(Received for publication, January 2, 1997, and in revised form, February 19, 1997)
From the Cardiology Division, Department of Medicine and the Gazes Cardiac Research Institute, Medical University of South Carolina, Charleston, South Carolina 29425-2221
The Na+-Ca2+
exchanger (NCX1) plays a major role in calcium efflux and therefore in
the control and regulation of intracellular calcium in the heart. The
exchanger has been shown to be regulated at several levels including
transcription. NCX1 mRNA levels are up-regulated in both cardiac
hypertrophy and failure. In this work, the 5-end of the
ncx1 gene has been cloned to study the mechanisms that
mediate hypertrophic stimulation and cardiac expression. The feline
ncx1 gene has three exons that encode 5
-untranslated sequences that are under the control of three tissue-specific promoters. The cardiac promoter drives expression in cardiocytes, but
not in mouse L cells. Although it contains at least one enhancer (
2000 to
1250 base pairs (bp)) and one or more negative elements (
1250 to
250 bp), a minimum promoter (
250 to +200 bp) is
sufficient for cardiac expression and
-adrenergic stimulation.
The Na+-Ca2+ exchanger is an integral membrane protein involved in the maintenance of calcium homeostasis in a number of cell types. Under physiological conditions, the exchanger functions as an antiporter, moving sodium and calcium in opposite directions across the cell membrane. The exchanger electrogenetically catalyzes the exchange of one intracellular Ca2+ ion for three extracellular Na+ ions (1, 2). The exchanger is reversible and can facilitate Ca2+ entry, which is capable of triggering calcium release from the sarcoplasmic reticulum. NCX1 is predominantly expressed in cardiocytes (3), neurons, and cells of the renal tubules (4-7). However, it is expressed at lower levels in a variety of other cell types, including skeletal muscle, smooth muscle, lung, and spleen. In the heart, the exchanger is the primary mechanism for calcium efflux (8) and therefore plays a major role in cardiac contractility. In the kidney, the exchanger is involved in the reabsorption of calcium and thus in maintenance of total body calcium homeostasis. The function of the exchanger in other tissues is less clear; however, in neurons, the exchanger has been implicated in the extrusion of calcium after neurotransmitter release (4).
The cDNA for the NCX1 Na+-Ca2+ exchanger
was originally cloned from canine heart (9) and more recently has been
cloned from other tissues, such as brain and kidney, and other species,
including the cow (10), rat (11), human (12), and cat (13). It shows high homology across species and tissues, and the predicted primary structure of NCX1 includes a 32-amino acid N-terminal signal sequence followed by 11 putative transmembrane domains. Between domains 5 and 6 lies a large intracytoplasmic loop that is involved in regulation of
the exchanger. Near the C-terminal end of the loop is a variable domain
of 110 amino acids that is generated by the tissue-specific alternative
splicing of six ncx1 exons (14). Eight tissue-specific
isoforms have been identified (14-16). In addition, three distinct
5-untranslated regions have been identified, each with a unique tissue
distribution (7, 17). Regulation of exchanger isoforms is made even
more complex by the discovery of two additional ncx genes in
mammals. In contrast to the more ubiquitous expression of
ncx1, the ncx2 (18) and ncx3 (19) genes are expressed only in brain and skeletal muscle. ncx1
expression appears to be regulated by three tissue-specific promoters,
which would play a role in determining the resulting splice variants in
the large cytoplasmic loop.
Exchange activity in cardiomyocytes is regulated by several factors. It is activated by cytosolic Ca2+ and MgATP (20) and inhibited by cytosolic sodium (21) and ATP depletion (22). A high affinity Ca2+-binding domain has been identified in the large cytoplasmic loop (residues 371-508) that is believed to be responsible for calcium regulation (23). It is also inhibited by the exchanger inhibitory peptide, XIP, that corresponds to a 20-amino acid segment at the N terminus of the large cytoplasmic loop (24). A recent study has demonstrated that the exchanger is phosphorylated via a protein kinase C-dependent pathway and that NCX1 phosphorylation appears to coincide with up-regulation of exchanger activity (25).
In addition, the exchanger is regulated at the transcriptional
level in cardiac hypertrophy, ischemia, and failure. We have demonstrated the rapid up-regulation of NCX1 mRNA in response to
pressure overload (26, 27). The exchanger is also up-regulated in
end-stage heart failure (28), but exchanger levels drop in ischemia
(29). In this report, we describe the molecular cloning of the cardiac,
brain, and kidney promoters; the exons encoding their respective
5-untranslated regions; and the first open reading frame exon of the
feline ncx1 gene. In addition, characterization of the
ncx1 cardiac promoter reveals regions required for cardiac expression and up-regulation. We define the 5
-flanking region containing the regulatory sequences required for cardiac expression and
up-regulation. This work provides a starting point for pursuing an
understanding of the processes by which
Na+-Ca2+ exchanger expression is regulated.
