Download SERCA pump level is a critical determinant of Ca2+ homeostasis

J Mol Cell Cardiol 33, 1053–1063 (2001)
doi:10.1006/jmcc.2001.1366, available online at http://www.idealibrary.com on
Review Article
SERCA Pump Level is a Critical
Determinant of Ca2+ Homeostasis and
Cardiac Contractility
Muthu Periasamy1 and Sabine Huke
Division of Cardiology, University of Cincinnati, College of Medicine, 231 Albert Sabin Way,
Cincinnati, Ohio 45267-0542, USA
(Received 7 February 2001, accepted 7 February 2001)
M. P  S. H. SERCA Pump Level is a Critical Determinant of Ca2+ Homeostasis and Cardiac
Contractility. Journal of Molecular and Cellular Cardiology (2001) 33, 1053–1063. The control of intracellular
calcium is central to regulation of cardiac contractility. A defect in SR Ca2+ transport and SR Ca2+ ATPase pump
activity and expression level has been implicated as a major player in cardiac dysfunction. However, a precise
cause–effect relationship between alterations in SERCA pump level and cardiac contractility could not be
established from these studies. Progress in transgenic mouse technology and adenoviral gene transfer has provided
new tools to investigate the role of SERCA pump level in the heart. This review focuses on how alterations in
SERCA level affect Ca2+ homeostasis and cardiac contractility. It discusses the consequences of altered SERCA
pump levels for the expression and activity of other Ca2+ handling proteins. Furthermore, the use of SERCA
pump as a therapeutic target for gene therapy of heart failure is evaluated.
 2001 Academic Press
K W: SR Ca2+ ATPase; Contractility; Transgenic; Gene therapy.
Introduction
The study of sarcoplasmic reticulum (SR) gene
expression and SR calcium transport function has
been an active area of research for decades because
of its importance to cardiac function and disease.
A regulated release and uptake of intracellular Ca2+
between SR and cytoplasm tightly controls the
contraction–relaxation cycle of the heart. Muscle
contraction is initiated when Ca2+ enters the cell
via L-type Ca2+ channels in the sarcolemma and,
as a consequence, triggers the release of a much
larger amount of Ca2+ from the SR via SR Ca2+
release channels (ryanodine receptor, RyR).1,2 The
free cytosolic Ca2+ concentration determines the
extent of muscle activation and therefore regulates
force development. The SR Ca2+ ATPase (SERCA)
pumps the Ca2+ back into the SR and is therefore
responsible for muscle relaxation and for replenishing Ca2+ stores needed for the next
1
contraction.3 SERCA pump activity is regulated by
the small 52-aminoacid phosphoprotein phospholamban (PLB), which in its unphosphorylated state
lowers the affinity of SERCA for Ca2+.4 Moreover,
Ca2+ removal by the Na+–Ca2+ exchanger in the
sarcolemma contributes to maintenance of intracellular Ca2+ homeostasis.5
A number of studies, conducted on animal models
of heart failure and human failing hearts, suggest
that alterations in SR Ca2+ handling are a critical
feature of the hypertrophied or failing myocardium.
Alterations in the expression of different SR proteins
and its associated Ca2+ transport abnormalities in
cardiac hypertrophy and heart failure have been
recently reviewed by Houser et al.6 The purpose of
this review is to highlight the role of SERCA in
intracellular Ca2+ homeostasis and cardiac contractility as learned from in vivo and in vitro genetic
manipulations of the SERCA pump. This review
focuses on the following major questions: (i) Is
E-mail: [email protected]
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 2001 Academic Press
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M. Periasamy and S. Huke
the protein level of SERCA pump critical for Ca2+
homeostasis and cardiac function? (ii) How do alterations in SERCA pump level affect other Ca2+
handling proteins? (iii) Can SERCA pump serve as
a therapeutic target to restore cardiac function in
failing myocardium?
