Calcium in atrial fibrillation — pulling the trigger or not?

Atrial fibrillation (AF) is the most common sustained arrhythmia disease. Current drug- and surgical-based therapies are ineffective in about 40% to 50% of AF patients; therefore, there is a great need to better understand the underlying mechanisms of this disease and identify potential therapeutic targets. In this issue of the JCI, Greiser and coworkers discovered that atrial remodeling in response to sustained tachycardia silences Ca²⁺ signaling in isolated rabbit and human atrial myocytes. This Ca²⁺ release silencing was attributable to a failure of subcellular propagated Ca²⁺ release due to an increased cytosolic buffering strength. The results from this study challenge the current paradigm that Ca²⁺ release instability underlies AF. Instead, Ca²⁺ silencing could be protective against the massive cellular Ca²⁺ loading that occurs during chronic AF.

Atrial fibrillation (AF) is the most common sustained arrhythmia disease, affecting 1% to 2% of the US population (1). Because current therapy — either with drugs or surgical ablation — remains ineffective in about 40% to 50% of AF patients (2), there is a great need to understand the underlying mechanisms of AF and identify better therapeutic targets. Previous work has shown that a rapid atrial activation rate induces electrical remodeling of the atria, which in turn increases the risk for AF (3). The remodeling consists of L-type Ca²⁺ current reduction (4), sodium-calcium exchanger upregulation, reduced Ca²⁺ transients (5), and altered sarcoplasmic reticulum (SR) function, which is characterized by increased spontaneous Ca²⁺ sparks and waves and has been attributed to hyperactive RyR2 channels, possibly as the result of increased phosphorylation at residues Ser2808 and Ser2815 (5–7). These so-called “leaky” RyR2 channels and increased Ca²⁺ waves are capable of triggering delayed afterdepolarizations and focal atrial electrical activity (8); therefore, hyperactive RyR2 channels are thought to be important contributors to the induction and maintenance of AF in humans (9).

It remains unclear whether the Ca²⁺ signaling remodeling observed in myocytes isolated from humans with AF is a consequence of a rapid activation rate or due to concomitant heart disease. In this issue of the JCI, Greiser et al. (10) attempt to answer this question by characterizing subcellular Ca²⁺ signaling in atrial myocytes harvested from rabbits that underwent 5 days of rapid atrial pacing (RAP) and compared them with atrial myocytes harvested from patients with chronic AF. Surprisingly, Greiser et al. did not observe changes in Ca²⁺ sparks, Ca²⁺ waves, or Ca²⁺ release instability, but rather they found that Ca²⁺ release was strongly suppressed in the center of rabbit atrial myocytes. Greiser and colleagues have termed this suppression “Ca²⁺ signaling silencing,” and a similar central Ca²⁺ release silencing also occurred in atrial myocytes harvested from patients with chronic AF. What causes the loss of central Ca²⁺ release? Because the density of transverse tubules is much lower than that of ventricular myocytes, atrial myocytes rely on Ca²⁺ diffusion to activate Ca²⁺ release in the myocyte core (11); consequently, the Ca²⁺ signal that is generated in the subsarcolemmal region from Ca²⁺ release triggered by L-type Ca²⁺ channels is propelled to the cell center by repetitive release from intracellular RyR2 clusters (Figure 1A). Greiser et al. determined that cytosolic Ca²⁺ buffering strength is markedly increased in RAP myocytes. This increased Ca²⁺ buffering capacity could be the consequence of reduced troponin I phosphorylation, which in turn would increase Ca²⁺ binding to troponin C in the myofilaments. Hence, increased Ca²⁺ binding to myofilaments may reduce the free Ca²⁺ available to activate neighboring RyR2 clusters. Together with the observed reduction in RyR2 expression, increased cytosolic buffering likely explains the failure in the centripetal Ca²⁺ propagation of RAP myocytes (Figure 1B), because SR Ca²⁺ content, RyR2 channel activity, and peripheral L-type current–induced Ca²⁺ release were all preserved.

Figure 1

Ca²⁺ signaling silencing and failure of centripetal Ca²⁺ propagation in RAP atrial myocytes. Atrial myocytes rely on Ca²⁺ diffusion to neighboring RyR2 clusters to activate Ca²⁺ release from the subsarcolemmal region to the core of the myocytes (A). In RAP atrial myocytes, increased Ca²⁺ binding to myofilaments reduces the free Ca²⁺ available to activate neighboring RyR2 clusters. This phenomenon, together with the reduction in RyR2 expression and RyR2 cluster size, could explain the failure in centripetal wave propagation (B). P, PKA phosphorylation at RyR2-2808; NCX, sodium-calcium exchanger.

