results
showed that a DR0 preceded by TCA (9 to
7 position) was preferred
for high affinity binding, indicating that the 5
-flanking region rather than the 3
-flanking region is important for optimal binding of
RTR. The
10 position did not reveal a strong requirement for a
particular nucleotide. In a second DNA binding site selection experiment, a mixture of 65-bp oligomers consisting of
5
-CGCGGATCCTGCAGCTCGAGN7N7GTCGACAAGCTTCTAGAGCA-3
was used as template. This experiment yielded the consensus
sequence NNTCAA and confirmed
that TCA in positions
9 to
7 is necessary for optimal binding of
RTR.
Based on the sequence analysis above, we synthesized the consensus RTR
response element GGAATCTAAT (conRTRE) (Fig.
2A) and analyzed its binding to RTR in EMSAs.
As shown in Fig. 2B, in vitro translated RTR
formed a complex with conRTRE, while increasing concentrations of the
unlabeled conRTRE competed effectively for the binding. Since conRTRE
consists of two overlapping TCAAGGTCA motifs, we next determined
whether RTR was able to bind to a single core motif (M3-RTRE)
consisting of the conRTRE in which the upstream half-site was destroyed
by mutating three different nucleotides (Fig. 2A). Although
this single core motif RTRE did compete with 32P-conRTRE
for RTR binding, it was not as effective as the conRTRE (Fig.
2B). These observations suggest that RTR can bind well to an
RTRE containing one core motif but does so with a lower affinity than
to an RTRE consisting of a DR0. A DR1-RTRE, identical to conRTRE except
that the two core motifs are separated by one nucleotide and a P0-RTRE
containing a palindromic repeat of the core motif (Fig. 2A),
also did not compete as well for binding as the conRTRE (Fig.
2B). It is likely that the binding of RTR to DR1-RTRE and P0-RTRE takes place through interaction with the single motif TCAAGGTCA
present in these elements. The apparent lower affinity of M3-RTRE,
DR1-RTRE, and P0-RTRE for RTR is in agreement with the results obtained
in Fig. 1, demonstrating that a DR0 is most optimal for RTR binding. In
addition, these results suggest that RTR very likely binds to DR0 as a
homodimer and as a monomer to the single motif TCAAGGTCA.
The hypothesis that RTR binds as a homodimer to conRTRE was further
investigated by analyzing the binding of the full-length RTR and
RTR406, a truncated form of RTR. EMSA with either RTR or
RTR406 yielded one single shifted band that migrated at
different positions. We then performed EMSA using a mixture of RTR and
RTR406. As can be observed in Fig. 3, EMSA
analysis of the binding of RTR and RTR406 to
32P-conRTRE yielded three shifted bands that would be
consistent with the formation of an RTR homodimer, an
RTR-RTR406 heterodimer, and an RTR406
homodimer. When EMSA was carried out with 32P-M3-RE, which
binds RTR or RTR406 as a monomer, only two shifted bands
were observed. These findings support the interpretation that RTR binds
to conRTRE as a homodimer.
Mutational Analysis of RTRE
As shown in Fig. 2B,
RTR is able to bind to a single core motif (1/2-RTRE). To define more
precisely the requirements for high affinity RTR binding, the effect of
several mutations in the 5-flanking sequence of 1/2-RTRE on the
ability of these response elements to compete with
32P-1/2-RTRE for RTR binding was examined (Fig.
4A). As shown in Fig. 4B, when one
or both of the adenosines at the
6 and
7 positions (H-M2, H-M3, and
H-M4) were replaced by cytidine, the ability of the oligonucleotides to
compete with 32P-1/2-RTRE for RTR binding was greatly
diminished (lanes 9-11, 12-14, and
15-17). A single mutation of the C into A at position
8
(H-M8) also reduced dramatically the ability to compete for RTR binding
(Fig. 4C, lanes 31-33). A single mutation of the
T into C (H-M5) or G (H-M7) at position
9 slightly reduced the ability to compete with 32P-1/2-RTR (Fig. 4, B,
lanes 18-20, and C, lanes 28-30,
respectively). The latter is in agreement with the lower requirement
for a specific nucleotide at this position as shown in Fig. 1, although
T appears to be the preferred nucleotide. Mutations at the positions
10,
11, and +1 (H-M1 and H-M6, respectively) did not significantly alter the affinity for RTR (lanes 6-8 and
21-23). Several mutations (H-M10 through H-M13) within the
core motif of the 1/2-RTRE had a dramatic effect on the binding of RTR
(Fig. 4D, lanes 41-52). The oligonucleotide
H-M9, which contains T instead of G at position
4, competed equally
well for binding as the 1/2-RTRE in agreement with obtained consensus
sequence (Fig. 1).