The SR Ca2+ ATPase Gene Family
The SERCA pump is a transmembrane protein of
>110 kDa and belongs to a family of highly conserved proteins. Molecular cloning analyses have
identified a family of SERCA pumps encoded by
three highly homologous genes (SERCA1, SERCA2
and SERCA3) (reviewed in Arai et al.7). The SERCA1
gene encodes two alternatively spliced transcripts,
SERCA1a and SERCA1b, which differ only by a few
amino acids in their carboxyl terminus.8 SERCA1b
is expressed in the fetal/neonatal stages of fast
twitch skeletal muscle development and is gradually
replaced by the SERCA1a isoform in the adult fast
twitch skeletal muscle.9 The SERCA2 gene is also
alternately spliced and encodes SERCA2a and SERCA2b isoforms.10 SERCA2a protein terminates with
four unique amino acids (Ala-ILe-Leu-Glu), whereas
SERCA2b has an extended hydrophobic sequence
of 49 amino acids at its carboxyl terminus.11,12 The
SERCA2b isoform is expressed ubiquitously and
is found to be associated with IP3 gated Ca2+
stores,11,13 whereas SERCA2a is the primary isoform
expressed in the heart.10,14,15
SERCA2a Pump Expression Level in the
Heart
The SERCA2a isoform plays a central role in SR
Ca2+ handling required for excitation–contraction
coupling in the heart. Although SERCA2a isoform
is expressed at high levels in cardiac myocytes, the
expression level of SERCA is not a fixed parameter.
There are naturally occurring regional differences,
developmental changes and aging-related effects as
well as alterations due to variations in thyroid
hormone levels. These changes can be drastic, as
shown for atrium in comparison to ventricle. The
expression level of SERCA in atrium v ventricle is
reported to be two-fold in most experimentally used
animals and in humans.16,17 With respect to the
functional differences, the higher SERCA pump
levels may account at least in part for the shorter
duration of contraction in atrial v ventricular tissue.16,18 Moreover, it was shown for mouse, rat and
rabbit that the expression of SERCA pump gradually
increases during development.16,19–22 This increase
was again accompanied by a shortening of relaxation time in adult v neonatal ventricle.22 In
addition also in adult hearts SERCA levels are not
steady, but are influenced by aging and fluctuations
in thyroid hormone levels. A decrease in content
and activity of SERCA was described in experimental
models of senescence and in senescent human myocardium.23,24 This decrease was associated with a
prolonged contraction time and depressed myocardial function. Hearts from rats and mice with
increased thyroxine levels showed increased
SERCA expression and significant increased contractility, whereas hypothyroid hearts displayed
decreased SERCA levels together with reduced
contractility.25,26
Therefore, several naturally occurring variations
in SERCA expression level correlate with the contractile status of the heart. The expression level
of SERCA pump protein appears to be a critical
determinant of cardiac contractility. In all these
studies the alteration in SERCA expression is in
consent with the accompanying functional
changes, but also changes in other factors might
contribute. Several additional tissue-specific (atrium
v ventricle), developmental and thyroxine-induced
changes may be equally responsible for the observed
changes in relaxation time and contractility.27–29
In the past three decades a decrease in SERCA
gene expression and activity was observed in a wide
variety of pathological conditions. Varying degrees
of defects in the SR Ca2+ uptake function have been
identified in animal models of heart disease and have
been shown to correlate with altered contractile
function (reviewed in Arai et al.7). Studies from
many laboratories have shown that the expression
level of SERCA is significantly decreased in pressure
overload (PO)-induced hypertrophy/heart failure.30–34 In these studies decreased SR calcium
transport and formation of the phosphoenzyme
intermediate, E-P, was observed.7,31–33,35
In addition to studies using animal models of
cardiac diseases, there are considerable data indicating that SR Ca2+ transport function is altered
in end-stage human heart failure. Intracellular Ca2+
measurements using aequorin and fura-2 showed
markedly prolonged Ca2+ transients in both Ca2+
release and uptake phases in muscle samples from
failing human hearts.36,37 It was shown that the
expression level of SR Ca2+ ATPase was decreased
both at the mRNA and protein level in end-stage
heart failure.38,39 Some other laboratories could
not observe a decrease in SERCA protein expression (for overview see Hasenfuss40). A decrease
SERCA Pump and Cardiac Contractility
in the level of SR Ca2+ ATPase (mRNA or protein
or activity) was closely correlated with decreased
myocardial function or altered force–frequency
response.38,39,41–43
The studies described above suggest that SERCA
pump level is critical for the maintenance of cardiac
function, but the true relationship between SERCA
levels and muscle function cannot be defined. Too
many changes occur in parallel within the heart
and myocytes during progression to heart failure.