RACE1 was carried out as
described previously (30-32). 1 µg of total RNA isolated from feline
brain, kidney, and heart was annealed to an antisense ncx1
gene-specific primer at positions +781 to +797
(5-TTGTAAAACAGCAGCCT-3
) and reverse-transcribed for 45 min at
42 °C using 1 µl of Superscript II reverse transcriptase (Life
Technologies, Inc.) in a 25-µl reaction mixture containing 0.05 M Tris-HCl, pH 8.5, 0.03 M KCl, 8 mM MgCl2, 0.25 mM dATP, 0.25 mM dCTP, 0.25 mM dGTP, 0.25 mM
dTTP, 1 mM dithiothreitol, and 2.5 pM primer.
The RNA was then degraded by addition of 1 µl of RNase H and
incubated for 10 min at 55 °C. The cDNA was purified using the
GlassMAX DNA purification system (Life Technologies, Inc.) and then
tailed with dCTP at 37 °C for 15 min using terminal dipeptidyltransferase (Promega). The tailed cDNA was amplified by
polymerase chain reaction (30 cycles at 94, 50, and 72 °C for 1 min
each) using a second antisense ncx1 gene-specific primer located at positions +207 to +224 (5
-CTCTGGCAATTTTGTCTC-3
) and an
anchor primer (5
-GGCCACGCGTCGACTAGTACGGGIIGGGIIGGGIIG-3
; Life
Technologies, Inc.). Products were subjected to Southern analysis and
hybridized to a [32P]dCTP (3000 Ci/mmol; DuPont
NEN)-labeled PCR probe generated from region
26 to +59 (sense primer,
5
-ACACTTGGAGGTCTAC-3
; and antisense primer,
5
-CAACGGCTAACAGATGACATC-3
). Products that hybridized to the probe
were ligated into the pT7Blue-T vector (Novagen) and transformed into
competent NovaBlue cells (Novagen). Minipreps were prepared from the
colonies and screened by Southern hybridization as described above.
Positives were sequenced either by the dideoxy chain termination method
using Sequenase 2.0 (U. S. Biochemical Corp.) or by cycle sequencing
using the Ampli-Cycle sequencing kit (Perkin-Elmer) as specified by the
manufacturer using an annealing temperature of 55 °C.
Feline brain, kidney, and heart tissues were dissected and immediately frozen in liquid nitrogen. Total RNA was then isolated from the tissue by the method of Chomczynski and Sacchi (33). Feline mRNA from heart, brain, and kidney was isolated using the FAST TRACK kit (Stratagene). Total RNA was isolated from rat neonatal cardiocytes using RNAzol (Cinna Biotex) as directed by the manufacturer. For Northern analysis, 15 µg of total RNA or 1.2 µg of mRNA was electrophoresed on each lane of a formaldehyde-agarose (1%) gel and transferred by the downward capillary method to a Duralon (Stratagene) membrane to which it was UV-cross-linked. The membrane was hybridized to [32P]dCTP-labeled PCR probes generated as described under "Results." Membranes were then washed to a stringency of 1 × SSC and 0.1% SDS at 42 °C.
Screening of cDNA and Genomic LibrariesBoth the feline cardiac cDNA and feline genomic libraries were screened using standard techniques (34). The libraries were plated on 10 150-mm plates at 30,000 plaques/plate. Duplicate lifts from each plate were hybridized to the [32P]dCTP-labeled PCR probes. Filters underwent four 20-min washes at 42 °C: the first two washes in 2 × SSC and 0.1% SDS and the last two in 0.2 × SSC and 0.1% SDS. The filters were then exposed to X-Omat AR film (Eastman Kodak Co.) overnight. Plaques that produced signals on both of the duplicate lifts were isolated and purified to homogeneity through three to four additional rounds of screening using the same probes and wash parameters.
The oligo(dT)-primed cDNA library was constructed in the vector
Lambda Zap using feline cardiac mRNA and Moloney murine leukemia virus reverse transcriptase (Stratagene). Filters were hybridized to a
[32P]dCTP-labeled PCR probe generated from region 26 to
+224 (sense primer, 5
-ACACTTGGAGGTCTAC-3
; and antisense primer,
5
-CTCTGGCAATTTTGTCTC-3
). Clones were sequenced as described for
5
-RACE experiments.