Analysis of the binding specificity of RTR to RTREs containing a single core motif (1/2-RTRE). A, H-M1-H-M13, sequences of 1/2-RTRE and mutated 1/2-RTREs. Arrows indicate the site of mutation. B-D, EMSA analysis of in vitro translated RTR using 32P-1/2-RTRE as a probe and unlabeled 1/2-RTRE and mutated 1/2-RTREs as competitors. Unlabeled oligonucleotides were used at 5-, 25-, and 100-fold excess (lanes 3, 4, and 5, etc., respectively). Lane 1, unprogrammed lysate was used. Lane 2, EMSA without competitors.
Supershift with RTR-specific Antiserum
An antibody, RTR-Ab2,
was raised against a specific RTR peptide and tested for its ability to
bind RTR. This antiserum, but not the preimmune serum, could
immunoprecipitate in vitro translated, 35S-labeled RTR, which migrated as a protein with a
molecular mass of about 58 kDa (Fig. 5A,
lanes 2 and 3). The addition of the immunizing
peptide blocked the immunoprecipitation of RTR (Fig. 5A,
lanes 4 and 5). These results demonstrate that
this antiserum is able to recognize RTR protein specifically. The
RTR-Ab2 antiserum was then used in a supershift assay. As shown in Fig.
5B, the RTR-RTRE complex migrates more slowly when the
reaction is carried out in the presence of RTR-Ab2 (lane 2),
whereas no change in migration was observed in the presence of
preimmune serum (lane 3). The presence of increasing amounts
of the immunizing peptide inhibited the supershift of the RTR-RTRE
complex (lanes 4 and 5).
Identification of an RTRE in the Promoter Regulatory Region of the Protamine 2 Gene
Previous observations demonstrated that RTR
mRNA is highly expressed in testis and was associated with a
specific stage of germ cells differentiation (27, 28). The highest
level of GCNF/RTR mRNA was detected in late round spermatids
(27).2 The association of GCNF/RTR mRNA
expression with late round spermatids suggests that GCNF/RTR regulates
gene expression during maturation of round spermatids to elongated
spermatids. This stage of differentiation is associated with many
changes in gene expression (29, 36). We, therefore, examined the
5-flanking promoter regions of several genes, including protamine 2, reported to be induced at this stage of differentiation for the
presence of RTREs. Based on the sequence of the conRTRE, we identified
a putative RTRE in the regulatory region of the mouse protamine 2 gene
between nucleotides
213 and
228 (30) (Fig.
6A). This RTRE, referred to as P2-RE,
consists of a DR0 and is almost identical to the conRTRE (Fig.
6A). Northern analysis using poly(A)+ RNA
isolated from mouse round spermatids confirmed that the RTR and
protamine 2 genes are highly expressed at this stage of spermatogenesis (not shown).
The putative P2-RE was synthesized and analyzed for RTR binding in EMSA. As shown in Fig. 6B, P2-RE could compete with 32P-conRTRE for RTR binding almost as well as the unlabeled conRTRE itself. We then performed EMSA with 32P-P2-RE in the absence or presence of a 5-, 25-, and 100-fold excess of unlabeled P2-RE or P2-mRE, which is mutated at two positions (Fig. 6A). These experiments demonstrated that 32P-P2-RE forms a complex with RTR (Fig. 6C, lane 2) and that unlabeled P2-RE competed effectively with this binding (lanes 3-5), while P2-mRE had a greatly reduced ability to compete with 32P-P2-RE for RTR binding (Fig. 6C, lanes 6-8). These results show that the P2-RE present in the promoter regulatory region of the protamine 2 gene can function as a high affinity binding site for RTR and as such could play a role in the regulation of the expression of this gene.
Specific RTRE Binding of RTR in Testis Nuclear ExtractsSince
RTR is highly expressed in testis, we examined whether proteins in
nuclear extracts from testis were able to bind to the P2-RE. When
testis nuclear extracts were incubated with 32P-P2-RE and
analyzed by EMSA, multiple DNA-protein complexes were observed (Fig.
7). This is not surprising, since a number of other nuclear receptors, including SF-1/FTZ-F1 (24-26), are expressed in the
testis and able to bind to the same response element. For example,
SF-1/FTZ-F1 has been reported to bind TCAAGGTCA (24), which is
contained in the P2-RE. We used the RTR-Ab2 antiserum to determine if
any of these DNA-protein complexes contained the RTR receptor and were
supershifted by the antiserum. As shown in Fig. 7A, the
RTR-Ab2 caused a supershift of one of the DNA-protein complexes
(lane 2). To determine the specificity of the interaction of
nuclear proteins with 32P-P2-RE, we analyzed nuclear
extracts from mouse liver, kidney, and brain that do not express
detectable levels of RTR (27, 28) in a supershift assay. In contrast to
nuclear extracts from testis, no supershift was observed with nuclear
extracts from brain (lanes 3 and 4), kidney
(lanes 5 and 6), and liver (lanes 7 and 8) even at longer exposures (not shown). These findings show that the presence of RTR demonstrated by the RTR-Ab2-induced supershift in EMSA correlates well with the tissue-specific expression pattern of RTR.
