Cardiovascular Research 2001 50(1):7-9; doi:10.1016/S0008-6363(01)00234-6
© 2001 by European Society of Cardiology
Copyright © 2001, European Society of Cardiology
Signalling in cardiac disease: the molecular deficit at the heart of the problem
Terence E Hébert*
Centre de recherche, Institut de cardiologie de Montréal et Département d'anésthesie-réanimation, Université de Montréal, 5000 rue Bélanger est, Montreal, PQ, Canada, H1T 1C8
* Tel.: +1-514-376-3330; fax: +1-514-376-1355 hebertt{at}icm.umontreal.ca
Received 1 February 2001;
KEYWORDS G Protein; Signalling; Adrenergic receptors; Heart failure; Cardiomyocyte
See article by Yoshida et al. [3] (pages 34–45) in this issue.
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1 Introduction
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Events leading to heart failure are characterized by a prolonged
action potential and a deficit in cardiac performance which
is paralleled by ionic remodelling and a loss of contractile
function at the level of the single cardiomyocyte (see Ref.
[1] for review). Increased sympathetic stimulation as well as
a number of paracrine and autocrine factors lead to cardiac
hypertrophy and an eventual decompensated failing phenotype
[2]. However, several issues remain to be resolved in terms
of a molecular mechanism for these events. First, is there a
deficit in the contractile apparatus of the failing myocyte
per se? Alternatively, are signalling pathways which modulate
contractility and are known to be altered in the failing heart
the trigger for these events? Further, the focus of work on
whole cardiac tissue to date has not resolved the role of non-myocyte
cells in the disease process. Finally, the subcellular localization
and compartmentalization of signalling pathways and the proteins
they modulate may be of critical importance as well. The study
by Yoshida et al.
[3] in this issue of Cardiovascular Research
begins to address some of these important issues. The authors
demonstrate that although contractile function is clearly maintained
in isolated cardiomyocytes from animals with coronary artery
ligation-induced congestive heart failure (CHF), their regulation
by the β-adrenergic system is significantly altered resolving
a dilemma in the literature about the actual deficit in failing
hearts. Further, they show alterations in levels of Gs
but not
Gi
subunits in cardiomyocytes, while in non-myocytes there are
significant alterations in both subtypes. These results are
important as they address the actual site of the molecular deficit
in cardiac tissue.
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2 Calcium-handling abnormalities
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The regulation of [Ca
2+]
i plays a central role in cardiac performance.
Ca
2+-dependent signalling is critically important in the development
of cardiac hypertrophy (see Refs.
[4,5] for reviews). Changes
in the expression or activities of Ca
2+ handling proteins have
been observed in both human dilated cardiomyopathy and experimental
models of heart failure
[6,7]. The sarcoplasmic reticulum (SR)
contains important components of the Ca
2+ handling machinery,
which may be functionally altered in the disease state. In the
rat infarct model, SR Ca
2+ ATPase (SERCA) 2 activity is increased
in the hyperfunctioning right ventricle during a compensatory
stage, but depressed at the heart failure stage
[8], and the
activities of SERCA2 and of phospholamban, which modulates SERCA2
function, correlate with the steady state levels of their respective
mRNAs
[6]. The levels of the SR Ca
2+ storage proteins, calreticulin
and calsequestrin, are also changed in some models. Overexpression
of calreticulin results in severe hypertrophy in young mice,
indicating that calsequestrin is both a storage and a regulatory
protein in myocardial Ca
2+ homeostasis
[9]. The density of the
cardiac ryanodine receptor isoform (RyR2) is decreased in response
to either increases in hemodynamic load
[10] or phospholamban
ablation
[11]. A number of sarcolemmal Ca
2+ handling proteins
are also implicated in hypertrophy and heart failure, among
them L-type Ca
2+ channels and the Na
+/Ca
2+ exchanger. However,
at the present time it remains unclear whether or not the levels
of any or all of these proteins are altered in heart failure
and to some extent depends on the actual model used
[1]. Regulatory
alterations in the function of various SR proteins may indeed
be responsible for these functional changes. In the study of
Yoshida et al.
[3] it is clearly demonstrated that contractile
function as measured by cell shortening remains intact in animals
with coronary artery ligation-induced heart failure although
there is a significant reduction in cardiac performance in the
working heart. In isolated cardiomyocytes from sham-operated
or infarcted animals, basal contractility, calcium transients
and receptor-independent stimulation by direct activation of
adenylyl cyclase or via an indirect ouabain-stimulated pathway
were similar between the two groups while β-adrenergic
receptor-stimulated responses (as measured with isoproterenol)
were blunted in the failing hearts. These results argue that
signalling pathways which control the cardiac contractility
are altered in disease and that these alterations are responsible
for reduced cardiac function, rather than a direct deficit in
the contractile apparatus itself.
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3 Alterations in signal transduction pathways: what and where
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In the failing heart it is known that both β
1AR and β
2AR
responses are significantly desensitized owing to higher sympathetic
drive
[12,13]. β
1AR density is reduced in animal models
of heart disease and in tissue from human patients while β
2AR
density tends to remain intact
[14,15]. These observations are
supported in the study of Yoshida et al.
