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Mol Cell Biol, May 1998, p. 2901-2911, Vol. 18, No. 5
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Distinct Functions Are Implicated for the GATA-4,
-5, and -6 Transcription Factors in the Regulation of
Intestine Epithelial Cell Differentiation
Xiaoping
Gao,1
Tiffany
Sedgwick,2
Yun-Bo
Shi,2 and
Todd
Evans1,*
Department of Development and Molecular
Biology, Albert Einstein College of Medicine, Bronx, New
York,1 and
Laboratory of Molecular
Embryology, National Institute of Child Health and Human
Development, National Institutes of Health, Bethesda,
Maryland2
Received 5 September 1997/Returned for modification 28 October
1997/Accepted 11 February 1998
 |
ABSTRACT |
Based on conserved expression patterns, three members of the GATA
family of transcriptional regulatory proteins, GATA-4, -5, and -6, are
thought to be involved in the regulation of cardiogenesis and gut
development. Functions for these factors are known in the heart, but
relatively little is understood regarding their possible roles in the
regulation of gut-specific gene expression. In this study, we analyze
the expression and function of GATA-4, -5, and -6 using three separate
but complementary vertebrate systems, and the results support a
function for these proteins in regulating the terminal-differentiation
program of intestinal epithelial cells. We show that xGATA-4, -5, and
-6 can stimulate directly activity of the promoter for the intestinal
fatty acid-binding protein (xIFABP) gene, which is a marker for
differentiated enterocytes. This is the first direct demonstration of a
target for GATA factors in the vertebrate intestinal epithelium.
Transactivation by xGATA-4, -5, and -6 is mediated at least in part by
a defined proximal IFABP promoter element. The expression patterns for
cGATA-4, -5, and -6 are markedly distinct along the proximal-distal
villus axis. Transcript levels for cGATA-4 increase along the axis
toward the villus tip; likewise, cGATA-5 transcripts are largely
restricted to the distal tip containing differentiated cells. In
contrast, the pattern of cGATA-6 transcripts is complementary to
cGATA-5, with highest levels detected in the region of proliferating
progenitor cells. Undifferentiated and proliferating human HT-29 cells
express hGATA-6 but not hGATA-4 or hGATA-5. Upon stimulation to
differentiate, the transcript levels for hGATA-5 increase, and this
occurs prior to increased transcription of the terminal differentiation
marker intestinal alkaline phosphatase. At the same time, hGATA-6
steady-state transcript levels decline appreciably. All of the data are
consistent with evolutionarily conserved but distinct roles for these
factors in regulating the differentiation program of intestinal
epithelium. Based on this data, we suggest that GATA-6 might function
primarily within the proliferating progenitor population, while GATA-4
and GATA-5 function during differentiation to activate
terminal-differentiation genes including IFABP.
 |
INTRODUCTION |
The intestinal epithelium provides
an excellent model system for investigating molecular mechanisms
regulating cell lineage establishment, stem cell proliferation,
morphogenesis, and the specialization of cell function during
terminal differentiation (see references 9 and
16 for reviews). In all vertebrates, the
embryonic intestinal lumen is lined by an endoderm-derived epithelial sheet, a monolayer consisting of four principal cell types
that are renewed from a proliferating stem cell population. Lineage
tracing experiments (7, 39) demonstrated that the four cell
types are derived from a small population of multipotential stem cells
present near the villus base (the crypt). The differentiating cells
migrate from the crypt toward the villus tip, where they eventually die
and are extruded into the lumen. Within the epithelium, absorptive
enterocytes are the predominant cell type, and they form a highly
organized apical brush border. In addition, goblet cells secrete mucus
through exported granules, and enteroendocrine cells secrete various
hormones and growth factors. The fourth cell type, the
lysozyme-producing Paneth cell, differentiates during migration toward
the base of the crypt. Although cell position along the polar crypt-tip
axis is clearly an important determinant, the mechanisms that regulate
the decision to differentiate (and the choice of pathway) are unknown.
Regulation of gut-specific differentiation programs is likely to
involve lineage-restricted transcription factors that control expression of terminal-differentiation genes. Experiments using transgenic mice and cell culture transfection have analyzed the promoters of gut-specific genes in order to identify regulatory elements that mediate lineage, temporal, and spatial control (see reference 52 for a review). In a few cases,
potential transcription factors have been identified that can interact
with defined regulatory elements. For example, the region including
nucleotides
103 to +28 of the murine intestinal fatty acid-binding
protein (IFABP) gene directs proper lineage-specific expression; a
repeated and conserved element in this region binds two members of the
steroid hormone receptor superfamily, HNF-4 and ARP-1 (38).
Other transcription factors implicated in the regulation of
differentiated epithelia include HNF-1 (5), HNF-3
(8), COUP-TF (31), and Cdx-2 (48).
Genetic studies with Caenorhabditis elegans and
Drosophila identified several additional genes that are
likely to have conserved functions in vertebrate gut development
(reviewed in reference 44).
GATA factors comprise a small family of transcriptional regulatory
proteins defined by a highly conserved DNA-binding domain that
interacts specifically with DNA cis elements containing a consensus WGATAR or related sequence. Six distinct vertebrate GATA
factors have been characterized, grouped into two subfamilies based on
structural and expression comparisons. The GATA-1, -2, and -3 genes
each function in the hematopoietic system (35). Genetic
studies defined a unique role for each in regulating the differentiation state of specific blood lineages (43). The
GATA-4, -5, and -6 genes are expressed with overlapping patterns in the developing cardiovascular system and in endoderm-derived tissues including the liver, lungs, pancreas, and gut (12). Several candidate target genes for GATA-4, -5, and -6 have been identified in
cardiomyocytes, and data from several experiments indicate a functional
role in regulating cardiac differentiation (17, 19, 24).
Disruption of GATA-4 function during embryogenesis demonstrated an
additional role regulating early morphogenesis critical for normal
heart development, although the relevant downstream target genes are
not known (26, 32). In addition, GATA-4 is required for the
development of visceral endoderm in embryonic stem cell culture
(46). In contrast, the function of the GATA-4, -5, and -6 genes in the differentiating gut is not known and is largely untested.
A possible role for GATA factors in regulating gut-specific gene