The expression of RTR is developmentally regulated.3 RTR is induced when germ cells differentiate into spermatids, a process that in mice is initiated at day 22 after birth. We therefore compared the binding of proteins of nuclear extracts isolated from testes of juvenile (17-day-old) and adult (3-month-old) mice in a supershift assay (Fig. 7B). In contrast to the nuclear extracts from testes of adult mice, no supershift was observed with RTR-Ab2 using nuclear extracts from testes of juvenile mice. These results are in agreement with the observed development-dependent expression of RTR.
The specificity of the supershift was further examined with preimmune serum and competition with unlabeled P2-RE (Fig. 7C). No supershift was noted with the preimmune serum (lane 3). The DNA-protein interactions and supershift were effectively displaced by a 100-fold excess of unlabeled P2-RE, which competed for the binding of almost all the bound protein and abolished the supershift (lanes 4 and 5).
Transcriptional Activation through RTRE and RTRTo examine
the transactivation activity of RTR, an RTR expression plasmid and
(conRTRE)3-CAT reporter DNA were cotransfected into CV-1
cells and assayed for CAT activity. No increase in CAT reporter gene
activity could be measured compared with cells transfected with
(conRTRE)3-CAT reporter DNA alone (Fig.
8A). One of the reasons for the lack of
transcriptional activity may be the absence of the proper ligand
required for the activation of RTR. To study the interaction of RTR
with RTRE in cells, we examined the ability of VP16-RTR, which encodes
a fusion protein consisting of the activating domain of VP16 and the
full coding region of RTR, to cause transactivation of CAT through
(conRTRE)3. As shown in Fig. 8A, pSG5-VP16-RTR
enhanced CAT activity about 5-fold, while no activation was observed in
cells cotransfected with pSG5-VP16. We then analyzed the
transactivation through several other response elements.
Transactivation through (P2-RE)3 was just as effective as
that through (conRTRE)3, while transactivation through
(1/2-RTRE)3 and (conRTRE)1 was greatly
diminished (Fig. 8B). The latter may be due to the
possibility that their orientation is not the most optimal for
promoting interactions with the protein TFIID complex at the TATA-box.
These observations support the hypothesis that binding of RTR to RTREs,
including conRTRE and P2-RE, can mediate changes in transcriptional
activation of target genes.
Initial studies have indicated that GCNF/RTR can bind effectively
to a response element consisting of a direct repeat of the core motif
RGGTCA (28).4 In this study, we
characterized in an unbiased manner the consensus sequence of the RTRE
by an EMSA/PCR-based strategy that selects for oligonucleotides with
highest affinity for RTR from a pool of degenerate oligonucleotides
(primer 1-N30-primer 2). The conRTRE, TCAAGG(T/T)CAAG(G/T)TCA, calculated from 37 independent sequences revealed the following. 1) RTR binds with highest affinity to a direct
repeat containing the core motif AG(G/T)TCA. 2) There is a strong
requirement for specific nucleotides in the 5-flanking region for
optimal RTR binding, while no strong requirements were revealed for the
3
-flanking region. 3) A single core motif, AGGTCA, preceded by TCA
(defined as 1/2-RTRE) is sufficient for RTR binding; however, the
apparent affinity of RTR for 1/2-RTRE is significantly lower than that
for conRTRE. It is likely that the moderate competition for RTR binding
displayed by DR1-RTRE and P0-RTRE occurs through the 1/2-RTRE present
in these response elements. Mutational analysis of the 5
-flanking
region of 1/2-RTRE supported the importance of A nucleotides at the
6
and
7 positions, a C at position
8, and to a lesser degree T at the
9 position for optimal binding of RTR and confirms that the sequence
TCAAGGTCA is necessary for optimal binding of RTR. The observed
differences in the affinity of RTR to conRTRE and 1/2-RTRE are
consistent with the hypothesis that RTR binds as a homodimer to conRTRE
and as a monomer to 1/2-RTRE. EMSA analysis using the full-length RTR
and a truncated form of RTR supports this concept. As expected for
dimer formation, this EMSA with 32P-conRTRE yielded three
shifted bands representing the homodimers (RTR)2 and
(RTR406)2 and the heterodimer
RTR-RTR406. These results support the preliminary
observations on the GCNF/RTR homodimerization (28).
Several other members of the steroid hormone superfamily have been
reported to bind as monomers to single core motifs (21, 24-26).
Members of the ROR/RZR family have been shown to bind to a single core
motif flanked at the 5-end by a 6-bp-long A/T-rich sequence (21, 23).
