CaMKIIδC slows [Ca]i decline in cardiac myocytes by promoting Ca sparks

Is CaMKII friend or foe for cell apoptosis in eye?: A narrative review

Ca2+/calmodulin-dependent protein kinase II (CaMKII) controls cell proliferation, differentiation, apoptosis, and other biological processes that have an essential role in eye diseases. However, it seems that previous studies have generated conflicting conclusions about the effect of CaMKII on cell apoptosis. In this review, we explore the positive and potentially deleterious effects of CaMKII on eye cell apoptosis. We can safely conclude that the early elevation of CaMKII could be viewed as a promoter of cell apoptosis. Overexpression of CaMKII by transfection or pretreatment with drugs helped combat cell apoptosis.

CaMKII-Dependent Contractile Dysfunction and Pro-Arrhythmic Activity in a Mouse Model of Obstructive Sleep Apnea

Left ventricular contractile dysfunction and arrhythmias frequently occur in patients with sleep-disordered breathing (SDB). The CaMKII-dependent dysregulation of cellular Ca homeostasis has recently been described in SDB patients, but these studies only partly explain the mechanism and are limited by the patients' heterogeneity. Here, we analyzed contractile function and Ca homeostasis in a mouse model of obstructive sleep apnea (OSA) that is not limited by confounding comorbidities. OSA was induced by artificial tongue enlargement with polytetrafluorethylene (PTFE) injection into the tongue of wildtype mice and mice with a genetic ablation of the oxidative activation sites of CaMKII (MMVV knock-in). After eight weeks, cardiac function was assessed with echocardiography. Reactive oxygen species (ROS) and Ca transients were measured using confocal and epifluorescence microscopy, respectively. Wildtype PTFE mice exhibited an impaired ejection fraction, while MMVV PTFE mice were fully protected. As expected, isolated cardiomyocytes from PTFE mice showed increased ROS production. We further observed decreased levels of steady-state Ca transients, decreased levels of caffeine-induced Ca transients, and increased pro-arrhythmic activity (defined as deviations from the diastolic Ca baseline) only in wildtype but not in MMVV PTFE mice. In summary, in the absence of any comorbidities, OSA was associated with contractile dysfunction and pro-arrhythmic activity and the inhibition of the oxidative activation of CaMKII conveyed cardioprotection, which may have therapeutic implications.

Cardiac function, structural, and electrical remodeling by microgravity exposure

Space medicine is key to the human exploration of outer space and pushes the boundaries of science, technology, and medicine. Because of harsh environmental conditions related to microgravity and other factors and hazards in outer space, astronauts and spaceflight participants face unique health and medical challenges, including those related to the heart. In this review, we summarize the literature regarding the effects of spaceflight on cardiac structure and function. We also provide an in-depth review of the literature regarding the effects of microgravity on cardiac calcium handling. Our review can inform future mechanistic and therapeutic studies and is applicable to other physiological states similar to microgravity such as prolonged horizontal bed rest and immobilization.

Molecular, Subcellular, and Arrhythmogenic Mechanisms in Genetic RyR2 Disease

The ryanodine receptor (RyR2) has a critical role in controlling Ca²⁺ release from the sarcoplasmic reticulum (SR) throughout the cardiac cycle. RyR2 protein has multiple functional domains with specific roles, and four of these RyR2 protomers are required to form the quaternary structure that comprises the functional channel. Numerous mutations in the gene encoding RyR2 protein have been identified and many are linked to a wide spectrum of arrhythmic heart disease. Gain of function mutations (GoF) result in a hyperactive channel that causes excessive spontaneous SR Ca²⁺ release. This is the predominant cause of the inherited syndrome catecholaminergic polymorphic ventricular tachycardia (CPVT). Recently, rare hypoactive loss of function (LoF) mutations have been identified that produce atypical effects on cardiac Ca²⁺ handling that has been termed calcium release deficiency syndrome (CRDS). Aberrant Ca²⁺ release resulting from both GoF and LoF mutations can result in arrhythmias through the Na⁺/Ca²⁺ exchange mechanism. This mini-review discusses recent findings regarding the role of RyR2 domains and endogenous regulators that influence RyR2 gating normally and with GoF/LoF mutations. The arrhythmogenic consequences of GoF/LoF mutations will then be discussed at the macromolecular and cellular level.

The oxidation-resistant CaMKII-MM281/282VV mutation does not prevent arrhythmias in CPVT1

Catecholaminergic polymorphic ventricular tachycardia type 1 (CPVT1) is an inherited arrhythmogenic disorder caused by missense mutations in the cardiac ryanodine receptors (RyR2), that result in increased β-adrenoceptor stimulation-induced diastolic Ca2+ leak. We have previously shown that exercise training prevents arrhythmias in CPVT1, potentially by reducing the oxidation of Ca2+ /calmodulin-dependent protein kinase type II (CaMKII). Therefore, we tested whether an oxidation-resistant form of CaMKII protects mice carrying the CPVT1-causative mutation RyR2-R2474S (RyR2-RS) against arrhythmias. Antioxidant treatment (N-acetyl-L-cysteine) reduced the frequency of β-adrenoceptor stimulation-induced arrhythmogenic Ca2+ waves in isolated cardiomyocytes from RyR2-RS mice. To test whether the prevention of CaMKII oxidation exerts an antiarrhythmic effect, mice expressing the oxidation-resistant CaMKII-MM281/282VV variant (MMVV) were crossed with RyR2-RS mice to create a double transgenic model (RyR2-RS/MMVV). Wild-type mice served as controls. Telemetric ECG surveillance revealed an increased incidence of ventricular tachycardia and an increased arrhythmia score in both RyR2-RS and RyR2-RS/MMVV compared to wild-type mice, both following a β-adrenoceptor challenge (isoprenaline i.p.), and following treadmill exercise combined with a β-adrenoceptor challenge. There were no differences in the incidence of arrhythmias between RyR2-RS and RyR2-RS/MMVV mice. Furthermore, no differences were observed in β-adrenoceptor stimulation-induced Ca2+ waves in RyR2-RS/MMVV compared to RyR2-RS. In conclusion, antioxidant treatment reduces β-adrenoceptor stimulation-induced Ca2+ waves in RyR2-RS cardiomyocytes. However, oxidation-resistant CaMKII-MM281/282VV does not protect RyR2-RS mice from β-adrenoceptor stimulation-induced Ca2+ waves or arrhythmias. Hence, alternative oxidation-sensitive targets need to be considered to explain the beneficial effect of antioxidant treatment on Ca2+ waves in cardiomyocytes from RyR2-RS mice.

