Download Atrial Chamber-specific Expression of Sarcolipin Is Regulated

THE JOURNAL OF BIOLOGICAL CHEMISTRY
© 2003 by The American Society for Biochemistry and Molecular Biology, Inc.
Vol. 278, No. 11, Issue of March 14, pp. 9570 –9575, 2003
Printed in U.S.A.
Atrial Chamber-specific Expression of Sarcolipin Is Regulated
during Development and Hypertrophic Remodeling*
Received for publication, December 23, 2002
Published, JBC Papers in Press, January 6, 2003, DOI 10.1074/jbc.M213132200
Susumu Minamisawa‡§¶, Yibin Wang储, Ju Chen**, Yoshihiro Ishikawa§‡‡, Kenneth R. Chien**,
and Rumiko Matsuoka‡¶§§
From the ‡Department of Pediatric Cardiology and the §§Division of Genomic Medicine, Institute of Advanced Biomedical
Engineering and Science, Graduate School of Medicine, Tokyo Women’s Medical University, Tokyo 162-8666, Japan, the
§Department of Physiology, Yokohama City University School of Medicine, Yokohama, 236-0004 Japan, the 储Department
of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201, the **Institute of Molecular
Medicine, University of California at San Diego, La Jolla, California 92093, and the ‡‡Department of Cell Biology and
Molecular Medicine, New Jersey Medical School, Newark, New Jersey 07103
Intracellular Ca2ⴙ regulation is critical in the normal
cardiac function and development of pathologic hearts.
Phospholamban, an endogenous inhibitor of sarcoplasmic reticulum Ca2ⴙ ATPase in the sarcoplasmic reticulum, plays an important role in Ca2ⴙ cycling in heart.
Recently, sarcolipin has been identified as having a similar function as phospholamban in skeletal muscle. Because phospholamban is differentially expressed in
atrial and ventricular myocardia and its expression is
often altered in diseased hearts, we investigated the
cardiac chamber specificity of sarcolipin expression
and its regulation during development and hypertrophic remodeling. Northern blot analysis revealed
that the expression of mouse sarcolipin mRNA was most
abundant in the atria and was undetectable in the ventricles, indicating an atrial chamber-specific expression
pattern. Atrial chamber-specific expression of sarcolipin mRNA was increased during development. These
findings were confirmed by in situ hybridization studies. In addition, sarcolipin expression was down-regulated in the atria of hypertrophic heart when induced by
ventricular specific overexpression of the activated
H-ras gene. In humans, sarcolipin mRNA was also expressed in the atria but not detected in the ventricles,
although sarcolipin expression was most abundant in
skeletal muscle. Taken together, sarcolipin is likely to
be an atrial chamber-specific regulator of Ca2ⴙ cycling
in heart.
Sarcolipin (SLN)1 is a 31-amino acid proteolipid in the sarcoplasmic reticulum (SR) (1) and is shown to be expressed most
abundantly in fast twitch skeletal muscle, less abundantly in
slow twitch skeletal muscle, and even less in human and rabbit
* This work was supported in part by an open research grant from the
Japan Research Promotion Society for Cardiovascular Diseases (2001).
The costs of publication of this article were defrayed in part by the
payment of page charges. This article must therefore be hereby marked
“advertisement” in accordance with 18 U.S.C. Section 1734 solely to
indicate this fact.
¶ To whom correspondence may be addressed: Dept. of Pediatric
Cardiology, The Heart Institute of Japan, Tokyo Women’s Medical
University, 8-1 Kawada-cho, Shinjuku-ku, Tokyo 162-8666, Japan. Tel.:
81-33353-8111, ext. 24064; Fax: 81-33352-3088; E-mail: sminamis@
med.yokohama-cu.ac.jp or [email protected].
1
The abbreviations used are: SLN, sarcolipin; SR, sarcoplasmic reticulum; SERCA, SR Ca2 ATPase; PLN, phospholamban; GFP, green
fluorescent protein; TES, 2-{[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]amino}ethanesulfonic acid; ANF, atrial natriuretic factor; BNP, brain
natriuretic peptide.
cardiac muscle (2). This tissue distribution corresponds to that
of the fast twitch skeletal muscle SR Ca2⫹ ATPase (SERCA1),
and SLN interacts with SERCA1 to modulate its activity (3, 4).