[3] where a significant
reduction in βAR-stimulated contractility in isolated myocytes
was demonstrated. The role of various G proteins in heart disease
has also been studied. Increases in relative levels of Gi proteins
in ventricular tissue have been noted by a number of studies
(see Refs.
[12,16] for reviews). However, are these alterations
in cardiomyocytes? This key question has been addressed by Yoshida
et al.
[3]. They observed no changes in levels (as assessed
by semi-quantitative Western blotting) of Gi
1, Gi
2 or Gi
3 proteins
in cardiomyocytes isolated from viable right and left ventricle
(RV, LV) or in the septum but significant increases in the general
tissue levels of these proteins in the infarct scar and in the
different regions of the heart. A concomitant increase in collagen
levels in left ventricle and the infarct scar suggests a proliferation
of fibroblasts in these regions. Alterations in Gi levels in
non-myocytes may lead to alterations in release of paracrine
or autocrine mediators which may have significant effects on
cardiomyocyte contractility and/or hypertrophy. Levels of Gs
were slightly increased in the infarct scar, unchanged in viable
LV but decreased in tissue RV and septum. Levels of Gs
were
decreased in cardiomyocytes isolated from all three regions
of viable cardiac tissue suggesting that they may play a role
in the reduced contractile response to isoproterenol. However,
it was not clear from this study whether or not modulation of
contractility or calcium handling by direct G protein activation
with GTP analogs was modified in the infarcted animals nor was
it determined if levels of βAR differed from control animals.
Thus it is difficult to conclude whether the alterations in
βAR or Gs levels in cardiomyocytes are responsible for
the observed contractile deficit. Basal levels of cAMP were
decreased in isolated LV cardiomyocytes in the infarcted animals.
However, no alterations in cAMP production by direct activation
of adenylyl cyclase (AC) with colforsin daropate were seen as
compared to control animals suggesting that adenylyl cyclase
itself in the infarcted animals was intact. Other studies have
demonstrated that forskolin-stimulated AC levels in tissue are
reduced in the failing heart
[17–19]. It remains to be
seen whether AC activity in non-myocytes is altered. Also the
specific alterations in signalling pathways in cardiomyocytes
versus non-myocytes which play roles in ionic remodelling leading
to action potential prolongation and ultimately myocyte hypertrophy
and failure, need to be more clearly delineated. The paper by
Yoshida et al.
[3] sets the stage for further dissection of
endogenous cardiomyocyte signalling pathways and paracrine factors
released by other cell types in the heart.
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4 Questions remaining
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This work opens several lines of inquiry that will need to be
pursued in the coming years. First, knowing that several autocrine
and paracrine regulators can be released by cardiomyocytes and
non-myocytes alike, it will be of critical importance to determine
if release of these mediators (including endothelin, epidermal
growth factor, cardiotrophin, etc.) is altered in heart failure
and to what extent alterations in G protein-coupled signalling
pathways are responsible. It is not clear what the specific
roles of β
1AR and β
2AR and possibly other βAR
subtypes play in this process. There is evidence that the β
1AR
and β
2AR are coupled to distinct signalling pathways and
much work remains to characterize their separate roles in the
disease process
[13,15]. Further studies are also needed to
resolve the relative roles of receptor versus G protein in development
of CHF. Are the responses desensitized in the failing heart
because of a combination of receptor- and G protein-specific
alterations? Are other signalling pathways coupled to G proteins
including
AR, M2 and M3 muscarinic receptors (among others)
also altered in a differential fashion between cardiomyocytes
and other cell types in the heart under normal and disease conditions?
Finally, as our appreciation of differences in the ultrastructural
organization and subcellular distribution of signalling pathways
increases, a number of issues relevant to heart failure are
evolving. We are beginning to perceive the players in these
pathways less like ships in the night who encounter each other
transiently after receptor activation but rather as parts of
more stable signalling complexes which are targetted to specific
regions of the cell. Caveolae and lipid rafts may be sites for
staging and assembly of these signalling complexes
[12,20,21].
Scaffolding proteins and adaptors such as PDZ proteins and SH2/SH3
domain-containing proteins are also involved in targetting of
signalling molecules
[22]. Stable interactions between receptors
and G proteins (and possibly the downstream effectors they modulate)
may represent a molecular mechanism for signalling specificity
in vivo
[23–26]. Is there a coordinate downregulation
of these signalling complexes in heart disease? Development
of more efficacious and specific therapies for heart failure
may depend on resolution of the organization of signalling pathways.
Knowing which cells contain them and how they are altered in
cardiac disease is an excellent place to start.
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Acknowledgements
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T.E.H. is a MacDonald Scholar of the Heart and Stroke Foundation
of Canada. This work was supported by grants from the Heart
and Stroke Foundation of Québec and the Canadian Institutes
for Health Research.
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1 Introduction
2 Calcium-handling abnormalities