CaMKII inhibition has dual effects on spontaneous Ca2+ release and Ca2+ alternans in ventricular cardiomyocytes from mice with a gain-of-function RyR2 mutation

In conditions with abnormally increased activity of the cardiac ryanodine receptor (RyR2), Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) can contribute to a further destabilization of RyR2 that results in triggered arrhythmias. Therefore, inhibition of CaMKII in such conditions has been suggested as a strategy to suppress RyR2 activity and arrhythmias. However, suppression of RyR2 activity can lead to the development of arrhythmogenic Ca²⁺ alternans. The aim of this study was to test whether the suppression of RyR2 activity caused by inhibition of CaMKII increases propensity for Ca²⁺ alternans. We studied spontaneous Ca²⁺ release events and Ca²⁺ alternans in isolated left ventricular cardiomyocytes from mice carrying the gain-of-function RyR2 mutation RyR2-R2474S and from wild-type mice. CaMKII inhibition by KN-93 effectively decreased the frequency of spontaneous Ca²⁺ release events in RyR2-R2474S cardiomyocytes exposed to the β-adrenoceptor agonist isoprenaline. However, KN-93-treated RyR2-R2474S cardiomyocytes also showed increased propensity for Ca²⁺ alternans and increased Ca²⁺ alternans ratio compared with both an inactive analog of KN-93 and with vehicle-treated controls. This increased propensity for Ca²⁺ alternans was explained by prolongation of Ca²⁺ release refractoriness. Importantly, the increased propensity for Ca²⁺ alternans in KN-93-treated RyR2-R2474S cardiomyocytes did not surpass that of wild type. In conclusion, inhibition of CaMKII efficiently reduces spontaneous Ca²⁺ release but promotes Ca²⁺ alternans in RyR2-R2474S cardiomyocytes with a gain-of-function RyR2 mutation. The dominant effect in RyR2-R2474S is to reduce spontaneous Ca²⁺ release, which supports this intervention as a therapeutic strategy in this specific condition. However, future studies on CaMKII inhibition in conditions with increased propensity for Ca²⁺ alternans should include investigation of both phenomena.NEW & NOTEWORTHY Genetically increased RyR2 activity promotes arrhythmogenic Ca²⁺ release. Inhibition of CaMKII suppresses RyR2 activity and arrhythmogenic Ca²⁺ release. Suppression of RyR2 activity prolongs refractoriness of Ca²⁺ release. Prolonged refractoriness of Ca²⁺ release leads to arrhythmogenic Ca²⁺ alternans. CaMKII inhibition promotes Ca²⁺ alternans by prolonging Ca²⁺ release refractoriness.

JNK2, a Newly-Identified SERCA2 Enhancer, Augments an Arrhythmic [Ca2+]SR Leak-Load Relationship

RATIONALE

We recently discovered pivotal contributions of stress kinase JNK2 (c-Jun N-terminal kinase isoform 2) in increased risk of atrial fibrillation through enhanced diastolic sarcoplasmic reticulum (SR) calcium (Ca2+) leak via RyR2 (ryanodine receptor isoform 2). However, the role of JNK2 in the function of the SERCA2 (SR Ca2+-ATPase), essential in maintaining SR Ca2+ content cycling during each heartbeat, is completely unknown.

OBJECTIVE

To test the hypothesis that JNK2 increases SERCA2 activity SR Ca2+ content and exacerbates an arrhythmic SR Ca2+ content leak-load relationship.

METHODS AND RESULTS

We used confocal Ca2+ imaging in myocytes and HEK-RyR2 (ryanodine receptor isoform 2-expressing human embryonic kidney 293 cells) cells, biochemistry, dual Ca2+/voltage optical mapping in intact hearts from alcohol-exposed or aged mice (where JNK2 is activated). We found that JNK2, but not JNK1 (c-Jun N-terminal kinase isoform 1), increased SERCA2 uptake and consequently elevated SR Ca2+ content load. JNK2 also associates with and phosphorylates SERCA2 proteins. JNK2 causally enhances SERCA2-ATPase activity via increased maximal rate, without altering Ca2+ affinity. Unlike the CaMKII (Ca2+/calmodulin-dependent kinase II)-dependent JNK2 action in SR Ca2+ leak, JNK2-driven SERCA2 function was CaMKII independent (not prevented by CaMKII inhibition). With CaMKII blocked, the JNK2-driven SR Ca2+ loading alone did not significantly raise leak. However, with JNK2-CaMKII-driven SR Ca2+ leak present, the JNK2-enhanced SR Ca2+ uptake limited leak-induced reduction in SR Ca2+, normalizing Ca2+ transient amplitude, but at a higher arrhythmogenic SR Ca2+ leak. JNK2-specific inhibition completely normalized SR Ca2+ handling, attenuated arrhythmic Ca2+ activities, and alleviated atrial fibrillation susceptibility in aged and alcohol-exposed myocytes and intact hearts.

