Dantrolene, a therapeutic agent for malignant hyperthermia, inhibits catecholaminergic polymorphic ventricular tachycardia in a RyR2(R2474S/+) knock-in mouse model

Antiarrhythmic effects of dantrolene in human diseased cardiomyocytes

BACKGROUND

Cardiac type 2 ryanodine receptors (RyR₂s) play a pivotal role in cellular electrophysiology and contractility. Increased RyR₂-mediated diastolic sarcoplasmic reticulum (SR) Ca²⁺ release is linked to heart failure (HF) and arrhythmias. Dantrolene, a drug used for the treatment of malignant hyperthermia, is known to stabilize RyRs in skeletal muscle.

OBJECTIVE

The purpose of this study was to investigate the effects of dantrolene on arrhythmogenic triggers and contractile function in human atrial fibrillation (AF) and HF cardiomyocytes (CM).

METHODS

Human CM were isolated from either patients with HF (ventricular) or patients with AF (atrial), and Ca²⁺ imaging, patch-clamp, or muscle strip experiments were performed.

RESULTS

After exposure to dantrolene, human atrial AF and left ventricular HF CM showed significant reductions in proarrhythmic SR Ca²⁺ spark frequency and diastolic SR Ca²⁺ leak. Moreover, dantrolene decreased the frequency of Ca²⁺ waves and spontaneous Ca²⁺ transients in HF CM. Patch-clamp experiments revealed that dantrolene significantly suppressed delayed afterdepolarizations in HF and AF CM. Importantly, dantrolene had no effect on action potential duration in AF or in HF CM. In addition, dantrolene had neutral effects on contractile force of human isometrically twitching ventricular HF trabeculae.

CONCLUSION

Our study showed that dantrolene beneficially influenced disrupted SR Ca²⁺ homeostasis in human HF and AF CM. Cellular arrhythmogenic triggers were potently suppressed by dantrolene, whereas action potential duration and contractility were not affected. As a clinically approved drug for the treatment of malignant hyperthermia, dantrolene may be a potential antiarrhythmic drug for patients with rhythm disorders and merits further clinical investigation.

CaMKII-dependent phosphorylation of RyR2 promotes targetable pathological RyR2 conformational shift

Diastolic calcium (Ca) leak via cardiac ryanodine receptors (RyR2) can cause arrhythmias and heart failure (HF). Ca/calmodulin (CaM)-dependent kinase II (CaMKII) is upregulated and more active in HF, promoting RyR2-mediated Ca leak by RyR2-Ser2814 phosphorylation. Here, we tested a mechanistic hypothesis that RyR2 phosphorylation by CaMKII increases Ca leak by promoting a pathological RyR2 conformation with reduced CaM affinity. Acute CaMKII activation in wild-type RyR2, and phosphomimetic RyR2-S2814D (vs. non-phosphorylatable RyR2-S2814A) knock-in mouse myocytes increased SR Ca leak, reduced CaM-RyR2 affinity, and caused a pathological shift in RyR2 conformation (detected via increased access of the RyR2 structural peptide DPc10). This same trio of effects was seen in myocytes from rabbits with pressure/volume-overload induced HF. Excess CaM quieted leak and restored control conformation, consistent with negative allosteric coupling between CaM affinity and DPc10 accessible conformation. Dantrolene (DAN) also restored CaM affinity, reduced DPc10 access, and suppressed RyR2-mediated Ca leak and ventricular tachycardia in RyR2-S2814D mice. We propose that a common pathological RyR2 conformational state (low CaM affinity, high DPc10 access, and elevated leak) may be caused by CaMKII-dependent phosphorylation, oxidation, and HF. Moreover, DAN (or excess CaM) can shift this pathological gating state back to the normal physiological conformation, a potentially important therapeutic approach.

Role of the dysfunctional ryanodine receptor - Na(+)-Ca(2+)exchanger axis in progression of cardiovascular diseases: What we can learn from pharmacological studies?

Abnormal Ca(2+)homeostasis is often associated with chronic cardiovascular diseases, such as hypertension, heart failure or cardiac arrhythmias, and typically contributes to the basic ethiology of the disease. Pharmacological targeting of cardiac Ca(2+)handling has great therapeutic potential offering invaluable options for the prevention, slowing down the progression or suppression of the harmful outcomes like life threatening cardiac arrhythmias. In this review we outline the existing knowledge on the involvement of malfunction of the ryanodine receptor and the Na(+)-Ca(2+)exchanger in disturbances of Ca(2+)homeostasis and discuss important proof of concept pharmacological studies targeting these mechanisms in context of hypertension, heart failure, atrial fibrillation and ventricular arrhythmias. We emphasize the promising results of preclinical studies underpinning the potential benefits of the therapeutic strategies based on ryanodine receptor or Na(+)-Ca(2+)exchanger inhibition.

Ion Channel Diseases: an Update for 2016

OPINION STATEMENT

Ion channelopathies are a frequent cause of sudden cardiac death (SCD) in patients with structurally normal hearts. These are generally Mendelian inherited electrical disorders with variable penetrance and expressivity. The ability to predict the development of life threatening arrhythmias in these patients is challenging. This chapter will present an update on the genetics, the role of genetic testing, and management of the inherited cardiac channelopathies with a focus on the relatively more common syndromes associated with an increased risk of SCD.