Similarly, physiological differences in SERCA expression (development, aging etc.) are accompanied
by simultaneous changes in other excitation–
contraction coupling proteins. Studies using these
models do not allow us to establish a precise causeeffect relationship between SERCA pump level and
cardiac contractility. Therefore, others and we have
taken a transgenic approach to specifically manipulate SERCA pump levels in vitro and in vivo.
Genetic Manipulation of SERCA Pump
Expression
To manipulate SERCA pump expression levels several investigators have utilized in vitro-adenoviralmediated gene transfer to produce targeted alterations in protein expression in cardiac myocytes.
A number of groups including us have generated
genetically altered mouse models to critically assess
the functional relevance of changes in SERCA levels
for contractile function. The usage of in vivo models
allows us to study the long-term changes in cardiovascular function at the levels of both the organ
and the whole animal. Transgenic animals with
increased SERCA pump levels in the heart (SERCA
level ↑) and a mouse model with decreased SERCA2
expression (SERCA level ↓) are powerful tools to
define the role of SERCA pump level for Ca2+ homeostasis and cardiac function. In this review we will
omit detailed description of already published data,
instead we will focus on what we have really learned
from these models so far.
Increased SERCA pump expression
Transgenic animals with increased SERCA pump
levels in the heart were generated by several investigators. Two independent mouse lines overexpressing SERCA2a protein (cardiac isoform) 1.2fold44 and 1.5-fold45 were described. One intriguing
finding of these experiments was that despite much
higher mRNA levels for SERCA2a (2.6-fold and
1055
eight-fold, respectively) the increase in SERCA2a
protein level was only modest above the endogenous
expression. The basis for this observation is still not
known. There might be competition between the
exogenous and the endogenous protein and/or
there are powerful post-transcriptional mechanisms
working to maintain a physiological SERCA level.
Still, the moderate increase in SERCA protein led
to significant functional changes in both models.
He et al.44 described faster rates of Ca2+ decline as
well as increased maximal rates of shortening and
relengthening in isolated myocytes. The SR Ca2+
content of caffeine-sensitive stores was increased
by 29%. The relaxation time of isolated papillary
muscles was shorter and maximal rates of contraction and relaxation, evaluated by in vivo cardiac
catheterization, were higher. Baker et al.45 described
an increase of 37% in the maximum velocity of
Ca2+ uptake function and SERCA level dependent
increased maximal rates of contraction and relaxation in the isolated work-performing heart preparation. Recently, SERCA2a overexpression in
transgenic rats was shown to increase parameters
of contraction and relaxation.46
Taken together these studies demonstrate that (i)
it is possible to increase the SERCA2a pump level
in vivo, (ii) increases in SERCA2a pump levels alter
Ca2+ homeostasis and enhance contractile function
of the heart and (iii) SERCA2a overexpression is
not pathological.
Furthermore, these findings are not limited to
increases in the expression of SERCA2a isoform.
Very similar results were obtained in transgenic
mice ectopically expressing the fast-twitch skeletal
muscle Ca2+ ATPase SERCA1a.47,48 The SERCA1a
pump is a pump with faster kinetics in vitro.49,50
Overexpression of this pump (under the MHCpromoter) increased SERCA levels to 2.5-fold and
Ca2+ uptake to 1.9-fold, but the apparent affinity
for Ca2+ was unchanged. The measured functional
parameters of these hearts were clearly enhanced,
but only slightly pronounced in comparison to the
mouse-line with 1.5-fold SERCA2a protein expression. It seems that the SERCA1a expressing
hearts reach a maximal contractile limit and other
factors do not allow further enhancement of cardiac
function, but this hypothesis needs to be tested
under varying experimental conditions. Interestingly, the expression of SERCA1a led to a 50%
downregulation of the endogenous SERCA2a pump,
as detected by isoform specific antibodies (see Fig.