CONCLUSIONS

We have identified a novel JNK2-induced activation of SERCA2. The dual action of JNK2 in CaMKII-dependent arrhythmic SR Ca2+ leak and a CaMKII-independent uptake exacerbates atrial arrhythmogenicity, while helping to maintain normal levels of Ca2+ transients and heart function. JNK2 modulation may be a novel therapeutic target for atrial fibrillation prevention and treatment.

Hyperglycemia Acutely Increases Cytosolic Reactive Oxygen Species via O-linked GlcNAcylation and CaMKII Activation in Mouse Ventricular Myocytes

RATIONALE

Diabetes mellitus is a complex, multisystem disease, affecting large populations worldwide. Chronic CaMKII (Ca²⁺/calmodulin-dependent kinase II) activation may occur in diabetes mellitus and be arrhythmogenic. Diabetic hyperglycemia was shown to activate CaMKII by (1) O-linked attachment of N-acetylglucosamine (O-GlcNAc) at S280 leading to arrhythmia and (2) a reactive oxygen species (ROS)-mediated oxidation of CaMKII that can increase postinfarction mortality.

OBJECTIVE

To test whether high extracellular glucose (Hi-Glu) promotes ventricular myocyte ROS generation and the role played by CaMKII.

METHODS AND RESULTS

We tested how extracellular Hi-Glu influences ROS production in adult ventricular myocytes, using DCF (2',7'-dichlorodihydrofluorescein diacetate) and genetically targeted Grx-roGFP2 redox sensors. Hi-Glu (30 mmol/L) significantly increased the rate of ROS generation-an effect prevented in myocytes pretreated with CaMKII inhibitor KN-93 or from either global or cardiac-specific CaMKIIδ KO (knockout) mice. CaMKII KO or inhibition also prevented Hi-Glu-induced sarcoplasmic reticulum Ca²⁺ release events (Ca²⁺ sparks). Thus, CaMKII activation is required for Hi-Glu-induced ROS generation and sarcoplasmic reticulum Ca²⁺ leak in cardiomyocytes. To test the involvement of O-GlcNAc-CaMKII pathway, we inhibited GlcNAcylation removal by Thiamet G (ThmG), which mimicked the Hi-Glu-induced ROS production. Conversely, inhibition of GlcNAcylation (OSMI-1 [(αR)-α-[[(1,2-dihydro-2-oxo-6-quinolinyl)sulfonyl]amino]-N-(2-furanylmethyl)-2-methoxy-N-(2-thienylmethyl)-benzeneacetamide]) prevented ROS induction in response to either Hi-Glu or ThmG. Moreover, in a CRSPR-based knock-in mouse in which the functional GlcNAcylation site on CaMKIIδ was ablated (S280A), neither Hi-Glu nor ThmG induced myocyte ROS generation. So CaMKIIδ-S280 is required for the Hi-Glu-induced (and GlcNAc dependent) ROS production. To identify the ROS source(s), we used different inhibitors of NOX (NADPH oxidase) 2 (Gp91ds-tat peptide), NOX4 (GKT137831), mitochondrial ROS (MitoTempo), and NOS (NO synthase) pathway inhibitors (L-NAME, L-NIO, and L-NPA). Only NOX2 inhibition or KO prevented Hi-Glu/ThmG-induced ROS generation.

CONCLUSIONS

Diabetic hyperglycemia induces acute cardiac myocyte ROS production by NOX2 that requires O-GlcNAcylation of CaMKIIδ at S280. This novel ROS induction may exacerbate pathological consequences of diabetic hyperglycemia.

The Control of Diastolic Calcium in the Heart: Basic Mechanisms and Functional Implications

Normal cardiac function requires that intracellular Ca²⁺ concentration be reduced to low levels in diastole so that the ventricle can relax and refill with blood. Heart failure is often associated with impaired cardiac relaxation. Little, however, is known about how diastolic intracellular Ca²⁺ concentration is regulated. This article first discusses the reasons for this ignorance before reviewing the basic mechanisms that control diastolic intracellular Ca²⁺ concentration. It then considers how the control of systolic and diastolic intracellular Ca²⁺ concentration is intimately connected. Finally, it discusses the changes that occur in heart failure and how these may result in heart failure with preserved versus reduced ejection fraction.

Oxidative CaMKII as a potential target for inflammatory disease (Review)

CaMKII is a calcium‑activated kinase, proved to be modulated by oxidation. Currently, the oxidative activation of CaMKII exists in several models of asthma, chronic rhinosinusitis with nasal polyps, cardiovascular disease, diabetes mellitus, acute ischemic stroke and cancer. Oxidized CaMKII (ox‑CaMKII) may be important in several of these diseases. The present review examines the mechanism underlying the oxidative activation of CaMKII and summarizes the current findings associated with the function of ox‑CaMKII in inflammatory diseases. Taken together, the findings of this review aim to improve current understanding of the function of ox‑CaMKII and provide novel insights for future research.

