Altered stoichiometry of FKBP12.6 versus ryanodine receptor as a cause of abnormal Ca(2+) leak through ryanodine receptor in heart failure

Role of chronic ryanodine receptor phosphorylation in heart failure and β-adrenergic receptor blockade in mice

Increased sarcoplasmic reticulum (SR) Ca2+ leak via the cardiac ryanodine receptor/calcium release channel (RyR2) is thought to play a role in heart failure (HF) progression. Inhibition of this leak is an emerging therapeutic strategy. To explore the role of chronic PKA phosphorylation of RyR2 in HF pathogenesis and treatment, we generated a knockin mouse with aspartic acid replacing serine 2808 (mice are referred to herein as RyR2-S2808D+/+ mice). This mutation mimics constitutive PKA hyperphosphorylation of RyR2, which causes depletion of the stabilizing subunit FKBP12.6 (also known as calstabin2), resulting in leaky RyR2. RyR2-S2808D+/+ mice developed age-dependent cardiomyopathy, elevated RyR2 oxidation and nitrosylation, reduced SR Ca2+ store content, and increased diastolic SR Ca2+ leak. After myocardial infarction, RyR2-S2808D+/+ mice exhibited increased mortality compared with WT littermates. Treatment with S107, a 1,4-benzothiazepine derivative that stabilizes RyR2-calstabin2 interactions, inhibited the RyR2-mediated diastolic SR Ca2+ leak and reduced HF progression in WT and RyR2-S2808D+/+ mice. In contrast, β-adrenergic receptor blockers improved cardiac function in WT but not in RyR2-S2808D+/+ mice.Thus, chronic PKA hyperphosphorylation of RyR2 results in a diastolic leak that causes cardiac dysfunction. Reversing PKA hyperphosphorylation of RyR2 is an important mechanism underlying the therapeutic action of β-blocker therapy in HF.

The ryanodine receptor in cardiac physiology and disease

According to the American Heart Association it is estimated that the United States will spend close to $39 billion in 2010 to treat over five million Americans suffering from heart failure. Patients with heart failure suffer from dyspnea and decreased exercised tolerance and are at increased risk for fatal ventricular arrhythmias. Food and Drug Administration -approved pharmacologic therapies for heart failure include diuretics, inhibitors of the renin-angiotensin system, and β-adrenergic receptor antagonists. Over the past 20 years advances in the field of ryanodine receptor (RyR2)/calcium release channel research have greatly advanced our understanding of cardiac physiology and the pathogenesis of heart failure and arrhythmias. Here we review the key observations, controversies, and discoveries that have led to the development of novel compounds targeting the RyR2/calcium release channel for treating heart failure and for preventing lethal arrhythmias.

Ryanodine receptor channelopathies

Ryanodine receptors (RyR) are intracellular Ca2+-permeable channels that provide the sarcoplasmic reticulum Ca2+ release required for skeletal and cardiac muscle contractions. RyR1 underlies skeletal muscle contraction, and RyR2 fulfills this role in cardiac muscle. Over the past 20 years, numerous mutations in both RyR isoforms have been identified and linked to skeletal and cardiac diseases. Malignant hyperthermia, central core disease, and catecholaminergic polymorphic ventricular tachycardia have been genetically linked to mutations in either RyR1 or RyR2. Thus, RyR channelopathies are both of interest because they cause significant human diseases and provide model systems that can be studied to elucidate important structure-function relationships of these ion channels.

Fixing ryanodine receptor Ca leak - a novel therapeutic strategy for contractile failure in heart and skeletal muscle

A critical component in regulating cardiac and skeletal muscle contractility is the release of Ca(2+) via ryanodine receptor (RyR) Ca(2+) release channels in the sarcoplasmic reticulum (SR). In heart failure and myopathy, the RyR has been found to be excessively phosphorylated or nitrosylated and depleted of the RyR-stabilizing protein calstabin (FK506 binding protein 12/12.6). This remodeling of the RyR channel complex results in an intracellular SR Ca(2+) leak and impaired contractility. Despite recent advances in heart failure treatment, there are still devastatingly high mortality rates with this disease. Moreover, pharmacological treatment for muscle weakness and myopathy is nearly nonexistent. A novel class of RyR-stabilizing drugs, rycals, which reduce Ca(2+) leak by stabilizing the RyR channels due to preservation of the RyR-calstabin interaction, have recently been shown to improve contractile function in both heart and skeletal muscle. This opens up a novel therapeutic strategy for the treatment of contractile failure in the cardiac and skeletal muscle.

Dissociation of calmodulin from cardiac ryanodine receptor causes aberrant Ca(2+) release in heart failure

AIMS

Calmodulin (CaM) is well known to modulate the channel function of the cardiac ryanodine receptor (RyR2). However, the possible role of CaM on the aberrant Ca(2+) release in diseased hearts remains unclear. In this study, we investigated the state of RyR2-bound CaM and channel dysfunctions in pacing-induced failing hearts.

METHODS AND RESULTS

The characteristics of CaM binding to RyR2 and the role of CaM on the aberrant Ca(2+) release were assessed in normal and failing canine hearts. The affinity of CaM binding to RyR2 was lower in failing sarcoplasmic reticulum (SR) than in normal SR. Addition of FK506, which dissociates FKBP12.6 from RyR2, to normal SR reduced the CaM-binding affinity. Dantrolene restored a normal level of the CaM-binding affinity in either FK506-treated (normal) SR or failing SR, suggesting that the defective inter-domain interaction between the N-terminal domain and the central domain of RyR2 (the therapeutic target of dantrolene) is involved in the reduction of the CaM-binding affinity in failing hearts. In saponin-permeabilized cardiomyocytes, the frequency of spontaneous Ca(2+) sparks was much more increased in failing cardiomyocytes than in normal cardiomyocytes, whereas the addition of a high concentration of CaM attenuated the aberrant increase of Ca(2+) sparks.

CONCLUSION

The defective inter-domain interaction between N-terminal and central domains within RyR2 reduces the binding affinity of CaM to RyR2, thereby causing the spontaneous Ca(2+) release events in failing hearts. Correction of the defective CaM binding may be a new strategy to protect against the aberrant Ca(2+) release in heart failure.

Atrial Ca2+ signaling in atrial fibrillation as an antiarrhythmic drug target

Atrial fibrillation (AF) is the most frequent arrhythmia and is associated with increased morbidity and mortality. Current drugs for AF treatment have moderate efficacy and increase the risk of life-threatening antiarrhythmias, making novel drug development crucial. Newer antiarrhythmic drugs like dronedarone and possibly vernakalant are efficient and may have less proarrhythmic potential. Emerging evidence suggests that abnormal intracellular Ca(2+) signaling is the key contributor to focal firing, substrate evolution, and atrial remodeling during AF. Accordingly, identification of the underlying atrial Ca(2+)-handling abnormalities is expected to discover novel mechanistically based therapeutic targets. This article reviews the molecular mechanisms of altered Ca(2+) signaling in AF and discusses the potential value of novel approaches targeting atrial Ca(2+)-handling abnormalities.