By contrast, phospholamban (PLN), an integral membrane protein in the SR, is expressed abundantly in cardiac and slow
twitch skeletal muscle wherein SERCA2a is the predominant
SERCA isoform (5). Thus, SLN expression is complementary to
PLN expression. The structure and protein sequence of SLN
are similar to those of PLN (2). Therefore, SLN and PLN may
belong to the same family, and SLN is likely to be an analog of
PLN in skeletal muscle.
PLN is an endogenous inhibitor of SERCA2 and plays a
central role in regulating cardiac contractility and relaxation
(6, 7). Studies using gene-targeted mice have revealed that
disruption of the PLN gene can prevent the progression of
cardiomyopathy (8, 9), suggesting that PLN plays a critical role
in the development of heart failure. The expression of PLN is
regulated by developmental, hormonal, and hemodynamic
changes (7). In the heart, PLN is expressed predominantly in
the ventricles and, to a lesser extent, in the atria (10). The
different levels of PLN expression in the heart are, in part,
responsible for the differences in the contractile properties
between the atria and the ventricles.
In contrast to PLN, the role of SLN in the heart remains
unknown, even though SLN may play an important role in
skeletal muscle contraction (11). Because SLN expression is
complementary to PLN expression, we hypothesize that SLN
expression in the heart is higher in the atria and is regulated in
a developmental stage-specific manner and/or by cardiac
stress. In addition, Asahi et al. (4) demonstrated recently that
SLN inhibited polymerization of PLN, resulting in the superinhibition of SERCA1 and SERCA2a. Therefore, characterizing
the expression pattern of SLN is important for establishing the
potential modulatory role in cardiac function.
In the present study, we examined the effects of development
and cardiac hypertrophy on the expression of SLN mRNA in
heart. We found that the expression of SLN mRNA was detected selectively in the atria. Furthermore, SLN was up-regulated during development and down-regulated by cardiac hypertrophy. SLN mRNA remained undetectable in the ventricles
during development or in the hypertrophic state, indicating
that the atrial chamber-specificity of SLN was conserved.
Taken together, we propose that SLN may serve an important
regulator of Ca2⫹ cycling in the atrium and may be an important factor in differentiating the contractile properties of the
atria from those of the ventricles.
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This paper is available on line at http://www.jbc.org
Atrial Chamber-specific Expression of Sarcolipin
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TABLE I
Primers
Gene
Mouse SLN
Human SLN
Mouse SERCA1
EMBL/GenBank™
accession number
NM_025540
U96094
NM_007504
Forward
Reverse
5⬘-CTGAGGTCCTTGGTAGCCTG-3⬘ 5⬘-GGTGTGTCAGGCATTGTGAG-3⬘
5⬘-GAGGTGAGGACAAGCCAGAG-3⬘ 5⬘-GTCTCAGGGCATAGAGCAGG-3⬘
5⬘-TGCTCGGAACTATCTGGAGG-3⬘ 5⬘-GAGTTCGGGAAGGGGATTTA-3⬘
Size of PCR product
331-bp
237-bp
258-bp
FIG. 1. Atrial chamber-specific expression of sarcolipin mRNA. A,
northern blot analysis for the various
mouse tissues. Using the full-length SLN
cDNA probe, a single transcript of 0.9 kb
was expressed most abundantly in the
atria of the heart, less abundantly in
esophageal muscle, and least abundantly
in skeletal muscle and bladder. No hybridized signal was detected in the ventricles. A small amount of SERCA1 transcript was detected in the atria. There
was robust SERCA1 expression in mouse
smooth muscle. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was used
for internal control. B, in situ hybridization analysis using an antisense SLN
RNA probe. SLN mRNA was expressed
restrictedly in the atria but not in the
ventricles. The localization of SLN transcript was distributed uniformly in both
the right and left atria. C, a high magnification image of the lesion indicated as a
rectangle in panel B. EDL, extensor digitorum longus muscle; LV, left ventricle;
RV, right ventricle; LA, left atrium; RA,
right atrium. Bar, 1 mm (B).
EXPERIMENTAL PROCEDURES
Animals and Tissue Samples—ICR mice obtained from Clea Japan
Inc. (Tokyo, Japan) were inbred at Tokyo Women’s Medical University.
Embryos from ICR mice were obtained from timed pregnant animals.