Calcium-Mediated Oscillation in Membrane Potentials and Atrial-Triggered Activity in Atrial Cells of Casq2R33Q/R33Q Mutation Mice

Aim: We investigated the underlying mechanisms in atrial fibrillation (AF) associated with R33Q mutation and Ca²⁺-triggered activity. Methods and Results: We examined AF susceptibility with intraesophageal burst pacing in the sarcoplasmic reticulum (SR) Ca²⁺ leak model calsequestrin 2 R33Q (Casq2^R33Q/R33Q) mice. Atrial trigger appeared in R33Q mice but not WT mice (17.24%, 5/29 vs. 0.00%, 0/32, P < 0.05). AF was induced by 25 Hz pacing in R33Q mice (48.27%, 14/29 vs. 6.25%, 2/32, P < 0.01). The mice were given 1.5 mg/kg isoproterenol (Iso), and the incidences of AF increased (65.51%, 19/29 vs. 9.21%, 3/32, P < 0.01). Electrophysiology experiments and the recording of intracellular Ca²⁺ indicated significant increases in the Ca²⁺ sparks (5.24 ± 0.75 100 μM^-1.s^-1 vs. 0.29 ± 0.04 100 μM^-1.s^-1, n = 20, P < 0.05), intracellular free Ca²⁺ (0.238 ± 0.009 μM vs. 0.172 ± 0.006 μM, n = 20, P < 0.05), Ca²⁺ wave (11.74% vs. 2.24%, n = 20, P < 0.05), transient inward current (ITi) (-0.56 ± 0.02 pA/pF vs. -0.42 ± 0.01 pA/pF, n = 10, P < 0.05), and oscillation in membrane potentials (10.71%, 3/28 vs. 4.16%, 1/24, P < 0.05) in the R33Q group, but there was no significant difference in the L-type calcium current. These effects were enhanced by Iso, and the inhibition of calmodulin-dependent protein kinase II (CaMKII) by 1 μM KN93 reversed the effects of Iso on Ca²⁺ sparks (5.01 ± 0.66 100 μm^-1.s^-1 vs. 11.33 ± 1.63 100 μm^-1.s^-1, P < 0.05), intracellular Ca²⁺ (0.245 ± 0.005 μM vs. 0.324 ± 0.008 μM, P < 0.05), Ca²⁺ wave (12.35% vs. 17.83%, P < 0.05), ITi (-0.61 ± 0.02 pA/pF vs. -0.78 ± 0.03 pA/pF, n = 10, P < 0.05), and oscillation in membrane potential (17.85% 5/28 vs. 32.17% 9/28, P < 0.05). The reduction of ryanodine receptor 2 (RyR2) stable subunits (Casq2, triadin, and junctin) rather than RYR2 and the increase in CaMKII, phosphor-CaMKII, phosphor-RyR2 (Ser 2814), SERCA, and NCX1.1 was reflected in the R33Q group. Conclusion: This study demonstrates that the increase in spontaneous calcium elevations corresponding to ITi that may trigger the oscillation in membrane potentials in the R33Q group, thereby increasing the risk of AF. The occurrence of spontaneous calcium elevations in R33Q atrial myocytes is due to the dysfunction of RyR2 stable subunits, CaMKII hyperactivity, and CaMKII-mediated RyR phosphorylation. An effective therapeutic strategy to intervene in Ca²⁺-induced AF associated with the R33Q mutation may be through CaMKII inhibition.

Arrhythmogenic Mechanisms in Heart Failure: Linking β-Adrenergic Stimulation, Stretch, and Calcium

Heart failure (HF) is associated with elevated sympathetic tone and mechanical load. Both systems activate signaling transduction pathways that increase cardiac output, but eventually become part of the disease process itself leading to further worsening of cardiac function. These alterations can adversely contribute to electrical instability, at least in part due to the modulation of Ca²⁺ handling at the level of the single cardiac myocyte. The major aim of this review is to provide a definitive overview of the links and cross talk between β-adrenergic stimulation, mechanical load, and arrhythmogenesis in the setting of HF. We will initially review the role of Ca²⁺ in the induction of both early and delayed afterdepolarizations, the role that β-adrenergic stimulation plays in the initiation of these and how the propensity for these may be altered in HF. We will then go onto reviewing the current data with regards to the link between mechanical load and afterdepolarizations, the associated mechano-sensitivity of the ryanodine receptor and other stretch activated channels that may be associated with HF-associated arrhythmias. Furthermore, we will discuss how alterations in local Ca²⁺ microdomains during the remodeling process associated the HF may contribute to the increased disposition for β-adrenergic or stretch induced arrhythmogenic triggers. Finally, the potential mechanisms linking β-adrenergic stimulation and mechanical stretch will be clarified, with the aim of finding common modalities of arrhythmogenesis that could be targeted by novel therapeutic agents in the setting of HF.

Unique Ca2+-Cycling Protein Abundance and Regulation Sustains Local Ca2+ Releases and Spontaneous Firing of Rabbit Sinoatrial Node Cells

Spontaneous beating of the heart pacemaker, the sinoatrial node, is generated by sinoatrial node cells (SANC) and caused by gradual change of the membrane potential called diastolic depolarization (DD). Submembrane local Ca²⁺ releases (LCR) from sarcoplasmic reticulum (SR) occur during late DD and activate an inward Na⁺/Ca²⁺ exchange current, which accelerates the DD rate leading to earlier occurrence of an action potential. A comparison of intrinsic SR Ca²⁺ cycling revealed that, at similar physiological Ca²⁺ concentrations, LCRs are large and rhythmic in permeabilized SANC, but small and random in permeabilized ventricular myocytes (VM). Permeabilized SANC spontaneously released more Ca²⁺ from SR than VM, despite comparable SR Ca²⁺ content in both cell types. In this review we discuss specific patterns of expression and distribution of SR Ca²⁺ cycling proteins (SR Ca²⁺ ATPase (SERCA2), phospholamban (PLB) and ryanodine receptors (RyR)) in SANC and ventricular myocytes. We link ability of SANC to generate larger and rhythmic LCRs with increased abundance of SERCA2, reduced abundance of the SERCA inhibitor PLB. In addition, an increase in intracellular [Ca²⁺] increases phosphorylation of both PLB and RyR exclusively in SANC. The differences in SR Ca²⁺ cycling protein expression between SANC and VM provide insights into diverse regulation of intrinsic SR Ca²⁺ cycling that drives automaticity of SANC.

