Cardiovascular Research

Cardiovascular Research 2007 75(1):7-9; doi:10.1016/j.cardiores.2007.04.029

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Copyright © 2007, European Society of Cardiology

Sarcoplasmic reticulum Ca²⁺ release channel complex and automatism: A matter of fine tuning

José W.M. Bassani^* and Rosana A. Bassani

Centro de Engenharia Biomédica and Dept. Engenharia Biomédica/FEEC, Universidade Estadual de Campinas, Campinas SP, Brazil

^* Centro de Engenharia Biomédica/UNICAMP, Caixa Postal 6040, 13084-971 Campinas, SP, Brazil. Tel.: +55 19 3521 9287; fax: +55 19 3289 3346. bassani{at}ceb.unicamp.br

Received 15 April 2007; accepted 26 April 2007

See article by Dirksen et al. [10] (pages 69–78) and Faber and Rudy [15] (pages 79–88) in this issue.

In heart muscle, excitation contraction coupling (ECC) relies on proper release of Ca²⁺ from the sarcoplasmic reticulum (SR), which is accomplished by the activation of SR Ca²⁺ release channels (also known as ryanodine receptors, RyR) upon Ca²⁺ binding, a process termed calcium-induced calcium release [1]. The spontaneous activation of a cluster of RyRs produces local Ca²⁺ release seen under confocal fluorescence microscopy as minuscule flashes of light called Ca²⁺ sparks, the elementary Ca²⁺ release events [2].

SR release units are never totally quiescent. They keep on releasing Ca²⁺ randomly even during diastole, when sparks are rare in time and spatially separable [3]. During ECC, thousands of intracellular release events are synchronized by Ca²⁺ influx during the action potential, which causes both spatial and temporal overlap of elementary events in a way to produce a global intracellular transient increase in Ca²⁺ concentration ([Ca²⁺]_i) that initiates cell contraction. For relaxation to occur, [Ca²⁺]_i must be reduced to the resting levels. This is accomplished by a number of transport systems that compete for Ca²⁺, mainly the SR Ca²⁺-ATPase and the Na⁺/Ca²⁺exchanger (NCX) [3].

The amount of Ca²⁺ released is strongly regulated by the SR Ca²⁺ content, both during systole [4] and diastole [5], so that release ceases at critically low SR loads, a condition in which a significant amount of Ca²⁺ (more than 40% of maximum load) can still be found in the organelle [4]. SR Ca²⁺ content, in turn, is regulated under normal physiological conditions by a pump-leak process as well as by Ca²⁺ binding to the intra-SR buffer calsequestrin (CSQ). Under steady-state conditions, the amount of Ca²⁺ released (either at rest or during ECC) matches SR Ca²⁺ uptake.

Alterations in Ca²⁺ regulation in cardiac myocytes may cause mechanical dysfunction and arrhythmias. Indirectly, the SR is a potential supplier of charge for cell membrane depolarization. During diastole, the typical transmembrane Ca²⁺ and Na⁺ concentration gradients, as well as membrane potential, favor NCX-mediated Ca²⁺ extrusion from the cell. Because this transport is electrogenic, a relatively small amount of Ca²⁺ extruded could give rise to sizable membrane depolarization that may trigger premature action potentials [6]. This is the accepted basis for the Ca²⁺-dependent delayed afterdepolarizations (DAD) observed in some pathological conditions.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a condition in which life-threatening arrhythmia may develop during sympathetic activation [7], which is the main physiological mechanism underlying circulatory support during stress and exercise. It is significant that this condition in humans may be associated with amino acid substitutions in SR proteins, and could be reproduced in animal models by point mutations in either RyR or CSQ [7]. This indicates a strong relationship between Ca²⁺ homeostasis and susceptibility to arrhythmia development, which represents a subversion of the natural order of physiological phenomena at the cellular level (i.e., Ca²⁺-dependent electrical activation during diastole vs. membrane depolarization triggering SR Ca²⁺ release during normal ECC), and resembles what is often observed in pacemaker cells from the right atrium [8,9].

In this issue of Cardiovascular Research, Dirksen et al. [10] demonstrate the occurrence of the CPVT phenotype in transgenic mice expressing a mutant cardiac CSQ in which a histidine replaces aspartate at position 307 (CASQ2^D307H), as observed in a familial form of CPVT in humans. The authors show that although the hearts of transgenic animals display normal myocardial structure and contractile performance in vivo, the incidence of complex ventricular arrhythmias following isoproterenol infusion is markedly higher than that seen in wild-type mice. Likewise, isolated ventricular myocytes from transgenic mice showed enhanced spontaneous Ca²⁺ release and DAD when challenged with isoproterenol, which suggests that this mutation causes facilitation of diastolic SR Ca²⁺ release, particularly during β-adrenergic stimulation.