1). This suggests a competition of SERCA2a and
1a for functional sites in the SR, but might also be
due to a compensatory downregulation of the endogenous SERCA2a isoform. The reduced SERCA2a
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M. Periasamy and S. Huke
Figure 1 Western blot analysis of SERCA1a TG mouse hearts. Increasing amounts of protein from TG and control
hearts were separated in 10% SDS-PAGE, transferred onto nitrocellulose membrane and probed with SERCA1a-,
SERCA2a-, calsequestrin-, -sarcomeric actin-, and tropomyosin-specific antibodies.47
levels and the enhanced cardiac function in these
mice clearly demonstrate that SERCA1a can functionally substitute for SERCA2a in the heart.
Recently, we generated transgenic mice with increased cardiac levels of SERCA2b pump.51 SERCA2b has a higher Ca2+ affinity than SERCA2a
and it is endogenously expressed in the heart, but
in a much smaller amount than SERCA2a. In these
mice the 8–10-fold overexpression of SERCA2b did
not cause a reduction of SERCA2a pump (unlike
SERCA1a overexpressing mice), most likely due to
a distinct distribution of SERCA2b within the SR.
This was further validated by immunostaining of
isolated myocytes. The distribution pattern shows
a preferential localization for SERCA2b around the
T-tubules, whereas SERCA2a is distributed transversely and longitudinally in the SR membrane.
The maximal velocity of Ca2+ uptake was not
changed in hearts from SERCA2b mice, but the
apparent Ca2+ affinity was increased. The heart
function determined by working heart preparations was enhanced, indicating that SERCA2b
contributes to SR Ca2+ transport on a beat-tobeat basis.
Decreased SERCA pump expression
To understand how decreased SERCA levels affect
cardiac function, we chose to disrupt the SERCA2
gene by homologous recombination.52 As expected,
the disruption of both copies of SERCA2 gene is
lethal, whereas heterozygous mice with a single
functional allele are alive and reproduce well. SERCA2a protein levels and the maximal velocity of
SR Ca2+ uptake were reduced by >35%. In spite
of that heterozygous mice do not develop cardiac
hypertrophy or other signs of heart failure, so far
examined for up to 12 months of age. In the absence
of cardiac pathology, the peak amplitude of the
Ca2+ transients in isolated single myocytes was
decreased by more than 30%, resulting in decreased
rates of cell shortening and relengthening in heterozygous mice.53 Measurements of in vivo cardiovascular function via transducers in the left
ventricle and right femoral artery of the anesthetized mouse revealed reductions in heart
rate, mean arterial pressure, systolic ventricular
pressure, and the absolute values of both maximal
rates of contraction and relaxation. These results
SERCA Pump and Cardiac Contractility
1057
Figure 2 Representative recordings of cardiac performance in isolated work-performing hearts from SERCA1a TG,
FVB/N controls (WT) and SERCA2 heterozygous (SERCA2-HET) mice. Mean aortic pressure was set to 50 mmHg and
venous return to 5 ml/min. Aortic-, left ventricular pressure and the rate of contraction (dP/dt) is shown.
demonstrate that two functional copies of the
SERCA2 gene are required to maintain “normal”
levels of SERCA2a protein, Ca2+ sequestering activity, and Ca2+ homeostasis, and that the loss
of one functional SERCA2 gene depresses cardiac
function.
These studies using transgenic models with
altered SERCA levels/activity suggest that there
is a direct correlation between SERCA level and
contractile state of the heart. Alterations in the
SERCA level, an increase or a decrease, modulate
cardiac contractility (see Fig. 2). Therefore we conclude that the level of functional SERCA protein in
the SR is one of the fundamental determinants of
cardiac contractility.