Stress Signaling JNK2 Crosstalk With CaMKII Underlies Enhanced Atrial Arrhythmogenesis

RATIONALE

Atrial fibrillation (AF) is the most common arrhythmia, and advanced age is an inevitable and predominant AF risk factor. However, the mechanisms that couple aging and AF propensity remain unclear, making targeted therapeutic interventions unattainable.

OBJECTIVE

To explore the functional role of an important stress response JNK (c-Jun N-terminal kinase) in sarcoplasmic reticulum Ca²⁺ handling and consequently Ca²⁺-mediated atrial arrhythmias.

METHODS AND RESULTS

We used a series of cutting-edge electrophysiological and molecular techniques, exploited the power of transgenic mouse models to detail the molecular mechanism, and verified its clinical applicability in parallel studies on donor human hearts. We discovered that significantly increased activity of the stress response kinase JNK2 (JNK isoform 2) in the aged atria is involved in arrhythmic remodeling. The JNK-driven atrial proarrhythmic mechanism is supported by a pathway linking JNK, CaMKII (Ca²⁺/calmodulin-dependent kinase II), and sarcoplasmic reticulum Ca²⁺ release RyR2 (ryanodine receptor) channels. JNK2 activates CaMKII, a critical proarrhythmic molecule in cardiac muscle. In turn, activated CaMKII upregulates diastolic sarcoplasmic reticulum Ca²⁺ leak mediated by RyR2 channels. This leads to aberrant intracellular Ca²⁺ waves and enhanced AF propensity. In contrast, this mechanism is absent in young atria. In JNK challenged animal models, this is eliminated by JNK2 ablation or CaMKII inhibition.

CONCLUSIONS

We have identified JNK2-driven CaMKII activation as a novel mode of kinase crosstalk and a causal factor in atrial arrhythmic remodeling, making JNK2 a compelling new therapeutic target for AF prevention and treatment.

Late Ca2+ Sparks and Ripples During the Systolic Ca2+ Transient in Heart Muscle Cells

Supplemental Digital Content is available in the text.

Rationale:

The development of a refractory period for Ca²⁺ spark initiation after Ca²⁺ release in cardiac myocytes should inhibit further Ca²⁺ release during the action potential plateau. However, Ca²⁺ release sites that did not initially activate or which have prematurely recovered from refractoriness might release Ca²⁺ later during the action potential and alter the cell-wide Ca²⁺ transient.

Objective:

To investigate the possibility of late Ca²⁺ spark (LCS) activity in intact isolated cardiac myocytes using fast confocal line scanning with improved confocality and signal to noise.

Methods and Results:

We recorded Ca²⁺ transients from cardiac ventricular myocytes isolated from rabbit hearts. Action potentials were produced by electric stimulation, and rapid solution changes were used to modify the L-type Ca²⁺ current. After the upstroke of the Ca²⁺ transient, LCSs were detected which had increased amplitude compared with diastolic Ca²⁺ sparks. LCS are triggered by both L-type Ca²⁺ channel activity during the action potential plateau, as well as by the increase of cytosolic Ca²⁺ associated with the Ca²⁺ transient itself. Importantly, a mismatch between sarcoplasmic reticulum load and L-type Ca²⁺ trigger can increase the number of LCS. The likelihood of triggering an LCS also depends on recovery from refractoriness that appears after prior activation. Consequences of LCS include a reduced rate of decline of the Ca²⁺ transient and, if frequent, formation of microscopic propagating Ca²⁺ release events (Ca²⁺ ripples). Ca²⁺ ripples resemble Ca²⁺ waves in terms of local propagation velocity but spread for only a short distance because of limited regeneration.

Conclusions:

These new types of Ca²⁺ signaling behavior extend our understanding of Ca²⁺-mediated signaling. LCS may provide an arrhythmogenic substrate by slowing the Ca²⁺ transient decline, as well as by amplifying maintained Ca²⁺ current effects on intracellular Ca²⁺ and consequently Na⁺/Ca²⁺ exchange current.

Computational modeling of amylin-induced calcium dysregulation in rat ventricular cardiomyocytes

Hyperamylinemia is a condition that accompanies obesity and precedes type II diabetes, and it is characterized by above-normal blood levels of amylin, the pancreas-derived peptide. Human amylin oligomerizes easily and can deposit in the pancreas [1], brain [2], and heart [3], where they have been associated with calcium dysregulation. In the heart, accumulating evidence suggests that human amylin oligomers form moderately cation-selective [4,5] channels that embed in the cell sarcolemma (SL). The oligomers increase membrane conductance in a concentration-dependent manner [5], which is correlated with elevated cytosolic Ca²⁺. These findings motivate our core hypothesis that non-selective inward Ca²⁺ conduction afforded by human amylin oligomers increase cytosolic and sarcoplasmic reticulum (SR) Ca²⁺ load, which thereby magnifies intracellular Ca²⁺ transients. Questions remain however regarding the mechanism of amylin-induced Ca²⁺ dysregulation, including whether enhanced SL Ca²⁺ influx is sufficient to elevate cytosolic Ca²⁺ load [6], and if so, how might amplified Ca²⁺ transients perturb Ca²⁺-dependent cardiac pathways. To investigate these questions, we modified a computational model of cardiomyocytes Ca²⁺ signaling to reflect experimentally-measured changes in SL membrane permeation and decreased sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) function stemming from acute and transgenic human amylin peptide exposure. With this model, we confirmed the hypothesis that increasing SL permeation alone was sufficient to enhance Ca²⁺ transient amplitudes. Our model indicated that amplified cytosolic transients are driven by increased Ca²⁺ loading of the SR and that greater fractional release may contribute to the Ca²⁺-dependent activation of calmodulin, which could prime the activation of myocyte remodeling pathways. Importantly, elevated Ca²⁺ in the SR and dyadic space collectively drive greater fractional SR Ca²⁺ release for human amylin expressing rats (HIP) and acute amylin-exposed rats (+Amylin) mice, which contributes to the inotropic rise in cytosolic Ca²⁺ transients. These findings suggest that increased membrane permeation induced by oligomeratization of amylin peptide in cell sarcolemma contributes to Ca²⁺ dysregulation in pre-diabetes.