A feline genomic library in the Lambda Fix II vector was purchased from
Stratagene. This library was screened using the same [32P]dCTP-labeled PCR probes used for Northern analysis.
To subclone fragments containing exons and 5-flanking regions, genomic
clones were digested with EcoRI (Promega) and ligated into
the pBluescript SK vector (Stratagene).
10 µg of mRNA was
reverse-transcribed at 42 °C for 1 h utilizing an antisense
oligonucleotide corresponding to 51-71 nucleotides 5 of the
translational start site. The reaction consisted of 1 × reverse
transcriptase buffer, 0.2 mM dTTP, 0.2 mM dATP,
0.2 mM dGTP, 0.02 mM dCTP, 4 mM
dithiothreitol, 200 units of Moloney murine leukemia virus reverse
transcriptase (Boehringer Mannheim), and 6.5 µl of
[32P]dCTP. The reaction was heat-inactivated at 90 °C.
After addition of 10 µl of stop solution (U. S. Biochemical Corp.),
each sample was heated at 95 °C for 3 min and run on a 7 M urea, 8% polyacrylamide sequencing gel. To determine the
size of primer extension products, a sequencing reaction was loaded on
the gel and run simultaneously. Sequencing was carried out as described
for 5
-RACE experiments. The gel was then dried and exposed to X-Omat
AR film for 3-6 h.
S1 nuclease protection was carried out according to previously published methods (35). A 57-bp end-labeled oligonucleotide was coprecipitated with 30 µg of cardiac total RNA or yeast tRNA. The pelleted nucleic acids were dissolved in 80% formamide, 0.4 M Pipes, pH 6.4, 0.4 M NaCl, and 1 mM EDTA; denatured for 10 min at 90 °C; and annealed for 3 h at 55-60 °C. The DNA-RNA hybrids were subjected to S1 nuclease digestion in 300 µl of 0.05 M ammonium acetate, pH 4.5, 0.25 M NaCl, and 0.01 M zinc chloride containing 6.25 µg of denatured salmon sperm DNA. 200-300 units of S1 nuclease was added, and samples were incubated for 30 min at 37 °C. Reactions were stopped by addition of EDTA to a concentration of 5 mM, extracted with phenol/chloroform, and precipitated with ethanol. S1 products were resuspended in 10 µl of distilled H2O and analyzed by polyacrylamide gel electrophoresis as with primer extension reactions.
Promoter-Reporter Plasmid ConstructsDigestion of the
genomic subclone containing exon H1 (pBH1) with SacI (New
England Biolabs Inc.) liberated a 2200-bp fragment encompassing the H1
exon and 2000 bp of 5-flanking region. This fragment was cloned into
the SacI site of the pGL2-Basic vector (Promega) in both
forward and reverse orientations. Deletion mutants were generated by a
PCR mutagenesis strategy (36). Sequencing was carried out as described
for 5
-RACE experiments.
Primary cardiocytes were obtained from day 2-3 neonatal rats by previously published methods (37). Briefly, ventricular myocardium was isolated from 40-50 neonatal rats. The tissue was finely minced, rinsed, and placed in a suspension culture flask. A mixture of 2.4 units/ml partially purified trypsin, 2.7 units/ml chymotrypsin, and 0.94 units/ml elastase in calcium- and magnesium-free Hanks' salt solution buffered with 30 mM Hepes, pH 7.4, was repeatedly added to the suspension culture flask for six 20-min incubations at 37 °C to dissociate the cardiocytes. After each incubation, dissociated cells were centrifuged at 500 × g and resuspended in modified Eagle's medium supplemented with 10% newborn calf serum. To enrich for cardiocytes, the cells were pooled and subjected to differential plating for 90 min. Cardiocytes were plated at a density of 1 × 105 cardiocytes/cm2 for future mRNA isolation or 1 × 106 cardiocytes/60-mm dish for transfection. The cardiocytes were maintained overnight in modified Eagle's medium supplemented with 10% newborn calf serum. The next day, the cells were switched to serum-free maintenance medium. Cells treated with phenylephrine prior to RNA extraction were treated for 2 h with 20 µg/ml after 24 h of serum-free medium. Mouse L cells were grown and passaged at low density in Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum. They were plated at a density of 1 × 106 cells/60-mm dish 24 h prior to transfection.
TransfectionsOne day prior to transfection, all cell types were plated at a density of 1 × 106 cells/60-mm dishes. The following day, the cardiocytes were placed in maintenance medium supplemented with 4% horse serum 1-4 h prior to transfection, while mouse L cells were placed in Dulbecco's modified Eagle's medium supplemented with 4% horse serum.
