The role of spatial organization of Ca2+ release sites in the generation of arrhythmogenic diastolic Ca2+ release in myocytes from failing hearts

The ryanodine receptor microdomain in cardiomyocytes

The ryanodine receptor type 2 (RyR) is a key player in Ca2+ handling during excitation-contraction coupling. During each heartbeat, RyR channels are responsible for linking the action potential with the contractile machinery of the cardiomyocyte by releasing Ca2+ from the sarcoplasmic reticulum. RyR function is fine-tuned by associated signalling molecules, arrangement in clusters and subcellular localization. These parameters together define RyR function within microdomains and are subject to disease remodelling. This review describes the latest findings on RyR microdomain organization, the alterations with disease which result in increased subcellular heterogeneity and emergence of microdomains with enhanced arrhythmogenic potential, and presents novel technologies that guide future research to study and target RyR channels within specific microdomains.

Acute Detubulation of Ventricular Myocytes Amplifies the Inhibitory Effect of Cholinergic Agonist on Intracellular Ca2+ Transients

Muscarinic receptors expressed in cardiac myocytes play a critical role in the regulation of heart function by the parasympathetic nervous system. How the structural organization of cardiac myocytes affects the regulation of Ca²⁺ handling by muscarinic receptors is not well-defined. Using confocal Ca²⁺ imaging, patch-clamp techniques, and immunocytochemistry, the relationship between t-tubule density and cholinergic regulation of intracellular Ca²⁺ in normal murine ventricular myocytes and myocytes with acute disruption of the t-tubule system caused by formamide treatment was studied. The inhibitory effect of muscarinic receptor agonist carbachol (CCh, 10 μM) on the amplitude of Ca²⁺ transients, evoked by field-stimulation in the presence of 100 nM isoproterenol (Iso), a β-adrenergic agonist, was directly proportional to the level of myocyte detubulation. The timing of the maximal rate of fluorescence increase of fluo-4, a Ca²⁺-sensitive dye, was used to classify image pixels into the regions functionally coupled or uncoupled to the sarcolemmal Ca²⁺ influx (I_Ca). CCh decreased the fraction of coupled regions and suppressed Ca²⁺ propagation from sarcolemma inside the cell. Formamide treatment reduced I_Ca density and decreased sarcoplasmic reticulum (SR) Ca²⁺ content. CCh did not change SR Ca²⁺ content in Iso-stimulated control and formamide-treated myocytes. CCh inhibited peak I_Ca recorded in the presence of Iso by ∼20% in both the control and detubulated myocytes. Reducing I_Ca amplitude up to 40% by changing the voltage step levels from 0 to –25 mV decreased Ca²⁺ transients in formamide-treated but not in control myocytes in the presence of Iso. CCh inhibited CaMKII activity, whereas CaMKII inhibition with KN93 mimicked the effect of CCh on Ca²⁺ transients in formamide-treated myocytes. It was concluded that the downregulation of t-tubules coupled with the diminished efficiency of excitation–contraction coupling, increases the sensitivity of Ca²⁺ release and propagation to muscarinic receptor-mediated inhibition of both I_Ca and CaMKII activity.

CaMKII inhibition reduces arrhythmogenic Ca2+ events in subendocardial cryoinjured rat living myocardial slices

Spontaneous Ca²⁺ release (SCR) can cause triggered activity and initiate arrhythmias. Intrinsic transmural heterogeneities in Ca²⁺ handling and their propensity to disease remodeling may differentially modulate SCR throughout the left ventricular (LV) wall and cause transmural differences in arrhythmia susceptibility. Here, we aimed to dissect the effect of cardiac injury on SCR in different regions in the intact LV myocardium using cryoinjury on rat living myocardial slices (LMS). We studied SCR under proarrhythmic conditions using a fluorescent Ca²⁺ indicator and high-resolution imaging in LMS from the subendocardium (ENDO) and subepicardium (EPI). Cryoinjury caused structural remodeling, with loss in T-tubule density and an increased time of Ca²⁺ transients to peak after injury. In ENDO LMS, the Ca²⁺ transient amplitude and decay phase were reduced, while these were not affected in EPI LMS after cryoinjury. The frequency of spontaneous whole-slice contractions increased in ENDO LMS without affecting EPI LMS after injury. Cryoinjury caused an increase in foci that generates SCR in both ENDO and EPI LMS. In ENDO LMS, SCRs were more closely distributed and had reduced latencies after cryoinjury, whereas this was not affected in EPI LMS. Inhibition of CaMKII reduced the number, distribution, and latencies of SCR, as well as whole-slice contractions in ENDO LMS, but not in EPI LMS after cryoinjury. Furthermore, CaMKII inhibition did not affect the excitation–contraction coupling in cryoinjured ENDO or EPI LMS. In conclusion, we demonstrate increased arrhythmogenic susceptibility in the injured ENDO. Our findings show involvement of CaMKII and highlight the need for region-specific targeting in cardiac therapies.