Depressed Ca²⁺-induced conformational change has been described in the CASQ2^D307H mutant protein when compared to the wild-type protein, which may result in decreased ability of polymerization and diminished Ca²⁺ binding capacity [11]. The role of CSQ in SR Ca²⁺ cycling does not seem to be restricted to intra-SR Ca²⁺ buffering, but also involves regulation of the RyR function. Available evidence points out that CSQ, by means of interaction with two other SR proteins, triadin and junctin, allows RyR activity to be modulated by the SR Ca²⁺ load, so that the channel is relatively inactive at low loads, but its open probability is greatly augmented when luminal [Ca²⁺] is high (e.g., [12,13]). Thus, one would expect that mutations that change the interaction among CSQ, junctin, triadin and RyR (which seems to be the case with CASQ2^D307H [11]) might primarily affect the way in which the channel reacts to changes in SR Ca²⁺ content. In the case of the CASQ2^D307H mouse model [10], this content was found to be comparable to that in wild-type mice, and the increase in diastolic channel activity, as inferred from Ca²⁺ spark frequency, was modest, albeit significant. On the other hand, β-adrenergic stimulation (which is known to enhance the rate of SR Ca²⁺ uptake, and thus SR Ca²⁺ load and leak [5]) was able to strongly promote DAD in vitro and ventricular arrhythmia in vivo, which might be explained by greater load sensitivity of the release mechanism in the transgenic mice. A recent report by Iyer et al. [14] shows that simulation of the functional consequences of CSQ mutations by disrupting RyR luminal [Ca²⁺] sensing reproduced the changes in Ca²⁺ transients and development of DAD in response to isoproterenol, as observed in myocytes from animal models of CPVT. Similar results are shown in a report by Faber and Rudy (see Ref. [15], also in this issue) in which the effects of this mutation on SR function are simulated as a decrease in intra-SR Ca²⁺ buffering capacity. The authors show that this change results in a diminished Ca²⁺ transient amplitude due to decreased SR Ca²⁺ content and that DAD occurrence requires both β-adrenergic stimulation and rapid pacing. This finding stresses the importance of the augmented cell (and SR) Ca²⁺ loading promoted by the two latter conditions for the buildup of luminal free [Ca²⁺] large enough to induce spontaneous Ca²⁺ release, which is in agreement with experimental results that ventricular preparations expressing CASQ2^D307H show rhythm abnormalities only in the presence of β-adrenoceptor agonists. This study also suggests that altered intra-SR Ca²⁺ homeostasis per se may, under high Ca²⁺ load conditions, lead to abnormally large diastolic Ca²⁺ release.

Increased SR Ca²⁺ content in response to β-adrenergic stimulation would thus exacerbate diastolic leak in cells with defective RyR regulation and/or impaired intra-SR Ca²⁺ buffering, which would further result in electrogenic Ca²⁺ extrusion by the NCX, leading to sarcolemmal depolarization. However, it is important to have in mind that, while enhanced diastolic SR Ca²⁺ release might be the primary factor responsible for the electrical disturbance, other factors might also contribute to translate the greater leak into spontaneous electrical activity during β-adrenergic stimulation. For instance, β-adrenoceptor activation is expected to increase membrane Cl^– conductance (both Ca²⁺-dependent and cAMP-activated currents), which, in addition to NCX-mediated current, may contribute to diastolic depolarization and DAD generation. Moreover, β-adrenergic stimulation and increased SR Ca²⁺ release itself decrease the inward-rectifying K⁺ current (I_K1) in ventricular myocytes [16,17]. Such a change should exert a depolarizing effect [15] and diminish the stability of diastolic membrane potential, thus providing an electrical substrate for greater membrane depolarization by relatively small currents [6], as seems to be the case in atrial pacemaker cells in which I_K1 density is low [18].

So far, the puzzle is still quite complex. Studies focusing on naturally occurring mutations of SR proteins will not only allow better characterization of the disease itself (as is the case with CPVT) and development of therapeutic approaches, but may also provide important insight into fundamental aspects of ECC. It is intriguing, however, that defective regulation of diastolic SR Ca²⁺ release (probably due to alteration of molecule-to-molecule interaction of SR proteins) may recapitulate the functional characteristics of embryonic (automatic) myocardial cells [19], which seem to be preserved in latent atrial pacemakers (i.e., higher rate of diastolic Ca²⁺ release) [8]. Investigation of the interaction of the SR proteins in pacemaker cells might shed more light on the relationship between regulation of diastolic RyR activity and automatism in the heart.