expression was proposed based on the expression of GATA-4 and GATA-5 in
the embryonic gut and the adult intestinal epithelium (2, 25,
27). Likewise, GATA-4 and GATA-6 are expressed in gastric
epithelium (51), and a few likely target genes for GATA
factors in the differentiated stomach have been proposed. A sequence
capable of binding GATA factors is present in the promoters of the
human and rat
- and
-subunit genes encoding the
H+/K+-ATPase (30, 33); this proton
pump is expressed in stomach parietal cells that also express GATA-4
and GATA-6 (GATA-5 was not analyzed). Similarly, a GATA-binding site is
present in the promoter of the rat histidine decarboxylase gene that is
expressed specifically in gastric endocrine cells containing abundant
GATA-6 transcripts (10). However, these and other putative
binding sites have not yet been shown to be functional regulatory
elements. Furthermore, candidate targets for GATA factors in intestinal epithelium have not been identified. To address the function and conservation of GATA factors in the gut, we studied the activity of
GATA-4, -5, and -6 on a putative downstream target promoter, and the
expression patterns for the GATA-4, -5, and -6 genes in chick
intestine, and in a human model cell culture system. The data support a
hypothesis that these genes have distinct functions in progenitor and
differentiated cell compartments of the intestinal epithelium. We
identify the gene encoding Xenopus IFABP as a downstream target for GATA factors in differentiating intestinal enterocytes. Therefore, GATA factors are likely to have distinct functions in the
regulation of the IFABP and other epithelium-specific genes.
 |
MATERIALS AND METHODS |
In situ hybridization.
Intestines were dissected from adult
Xenopus females or 1-month-old chicks, washed thoroughly in
phosphate-buffered saline (PBS), and fixed in a solution of 4%
formaldehyde-2 mM EDTA-PBS at room temperature for 3 h.
Pretreatment and hybridization were carried out essentially as
described elsewhere (20) with some modifications. Small
pieces of intestine were treated with proteinase K for 30 min at room
temperature (100 µg/ml for Xenopus, 50 µg/ml for
chicks). Color development solutions included 10% polyvinyl alcohol
(4). The antisense RNA probes were generated by in vitro
transcription using bacteriophage polymerases. The cDNA templates used
to generate the probes were described previously and shown to be
specific in Northern blotting experiments (24, 27).
Hybridization solutions contained 0.5 or 0.2 µg/ml of
digoxigenin-labeled probes for Xenopus and chick samples,
respectively. Following development of the alkaline phosphatase
reaction, the embryos were refixed in 4% formaldehyde-0.1%
glutaraldehyde, dehydrated, embedded in paraplast, and sectioned (5 µm).
Isolation of the xIFABP genomic clone, characterization of the
genomic structure, and mapping of the transcriptional start site.
A lambda genomic library of Xenopus laevis DNA was screened
with the xIFABP cDNA probe, as described elsewhere (36, 42). Positive clones were isolated following secondary and tertiary screening. The intron-exon boundaries were determined by direct sequencing of the inserts in purified lambda DNA using end-labeled exon-specific primers (37). To determine intron sizes, a
lambda genomic clone encompassing the entire coding region and the 3' untranslated region was analyzed in detail. Restriction analysis showed
that the introns were small enough to be amplified by PCR. Therefore, 1 µg of the lambda DNA was used as a template in a 25-µl PCR mixture
using two primers (16 to 20 bp; 0.18 µg each) located in neighboring
exons. The amplified product therefore contains a defined sequence
derived from each of the two exons plus the intervening sequences. The
products were analyzed by agarose gel electrophoresis to determine the
sizes of the introns, by using known molecular size standards.
For primer extension experiments, 20 µg of total RNA isolated from
stage 62 tadpoles (metamorphic climax) or stage 66 frogs (postmetamorphic) was annealed with 1 ng of a
[
-32P]ATP-end-labeled primer in 10 µl of annealing
buffer (20 mM Tris [pH 8.3], 0.4 M KCl) by incubating the mixture
sequentially at 65°C for 10 min, 55°C for 25 min, and finally,
45°C for 10 min. Following annealing, the reaction mixture was
supplemented with 4 µl of 10× reverse transcription buffer (0.5 M
Tris [pH 8.3], 60 mM MgCl2), 1 µl of a deoxynucleoside
triphosphate mix (25 mM each nucleotide), 0.1 µl of an RNase
inhibitor (Gibco/BRL), 1 µl of dithiothreitol (0.1 M), 1 µl of
actinomycin D (1 mg/ml; Sigma), 0.2 µl of superscript II reverse
transcriptase (Gibco/BRL), and 23 µl of distilled water. The reaction
mixture was incubated at 42°C for 1.5 h. Extension products were
ethanol precipitated and analyzed by denaturing polyacrylamide
electrophoresis, alongside sequencing ladders generated with the same
primer and a genomic clone template containing the promoter region. The
sequence of the primer is 5' CCA TAA CTT CCA TGA ATT T (+116
to +98, relative to the transcription start site).
Transient transfection and transactivation assays.
The
xIFABP promoter was isolated by PCR using the genomic clone as a
template. The 7-kb XbaI fragment was first subcloned into
the pBluescript SK
vector. The two primers used for PCR were T7
(forward, complementary to the T7 promoter in the cloning vector,
upstream of the XbaI site) and TE356 (reverse, complementary to sequences from +34 to +50 of the xIFABP gene, relative to the transcriptional start site, and including an XbaI
restriction site). The PCR product was digested with XbaI,
and the 1,024-bp fragment was purified and subcloned into the
NheI site of the pGL3-Basic luciferase reporter plasmid
(Promega). The insert orientation was determined, and the entire
sequence was confirmed. The
233 mutant promoter (deleting several
upstream potential GATA-binding sites) was also generated by PCR, using
as primers FABP10 (forward, complementary to nucleotides
233 to
217) and TE356. The product was subcloned into pGL3-Basic as
described above. The pm reporter, containing a mutation of the proximal
GATA-binding site, was generated by site-directed mutagenesis
(QuickChange kit; Stratagene), using the full-length (wild-type [WT])
xIFABP promoter as a template, according to the manufacturer's
instructions. The mutagenic primers (TE452 and TE453) introduced by
this procedure a 2-bp change within the core of the GATA consensus (GA
to CT). The
233m reporter was generated similarly, except that the
233 reporter was used as a template for mutagenesis. Each of the
mutant promoter constructs was confirmed by manual sequencing.
For transactivation assays, the quail fibroblast cell line QT6 was
transfected essentially as described elsewhere (
14).
The
evening before transfection, 35-mm dishes were seeded with
2 × 10
5 cells. Each transfection mixture contained 3 µg of
luciferase
reporter plasmid, 1 µg of a cytomegalovirus-regulated
expression
plasmid (either control pCDNA3 vector [Invitrogen] or
pCDNA3 containing
a full-length xGATA-4, -5, or -6 cDNA), 0.25 µg of
the pCH110

