Some 'brain' in the heart: a novel microdomain with neuronal Na channels responsible for arrhythmias?

This editorial refers to ‘Neuronal Na+ channel blockade suppresses arrhythmogenic diastolic Ca2+ release’ by P.B. Radwański et al., pp. 143–152. Arrhythmias are a well-accepted cause for morbidity and mortality. This is true both for atrial as well as ventricular arrhythmias. In general, atrial fibrillation can lead to the progression of cardiac diseases such as heart failure whereas ventricular arrhythmias can lead to sudden cardiac death. Although several antiarrhythmic drugs are currently available and invasive ablation therapies have been established throughout the recent years both for atrial as well as ventricular arrhythmias, the prognosis has not been improved except for ICD therapy. One reason for this caveat may be that the basic mechanisms for arrhythmias are still poorly understood. Moreover, some of the novel mechanisms for arrhythmias have not been translated into clinical routine yet. Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare congenital arrhythmogenic disease leading to sudden cardiac death without structural heart disease. Several mutations were described including the RyR2 gene1 encoding the cardiac isoform of the sarcoplasmic reticulum (SR) Ca-release channel (ryanodine receptor) as well as the SR Ca-binding protein calsequestrin (CASQ2).2,3 Radwanski et al.4 describe a new proarrhythmogenic mechanism in the well-known CASQ2 (R33Q) CPVT mouse model3 that seems clinically relevant. The authors report that a certain subgroup of voltage-dependent Na channels which are called nNaV and which were initially thought to be mainly present in neurons may be co-localized with RyRs of the SR. In this microdomain (see Fig. 7F, Ref. 4), isoproterenol increases local Na concentrations (the exact magnitude unfortunately is unclear) specifically through these nNaV. In consequence, Na extrusion through the sarcolemmal Na/Ca exchanger (NCX) occurs and Ca influx (reverse mode NCX) is preferred thus leading to spontaneous Ca release through RyRs from the SR. This diastolic SR Ca leak may lead to delayed after depolarizations (DADs), a well-known mechanism that can trigger arrhythmias. This elegant report underlines again the importance and the close relationship of intracellular Na and Ca handling for excitation–contraction coupling and arrhythmias.5,6 Whether the current findings in CPVT mice can be generalized to other species and diseases, e.g. human heart failure remains unclear. Also, the current findings do not exclude cardiac Na channels (NaV1.5) and late Na current from contributing to arrhythmias.7,8 This would be possible by experiments using myocyte-specific KO-experiments for neuronal Na channels. Late Na current inhibitors are currently under clinical investigation in phase II trials for atrial as well as for ventricular arrhythmias (RAFFAELLO, EudraCT-Nr: 2011-002789-189; HARMONY, EudraCT-Nr: 2011-001134-42; TEMPO trials, EudraCT Nr: 2013-004430-15). Of note, the general importance of intracellular Na handling for contractility has been known for many years.10–13 As mentioned above, in the current study, immunohistochemical staining using confocal microscopy suggests the localized expression of neuronal Na channels (i.e. NaV1.3, 1.1., and 1.6), which occurs close to the SR whereas the cardiac isoform (NaV1.5) is found ‘far’ away from this microdomain. The fact that NaV1.5 when compared with neuronal Na channels are preferentially localized in intercalated disks has been previously described by others.14 However, Radwanski et al.4 manage to go beyond this former study adding functional data showing that riluzole, a specific nNaV inhibitor which is currently used to treat amyotrophic lateral sclerosis, reduces arrhythmogenic diastolic SR Ca leak. A few questions remain unanswered and should be investigated in the near future, e.g. whether neuronal Na channels play only a role for CPVT in this CSQ mutation or whether it also occurs with mutations in the RyR2 associated with CPVT. Even more important, whether nNaV are functionally important in structural heart disease, e.g. human heart failure. This would be of great interest with a first approach to study samples from patients with heart failure. One set of experiments in the current report4 convincingly points in this general direction, i.e. riluzole also reduced diastolic SR Ca leak in WT myocytes (although co-localization studies for nNaV and RyR were only performed in R33Q myocytes and not in WT). Moreover, these functional findings were described even without the presence of isoproterenol which is usually needed to trigger arrhythmias in CPVT. It remains unclear, however, why SR Ca content does not change since a reduction in diastolic SR Ca leak in the presence of riluzole should actually increase SR Ca load (and thus intracellular Ca transients). This was not the case in the current study although there were some tendencies. The authors argue that this was due to the very local signalling which did not affect overall SR Ca load. Nevertheless, unchanged global Ca handling as suggested may further support the antiarrhythmic effect of riluzole. Of note, unspecific off-target effects of riluzole cannot be excluded completely since riluzole was more potent than TTX and SN-6 (NCX inhibitor) to reduce SR Ca sparks. Nevertheless, experiments in permeabilized myocytes convincingly showed no direct effects of riluzole on RyR2 function. The more general importance is also underlined by other findings including Sedej et al.15 who reported in CPVT mice (RyR2R4496C+/− mutation) that increasing intracellular Na concentration by using ouabain leads to an increased diastolic SR Ca leak and thus arrhythmogenic events including Ca waves, DADs, and spontaneous APs. In addition, increased late INa either by pharmacological stimulation7 or reactive oxygen species (ROS) increased intracellular Na and SR Ca leak mainly through NCX.16,17 A final thought about the role of the NCX18,19 should also be allowed. In the current manuscript, inhibiting NCX using SN-6 has very similar antiarrhythmic effects when compared with inhibiting nNaV with riluzole — even in the in vivo experiments in mice. Therefore, it may be possible that beside the role of nNaV in CPVT the NCX acts as a more general target and may also be of further clinical importance. I would like to add the NCX to the nNaV/RyR microdomain the authors suggested (see Fig. 7F, Ref. 4), because NCX reverse and forward mode inhibition may be at least equally important.18 Whether the antiarrhythmic effect suggested here, however, translates into improved clinically relevant morbidity and mortality should be studied in the near future. L.S.M. is funded by Deutsche Forschungsgemeinschaft (DFG) through an International Research Training Group GRK 1816, the Fondation Leducq Transatlantic Network on ‘Redox and Nitrosative Regulation of Cardiac Remodeling’, as well as the DZHK (Deutsches Zentrum für Herz-Kreislauf-Forschung – German Centre for Cardiovascular Research). Conflict of interest: L.S.M. receives research grants from Gilead and is involved in clinical trials with Gilead and Menarini and received speaker's honoraria from Berlin-Chemie. These companies are involved in late Na channel research and distribution of ranolazine, the only late Na channel inhibitor on the market so far.