Calcium and Excitation-Contraction Coupling in the Heart

Cardiac contractility is regulated by changes in intracellular Ca concentration ([Ca²⁺]_i). Normal function requires that [Ca²⁺]_i be sufficiently high in systole and low in diastole. Much of the Ca needed for contraction comes from the sarcoplasmic reticulum and is released by the process of calcium-induced calcium release. The factors that regulate and fine-tune the initiation and termination of release are reviewed. The precise control of intracellular Ca cycling depends on the relationships between the various channels and pumps that are involved. We consider 2 aspects: (1) structural coupling: the transporters are organized within the dyad, linking the transverse tubule and sarcoplasmic reticulum and ensuring close proximity of Ca entry to sites of release. (2) Functional coupling: where the fluxes across all membranes must be balanced such that, in the steady state, Ca influx equals Ca efflux on every beat. The remainder of the review considers specific aspects of Ca signaling, including the role of Ca buffers, mitochondria, Ca leak, and regulation of diastolic [Ca²⁺]_i.

Ambiguous interactions between diastolic and SR Ca2+ in the regulation of cardiac Ca2+ release

Sobie et al. highlight unresolved issues concerning the regulation of sarcoplasmic reticulum calcium release in cardiac myocytes.

Glycosylated Chromogranin A in Heart Failure: Implications for Processing and Cardiomyocyte Calcium Homeostasis

BACKGROUND

Chromogranin A (CgA) levels have previously been found to predict mortality in heart failure (HF), but currently no information is available regarding CgA processing in HF and whether the CgA fragment catestatin (CST) may directly influence cardiomyocyte function.

METHODS AND RESULTS

CgA processing was characterized in postinfarction HF mice and in patients with acute HF, and the functional role of CST was explored in experimental models. Myocardial biopsies from HF, but not sham-operated mice, demonstrated high molecular weight CgA bands. Deglycosylation treatment attenuated high molecular weight bands, induced a mobility shift, and increased shorter CgA fragments. Adjusting for established risk indices and biomarkers, circulating CgA levels were found to be associated with mortality in patients with acute HF, but not in patients with acute exacerbation of chronic obstructive pulmonary disease. Low CgA-to-CST conversion was also associated with increased mortality in acute HF, thus, supporting functional relevance of impaired CgA processing in cardiovascular disease. CST was identified as a direct inhibitor of CaMKIIδ (Ca²⁺/calmodulin-dependent protein kinase IIδ) activity, and CST reduced CaMKIIδ-dependent phosphorylation of phospholamban and the ryanodine receptor 2. In line with CaMKIIδ inhibition, CST reduced Ca²⁺ spark and wave frequency, reduced Ca²⁺ spark dimensions, increased sarcoplasmic reticulum Ca²⁺ content, and augmented the magnitude and kinetics of cardiomyocyte Ca²⁺ transients and contractions.

CONCLUSIONS

CgA-to-CST conversion in HF is impaired because of hyperglycosylation, which is associated with clinical outcomes in acute HF. The mechanism for increased mortality may be dysregulated cardiomyocyte Ca²⁺ handling because of reduced CaMKIIδ inhibition.

Oxidative stress and ca(2+) release events in mouse cardiomyocytes

Cellular oxidative stress, associated with a variety of common cardiac diseases, is well recognized to affect the function of several key proteins involved in Ca(2+) signaling and excitation-contraction coupling, which are known to be exquisitely sensitive to reactive oxygen species. These include the Ca(2+) release channels of the sarcoplasmic reticulum (ryanodine receptors or RyR2s) and the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Oxidation of RyR2s was found to increase the open probability of the channel, whereas CaMKII can be activated independent of Ca(2+) through oxidation. Here, we investigated how oxidative stress affects RyR2 function and SR Ca(2+) signaling in situ, by analyzing Ca(2+) sparks in permeabilized mouse cardiomyocytes under a broad range of oxidative conditions. The results show that with increasing oxidative stress Ca(2+) spark duration is prolonged. In addition, long and very long-lasting (up to hundreds of milliseconds) localized Ca(2+) release events started to appear, eventually leading to sarcoplasmic reticulum (SR) Ca(2+) depletion. These changes of release duration could be prevented by the CaMKII inhibitor KN93 and did not occur in mice lacking the CaMKII-specific S2814 phosphorylation site on RyR2. The appearance of long-lasting Ca(2+) release events was paralleled by an increase of RyR2 oxidation, but also by RyR-S2814 phosphorylation, and by CaMKII oxidation. Our results suggest that in a strongly oxidative environment oxidation-dependent activation of CaMKII leads to RyR2 phosphorylation and thereby contributes to the massive prolongation of SR Ca(2+) release events.

Biphasic decay of the Ca transient results from increased sarcoplasmic reticulum Ca leak

Key points

Ca leak from the sarcoplasmic reticulum through the ryanodine receptor (RyR) reduces the amplitude of the Ca transient and slows its rate of decay.

In the presence of β‐adrenergic stimulation, RyR‐mediated Ca leak produces a biphasic decay of the Ca transient with a fast early phase and a slow late phase.