Structural and functional interactions within ryanodine receptor

The ryanodine receptor/Ca2+ release channel plays a pivotal role in skeletal and cardiac muscle excitation-contraction coupling. Defective regulation leads to neuromuscular disorders and arrhythmogenic cardiac disease. This mini-review focuses on channel regulation through structural intra- and inter-subunit interactions and their implications in ryanodine receptor pathophysiology.

Role of pharmacotherapy in cardiac ion channelopathies

In the last decade, there have been considerable advances in the understanding of the pathophysiology of malignant ventricular tachyarrhythmias (VT) and sudden cardiac death (SCD). Over 80% of SCD occurs in patients with organic heart disease. However, approximately 10%-15% of SCD occurs in the presence of structurally normal heart, and the majority of these patients are young. In this group of patients, changes in genes encoding cardiac ion channels produce modifications of the function of the channel resulting in an electrophysiological substrate of VT and SCD. Collectively, these disorders are referred to as cardiac ion channelopathies. The four major syndromes in this group are: the long QT syndrome (LQTS), the Brugada syndrome (BrS), the short QT syndrome (SQTS), and the catecholaminergic polymorphic ventricular tachycardia (CPVT). Each of these syndromes includes multiple subtypes with different and sometimes complex cardiac ion channel genetic abnormalities. Many are associated with other somatic and neurological abnormalities besides the risk of VT and SCD. The current management of cardiac ion channelopathies can be summarized as follows: (1) in symptomatic patients, the implantable cardioverter defibrillator (ICD) is the only viable option; (2) in asymptomatic patients, risk stratification is necessary, followed by either the ICD, pharmacotherapy, or a combination of both. A genotype-specific approach to pharmacotherapy requires a thorough understanding of the molecular-cellular basis of arrhythmogenesis in cardiac ion channelopathies as well as the specific drug profile.

Oxidation of ryanodine receptor (RyR) and calmodulin enhance Ca release and pathologically alter, RyR structure and calmodulin affinity

Oxidative stress may contribute to cardiac ryanodine receptor (RyR2) dysfunction in heart failure (HF) and arrhythmias. Altered RyR2 domain-domain interaction (domain unzipping) and calmodulin (CaM) binding affinity are allosterically coupled indices of RyR2 conformation. In HF RyR2 exhibits reduced CaM binding, increased domain unzipping and greater SR Ca leak, and dantrolene can reverse these changes. However, effects of oxidative stress on RyR2 conformation and leak in myocytes are poorly understood. We used fluorescent CaM, FKBP12.6, and domain-peptide biosensor (F-DPc10) to measure, directly in cardiac myocytes, (1) RyR2 activation by hydrogen peroxide (H2O2)-induced oxidation, (2) RyR2 conformation change caused by oxidation, (3) CaM-RyR2 and FK506-binding protein (FKBP12.6)-RyR2 interaction upon oxidation, and (4) whether dantrolene affects 1-3. H2O2 was used to mimic oxidative stress. H2O2 significantly increased the frequency of Ca(2+) sparks and spontaneous Ca(2+) waves, and dantrolene almost completely blocked these effects. H2O2 pretreatment significantly reduced CaM-RyR2 binding, but had no effect on FKBP12.6-RyR2 binding. Dantrolene restored CaM-RyR2 binding but had no effect on intracellular and RyR2 oxidation levels. H2O2 also accelerated F-DPc10-RyR2 association while dantrolene slowed it. Thus, H2O2 causes conformational changes (sensed by CaM and DPc10 binding) associated with Ca leak, and dantrolene reverses these RyR2 effects. In conclusion, in cardiomyocytes, H2O2 treatment markedly reduces the CaM-RyR2 affinity, has no effect on FKBP12.6-RyR2 affinity, and causes domain unzipping. Dantrolene can correct domain unzipping, restore CaM-RyR2 affinity, and quiet pathological RyR2 channel gating. F-DPc10 and CaM are useful biosensors of a pathophysiological RyR2 state.

Antiarrhythmic Effects of Dantrolene in Patients with Catecholaminergic Polymorphic Ventricular Tachycardia and Replication of the Responses Using iPSC Models

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a highly malignant inherited arrhythmogenic disorder. Type 1 CPVT (CPVT1) is caused by cardiac ryanodine receptor (RyR2) gene mutations resulting in abnormal calcium release from sarcoplasmic reticulum. Dantrolene, an inhibitor of sarcoplasmic Ca²⁺ release, has been shown to rescue this abnormal Ca²⁺ release in vitro. We assessed the antiarrhythmic efficacy of dantrolene in six patients carrying various RyR2 mutations causing CPVT. The patients underwent exercise stress test before and after dantrolene infusion. Dantrolene reduced the number of premature ventricular complexes (PVCs) on average by 74% (range 33-97) in four patients with N-terminal or central mutations in the cytosolic region of the RyR2 protein, while dantrolene had no effect in two patients with mutations in or near the transmembrane domain. Induced pluripotent stem cells (iPSCs) were generated from all the patients and differentiated into spontaneously beating cardiomyocytes (CMs). The antiarrhythmic effect of dantrolene was studied in CMs after adrenaline stimulation by Ca²⁺ imaging. In iPSC derived CMs with RyR2 mutations in the N-terminal or central region, dantrolene suppressed the Ca²⁺ cycling abnormalities in 80% (range 65-97) of cells while with mutations in or near the transmembrane domain only in 23 or 32% of cells. In conclusion, we demonstrate that dantrolene given intravenously shows antiarrhythmic effects in a portion of CPVT1 patients and that iPSC derived CM models replicate these individual drug responses. These findings illustrate the potential of iPSC models to individualize drug therapy of inherited diseases.