All transfections were carried out in triplicate using 16 µg of
ncx-luciferase construct cotransfected with 8 µg of
cytomegalovirus promoter-driven -galactosidase expression plasmid.
Cells were transfected by a modified calcium phosphate precipitation
technique (38). Cells were incubated with the DNA/calcium phosphate
precipitate for 16 h and then washed and kept in either
maintenance medium (cardiocytes) or Dulbecco's modified Eagle's
medium supplemented with 10% horse serum (mouse L cells). Immediately
after being placed in maintenance medium, transfected cardiocytes were
treated with 100 µM phenylephrine or 10 µM
verapamil or left untreated. After 48 h, all transfected cells
were washed twice with phosphate-buffered saline and scraped in 400 µl of 1 × Reporter Lysis Buffer (Promega). Lysates were
quick-frozen and stored at
70 °C until they were assayed for
luciferase and
-galactosidase activities.
To determine luciferase activity, 5 µl of crude lysate was added to
50 µl of luciferin mixture containing 75 mM luciferin, 5.5 mM ATP, 10 mM MgSO4, 100 mM Tricine, pH 8, and 2 mM EDTA. Light emission
was measured using an Auto Lumat LB 953 luminometer. -Galactosidase
activity was determined by addition of 100 µl of crude lysate to 200 ml of a mixture containing 0.1 M phosphate buffer, pH 7.3, 2 mg/ml o-nitrophenyl
-D-galactopyranoside,
75 mM 2-mercaptoethanol, and 1.5 mM
MgCl2. The reaction was allowed to proceed for 2-5 h at
37 °C before it was terminated by addition of 500 µl of 0.5 M NaCO3. Absorbance was then measured
spectrophotometrically at a wavelength of 410 nm.
5-RACE was utilized to determine whether the 5
-UTR of the feline
ncx1 gene demonstrated the same tissue-specific
heterogeneity identified in the rat (7). After two rounds of
amplification, four major products were clearly present from brain, two
from heart, and two from kidney (Fig. 1). RACE was
repeated at least three times from different tissue samples, and
separate RNA isolation yielded similar results. The 5
-RACE was
designed so that positive clones should contain ncx1
sequence from
33 to +250 bp, allowing for the screening of positive
clones by Southern blotting. Interestingly, all four major products
from the brain showed positive hybridization in the Southern blot, and
sequencing revealed that each had the correct ncx1 sequence
from
33 to +250 bp, but unique 5
-ends. Each of these unique 5
-UTR
sequences was used to generate PCR probes, and Northern blot analysis
revealed that only the 350-bp 5
-RACE NCX1 cDNA clone was present
at a detectable level in the brain (Fig. 2). This clone,
Br1, contains 139 bp 5
of position
33, has a very high GC content,
and has a 65% homology to the rat brain 5
-UTR (7). Both products
found in the kidney were positive by Southern blotting and have the
correct ncx1 sequence from
250 to
33 bp, where they
diverge into unique sequences. Again, only one, K1, was present in the
kidney Northern analysis (Fig. 2). This clone contained 278 bp 5
of
position
33. K1 contains an out-of-frame initiation codon within a
nucleotide environment showing reasonable conformity to the Kozak
consensus sequence (39). A polypeptide initiated at this site would
precede 17 amino acids before terminating at a stop codon. The presence
of alternative upstream initiation sites has been demonstrated to reduce the efficiency of mRNA translation (39, 40). This suggests that ncx1 isoforms containing K1 may be under some level of
translational control. Surprisingly, the feline kidney 5
-UTR shows
no sequence homology within the rat kidney clone (7).
In the heart, both 5-RACE products were positive on Southern analysis.
Extensive sequencing of numerous clones showed that the products
consisted of two separate isotypes, H1 and H2. The H1 product contained
110 bases 5
of the
33 splice site. This isoform showed significant
homology to the rat H1 and bovine P13 clones, 68 and 85%,
respectively. The H2 product consisted of the entirety (278 bp) of K1
sequence 5
of position
33 and then continued 5
to include the
entire 110 bp of H1 (Fig. 1). Northern analysis indicates that both H1
and K1 are present in the heart (Fig. 2). More important, 5
-RACE with
primers within the K1 sequence always gave a product that contained all
or a substantial portion of the H1 sequence. Therefore, in the normal
heart, K1 is only expressed as the H2 5
-UTR and is never detected
alone. The existence of two feline cardiac 5
-UTR isoforms differing
only in the splicing of the intervening sequence (K1) is consistent
with reports concerning bovine cardiac ncx1 (10), but
differs from what has been found in the rat (7). Extensive screening of
a feline cardiac cDNA library resulted in confirmation of H1 and H2
and uncovered no new alternative 5
-UTRs. The library yielded a ratio
of three clones containing H1 to one containing H2 (out of 23 positive clones). While not quantitative, the relative number of H1 clones compared with H2 clones suggests that H1 may be the more abundant cardiac isoform. This is further substantiated by Northern analysis, as
is shown below.