Greiser and colleagues (10) also report that, consistent with previous studies, the remaining RyR2 clusters were hyperphosphorylated at the protein kinase A (PKA) phosphorylation site (Ser2808), which may compensate for the reduction in RyR2 protein expression and help sustain subsarcolemmal Ca²⁺ release despite reduced L-type Ca²⁺ currents (Figure 1B). However, RAP myocytes exhibited reduced RyR2 phosphorylation at the calmodulin-dependent protein kinase II (CaMKII) phosphorylation site (Ser2815) and no changes in CaMKII activity. This finding contrasts with previous studies that reported increased atrial CaMKII activity and CaMKII-dependent RyR2-Ser2815 phosphorylation in human AF (5). Moreover, other studies have shown that treatment with CaMKII inhibitors or selective disruption of the Ser2815 CaMKII phosphorylation site prevented AF in animal models through a reduction of SR Ca²⁺ leak (12). One explanation for this discrepancy could be the limited duration of pacing in the rabbit model used by Greiser and colleagues, in which none of the animals exhibited spontaneous AF after the 5 days of rapid pacing. It remains unclear whether the effects observed in response to limited atrial pacing are just transient changes due to the short period of this protocol, or whether, as suggested by Greiser et al., Ca²⁺ signaling silencing represents a maintained response of atrial myocytes to chronic rapid activation. Strikingly, but consistent with the unaltered CaMKII activity, Greiser et al. did not observe changes in phospholamban activity. Rather, they found decreased expression of the SR Ca²⁺ ATPase (SERCA), despite the fact that the Ca²⁺ load in the SR was also unchanged. This reduction in SERCA expression with no changes in SR Ca²⁺ load can be explained by the reduction in Ca²⁺ release from the SR due to Ca²⁺ release silencing; however, the role of SERCA remodeling during AF remains unclear. Recent studies suggest that in atrial muscle, SERCA activity is also regulated by sarcolipin (13, 14), and it remains to be tested whether changes in sarcolipin contribute to altered SERCA activity in the rabbit model of RAP.

The study by Greiser et al. (10) elegantly provides evidence of altered atrial Ca²⁺ handling in response to tachycardia, but also raises new questions. It is well established that Ca²⁺-dependent signaling affects atrial remodeling: cellular Ca²⁺ loading during rapid pacing activates calcineurin, which in turn dephosphorylates, for example, nuclear factor of activated T cells (NFAT) and promotes its translocation to the nucleus. In the nucleus, NFAT regulates several targets at the transcriptional level, including the L-type Ca²⁺ channel, which is reduced by NFAT (15). In this regard, it would be interesting to further analyze whether atrial Ca²⁺ signaling silencing reduces NFAT translocation to the nucleus and thus limits the electrical and structural remodeling that occurs after the Ca²⁺ overload in AF. This structural remodeling contributes to the reinduction of AF. Although it takes place after the electrical remodeling, the Ca²⁺ overload induced by rapid atrial activation rates is one of the main signals that triggers the remodeling process (16). Thus, while the Ca²⁺ signaling silencing discovered by Greiser et al. (10) appears to help limit the consequence of rapid pacing–induced Ca²⁺ overload, the extent of this protective effect in the intact organ during AF or in preventing AF triggering is not clear. Studies in the intact atria during and after AF will be needed to better understand the role of Ca²⁺ signaling silencing in the pathophysiology of AF.

This work was supported in part by NIH grants HL88635 and HL71670.

Address correspondence to: Björn C. Knollmann, Professor of Medicine and Pharmacology, Division of Clinical Pharmacology, Vanderbilt University School of Medicine, Medical Research Building IV, Rm. 1265, 2215B Garland Ave., Nashville, Tennessee 37232-0575, USA. Phone: 615.343.6493; E-mail: bjorn.knollmann@vanderbilt.edu.

Conflict of interest: The authors have declared that no conflict of interest exists.

Reference information: J Clin Invest. 2014;124(11):4684–4686. doi:10.1172/JCI77986.