K201 improves aspects of the contractile performance of human failing myocardium via reduction in Ca2+ leak from the sarcoplasmic reticulum

In heart failure, intracellular Ca2+ leak from cardiac ryanodine receptors (RyR2s) leads to a loss of Ca2+ from the sarcoplasmic reticulum (SR) potentially contributing to decreased function. Experimental data suggest that the 1,4-benzothiazepine K201 (JTV-519) may stabilise RyR2s and thereby reduce detrimental intracellular Ca2+ leak. Whether K201 exerts beneficial effects in human failing myocardium is unknown. Therefore, we have studied the effects of K201 on muscle preparations from failing human hearts. K201 (0.3 microM; extracellular [Ca2+]e 1.25 mM) showed no effects on contractile function and micromolar concentrations resulted in negative inotropic effects (K201 1 microM; developed tension -9.8 +/- 2.5% compared to control group; P < 0.05). Interestingly, K201 (0.3 microM) increased the post-rest potentiation (PRP) of failing myocardium after 120 s, indicating an increased SR Ca2+ load. At high [Ca2+]e concentrations (5 mmol/L), K201 increased PRP already at shorter rest intervals (30 s). Strikingly, treatment with K201 (0.3 microM) prevented diastolic dysfunction (diastolic tension at 5 mmol/L [Ca2+]e normalised to 1 mmol/L [Ca2+]e: control 1.26 +/- 0.06, K201 1.01 +/- 0.03, P < 0.01). In addition at high [Ca2+]e) K201 (0.3 microM) treatment significantly improved systolic function [developed tension +27 +/- 8% (K201 vs. control); P < 0.05]. The beneficial effects on diastolic and systolic functions occurred throughout the physiological frequency range of the human heart rate from 1 to 3 Hz. Upon elevated intracellular Ca2+ concentration, systolic and diastolic contractile functions of terminally failing human myocardium are improved by K201.

Decreased triadin and increased calstabin2 expression in Great Danes with dilated cardiomyopathy

BACKGROUND

Dilated cardiomyopathy (DCM) is a common cardiac disease of Great Dane dogs, yet very little is known about the underlying molecular abnormalities that contribute to disease.

OBJECTIVE

Discover a set of genes that are differentially expressed in Great Dane dogs with DCM as a way to identify candidate genes for further study as well as to better understand the molecular abnormalities that underlie the disease.

ANIMALS

Three Great Dane dogs with end-stage DCM and 3 large breed control dogs.

METHODS

Prospective study. Transcriptional activity of 42,869 canine DNA sequences was determined with a canine-specific oligonucleotide microarray. Genome expression patterns of left ventricular tissue samples from affected Great Dane dogs were evaluated by measuring the relative amount of complementary RNA hybridization to the microarray probes and comparing it with expression from large breed dogs with noncardiac disease.

RESULTS

Three hundred and twenty-three transcripts were differentially expressed (> or = 2-fold change). The transcript with the greatest degree of upregulation (+61.3-fold) was calstabin2 (FKBP12.6), whereas the transcript with the greatest degree of downregulation (-9.07-fold) was triadin. Calstabin2 and triadin are both regulatory components of the cardiac ryanodine receptor (RyR2) and are critical to normal intracellular Ca2+ release and excitation-contraction coupling.

CONCLUSION AND CLINICAL IMPORTANCE

Great Dane dogs with DCM demonstrate abnormal calstabin2 and triadin expression. These changes likely affect Ca2+ flux within cardiac cells and may contribute to the pathophysiology of disease. Microarray-based analysis identifies calstabin2, triadin, and RyR2 function as targets of future study.

Effects of therapy using the Celacade system on structural and functional cardiac remodelling in rats following myocardial infarction

BACKGROUND

Immune modulation by the Celacade system (Vasogen Inc, Canada) decreases mortality and hospitalization in human heart failure.

OBJECTIVES

To study the effects of Celacade in rats on acute cytokine expression after coronary artery ligation, cardiac dimensions following myocardial infarction (MI), and systolic and diastolic function of cardiac muscle in MI.

METHODS

Celacade treatment was administered 14 days before coronary artery ligation and monthly after the surgery. Cytokine expression in cardiac tissue was measured on days 1 and 7 by ELISA in sham rats and in rats with MI (with or without Celacade treatment). Echocardiograms were obtained serially for 16 weeks. Force and sarcomere length (SL) were measured by strain gauge and laser diffraction in isolated right ventricle trabeculas at 16 weeks. The inotropic effect of pacing on force was quantified as F5 Hz/0.5 Hz. Diastolic dysfunction was quantified as the root mean square of spontaneous SL fluctuations.

RESULTS

Celacade inhibited transforming growth factor beta-1 production in the infarct area on day 7 (191.6+/-22.6 pg/mg versus 275.4+/-30.1 pg/mg; P<0.05), but did not attenuate cardiac dilation in MI. Celacade restored positive inotropism of pacing in MI (F5 Hz/0.5 Hz in Celacade, 219.1+/-46.7%; MI, 148.1+/-27.1% [P<0.05 compared with 211.4+/-37.9% in sham]). Celacade reduced diastolic dysfunction in MI (root mean square of spontaneous SL fluctuations: 121+/-15% and 143+/-19% with Celacade versus 184+/-19% and 190+/-26% without Celacade at 26 degrees C and 36 degrees C, respectively) compared with sham (100%; P<0.05).

CONCLUSIONS

Celacade reduces the increase of transforming growth factor beta-1 expression during the acute stage of MI in rats, but does not prevent chronic cardiac dilation. Celacade restores the positive inotropic effect of increased pacing rate in trabeculas from rat right ventricles with large MIs and reduces diastolic dysfunction.

Global gene expression profiling in the failing myocardium

Heart failure (HF) is a syndrome that involves multiple cellular mechanisms leading to a common phenotype of reduced ventricular contraction and cardiac chamber dilation. To clarify the mechanisms, a number of microarray analyses of the failing myocardium have been conducted. Gene expression profiles are usually compared between opposing pairs of samples, such as non-failing vs failing hearts, ischemic vs non-ischemic hearts, male vs female failing hearts or atria vs ventricles of failing hearts. Apart from these conventional methods, a different novel approach identified cardiac myosin light chain kinase (MLCK) as a HF-related gene by the comprehensive search for the genes that had an expression level that strongly correlated with the severity of HF; further investigations proved the important role of cardiac MLCK in HF. Moreover, a robust gene expression signature composed of 27 genes was revealed on analysis of 4 independent microarray data sets from the failing myocardium of dilated cardiomyopathy. The authors newly demonstrate 107 HF-related genes that were listed in 2 or more of 7 microarray data sets previously reported. Among these genes, many were observed to be involved in mitochondrial dysfunction and oxidative phosphorylation and 3 extracellular molecules, including periostin, pleiotrophin, and SERPINA3, which might become novel diagnostic and therapeutic targets for HF. These novel strategies warrant the new identification of specific genes that are linked to the pathophysiology of HF.

Dantrolene, a therapeutic agent for malignant hyperthermia, markedly improves the function of failing cardiomyocytes by stabilizing interdomain interactions within the ryanodine receptor

OBJECTIVES

We sought to investigate the effect of dantrolene, a drug generally used to treat malignant hyperthermia, on the Ca2+ release and cardiomyocyte function in failing hearts.

BACKGROUND

The N-terminal (N: 1-600) and central (C: 2000-2500) domains of the ryanodine receptor (RyR) harbor many mutations associated with malignant hyperthermia in skeletal muscle RyR (RyR1) and polymorphic ventricular tachycardia in cardiac RyR (RyR2). There is strong evidence that interdomain interaction between these regions plays an important role in the mechanism of channel regulation.

METHODS

Sarcoplasmic reticulum vesicles and cardiomyocytes were isolated from the left ventricular muscles of dogs (normal or rapid ventricular pacing for 4 weeks), for Ca2+ leak, transient, and spark assays. To assess the zipped or unzipped state of the interacting domains, the RyR was labeled fluorescently with methylcoumarin acetate in a site-directed manner. We used a quartz-crystal microbalance technique to identify the dantrolene binding site within the RyR2.

RESULTS

Dantrolene specifically bound to domain 601-620 in RyR2. In the sarcoplasmic reticulum isolated from pacing-induced failing dog hearts, the defective interdomain interaction (domain unzipping) had already occurred, causing spontaneous Ca2+ leak. Dantrolene suppressed both domain unzipping and the Ca2+ leak, demonstrating identical drug concentration-dependence (IC50 = 0.3 micromol/l). In failing cardiomyocytes, both diastolic Ca2+ sparks and delayed afterdepolarization were observed frequently, but 1 micromol/l dantrolene inhibited both events.