Two forms of Ca leak have been studied, Ca‐sensitising (induced by caffeine) and non‐sensitising (induced by ryanodine) and both induce biphasic decay of the Ca transient.

Only Ca‐sensitising leak can be reversed by traditional RyR inhibitors such as tetracaine.

Ca leak can also induce Ca waves. At low levels of leak, waves occur. As leak is increased, first biphasic decay and then slowed monophasic decay is seen. The level of leak has major effects on the shape of the Ca transient.

Abstract

In heart failure, a reduction in Ca transient amplitude and contractile dysfunction can by caused by Ca leak through the sarcoplasmic reticulum (SR) Ca channel (ryanodine receptor, RyR) and/or decreased activity of the SR Ca ATPase (SERCA). We have characterised the effects of two forms of Ca leak (Ca‐sensitising and non‐sensitising) on calcium cycling and compared with those of SERCA inhibition. We measured [Ca²⁺]_i with fluo‐3 in voltage‐clamped rat ventricular myocytes. Increasing SR leak with either caffeine (to sensitise the RyR to Ca activation) or ryanodine (non‐sensitising) had similar effects to SERCA inhibition: decreased systolic [Ca²⁺]_i, increased diastolic [Ca²⁺]_i and slowed decay. However, in the presence of isoproterenol, leak produced a biphasic decay of the Ca transient in the majority of cells while SERCA inhibition produced monophasic decay. Tetracaine reversed the effects of caffeine but not of ryanodine. When caffeine (1 mmol l⁻¹) was added to a cell which displayed Ca waves, the wave frequency initially increased before waves disappeared and biphasic decay developed. Eventually (at higher caffeine concentrations), the biphasic decay was replaced by slow decay. We conclude that, in the presence of adrenergic stimulation, Ca leak can produce biphasic decay; the slow phase results from the leak opposing Ca uptake by SERCA. The degree of leak determines whether decay of Ca waves, biphasic or monophasic, occurs.

Ca leak from the sarcoplasmic reticulum through the ryanodine receptor (RyR) reduces the amplitude of the Ca transient and slows its rate of decay.

In the presence of β‐adrenergic stimulation, RyR‐mediated Ca leak produces a biphasic decay of the Ca transient with a fast early phase and a slow late phase.

Two forms of Ca leak have been studied, Ca‐sensitising (induced by caffeine) and non‐sensitising (induced by ryanodine) and both induce biphasic decay of the Ca transient.

Only Ca‐sensitising leak can be reversed by traditional RyR inhibitors such as tetracaine.

Ca leak can also induce Ca waves. At low levels of leak, waves occur. As leak is increased, first biphasic decay and then slowed monophasic decay is seen. The level of leak has major effects on the shape of the Ca transient.

Selective and non-selective non-steroidal anti-inflammatory drugs differentially regulate pulmonary vein and atrial arrhythmogenesis

BACKGROUND

Non-steroidal anti-inflammatory drugs (NSAIDs) increase the risk of atrial fibrillation (AF). This study investigated whether selective and non-selective NSAIDs differentially regulate the arrhythmogenesis of pulmonary veins and atria.

METHODS

Conventional microelectrodes were used to record action potentials (APs) in isolated rabbit PVs, sinoatrial node (SAN), left atrium (LA), and right atrium (RA) preparations before and after celecoxib or indomethacin administration. A whole-cell patch clamp was used to record the sodium-calcium exchanger (NCX) current, L-type calcium current (ICa-L), and late sodium current (INa-late) before and after celecoxib administration in isolated PV cardiomyocytes.

RESULTS

Celecoxib (0.3, 1, and 3 μM) reduced PV spontaneous beating rates, and induced delayed afterdepolarizations and burst firings in four of eight PV preparations (50%, p<0.05). Celecoxib also reduced SAN beating rates and decreased AP durations (APDs) in RA and LA, but did not change the resting membrane potential. Indomethacin (0.3, 1, 3, and 10 μM) changed neither the PV or SAN beating rates nor RA APDs, but it reduced LA APDs. Celecoxib (3 μM) significantly increased the NCX current and decreased the ICa-L, but did not change the INa-late. Ranolazine (10 μM) suppressed celecoxib (3 μM)-induced PV burst firings in 6 (86%, p<0.05) of 7 PVs. KB-R7943 (10 μM) suppressed celecoxib (3 μM)-induced PV burst firings in 5 (71%, p<0.05) of 7 PVs.

CONCLUSIONS

Selective and non-selective NSAIDs differentially modulate PV and atrial electrophysiological characteristics. Celecoxib increased PV triggered activity through enhancement of the NCX current, which contributed to its arrhythmogenesis.

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.

Cardiac myocyte Z-line calmodulin is mainly RyR2-bound, and reduction is arrhythmogenic and occurs in heart failure

RATIONALE

Calmodulin (CaM) associates with cardiac ryanodine receptor type-2 (RyR2) as an important regulator. Defective CaM-RyR2 interaction may occur in heart failure, cardiac hypertrophy, and catecholaminergic polymorphic ventricular tachycardia. However, the in situ binding properties for CaM-RyR2 are unknown.

OBJECTIVE

We sought to measure the in situ binding affinity and kinetics for CaM-RyR2 in normal and heart failure ventricular myocytes, estimate the percentage of Z-line-localized CaM that is RyR2-bound, and test cellular function of defective CaM-RyR2 interaction.