Essential Role of Calmodulin in RyR Inhibition by Dantrolene

Dantrolene is the first line therapy of malignant hyperthermia. Animal studies suggest that dantrolene also protects against heart failure and arrhythmias caused by spontaneous Ca(2+) release. Although dantrolene inhibits Ca(2+) release from the sarcoplasmic reticulum of skeletal and cardiac muscle preparations, its mechanism of action has remained controversial, because dantrolene does not inhibit single ryanodine receptor (RyR) Ca(2+) release channels in lipid bilayers. Here we test the hypothesis that calmodulin (CaM), a physiologic RyR binding partner that is lost during incorporation into lipid bilayers, is required for dantrolene inhibition of RyR channels. In single channel recordings (100 nM cytoplasmic [Ca(2+)] + 2 mM ATP), dantrolene caused inhibition of RyR1 (rabbit skeletal muscle) and RyR2 (sheep) with a maximal inhibition of Po (Emax) to 52 ± 4% of control only after adding physiologic [CaM] = 100 nM. Dantrolene inhibited RyR2 with an IC50 of 0.16 ± 0.03 µM. Mutant N98S-CaM facilitated dantrolene inhibition with an IC50 = 5.9 ± 0.3 nM. In mouse cardiomyocytes, dantrolene had no effect on cardiac Ca(2+) release in the absence of CaM, but reduced Ca(2+) wave frequency (IC50 = 0.42 ± 0.18 µM, Emax = 47 ± 4%) and amplitude (IC50 = 0.19 ± 0.04 µM, Emax = 66 ± 4%) in the presence of 100 nM CaM. We conclude that CaM is essential for dantrolene inhibition of RyR1 and RyR2. Its absence explains why dantrolene inhibition of single RyR channels has not been previously observed.

Effects of Dantrolene Treatment on Ventricular Electrophysiology and Arrhythmogenesis in Rats With Chronic β-Adrenergic Receptor Activation

Dantrolene, which is primarily used to treat malignant hyperthermia, has recently been suggested for the prevention of arrhythmogenesis in various animal models. In this study, the effects of dantrolene treatment on electrophysiological properties and ventricular arrhythmias (VAs) in rats with chronic β-adrenergic receptor (β-AR) activation were investigated. Rats were randomized to treatment with saline (control group), isoproterenol (ISO; ISO group), or ISO + dantrolene (ID group) for 2 weeks. An electrophysiological study was performed to assess action potential duration restitution (APDR) and induce action potential duration (APD) alternans or VA in vitro. The protein levels of Cav1.2, sarcoplasmic reticulum Ca2+-ATPase (SERCA2a), and ryanodine receptor 2 (RyR2) were detected by Western blot. Compared with the control group, chronic administration of ISO significantly increased APD, the maximum slope (Smax) of APDR curve, and the spatial dispersions of Smax and APD (all P < .01), and all effects were attenuated by dantrolene treatment (all P < .05). Additionally, chronic ISO administration significantly reduced the protein levels of SERCA2 and RyR2, but increased the Cav1.2 protein expression (all P < .05). However, compared with the ISO group, dantrolene treatment preserved SERCA2a and RyR2 protein levels and decreased Cav1.2 protein levels in the ID group (all P < .05). The intracellular Ca2+ ([Ca2+]i) levels measured by incubating isolated cardiomyocytes with Fluo-3/alveolar macrophages were significantly increased in the ISO group compared with the control group ( P < .01). Dantrolene treatment markedly reduced the rise of [Ca2+]i levels caused by chronic administration of ISO ( P < .05). Dantrolene treatment also prevented the reductions in the APD alternans and VA thresholds induced by chronic ISO stimulation (all P < .05). These data suggest that dantrolene stabilizes ventricular electrophysiological characteristics and increases the expression of key sarcoplasmic reticulum calcium cycling proteins to reduce vulnerability to VA in rats with chronic β-AR activation.

Dantrolene rescues aberrant N-terminus intersubunit interactions in mutant pro-arrhythmic cardiac ryanodine receptors

AIMS

The ryanodine receptor (RyR2) is an intracellular Ca(2+) release channel essential for cardiac excitation-contraction coupling. Abnormal RyR2 channel function results in the generation of arrhythmias and sudden cardiac death. The present study was undertaken to investigate the mechanistic basis of RyR2 dysfunction in inherited arrhythmogenic cardiac disease.

METHODS AND RESULTS

We present several lines of complementary evidence, indicating that the arrhythmia-associated L433P mutation disrupts RyR2 N-terminus self-association. A combination of yeast two-hybrid, co-immunoprecipitation, and chemical cross-linking assays collectively demonstrate that a RyR2 N-terminal fragment carrying the L433P mutation displays substantially reduced self-interaction compared with wild type. Moreover, sucrose density gradient centrifugation reveals that the L433P mutation impairs tetramerization of the full-length channel. [(3)H]Ryanodine-binding assays demonstrate that disrupted N-terminal intersubunit interactions within RyR2(L433P) confer an altered sensitivity to Ca(2+) activation. Calcium imaging of RyR2(L433P)-expressing cells reveals substantially prolonged Ca(2+) transients and reduced Ca(2+) store content indicating defective channel closure. Importantly, dantrolene treatment reverses the L433P mutation-induced impairment and restores channel function.