CONCLUSIONS

Dantrolene corrects defective interdomain interactions within RyR2 in failing hearts, inhibits spontaneous Ca2+ leak, and in turn improves cardiomyocyte function in failing hearts. Thus, dantrolene may have a potential to treat heart failure, specifically targeting the RyR2.

Role of protein phosphatase-1 inhibitor-1 in cardiac physiology and pathophysiology

The type 1 protein phosphatase (PP1) is a critical negative regulator of Ca(2+) cycling and contractility in the cardiomyocyte. In particular, it mediates restoration of cardiac function to basal levels, after beta-adrenergic stimulation, by dephosphorylating key phospho-proteins. PP1 is a holoenzyme comprised of its catalytic and auxiliary subunits. These regulatory proteins dictate PP1's subcellular localization, substrate specificity and activity. Amongst them, inhibitor-1 is of particular importance since it has been implicated as an integrator of multiple neurohormonal pathways, which finely regulate PP1 activity, at the level of the sarcoplasmic reticulum (SR). In fact, perturbations in the regulation of PP1 by inhibitor-1 have been implicated in the pathogenesis of heart failure, suggesting that inhibitor-1-based therapeutic interventions may ameliorate cardiac dysfunction and remodeling in the failing heart. This review will discuss the current views on the role of inhibitor-1 in cardiac physiology, its possible contribution to cardiac disease and its potential as a novel therapeutic strategy.

Protein phosphatase inhibitor-1 augments a protein kinase A-dependent increase in the Ca2+ loading of the sarcoplasmic reticulum without changing its Ca2+ release

BACKGROUND

An increase in cytosolic protein phosphatases (PPs) de-phosphorylates phospholamban, decreasing the Ca(2+) uptake of the sarcoplasmic reticulum (SR). The effects of PP inhibitors on cellular Ca(2+) handling were investigated.

METHODS AND RESULTS

Twitch Ca(2+) transients (CaTs) and cell shortening were measured in intact rat cardiac myocytes, and caffeine-induced Ca(2+) transients (CaffCaTs) and Ca(2+) sparks were studied in saponin-permeabilized cells. Calyculin A augmented isoproterenol-induced increases in CaTs and cell shortening without altering the diastolic [Ca(2+)](i) and twitch [Ca(2+)](i) decay. The protein kinase A catalytic subunit (PKA(cat)) increased the peak of CaffCaTs between 5 and 50 U/ml, and the addition of inhibitor-1 (I-1) augmented the increase. PKA(cat) increased Ca(2+) spark frequency and the addition of I-1 increased it further. PKA(cat) at 50 U/ml amplified the peak and prolonged the duration of Ca(2+) sparks, whereas the addition of I-1 did not alter them. An abrupt inhibition of SR Ca(2+) uptake following exposure to PKA(cat) caused a gradual decrease in Ca(2+) spark frequency, but the addition of I-1 did not accelerate the decline of Ca(2+) spark frequency or CaffCaTs.

CONCLUSIONS

Inhibition of PPs augmented the inotropic effect of isoproterenol. Specific inhibition of PP1 could stimulate the Ca(2+) uptake of the SR with less significant effects on the Ca(2+) release.

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.

Mechanical work and energetic analysis of eccentric cardiac remodeling in a volume overload heart failure in rats

Eccentric cardiac remodeling seen in dilated cardiomyopathy or regurgitant valvular disease is a well-known process of heart failure progression, but its mechanoenergetic profile has not been yet established. We made a volume overload (VO) heart failure model in rats and for the first time investigated left ventricular (LV) mechanical work and energetics in cross-circulated whole heart preparations. Laparotomy was performed in 14 Wistar male rats, and abdominal aortic-inferior vena caval shunt was created in seven rats (VO group). Another seven rats underwent a sham operation without functional shunt (Sham group). LV dimensions changes were followed with weekly transthoracic echocardiography. Three months after surgery, we measured LV pressure and volume and myocardial O(2) consumption in isolated heart cross circulation. LV internal dimensions in both systolic and diastolic phases were significantly increased in the VO group versus the Sham group (P < 0.05). LV pressure was markedly decreased in the VO group versus in the Sham group (P < 0.05). LV end-systolic pressure-volume relation shifted downward, and myocardial O(2) consumption related to Ca(2+) handling significantly decreased. The contractile response to Ca(2+) infusion was attenuated. Nevertheless, the increase in Ca(2+) handling-related O(2) consumption per unit change in LV contractility in the VO group was significantly higher than that in the Sham group (P < 0.05). The levels of sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a protein were reduced in the VO group (P < 0.01). In conclusion, VO failing rat hearts had a character of marked contractile dysfunction accompanied with less efficient energy utilization in the Ca(2+) handling processes. These results suggest that restoring Ca(2+) handling in excitation-contraction coupling would improve the contractility of the myocardium after eccentric cardiac remodeling.

Calcium handling abnormalities in atrial fibrillation as a target for innovative therapeutics

Atrial electrical and structural alterations (remodeling) have emerged as key elements in the development of the atrial fibrillation (AF) substrate. Evidence points to abnormalities in intracellular Ca (calcium) handling as crucial links in AF-initiating focal activity and in perpetuation by rapidly firing foci and reentry. This review focuses on the molecular basis of altered Ca handling in AF, with the goal of providing new insights into molecular effective antiarrhythmic therapy.

Role of ryanodine receptor as a Ca²(+) regulatory center in normal and failing hearts

Abnormal Ca²(+) cycling is important in various cardiac diseases. Evidence has accumulated that dysregulation of Ca²(+) release from the ryanodine receptor (RyR2) plays a significant role in cardiac dysfunction. Spontaneous Ca²(+) release through RyR2 during diastole decreases sarcoplasmic reticulum (SR) Ca²(+) content, and also induces delayed after depolarization (DAD) as a substrate for lethal arrhythmia. Several disease-linked mutations in the RyR have been reported in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT) or arrythmogenic right ventricular cardiomyopathy type 2 (ARVC2). The unique distribution of these mutation sites has produced the concept that the interaction among the putative regulatory domains within the RyR may play a key role in regulating the channel opening, and that there seems to be a common abnormality in the channel disorder between heart failure and CPVT/ARVC2. We review here the considerable body of evidence regarding defective channel gating of RyR2 in the pathogenesis of heart failure and lethal arrhythmia.

Defective domain-domain interactions within the ryanodine receptor as a critical cause of diastolic Ca2+ leak in failing hearts

AIMS

A domain peptide (DP) matching the Gly(2460)-Pro(2495) region of the cardiac type-2 ryanodine receptor (RyR2), DPc10, is known to mimic channel dysfunction associated with catecholaminergic polymorphic ventricular tachycardia (CPVT), owing to its interference in a normal interaction of the N-terminal (1-600) and central (2000-2500) domains (viz. domain unzipping). Using DPc10 and two other DPs harboring different mutation sites, we investigated the underlying mechanism of abnormal Ca(2+) cycling in failing hearts.

METHODS AND RESULTS

Sarcoplasmic reticulum (SR) vesicles and cardiomyocytes were isolated from dog left ventricular muscles for Ca(2+) leak and spark assays. The RyR2 moiety of the SR was fluorescently labelled with methylcoumarin acetate (MCA) using DPs corresponding to the 163-195 and 4090-4123 regions of RyR2 (DP163-195 and DP4090-4123, respectively) as site-directed carriers. Both DPs mediated a specific MCA fluorescence labelling of RyR2. Addition of either DP to the MCA-labelled SR induced domain unzipping, as evidenced by an increased accessibility of the bound MCA to a large-size fluorescence quencher. Both SR Ca(2+) leak and Ca(2+) spark frequency (SpF) were markedly increased in failing cardiomyocytes. Upon introduction of DP163-195 or DP4090-4123 into normal SR or cardiomyocytes, both Ca(2+) leak and SpF increased to the levels comparable with those of failing myocytes. K201 (JTV519) suppressed all of the effects induced by DP163-195 (domain unzipping and increased Ca(2+) leak and SpF) or those in failing cardiomyocytes, but did not suppress the effects induced by DP4090-4123.

