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Cutting Edge |
Department of Pathology and Immunology, Howard Hughes Medical Institute, Washington University School of Medicine, St. Louis, MO 63110
| Abstract |
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| Introduction |
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Polarization of CD4+ T cells into distinct
cytokine-secreting subsets is controlled by various transcription
factors, with GATA-3 inducing Th2 cytokines and inhibiting Th1
cytokines (8). In naive CD4+ T
cells, the TCR and JAK/STAT signaling pathways influence GATA-3
expression (9). Initially low GATA-3 expression is
augmented on T cell activation by IL-4 and decreased by IFN-
and
IL-12. Ectopic GATA-3 expression induced full Th2 development even in
Stat6-/- T cells (10). Thus,
GATA-3 expression is also critical for commitment to the Th2 phenotype
of CD4+ Th cells.
GATA-3 is the only GATA factor expressed in T cells. Yet ectopic expression of GATA-1, -2, and -4 into naive CD4+ T cells also induced Th2 development in Stat6-deficient T cells (11) but worked by inducing endogenous GATA-3 (10, 11). Thus, T cells appear to regulate GATA-3 expression both by an IL-4/Stat6-dependent pathway and by a separate GATA-dependent pathway, suggesting two distinct patterns of transcriptional regulation for GATA-3. The GATA family gene organization is highly conserved (12, 13). Murine GATA-1 (14), mouse and human GATA-2 (13, 15), chicken GATA-5 (16, 17), and GATA-6 (18) each contains two distinct promoters and alternate first exons that independently regulate gene expression in distinct tissues or cell lineages.
This study describes two modes of GATA-3 expression in T cells, an IL-4-dependent mode operating in naive CD4+ T cells and an IL-4-independent mode operating in Th2 effector cells. Despite the lack of reported alternative promoters described for GATA-3 (1, 12, 19), we asked whether alternative promoter usage could underlie these dual modes of GATA-3 expression. In this report, we describe the identification of an alternative GATA-3 promoter and first exon that is conserved between human and mouse genomic sequences. We find that these two promoters are used differentially between brain and thymus and between naive and fully differentiated Th2 cells. Therefore, this report establishes an important physical basis for differential regulation of GATA-3 expression in distinct tissues and during distinct phases of Th2 development.
| Materials and Methods |
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DO11.10 
TCR-transgenic mice (9, 20), murine
rIL-4, and rIL-12 (21) have been described. Anti-CD28
(37.51) and CD3 (500A2) were gifts from Dr. J. P. Allison
(Berkeley, CA) and were purified by affinity chromatography from
culture supernatant or used as ascites.
T cell differentiation and restimulation
Sorted
CD4+Mel-14high DO11.10 T
cells were stimulated with 0.3 µM OVA, under Th1 or Th2 conditions as
described (10). The Th0 condition refers to the priming of
T cells with Ag/APCs in the presence of neutralization Abs against IL-
4 (11B11) (22), IL-12 (Tosh) (23), and
IFN-
(H22) (24). Naive or differentiated T cells were
reactivated with plate-bound anti-CD3 (1/1000) and anti-CD28 (1
µg/ml) for 48 h, and RNA was prepared.
RNA, Northern blots
GATA-3 Northern analysis was done as described (9). RNA tissues from thymus and brain were from Ambion (Austin, TX).
5'-RACE and RT-PCR
Total RNA was prepared from Th0 or Th2 effector cells 48 h
after stimulation with Ag/APC. 5'-RACE was conducted using the SMART
RACE kit (Clontech Laboratories, Palo Alto, CA). GATA-3 primers were
against the murine GATA-3 exon 2 (1): primer RT,
CGATGTTAAAAAGTACGTCCACCTCT, for the reverse transcription reaction;
primer b, CTTTGCGGGATAGTTTAGCAA, for the PCR amplification reaction
(Fig. 3
A). PCR fragments were cloned into pGEM-T Easy vector
(Promega, Madison, WI) and screened by Southern blot with primer b to
identify exon 2. We excluded exon 1 using an exon 1-specific probe,
generated by PCR using the following primer pair: GPE1,
GAGTAGCAAGGAGCGTAGAGGAGG (1); and primer c,
GAACACTGAGCTGCCTGGCGCCGT, an exon 1-specific antisense primer. Clones
hybridizing to the exon 2 but not to the exon 1 probe were
sequenced.