Northern analysis indicates a
tissue-selective expression of the isoforms (Fig. 2). The blot was
sequentially probed with sequences corresponding to the ORF, K1, Br1,
and finally the H1 exons. Comparing expression levels using a common
probe generated from the ORF, the exchanger appears to be most abundant
in the heart, less abundant in the brain, and still less in the kidney. The Br1 5-UTR is specifically expressed only in the brain, while the
other 5
-UTR exons show more promiscuity. The H1 5
-UTR is expressed in
both the heart and brain. The K1 5
-UTR sequence is expressed in both
the kidney and heart. More important, the ORF-probed blot was exposed
to film for 1 day, the H1-probed blot for 2 days, and the Br1 and K1
blots for 5 days. Therefore, the relative band intensity indicates
that, in the heart, the K1 sequence (H2) is expressed at very low
levels compared with H1, again suggesting that H1 is the predominant
cardiac isoform. The ORF probe also recognizes a transcript at 1.8 kb.
Other probes corresponding to sequences in the 5
-end of the
ncx1 ORF label a band at 1.8 kb (data not shown). Although
these data are intriguing, the relationship of the 1.8-kb transcript to
ncx1 is not clear.
To determine the structure of the 5-end of
the exchanger gene and to isolate its cardiac promoter region, a feline
Lambda Fix II genomic library was screened using PCR probes generated from the open reading frame and each of the unique 5
-UTRs. Screening with a probe generated from region +152 to +484 yielded three clones
(P1, P2, and P3) ~17 kb in size, each containing a 1.8-kb exon
encoding nucleotides
33 to +1826 (Fig. 3). Screening
with probes generated from the K1 5
-UTR yielded one 18-kb clone (M1) that encoded both K1 and Br1 sequences. Screening with probes from the
H1 5
-UTR yielded three identical clones ~15 kb in length. To further
characterize the clones, each was mapped through restriction analysis.
EcoRI fragments of the genomic clones were subcloned into
pBluescript. These subclones were then restricted and subjected to
Southern analysis, which in combination with PCR between exons and
sequencing, was used to determine the arrangement of exons, distances
between exons, and the ends of the clones as well as exon-intron
boundaries. The data were combined to prepare the map shown in Fig. 3.
The 1.8-kb exon 2 lies alone near the center of the P clones, which
contain at least 6 kb of intron upstream and at least 6 kb of intron
downstream. The M clone does not overlap with any of the P clones, but
encodes both K1 and Br1 exons. K1 lies 1 kb upstream from Br1, with 6 kb of intron upstream and 6 kb downstream of the pair. The R clones
contain only the H1 5
-UTR encoded as one exon located 3 kb from the
3
-end. The structure of the ncx1 gene is rather unique,
with three upstream UTR exons, each expressed in a tissue-specific
manner, followed by a 1.8-kb exon encoding over 60% of the total ORF
of the transcript. There is no overlap between the R and M clones. At
least 10 kb of intron lie between the 1.8-kb exon 2 and Br1 and >10 kb
between K1 and H1. Hence, the 5
-UTR exons are spread over at least 25 kb of genomic sequence. Each exon conforms in sequence with the
cDNA clones, and all were bound by conventional splice consensus
sequences (Fig. 4). The recently isolated human
NCX1 gene has 25 kb between what corresponds to the feline
Br1 exon (exon 1) and the 1.8-kb exon (41). No other 5
-UTR exons were
identified in the human genomic clones.
DNA sequence of first four exons and
immediate flanking regions of the H1, K1, and Br1 exons. A,
the H1 exon (upper-case) along with 375 bp of 5-flanking
sequence and the first 49 nucleotides of intron A. Numbering is relative to the start of
transcription. H1 Px Primer is the oligonucleotide primer
that was used to map the start site of transcription, and the
dashed line indicates the oligonucleotide used as a probe
for the S1 mapping experiment (see Fig. 5). nt, nucleotide.
The asterisks indicate transcriptional start sites. The H1
5
-UTR contains a single alternative upstream ORF ().