CONCLUSION

Defective inter-domain interaction between N-terminal and central domains induces diastolic Ca(2+) leak, leading to heart failure and lethal arrhythmia. Mutation at the C-terminal region seen in CPVT does not seem to communicate with the aforementioned N-terminal and central inter-domain interaction, although spontaneous Ca(2+) leak is similarly induced.

Decreased FKBP12.6 expression and enhanced endothelin receptor signaling associated with arrhymogenesis in myocardial infarction rats

An increased propensity towards cardiac arrhythmias and aggravated heart function is observed in myocardial infarction (MI), the development of which is associated with the calcium handling system in the myocardium. It was hypothesized that the abnormal changes in the MI model may be a consequence of the abnormal expression and function of the RyR2-FKBP12.6 channel complex and that these abnormalities may be related to an over-activated endothelin (ET) system. Salvianolic acid B is expected to suppress life-threatening arrhythmias and to restore the abnormality of the RyR2-FKBP12.6 complex in rats. MI was produced by ligating the coronary artery for 4 weeks. Salvianolic acid B (100 mg/kg/day, p.o. for 4 weeks) was administered to rats 0.5 h before surgery. Measurements of cardiac arrhythmias, cardiac function, calcium transient, cardiac calcium release channel handling proteins and the endothelin system were conducted. The aggravated arrhythmia and compromised cardiac function in MI rats was accompanied by elevated diastolic Ca(2+) levels in the cytosol and a significant down-regulation of expression of RyR2-FKBP12.6. These were closely linked with an over-activated ET pathway in the myocardium. After a 4-week treatment with salvianolic acid B, all abnormalities were reversed significantly. Salvianolic acid B was capable of normalizing FKBP12.6 expression levels and decreasing the propensity towards arrhythmias by attenuating the up-regulated ET pathway.

Physiological consequences of the P2328S mutation in the ryanodine receptor (RyR2) gene in genetically modified murine hearts

Aim

To explore the physiological consequences of the ryanodine receptor (RyR2)-P2328S mutation associated with catecholaminergic polymorphic ventricular tachycardia (CPVT).

Methods

We generated heterozygotic (RyR2^p/s) and homozygotic (RyR2^s/s) transgenic mice and studied Ca²⁺ signals from regularly stimulated, Fluo-3-loaded, cardiac myocytes. Results were compared with monophasic action potentials (MAPs) in Langendorff-perfused hearts under both regular and programmed electrical stimulation (PES).

Results

Evoked Ca²⁺ transients from wild-type (WT), heterozygote (RyR2^p/s) and homozygote (RyR2^s/s) myocytes had indistinguishable peak amplitudes with RyR2^s/s showing subsidiary events. Adding 100 nm isoproterenol produced both ectopic peaks and subsidiary events in WT but not RyR2^p/s and ectopic peaks and reduced amplitudes of evoked peaks in RyR2^s/s. Regularly stimulated WT, RyR2^p/s and RyR2^s/s hearts showed indistinguishable MAP durations and refractory periods. RyR2^p/s hearts showed non-sustained ventricular tachycardias (nsVTs) only with PES. Both nsVTs and sustained VTs (sVTs) occurred with regular stimuli and PES with isoproterenol treatment. RyR2^s/s hearts showed higher incidences of nsVTs before but mainly sVTs after introduction of isoproterenol with both regular stimuli and PES, particularly at higher pacing frequencies. Additionally, intrinsically beating RyR2^s/s showed extrasystolic events often followed by spontaneous sVT.

Conclusion

The RyR2-P2328S mutation results in marked alterations in cellular Ca²⁺ homeostasis and arrhythmogenic properties resembling CPVT with greater effects in the homozygote than the heterozygote demonstrating an important gene dosage effect.

Defective Ca2+ cycling as a key pathogenic mechanism of heart failure

Structural and functional alterations in the Ca(2+) regulatory proteins present in the sarcoplasmic reticulum (SR) have recently been shown to play a crucial role in the pathogenesis of heart failure (HF), and lethal arrhythmia as well. Chronic activation of the sympathetic nervous system induces abnormalities in both the function and structure of these proteins. For instance, the diastolic Ca(2+) leak through the SR Ca(2+) release channel (ryanodine receptor) reduces the SR Ca(2+) content, inducing contractile dysfunction. Moreover, the Ca(2+) leak provides a substrate for delayed after depolarization that leads to lethal arrhythmia. There is a considerable body of evidence regarding the role of Ca(2+) cycling abnormality in HF.

Downregulated FKBP12.6 expression and upregulated endothelin signaling contribute to elevated diastolic calcium and arrhythmogenesis in rat cardiomyopathy produced by l-thyroxin

BACKGROUND

Dissociation of FKBP12.6 from RyR2 is considered as an important molecular event resulting in calcium leak and an increased risk in arrhythmogenesis. We hypothesized that augmented ventricular fibrillation (VF) on reperfusion of rat cardiomyopathy induced by l-thyroxin may result from elevated diastolic Ca(2+) levels due to dissociation (downregulation) of FKBP12.6 and upregulation of endothelin (ET-1) signaling pathway.

METHODS

Rats were treated with l-thyroxin (0.4 mg/kg, s.c.) for 10 days. Dajisentan (CPU0213), a dual endothelin receptor antagonist (100 mg/kg p.o.), or propranolol was administered on day 6 to 10. Susceptibility to VF was evaluated on ischemia/reperfusion episode. mRNA expression of FKBP12.6, and ET-1 levels were determined. Calcium transients and FKBP12.6 immunohistochemistry were measured by confocal microscopy in isolated cardiomyocytes from cardiomyopathy.

RESULTS

Cardiomyopathy induced by l-thyroxin resulted in an increased susceptibility to VF on ischemia/reperfusion. Upregulated mRNA expression of RyR2 and PKA in association with downregulated FKBP12.6 expression was found in l-thyroxin-treated rats compared to controls. Calcium transients evoked by field electrical stimulation showed an increase in Ca(2+) by +75% during diastole. An increase in ET-1 (ng/mg protein) (+36.6%) and mRNA abundance of preproET-1 were found in the left ventricle. A decreased mRNA ratio of FKBP12.6 to RyR2 likely reflected dissociation of FKBP12.6 in cardiomyopathy. These changes were normalized by Dajisentan, comparable to propranolol.

CONCLUSION

Increased susceptibility to VF in l-thyroxin-induced cardiomyopathy is related to increase in diastolic Ca(2+) levels, resulting from downregulated FKBP12.6 and upregulated ET system. ET antagonism might be useful in settings of FKBP12.6 dissociation.

Ryanodine receptor as a new therapeutic target of heart failure and lethal arrhythmia

Abnormal intracellular Ca(2+) handling by the sarcoplasmic reticulum (SR) is a critical factor in the development of heart failure (HF). Not only decreased Ca(2+) uptake, but also uncoordinated Ca(2+) release plays a significant role in contractile and relaxation dysfunction. Spontaneous Ca(2+) release through ryanodine receptor (RyR) 2, a huge tetrameric protein, during diastole leads to a decrease in the SR Ca(2+) content, and also triggers delayed after depolarization that is a substrate for lethal arrhythmia. Several disease-linked mutations of RyR have been reported in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT) or arrhythmogenic right ventricular cardiomyopathy type 2 (ARVC2). The unique distribution of these mutation sites has lead to the concept that an interaction among the putative regulatory domains within RyR may play a key role in regulating channel opening, and that there seems to be a common abnormality in the channel disorder of HF and CPVT/ARVC2. Recent knowledge gained from pathological conditions may lead to the development of a new therapeutic strategy for the treatment of HF or cardiac arrhythmia.