-galactosidase expression plasmid (Pharmacia) as an
internal
control, and 10 µg of Lipofectamine (Gibco/BRL). Cells were
harvested
48 h following transfection, and luciferase activity was
measured
by using the luciferase assay system (Promega) as described by
the manufacturer, in a Turner TD-20e luminometer. The luciferase
activity was normalized to

-galactosidase and expressed relative
to
reporter activity in the control (pCDNA3) transfections. Each
transfection was performed in duplicate, and the data in Fig.
4 were
averaged from at least four independent experiments. Preliminary
experiments were performed to ensure that under these transfection
conditions the assay results are linear with respect to reporter
input
and that the expression of GATA factors is not limiting.
In some cases, transfected cells were harvested and used to prepare
nuclear extracts for gel mobility shift experiments, as
described
elsewhere (
56). The probe was an end-labeled double-stranded
oligomer generated by hybridizing TE492 and TE493, containing
the
sequence of the xIFABP promoter from

58 to

31 (indicated
in Fig.
2c), including the proximal consensus GATA-binding site.
In competition
experiments, unlabeled double-stranded oligomer
DNA was added (50-fold
molar excess). The competitor DNA either
was the same as the probe
(TE492/TE493), generated with the TE452/TE453
mutant oligomers, or
contained the sequence of the GATA-binding
site derived from the chick
D-globin promoter (TE72/TE73 [
14]).
The sequences of the primers used are as follows: TE356, 5' GCG
TCT AGA TGA TTG GTG GAG AGA (
XbaI site
underlined); FABP10,
5' ATA TGC CCT TCC TAA TG (

233 to

217); TE452, 5' GGA GAT CCC
TGT ACA
CTT ATG GGG AGA
C (mutation underlined); TE453, 5' GTC
TCC CCA TA
A
GTG TAC AGG GAT CTC C (mutation underlined); TE492,
5'
GGA GAT CCC TGT AC
A GAT ATG GGG AGA C (GATA consensus
underlined);
TE493, 5' GTC TCC CC
A TAT CTG TAC AGG GAT
CTC C (GATA consensus
underlined).
Cell culture and reverse transcription-PCR (RT-PCR).
The
human cell lines CaCo-2, HT-29, and SW1417 were obtained from the
American Type Culture Collection and maintained according to the
supplier's instructions. For HT-29 cells, sodium bicarbonate was added
to the medium (0.35 g/liter). Media were changed every day, and cells
were passaged to avoid confluence. To induce differentiation, sodium
butyrate was added (5 mM) after the cultures reached 80% confluence.
Cells were harvested at specific time points up to 48 h (medium
was changed at 24 h). In control cultures, PBS was added in place
of sodium butyrate and the cells were cultured for the same times
(usually an additional 48 h).
Cells were harvested as described elsewhere (
14), and total
RNA was prepared by using the SNAP RNA isolation kit (Invitrogen)
as
instructed by the manufacturer. For each sample, cDNA was prepared
in a
50-µl RT reaction mixture with 1.25 µg of total RNA, random
hexanucleotide primers, and Moloney murine leukemia virus reverse
transcriptase. Two microliters of this RT reaction mixture was
used
directly for semiquantitative PCR, essentially as described
elsewhere
(
57). Preliminary experiments confirmed that the reactions
were entirely dependent on the RT reaction and that product accumulates
linearly with respect to input RNA and cycle number. In some
experiments,
reaction mixtures contained trace
[

-
32P]dCTP; following gel electrophoresis in
nondenaturing polyacrylamide
gels, the products were analyzed by
autoradiography. Reaction
conditions were as follows: S14, 1.25 mM
MgCl
2 and 20 cycles;
hGATA-4, 1.0 mM MgCl
2 and
28 cycles; hGATA-5, 0.8 mM MgCl
2 and
28 cycles; hGATA-6,
1.25 mM MgCl
2 and 28 cycles; intestinal alkaline
phosphatase (IAP), 0.7 mM MgCl
2 and 22 cycles. The
following primers
were used for PCR: TE442 (5' GGC AGA CCG AGA TGA
ATC CTC A) and
TE443 (5' CAG GTC CAG GGG TCT TGG TCC)
for S14, TE444 (5' AAC
GGA AGC CCA AGA ACC TG) and
TE445 (5' TTG TTC TCA GAT CCT TCG
GTG C) for hGATA-4, TE454
(5' GAA CAG CCT GGA ACA GAC CA) and
TE451 (5' TCC CTC
ACC AGC CTT CTT GC) for hGATA-5, TE446 (5' AGG
CCA TTT GGT
ACA CAT CTC TG) and TE447 (5' TAA TGT AAA CCA ACC
TGC CTG
TG) for hGATA6, and TE464 (5'- GCC AGA CAG CGC AGC CAC
AGC)
and TE465 (5'
TGC ACC AGG TTC TTC CCG TCC AG) for IAP.
Northern blotting experiments were performed as described elsewhere
(
13) by using total RNA isolated from control or
butyrate-induced
cultures of HT-29 cells. For gel mobility shift
experiments, nuclear
extracts were prepared and DNA-binding assays were
performed as
described above. The antiserum used in gel mobility shift
experiments
was from rabbits immunized with three synthetic peptides
consistent
with the hGATA-6 coding sequence. These peptides include
sequences
from highly conserved regions of GATA factors and therefore
may
contain antibodies to GATA-4 and/or GATA-5 in addition to GATA-6.
 |
RESULTS |
The xIFABP promoter is likely a direct target for transcriptional
activation by GATA factors in the intestinal epithelium.
The cDNA
encoding Xenopus IFABP was isolated by a differential
hybridization screen used to identify targets of thyroid
hormone-mediated intestinal remodeling (40). The gene
product is intestine specific and provides a marker for absorptive
enterocytes of the epithelium (23), a pattern of
tissue-specific expression that is conserved in mammalian systems
(18, 50). Our previous data indicated that the xGATA-4, -5, and -6 genes are expressed in frog gut and that xGATA-5 transcripts are
localized within the stomach and intestine to cells of the epithelium
(24, 25). Therefore, we considered whether IFABP might be a
direct downstream target for regulation by GATA factors in the gut.
However, the expression patterns for xGATA factors were reported
previously for adult gut (24, 25), while xIFABP was studied
in tadpoles (23). We first confirmed the expression of
xIFABP in intestinal epithelium of adult frogs, using a whole-mount in
situ hybridization assay. Isolated adult frog intestine was fixed and
permeabilized prior to incubation with a digoxigenin-labeled antisense
RNA probe synthesized in vitro from the xIFABP cDNA template. Following
washing, and detection of the hybridized probe using an alkaline
phosphatase-conjugated antibody specific to digoxigenin, the tissue was
embedded and sectioned. As shown in Fig.
1a, the xIFABP gene is transcribed in the
gut and transcripts are localized to the differentiating epithelium
along the distal tip of the villus. No signal is detected in the crypt
region or in underlying smooth-muscle layers. The xIFABP transcripts
are first detected by this methodology at about 50% of the distance
from the crypt zone, and the levels increase along the axis to the
villus tip (Fig. 1b). No signal is present in control experiments using
the sense strand of the xIFABP cDNA as a probe (Fig. 1c). This
experiment confirms that transcription of the xIFABP gene is restricted
throughout development to differentiated intestinal epithelium and is
therefore a potential downstream target for GATA factors in the gut.