METHODS AND RESULTS

Using fluorescence resonance energy transfer in permeabilized myocytes, we specifically resolved RyR2-bound CaM from other potential binding targets and measured CaM-RyR2 binding affinity in situ (Kd=10-20 nmol/L). Using RyR2(ADA/+) knock-in mice, in which half of the CaM-RyR2 binding is suppressed, we estimated that >90% of Z-line CaM is RyR2-bound. Functional tests indicated a higher propensity for Ca2+ wave production and stress-induced ventricular arrhythmia in RyR2(ADA/+) mice. In a post-myocardial infarction rat heart failure model, we detected a decrease in the CaM-RyR2 binding affinity (Kd≈51 nmol/L; ≈3-fold increase) and unaltered RyR2 affinity for the FK506-binding protein FKBP12.6 (Kd~0.8 nmol/L).

CONCLUSIONS

CaM binds to RyR2 with high affinity in cardiac myocytes. Physiologically, CaM is bound to >70% of RyR2 monomers and inhibits sarcoplasmic reticulum Ca2+ release. RyR2 is the major binding site for CaM along the Z-line in cardiomyocytes, and dissociating CaM from RyR2 can cause severe ventricular arrhythmia. In heart failure, RyR2 shows decreased CaM affinity, but unaltered FKBP 12.6 affinity.

The arrhythmogenic consequences of increasing late INa in the cardiomyocyte

This review presents the roles of cardiac sodium channel NaV1.5 late current (late INa) in generation of arrhythmic activity. The assumption of the authors is that proper Na(+) channel function is necessary to the maintenance of the transmembrane electrochemical gradient of Na(+) and regulation of cardiac electrical activity. Myocyte Na(+) channels' openings during the brief action potential upstroke contribute to peak INa and initiate excitation-contraction coupling. Openings of Na(+) channels outside the upstroke contribute to late INa, a depolarizing current that persists throughout the action potential plateau. The small, physiological late INa does not appear to be critical for normal electrical or contractile function in the heart. Late INa does, however, reduce the net repolarizing current, prolongs action potential duration, and increases cellular Na(+) loading. An increase of late INa, due to acquired conditions (e.g. heart failure) or inherited Na(+) channelopathies, facilitates the formation of early and delayed afterpolarizations and triggered arrhythmias, spontaneous diastolic depolarization, and cellular Ca(2+) loading. These in turn increase the spatial and temporal dispersion of repolarization time and may lead to reentrant arrhythmias.

Abnormal propagation of calcium waves and ultrastructural remodeling in recessive catecholaminergic polymorphic ventricular tachycardia

RATIONALE

The recessive form of catecholaminergic polymorphic ventricular tachycardia is caused by mutations in the cardiac calsequestrin-2 gene; this variant of catecholaminergic polymorphic ventricular tachycardia is less well characterized than the autosomal-dominant form caused by mutations in the ryanodine receptor-2 gene.

OBJECTIVE

We characterized the intracellular Ca²⁺ homeostasis, electrophysiological properties, and ultrastructural features of the Ca²⁺ release units in the homozygous calsequestrin 2-R33Q knock-in mouse model (R33Q) R33Q knock-in mouse model.

METHODS AND RESULTS

We studied isolated R33Q and wild-type ventricular myocytes and observed properties not previously identified in a catecholaminergic polymorphic ventricular tachycardia model. As compared with wild-type cells, R33Q myocytes (1) show spontaneous Ca²⁺ waves unable to propagate as cell-wide waves; (2) show smaller Ca²⁺sparks with shortened coupling intervals, suggesting a reduced refractoriness of Ca²⁺ release events; (3) have a reduction of the area of membrane contact, of the junctions between junctional sarcoplasmic reticulum and T tubules (couplons), and of junctional sarcoplasmic reticulum volume; (4) have a propensity to develop phase 2 to 4 afterdepolarizations that can elicit triggered beats; and (5) involve viral gene transfer with wild-type cardiac calsequestrin-2 that is able to normalize structural abnormalities and to restore cell-wide calcium wave propagation.

CONCLUSIONS

Our data show that homozygous cardiac calsequestrin-2-R33Q myocytes develop spontaneous Ca²⁺ release events with a broad range of intervals coupled to preceding beats, leading to the formation of early and delayed afterdepolarizations. They also display a major disruption of the Ca²⁺ release unit architecture that leads to fragmentation of spontaneous Ca²⁺ waves. We propose that these 2 substrates in R33Q myocytes synergize to provide a new arrhythmogenic mechanism for catecholaminergic polymorphic ventricular tachycardia.

Phosphorylation at Ser²⁶ in the ATP-binding site of Ca²⁺/calmodulin-dependent kinase II as a mechanism for switching off the kinase activity

CaMKII (Ca²⁺/calmodulin-dependent kinase II) is a serine/threonine phosphotransferase that is capable of long-term retention of activity due to autophosphorylation at a specific threonine residue within each subunit of its oligomeric structure. The γ isoform of CaMKII is a significant regulator of vascular contractility. Here, we show that phosphorylation of CaMKII γ at Ser²⁶, a residue located within the ATP-binding site, terminates the sustained activity of the enzyme. To test the physiological importance of phosphorylation at Ser²⁶, we generated a phosphospecific Ser²⁶ antibody and demonstrated an increase in Ser²⁶ phosphorylation upon depolarization and contraction of blood vessels. To determine if the phosphorylation of Ser²⁶ affects the kinase activity, we mutated Ser²⁶ to alanine or aspartic acid. The S26D mutation mimicking the phosphorylated state of CaMKII causes a dramatic decrease in Thr²⁸⁷ autophosphorylation levels and greatly reduces the catalytic activity towards an exogenous substrate (autocamtide-3), whereas the S26A mutation has no effect. These data combined with molecular modelling indicate that a negative charge at Ser²⁶ of CaMKII γ inhibits the catalytic activity of the enzyme towards its autophosphorylation site at Thr²⁸⁷ most probably by blocking ATP binding. We propose that Ser²⁶ phosphorylation constitutes an important mechanism for switching off CaMKII activity.