Background-Approximately half of patients with heart failure die suddenly as a result of ventricular arrhythmias.Although abnormal Ca 2ϩ release from the sarcoplasmic reticulum through ryanodine receptors (RyR2) has been linked to arrhythmogenesis, the molecular mechanisms triggering release of arrhythmogenic Ca 2ϩ remain unknown. We tested the hypothesis that increased RyR2 phosphorylation by Ca 2ϩ /calmodulin-dependent protein kinase II is both necessary and sufficient to promote lethal ventricular arrhythmias. Methods and Results-Mice in which the S2814 Ca 2ϩ /calmodulin-dependent protein kinase II site on RyR2 is constitutively activated (S2814D) develop pathological sarcoplasmic reticulum Ca 2ϩ release events, resulting in reduced sarcoplasmic reticulum Ca 2ϩ load on confocal microscopy. These Ca 2ϩ release events are associated with increased RyR2 open probability in lipid bilayer preparations. At baseline, young S2814D mice have structurally and functionally normal hearts without arrhythmias; however, they develop sustained ventricular tachycardia and sudden cardiac death on catecholaminergic provocation by caffeine/epinephrine or programmed electric stimulation. Young S2814D mice have a significant predisposition to sudden arrhythmogenic death after transverse aortic constriction surgery. Finally, genetic ablation of the Ca 2ϩ /calmodulin-dependent protein kinase II site on RyR2 (S2814A) protects mutant mice from pacing-induced arrhythmias versus wild-type mice after transverse aortic constriction surgery. Conclusions-Our results suggest that Ca 2ϩ /calmodulin-dependent protein kinase II phosphorylation of RyR2 Ca 2ϩ release channels at S2814 plays an important role in arrhythmogenesis and sudden cardiac death in mice with heart failure. (Circulation. 2010;122:2669-2679.)Key Words: arrhythmia Ⅲ calcium Ⅲ calcium-calmodulin-dependent protein kinase type 2 Ⅲ heart failure Ⅲ ryanodine receptor calcium release channel Ⅲ sarcoplasmic reticulum C ongestive heart failure (HF) is a leading cause of mortality and morbidity worldwide. Approximately 50% of HF patients die of sudden cardiac death (SCD) attributed to ventricular arrhythmias (Ͼ300 000 in the United States annually). 1,2 A large fraction of these arrhythmias are thought to be initiated by focal triggered mechanisms, such as spontaneous diastolic Ca 2ϩ release from cardiac myocyte ryanodine receptors (RyR2) on the sarcoplasmic reticulum (SR), which activates an arrhythmogenic depolarizing inward Na ϩ /Ca 2ϩ exchange (NCX) current. 3,4 Indeed, in HF there is enhanced diastolic SR Ca 2ϩ release and other changes in electrophysiological substrate that greatly enhance the propensity for triggered arrhythmias. Likewise, patients with inherited RyR2 point mutations exhibit catecholaminergic polymorphic ventricular tachycardia, a known cause of SCD with sensitivity to adrenergic conditions such as exercise or stress. 5,6 HF is a chronic hyperadrenergic state, and a prominent theory suggested that ␤-adrenergic activation of protein kinase A (PKA) destabilized ...

Sudden cardiac death exhibits diurnal variation in both acquired and hereditary forms of heart disease 1, 2, but the molecular basis is unknown. A common mechanism that underlies susceptibility to ventricular arrhythmias is abnormalities in the duration (e.g. short or long QT syndromes, heart failure) 3-5 or pattern (e.g. Brugada syndrome) 6 of myocardial repolarization. Here we provide the first molecular evidence that links circadian rhythms to vulnerability in ventricular arrhythmias in mice. Specifically, we show that cardiac ion channel expression and QT interval duration (an index of myocardial repolarization) exhibit endogenous circadian rhythmicity under the control of a novel clock-dependent oscillator, Krüppel-like factor 15 (Klf15). Klf15 transcriptionally controls rhythmic expression of KChIP2, a critical subunit required for generating the transient outward potassium current (Ito). 7 Deficiency or excess of Klf15 causes loss of rhythmic QT variation, abnormal repolarization and enhanced susceptibility to ventricular arrhythmias. In sum, these findings identify circadian transcription of ion channels as a novel mechanism for cardiac arrhythmogenesis.