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FIG. 1.
The xIFABP gene is transcribed in adult
Xenopus gut specifically in the differentiating epithelium.
Xenopus intestine was analyzed by whole-mount in situ
hybridization. Sections of processed tissue are shown. (a) Villus
architecture, with smooth-muscle (SM) layers on the left and the villus
proximal-distal axis running left to right toward the tip (T). The dark
strain indicates the pattern of xIFABP transcripts following
development of the alkaline phosphatase reaction to detect the
hybridized antisense xIFABP RNA probe. (b) Higher-magnification view of
a villus section. Arrows in both panels indicate the positions along
the villus axis that transcripts are first detected in the epithelium.
The signal increases in intensity along the proximal-distal axis and is
strongest at the distal tip (T). (c) Section from similarly processed
tissue that was hybridized with a control sense-strand RNA probe.
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In order to identify upstream regulatory sequences controlling the
tissue-specific expression of the xIFABP gene, a genomic
clone was
isolated by screening a bacteriophage lambda library
with the xIFABP
cDNA insert as the probe. A 7-kb
XbaI fragment
containing
sequences for the entire coding region was isolated
and used to
characterize the structure of the xIFABP gene. The
gene is encoded by
four exons interrupted by three introns of
approximately 250, 1,800, and 620 bp (Fig.
2a). The intron
positions
are entirely conserved in
Xenopus (Fig.
2b), mouse
(
18), rat,
and humans (
50). The genomic clone
includes approximately 1
kb of sequence upstream of the ATG initiation
codon, and the primary
structure of this entire region was determined
(Fig.
2c). An oligonucleotide
primer complementary to nucleotides +49
to +67 (relative to the
ATG initiation codon) was used in primer
extension experiments
to determine the transcriptional start site of
the gene. As shown
in Fig.
3, a single
extension product is detected in RNA isolated
from stage 66 adult
intestine but not in RNA isolated from stage
62 tadpoles. Stage 62 is
the climax of metamorphosis, when the
xIFABP gene is not expressed due
to intestinal remodeling (
23,
41). The position of the
single start site maps 49 nucleotides
upstream of the translational
initiator ATG, consistent with the
size of the cDNA clone. PCR products
obtained with IFABP-specific
primers in a 5' rapid amplification of
cDNA ends procedure were
sequenced, and the ends were found to be
consistent with the primer
extension results (not shown). The
transcriptional start site
(+1) is indicated on the sequence shown in
Fig.
2c.

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FIG. 2.
Structure of the xIFABP gene and sequence of the
promoter region. (a) Genomic organization of exons along a 7-kb
XbaI fragment. The positions of BglII and
SacI restriction sites are also indicated. Solid boxes
represent the four exons, and the arrow and arrowhead indicate the
positions of the ATG translational initiation codon and stop codon,
respectively. The positions of the exons, placement of the introns, and
sizes of the introns separating the exons were determined by PCR and
direct sequencing. (b) Sequence encompassing the exon/intron
boundaries, relative to the xIFABP coding sequence. (c) Sequence of the
xIFABP gene from 969 (around the upstream XbaI site) to
+75, relative to the transcriptional start site (see Fig. 3). Small
brackets (three located between 360 and 280 and one located around
42) indicate several sequences consistent with the GATA consensus
binding sites. The larger bracket around 290 encompasses a conserved
sequence shown in previous experiments to be important for regulating
IFABP expression in transgenic mice (45). Note that the
deviated GATA site in this sequence (TGATG) is in the reverse
orientation. The vertical brace indicates the position at which
upstream sequences were deleted in the 233 reporter. The long
double-headed arrow indicates the sequence of the oligomer probe used
in the gel mobility shift experiments (Fig. 5 and 8), and the small
arrowheads point to the bases mutated in the pm reporter (Fig. 4) and
in the mutant oligomers used in competition experiments (Fig. 5 and 8).
The angled arrow indicates the transcriptional start site designated +1
(from the data in Fig. 3), and the solid triangle identifies the ATG
translational initiation codon.
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FIG. 3.
Primer extension analysis mapping of the xIFABP
transcriptional start site. RNA from stage (st.) 62 tadpoles or stage
66 adult frogs was analyzed with a specific labeled primer. The
reaction products were analyzed on a denaturing polyacrylamide gel
alongside sequencing ladders made with the same primer and the genomic
clone as a template. The arrow indicates the position of the single
extension product.
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The promoter region upstream of the xIFABP start site contains
potential regulatory elements that might restrict expression
of the
gene to differentiated intestinal epithelium. The consensus
binding
site for GATA factors is (A/T) GATA (A/G) (
15). Several
sequences consistent with this consensus, representing potential
binding sites for GATA factors, are present in the promoter region,
indicated by brackets over the 6-bp sites in Fig.
2c. A fragment
including all of the upstream sequences derived from the genomic
clone
(down to but not including the ATG initiation codon) was
isolated by
using specific primers in a PCR, and the resulting
product was fused in
frame with a luciferase reporter gene. The
recombinant reporter gene
therefore contains 969 bp of the xIFABP
promoter regulating expression
of the luciferase coding sequence
(the resulting RNA includes the
49-nucleotide xIFABP 5' untranslated
region but contains the ATG
initiation codon of the luciferase
gene). This reporter plasmid was
used to test directly the ability
of
Xenopus GATA factors to
transactivate the xIFABP promoter.
We used a QT6 fibroblast cell line
to express ectopically xGATA-4,
-5, and -6 using expression plasmids
regulated by a cytomegalovirus
promoter. This system was used
previously to analyze the ability
of GATA-1 to activate expression of a
target globin promoter (
14),
and to define functional
domains of GATA-1 (
55,
56), because
of the low levels or
absence of endogenous GATA factors.
The luciferase reporter regulated by the xIFABP promoter was
cotransfected into QT6 cells with the expression vector alone
or
containing an xGATA factor cDNA, and with a plasmid containing
the
lacZ gene as an internal control for transfection efficiency
and lysate recovery. After 2 days in culture, cell lysates were
harvested from transfected cells and luciferase activities were
measured relative to