Pyridostigmine improves cardiac function and rhythmicity through RyR2 stabilization and inhibition of STIM1-mediated calcium entry in heart failure

Heart failure (HF) is characterized by asymmetrical autonomic balance. Treatments to restore parasympathetic activity in human heart failure trials have shown beneficial effects. However, mechanisms of parasympathetic-mediated improvement in cardiac function remain unclear. The present study examined the effects and underpinning mechanisms of chronic treatment with the cholinesterase inhibitor, pyridostigmine (PYR), in pressure overload HF induced by transverse aortic constriction (TAC) in mice. TAC mice exhibited characteristic adverse structural (left ventricular hypertrophy) and functional remodelling (reduced ejection fraction, altered myocyte calcium (Ca) handling, increased arrhythmogenesis) with enhanced predisposition to arrhythmogenic aberrant sarcoplasmic reticulum (SR) Ca release, cardiac ryanodine receptor (RyR2) hyper-phosphorylation and up-regulated store-operated Ca entry (SOCE). PYR treatment resulted in improved cardiac contractile performance and rhythmic activity relative to untreated TAC mice. Chronic PYR treatment inhibited altered intracellular Ca handling by alleviating aberrant Ca release and diminishing pathologically enhanced SOCE in TAC myocytes. At the molecular level, these PYR-induced changes in Ca handling were associated with reductions of pathologically enhanced phosphorylation of RyR2 serine-2814 and STIM1 expression in HF myocytes. These results suggest that chronic cholinergic augmentation alleviates HF via normalization of both canonical RyR2-mediated SR Ca release and non-canonical hypertrophic Ca signaling via STIM1-dependent SOCE.

MD1 Depletion Predisposes to Ventricular Arrhythmias in the Setting of Myocardial Infarction

BACKGROUND

Myeloid differentiation protein 1 (MD1) is expressed in the human heart and is a negative regulator of Toll-like receptor 4 (TLR4) signalling. MD1 exerts anti-arrhythmic effects.

AIM

The aim of this study was to determine the role of MD1 in myocardial infarction (MI)-related ventricular arrhythmias (VAs).

METHOD

Myocardial infarction was induced by surgical ligation of the left anterior coronary artery in MD1 knockout (KO) mice and their wild-type littermates. Myocardial infarction-induced vulnerability to VAs and its underlying mechanisms were evaluated.

RESULTS

Myeloid differentiation protein 1 was downregulated in the MI mice. Myeloid differentiation protein 1 deficiency decreased post-MI left ventricular (LV) function and increased the infarct size. The MI mice exhibited prolonged action potential duration (APD), enhanced APD alternans thresholds, and a higher incidence of VAs. Myocardial infarction-induced LV fibrosis and inflammation decreased the expression levels of Kv4.2, Kv4.3, Kv1.5, and Kv2.1, increased Cav1.2 expression, and disturbed Ca²⁺ handling protein expression. These MI-induced adverse effects were further exacerbated in KO mice. Mechanistically, MD1 deletion markedly enhanced the activation of the TLR4/calmodulin-dependent protein kinase II (CaMKII) signalling pathway in post-MI mice.

CONCLUSIONS

Myeloid differentiation protein 1 deletion increases the vulnerability to VAs in post-MI mice. This is mainly caused by the aggravated maladaptive LV fibrosis and inflammation and interference with the expressions of ion channels and Ca²⁺ handling proteins, which is related to enhanced activation of the TLR4/CaMKII signalling pathway.