Rett Syndrome is a neurodevelopmental disorder typically caused by mutations in Methyl-CpG-Binding Protein 2 (MECP2) in which 26% of deaths are sudden and of unknown cause. To explore the hypothesis that these deaths may be due to cardiac dysfunction, we characterized the electrocardiograms (ECGs) in 379 people with Rett syndrome and found that 18.5% show prolongation of the corrected QT interval (QTc), indicating a repolarization abnormality that can predispose to the development of an unstable fatal cardiac rhythm. Male mice lacking MeCP2 function, Mecp2Null/Y, also have prolonged QTc and show increased susceptibility to induced ventricular tachycardia. Female heterozygous null mice, Mecp2Null/+, show an age-dependent prolongation of QTc associated with ventricular tachycardia and cardiac-related death. Genetic deletion of MeCP2 function in only the nervous system was sufficient to cause long QTc and ventricular tachycardia, implicating neuronally-mediated changes to cardiac electrical conduction as a potential cause of ventricular tachycardia in Rett syndrome. The standard therapy for prolonged QTc in Rett syndrome, β-adrenergic receptor blockers, did not prevent ventricular tachycardia in Mecp2Null/Y mice. To determine whether an alternative therapy would be more appropriate, we characterized cardiomyocytes from Mecp2Null/Y mice and found increased persistent sodium current, which was normalized when cells were treated with the sodium channel-blocking anti-seizure drug phenytoin. Treatment with phenytoin reduced both QTc and sustained ventricular tachycardia in Mecp2Null/Y mice. These results demonstrate that cardiac abnormalities in Rett syndrome are secondary to abnormal nervous system control, which leads to increased persistent sodium current. Our findings suggest that treatment in people with Rett syndrome would be more effective if it targeted the increased persistent sodium current in order to prevent lethal cardiac arrhythmias.

Ecological and evolutionary processes are affected by forces acting at both local and regional scales, yet our understanding of how these scales interact has remained limited. These processes are fundamentally linked through individuals that develop as juve niles in one environment and then either remain in the natal habitat or disperse to new environments. Empirical studies in a diverse range of organisms have demonstrated that the conditions experienced in the natal habitat can have profound effects on the adult phenotype. This environmentally induced phenotypic variation can in turn affect the probability that an individual will disperse to a new environment and the ecological and evolutionary impact of that individual in the new environment. We synthesize the literature on this process and propose a framework for exploring the linkage between local devel opmental environment and dispersal. We then discuss the ecological and evolutionary implications of dispersal asymmetries generated by the effects of natal habitat conditions on individual phenotypes. Our review indicates that the influence of natal habitat conditions on adult phenotypes may be a highly general mechanism affecting the flow of individuals between populations. The wealth of information already gathered on how local conditions affect adult phenotype can and should be integrated into the study of dispersal as a critical force in ecology and evolution.

Rationale Junctional membrane complexes (JMC) in myocytes are critical microdomains, in which excitation-contraction coupling occurs. Structural and functional disruption of JMCs underlies contractile dysfunction in failing hearts. However, the role of newly identified JMC protein ‘striated muscle preferentially expressed gene’ (SPEG) remains unclear. Objective To determine the role of SPEG in healthy and failing adult hearts. Methods and Results Proteomic analysis of immunoprecipatated JMC-proteins ryanodine receptor type-2 (RyR2) and junctophilin-2 (JPH2) followed by mass spectrometry identified the serine-threonine kinase SPEG as the only novel binding partner for both proteins. Real-time PCR revealed downregulation of SPEG mRNA levels in failing human hearts. A novel cardiac myocyte-specific Speg conditional knockout (MCM-Spegfl/fl) model revealed that adult-onset SPEG-deficiency results in heart failure. Calcium (Ca2+) and transverse-tubule (TT) imaging of ventricular myocytes from MCM-Spegfl/fl mice post heart failure revealed both increased SR Ca2+ spark frequency and disrupted JMC integrity. Additional studies revealed that TT disruption precedes the development of heart failure development in MCM-Spegfl/fl mice. Although total JPH2 levels were unaltered, JPH2 phosphorylation levels were found to be reduced in MCM-Spegfl/fl mice, suggesting that loss of SPEG phosphorylation of JPH2 led to TT disruption, a precursor of heart failure development in SPEG deficient mice. Conclusion The novel JMC protein SPEG is downregulated in human failing hearts. Acute loss of SPEG in mouse hearts causes JPH2 dephosphorylation and TT loss associated with downstream Ca2+ mishandling leading to heart failure. Our study suggests that SPEG could be a novel target for the treatment of heart failure.