-galactosidase enzyme levels. As shown in
Fig.
4A, each of the GATA-4, -5, and -6 factors is able to transactivate
the luciferase reporter containing the
xIFABP promoter, although
at different levels. Specifically, GATA-4 and
GATA-5 expression
activates the promoter between five- and ninefold.
GATA-6 is less
potent, resulting in only a two- to threefold activation
relative
to the basal expression level of the xIFABP promoter in the
QT6
cells. Expression of the luciferase reporter is entirely dependent
on the presence of the xIFABP promoter sequences (not shown).
Coexpression of the GATA factors in various combinations did not
further augment the activity of the luciferase reporter over that
of
GATA-4 alone (not shown). Because specific antibodies are not
available
to quantify ectopic expression levels, it is possible
that xGATA-6
binding activity does not accumulate at the same
level in the transient
transfection system, relative to xGATA-4
and -5. Analysis of binding
activities derived from transfected
cells is consistent with this
possibility (see Fig.
5). Nevertheless,
GATA factors known to be
expressed in gut epithelium are capable
of activating directly the
xIFABP promoter in a nonintestinal
cell type, in some cases nearly
10-fold.

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FIG. 4.
The xIFABP promoter is a direct target for
transactivation by GATA factors. The graphs show the relative activity
of a reporter gene regulated by the xIFABP promoter. (A) The basal
activity of the full length ( 1 to 969) promoter when cotransfected
with the control (empty) expression vector was arbitrarily assigned a
value of 1 (Vec). The relative activity of this promoter when
cotransfected with expression vectors for xGATA-4 (G4), xGATA-5 (G5),
or xGATA-6 (G6) is shown. The data were averaged from at least four
independent experiments, and error bars indicate the standard
deviations. In all cases, the transfection includes saturating amounts
of the GATA factor expression vector, determined in preliminary
transfections. (B) Similar transfections were performed with the
xGATA-4 expression vector but with either the full-length reporter (WT)
(presented as 100% activity) or reporters containing mutated promoters
as indicated. The activity of a control GATA-dependent promoter ( D3
[14]) is shown for comparison. Note that the majority
(but not all) of the reporter activity in the presence of GATA-4 is
eliminated when the proximal GATA site is mutated (pm).
|
|
Specific mutations of the xIFABP promoter were generated and analyzed
to test whether specific GATA consensus sites mediate
promoter activity
in the presence of ectopic GATA factors. The
transactivation data for
xGATA-4 regarding these mutated reporters
is shown in Fig.
4B.
Essentially the same results were obtained
for xGATA-5 and xGATA-6,
although the reporter activities are
relatively lower (not shown). Two
regions were targeted for analysis.
First, several consensus
GATA-binding sites are clustered between
nucleotides

270 and

350
(Fig.
2c). Of particular note, a putative
GATA-binding site centered at

285 lies within a sequence shown
previously to be important for
regulating lineage-specific expression
of the murine gene in transgenic
mice (
45). The
Xenopus promoter
contains a 12/15
match with the mouse sequence at this position.
PCR was used to delete
sequences from the promoter upstream of
nucleotide

233, relative to
the transcriptional start site. Deletion
of these sequences, including
the potential GATA-binding sites,
had minimal effect on transactivation
by GATA factors (Fig.
4B,

233). A second region containing a
consensus GATA-binding site,
centered around

42 relative to the start
site, was next targeted
for mutation. Mutation of this proximal
GATA-binding site had
a significant effect on the function of ectopic
GATA factors (Fig.
4B, pm); the 2-bp mutation in the context of an
otherwise wild-type
promoter results in an 80% decrease in the level
of transactivation.
Deletion of the upstream sequences in addition to
mutation of
the proximal GATA site (Fig.
4B,

233m) did not have a
significant
effect on transactivation, relative to mutation of the
proximal
site alone. A low level of residual transactivation by GATA
factors
occurs with the

233m reporter. It is not clear if this
results
from binding at a cryptic GATA element elsewhere in the
promoter
or plasmid or is due to GATA-dependent activation that is not
dependent on specific DNA binding. However, this low-level (twofold)
transactivation has been noted in previous transfection experiments
(
14) and may not be relevant to the normal regulation of the
gene by GATA factors.
Following similar transfections, cell lysates were harvested and
nuclear proteins were extracted. These extracts were used
in gel
mobility shift experiments, as shown in Fig.
5. The labeled
probe was a
double-stranded oligomer sequence of the xIFABP promoter,
centered
around the proximal (

42) consensus GATA site. Lysates
from cells
transfected with GATA-4, -5, and -6 expression plasmids
contained
binding activity that was specific for the xIFABP promoter
sequence,
generating a single major complex, not present in cells
transfected
with the control expression vector (Fig.
5, lanes
1, arrow). This
complex was inhibited by an excess of unlabeled
oligomer (lanes 2) but
not by an oligomer containing the 2-bp
mutation present in the pm
construct, which changes the GATA consensus
binding site (lanes 3).
Therefore, the same mutation that decreases
the transactivation of the
xIFABP promoter also abolishes the
ability of ectopic GATA factors to
bind to this promoter sequence.
The complex is similarly inhibited by
unlabeled excess oligomer
containing a well-characterized consensus
GATA-binding site from
the chick
D-globin promoter
(lanes 4). We conclude that GATA factors are
able to activate directly
the xIFABP promoter, that a specific
GATA-binding site around

42 can
mediate much of this response,
and that this site is therefore likely
to be a target for GATA
factors during gut epithelium differentiation.
Multiple GATA-binding
sites (in addition to other gut-specific factors)
might contribute
to full promoter activity, particularly in vivo under
conditions
of potentially limiting factors. Also, the more distal sites
(perhaps
including the conserved region around

285) might not be
functional
in the cell culture system or in the absence of additional
cell-specific
factors.