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For GATA-3 exon 1a, the primer pairs were primer a,
CATCAGCCAGGTTTTACC, and exon 2 antisense primer b (above). For
GATA-3 exon 1b, primer c and primer b above were used. PCR conditions
were denaturation at 94°C for 4 min, followed by 2035 cycles at
94°C for 20 s, 60°C for 20 s, and 72°C for 2 min, with
final elongation at 72°C for 4 min using the Taq
polymerase (Promega). PCR products were analyzed by Southern blot with
32P-end-labeled oligonucleotides: for GATA-3 exon
1a, primer 1a-AS, CAGCTGGTTTGGTTTTTGTTTTTTT; for GATA-3 exon 1b, primer
1b-AS, GAGTAGCAAGGAGCGTAGAGGAGGA (Fig. 3
A).
For comparisons of human and murine genomic sequences, we used the Celera Discovery System (San Francisco, CA).
| Results and Discussion |
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In contrast, T cells previously exposed to IL-4 such as Th2 cells
exhibited a distinct pattern of GATA-3 inducibility (Fig. 1
A, lanes 6 and 7). In resting Th2
cells, GATA-3 mRNA was expressed and was inducible by TCR. Further,
addition of IL-4 during TCR signaling did not augment GATA-3 induction.
Finally, IL-4 treatment of resting Th2 cells did not induce GATA-3
expression (Fig. 1
B). Therefore, in differentiated Th2
cells, TCR signaling induces GATA-3 expression without an apparent
requirement for IL-4. In summary, naive T cells clearly require both
TCR and IL-4 signaling to induce GATA-3 expression, but differentiated
Th2 cells can induce GATA-3 in response to TCR signaling alone.
Because several GATA family members have two alternative promoters that
provide distinct patterns of gene expression, we were interested in
directly testing for the existence of an alternative promoter and first
exon for GATA-3. Not knowing which tissues or conditions would use such
a transcript, we obtained RNA from T cells activated under a variety of
conditions including TCR-stimulated effector Th2 cells and TCR plus
IL-4-stimulated Th0 cells. 5' RACE analysis was done with reverse
transcription of mRNA primed either with oligo(dT) or exon 2-specific
primers. We identified two classes of 5' RACE products. The first
represented the published GATA-3 transcript using the recognized GATA-3
promoter and first exon (1). A second product class
contained 3'-GATA-3 exon 2 sequences but had a unique 5' 195-nucleotide
sequence, similar in size to other recognized first exons of GATA
factors. We used a bacterial artificial chromosome-containing murine
GATA-3 gene as a genomic template for PCR analysis using
primers specific for this novel sequence. PCR with these primers
generated a specific 9.5-kb product (data not shown). Thus, we have
identified an putative novel exon for GATA-3 located
10 kb upstream
of the recognized exon 1. In this study, we refer to the previously
recognized exon 1 as exon 1b and to the new exon 1 as exon 1a.
We compared the murine and human genomic sequences (Fig. 2
). A blast search using the murine exon
1a (RACE) against the murine genome identified a region of 99%
homology on mouse chromosome 2 located 9.4 kb upstream of GATA-3 exon
2, consistent with our genomic PCR analysis. A blast search using the
murine exon 1a against the human genome identified a region of 96%
homology on human chromosome 10 positions located 9.4 kb upstream of
the human GATA-3 exon 2. Thus, exon 1a is conserved between human and
murine GATA-3 genes in a configuration similar to the
conserved upstream exons described for GATA-1
(14) and GATA-2 (13, 15) genes. We
also compared the genomic flanking sequences surrounding exon 1a for
both murine and human genomes. A conserved GT dinucleotide is found
immediately downstream of the human and murine transcribed exon 1a
sequences, suggesting a conserved splice donor site for exon 1a. Also,
the 200 nucleotides upstream of exon 1a are very highly conserved
between murine and human, particularly a GC box, an E-26-binding site,
and a nonconsensus GATA site which are perfectly conserved between the
human and murine genomes, suggesting potential regulatory elements
(Fig. 2
).