Key pointsr Mice with Ca 2+ -calmodulin-dependent protein kinase (CaMKII) constitutive pseudo-phosphorylation of the ryanodine receptor RyR2 at Ser2814 (S2814D +/+ mice) exhibit a higher open probability of RyR2, higher sarcoplasmic reticulum (SR) Ca 2+ leak in diastole and increased propensity to arrhythmias under stress conditions. r We generated phospholamban (PLN)-deficient S2814D r A mathematical human myocyte model replicates these results and predicts the increase in SR Ca 2+ uptake required to prevent the arrhythmias induced by a CaMKII-dependent leaky RyR2.Abstract Mice with constitutive pseudo-phosphorylation at Ser2814-RyR2 (S2814D +/+ ) have increased propensity to arrhythmias under β-adrenergic stress conditions. Although abnormal Ca 2+ release from the sarcoplasmic reticulum (SR) has been linked to arrhythmogenesis, the role played by SR Ca 2+ uptake remains controversial. We tested the hypothesis that an increase in SR Ca 2+ uptake is able to rescue the increased arrhythmia propensity of S2814D +/+ mice. We generated phospholamban (PLN)-deficient/S2814D +/+ knock-in mice by crossing two colonies, S2814D

Voltage-gated Kv1.1 channels encoded by the Kcna1 gene are traditionally regarded as being neural-specific with no known expression or intrinsic functional role in the heart. However, recent studies in mice reveal low-level Kv1.1 expression in heart and cardiac abnormalities associated with Kv1.1-deficiency suggesting that the channel may have a previously unrecognized cardiac role. Therefore, this study tests the hypothesis that Kv1.1 channels are associated with arrhythmogenesis and contribute to intrinsic cardiac function. In intra-atrial burst pacing experiments, Kcna1-null mice exhibited increased susceptibility to atrial fibrillation (AF). The atria of Kcna1-null mice showed minimal Kv1 family ion channel remodeling and fibrosis as measured by qRT-PCR and Masson’s trichrome histology, respectively. Using RT-PCR, immunocytochemistry, and immunoblotting, KCNA1 mRNA and protein were detected in isolated mouse cardiomyocytes and human atria for the first time. Patients with chronic AF (cAF) showed no changes in KCNA1 mRNA levels relative to controls; however, they exhibited increases in atrial Kv1.1 protein levels, not seen in paroxysmal AF patients. Patch-clamp recordings of isolated human atrial myocytes revealed significant dendrotoxin-K (DTX-K)-sensitive outward current components that were significantly increased in cAF patients, reflecting a contribution by Kv1.1 channels. The concomitant increases in Kv1.1 protein and DTX-K-sensitive currents in atria of cAF patients suggest that the channel contributes to the pathological mechanisms of persistent AF. These findings provide evidence of an intrinsic cardiac role of Kv1.1 channels and indicate that they may contribute to atrial repolarization and AF susceptibility.

Background: Epidemiological studies have established obesity as an independent risk factor for atrial fibrillation (AF), but the underlying pathophysiological mechanisms remain unclear. Reduced cardiac sodium channel expression is a known causal mechanism in AF. We hypothesized that obesity decreases Nav1.5 expression via enhanced oxidative stress, thus reducing I Na , and enhancing susceptibility to AF. Methods: To elucidate the underlying electrophysiological mechanisms a diet-induced obese mouse model was used. Weight, blood pressure, glucose, F 2 -isoprostanes, NOX2 (NADPH oxidase 2), and PKC (protein kinase C) were measured in obese mice and compared with lean controls. Invasive electrophysiological, immunohistochemistry, Western blotting, and patch clamping of membrane potentials was performed to evaluate the molecular and electrophysiological phenotype of atrial myocytes. Results: Pacing-induced AF in 100% of diet-induced obese mice versus 25% in controls ( P <0.01) with increased AF burden. Cardiac sodium channel expression, I Na and atrial action potential duration were reduced and potassium channel expression (Kv1.5) and current ( I Kur ) and F 2 -isoprostanes, NOX2, and PKC-α/δ expression and atrial fibrosis were significantly increased in diet-induced obese mice as compared with controls. A mitochondrial antioxidant reduced AF burden, restored I Na , I Ca,L , I Kur , action potential duration, and reversed atrial fibrosis in diet-induced obese mice as compared with controls. Conclusions: Inducible AF in obese mice is mediated, in part, by a combined effect of sodium, potassium, and calcium channel remodeling and atrial fibrosis. Mitochondrial antioxidant therapy abrogated the ion channel and structural remodeling and reversed the obesity-induced AF burden. Our findings have important implications for the management of obesity-mediated AF in patients. Graphic Abstract: A graphic abstract is available for this article.