Is there a cross-talk between SERCA and other Ca2+
transport proteins?
SERCA pump is a major determinant of SR Ca2+
transport and changes in its level alter intracellular
Ca2+ homeostasis. Thus, this could have a profound
effect on the expression or activity of other Ca2+
handling proteins that participate in the maintenance of the intracellular Ca2+ homeostasis. These
include proteins in the SR membrane, such as
phospholamban (PLB), an inhibitory regulator of
SERCA pump and the SR Ca2+ release channel
(ryanodine receptor, RyR). In addition, sarcolemmal
Ca2+ handling proteins, the Na+–Ca2+ exchanger
(NCX) and the L-type Ca2+ channel could be altered
due to changes in SERCA expression. Some of the
following interpretations are supported by a limited
amount of data and need further confirmation.
Does the SERCA level affect the PLB protein levels and
its interaction with SERCA pump?
A change in SERCA level could affect the dynamics
of SERCA/PLB interaction. An increase in SERCA
pump may shift the equilibrium of SERCA/PLB ratio
in favor of more free pump. On the other hand, a
decrease in SERCA may result in higher PLB ratio
to SERCA resulting in maximal inhibition of SERCA
pump. However, it is not yet clear if the expression
of one protein determines the expression and/or
activity of the other.
Recently, we reported that chronic alterations in
SERCA2a pump levels modify the PLB protein
level.53 We found that in SERCA2 heterozygous
hearts, where SERCA2a pump levels are decreased
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M. Periasamy and S. Huke
by >35%, PLB protein levels are decreased by
>40%. It seems possible that a cross-talk exists
between these two proteins to maintain a constant
PLB:SERCA2a ratio. Similarly, in mice ectopically
expressing SERCA1a in the heart (total SERCA level
2.5-fold) the PLB protein levels were decreased
by >50%, in line with a 50% reduction in the
endogenous SERCA2a pump levels (Prasad et al.,
unpublished data). This rather intriguing finding
suggests that SERCA2a pump level, but not SERCA1a level, may modulate PLB protein level. Recently, Ver Heyen et al. generated a mouse model
in which the SERCA2a isoform is completely replaced by SERCA2b by exon-specific gene targeting
methods.54 In this model PLB protein levels are
increased over two-fold despite a decrease in total
SERCA protein (SERCA2b) to around 50% of nontransgenic controls. This was most intriguing considering that SERCA levels are drastically reduced
in these hearts. This leaves the impression that in
the absence of its endogenous partner SERCA2a
the regulation of PLB protein level is lost. Based on
these studies one could speculate that SERCA2a is
the virtual partner for PLB. However, studies have
shown that PLB can interact with SERCA1 and
other SERCA2 isoforms when co-expressed.55–57
These data could provide support to the idea that
chronic alterations in SERCA2a pump levels can
influence PLB levels thereby maintaining an optimal
PLB:SERCA2a ratio.
It is unclear whether simple protein ratio can be
equated to a functional ratio of PLB to SERCA,
since PLB exists both as a monomer and pentamer
and the PLB monomer is shown to be the more
active inhibitor.58,59 Changes in SERCA pump level
could influence the pentamer to monomer ratio,
although this has not been carefully studied. Additionally, it has been well established that phosphorylation of PLB relieves the inhibition and
therefore accelerates SERCA pump activity.60,61 An
important finding in SERCA2 heterozygous hearts
is the enhanced basal phosphorylation status of
PLB (in vivo) at both Ser-16 and Thr-17 in spite of
decreased PLB protein level.53 These data suggest
that a decrease in PLB protein level and an increase
in PLB phosphorylation are adaptive changes to
partially compensate for the decrease in SERCA
pump level and Ca2+ uptake function. The phosphorylation status of PLB in models where SERCA
pump is increased is not known.
shown by an increase in the amplitude of the Ca2+
transients.48 This is likely due to an increase in SR
Ca2+ load.62 Interestingly, in SERCA1a expressing
hearts the amount of Ca2+ release channel (RyR)
protein is decreased (>30%). This rather indicates
a compensatory downregulation in the RyR protein
expression to regulate the amount of Ca2+ released
during each contraction.62 A similar finding was
described for PLB-null mice, where RyR protein
levels are decreased (25%) while mRNA levels are
unchanged.63 Taken together these findings suggest
that alterations in the level of ryanodine receptors
(likely due to post-transcriptional regulation) may
be a general protective mechanism to regulate Ca2+
release from the SR in models with increased SR
Ca2+ load.