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Next we examined differential exon usage in the thymus and brain
tissues from adult mice. We found clear evidence for distinct exon
usage in these tissues. Whole murine thymus mRNA was weakly positive
for exon 1b but essentially negative for expression of exon 1a (Fig. 3
B, compare lane 1, upper and
middle panels; Fig. 3
D, lane 7,
compare upper and middle panels). In
contrast, whole brain mRNA was positive for exon 1a but negative for
detectable usage of exon 1b (Fig. 3
B, lane 2;
Fig. 3
D, lane 8). Thus, whereas in vitro-adapted
cell lines show no distinct alternative exon usage, whole tissues
clearly indicate a differential usage of GATA-3 exons 1a and 1b between
thymus and brain.
We also examined the potential for differential exon usage in T cells
at different stages of Th cell development. Naive
CD4+ T cells were primed in vitro with
anti-CD3 and anti-CD28 Abs in either the presence or absence of
IL-4 and analyzed at 48 h after activation (Fig. 3
D).
In the absence of IL-4, exon 1b transcripts were only weakly
detectable, and exon 1a transcripts were essentially undetectable (Fig. 3
D, lane 1), whereas in the presence of IL-4, transcripts
from both exon 1a and exon 1b were detected (Fig. 3
D,
lane 2). By comparison, fully differentiated Th2 cells
strongly expressed exons 1a and 1b. Transcripts from both exons were
detected in resting cells and were moderately induced on stimulation
through the TCR activation in either the presence or absence of IL-4
(Fig. 3
D, lanes 3 and 5). By contrast,
TCR-stimulated Th1 cells showed very weak usage of either transcript
(Fig. 3
C, lanes 6; Fig. 3
D, lane 6).
Finally whole thymus RNA showed selective usage of exon 1b, whereas
whole brain RNA showed selective usage of exon 1a. Interestingly, there
appeared to be a gradual shift is usage during the time course of Th2
development from naive CD4+ T cells from day 2 to
day 7 after primary activation (Fig. 3
C). Here, exon 1a,
which is absent in thymus, was weakly expressed at 2 days of Th2
development but gradually increased to a maximum at day 7. By contrast,
exon 1b, which is expressed in thymus, showed a peak at day 3 followed
by a gradual decrease over the same time period.
In summary, we report the existence of a novel alternative GATA-3 promoter/exon located 10 kb upstream of the recognized GATA-3 exon 2 that is conserved between mouse and human, bringing GATA-3 gene organization into a pattern recognized for several other GATA factors. Even though tissue culture adapted cell lines showed usage of both exons, examination of freshly isolated thymus and brain tissues indicated a distinct differential pattern of exon usage. A previous study attempted to identify GATA-3 transcripts by primer extension analysis of the murine BW5147 thymoma and MEL erythroleukemia but did not included nontransformed T cell or tissue (1). Interestingly, exon 1a is located near a previously identified DNase-hypersensitive site that is present in BW5147 but not in C1300 (26).
Important developmental roles for GATA-3 are recognized both in the sympathetic nervous system and in T cells, so that our finding of selective exon 1a expression in brain tissue and exon 1b in thymus fits into the paradigm of alternative promoter usage in distinct developmental contexts. In T cells, GATA-3 is also regulated by different pathways at distinct stages of Th2 development, suggesting a role for these two promoters here as well. It is important next to examine differential promoter usage in vivo during embryogenesis and during stages of Th2 development, which will likely require the generation of selective gene-targeting approaches to mark the usage of each promoter individually.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Kenneth M. Murphy, Department of Pathology and Immunology, Howard Hughes Medical Institute, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110. E-mail address: murphy{at}immunology.wustl.edu ![]()
Received for publication January 15, 2002. Accepted for publication February 27, 2002.
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