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FIG. 5.
GATA factors bind specifically to the xIFABP proximal
promoter element. Gel mobility shift assays were performed by using as
probe an oligomer containing sequences from 58 to 31 (Fig. 2c),
including the proximal consensus GATA-binding site. Extracts were
derived from cells transfected with the expression vector alone or the
xGATA-4, xGATA-5, or xGATA-6 expression vector, as indicated. In
addition to the probe and extract, incubation mixtures contained no
competitor (lanes 1), competitor containing the WT promoter sequence
(TE492/TE493) (lanes 2), competitor containing the pm mutated sequence
that alters the GA of the consensus GATA-binding site (TE452/TE453)
(lanes 3), or competitor containing a well-characterized GATA-binding
site from the chicken D-globin promoter TE72/TE73 (lanes
4). The position of the major specific complex (arrow) and the position
of the labeled probe (P) are indicated. The GATA-6 reactions
consistently generate a less-abundant complex; it is not known if this
is due to lower expression levels or binding affinity.
|
|
GATA-4 and GATA-5 transcript levels correlate with terminal
differentiation, while GATA-6 mRNA is the predominant GATA factor
message in proliferating progenitor cells.
The ability of
different GATA factors to function in the transactivation assay raises
the question of whether there is any specificity for GATA-4, -5, and -6 regarding regulation of the xIFABP and other terminal-differentiation
genes. Therefore, we determined the expression patterns for these genes
within the intestine as an indication of potential gene function. For
this purpose, given the high degrees of conservation for the sequence and expression of the IFABP gene in vertebrate evolution, we analyzed the expression patterns of GATA factors in chick gut, because the
villus crypt-tip axis is particularly distinguished in the chick
relative to that in the frog. Intestine was isolated from 1-month-old
chicks and fixed prior to incubation with antisense chick GATA-4, -5, and -6 RNA probes in whole-mount in situ hybridization experiments, and
the relative transcript patterns for these three genes were compared.
As shown in Fig.
6, the transcript
patterns for cGATA-4 and (particularly) cGATA-5 are consistent with a
function in regulating
terminal differentiation. Transcripts for
cGATA-4 (Fig.
6a, upper
panel) are detected at a very low level in the
crypt cell zone,
and levels increase substantially towards the villus
tip. Transcripts
are also detected below the crypt in the region where
Paneth cells
reside. The transcripts encoding cGATA-5 are first
detected about
50% along the villus axis, with highest levels toward
the distal
end (Fig.
6a, middle panel). Note the similarity of this
pattern
with the xIFABP pattern shown in Fig.
1. The transcripts for
cGATA-4
and cGATA-5 are not generally detected at the extreme tip of
the
villus (seen only in certain correctly oriented sections [T in
Fig.
6a]); these cells do express high levels of alkaline phosphatase
activity (specific to differentiated epithelial cells [not shown])
but are in the process of dying prior to being extruded into the
lumen.
In contrast to cGATA-4 and cGATA-5, the cGATA-6 gene is
regulated very
differently along the crypt-tip axis (Fig.
6a,
lower panel). The
cGATA-6 gene is not expressed in the differentiated
epithelial cells
near the tip of the villus. Instead, relatively
high transcript levels
are present in the proximal region of the
villus associated with
proliferating cells. As shown in Fig.
6b,
the cGATA-6 pattern is
essentially complementary to the cGATA-5
pattern, along the villus
epithelium. Therefore, cGATA-6 is the
predominant GATA factor expressed
in the less differentiated,
proliferating progenitor cell population,
while cGATA-4 and cGATA-5
are expressed abundantly in the
differentiated epithelium.

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FIG. 6.
GATA factors are differentially regulated along the
villus axis within intestinal epithelium. (a) Intestine was isolated
from 1-month-old chicks and processed by in situ whole-mount
hybridization. Sections from tissues incubated with antisense RNA
probes for cGATA-4 (upper panel), cGATA-5 (middle panel), or cGATA-6
(lower panel) are shown. The dark signal indicates the pattern of GATA
factor transcripts, as in Fig. 1. The relative positions of the crypt
(c) progenitor cells, the Paneth (p) cells, and the distal-tip (T)
cells are indicated. Note that GATA-4 transcripts are distributed from
the crypt to the tip, with increasing levels accumulating toward the
distal tip. In contrast, GATA-5 transcripts are highly localized within
differentiating cells of the tip, and GATA-6 transcript levels are
highest in the less differentiated region located closer to the crypt
zone. (b) Higher-magnification views of sections derived from tissue
hybridized to probes for cGATA-5 (upper panel) or cGATA-6 (lower
panel). Note that the transcript patterns are essentially
complementary.
|
|
Induction of terminal differentiation leads to an early increase in
GATA-5 mRNA levels and a concomitant decrease in levels of transcripts
encoding GATA-6.
To provide further evidence that different GATA
factors have distinct functions in intestine epithelial cell
development, we analyzed the expression of GATA factors during in vitro
cell differentiation. Therefore, we studied human
adenocarcinoma-derived cell lines that are characterized as inducible
models for gut cell differentiation. The sequences for the human GATA-4
(22, 54) and GATA-6 (49) genes were described
previously. These sequences, and a partial sequence deposited in the
GenBank database for human GATA-5, were used to design RT-PCR primers
to measure in a semiquantitative assay transcripts for human GATA
factors in RNA isolated from various cell lines. In preliminary
experiments, RNA was isolated from growing and subconfluent (uninduced)
CaCo-2, HT-29, or SW1417 adenocarcinoma-derived cells. As shown in Fig. 7a, CaCo-2 cells contain transcripts for
each of the GATA-4, -5, and -6 genes. The PCR analysis detects very low
levels of hGATA-4 and hGATA-5 mRNA in uninduced cells and relatively
higher levels of transcripts for hGATA-6. Transcripts for hGATA-4 were
not detected in the undifferentiated HT-29 or SW1417 cells. The SW1417
cells express relatively abundant levels of hGATA-5 mRNA, while HT-29 cells contain only hGATA-6 transcripts. These results are consistent with previous studies that indicate that uninduced HT-29 cells represent a relatively early or crypt-like cell type (58);
based on our hypothesis (see below), the SW1417 cells may therefore represent a more differentiated epithelial cell.