The sequence of the consensus half-site RTRE is identical to that
reported for the SF-1/FTZ-F1 receptor (24-26). Interestingly,
comparison of the sequences in the T- and A-box of RTR with those of
SF-1/FTZ (FTZ-F1 box) shows the presence of a highly homologous region
in which 8 of 11 amino acids are identical (26, 27). Since the FTZ-F1
box has been implicated in the recognition of the SF-1/FTZ response
element (26), this similarity is consistent with observations showing
that RTR and SF-1 bind to very similar response elements.
To determine the ability of RTR to transactivate a CAT reporter gene that is under the regulation of an RTRE, CV-1 cells were transfected with an RTR expression plasmid and (RTRE)3-CAT. No transactivation of the CAT reporter gene was observed. However, the fusion protein RTR-VP16, consisting of the full-length RTR fused to the 80-amino acid-long activating domain of VP16 was able to cause RTRE-dependent transactivation of CAT. These results demonstrate that the fusion protein was able to bind to the RTRE through RTR and to cause transactivation of the reporter via the activation domain of VP16. The lack of transactivation by RTR could be attributed to several factors. First, the activation of RTR may require the presence of its ligand, which has not yet been identified. Alternatively, the presence of a ligand in the serum could repress the transactivation activity of RTR. We have found that neither the addition of lipid extracts isolated from testes nor the presence of delipidized serum had any effect on the ability of RTR to cause RTRE-dependent transactivation.2 Second, transactivation by nuclear receptors is mediated through interactions with other nuclear proteins that function either as repressors or as co-activators (18, 37). Some of these interactions may exhibit a high specificity. It is therefore possible that RTR needs a co-activator that is highly testis-specific and absent in CV-1 cells. Another possibility is that RTR functions as a negative transcriptional regulator.
Previous studies reported that the orphan receptor GCNF/RTR is
predominantly expressed in the testis and induced during a specific
stage of spermatogenesis (27, 28). The highest level of GCNF/RTR
mRNA was detected in late round spermatids (27),2
suggesting that GCNF/RTR regulates gene expression during maturation of
round spermatids to elongated spermatids. This stage of differentiation is accompanied by an induction in the expression of many genes, including protamine 1 and 2, c-abl, transition nuclear
proteins 1 and 2, c-pim-1, hsp-70.1 and histone
2b (29, 36). To identify candidate target genes that are regulated by
RTR, the reported promoter regions of several of these genes were
examined for the presence of RTREs. The 5 promoter-flanking region of
the mouse protamine 2 gene (30) was found to contain the sequence
AGAAGTTCAAGGTCAT, referred to as P2-RE, between nucleotides
213 and
228. This sequence is highly homologous to the conRTRE. We
demonstrate by EMSA analysis that RTR is able to bind to this element
almost as effectively as to the conRTRE. EMSA analysis of nuclear
extracts isolated from testis, brain, liver, and kidney demonstrated
that the RTR-specific antiserum was able to cause supershift of an RTR-DNA complex only with extracts from testis. We have
demonstrated3 that the expression of RTR is
initiated at day 17 after birth at a time that mouse germ cells
differentiate into spermatocytes and that it is highly expressed at day
22 when spermatids appear. The RTR-specific antiserum does not cause a
supershift with nuclear extracts from testes of juvenile (17-day-old)
mice in contrast to nuclear extracts from testes of adult (3-month-old)
mice. These findings indicate that the observed supershift by the
RTR-specific antibody correlates well with the tissue-specific and
development-dependent expression of RTR. Comparison of this
sequence between the mouse and rat protamine 2 gene shows that the
mouse gene contains two core motifs (P2-RE) but that the rat gene
contains only a single core motif. It is interesting to note that
protamine 2 levels in rat testis have been reported to be only 5% of
that in mouse (38). One could speculate that this may be related at
least in part to the differences in the affinity of conRTRE and
1/2-RTRE for RTR.
In summary, in this study we provide evidence consistent with the
conclusion that the orphan receptor GCNF/RTR is able to bind as a
homodimer to a DR0 and as a monomer to a 1/2-RTRE. The sequence
preceding the core motif is important for determining the affinity of
GCNF/RTR to RTREs and consists of the consensus TCA. GCNF/RTR is unable
to cause transactivation of a reporter gene through the conRTRE in CV-1
cells. This may be due to either the lack of its ligand or a proper
co-activator. Alternatively, GCNF/RTR may function as a negative
regulator of transcription. An RTRE was identified in the 5
promoter-flanking region of the protamine 2 gene that is induced during
the same stage of spermatogenesis as GCNF/RTR. This gene may belong to
a subset of genes that are regulated by GCNF/RTR. Studies are in
progress to obtain further evidence in support of this hypothesis.
We thank Drs. T. Teng and B. Harvat for comments on the manuscript.