The transgenic model of cardiac hypertrophy was established by ventricular specific expression of activated H-ras transgene. In this study, we
applied the same “gene-switch” transgenic strategy as reported previously
(12). Briefly, a green fluorescence protein (GFP) coding sequence and
poly(A) regions were cloned from pEGFP-N1 (Invitrogen) and inserted
between the two loxP sequences of the pUC1015loxlox vector (a gift from
Dr. Jamy Marth, University of California, San Diego, CA) to generate the
pfloxGFP vector. The XbaI/SmaI fragment from pfloxGFP, containing the
GFP-poly(A) sequences flanked by two copies of loxP, was then inserted at
the HindIII site of the pMHC vector behind the 5.5 kilobase mouse
␣-MHC promoter (a gift from Dr. Jeffery Robbins, University of Cincinnati) to generate the pMHC-floxGFP vector. The cDNA fragment coding
for the activated H-ras (V15) mutant was then inserted at the EcoRV site
behind the loxP-GFP-loxP fragment to generate the final pMHC-floxRas
vector. Transgenic mice (floxRas) were generated by established intranuclear injection methods using a KpnI/SstI fragment digested from the
pMHC-floxRas vector. The GFP marker gene was expressed in both the
atria and ventricles. The floxRas mice were crossed with MLC-2v/Cre
mice in which the Cre enzyme was only expressed in the ventricular
muscle cells. The GFP expression cassette was floxed out, and the activated H-ras mutant was led to be expressed in the ventricles of double
transgenic mice. The effect of the H-ras mutant on cardiac hypertrophy
has been validated (13).
Human tissues were obtained from specimens at autopsy. Informed
consent was obtained from the patients and/or their families.
Total RNA Isolation—Total RNA was isolated from various tissues
using TRIzol reagent (Invitrogen) as recommended by the manufacturer. For the developmental study, atrial and left ventricular tissues
from embryos and neonates were pooled. The isolated RNA was quantified by spectrophotometry.
cDNA and RNA Probes—To obtain the mouse and human full-length
SLN cDNAs and mouse partial-length SERCA1 cDNA, a set of primers
was designed based on the reported sequence (Table I). The mouse
SERCA1 cDNA included the end of the coding sequence and 3⬘-untranslated region. Reverse transcription PCR was performed using the Superscript preamplification system (Invitrogen) as recommended by the
manufacturer. The reaction of PCR was set to denature at 94 °C for
30 s, anneal at 55 °C for 30 s, and extend at 72 °C for 45 s for 30 cycles.
The obtained PCR fragments were subcloned in pCR II vector (Invitrogen). The antisense mouse SLN digoxygenin-labeled RNA probe was
created from 1 ␮g of HindIII-linearized mouse SLN pCRII vector using
T3 RNA polymerase.
Mouse SERCA2a and PLN cDNA probes were kindly provided by Dr.
Wolfgang H. Dillmann (University of California, San Diego, CA) and
Dr. Evangelia G. Kranias (University of Cincinnati College of Medicine), respectively. We confirmed the nucleotide sequences of these
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Atrial Chamber-specific Expression of Sarcolipin
probes compared with the reported sequences by direct DNA
sequencing.
Northern Blot Analysis—Northern blot analysis was performed as
described previously with modification (14). Total RNA was electrophoresed on a 1% formaldehyde-agarose gel and transferred onto Nytran membranes (Schleicher & Schuell). Blots were hybridized at 68 °C
with radiolabeled probes in Quickhyb solution (Stratagene, La Jolla,
CA) as recommended by the manufacturer. Membranes were washed
under high stringency conditions (0.1⫻ SSC, 0.1% SDS) at 60 °C.
In Situ Hybridization—In situ hybridization was performed as described previously (15). Paraffin sections were prepared using a standard
protocol.
SERCA Activity—Microsomes enriched in SR membranes were obtained as described previously (16). SR Ca2⫹ ATPase activity was measured as described previously with modification (17). Using 5 ␮g of SR
protein, the reaction was carried out at 37 °C in the reaction medium
containing 30 mM TES, 100 mM KCl, 5 mM NaN3, 5 mM MgCl2, 0.5 mM
EGTA, and 4 mM ATP with or without 0.5 CaCl2. Data are expressed as
mean ⫾ S.E. Statistical significance was analyzed using Student’s
unpaired t test.
RESULTS
SLN mRNA Was Expressed Abundantly in the Murine
Atria—The distribution of SLN mRNA in various mouse tissues was analyzed by Northern blot using a PCR-amplified
cDNA probe as described under “Experimental Procedures.” A
single 0.9-kb transcript was detected. Specifically, SLN mRNA
expression was most abundant in the atria of the heart, less
abundant in esophageal muscle, and least abundant in skeletal
muscle and bladder (Fig. 1A). In murine myocardium, the expression of SLN mRNA was restricted to the atria and was not
present in the ventricles. In skeletal muscle, the similar level of
SLN mRNA expression was detected in fast twitch skeletal
muscle (extensor digitorum longus muscle) and slow twitch
skeletal muscle (soleus muscle). In smooth muscle, SLN mRNA
expression was much greater in esophagus than in bladder.