The 5
-flanking region contains two E boxes, two GATA elements, an
MEF-2 recognition sequence, and one M-CAT-binding site. B,
sequence of the K1 exon and 5
-flanking region. The K1 5
-UTR contains
four additional methionine-initiated ORFs, each followed by at least
one in-frame stop codon (underlined). The kidney 5
-flanking
region encodes consensus CAAT and TATA boxes at
120 and
28 bp,
respectively. All other features are similar to those described for the
H1 sequence. C, sequence of the Br1 exon 5
-flanking region.
All features shown are similar to those described for the H1 sequence.
D, sequence of the 1.8-kb exon 2. Numbering in this case is
relative to the start of translation. The genomic sequence is identical
to the feline heart cDNA sequence (13), and bases 113-1719 have
not been included.
The start sites of the heart isoforms have been mapped by primer
extension. Three transcriptional start sites were identified in the
heart corresponding to 131, 139, and 143 bp 5 of the AUG codon (Fig.
5) using an antisense oligonucleotide corresponding to
51-71 nucleotides 5
of the AUG codon. Primer extension with a second
oligonucleotide yielded products corresponding to the same initiation
sites (data not shown). These were verified through S1 nuclease
protection using an antisense end-labeled oligonucleotide spanning the
H1 sequence from nucleotides
111 to
179 5
of the translational
start site. S1 nuclease digestion produced protected fragments 33, 27, and 22 nucleotides in length (Fig. 5), confirming sites identified
through primer extension. Control primer extension and S1 nuclease
protection reaction using yeast tRNA as a template showed no products
(data not shown).
5
The sequence surrounding the cardiac
transcriptional start site contains no apparent TATAA sequence or CCAAT
box. In the first 350 nucleotides of the cardiac promoter, there are
two CANNTG motifs (E boxes) at positions 173 and
155 that are
potential target sites for the helix-loop-helix family of transcription factors (Fig. 4). A single M-CAT consensus element is present at
position
325. M-CAT elements govern MyoD-independent cardiac transcription of several cardiac genes, including cardiac troponins T
(42) and C (43),
-myosin heavy chain (44), and skeletal
-actin
(45). The promoter region contains consensus sequence for two GATA
boxes at positions
49 and
124. Several cardiac-specific genes such
as myosin light chains IA and IV and
-myosin heavy chain (46, 47)
contain conserved GATA-binding motifs. The GATA elements in atrial
natriuretic peptide (48) and
-myosin heavy chain (49) genes have
been shown to be critical for cardiac expression. The first 350 nucleotides of the cardiac promoter also contain a single MEF-2 element
at position
168. An MEF-2-like motif appears to be required for
cardiac-specific expression of the rat cardiac troponin T gene.
Sequencing the 5-flanking region 5
of the K1 exon revealed a
TATAA-like motif at position
30 and a CATT box at position
133
(Fig. 4B). A consensus site for C/EBP was found at position
204. Farther upstream are putative sites for P2, AP-2, GATA-1, and
two E boxes. To identify consensus sequences possibly involved in
transcriptional regulation of the brain isoform, we have sequenced a
portion of the 5
-flanking region. Sequencing proved difficult presumably due to a GC content of >75% in the region. No TATAA or
CAAT boxes were evident. We have identified two overlapping Sp1 sites
at positions
54 and
53, which are common in housekeeping genes
(Fig. 4C). An EGR-1 consensus site begins at position
54, encompassing both Sp1 sites. At position
7 there is an E box, which
is the consensus sequence for the helix-loop-helix family of
transcription factors. Members of the helix-loop-helix transcription factor family have been implicated in the regulation of other neuronal
genes. An AP-2 site has also been identified at position +13.
Transient transfection of freshly
isolated rat neonatal cardiocytes confirmed the identity of the cardiac
promoter. A fragment containing the complete H1 exon and 2 kb of the
5-flanking sequence was cloned in both the forward and reverse
directions into the pGL2-Basic vector. As shown in Fig.
6A, the forward construct (pH2000)
reproducibly drove levels of luciferase expression 24-55 times that of
the pGL2-Basic vector alone. The reverse construct had no activity
over vector alone. The chimeric luciferase gene construct
containing the cardiac promoter was also transfected into mouse L
cells. Luciferase activity in the transfected L cell extract was just
above that of the pGL2-Basic vector alone (Fig. 6B). In
addition, 1 kb of the kidney 5
-flanking sequence was also ligated into
the pGL2-Basic vector. The putative kidney promoter did not drive
luciferase expression significantly above background levels when
transfected into neonatal cardiocytes (data not shown).
We have previously shown (26) that, in the intact heart, the levels of
Na+-Ca2+ exchanger mRNA and protein are
significantly elevated in early hypertrophy produced by pressure
overload. This increase in mRNA is at least 2-fold and occurs
within 1 h of pressure overload, indicating regulation of the
exchanger in the manner of an immediate-early gene.