The Protective Effect of Qishen Granule on Heart Failure after Myocardial Infarction through Regulation of Calcium Homeostasis

Qishen granule (QSG) is a frequently prescribed traditional Chinese medicine formula, which improves heart function in patients with heart failure (HF). However, the cardioprotective mechanisms of QSG have not been fully understood. The current study aimed to elucidate whether the effect of QSG is mediated by ameliorating cytoplasmic calcium (Ca²⁺) overload in cardiomyocytes. The HF rat model was induced by left anterior descending (LAD) artery ligation surgery. Rats were randomly divided into sham, model, QSG-low dosage (QSG-L) treatment, QSG-high dosage (QSG-H) treatment, and positive drug (diltiazem) treatment groups. 28 days after surgery, cardiac functions were assessed by echocardiography. Levels of norepinephrine (NE) and angiotensin II (AngII) in the plasma were evaluated. Expressions of critical proteins in the calcium signaling pathway, including cell membrane calcium channel CaV1.2, sarcoendoplasmic reticulum ATPase 2a (SERCA2a), calcium/calmodulin-dependent protein kinase type II (CaMKII), and protein phosphatase calcineurin (CaN), were measured by Western blotting (WB) and immunohistochemistry (IHC). Echocardiography showed that left ventricular ejection fraction (EF) and fractional shortening (FS) value significantly decreased in the model group compared to the sham group, and illustrating heart function was severely impaired. Furthermore, levels of NE and AngII in the plasma were dramatically increased. Expressions of CaV1.2, CaMKII, and CaN in the cardiomyocytes were upregulated, and expressions of SERCA2a were downregulated in the model group. After treatment with QSG, both EF and FS values were increased. QSG significantly reduced levels of NE and AngII in the plasma. In particular, QSG prevented cytoplasmic Ca²⁺ overload by downregulating expression of CaV1.2 and upregulating expression of SERCA2a. Meanwhile, expressions of CaMKII and CaN were inhibited by QSG treatment. In conclusion, QSG could effectively promote heart function in HF rats by restoring cardiac Ca²⁺ homeostasis. These findings revealed novel therapeutic mechanisms of QSG and provided potential targets in the treatment of HF.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Xanthine oxidase inhibitor allopurinol improves atrial electrical remodeling in diabetic rats by inhibiting CaMKII/NCX signaling

AIMS

Atrial fibrillation (AF) is a common arrhythmia which is associated with higher risk of stroke, heart failure and all-cause mortality. Abnormal Ca²⁺ handling in diabetes mellitus (DM) can cause delayed depolarization involved with increased NCX activity. Complicated mechanisms are involved in atrial remodeling, of which CaMKII may be a key node signal. Therefore, we intend to explore whether CaMKII activation induces atrial electrical remodeling by regulating NCX expression in this study.

MAIN METHODS

Adult male SD rats were used to establish a diabetic rat model, divided into three groups: the control group, DM group and allopurinol group. Hemodynamic and ECG indicators were recorded, after which electrophysiological studies were conducted. The protein expression of CaMKII, p-CaMKII, XO, MnSOD and NCX was measured by Western blot and immunohistochemistry. H&E and Masson staining were applied for observing myocardial fibrosis. HL-1 cells were cultured for the measurement of ROS generation.

KEY FINDINGS

The arrangement of atrial myocytes was disordered and the collagen volume fraction of the atrium tissue was elevated in the DM group compared with the control group, and improved by allopurinol. Higher incidence of inducible AF, reduced conduction velocity and higher conduction inhomogeneity were observed in diabetic rats. These electrophysiological abnormalities were accompanied by higher oxidative stress and protein expression of p-CaMKII and NCX. Allopurinol prevented the development of these abnormal changes.

SIGNIFICANCE

Allopurinol can improve atrial electrical remodeling by inhibiting CaMKII activity and protein expression of NCX. These data indicate xanthine oxidase inhibition can reduce oxidative stress and ameliorate atrial electrical remodeling.

Sacubitrilat reduces pro-arrhythmogenic sarcoplasmic reticulum Ca2+ leak in human ventricular cardiomyocytes of patients with end-stage heart failure

Aims

Inhibition of neprilysin and angiotensin II receptor by sacubitril/valsartan (Val) (LCZ696) reduces mortality in heart failure (HF) patients compared with sole inhibition of renin–angiotensin system. Beneficial effects of increased natriuretic peptide levels upon neprilysin inhibition have been proposed, whereas direct effects of sacubitrilat (Sac) (LBQ657) on myocardial Ca²⁺ cycling remain elusive.