CONCLUSION

The N-terminus domain constitutes an important structural determinant for the functional oligomerization of RyR2. Our findings are consistent with defective N-terminus self-association as a molecular mechanism underlying RyR2 channel deregulation in inherited arrhythmogenic cardiac disease. Significantly, the therapeutic action of dantrolene may occur via the restoration of normal RyR2 N-terminal intersubunit interactions.

[Ryanodine receptor, calcium leak and arrhythmias]

The participation of the ionic Ca(2+) release channel/ryanodine receptor in cardiac excitation-contraction coupling is well known since the late '80s, when various seminal papers communicated its purification for the first time and its identity with the "foot" structures located at the terminal cisternae of the sarcoplasmic reticulum. In addition to its main role as the Ca(2+) channel responsible for the transient Ca(2+) increase that activates the contractile machinery of the cardiomyocytes, the ryanodine receptor releases Ca(2+) during the relaxation phase of the cardiac cycle, giving rise to a diastolic Ca(2+) leak. In normal physiological conditions, diastolic Ca(2+) leak regulates the proper level of luminal Ca(2+), but in pathological conditions it participates in the generation of both, acquired and hereditary arrhythmias. Very recently, several groups have focused their efforts into the development of pharmacological tools to control the altered diastolic Ca(2+) leak via ryanodine receptors. In this review, we focus our interest on describing the participation of cardiac ryanodine receptor in the diastolic Ca(2+) leak under physiological or pathological conditions and also on the therapeutic approaches to control its undesired exacerbated activity during diastole.

Concise review: drug discovery in the age of the induced pluripotent stem cell

For decades, the paradigm of drug discovery and development has relied on immortalized cell lines, animal models of human disease, and clinical trials. With the discovery of induced pluripotent stem cell (iPSC) technology in 2007, a new human in vitro drug testing platform has potentially augmented this set of tools by providing additional ways to screen compounds for safety and efficacy. The growing number of human disease models made with patient-specific iPSCs has made it possible to conduct research on a wide range of disorders, including rare diseases and those with multifactorial origin, as well as to simulate drug effects on difficult-to-obtain tissues such as brain and cardiac muscle. Toxicity and teratogenicity assays developed with iPSC-derived cells can also provide an additional layer of safety before advancing drugs to clinical trials. The incorporation of iPSC technology into drug therapy development holds promise as a more powerful and nuanced approach to personalized medicine.

Dantrolene improves survival after ventricular fibrillation by mitigating impaired calcium handling in animal models

BACKGROUND

Resistant ventricular fibrillation, refibrillation. and diminished myocardial contractility are important factors leading to poor survival after cardiac arrest. We hypothesized that dantrolene improves survival after ventricular fibrillation (VF) by rectifying the calcium dysregulation caused by VF.

METHODS AND RESULTS

VF was induced in 26 Yorkshire pigs for 4 minutes. Cardiopulmonary resuscitation was then commenced for 3 minutes, and dantrolene or isotonic saline was infused at the onset of cardiopulmonary resuscitation. Animals were defibrillated and observed for 30 minutes. To study the effect of VF on calcium handling and its modulation by dantrolene, hearts from 14 New Zealand rabbits were Langendorff-perfused. The inducibility of VF after dantrolene administration was documented. Optical mapping was performed to evaluate diastolic spontaneous calcium elevations as a measure of cytosolic calcium leak. The sustained return of spontaneous circulation (systolic blood pressure ≥60 mm Hg) was achieved in 85% of the dantrolene group in comparison with 39% of controls (P=0.02). return of spontaneous circulation was achieved earlier in dantrolene-treated pigs after successful defibrillation (21 ± 6 s versus 181 ± 57 s in controls, P=0.005). The median number of refibrillation episodes was lower in the dantrolene group (0 versus 1, P=0.04). In isolated rabbit hearts, the successful induction of VF was achieved in 83% of attempts in controls versus 41% in dantrolene-treated hearts (P=0.007). VF caused diastolic calcium leaks in the form of spontaneous calcium elevations. Administration of 20 μmol/L dantrolene significantly decreased spontaneous calcium elevation amplitude versus controls. (0.024 ± 0.013 versus 0.12 ± 0.02 arbitrary unit [200-ms cycle length], P=0.001).

CONCLUSIONS

Dantrolene infusion during cardiopulmonary resuscitation facilitates successful defibrillation, improves hemodynamics postdefibrillation, decreases refibrillation, and thus improves survival after cardiac arrest. The effects are mediated through normalizing VF-induced dysfunctional calcium cycling.