SLN mRNA was not detected in brain, kidney, liver, spleen,
thymus, and lung (liver, spleen, thymus, and lung, data not
shown) by Northern blot analysis.
In situ hybridization analysis using an antisense SLN RNA
probe also demonstrated that SLN mRNA was expressed only
in the atria and not in the ventricles of the heart (Fig. 1B). The
localization of SLN transcript was distributed uniformly in
both right and left atria.
We also examined the expression of SERCA1 mRNA in the
mouse heart, because SLN is believed to interact with
SERCA1. As shown in Fig. 1A, a very small amount of the
SERCA1 transcript was detected in the atria. The level of
SERCA2 mRNA in the atria was comparable with that in the
ventricles. At the protein level, only SERCA2a but not SERCA1
was detected in the atria, using SERCA isoform-specific antibodies (data not shown; SERCA1 antibody (MA3–912) and
SERCA2 antibody (MA3–919) were from Affinity Bioreagents,
Inc.). There was abundant SERCA1 expression in mouse
smooth muscle. SERCA1 mRNA levels were similar in esophagus and bladder. The tissue distribution of PLN and
SERCA2a was consistent with previous reports (18, 19). SR
Ca2⫹ ATPase activity in the atria (n ⫽ 5) was significantly
higher than that in the ventricles (n ⫽ 7) (1573 ⫾ 85 nmol/mg
of protein/min and 348 ⫾ 35 nmol/mg of protein/min, respectively) in adult murine hearts.
SLN Was Developmentally Up-regulated in the Atria—The
developmental changes of SLN and SERCA1 mRNA in the atria
were also determined by Northern blot analysis. Fig. 2A demonstrates that the expression of SLN and SERCA1 mRNA was
increased over time in the atria and not detected in the ventricles
at any of the developmental stages. Interestingly, the expression
of SERCA1 mRNA became very low in the atria in older mice (2
years old), suggesting that SERCA1 expression is down-regulated in the atria with aging. The expression of ANF transcript
FIG. 2. Developmental up-regulation of sarcolipin mRNA in
the atria. A, the developmental change in the expression of SLN,
SERCA1, and ANF mRNA in the atrial and ventricular myocardium.
Northern blot analysis revealed that these transcripts are up-regulated
during development in the atria. The expression of ANF transcript was
also detected in the ventricles of embryos at embryonic day 17.5 and in
older mice (at 2 years old). B, the developmental changes in the atrial
expression of SLN and SERCA1 mRNA in detail. No SLN transcript
was detected in embryos at embryonic day (ed) 10.5 but became detectable in the atria at embryonic day 12.5. After embryonic day 16.5, the
expression of SLN mRNA was abruptly increased.
was also increased over time in the atria. In contrast to SLN,
ANF transcripts were detected in the ventricles of embryos at
embryonic day 17.5 and at 2 years old of age. We further examined the developmental changes of SLN and SERCA1 mRNA
expression in detail. No SLN transcript was found in embryos at
embryonic day 10.5 but became detectable in the atria at embryonic day 12.5 (Fig. 2B). After embryonic day 16.5, the expression
of SLN mRNA was abruptly increased in the atria. These
changes were confirmed by in situ hybridization studies using
embryos at specific developmental stages. At embryonic day 9.5
no SLN transcript was detected (data not shown), whereas at
embryonic day 12.5 SLN mRNA was present at a low level in the
atria and expressed abundantly in the progenitors of skeletal
muscle (Fig. 3).
SERCA1 mRNA was expressed weakly in the atria at embryonic day 10.5 and 12.5 but was not detectable after embryonic day 16.5 (Fig. 2B). The SERCA1 transcript was detected
again at 21 days after birth. Thus, the developmental changes
in SLN and SERCA1 transcripts were not exactly coordinated.
