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Cardiovascular Research 2001 49(2):249-252; doi:10.1016/S0008-6363(00)00275-3
© 2001 by European Society of Cardiology
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Copyright © 2000, European Society of Cardiology

Adenovirus gene transfer of SERCA in heart failure. A promising therapeutic approach ?

A Baartscheer*

Department of Experimental Cardiology, Academic Medical Center, University of Amsterdam, P.O. Box 22700, 1100 DE, Amsterdam, The Netherlands

* Tel.: +31-20-566-3265; fax: +31-20-697-5458 a.baartscheer{at}amc.uva.nl

Received 30 October 2000; accepted 30 October 2000

See article by Chossat et al. [10] (pages 288–297) in this issue.


   1 Introduction
Top
 1 Introduction
2 Present limitation of...
 3 Metabolic and...
 4 Concluding remark
 References
 
Abnormal and reduced contractile function is characteristic in patients suffering from heart failure. Therefore, it seems obvious that inotropic therapy could potentially be beneficial in the treatment of heart failure. However, a number of studies show short-term beneficial effects with progressive myocardial dysfunction in long-term treatment with inotropic interventions in chronic heart failure [1,2]. Inotropic interventions not only enhance contractility and cytosolic systole calcium concentration ([Ca2+]), but also potentially adversely affect diastolic [Ca2+].

Functional abnormalities commonly observed in the failing heart include a negative force frequency relationship secondary to disturbed calcium handling [3–6] characterized by increased diastolic [Ca2+], increased duration and reduced amplitude of the calcium transient and reduction of the SR calcium content [7,8]. Available evidence strongly suggests that down regulation of the sarcoplasmic reticular SR Ca-ATPase (SERCA) underlies these abnormalities possibly in combination with increased sarcolemmal Na+/Ca2+-exchange activity. Consequently, therapies aiming to enhance SERCA activity might prove potentially beneficial in the treatment of heart failure.

A promising approach to this end could be adenoviral gene therapy to compensate for compromised SERCA synthesis. Indeed, one of the first studies using adenoviral gene transfer in isolated neonatal cardiac myocytes reported a successful 7.5 fold increase of total SERCA [9]. However, in similar later studies in adult normal and failing myocytes increase of SERCA was limited to maximally 1.5 to 2 fold [10–12]. Similar results were reported in transgenic animals over-expressing SERCA [12]. Over-expression of SERCA appears limited by post-transcriptional events. Although SERCA mRNA levels continuously increase with increasing viral titer, protein levels become saturated [13]. Both in healthy transgenic mice and in adenovirus treated non-failing cardiac myocytes over expression of SERCA resulted in increased contractility, calcium transient amplitude and SR calcium content and in shortening of the calcium transient [10,12,13]. In myocytes isolated from failing hearts adenoviral gene transfer recovered these parameters to normal values [11].

The study by Chossat et al. published in this issue of Cardiovascular Research [10] closely examines the relation between levels of SERCA and the kinetic parameters of the cytosolic calcium transient and SR calcium content. To obtain reliable quantitative estimates of change of total SERCA protein they deliberately used gene transfer of skeletal muscle specific SERCA (SERC1a) rather than the cardio-specific SERCA (SERC2a).


   2 Present limitation of adenoviral SERCA gene transfer
Top
 1 Introduction
 2 Present limitation of...
3 Metabolic and...
 4 Concluding remark
 References
 
Despite promising results of adenoviral SERCA gene transfer in experimental models, before successful application in vivo a number of potential problems have to be resolved, among which inflammatory responses, insufficient infection efficacy, loss of transgene expression and organ specific delivery [11]. Another at least equally important complication could be inhomogeneous gene transfer in the heart [11]. Inhomogeneous up-regulation of SERCA will cause dispersion of diastolic and systolic calcium handling and contractile abnormalities. Altered cytosolic calcium dynamics affect calcium dependent sarcolemmal currents and, consequently, action potential configuration. In the failing heart action potential is in-homogeneously prolonged, which might provide a substrate for the creation of areas of block and abnormal conduction [14,15]. Inhomogeneous up-regulation of SERCA in the failing heart aggravates the dispersion and the propensity to arrhythmias.


   3 Metabolic and electrophysiological effects of up-regulation of SERCA
Top
 1 Introduction
 2 Present limitation of...
 3 Metabolic and...
4 Concluding remark
 References
 
3.1 Metabolic effects
It has been suggested that pathologically increased energy demand sets off adaptive processes leading to hypertrophy, which and eventually may develop into heart failure [16]. This would be in agreement with studies in patients treated with β-blockers; progression of clinical heart failure was slowed, however without apparent recovery of impaired myocardium, which was attributed to reduction of energy demand during β-blockade [17,18]. Conversely, positive inotropic interventions in the failing heart, e.g. by up-regulation of SERCA, inevitably speeds up the rate of energy expenditure and would be expected to accelerate progression of clinical heart failure. Presently available evidence indeed indicates that high-energy phosphates are reduced in hypertrophy and heart failure [19–21]. Correspondingly, the free energy of ATP hydrolysis ({Delta}GATP), the ultimate driving force for all energy depended metabolic and ion transport processes, becomes reduced [22]. Un-proportional growth of cardiac structures may underlie this reduction; capillary density decreases and inter capillary distance increases resulting in an impaired diffusion of substrates and oxygen [23,24]. For thermodynamic reasons trans-membrane ion gradients have to adapt to reduced {Delta}GATP in the failing heart. Increased energy expenditure induced by up-regulation of SERCA may further depress {Delta}GATP and, consequently, the magnitude of trans-membrane ion gradients become further reduced. This could explain increased cytosolic [Na+] and [Ca2+] in heart failure. In addition elevated cytosolic [Ca2+] is related to the respiratory rate [25].