Methods and results

Confocal microscopy (Fluo‐4 AM) was used to investigate pro‐arrhythmogenic sarcoplasmic reticulum (SR) Ca²⁺ leak in freshly isolated murine and human ventricular cardiomyocytes (CMs) upon Sac (40 μmol/L)/Val (13 μmol/L) treatment. The concentrations of Sac and Val equalled plasma concentrations of LCZ696 treatment used in PARADIGM‐HF trial. Epifluorescence microscopy measurements (Fura‐2 AM) were performed to investigate effects on systolic Ca²⁺ release, SR Ca²⁺ load, and Ca²⁺‐transient kinetics in freshly isolated murine ventricular CMs. The impact of Sac on myocardial contractility was evaluated using in toto‐isolated, isometrically twitching ventricular trabeculae from human hearts with end‐stage HF. Under basal conditions, the combination of Sac/Val did not influence diastolic Ca²⁺‐spark frequency (CaSpF) nor pro‐arrhythmogenic SR Ca² leak in isolated murine ventricular CMs (n CMs/hearts = 80/7 vs. 100/7, P = 0.91/0.99). In contrast, Sac/Val treatment reduced CaSpF by 35 ± 9% and SR Ca²⁺ leak by 45 ± 9% in CMs put under catecholaminergic stress (isoproterenol 30 nmol/L, n = 81/7 vs. 62/7, P < 0.001 each). This could be attributed to Sac, as sole Sac treatment also reduced both parameters by similar degrees (reduction of CaSpF by 57 ± 7% and SR Ca²⁺ leak by 76 ± 5%; n = 101/4 vs. 108/4, P < 0.01 each), whereas sole Val treatment did not. Systolic Ca²⁺ release, SR Ca²⁺ load, and Ca²⁺‐transient kinetics including SERCA activity (k_SERCA) were not compromised by Sac in isolated murine CMs (n = 41/6 vs. 39/6). Importantly, the combination of Sac/Val and Sac alone also reduced diastolic CaSpF and SR Ca²⁺ leak (reduction by 74 ± 7%) in human left ventricular CMs from patients with end‐stage HF (n = 71/8 vs. 78/8, P < 0.05 each). Myocardial contractility of human ventricular trabeculae was not acutely affected by Sac treatment as the developed force remained unchanged over a time course of 30 min (n trabeculae/hearts = 3/3 vs. 4/3).

Conclusion

This study demonstrates that neprilysin inhibitor Sac directly improves Ca²⁺ homeostasis in human end‐stage HF by reducing pro‐arrhythmogenic SR Ca²⁺ leak without acutely affecting systolic Ca²⁺ release and inotropy. These effects might contribute to the mortality benefits observed in the PARADIGM‐HF trial.

Tetrodotoxin-Sensitive Neuronal-Type Na+ Channels: A Novel and Druggable Target for Prevention of Atrial Fibrillation

Background

Atrial fibrillation (AF) is a comorbidity associated with heart failure and catecholaminergic polymorphic ventricular tachycardia. Despite the Ca²⁺‐dependent nature of both of these pathologies, AF often responds to Na⁺ channel blockers. We investigated how targeting interdependent Na⁺/Ca²⁺ dysregulation might prevent focal activity and control AF.

Methods and Results

We studied AF in 2 models of Ca²⁺‐dependent disorders, a murine model of catecholaminergic polymorphic ventricular tachycardia and a canine model of chronic tachypacing‐induced heart failure. Imaging studies revealed close association of neuronal‐type Na⁺ channels (nNa_v) with ryanodine receptors and Na⁺/Ca²⁺ exchanger. Catecholamine stimulation induced cellular and in vivo atrial arrhythmias in wild‐type mice only during pharmacological augmentation of nNa_v activity. In contrast, catecholamine stimulation alone was sufficient to elicit atrial arrhythmias in catecholaminergic polymorphic ventricular tachycardia mice and failing canine atria. Importantly, these were abolished by acute nNa_v inhibition (tetrodotoxin or riluzole) implicating Na⁺/Ca²⁺ dysregulation in AF. These findings were then tested in 2 nonrandomized retrospective cohorts: an amyotrophic lateral sclerosis clinic and an academic medical center. Riluzole‐treated patients adjusted for baseline characteristics evidenced significantly lower incidence of arrhythmias including new‐onset AF, supporting the preclinical results.

Conclusions

These data suggest that nNa_Vs mediate Na⁺‐Ca²⁺ crosstalk within nanodomains containing Ca²⁺ release machinery and, thereby, contribute to AF triggers. Disruption of this mechanism by nNa_v inhibition can effectively prevent AF arising from diverse causes.