See the related article at Tachycardia-induced silencing of subcellular Ca2+ signaling in atrial myocytes.

1. Friberg L, Bergfeldt L. Atrial fibrillation prevalence revisited. J Intern Med. 2013;274(5):461–468.
   View this article via: PubMed CrossRef Google Scholar
2. Nieuwlaat R, et al. Prognosis, disease progression, and treatment of atrial fibrillation patients during 1 year: follow-up of the Euro Heart Survey on atrial fibrillation. Eur Heart J. 2008;29(9):1181–1189.
   View this article via: PubMed CrossRef Google Scholar
3. Wijffels MC, Kirchhof CJ, Dorland R, Allessie MA. Atrial fibrillation begets atrial fibrillation. A study in awake chronically instrumented goats. Circulation. 1995;92(7):1954–1968.
   View this article via: PubMed CrossRef Google Scholar
4. Van Wagoner DR, Pond AL, Lamorgese M, Rossie SS, McCarthy PM, Nerbonne JM. Atrial L-type Ca²⁺ currents and human atrial fibrillation. Circ Res. 1999;85(5):428–436.
   View this article via: PubMed CrossRef Google Scholar
5. Voigt N, et al. Enhanced sarcoplasmic reticulum Ca²⁺ leak and increased Na⁺-Ca²⁺ exchanger function underlie delayed afterdepolarizations in patients with chronic atrial fibrillation. Circulation. 2012;125(17):2059–2070.
   View this article via: PubMed CrossRef Google Scholar
6. Neef S, et al. CaMKII-dependent diastolic SR Ca²⁺ leak and elevated diastolic Ca²⁺ levels in right atrial myocardium of patients with atrial fibrillation. Circ Res. 2010;106(6):1134–1144.
   View this article via: PubMed CrossRef Google Scholar
7. Vest JA, et al. Defective cardiac ryanodine receptor regulation during atrial fibrillation. Circulation. 2005;111(16):2025–2032.
   View this article via: PubMed CrossRef Google Scholar
8. Faggioni M, et al. Suppression of spontaneous ca elevations prevents atrial fibrillation in calsequestrin 2-null hearts. Circ Arrhythm Electrophysiol. 2014;7(2):313–320.
   View this article via: PubMed CrossRef Google Scholar
9. Wakili R, Voigt N, Kääb S, Dobrev D, Nattel S. Recent advances in the molecular pathophysiology of atrial fibrillation. J Clin Invest. 2011;121(8):2955–2968.
   View this article via: JCI PubMed CrossRef Google Scholar
10. Greiser M, et al. Tachycardia-induced silencing of subcellular Ca²⁺ signaling in atrial myocytes. J Clin Invest. 2014;124(11):4759–4772.
    View this article via: JCI CrossRef Google Scholar
11. Hüser J, Lipsius SL, Blatter LA. Calcium gradients during excitation-contraction coupling in cat atrial myocytes. J Physiol. 1996;494(pt 3):641–651.
    View this article via: PubMed Google Scholar
12. Chelu MG, et al. Calmodulin kinase II-mediated sarcoplasmic reticulum Ca²⁺ leak promotes atrial fibrillation in mice. J Clin Invest. 2009;119(7):1940–1951.
    View this article via: JCI PubMed CrossRef Google Scholar
13. Xie LH, et al. Ablation of sarcolipin results in atrial remodeling. Am J Physiol Cell Physiol. 2012;302(12):C1762–C1771.
    View this article via: PubMed CrossRef Google Scholar
14. Shanmugam M, Molina CE, Gao S, Severac-Bastide R, Fischmeister R, Babu GJ. Decreased sarcolipin protein expression and enhanced sarco(endo)plasmic reticulum Ca²⁺ uptake in human atrial fibrillation. Biochem Biophys Res Commun. 2011;410(1):97–101.
    View this article via: PubMed CrossRef Google Scholar
15. Qi XY, et al. Cellular signaling underlying atrial tachycardia remodeling of L-type calcium current. Circ Res. 2008;103(8):845–854.
    View this article via: PubMed CrossRef Google Scholar
16. Andrade J, Khairy P, Dobrev D, Nattel S. The clinical profile and pathophysiology of atrial fibrillation: relationships among clinical features, epidemiology, and mechanisms. Circ Res. 2014;114(9):1453–1468.
    View this article via: PubMed CrossRef Google Scholar