SLN and SERCA1 mRNA Were Down-regulated in the Atria
in Hypertrophic Hearts—The transgenic mice overexpressing
activated H-ras developed severe concentric ventricular hypertrophy with concomitant obstruction of forward blood flow from
the atria to the ventricles, resulting in marked enlargement of
atrial chambers (Fig. 4). The levels of SLN, PLN, SERCA1 and
SERCA2a mRNA were all decreased in the atria of the transgenic mice at 3 months old (Fig. 5). Although ANF and BNP
Atrial Chamber-specific Expression of Sarcolipin
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FIG. 3. In situ hybridization analysis of sarcolipin mRNA in an embryo
at embryonic day 12.5. Using an antisense SLN riboprobe, SLN mRNA was detected at low levels in the atria and was
much higher in the progenitors of skeletal
muscle. A, atrium; L, liver; T, tongue; V,
ventricle. Bar, 1 mm.
FIG. 4. Marked ventricular hypertrophy and dilated atria in Ras transgenic mice. The activated H-ras transgenic mice developed severe
concentric ventricular hypertrophy and marked enlargement of atrial chambers (A, B, and C, non-transgenic mice; D, E, F, and H, activated H-ras
transgenic mice). Panels A and D are the cross-sections of the ventricles. Severe concentric ventricular hypertrophy was detected in the activated
H-ras transgenic mice (D). Panel E is a high magnification image of the left ventricle of the activated H-ras transgenic mice demonstrating
disorganized myofibrils. Organized thrombi were found frequently in the atria of the activated H-ras transgenic mice (panels F and H). Panel H
is the frontal section of a H-ras transgenic heart.
transcripts were increased in both the atria and ventricles of
the transgenic mice, SLN mRNA remained undetectable in the
hypertrophic ventricles.
SLN mRNA Was Also Expressed Specifically in the Human
Atria—Although a previous study showed that SLN expression
was low in human heart, its chamber-specific localization has
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Atrial Chamber-specific Expression of Sarcolipin
FIG. 5. Down-regulation of sarcolipin and SERCA1 mRNA in H-ras
transgenic mice. The representative
Northern blot analysis of the activated
H-ras transgenic mice. The expressions of
SLN, PLN, SERCA1, and SERCA2a
mRNA were all down-regulated in the
atria of the H-ras transgenic mice at 3
months old. ANF and BNP were induced
in the atria and the ventricles of the H-ras
transgenic mice. RasTG, the activated Hras transgenic mice; NTG, non-transgenic
mice as control. GAPDH, glyceraldehyde3-phosphate dehydrogenase.
remained undetermined. We found that SLN mRNA was expressed abundantly in human atria and was undetectable in
ventricles (Fig. 6). In contrast to mouse, the expression of SLN
in the atria was lower than in skeletal muscle in human. Using
the mouse SERCA1 cDNA probe that has 75% homology with
human SERCA1, no hybridization signal was detected in the
human atria, even under low stringent conditions.
DISCUSSION
The present study revealed that SLN mRNA was expressed
abundantly in the murine and human atria but was not detected in the murine and human ventricles by Northern blot
analysis. In mice, the expression of SLN mRNA was most
abundant in the atria, and it was lower in skeletal muscle than
in the atria and smooth muscle. By contrast, in humans the
expression of SLN mRNA was weaker in the atria than that in
skeletal muscle. Previous studies have demonstrated the predominant expression of SLN mRNA in human and rabbit fast
skeletal muscle (2) and the very weak expression of SLN
mRNA in rat extensor digitorum longus (EDL) muscle (20).
These data are consistent with the findings in the present
study, suggesting that the tissue distribution of sarcolipin
mRNA is regulated differently between humans and rodents.
The fact that the nucleotide sequence of SLN cDNA is not well
conserved between mice and humans and that their polyadenylation signal sequences are different (2) may account for
the different tissue distribution between rodents and humans.
Importantly, the atrial chamber-specific expression of SLN
was developmentally up-regulated, and it was down-regulated
in hypertrophic remodeling at the transcriptional level. These
changes are similar to those of PLN in the ventricles (21) (22).
No ventricular SLN expression was detected at any stage of
development or cardiac hypertrophy. We did not investigate
the protein levels of SLN, because a SLN antibody was not
readily available. However, a previous study (2) demonstrated
that changes in the SLN levels of mRNA and protein are
proportional, although the magnitude of the changes in SLN
protein was greater than that in SLN mRNA in the skeletal
muscle. Therefore, it is likely that the SLN protein is also
expressed abundantly in the murine atria.
PLN decreases the apparent affinity of SERCA2a for Ca2⫹
and inhibits Ca2⫹ uptake into the SR. The expression level of
FIG. 6. The high expression of sarcolipin mRNA in the human
atria. Abundant SLN expression was also detected in the human atria
but not in the ventricles. The expression was most abundant in skeletal
muscle. Using a mouse SERCA1 cDNA probe, no hybrydization signal
was detected in the human atria. GAPDH, glyceraldehyde-3-phosphate
dehydrogenase.
