Paradoxical effect of increased diastolic Ca(2+) release and decreased sinoatrial node activity in a mouse model of catecholaminergic polymorphic ventricular tachycardia

Genetic basis and causal relationship between atrial fibrillation and sinus node dysfunction: Evidence from comprehensive genetic analysis

BACKGROUND

Atrial fibrillation (AF) and sinus node dysfunction (SND) are commonly observed together clinically. However, little is known about the genetic background and causal relationship between the two.

METHODS

Firstly, we investigated the global and local genetic correlations between AF and SND using LDSC and HESS. Then, we identified shared "Novel SNPs" between AF and SND through two complementary cross-trait meta-analyses and mapped the "pleiotropic genes" behind these SNPs, validated by colocalization analysis. Additionally, we explored the degree of genetic enrichment of SNPs in specific tissues using LDSC-SEG and MAGMA, and identified potential functional genes in tissues using summary data-based Mendelian randomization (SMR). Finally, two-sample Mendelian randomization (TSMR) and multivariable Mendelian randomization (MVMR) were used to explore the causal relationship between AF and SND.

RESULTS

Both global and local genetic correlation analyses revealed a high positive genetic correlation between AF and SND. HESS identified 9 shared loci, with chr4(q25-q26) and chr11(p11.12-q11) being prominent. Cross-trait meta-analysis and colocalization analysis identified ENPEP and PITX2 as novel pleiotropic genes. MAGMA revealed genetic enrichment of SNPs for AF and SND in the "Heart Left Ventricle" and "Heart Atrial Appendage" tissues, with CEP68 and BEST3 identified as potential functional genes through SMR. MR analysis indicated that AF increases the risk of SND, even after adjusting for confounding factors.

CONCLUSION

This study provides genetic evidence for the increased risk of SND associated with AF, identifying multiple shared risk loci and enriched tissues, and discovering 2 novel pleiotropic genes and 2 new functional genes.

Dual effect of cardiac FKBP12.6 overexpression on excitation-contraction coupling and the incidence of ventricular arrhythmia depending on its expression level

FKBP12.6, a binding protein to the immunosuppressant FK506, which also binds the ryanodine receptor (RyR2) in the heart, has been proposed to regulate RyR2 function and to have antiarrhythmic properties. However, the level of FKBP12.6 expression in normal hearts remains elusive and some controversies still persist regarding its effects, both in basal conditions and during β-adrenergic stimulation. We quantified FKBP12.6 in the left ventricles (LV) of WT (wild-type) mice and in two novel transgenic models expressing distinct levels of FKBP12.6, using a custom-made specific anti-FKBP12.6 antibody and a recombinant protein. FKBP12.6 level in WT LV was very low (0.16 ± 0.02 nmol/g of LV), indicating that <15% RyR2 monomers are bound to the protein. Mice with 14.1 ± 0.2 nmol of FKBP12.6 per g of LV (TG1) had mild cardiac hypertrophy and normal function and were protected against epinephrine/caffeine-evoked arrhythmias. The ventricular myocytes showed higher [Ca2+]i transient amplitudes than WT myocytes and normal SR-Ca2+ load, while fewer myocytes showed Ca2+ sparks. TG1 cardiomyocytes responded to 50 nM Isoproterenol increasing these [Ca2+]i parameters and producing RyR2-Ser2808 phosphorylation. Mice with more than twice the TG1 FKBP12.6 value (TG2) showed marked cardiac hypertrophy with calcineurin activation and more arrhythmias than WT mice during β-adrenergic stimulation, challenging the protective potential of high FKBP12.6. RyR2R420Q CPVT mice overexpressing FKBP12.6 showed fewer proarrhythmic events and decreased incidence and duration of stress-induced bidirectional ventricular tachycardia. Our study, therefore, quantifies for the first time endogenous FKBP12.6 in the mouse heart, questioning its physiological relevance, at least at rest due its low level. By contrast, our work demonstrates that with caution FKBP12.6 remains an interesting target for the development of new antiarrhythmic therapies.

Synergy between Membrane Currents Prevents Severe Bradycardia in Mouse Sinoatrial Node Tissue

Bradycardia is initiated by the sinoatrial node (SAN), which is regulated by a coupled-clock system. Due to the clock coupling, reduction in the 'funny' current (I_f), which affects SAN automaticity, can be compensated, thus preventing severe bradycardia. We hypothesize that this fail-safe system is an inherent feature of SAN pacemaker cells and is driven by synergy between I_f and other ion channels. This work aimed to characterize the connection between membrane currents and their underlying mechanisms in SAN cells. SAN tissues were isolated from C57BL mice and Ca²⁺ signaling was measured in pacemaker cells within them. A computational model of SAN cells was used to understand the interactions between cell components. Beat interval (BI) was prolonged by 54 ± 18% (N = 16) and 30 ± 9% (N = 21) in response to I_f blockade, by ivabradine, or sodium current (I_Na) blockade, by tetrodotoxin, respectively. Combined drug application had a synergistic effect, manifested by a BI prolonged by 143 ± 25% (N = 18). A prolongation in the local Ca²⁺ release period, which reports on the level of crosstalk within the coupled-clock system, was measured and correlated with the prolongation in BI. The computational model predicted that I_Na increases in response to I_f blockade and that this connection is mediated by changes in T and L-type Ca²⁺ channels.