Cardiac sarcoplasmic reticulum calcium leak: basis and roles in cardiac dysfunction

Synchronized SR calcium (Ca) release is critical to normal cardiac myocyte excitation-contraction coupling, and ideally this release shuts off completely between heartbeats. However, other SR Ca release events are referred to collectively as SR Ca leak (which includes Ca sparks and waves as well as smaller events not detectable as Ca sparks). Much, but not all, of the SR Ca leak occurs via ryanodine receptors and can be exacerbated in pathological states such as heart failure. The extent of SR Ca leak is important because it can (a) reduce SR Ca available for release, causing systolic dysfunction; (b) elevate diastolic [Ca]i, contributing to diastolic dysfunction; (c) cause triggered arrhythmias; and (d) be energetically costly because of extra ATP used to repump Ca. This review addresses quantitative aspects and manifestations of SR Ca leak and its measurement, and how leak is modulated by Ca, associated proteins, and posttranslational modifications in health and disease.

N-terminus oligomerization regulates the function of cardiac ryanodine receptors

The ryanodine receptor (RyR) is an ion channel composed of four identical subunits mediating calcium efflux from the endo/sarcoplasmic reticulum of excitable and non-excitable cells. We present several lines of evidence indicating that the RyR2 N-terminus is capable of self-association. A combination of yeast two-hybrid screens, co-immunoprecipitation analysis, chemical crosslinking and gel filtration assays collectively demonstrate that a RyR2 N-terminal fragment possesses the intrinsic ability to oligomerize, enabling apparent tetramer formation. Interestingly, N-terminus tetramerization mediated by endogenous disulfide bond formation occurs in native RyR2, but notably not in RyR1. Disruption of N-terminal inter-subunit interactions within RyR2 results in dysregulation of channel activation at diastolic Ca(2+) concentrations from ryanodine binding and single channel measurements. Our findings suggest that the N-terminus interactions mediating tetramer assembly are involved in RyR channel closure, identifying a crucial role for this structural association in the dynamic regulation of intracellular Ca(2+) release.

Decreased RyR2 refractoriness determines myocardial synchronization of aberrant Ca2+ release in a genetic model of arrhythmia

Dysregulated intracellular Ca(2+) signaling is implicated in a variety of cardiac arrhythmias, including catecholaminergic polymorphic ventricular tachycardia. Spontaneous diastolic Ca(2+) release (DCR) can induce arrhythmogenic plasma membrane depolarizations, although the mechanism responsible for DCR synchronization among adjacent myocytes required for ectopic activity remains unclear. We investigated the synchronization mechanism(s) of DCR underlying untimely action potentials and diastolic contractions (DCs) in a catecholaminergic polymorphic ventricular tachycardia mouse model with a mutation in cardiac calsequestrin. We used a combination of different approaches including single ryanodine receptor channel recording, optical imaging (Ca(2+) and membrane potential), and contractile force measurements in ventricular myocytes and intact cardiac muscles. We demonstrate that DCR occurs in a temporally and spatially uniform manner in both myocytes and intact myocardial tissue isolated from cardiac calsequestrin mutation mice. Such synchronized DCR events give rise to triggered electrical activity that results in synchronous DCs in the myocardium. Importantly, we establish that synchronization of DCR is a result of a combination of abbreviated ryanodine receptor channel refractoriness and the preceding synchronous stimulated Ca(2+) release/reuptake dynamics. Our study reveals how aberrant DCR events can become synchronized in the intact myocardium, leading to triggered activity and the resultant DCs in the settings of a cardiac rhythm disorder.

Calcium cycling proteins and heart failure: mechanisms and therapeutics

Ca2+-dependent signaling is highly regulated in cardiomyocytes and determines the force of cardiac muscle contraction. Ca2+ cycling refers to the release and reuptake of intracellular Ca2+ that drives muscle contraction and relaxation. In failing hearts, Ca2+ cycling is profoundly altered, resulting in impaired contractility and fatal cardiac arrhythmias. The key defects in Ca2+ cycling occur at the level of the sarcoplasmic reticulum (SR), a Ca2+ storage organelle in muscle. Defects in the regulation of Ca2+ cycling proteins including the ryanodine receptor 2, cardiac (RyR2)/Ca2+ release channel macromolecular complexes and the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase 2a (SERCA2a)/phospholamban complex contribute to heart failure. RyR2s are oxidized, nitrosylated, and PKA hyperphosphorylated, resulting in "leaky" channels in failing hearts. These leaky RyR2s contribute to depletion of Ca2+ from the SR, and the leaking Ca2+ depolarizes cardiomyocytes and triggers fatal arrhythmias. SERCA2a is downregulated and phospholamban is hypophosphorylated in failing hearts, resulting in impaired SR Ca2+ reuptake that conspires with leaky RyR2 to deplete SR Ca2+. Two new therapeutic strategies for heart failure (HF) are now being tested in clinical trials: (a) fixing the leak in RyR2 channels with a novel class of Ca2+-release channel stabilizers called Rycals and (b) increasing expression of SERCA2a to improve SR Ca2+ reuptake with viral-mediated gene therapy. There are many potential opportunities for additional mechanism-based therapeutics involving the machinery that regulates Ca2+ cycling in the heart.