Alterations in sarcolemmal proteins, Na+–Ca2+
exchanger and L-type Ca2+ channel activity
Previous studies have shown that there is a reciprocal/inverse relationship between SERCA and
Na+–Ca2+ exchanger protein expression. This is
particularly observed during heart development, in
hyper- and hypothyroid hearts and in heart failure.19,64–66 It is generally believed that the Na+–Ca2+
exchanger might play a compensatory role for loss
of SERCA pump activity. Indeed, in SERCA2 heterozygous hearts, the Na+–Ca2+ exchanger protein
level and the exchanger current activity were
increased53 (see Fig. 3). The exact physiological
significance of this finding is not completely understood. On the other hand, in mice overexpressing
SERCA pump, the Na+–Ca2+ exchanger levels are
not altered, but functional measurements are
needed.62
Our preliminary studies on L-type Ca2+ channel
activity show that SERCA overexpression results
in a significant reduction in L-type Ca2+ current
density, suggesting altered Ca2+ channel activity or
expression.62 Experiments are in progress to determine the Ca2+ channel protein expression in transgenic models with increased or decreased SERCA
protein expression. It is noteworthy to mention that
in myocytes from PLB ablated mice the decay of Ltype Ca2+ current was faster than in control cells.67
These and other studies suggest that alterations in
SERCA expression and activity can affect sarcolemmal Ca2+ ion transporters, but the exact mechanism remains to be understood.
How do SERCA levels affect SR Ca2+ release function?
SERCA as a Therapeutic Agent
In SERCA1a transgenic myocytes more Ca2+ is
released from the SR during each contraction, as
In recent years gene therapy for heart diseases has
become a promising area of research. Heart failure
SERCA Pump and Cardiac Contractility
Figure 3 Western blot analysis of SERCA2 heterozygous
hearts. Increasing amounts of protein were separated by
SDS-PAGE and probed with a Na+–Ca2+ exchanger (NCX)
specific antibody. A representative immunoblot is shown.
The bar graph shows the mean±... of six individual
hearts.53
provides an attractive candidate for gene therapy,
because a number of protein targets have been
identified as defective or functionally impaired.68 In
particular, abnormalities in excitation–contraction
coupling have received much attention. A decrease
in SR Ca2+ ATPase expression and/or activity seems
to be a major defect responsible for impaired function of the failing heart. Therefore, a number of
recent studies were focused on restoring SERCA
pump activity by adenoviral mediated gene transfer.
Hajjar et al.69 and Giordano et al.70 have elegantly
demonstrated that the overexpression of SERCA2a
in isolated myocytes through adenoviral gene transfer results in increased contractility and a faster
decay of the cytosolic Ca2+ transient. Inesi and
colleagues71,72 have further documented that both
SERCA2a and SERCA1a can be expressed at high
levels in embryonic chicken and neonatal rat cardiomyocytes by adenoviral vectors. These authors
showed that despite identical protein expression
1059
levels SERCA1a activity was two-fold greater than
SERCA2a activity due to intrinsic differences in
turnover rates. The decay of cytosolic Ca2+ transients in cells expressing SERCA1a was faster without a significant change in resting Ca2+ or peak
amplitudes.
Furthermore, two independent studies tested the
benefit of higher SERCA levels during pressureoverload induced cardiac remodeling in genetically
altered animal models.46,73 Transgenic mice and
rats overexpressing SERCA2a were subjected to
aortic stenosis for 7 weeks or abdominal aortic
banding for 10 weeks, respectively. Ito et al.73 found
decreased mortality in banded transgenic mice v
banded controls and both studies demonstrate a
beneficial effect of increased SERCA levels in terms
of enhanced contractile reserve or higher cardiac
contractility after the treatment.