3.2 Electrophysiological effects
Clinical heart failure is strongly associated with arrhythmogenesis. The incidence and complexity of arrhythmias augment with progression of heart failure and are related to mortality [26–29]. A major mechanism involved in the initiation of arrhythmias is triggered activity arising from early (EAD) or delayed (DAD) after depolarizations. After-potentials occur when abnormal inward current causes depolarization of the sarcolemmal membrane during or immediately following the action potential. The stage for the occurrence of EADs and/or DAD's can be set by conditions of calcium overload or prolonged action potential duration [30]. In hypertrophied myocardium the inward rectifier (Ik1), the delayed rectifier (Ik) and the transient outward current (Ito) are decreased, which results in net reduction of outward current and prolongation of the action potential [31,32]. Up-regulation of SERCA increases SR calcium content and the amplitude of the calcium transient, which causes enhanced depolarizing Na+/Ca2+-exchanger current, associated with calcium outward transport, during the plateau phase of the action potential. This is especially relevant in failing myocardium in which Na+/Ca2+-exchanger is up regulated [33] and action potential duration is prolonged. The increased Na+/Ca2+-exchanger current causes further increase of the action potential duration and may lead to the occurrence of EAD's. Such a mechanism has been proposed to be operative in isolated myocytes from healthy myocardium with artificially induced calcium overload [34,35]. Another possible mechanism to explain the occurrence of EAD's is activation of stretch regulated channels in the dilated heart. Stretch activated channels may contribute to increased depolarizing current during the plateau phase of the action potential and favor the development of EAD's. It has indeed been shown that EAD's could be elicited more easily in failing myocardium than healthy myocardium [36]. Positive inotropic interventions, such as up-regulation of SERCA, might be expected to enhance activation of stretch regulated channels in the dilated heart.

There is an increasing body of evidence that the occurrence of DAD's is related to SR calcium release function. Open probability of SR calcium release channels increases with calcium concentration at either side of the SR membrane, but may also depend on the magnitude of the calcium gradient across the membrane [37–39]. Increased open probability may lead to a transient elevation of intracellular calcium due to spontaneous release of calcium from SR, the so-called calcium sparks, which trigger further release through neighboring release channels [40]. Extrusion of calcium by Na+/Ca2+-exchanger associated with a depolarizing current may generate DAD's [41,42]. The propensity for the occurrence of DAD's is particularly enhanced in heart failure, because the Na+/Ca2+-exchanger is up regulated. In addition, it has been reported that in heart failure the open probability of calcium release channels is increased due to an altered stoichiometry between FK binding proteins and calcium release channels [43], an increased cytosolic calcium [39,44] and an increase activity of inositol 1,4,5,-trisphosphate regulated release channels [45]. However, in heart failure increased open probability of release channels in combination with down regulated SERCA causes a decrease of SR calcium content [46]. Decreased SR calcium content would counteract spontaneous release of calcium by increased open probability. Despite decreased SR calcium content, DAD's, and calcium aftertransients more frequently occur in myocytes isolated from failing hearts [47]. Occurrence of calcium aftertransients measured in vitro are related to the occurrence of ventricular arrhythmias in vivo [48].

Up-regulation of SERCA by adenovirus gene transfer causes an increase of SR calcium content, an increase of the open probability of the calcium release channels and the occurrence of DAD's, particularly in the failing heart. However, it probably does not change diastolic calcium, because of other regulating processes, i.e., Na+/Ca2+-exchanger and indirectly Na+/K+-pump [49]. For long-lasting changes of [Ca2+]i either the activity or the driving force of these regulators have to be affected [50]. Indeed, diastolic function is reported to be unchanged [11] or improved after up-regulation of SERCA [10]. This unchanged or even decrease of diastolic calcium might be beneficial with respect to increase of the open probability of the calcium release channels and the occurrence of DAD's. Especially compared to inotropic interventions, which not only enhance contractility but also increase diastolic [Ca2+].


   4 Concluding remark
Top
 1 Introduction
 2 Present limitation of...
 3 Metabolic and...
 4 Concluding remark
References
 
Adenoviral gene transfer of SERCA may prove relevant to treatment of heart failure provided the technical limitations can be resolved. However, it should be realized, that targeting only one out off all potential patho-physiological mechanisms in heart failure, might turn out to have just adverse or only temporal beneficial effects.


   References
Top
 1 Introduction
 2 Present limitation of...
 3 Metabolic and...
 4 Concluding remark
 References
 

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