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FIG. 7.
Changes in GATA factor transcript levels correlate with
induction of terminal differentiation in human gut epithelium cell
lines. (a) RT-PCR analysis was used to measure the relative levels of
RNA encoding human GATA-4, -5, and -6 (G4, G5, and G6, respectively) in
samples derived from uninduced and proliferating CaCo-2, HT-29, or
SW1417 cells. The S14 gene encodes a relatively abundant rRNA protein
message that does not change significantly in different samples and is
used as a positive control for the RT reactions. PCR products were
labeled by including trace radiolabeled nucleotides in the reaction.
The products were detected after gel electrophoresis by
autoradiography. (b) HT-29 cells were either uninduced (lane 0) or
induced with 5 mM sodium butyrate, and RNA was harvested at various
times (in hours) to measure relative transcript levels by
semiquantitative RT-PCR analysis. A representative autoradiograph
following gel electrophoresis of the PCR products is shown. The IAP
gene is a terminal-differentiation marker that is induced to high
levels by 48 h. The hGATA-5 gene is an early target for activation
by sodium butyrate, while the hGATA-6 transcript levels decline
initially during differentiation before recovering at later time
points. Similar kinetics were consistently noted in multiple
experiments. (c) The same RNA as that used for the RT-PCR analysis in
panel b was analyzed for hGATA-6 mRNA in a Northern blotting
experiment. Total RNA was electrophoresed, blotted, and hybridized to
an hGATA-6 cDNA probe. As shown in the upper panel, the RNA levels
decrease prior to reaccumulating by 48 h, confirming the RT-PCR
results. The lower panel shows the ethidium bromide-stained 28S rRNA,
demonstrating equal RNA loading for each lane.
|
|
We used the HT-29 cells to analyze changes over time in GATA factor
transcript levels during sodium butyrate-induced differentiation.
When
undifferentiated HT-29 cells are grown to near confluence
and then
treated with sodium butyrate, they undergo differentiation
and
eventually develop a well-organized brush border morphology
(not
shown). At various times following induction, cells were
harvested and
RNA was prepared and analyzed for levels of GATA
factor transcripts,
relative to those encoding the terminal differentiation
marker IAP and
a control mRNA encoding the S14 ribosomal protein
(
28).
Transcript levels were measured by a semiquantitative
RT-PCR assay,
under conditions that ensured a linear accumulation
of signal with
respect to input RNA and cycle number. As shown
in Fig.
7b, sodium
butyrate induction leads to significant changes
in GATA factor
transcript levels, prior to changes in expression
of
terminal-differentiation markers. In particular, the hGATA-5
transcript
levels are increased substantially by 2 h. Transcript
levels
continue to rise relative to the control S14 transcript
levels during
the 48-h induction period. The IAP levels also rise
dramatically as the
cells initiate differentiation, but these
changes are not significant
until 24 h after induction. In marked
contrast, GATA-6 transcript
levels decline appreciably between
1 and 24 h, before eventually
recovering to the initial levels
by 48 h. Although the
significance of the recovery of GATA-6 transcript
levels is not clear,
it is confirmed by Northern blotting experiments
(Fig.
7c) and probably
indicates a proliferating subset of cells
that escaped butyrate-induced
differentiation. The hGATA-4 mRNA
was not detected in HT-29 cells. The
timing of IAP transcript
accumulation is consistent with that described
previously in Northern
blotting experiments (
3,
21). The
rapid accumulation of GATA-5
indicates that it might function
relatively early in the differentiation
program induced by sodium
butyrate.
Nuclear extracts were prepared from uninduced or induced HT-29 cells;
gel mobility shift analysis demonstrates that HT-29
cells express
GATA-binding factors that interact specifically
with a DNA probe
containing the IFABP proximal GATA element (Fig.
8a). Most interestingly, the abundance of
this specific binding
activity increases during the 48-h induction
period (compare lanes
1 in Fig.
8a) following an initial decrease at
24 h (Fig.
8b).
We prepared polyclonal antibodies by injecting
rabbits with peptides
derived from the predicted human GATA-6 protein
sequence. These
antibodies interfere specifically with binding of the
GATA activity
to the IFABP element-containing probe (Fig.
8b, lanes 3).
Because
the human GATA-5 cDNA is not available, we cannot test if the
antibodies cross-react with the GATA-5 protein, but this is likely,
considering that at least some of the antigen is derived from
highly
conserved regions. We conclude that GATA-binding activity
is present in
uninduced HT-29 cells and that this is probably
predominantly GATA-6
(based on the RNA studies). GATA-6 transcript
levels and total
GATA-binding activities decline during earlier
stages of
butyrate-induced differentiation (Fig.
8b, 24 h, lane
1). By
48 h, after GATA-5 transcript levels are increased, there
is a
corresponding increase in GATA-binding activity, although
this might
also include reaccumulating levels of GATA-6.