Recently, the feasibility of restoring function both
in failing cardiac myocytes and in intact animal
hearts subjected to experimental heart failure was
tested. Del Monte et al.74 showed that overexpression
of SERCA2a in human ventricular myocytes from
patients with end-stage heart failure can increase
SERCA pump activity and enhance contraction
and relaxation velocity. The negative frequencyresponse was normalized in cardiomyocytes overexpressing SERCA2a. In addition, Hajjar and colleagues have used a catheter-based technique of
adenoviral gene transfer to achieve global myocardial transduction of SERCA2a in vivo.75,76 The
authors chose to restore SERCA2a activity in an
animal model of pressure-overload hypertrophy in
transition to failure. SERCA2a levels and activity
were decreased and severe contractile dysfunction
was evident. Overexpression of SERCA2a by gene
transfer in vivo restored both systolic and diastolic
dysfunction to normal levels. SERCA overexpression
decreased left ventricular size and restored the slope
of the end-diastolic pressure-dimension relationship
to control levels. Similarly, infection of senescent
rat hearts with adenovirus carrying SERCA2a increased Ca2+ uptake activity and improved ratedependent contractility and diastolic function.75
These studies provide strong evidence that increased
SERCA expression can be used to restore Ca2+
transport and contractility. Furthermore, these data
suggest the feasibility of cardiac gene transfer into
failing hearts as a therapeutic intervention.
Summary and Conclusions
The SERCA manipulation studies described above
in mice and using adenoviral gene transfer clearly
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M. Periasamy and S. Huke
demonstrate that it is possible to increase SERCA
protein level in cardiac myocytes. The SR membrane
is not fully saturated and can incorporate additional
Ca2+ pumps. A pump expression of up to 2.5-fold
is not pathological in the heart. The exogenously
expressed pumps are functional and can alter Ca2+
transport activity. Increased pump levels lead to
increases in SR Ca2+ uptake, SR Ca2+ load and
enhanced contractile function. A decrease in SERCA
level elicits the opposite effects. These data convincingly demonstrate that SERCA protein level
is a critical determinant of intracellular calcium
homeostasis and contractility in the mammalian
heart.
However, alterations in SERCA expression are
not without consequences. Several compensatory
adaptations do occur in transgenic mice at various
levels both in sarcolemmal and SR membrane.
Alterations in the expression and/or activity of
other Ca2+ regulatory proteins, including RyR, PLB,
Na+–Ca2+ exchanger and L-type Ca2+ channels,
were found, suggesting that there is a cross-talk
and/or functional dependence between different
Ca2+ handling proteins. In particular, such compensatory changes might be needed to maintain
sufficient cardiac function and prevent the development of cardiac hypertrophy in SERCA2
heterozygous mice.
The SERCA manipulation studies also raise many
additional questions: What is the “normal” physiological SERCA level and how is it maintained? What
are the effects of long-term changes in SERCA pump
level over the physiological range? What is the
mechanism for sensing perturbations in SERCA
pump level/activity? How do alterations in SERCA
affect the dynamics of SERCA/PLB interaction?
What is the relationship, if any, between the SERCA
and the Na+–Ca2+ exchanger? Can data from transgenic mice be extrapolated to higher mammals,
including humans? Nevertheless, the SERCA
manipulation studies provide us with promising
results and move us a step forward towards employing SERCA protein as a therapeutic agent to
rescue function in human failing hearts.
Acknowledgements
The authors wish to thank Lynne H. Liu, Yong Ji,
M. Jane Lalli, Vikram Prasad, Tom Reed, Kalpana
Nattamai, Gopal J. Babu, Atai Watanabe and Pei
Hong Dong for their contributions and for helpful
discussions. We are grateful to Roger Hajjar and
Guoxiang Chu for critical reading of this manuscript. M. Periasamy is supported by grant RO1 HL
64140-02 and S. Huke is supported by the DFG.
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