Induced pluripotent stem cell-derived cardiomyocytes in studies of inherited arrhythmias

The discovery of the genetic basis of inherited arrhythmias has paved the way for an improved understanding of arrhythmogenesis in a wide spectrum of life-threatening conditions. In vitro expression of mutations and transgenic animal models have been instrumental in enhancing this understanding, but the applicability of results to the human heart remains unknown. The ability to differentiate induced pluripotent stem cells (iPSs) into cardiomyocytes enables the potential to generate patient-specific myocytes, which could be used to recapitulate the features of inherited arrhythmias in the context of the patient's genetic background. Few studies have been reported on iPS-derived myocytes obtained from patients with heritable arrhythmias, but they have demonstrated the applicability of this innovative approach to the study of inherited arrhythmias. Here we review the results achieved by iPS investigations in arrhythmogenic syndromes and discuss the existing challenges to be addressed before the use of iPS-derived myocytes can become a part of personalized management of inherited arrhythmias.

Calcium leak through ryanodine receptors leads to atrial fibrillation in 3 mouse models of catecholaminergic polymorphic ventricular tachycardia

RATIONALE

Atrial fibrillation (AF) is the most common cardiac arrhythmia, however the mechanism(s) causing AF remain poorly understood and therapy is suboptimal. The ryanodine receptor (RyR2) is the major calcium (Ca2+) release channel on the sarcoplasmic reticulum (SR) required for excitation-contraction coupling in cardiac muscle.

OBJECTIVE

In the present study, we sought to determine whether intracellular diastolic SR Ca2+ leak via RyR2 plays a role in triggering AF and whether inhibiting this leak can prevent AF.

METHODS AND RESULTS

We generated 3 knock-in mice with mutations introduced into RyR2 that result in leaky channels and cause exercise induced polymorphic ventricular tachycardia in humans [catecholaminergic polymorphic ventricular tachycardia (CPVT)]. We examined AF susceptibility in these three CPVT mouse models harboring RyR2 mutations to explore the role of diastolic SR Ca2+ leak in AF. AF was stimulated with an intra-esophageal burst pacing protocol in the 3 CPVT mouse models (RyR2-R2474S+/-, 70%; RyR2-N2386I+/-, 60%; RyR2-L433P+/-, 35.71%) but not in wild-type (WT) mice (P<0.05). Consistent with these in vivo results, there was a significant diastolic SR Ca2+ leak in atrial myocytes isolated from the CPVT mouse models. Calstabin2 (FKBP12.6) is an RyR2 subunit that stabilizes the closed state of RyR2 and prevents a Ca2+ leak through the channel. Atrial RyR2 from RyR2-R2474S+/- mice were oxidized, and the RyR2 macromolecular complex was depleted of calstabin2. The Rycal drug S107 stabilizes the closed state of RyR2 by inhibiting the oxidation/phosphorylation induced dissociation of calstabin2 from the channel. S107 reduced the diastolic SR Ca2+ leak in atrial myocytes and decreased burst pacing-induced AF in vivo. S107 did not reduce the increased prevalence of burst pacing-induced AF in calstabin2-deficient mice, confirming that calstabin2 is required for the mechanism of action of the drug.

CONCLUSIONS

The present study demonstrates that RyR2-mediated diastolic SR Ca2+ leak in atrial myocytes is associated with AF in CPVT mice. Moreover, the Rycal S107 inhibited diastolic SR Ca2+ leak through RyR2 and pacing-induced AF associated with CPVT mutations.

Dantrolene rescues arrhythmogenic RYR2 defect in a patient-specific stem cell model of catecholaminergic polymorphic ventricular tachycardia

Coordinated release of calcium (Ca²⁺) from the sarcoplasmic reticulum (SR) through cardiac ryanodine receptor (RYR2) channels is essential for cardiomyocyte function. In catecholaminergic polymorphic ventricular tachycardia (CPVT), an inherited disease characterized by stress-induced ventricular arrhythmias in young patients with structurally normal hearts, autosomal dominant mutations in RYR2 or recessive mutations in calsequestrin lead to aberrant diastolic Ca²⁺ release from the SR causing arrhythmogenic delayed after depolarizations (DADs). Here, we report the generation of induced pluripotent stem cells (iPSCs) from a CPVT patient carrying a novel RYR2 S406L mutation. In patient iPSC-derived cardiomyocytes, catecholaminergic stress led to elevated diastolic Ca²⁺ concentrations, a reduced SR Ca²⁺ content and an increased susceptibility to DADs and arrhythmia as compared to control myocytes. This was due to increased frequency and duration of elementary Ca²⁺ release events (Ca²⁺ sparks). Dantrolene, a drug effective on malignant hyperthermia, restored normal Ca²⁺ spark properties and rescued the arrhythmogenic phenotype. This suggests defective inter-domain interactions within the RYR2 channel as the pathomechanism of the S406L mutation. Our work provides a new in vitro model to study the pathogenesis of human cardiac arrhythmias and develop novel therapies for CPVT.