PLN directly affects cardiac contractility and relaxation in the
heart (23, 24). SLN also inhibits Ca2⫹ uptake at low Ca2⫹
concentrations like PLN but enhances Ca2⫹ uptake at high
Ca2⫹ concentrations (3, 25). Therefore, the atrial chamberspecific expression of SLN may be related to functional differences between the atria and ventricles, and the abundant ex-
Atrial Chamber-specific Expression of Sarcolipin
pression of SLN may play an important role in the regulation of
Ca2⫹ cycling in the atrial myocardium.
The coordinated contraction of the atria with the ventricles is
essential for normal cardiac output. However, the physiological
characteristics are quite different between atrial and ventricular myocardium. For examples, muscle contraction time,
measured as a time to peak tension, and the duration of the
myocyte Ca2⫹ transients are shorter in atrial muscle compared
with ventricular muscle. In the present study, we also found
that SR Ca2⫹ ATPase activity is significantly higher in the
atria than in the ventricles. This may be due to different levels
of PLN expression between atria and ventricles (10). In addition to the lower expression of PLN, SLN may also contribute to
the chamber-specific physiological properties of the atria. Furthermore, we found that SERCA1 mRNA was expressed
weakly in the mouse atria and that its expression was also
regulated during development and hypertrophic remodeling.
However, it is unlikely that SERCA1 contributes to the higher
ATPase activity in the atria, because SERCA2a is the predominant isoform, and the SERCA1 protein was not detected in
the atria.
In skeletal muscle, SLN expression is regulated by chronic
low frequency stimulation (3) or corticosteroids (20). Odermatt
et al. (3) suggested that reduced SLN expression associated
with the decrease in SERCA1 activity represented an early
functional adaptation to chronic low frequency stimulation.
They proposed that SLN provides a constant stimulation of
SERCA Vmax to increase the activity of SERCA. Thus, the
abundant expression of SLN with an increase in the activity of
SERCA and faster contraction in the atria is consistent with
this model. However, these investigators demonstrated recently that SLN inhibited both SERCA1 and SERCA2a activities in human embryonic kidney 293 cells (4) and in in vivo slow
skeletal muscle (11). Moreover, they suggested that SLN binds
directly to PLN and increases the formation of PLN monomers
that inhibits SERCA activity (4). If these data are true in the in
vivo heart, overexpression of SLN should result in the depression of SERCA activity and slow muscle contraction. Further
studies using transgenic mice overexpressing SLN in heart are
needed to address these issues.
Using a Ras transgenic mouse model for hypertrophy, we
demonstrated that SLN mRNA was down-regulated in cardiac
hypertrophy. Isolated cardiomyocytes from Ras transgenic
mice displayed reduced contraction and prolonged Ca2⫹ transients compared with wild-type myocytes2 despite the reduction of PLN expression. Other studies also demonstrated that
Ca2⫹ cycling in the atria is impaired in cardiac hypertrophy or
chronic atrial stimulation such as atrial fibrillation. Therefore,
decreases in the expression of SLN, SERCA1, and SERCA2
may result in the reduction of Ca2⫹ uptake in the atria and
may play a more important role in maintaining the atrial
function than the level of PLN expression.
Another important finding in the present study is that SLN
mRNA was not expressed in the ventricular myocardium during any stage of development or in a cardiac hypertrophic state,
indicating that SLN is an atrial chamber-specific molecular
2
S. Miamisawa, Y. Wang, and K. R. Chen, unpublished observations.
9575
marker. In contrast to SLN, other atrial chamber-specific molecular markers such as ANF and atrial type of myosin light
chain 1 are induced in ventricles by cardiac stresses. Therefore,
a putative promoter/enhancer gene fragment of SLN may be
useful for atrial chamber-specific gene targeting or gene
delivery.
In conclusion, we found that SLN, a counterpart of PLN, was
expressed specifically in the atrial myocardium. The atrial
chamber-specific expression of SLN was up-regulated during
development and down-regulated in hypertrophic remodeling.
The present study suggests that SLN is an important regulator
of Ca2⫹ cycling in the atrium.
Acknowledgments—We thank Dr. Tomoyuki Nakamura and Dr. Yoji
Sato for stimulating discussion, Dr. Paul Grossfeld for critical reading
of the manuscript, Ms. Barbara Levene for language editing of the
manuscript, and Ms. Keiko Komatsu for preparation of paraffin
sections.
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