	References

Top References

 

1. Fabiato A. Calcium-induced calcium release of calcium from the cardiac sarcoplasmic reticulum. Am J Physiol (1983) 245:C1–C14.[ISI][Medline]
2. Cheng H., Lederer W.J., Canell M.B. Calcium sparks: elementary events underlying excitation–contraction coupling in heart muscle. Science (1993) 262:740–744.[Abstract/Free Full Text]
3. Bers D.M. Cardiac excitation–contraction coupling. Nature (2002) 415:198–205.[CrossRef][Medline]
4. Bassani J.W.M., Yuan W., Bers D.M., Fractional S.R. Ca release is regulated by trigger Ca and SR Ca content in cardiac myocytes. Am J Physiol (1995) 268:C1313–C1319.[ISI][Medline]
5. Shannon T.R., Ginsburg K.S., Bers D.M. Quantitative assessment of the SR Ca²⁺ leak-load relationship. Circ Res (2002) 91:594–600.[Abstract/Free Full Text]
6. Pogwizd S.M., Schlotthauer K., Li L., Yuan W., Bers D.M. Arrhythmogenesis and contractile dysfunction in heart failure: roles of sodium–calcium exchanger, inward rectifier potassium channels and β-adrenergic responsiveness. Circ Res (2001) 88:1159–1169.[Abstract/Free Full Text]
7. Kontula K., Laitinen P.J., Lehtonen A., Toivonen L., Viitasalo M., Swan H. Catecholaminergic polymorphic ventricular tachycardia: recent mechanistic insights. Cardiovasc Res (2005) 67:379–387.[Abstract/Free Full Text]
8. Bassani R.A., Bassani J.W.M., Lipsius S.L., Bers D.M. Diastolic SR Ca efflux in atrial pacemaker cells and Ca-overloaded myocytes. Am J Physiol (1997) 273:H886–H892.[ISI][Medline]
9. Bogdanov K.Y., Vinogradova T.M., Lakatta E.G. Sinoatrial nodal cell ryanodine receptor and Na⁺–Ca²⁺ exchanger: molecular partners in pacemaker regulation. Circ Res (2001) 88:1254–1258.[Abstract/Free Full Text]
10. Dirksen W.P., Lacombe V.A., Chi M., Kalyanasundaram A., Viatchenko-Karpinski S., Terentyev D., et al. A mutation in calsequestrin, CASQ2D307H, impairs SR Ca²⁺ handling and causes complex ventricular arrhythmias in mice. Cardiovasc Res (2007) 75:69–78.[Abstract/Free Full Text]
11. Houle T.D., Ram M.L., Cala S.E. Calsequestrin mutant D307H exhibits depressed binding to its protein targets and a depressed response to calcium. Cardiovasc Res (2004) 64:227–233.[Abstract/Free Full Text]
12. Terentyev D., Viatchenko-Karpinski S., Györke I., Volpe P., Williams S.C., Györke S. Calsequestrin determines the functional size and stability of cardiac intracellular calcium stores: mechanism for hereditary arrhythmia. Proc Natl Acad Sci USA (2003) 100:11759–11764.[Abstract/Free Full Text]
13. Györke I., Hester N., Jones L.R., Györke S. The role of calsequestrin, triadin, and junctin in conferring cardiac ryanodine receptor responsiveness to luminal calcium. Biophys J (2004) 86:2121–2128.[Abstract/Free Full Text]
14. Iyer V., Hajjar R.J., Armoundas A.A. Mechanisms of abnormal calcium homeostasis in mutations responsible for catecholaminergic polymorphic ventricular tachycardia. Circ Res (2007) 100:e22–e31.[Abstract/Free Full Text]
15. Faber G.M., Rudy Y. Calsequestrin mutation and catecholaminergic 210 polymorphic ventricular tachycardia: a simulation study of cellular 211 mechanism. Cardiovasc Res (2007) 75:79–88.[Abstract/Free Full Text]
16. Koumi S., Wasserstrom J.A., Ten Eick R.E. Beta-adrenergic and cholinergic modulation of inward rectifier K⁺ channel function and phosphorylation in guinea-pig ventricle. J Physiol (1995) 486:661–678.[Abstract/Free Full Text]
17. Fauconnier J., Lacampagne A., Rauzier J.M., Vassort G., Richard S. Ca²⁺-dependent reduction of I_K1 in rat ventricular cells: a novel paradigm for arrhythmia in heart failure? Cardiovasc Res (2005) 68:204–212.[Abstract/Free Full Text]
18. Cho H.S., Takano M., Noma A. The electrophysiological properties of spontaneously beating pacemaker cells isolated from mouse sinoatrial node. J Physiol (2003) 550:169–180.[Abstract/Free Full Text]
19. Viatchenko-Karpinsky S., Fleischmann B.K., Liu Q., Sauer H., Gryshchenko G.J.J., Hescheler J. Intracellular Ca²⁺ oscillations drive spontaneous contractions in cardiomyocytes during early development. Proc Natl Acad Sci USA (1999) 96:8259–8264.[Abstract/Free Full Text]

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