1-Adrenergic stimulated neonatal cardiomyocytes have
several structural, morphological, and genetic markers of hypertrophy and provide a good starting point for our investigation of candidate cis-elements involved in the hypertrophic up-regulation of
ncx1. We have already demonstrated that, within 2 h,
1-adrenergic stimulation induces a 2-3-fold increase in
NCX1 mRNA in neonatal rat cardiomyocytes plated at low density or
pretreated with verapamil to prevent spontaneous beating (15). To
determine if the rise in mRNA seen under phenylephrine stimulation
is transcriptionally regulated by elements within the pH2000 construct,
we treated cells transiently transfected with pH2000 with 100 µM phenylephrine for 48 h. All dishes were
cotransfected with a constant amount of
cytomegalovirus-
-galactosidase vector as a control for transfection
efficiency. Phenylephrine treatment induced a 1.5-3-fold rise in
luciferase activity (Fig. 6A) over that of untreated or
verapamil-treated transfected cells, consistent with data from Northern
analysis (15).
More important, cell density was an important factor in the transfection experiments. All data reported above were derived from cardiocytes plated at 1.0 × 106 cardiocytes/60-mm plate. At this density, there is very little cell contact, and spontaneous beating is rare. Several experiments included pH2000 transfection of cells plated at higher densities. When cells were plated at densities high enough to produce rhythmic spontaneous beating, the basal level of activity of pH2000-transfected cardiocytes was measured as 1.5-2-fold greater than with transfection of less densely plated cells. However, in these experiments, the level of phenylephrine induction decreased and, in several of these experiments, was undetectable. If beating of control cells was inhibited by 10 µM verapamil, basal activity decreased to levels just below that of the more sparsely plated cells, and phenylephrine up-regulation was restored.
To further delineate the promoter regions directing cardiac
ncx1 gene transcription and phenylephrine-induced
up-regulation, luciferase reporter constructs containing shorter
portions of the cardiac 5-flanking region were created through a
PCR-based method (see "Experimental Procedures"). Clones containing
1250 bp (pH1250), 500 bp (pH500), and 250 bp (pH250) of 5
-flanking region fused to the luciferase reporter gene were generated and transfected into rat neonatal cardiocytes. In all experiments, transfection efficiency was monitored by cotransfection of a
cytomegalovirus-
-galactosidase vector, and the luciferase activity
was normalized to
-galactosidase activity. The deletion of
2000 to
1250 bp yields a dramatic decrease in basal promoter activity. The
pH1250 construct had luciferase activity just above that of the control
promoterless plasmid pGL2-Basic. Constructs truncated at
500 (pH500)
or
250 (pH250) bp yield luciferase activity at 15 and >50%,
respectively, of the pH2000 construct. More important, both the pH250
and pH500 construct promoter activities were up-regulated at least
2-fold by
1-adrenergic treatment. Thus, sequences within
region
250 to +200 of the feline ncx1 cardiac promoter
confer cardiac expression and
1-adrenergic induction.
There also appears to be a cardiac-specific enhancer between
2000 and
1250 bp and one or more negative regulatory elements between
1250
and
200 bp. But these elements do not appear to be required for
1-adrenergic stimulation.
The ncx1 gene is unusual in that it contains three
promoter regions and three 5-UTR exons upstream of the coding region. As a result of alternative promoter usage and alternative splicing of
these 5
-UTR exons, ncx1 transcripts in the brain, heart,
and kidney possess at least four different 5
-UTRs. The Br1 5
-UTR is
expressed predominantly in the brain, but is not detectable (by
Northern analysis) in the heart or kidney. Unlike the rat heart, in
which only a single species of 5
-UTR is expressed, we have discovered
two forms in the feline heart. The first isoform contains the H1 exon
spliced to the common acceptor site (
33) at the beginning of exon 2. This is the predominant transcript found in the heart. The second
transcript is derived by an alternative splice receptor in the K1 exon,
which results in the inclusion of the entire K1 exon between the H1
exon and exon 2.
The existence of two feline cardiac 5-UTR isoforms differing only in
the splicing of the intervening sequence (K1 exon) is consistent with
what was reported for the bovine cardiac ncx1 p17 clone
(10). Bovine p13 and the 5
-end of the bovine p17 clone are identical
and have 85% homology to the feline H1 sequence. Interestingly, the
p17 clone contains a 156-bp insert that, although not homologous to the
feline K1 sequence, has significant homology to the rat kidney 5
-UTR
sequence (7). The K1 sequence was always found as an alternative exon
inclusion with H1 and was never detected alone. In addition,
transfection of neonatal cardiocytes with the kidney
promoter-luciferase fusion construct never gave luciferase expression
above background levels. Therefore, K1 expression in the heart is not
regulated by the kidney promoter. Only one species of transcript is
present in the kidney, in which the K1 exon is spliced directly to the
common receptor site (
33) at the beginning of exon 2.