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FIG. 8.
Nuclear extracts derived from HT-29 cells contain
binding activity that interacts specifically with the IFABP proximal
GATA element, and the levels of this activity increase following sodium
butyrate-induced differentiation. (a) The oligomer probe containing the
xIFABP proximal promoter element (TE492/TE493, as in Fig. 5) was end
labeled and incubated with nuclear extracts derived from uninduced
(control) or induced cells cultured for 48 h in the presence of
butyrate. Control cultures were incubated for 48 h in a mock
induction, and equal amounts of total protein lysates were used in
mobility shift assays. The positions of the free probe (P) and a
complex formed with a nonspecific DNA-binding activity (ns) are
indicated. The first lane contains the probe incubated in buffer alone.
Lanes: 1, no specific competitor DNA; 2, 100-fold-excess
unlabeled-probe competitor; 3, 100-fold-excess competitor DNA
containing a specific mutation of the GATA cis element
(TE452/TE453); 4, 100-fold-excess competitor containing the
GATA-binding site from the chicken D-globin promoter
(TE72/TE73). Note that the specific complex (large arrow), but not the
nonspecific complex, is increased in abundance severalfold in extracts
from induced cells. Lane xG4, extract from QT6 cells transfected with
the xGATA-4 expression vector (as in Fig. 5) as a positive control for
a specific complex (small arrow). (b) Gel mobility shift experiments
were performed as for panel a with extracts derived from uninduced
HT-29 cells (0 h) or cells induced for 24 or 48 h. In all cases,
cells were cultured for 48 h following addition of butyrate to the
48-h sample; equal amounts of total nuclear proteins were incubated
with the probe. Lanes: 1, no additional competitor; 2, 1 µl of
preimmune rabbit serum; 3, 1 µl of immune serum derived from rabbits
injected with synthetic peptides consistent with several regions of the
hGATA-6 sequence. Note that the antibodies specifically inhibit or
disrupt formation of the complex (arrow), while preimmune serum
actually enhances binding. Although it is not known, the antibody is
likely to interfere with binding in all GATA-4, -5, and -6 interactions
in this experiment. P, free probe.
|
|
 |
DISCUSSION |
Our results provide strong evidence for evolutionarily conserved
but distinct functions for different GATA factors in the regulation of
intestinal epithelial differentiation. We used three different
vertebrate developmental systems that provide distinct but
complementary information on the function and regulation of GATA-4, -5, and -6 during intestinal epithelium development. In each system we find
evidence correlating the expression of GATA factors with changes in the
differentiation state of gut epithelium. First, each of the xGATA-4,
-5, and -6 factors, when expressed ectopically, can activate the
promoter for the terminal-differentiation marker xIFABP in transient
transfection experiments. This appears to be a direct function of
binding to GATA consensus binding sites in the promoter. In particular,
mutation of a proximal GATA consensus site affects significantly the
ability of expressed GATA factors to activate the xIFABP promoter. This
proximal GATA-binding site is approximately 10 bp upstream of a
potential TATA box; a similar arrangement of regulatory elements is
present in the chick
-globin gene that is activated in similar
experiments by ectopic GATA-1 expression (14). Ectopically
expressed xGATA-6 is less active on the xIFABP promoter relative to
xGATA-4 or xGATA-5. Although we cannot rule out that this is due to
differences in protein accumulation, stability, or binding affinity,
this might indicate that GATA-4 and/or GATA-5 are primary regulators of
IFABP transcription. The activation of the xIFABP promoter by xGATA-4,
-5, and -6 in the cell culture system is relatively modest (no more
than 10-fold), indicating that additional cell-specific factors are
likely required for regulation in vivo. Second, the expression patterns
in chick intestine are consistent with a model in which GATA-4 and/or
GATA-5 regulate the expression of terminal-differentiation genes
including IFABP and IAP. Both GATA-4 and GATA-5 are transcribed at
highest levels in a zone of epithelial cells apical of the progenitor crypt cells. The patterns are consistent with a "gradient" of increasing GATA-4 and -5 expression as the cells migrate from the crypt
to the villus tip, where they are fully differentiated and, after a
finite period of time, eventually die by apoptosis. In contrast, GATA-6
levels are relatively high in the proliferating and less differentiated
cell population and transcript abundance decreases in the
differentiating cells. As a consequence, this gene is relatively
abundant in the progenitor cell population, suggesting a role in
proliferation or early differentiation. Third, in the human HT-29 cell
line, induction of differentiation leads to rapid changes in
steady-state mRNA levels that are distinct for different GATA factors:
GATA-5 transcript levels increase, while GATA-6 levels decline (but
eventually recover), prior to activation of the
terminal-differentiation marker IAP. There is, after 48 h of
induction, a corresponding increase in GATA-binding activity that can
interact specifically with the xIFABP proximal promoter element.
An important question regarding the function of GATA factors in
regulating differentiation programs is whether coexpressed genes
perform similar or distinct functions. The GATA-1, -2, and -3 genes are
each expressed in the hematopoietic system and may be coexpressed in
certain cell types. However, GATA-2 is implicated in the regulation of
proliferating stem cell populations (6, 53), while GATA-1
and GATA-3 are critical for differentiation of specific cell lineages
(34, 43). In this context, the genes are regulated
differentially within a single lineage with regard to the developmental
time frame and presumably activate distinct sets of target genes. By
analogy, it is possible that similar specialization of function occurs
in the gut epithelial system. In this case, GATA-6 might be important
within a proliferating cell population for regulating the transition to
a differentiating cell type, while GATA-4 and/or GATA-5 might function
later, upon an environmental cue, to activate directly the
differentiation program. Further evidence for specific and distinct
functions of GATA factors will require additional experiments in which
specific expression patterns are altered during development. Different cells within the epithelium might express distinct subsets or levels of
GATA factors at various stages of development and differentiation, and
this might be relevant to cell type identity.
A highly conserved function for GATA factors in intestinal epithelium
is consistent with recent results regarding roles for GATA factors
during invertebrate development. At least one GATA factor is involved
in intestine-specific gene expression in C. elegans
(11). McGhee and colleagues identified a cell-specific enhancer of the gut esterase 1 (ges-1) gene located 1,100 bp upstream of the transcriptional start site. A 36-bp region containing a tandem
GATA binding site is critical for directing gut-specific transcription
and for efficient repression of the gene in the tail or pharynx. The
tandem GATA enhancer directs gut-specific expression of a linked
reporter gene; the factor that interacts with these GATA sites is
apparently distinct from a previously identified C. elegans
GATA factor (47). A Drosophila GATA factor called
"serpent" (srp) is required for midgut development in
addition to hematopoiesis. srp can activate the expression
of downstream fat body genes (1). It is unlikely that
srp is involved directly in terminal differentiation because
the gene is expressed prior to this and transiently (before the end of
germ band extension). However, srp might be important for
regulation of primordial development, while other GATA factors that are
expressed in the midgut (dGATA-c) might regulate terminal
differentiation (29). Therefore, distinct functions for GATA
factors in regulating different developmental compartments of the gut
might be a highly conserved mechanism of regulation.
Understanding the mechanism for GATA factor function in development
requires identification of the genes that they regulate. While several
potential targets for GATA factors in the gut have been described, our
data indicate that the IFABP gene is a direct target. However,
transcriptional regulation in the gut is highly complex, involving
numerous changes along both the crypt-villus axis and the
proximal-distal axis. In addition, expression is regulated
developmentally and is lineage specific within the four distinct
subpopulations of epithelial cells. Thus, it will be of interest to
determine how GATA factors cooperate with other transcription factors
to achieve temporal and spatial regulation of the IFABP gene during
development. While our studies provide data implicating GATA factor
function in differentiating intestinal enterocytes, others have
reported possible functions for these factors in gastric parietal cells
(33) and gastric endocrine cells (10). Therefore,
the functions of each GATA factor are likely diverse and largely
dependent on the relative levels of each GATA factor and the complement
of available additional transcription factors within a specific cell
type.
 |
ACKNOWLEDGMENTS |
We thank Yongmei Jiang (AECOM) for providing the cGATA-4, -5, and
-6 probes and for help in the in situ hybridization studies and Jiemin
Wong (NIH) for help with the primer extension analysis.
This work was supported by a grant to T.E. from the March of Dimes
Birth Defects Foundation.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: 1300 Morris Park
Ave., Chanin 503, Bronx, NY 10461. Phone: (718) 430-3506. Fax: (718) 430-8988. E-mail: tevans{at}aecom.yu.edu.
 |
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