Dantrolene prevents arrhythmogenic Ca2+ release in heart failure

In heart failure (HF), arrhythmogenic Ca(2+) release and chronic Ca(2+) depletion of the sarcoplasmic reticulum (SR) arise due to altered function of the ryanodine receptor (RyR) SR Ca(2+)-release channel. Dantrolene, a therapeutic agent used to treat malignant hyperthermia associated with mutations of the skeletal muscle type 1 RyR (RyR1), has recently been suggested to have effects on the cardiac type 2 RyR (RyR2). In this investigation, we tested the hypothesis that dantrolene exerts antiarrhythmic and inotropic effects on HF ventricular myocytes by examining multiple aspects of intracellular Ca(2+) handling. In normal rabbit myocytes, dantrolene (1 μM) had no effect on SR Ca(2+) load, postrest decay of SR Ca(2+) content, the threshold for spontaneous Ca(2+) wave initiation (i.e., the SR Ca(2+) content at which spontaneous waves initiate) and Ca(2+) spark frequency. In cardiomyocytes from failing rabbit hearts, SR Ca(2+) load and the wave initiation threshold were decreased compared with normal myocytes, Ca(2+) spark frequency was increased, and the postrest decay was potentiated. Using a novel approach of measuring cytosolic and intra-SR Ca(2+) concentration (using the low-affinity Ca(2+) indicator fluo-5N entrapped within the SR), we showed that treatment of HF cardiomyocytes with dantrolene rescued postrest decay and increased the wave initiation threshold. Additionally, dantrolene decreased Ca(2+) spark frequency while increasing the SR Ca(2+) content in HF myocytes. These data suggest that dantrolene exerts antiarrhythmic effects and preserves inotropy in HF cardiomyocytes by decreasing the incidence of diastolic Ca(2+) sparks, increasing the intra-SR Ca(2+) threshold at which spontaneous Ca(2+) waves occur, and decreasing the loss of Ca(2+) from the SR. Furthermore, the observation that dantrolene reduces arrhythmogenicity while at the same time preserves inotropy suggests that dantrolene is a potentially useful drug in the treatment of arrhythmia associated with HF.

In vitro modeling of ryanodine receptor 2 dysfunction using human induced pluripotent stem cells

BACKGROUND/AIMS

Induced pluripotent stem (iPS) cells generated from accessible adult cells of patients with genetic diseases open unprecedented opportunities for exploring the pathophysiology of human diseases in vitro. Catecholaminergic polymorphic ventricular tachycardia type 1 (CPVT1) is an inherited cardiac disorder that is caused by mutations in the cardiac ryanodine receptor type 2 gene (RYR2) and is characterized by stress-induced ventricular arrhythmia that can lead to sudden cardiac death in young individuals. The aim of this study was to generate iPS cells from a patient with CPVT1 and determine whether iPS cell-derived cardiomyocytes carrying patient specific RYR2 mutation recapitulate the disease phenotype in vitro.

METHODS

iPS cells were derived from dermal fibroblasts of healthy donors and a patient with CPVT1 carrying the novel heterozygous autosomal dominant mutation p.F2483I in the RYR2. Functional properties of iPS cell derived-cardiomyocytes were analyzed by using whole-cell current and voltage clamp and calcium imaging techniques.

RESULTS

Patch-clamp recordings revealed arrhythmias and delayed afterdepolarizations (DADs) after catecholaminergic stimulation of CPVT1-iPS cell-derived cardiomyocytes. Calcium imaging studies showed that, compared to healthy cardiomyocytes, CPVT1-cardiomyocytes exhibit higher amplitudes and longer durations of spontaneous Ca(2+) release events at basal state. In addition, in CPVT1-cardiomyocytes the Ca(2+)-induced Ca(2+)-release events continued after repolarization and were abolished by increasing the cytosolic cAMP levels with forskolin.

CONCLUSION

This study demonstrates the suitability of iPS cells in modeling RYR2-related cardiac disorders in vitro and opens new opportunities for investigating the disease mechanism in vitro, developing new drugs, predicting their toxicity, and optimizing current treatment strategies.

Mutation-linked defective interdomain interactions within ryanodine receptor cause aberrant Ca²⁺release leading to catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

The molecular mechanism by which catecholaminergic polymorphic ventricular tachycardia is induced by single amino acid mutations within the cardiac ryanodine receptor (RyR2) remains elusive. In the present study, we investigated mutation-induced conformational defects of RyR2 using a knockin mouse model expressing the human catecholaminergic polymorphic ventricular tachycardia-associated RyR2 mutant (S2246L; serine to leucine mutation at the residue 2246).

METHODS AND RESULTS

All knockin mice we examined produced ventricular tachycardia after exercise on a treadmill. cAMP-dependent increase in the frequency of Ca²⁺ sparks was more pronounced in saponin-permeabilized knockin cardiomyocytes than in wild-type cardiomyocytes. Site-directed fluorescent labeling and quartz microbalance assays of the specific binding of DP2246 (a peptide corresponding to the 2232 to 2266 region: the 2246 domain) showed that DP2246 binds with the K201-binding sequence of RyR2 (1741 to 2270). Introduction of S2246L mutation into the DP2246 increased the affinity of peptide binding. Fluorescence quench assays of interdomain interactions within RyR2 showed that tight interaction of the 2246 domain/K201-binding domain is coupled with domain unzipping of the N-terminal (1 to 600)/central (2000 to 2500) domain pair in an allosteric manner. Dantrolene corrected the mutation-caused domain unzipping of the domain switch and stopped the exercise-induced ventricular tachycardia.

CONCLUSIONS

The catecholaminergic polymorphic ventricular tachycardia-linked mutation of RyR2, S2246L, causes an abnormally tight local subdomain-subdomain interaction within the central domain involving the mutation site, which induces defective interaction between the N-terminal and central domains. This results in an erroneous activation of Ca²⁺ channel in a diastolic state reflecting on the increased Ca²⁺ spark frequency, which then leads to lethal arrhythmia.