Several other genes have multiple promoters and undergo complex
alternative splicing events that are regulated in a tissue-specific or
developmental manner. In some cases, the alternative promoters control
transcription of the same protein, but diverge only in the 5-UTR
sequences, such as the human mineralocorticoid receptor gene (50) and
lck gene (51). The parathyroid hormone receptor gene (52) is
regulated by two promoters, one tissue-specific and the second
controlling ubiquitous expression. The pem homeobox gene is
regulated by either an androgen-dependent or -independent promoter in a tissue-specific manner (53). In some cases, the alternative splicing events alter both the 5
-UTR and coding regions. This is what is occurring with the ncx1 gene (Refs. 7 and 15 and this work). Schulze and co-workers (15) have identified several
tissue-specific isoforms of ncx1. These variants are
produced by the alternative splicing of a cluster of six exons (termed A-F) coding for the variable region in the C terminus of the large intracellular loop. To date, nine isoforms have been identified from
cDNAs and reverse transcription-PCR in a wide variety of cells and
tissue (7, 15, 19). The role of each promoter and 5
-UTR splice variant
in determining alternative splicing in the variable region of the large
cytoplasmic loop is currently being investigated. More important, the
two alternatively spliced 5
-UTR species found in the heart yield two
loop isoforms of the exchanger.2 Therefore,
unlike the rodent (7, 15), the cat expresses two distinct
ncx1 isoforms in cardiac tissue. The cytoplasmic loop has
been demonstrated to contain domains involved with intracellular calcium regulation and exchanger-inhibitor peptide interaction. Given
the diverse roles that the exchanger plays in the kidney, brain, and
heart, it is believed that its regulation would differ between tissues.
Thus, the variable region may play a role in the tissue-specific
regulation (7, 15) or possibly in subcellular targeting of the
exchanger.
The ncx1 brain and heart promoters have several features
common to a number of housekeeping genes, including a high GC content, a high frequency of the dinucleotide CpG, and the absence of TATA and
CAAT boxes. Only the kidney promoter contains canonical CAAT and TATA
boxes. This is consistent with the finding that the brain and heart
isoforms are expressed at very low levels in several tissues (7).
Although not conclusive, 5-RACE products and transfections of the
kidney promoter support the premise that kidney promoter expression is
tissue-specific. The kidney promoter has several consensus sequences,
including two E boxes and C/EBP, P2, AP-2, and GATA-1 sites. Only 88 bp
of the brain promoter sequence have been obtained presumably because of
extensive secondary structure. This proximal end of the brain promoter
has a very high GC content (>75%) and two Sp1 elements. As discussed
above, although the heart promoter has several features of a
housekeeping gene, it also has several elements that have been
demonstrated to be required or important for cardiac expression of
other genes. These include two GATA elements, two E boxes, and an M-CAT
element.
To begin to characterize the ncx1 cardiac promoter, a DNA
fragment containing 2000 bases of the 5-flanking region and 122 bases
of the H1 exon plus 80 bp of the first intron was fused to the
luciferase reporter gene and transfected into rat neonatal cardiocytes.
Deletion analysis demonstrated that there may be one or more enhancer
elements between
2000 and
1250 bp and at least one negative element
between
1250 and
250 bp. But more important, the first 250 bp of
the 5
-flanking region contain all that is requisite for
cardiac-directed expression and
1-adrenergic stimulation
of the ncx1 gene. This region contains the two GATA elements, an Sp1 site, a single AP-2 element, and two E boxes. More
important, the M-CAT element at position
380 is not required for
basal expression or
1-adrenergic stimulation. This is
clearly different from what has been reported for both the
-myosin
heavy chain (54) and skeletal
-actin (55). The
-adrenergic
induction of the the
-myosin heavy chain requires M-CAT and possible
AP-1 elements, and the induction of the skeletal
-actin promoter
also requires M-CAT as well as c, AT-rich G sequence (CArg) and Sp1 elements. Studies are currently in progress to identify the specific elements required for cardiac expression and the up-regulation of the
ncx1 gene in cardiac hypertrophy.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) U67072[GenBank], U67073[GenBank], U67074[GenBank], U67075[GenBank].
We thank Dr. Kenneth Philipson for sharing unpublished work and Linda Paddock for typing the manuscript.