The cardiac sarcoplasmic/endoplasmic reticulum calcium ATPase: a potent target for cardiovascular diseases

The cardiac isoform of the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA2a) is a calcium ion (Ca(2+)) pump powered by ATP hydrolysis. SERCA2a transfers Ca(2+) from the cytosol of the cardiomyocyte to the lumen of the sarcoplasmic reticulum during muscle relaxation. As such, this transporter has a key role in cardiomyocyte Ca(2+) regulation. In both experimental models and human heart failure, SERCA2a expression is significantly decreased, which leads to abnormal Ca(2+) handling and a deficient contractile state. Following a long line of investigations in isolated cardiac myocytes and small and large animal models, a clinical trial is underway that is restoring SERCA2a expression in patients with heart failure by use of adeno-associated virus type 1. Beyond its role in contractile abnormalities in heart failure, SERCA2a overexpression has beneficial effects in a host of other cardiovascular diseases. Here we describe the mechanism of Ca(2+) regulation by SERCA2a, examine the beneficial effects as well as the failures, risks and complexities associated with SERCA2a overexpression, and discuss the potential of SERCA2a as a target for the treatment of cardiovascular disease.

Arrhythmogenic right ventricular cardiomyopathy in Boxer dogs is associated with calstabin2 deficiency

OBJECTIVE

To examine the presence and effect of calstabin2-deficiency in Boxer dogs with arrhythmogenic right ventricular cardiomyopathy (ARVC).

ANIMALS

Thirteen Boxer dogs with ARVC.

MATERIALS AND METHODS

Tissue samples were collected for histopathology, oligonucleotide microarray, PCR, immunoelectrophoresis, ryanodine channel immunoprecipitation and single-channel recordings, and calstabin2 DNA sequencing.

RESULTS

In cardiomyopathic Boxer dogs, myocardial calstabin2 mRNA and protein were significantly decreased as compared to healthy control dogs (calstabin2 protein normalized to tetrameric cardiac ryanodine receptor (RyR2) complex: affected, 0.51+/-0.04; control, 3.81+/-0.22; P<0.0001). Calstabin2 deficiency in diseased dog hearts was associated with a significantly increased open probability of single RyR2 channels indicating intracellular Ca(2+) leak. PCR-based sequencing of the promoter, exonic and splice site regions of the canine calstabin2 gene did not identify any causative mutations.

CONCLUSIONS

Calstabin2 deficiency is a potential mechanism of Ca(2+) leak-induced ventricular arrhythmias and heart disease in Boxer dogs with ARVC.

Toward biologically targeted therapy of calcium cycling defects in heart failure

A growing body of evidence indicates that heart failure progression is tightly associated with dysregulation of phosphorylation of Ca2+ regulators localized in the sub-cellular microdomain of the sarcoplasmic reticulum. Chemical or genetic correction of abnormalities in cardiac phosphorylation cascades is emerging as a potential target in the treatment of heart failure. Here, we review how specific kinases and phosphatases finely tune Ca2+ cycling and regulate excitation-contraction (E-C) coupling in cardiomyocytes.

Adrenergic regulation of cardiac contractility does not involve phosphorylation of the cardiac ryanodine receptor at serine 2808

The sympathetic nervous system is a critical regulator of cardiac function (heart rate and contractility) in health and disease. Sympathetic nervous system agonists bind to adrenergic receptors that are known to activate protein kinase A, which phosphorylates target proteins and enhances cardiac performance. Recently, it has been proposed that protein kinase A-mediated phosphorylation of the cardiac ryanodine receptor (the Ca(2+) release channel of the sarcoplasmic reticulum at a single residue, Ser2808) is a critical component of sympathetic nervous system regulation of cardiac function. This is a highly controversial hypothesis that has not been confirmed by several independent laboratories. The present study used a genetically modified mouse in which Ser2808 was replaced by alanine (S2808A) to prevent phosphorylation at this site. The effects of isoproterenol (a sympathetic agonist) on ventricular performance were compared in wild-type and S2808A hearts, both in vivo and in isolated hearts. Isoproterenol effects on L-type Ca(2+) current (I(CaL)), sarcoplasmic reticulum Ca(2+) release, and excitation-contraction coupling gain were also measured. Our results showed that isoproterenol caused significant increases in cardiac function, both in vivo and in isolated hearts, and there were no differences in these contractile effects in wild-type and S2808A hearts. Isoproterenol increased I(CaL), the amplitude of the Ca(2+) transient and excitation-contraction coupling gain, but, again, there were no significant differences between wild-type and S2808A myocytes. These results show that protein kinase A phosphorylation of ryanodine receptor Ser2808 does not have a major role in sympathetic nervous system regulation of normal cardiac function.

Effects of FK506 on ca release channels (review)

Tacrolimus (FK506), which was isolated from the fermentation broth of Streptomyces tsukubaensis No. 9993, has an immunosuppressive effect. In T-lymphocytes, FK506 binds to the intracellular receptor, a 12-kDa FK506-binding protein (FKBP12). The FK506-FKBP12 complex binds to the phosphatase calcineurin (CN) and inhibits the activity of CN. By inhibition of the activity of CN, dephosphorylation of a nuclear factor of activated T-cells (NFAT) is inhibited, and translocation of the NFAT to the nucleus is suppressed. Thereby, the production of T-cell-derived mediators such as interleukin 2 (IL-2) is inhibited, and the proliferation of cytotoxic T-cells is suppressed. In muscle cells, FKBP12 and FKBP12.6 are associated with ryanodine-sensitive Ca²⁺ release channels (ryanodine receptors: RyRs) on the skeletal and cardiac muscle sarcoplasmic reticulum (SR), respectively. FK506 modulates the RyR by dissociating FKBP12 or FKBP12.6 from the RyR complex. FKBP12 is also associated with inositol 1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ release channels (IP₃ receptors: IP₃Rs) on the endoplasmic reticulum (ER) of non-muscle cells. The IP₃R-FKBP12 complex binds to CN, which dephosphorylates the protein kinase C (PKC) phosphorylation site on the receptor. When FKBP12 is dissociated from the IP₃R complex by FK506, CN is also dissociated from the IP₃R. Thereby, the IP₃R is phosphorylated by PKC, and the receptor is modulated. Recently, it was found that FK506 itself induces Ca²⁺ release through RyRs in some tissues.

Cellular mechanisms of arrhythmogenic cardiac alternans

Despite the strong association between mechanical dysfunction of the heart and sudden death due to arrhythmias, the causal relationship is not well understood. Cardiac alternans has been linked to arrhythmogenesis and can be mediated by intracellular calcium handling. Given the integral role intracellular calcium plays in contractile function, calcium-mediated alternans may represent an important mechanistic link between mechanical dysfunction and electrical instability. This relationship, however, is not well understood due to complex feedback between membrane currents, intracellular calcium, and contraction. This manuscript describes the cellular mechanisms of cardiac alternans. Through several pathways, calcium transient alternans is coupled to repolarization alternans that can form a substrate for reentrant excitation. Abnormal intracellular calcium cycling, either impaired release or impaired reuptake of sarcoplasmic reticulum calcium, is a cellular mechanism of calcium transient alternans. Thus, cardiac alternans is an important mechanistic link between mechanical dysfunction and sudden cardiac death.