Special article: Future directions in malignant hyperthermia research and patient care

Malignant hyperthermia (MH) is a complex pharmacogenetic disorder of muscle metabolism. To more closely examine the complexities of MH and other related muscle disorders, the Malignant Hyperthermia Association of the United States (MHAUS) recently sponsored a scientific conference at which an interdisciplinary group of experts gathered to share new information and ideas. In this Special Article, we highlight key concepts and theories presented at the conference along with exciting new trends and challenges in MH research and patient care.

Possible targets of therapy for catecholaminergic polymorphic ventricular tachycardia. - Insight from a theoretical model -

BACKGROUND

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a serious disease with a high mortality but its management is limited. The aim of this study was to investigate specific target sites for therapy in order to find potential management strategies for CPVT.

METHODS AND RESULTS

The mutant Ryanodine receptor 2 (RyR2) with reduced stored-overloaded-induced Ca²⁺ release (SOICR) threshold was incorporated into the Luo-Rudy dynamic (LRd) cell model to elucidate the underlying pathologies of CPVT. The simulations reveal that β-adrenergic stimulation increased the Ca²⁺ load in cardiac myocyte, which facilitates spontaneous SR Ca²⁺ leakage, resulting in triggered arrhythmias. Varied blockade (from 0% to 90%) in specific ion channels, including the Na⁺/Ca²⁺ exchanger (I(NaCa)), fast Na⁺ channel (I(Na)), RyR2 receptor (I(rel)), Ca²⁺-ATPase (SERCA) (I(up)) or L-type Ca²⁺channel (I(Ca(L))),was performed to simulate the action of specific drugs on target sites. Blockade of the I(NaCa) (≤ 10% blockade), in contrast to the I(up) (≤ 30% blockade), I(Ca(L)) and I(Na) (≤40% blockade), and followed by I(rel) (≤ 80% blockade), was most effective in suppressing the triggered arrhythmias in CPVT. Specifically, dual blockade of I(Ca(L))/I(up), I(Na)/I(rel) or I(Ca(L))/I(rel) had a synergistic effect in CPVT management.

CONCLUSIONS

Blockade of I(NaCa) appears to be the most efficacious target for CPVT management. Dual blockade of I(Ca(L))/I(up), I(Na)/I(rel) or I(Ca(L))/I(rel) has a synergistic effect in CPVT treatment.

Targeting ryanodine receptors for anti-arrhythmic therapy

Antiarrhythmic drugs are a group of pharmaceuticals that suppress or prevent abnormal heart rhythms, which are often associated with substantial morbidity and mortality. Current antiarrhythmic drugs that typically target plasma membrane ion channels have limited clinical success and in some cases have been described as being pro-arrhythmic. However, recent studies suggest that pathological release of calcium (Ca(2+)) from the sarcoplasmic reticulum via cardiac ryanodine receptors (RyR2) could represent a promising target for antiarrhythmic therapy. Diastolic SR Ca(2+) release has been linked to arrhythmogenesis in both the inherited arrhythmia syndrome 'catecholaminergic polymorphic ventricular tachycardia' and acquired forms of heart disease (eg, atrial fibrillation, heart failure). Several classes of pharmaceuticals have been shown to reduce abnormal RyR2 activity and may confer protection against triggered arrhythmias through reduction of SR Ca(2+) leak. In this review, we will evaluate the current pharmacological methods for stabilizing RyR2 and suggest treatment modalities based on current evidence of molecular mechanisms.

New drugs vs. old concepts: a fresh look at antiarrhythmics

Common arrhythmias, particularly atrial fibrillation (AF) and ventricular tachycardia/fibrillation (VT/VF) are a major public health concern. Classic antiarrhythmic (AA) drugs for AF are of limited effectiveness, and pose the risk of life-threatening VT/VF. For VT/VF, implantable cardiac defibrillators appear to be the unique, yet unsatisfactory, solution. Very few AA drugs have been successful in the last few decades, due to safety concerns or limited benefits in comparison to existing therapy. The Vaughan-Williams classification (one drug for one molecular target) appears too restrictive in light of current knowledge of molecular and cellular mechanisms. New AA drugs such as atrial-specific and/or multichannel blockers, upstream therapy and anti-remodeling drugs, are emerging. We focus on the cellular mechanisms related to abnormal Na⁺ and Ca²⁺ handling in AF, heart failure, and inherited arrhythmias, and on novel strategies aimed at normalizing ionic homeostasis. Drugs that prevent excessive Na⁺ entry (ranolazine) and aberrant diastolic Ca²⁺ release via the ryanodine receptor RyR2 (rycals, dantrolene, and flecainide) exhibit very interesting antiarrhythmic properties. These drugs act by normalizing, rather than blocking, channel activity. Ranolazine preferentially blocks abnormal persistent (vs. normal peak) Na⁺ currents, with minimal effects on normal channel function (cell excitability, and conduction). A similar "normalization" concept also applies to RyR2 stabilizers, which only prevent aberrant opening and diastolic Ca²⁺ leakage in diseased tissues, with no effect on normal function during systole. The different mechanisms of action of AA drugs may increase the therapeutic options available for the safe treatment of arrhythmias in a wide variety of pathophysiological situations.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.