Hyperactive ryanodine receptors in human heart failure and ischaemic cardiomyopathy reside outside of couplons

Aims

In ventricular myocytes from humans and large mammals, the transverse and axial tubular system (TATS) network is less extensive than in rodents with consequently a greater proportion of ryanodine receptors (RyRs) not coupled to this membrane system. TATS remodelling in heart failure (HF) and after myocardial infarction (MI) increases the fraction of non-coupled RyRs. Here we investigate whether this remodelling alters the activity of coupled and non-coupled RyR sub-populations through changes in local signalling. We study myocytes from patients with end-stage HF, compared with non-failing (non-HF), and myocytes from pigs with MI and reduced left ventricular (LV) function, compared with sham intervention (SHAM).

Methods and results

Single LV myocytes for functional studies were isolated according to standard protocols. Immunofluorescent staining visualized organization of TATS and RyRs. Ca²⁺ was measured by confocal imaging (fluo-4 as indicator) and using whole-cell patch-clamp (37°C). Spontaneous Ca²⁺ release events, Ca²⁺ sparks, as a readout for RyR activity were recorded during a 15 s period following conditioning stimulation at 2 Hz. Sparks were assigned to cell regions categorized as coupled or non-coupled sites according to a previously developed method. Human HF myocytes had more non-coupled sites and these had more spontaneous activity than in non-HF. Hyperactivity of these non-coupled RyRs was reduced by Ca²⁺/calmodulin-dependent kinase II (CaMKII) inhibition. Myocytes from MI pigs had similar changes compared with SHAM controls as seen in human HF myocytes. As well as by CaMKII inhibition, in MI, the increased activity of non-coupled sites was inhibited by mitochondrial reactive oxygen species (mito-ROS) scavenging. Under adrenergic stimulation, Ca²⁺ waves were more frequent and originated at non-coupled sites, generating larger Na⁺/Ca²⁺ exchange currents in MI than in SHAM. Inhibition of CaMKII or mito-ROS scavenging reduced spontaneous Ca²⁺ waves, and improved excitation–contraction coupling.

Conclusions

In HF and after MI, RyR microdomain re-organization enhances spontaneous Ca²⁺ release at non-coupled sites in a manner dependent on CaMKII activation and mito-ROS production. This specific modulation generates a substrate for arrhythmia that appears to be responsive to selective pharmacologic modulation.

Gene Transfer of Engineered Calmodulin Alleviates Ventricular Arrhythmias in a Calsequestrin-Associated Mouse Model of Catecholaminergic Polymorphic Ventricular Tachycardia

Background

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a familial arrhythmogenic syndrome characterized by sudden death. There are several genetic forms of CPVT associated with mutations in genes encoding the cardiac ryanodine receptor (RyR2) and its auxiliary proteins including calsequestrin (CASQ2) and calmodulin (CaM). It has been suggested that impairment of the ability of RyR2 to stay closed (ie, refractory) during diastole may be a common mechanism for these diseases. Here, we explore the possibility of engineering CaM variants that normalize abbreviated RyR2 refractoriness for subsequent viral‐mediated delivery to alleviate arrhythmias in non–CaM‐related CPVT.

Methods and Results

To that end, we have designed a CaM protein (GSH‐M37Q; dubbed as therapeutic CaM or T‐CaM) that exhibited a slowed N‐terminal Ca dissociation rate and prolonged RyR2 refractoriness in permeabilized myocytes derived from CPVT mice carrying the CASQ2 mutation R33Q. This T‐CaM was introduced to the heart of R33Q mice through recombinant adeno‐associated viral vector serotype 9. Eight weeks postinfection, we performed confocal microscopy to assess Ca handling and recorded surface ECGs to assess susceptibility to arrhythmias in vivo. During catecholamine stimulation with isoproterenol, T‐CaM reduced isoproterenol‐promoted diastolic Ca waves in isolated CPVT cardiomyocytes. Importantly, T‐CaM exposure abolished ventricular tachycardia in CPVT mice challenged with catecholamines.

Conclusions

Our results suggest that gene transfer of T‐CaM by adeno‐associated viral vector serotype 9 improves myocyte Ca handling and alleviates arrhythmias in a calsequestrin‐associated CPVT model, thus supporting the potential of a CaM‐based antiarrhythmic approach as a therapeutic avenue for genetically distinct forms of CPVT.

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.

The role of luminal Ca regulation in Ca signaling refractoriness and cardiac arrhythmogenesis

Györke et al. discuss the role of sarcoplasmic reticulum Ca2+ in cardiac refractoriness and pathological implications.