Sarcoplasmic reticulum calcium leak and cardiac arrhythmias

Ventricular arrhythmias deteriorating into sudden cardiac death are a major cause of mortality worldwide. The recent linkage of a genetic form of cardiac arrhythmia to mutations in the gene encoding RyR2 (ryanodine receptor 2) has uncovered an important role of this SR (sarcoplasmic reticulum) calcium release channel in triggering arrhythmias. Mutant RyR2 channels give rise to spontaneous release of calcium (Ca(2+)) from the SR during diastole, which enhances the probability of ventricular arrhythmias. Several molecular mechanisms have been proposed to explain the gain-of-function phenotype observed in mutant RyR2 channels. Despite considerable differences between the models discussed in the present review, each predicts spontaneous diastolic Ca(2+) leak from the SR due to incomplete closure of the RyR2 channel. Enhanced SR Ca(2+) leak is also observed in common structural diseases of the heart, such as heart failure. In heart failure, defective channel regulation in the absence of inherited mutations may also increase SR Ca(2+) leak and initiate cardiac arrhythmias. Therefore inhibition of diastolic Ca(2+) leak through SR Ca(2+) release channels has emerged as a new and promising therapeutic target for cardiac arrhythmias.

Targeting altered calcium physiology in the heart: translational approaches to excitation, contraction, and transcription

Calcium (Ca) is essential for excitation-contraction coupling. At the same time, Ca is of pivotal importance as a second messenger in cardiac signal transduction, where it regulates cardiac growth and function by activation of kinases and phosphatases, ultimately driving transcriptional responses and feeding back on Ca handling proteins, a phenomenon termed excitation-transcription coupling. Cardiac Ca homeostasis thus needs to be maintained via a delicate interplay of proteins to allow physiological function and adaptation, whereas disturbed Ca-handling and Ca-dependent signaling are hallmarks of heart failure. In this review, we will discuss the most recent mechanistic findings in Ca-handling and Ca-signaling proteins in the development of cardiac pathology with a focus on translational aspects.

Fibrinogen decreases cardiomyocyte contractility through an ICAM-1-dependent mechanism

Introduction

Cardiomyocytes exposed to inflammatory processes express intracellular adhesion molecule-1 (ICAM-1). We investigated whether fibrinogen and fibrinogen degradation products, including D-dimer, could alter cardiomyocyte contractile function through interaction with ICAM-1 found on inflamed cardiomyocytes.

Methods

In vivo, rats were injected with endotoxin to model systemic inflammation, whereas isolated rat cardiomyocytes were treated with tumor necrosis factor-alpha to model the inflammatory environment seen following exposure to bacterial products such as lipopolysaccharide.

Results

In vivo, endotoxin administration profoundly decreased cardiac contractile function associated with a large increase in intracardiac ICAM-1 and perivascular fibrinogen. Confocal microscopy with double-staining of isolated rat cardiomyocytes demonstrated colocalization of ICAM-1 and fibrinogen. This interaction was disrupted through pre-treatment of the cells with an ICAM-1-blocking antibody. Functionally, isolated rat cardiomyocyte preparations exhibited decreased fractional shortening when incubated with fibrinogen, and through the use of synthetic peptides, we determined that residues 117–133 of the fibrinogen gamma chain are responsible for this interaction with ICAM-1. Despite having crosslinked gamma chains, D-dimer retained the ability to decrease cardiomyocyte contractility.

Conclusion

Site 117–133 of the fibrinogen gamma chain is able to depress cardiomyocyte contractility through binding ICAM-1.

Mechanisms and potential therapeutic targets for ventricular arrhythmias associated with impaired cardiac calcium cycling

The close relationship between life-threatening ventricular arrhythmias and contractile dysfunction in the heart implicates intracellular calcium cycling as an important underlying mechanism of arrhythmogenesis. Despite this close association, however, the mechanisms of arrhythmogenesis attributable to impaired calcium cycling are not fully appreciated or understood. In this report we review some of the current thinking regarding arrhythmia mechanisms associated with either abnormal impulse initiation (i.e. arrhythmia triggers) or impulse propagation (i.e. arrhythmia substrates). In all cases, the mechanisms are primarily related to dysfunction of calcium regulatory proteins associated with the sarcomere. These findings highlight the broad scope of arrhythmias associated with abnormal calcium cycling, and provide a basis for a causal relationship between cardiac electrical instability and contractile dysfunction. Moreover, calcium cycling proteins may provide much needed targets for novel antiarrhythmic therapies.

CaMKIIdelta isoforms differentially affect calcium handling but similarly regulate HDAC/MEF2 transcriptional responses

The delta(B) and delta(C) splice variants of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), which differ by the presence of a nuclear localization sequence, are both expressed in cardiomyocytes. We used transgenic (TG) mice and CaMKII expression in cardiomyocytes to test the hypothesis that the CaMKIIdelta(C) isoform regulates cytosolic Ca(2+) handling and the delta(B) isoform, which localizes to the nucleus, regulates gene transcription. Phosphorylation of CaMKII sites on the ryanodine receptor (RyR) and on phospholamban (PLB) were increased in CaMKIIdelta(C) TG. This was associated with markedly enhanced sarcoplasmic reticulum (SR) Ca(2+) spark frequency and decreased SR Ca(2+) content in cardiomyocytes. None of these parameters were altered in TG mice expressing the nuclear-targeted CaMKIIdelta(B). In contrast, cardiac expression of either CaMKIIdelta(B) or delta(C) induced transactivation of myocyte enhancer factor 2 (MEF2) gene expression and up-regulated hypertrophic marker genes. Studies using rat ventricular cardiomyocytes confirmed that CaMKIIdelta(B) and delta(C) both regulate MEF2-luciferase gene expression, increase histone deacetylase 4 (HDAC4) association with 14-3-3, and induce HDAC4 translocation from nucleus to cytoplasm, indicating that either isoform can stimulate HDAC4 phosphorylation. Finally, HDAC4 kinase activity was shown to be increased in cardiac homogenates from either CaMKIIdelta(B) or delta(C) TG mice. Thus CaMKIIdelta isoforms have similar effects on hypertrophic gene expression but disparate effects on Ca(2+) handling, suggesting distinct roles for CaMKIIdelta isoform activation in the pathogenesis of cardiac hypertrophy versus heart failure.

Removal of FKBP12.6 does not alter the conductance and activation of the cardiac ryanodine receptor or the susceptibility to stress-induced ventricular arrhythmias

The 12.6-kDa FK506-binding protein (FKBP12.6) is considered to be a key regulator of the cardiac ryanodine receptor (RyR2), but its precise role in RyR2 function is complex and controversial. In the present study we investigated the impact of FKBP12.6 removal on the properties of the RyR2 channel and the propensity for spontaneous Ca(2+) release and the occurrence of ventricular arrhythmias. Single channel recordings in lipid bilayers showed that FK506 treatment of recombinant RyR2 co-expressed with or without FKBP12.6 or native canine RyR2 did not induce long-lived subconductance states. [(3)H]Ryanodine binding studies revealed that coexpression with or without FKBP12.6 or treatment with or without FK506 did not alter the sensitivity of RyR2 to activation by Ca(2+) or caffeine. Furthermore, single cell Ca(2+) imaging analyses demonstrated that HEK293 cells co-expressing RyR2 and FKBP12.6 or expressing RyR2 alone displayed the same propensity for spontaneous Ca(2+) release or store overload-induced Ca(2+) release (SOICR). FK506 increased the amplitude and decreased the frequency of SOICR in HEK293 cells expressing RyR2 with or without FKBP12.6, indicating that the action of FK506 on SOICR is independent of FKBP12.6. As with recombinant RyR2, the conductance and ligand-gating properties of single RyR2 channels from FKBP12.6-null mice were indistinguishable from those of single wild type channels. Moreover, FKBP12.6-null mice did not exhibit enhanced susceptibility to stress-induced ventricular arrhythmias, in contrast to previous reports. Collectively, our results demonstrate that the loss of FKBP12.6 has no significant effect on the conduction and activation of RyR2 or the propensity for spontaneous Ca(2+) release and stress-induced ventricular arrhythmias.

Calcium and cardiomyopathies

Regulation of Calcium (Ca) cycling by the sarcoplasmic reticulum (SR) underlies the control of cardiac contraction during excitation-contraction (E-C) coupling. Moreover, alterations in E-C coupling occurring in cardiac hypertrophy and heart failure are characterized by abnormal Ca-cycling through the SR network. A large body of evidence points to the central role of: a) SERCA and its regulator phospholamban (PLN) in the modulation of cardiac relaxation; b) calsequestrin in the regulation of SR Ca-load; and c) the ryanodine receptor (RyR) Ca-channel in the control of SR Ca-release. The levels or activity of these key Ca-handling proteins are altered in cardiomyopathies, and these changes have been linked to the deteriorated cardiac function and remodeling. Furthermore, genetic variants in these SR Ca-cycling proteins have been identified, which may predispose to heart failure or fatal arrhythmias. This chapter concentrates on the pivotal role of SR Ca-cycling proteins in health and disease with specific emphasis on their recently reported genetic modifiers.

Ryanodine receptors as pharmacological targets for heart disease

Calcium release from intracellular stores plays an important role in the regulation of muscle contraction and electrical signals that determine the heart rhythm. The ryanodine receptor (RyR) is the major calcium (Ca2+) release channel required for excitation-contraction coupling in the heart. Recent studies have demonstrated that RyR are macromolecular complexes comprising of 4 pore-forming channel subunits, each of which is associated with regulatory subunits. Clinical and experimental studies over the past 5 years have provided compelling evidence that intracellular Ca2+ release channels play a pivotal role in the development of cardiac arrhythmias and heart failure. Changes in the channel regulation and subunit composition are believed to cause diastolic calcium leakage from the sarcoplasmic reticulum, which could trigger arrhythmias and weaken cardiac contractility. Therefore, cardiac RyR have emerged as potential therapeutic targets for the treatment of heart disease. Consequently, there is a strong desire to identify and/or develop novel pharmacological agents that may target these Ca2+ signaling pathways. Pharmacological agents known to modulate RyR in the heart, and their potential application towards the treatment of heart disease are discussed in this review.

Modulation of the Ryanodine Receptor and Intracellular Calcium

Ryanodine receptors (RyRs)/Ca2+ release channels, on the endoplasmic and sarcoplasmic reticulum of most cell types, are required for intracellular Ca2+ release involved in diverse cellular functions, including muscle contraction and neurotransmitter release. The large cytoplasmic domain of the RyR serves as a scaffold for proteins that bind to and modulate the channel's function and that comprise a macromolecular signaling complex. These proteins include calstabins [FK506-binding proteins (FKBPs)], calmodulin (CaM), phosphodiesterase, kinases, phosphatases, and their cognate targeting proteins. This review focuses on recent progress in the understanding of RyR regulation and disease mechanisms that are associated with channel dysfunction.

Cellular alternans: a mechanism linking calcium cycling proteins to cardiac arrhythmogenesis

Essentially all previous research on alternans has been restricted to normal myocardium, whereas sudden cardiac death (SCD) occurs most commonly in patients with ventricular dysfunction (i.e., heart failure), which is associated with marked disruption of proteins responsible for normal calcium cycling in myocytes. Several lines of evidence from studies in normal hearts suggest a link between impaired calcium cycling which characterizes ventricular mechanical dysfunction and impaired calcium cycling that is responsible for alternans. In normal myocardium, cells which exhibit the slowest calcium cycling, and not the slowest repolarization, are most susceptible to alternans. Decreased expression of key calcium cycling proteins is observed in alternans-prone cells. Sarcoplasmic reticulum ATPase (SERCA2a) expression is decreased, suggesting a mechanism for the slower sarcoplasmic reticulum (SR) calcium reuptake observed in alternans-prone cells. In addition, diminished ryanodine receptor (RyR) function leading to abnormal calcium release from the SR is also linked to cellular alternans. Although impaired contractile function clearly predisposes to SCD, the mechanisms linking mechanical to electrophysiological dysfunction in the heart are unclear. We propose that cellular calcium alternans may be an important mechanism linking mechanical dysfunction to cardiac arrhythmogenesis.

Scavenging free radicals by low-dose carvedilol prevents redox-dependent Ca2+ leak via stabilization of ryanodine receptor in heart failure

OBJECTIVES

We investigated whether defective intracellular Ca2+ handling is corrected by carvedilol in heart failure.

BACKGROUND

In heart failure, the interaction between the N-terminal and central domains of the ryanodine receptor (RyR), the domains where many mutations have been found in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), is defective, as shown in our recent report.

METHODS

Sarcoplasmic reticulum vesicles were isolated from canine left ventricular muscle (normal or 4-weeks rapid ventricular pacing). The RyR was labeled with the fluorescent conformational probe methylcoumarin acetate (MCA) with DPc10 (a synthetic peptide corresponding to Gly2460-Pro2495 of RyR, one of the mutable domains in CPVT) as a site-direction carrier.

RESULTS

Normal cardiac function was well preserved in carvedilol-treated/paced dogs (CV+) but not in the untreated/paced dogs (CV-). In CV-, the interdomain interaction within RyR was defective (i.e., in an unzipped state), as determined by the fluorescence quenching technique. However, in CV+, the domain interaction remained normal (i.e., in a zipped state). In CV-, oxidative stress of RyR (reduction in the number of free thiols) was severe, but it was negligible in CV+. In (CV-) failing cardiomyocytes, incubation with low-dose CV (30 nmol/l), which eliminated intracellular reactive oxygen species with no acute effect on cell shortening, markedly improved the contractile function and Ca2+ transient. However, after domain unzipping by DPc10, CV was without effect.

CONCLUSIONS

Carvedilol, at a concentration that is sufficient to produce antioxidant effect, improves the intracellular Ca2+ handling and contractile dysfunction by correcting defective interdomain interaction within the RyR in the failing heart.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Arrhythmogenic Ion-Channel Remodeling in the Heart: Heart Failure, Myocardial Infarction, and Atrial Fibrillation

Rhythmic and effective cardiac contraction depends on appropriately timed generation and spread of cardiac electrical activity. The basic cellular unit of such activity is the action potential, which is shaped by specialized proteins (channels and transporters) that control the movement of ions across cardiac cell membranes in a highly regulated fashion. Cardiac disease modifies the operation of ion channels and transporters in a way that promotes the occurrence of cardiac rhythm disturbances, a process called “arrhythmogenic remodeling.” Arrhythmogenic remodeling involves alterations in ion channel and transporter expression, regulation and association with important protein partners, and has important pathophysiological implications that contribute in major ways to cardiac morbidity and mortality. We review the changes in ion channel and transporter properties associated with three important clinical and experimental paradigms: congestive heart failure, myocardial infarction, and atrial fibrillation. We pay particular attention to K+, Na+, and Ca2+channels; Ca2+transporters; connexins; and hyperpolarization-activated nonselective cation channels and discuss the mechanisms through which changes in ion handling processes lead to cardiac arrhythmias. We highlight areas of future investigation, as well as important opportunities for improved therapeutic approaches that are being opened by an improved understanding of the mechanisms of arrhythmogenic remodeling.

Agonists and antagonists of the cardiac ryanodine receptor: Potential therapeutic agents?

This review addresses the potential use of the intracellular ryanodine receptor (RyR) Ca(2+) release channel as a therapeutic target in heart disease. Heart disease encompasses a wide range of conditions with the major contributors to mortality and morbidity being ischaemic heart disease and heart failure (HF). In addition there are many rare, but devastating conditions, some of which are either genetically linked to the RyR and its regulatory proteins or involve drug-induced modification of the proteins. The defects in Ca(2+) signalling vary with the nature of the heart disease and the stage in its progress and therefore specific corrections require different modifications of Ca(2+) signalling. Compounds that activate the RyR are potential inotropic agents to increase the Ca(2+) transient and strength of contraction. Compounds that reduce RyR activity are potentially useful in conditions where excess RyR activity initiates arrhythmias, or depletes the Ca(2+) store, as in end stage HF. It has recently been discovered that the cardio-protective action of the drug JTV519 can be attributed partly to its ability to stabilise the interaction between the RyR and the 12.6 kDa binding protein for the commonly used immunosuppressive drug FK506 (FKBP12.6, known as tacrolimus). This has established the credibility of the RyR as a therapeutic target. We explore the possibility that mutations causing the rare RyR-linked arrhythmias will open the door to identification of novel RyR-based therapeutic agents. The use of regulatory binding sites within the RyR complex or on its associated proteins as templates for drug design is discussed.

Restoration of mechanical and energetic function in failing aortic-banded rat hearts by gene transfer of calcium cycling proteins

The aim of this study was to examine whether short- and long-term gene transfer of Ca(2+) handling proteins restore left ventricular (LV) mechanoenergetics in aortic banding-induced failing hearts. Aortic-banded rats received recombinant adenoviruses carrying sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) (Banding+SERCA), parvalbumin (Banding+Parv) or beta-galactosidase (Banding+betagal), or an adeno-associated virus carrying SERCA2a (Banding+AAV.SERCA) by a catheter-based technique. LV mechanoenergetic function was measured in cross-circulated hearts. "Banding", "Banding+betagal" and "Banding+saline" groups showed lower end-systolic pressure at 0.1 ml intraballoon water (ESP(0.1)), higher end-diastolic pressure at 0.1 ml intraballoon water (EDP(0.1)) and slower LV relaxation rate, compared with "Normal" and "Sham". However, "Banding+SERCA" and "Banding+Parv" showed high ESP(0.1), low EDP(0.1) and fast LV relaxation rate. In "Banding", "Banding+betagal" and "Banding+saline", slope of relation between cardiac oxygen consumption and systolic pressure-volume area, O(2) cost of total mechanical energy, was twice higher than normal value, whereas slope in "Baning+SERCA" and "Banding+Parv" was similar to normal value. Furthermore, O(2) cost of LV contractility in the 3 control banding groups was approximately 3 times higher than normal value, whereas O(2) cost of contractility in "Banding+SERCA", "Banding+AAV.SERCA" and "Banding+Parv" was as low as normal value. Thus, high O(2) costs of total mechanical energy and of LV contractility in failing hearts indicate energy wasting both in chemomechanical energy transduction and in calcium handling. Improved calcium handling by both short- and long-term overexpression of SERCA2a and parvalbumin transforms the inefficient energy utilization into a more efficient state. Therefore enhancement of calcium handling either by resequestration into the SR or by intracellular buffering improves not only mechanical but energetic function in failing hearts.

Ryanodine receptor dysfunction and triggered activity in the heart

Arrhythmogenesis has been increasingly linked to cardiac ryanodine receptor (RyR) dysfunction. However, the mechanistic relationship between abnormal RyR function and arrhythmogenesis in the heart is not clear. We hypothesize that, under abnormal RyR conditions, triggered activity will be caused by spontaneous calcium release (SCR) events that depend on transmural heterogeneities of calcium handling. We performed high-resolution optical mapping of intracellular calcium and transmembrane potential in the canine left ventricular wedge preparation (n = 28). Rapid pacing was used to initiate triggered activity under normal and abnormal RyR conditions induced by FKBP12.6 dissociation and beta-adrenergic stimulation (20-150 microM rapamycin, 0.2 microM isoproterenol). Under abnormal RyR conditions, almost all preparations experienced SCRs and triggered activity, in contrast to control, rapamycin, or isoproterenol conditions alone. Furthermore, under abnormal RyR conditions, complex arrhythmias (monomorphic and polymorphic tachycardia) were commonly observed. After washout of rapamycin and isoproterenol, no triggered activity was observed. Surprisingly, triggered activity and SCRs occurred preferentially near the epicardium but not the endocardium (P < 0.01). Interestingly, the occurrence of triggered activity and SCR events could not be explained by cytoplasmic calcium levels, but rather by fast calcium reuptake kinetics. These data suggest that, under abnormal RyR conditions, triggered activity is caused by multiple SCR events that depend on the faster calcium reuptake kinetics near the epicardium. Furthermore, multiple regions of SCR may be a mechanism for multifocal arrhythmias associated with RyR dysfunction.

Calcium signaling phenomena in heart diseases: a perspective

Ca(2+) is a major intracellular messenger and nature has evolved multiple mechanisms to regulate free intracellular (Ca(2+))(i) level in situ. The Ca(2+) signal inducing contraction in cardiac muscle originates from two sources. Ca(2+) enters the cell through voltage dependent Ca(2+) channels. This Ca(2+) binds to and activates Ca(2+) release channels (ryanodine receptors) of the sarcoplasmic reticulum (SR) through a Ca(2+) induced Ca(2+) release (CICR) process. Entry of Ca(2+) with each contraction requires an equal amount of Ca(2+) extrusion within a single heartbeat to maintain Ca(2+) homeostasis and to ensure relaxation. Cardiac Ca(2+) extrusion mechanisms are mainly contributed by Na(+)/Ca(2+) exchanger and ATP dependent Ca(2+) pump (Ca(2+)-ATPase). These transport systems are important determinants of (Ca(2+))(i) level and cardiac contractility. Altered intracellular Ca(2+) handling importantly contributes to impaired contractility in heart failure. Chronic hyperactivity of the beta-adrenergic signaling pathway results in PKA-hyperphosphorylation of the cardiac RyR/intracellular Ca(2+) release channels. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, protein kinase C, Gq, and the down stream effectors such as mitogen activated protein kinases pathways, and the Ca(2+) regulated phosphatase calcineurin. A number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocytes. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underlie heart diseases. Recent progress in molecular cardiology makes it possible to envision a new therapeutic approach to heart failure (HF), targeting key molecules involved in intracellular Ca(2+) handling such as RyR, SERCA2a, and PLN. Controlling these molecular functions by different agents have been found to be beneficial in some experimental conditions.

Ryanodine receptors and ventricular arrhythmias: emerging trends in mutations, mechanisms and therapies

It has been six years since the first reported link between mutations in the cardiac ryanodine receptor Ca(2+) release channel (RyR2) and catecholaminergic polymorphic ventricular tachycardia (CPVT), a malignant stress-induced arrhythmia. In this time, rapid advances have been made in identifying new mutations, and in understanding how these mutations disrupt normal channel function to cause VT that frequently degenerates into ventricular fibrillation (VF) and sudden death. Functional characterisation of these RyR2 Ca(2+) channelopathies suggests that mutations alter the ability of RyR2 to sense its intracellular environment, and that channel modulation via covalent modification, Ca(2+)- and Mg(2+)-dependent regulation and structural feedback mechanisms are catastrophically disturbed. This review reconciles the current status of RyR2 mutation-linked etiopathology, the significance of mutational clustering within the RyR2 polypeptide and the mechanisms underlying channel dysfunction. We will also review new data that explores the link between abnormal Ca(2+) release and the resultant cardiac electrical instability in VT and VF, and how these recent developments impact on novel anti-arrhythmic therapies. Finally, we evaluate the concept that mechanistic differences between CPVT and other arrhythmogenic disorders may preclude a common therapeutic strategy to normalise RyR2 function in cardiac disease.