Sinus node dysfunction: current understanding and future directions

The sinoatrial node (SAN) is the primary pacemaker of the heart. Normal SAN function is crucial in maintaining proper cardiac rhythm and contraction. Sinus node dysfunction (SND) is due to abnormalities within the SAN, which can affect the heartbeat frequency, regularity, and the propagation of electrical pulses through the cardiac conduction system. As a result, SND often increases the risk of cardiac arrhythmias. SND is most commonly seen as a disease of the elderly given the role of degenerative fibrosis as well as other age-dependent changes in its pathogenesis. Despite the prevalence of SND, current treatment is limited to pacemaker implantation, which is associated with substantial medical costs and complications. Emerging evidence has identified various genetic abnormalities that can cause SND, shedding light on the molecular underpinnings of SND. Identification of these molecular mechanisms and pathways implicated in the pathogenesis of SND is hoped to identify novel therapeutic targets for the development of more effective therapies for this disease. In this review article, we examine the anatomy of the SAN and the pathophysiology and epidemiology of SND. We then discuss in detail the most common genetic mutations correlated with SND and provide our perspectives on future research and therapeutic opportunities in this field.

Characterization of sinoatrial automaticity in Microcebus murinus to study the effect of aging on cardiac activity and the correlation with longevity

Microcebus murinus, or gray mouse lemur (GML), is one of the smallest primates known, with a size in between mice and rats. The small size, genetic proximity to humans and prolonged senescence, make this lemur an emerging model for neurodegenerative diseases. For the same reasons, it could help understand how aging affects cardiac activity. Here, we provide the first characterization of sinoatrial (SAN) pacemaker activity and of the effect of aging on GML heart rate (HR). According to GML size, its heartbeat and intrinsic pacemaker frequencies lie in between those of mice and rats. To sustain this fast automaticity the GML SAN expresses funny and Ca²⁺ currents (I_f, I_Ca,L and I_Ca,T) at densities similar to that of small rodents. SAN automaticity was also responsive to β-adrenergic and cholinergic pharmacological stimulation, showing a consequent shift in the localization of the origin of pacemaker activity. We found that aging causes decrease of basal HR and atrial remodeling in GML. We also estimated that, over 12 years of a lifetime, GML generates about 3 billion heartbeats, thus, as many as humans and three times more than rodents of equivalent size. In addition, we estimated that the high number of heartbeats per lifetime is a characteristic that distinguishes primates from rodents or other eutherian mammals, independently from body size. Thus, cardiac endurance could contribute to the exceptional longevity of GML and other primates, suggesting that GML's heart sustains a workload comparable to that of humans in a lifetime. In conclusion, despite the fast HR, GML replicates some of the cardiac deficiencies reported in old people, providing a suitable model to study heart rhythm impairment in aging. Moreover, we estimated that, along with humans and other primates, GML presents a remarkable cardiac longevity, enabling longer life span than other mammals of equivalent size.

Reduction in Junctophilin 2 Expression in Cardiac Nodal Tissue Results in Intracellular Calcium-Driven Increase in Nodal Cell Automaticity

BACKGROUND

Spontaneously depolarizing nodal cells comprise the pacemaker of the heart. Intracellular calcium (Ca2+) plays a critical role in mediating nodal cell automaticity and understanding this so-called Ca2+ clock is critical to understanding nodal arrhythmias. We previously demonstrated a role for Jph2 (junctophilin 2) in regulating Ca2+-signaling through inhibition of RyR2 (ryanodine receptor 2) Ca2+ leak in cardiac myocytes; however, its role in pacemaker function and nodal arrhythmias remains unknown. We sought to determine whether nodal Jph2 expression silencing causes increased sinoatrial and atrioventricular nodal cell automaticity due to aberrant RyR2 Ca2+ leak.

METHODS

A tamoxifen-inducible, nodal tissue-specific, knockdown mouse of Jph2 was achieved using a Cre-recombinase-triggered short RNA hairpin directed against Jph2 (Hcn4:shJph2). In vivo cardiac rhythm was monitored by surface ECG, implantable cardiac telemetry, and intracardiac electrophysiology studies. Intracellular Ca2+ imaging was performed using confocal-based line scans of isolated nodal cells loaded with fluorescent Ca2+ reporter Cal-520. Whole cell patch clamp was conducted on isolated nodal cells to determine action potential kinetics and sodium-calcium exchanger function.

RESULTS

Hcn4:shJph2 mice demonstrated a 40% reduction in nodal Jph2 expression, resting sinus tachycardia, and impaired heart rate response to pharmacologic stress. In vivo intracardiac electrophysiology studies and ex vivo optical mapping demonstrated accelerated junctional rhythm originating from the atrioventricular node. Hcn4:shJph2 nodal cells demonstrated increased and irregular Ca2+ transient generation with increased Ca2+ spark frequency and Ca2+ leak from the sarcoplasmic reticulum. This was associated with increased nodal cell AP firing rate, faster diastolic repolarization rate, and reduced sodium-calcium exchanger activity during repolarized states compared to control. Phenome-wide association studies of the JPH2 locus identified an association with sinoatrial nodal disease and atrioventricular nodal block.

CONCLUSIONS

Nodal-specific Jph2 knockdown causes increased nodal automaticity through increased Ca2+ leak from intracellular stores. Dysregulated intracellular Ca2+ underlies nodal arrhythmogenesis in this mouse model.

cAMP-PKA signaling modulates the automaticity of human iPSC-derived cardiomyocytes

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have been used to screen and characterize drugs and to reveal mechanisms underlying cardiac diseases. However, before hiPSC-CMs can be used as a reliable experimental model, the physiological mechanisms underlying their normal function should be further explored. Accordingly, a major feature of hiPSC-CMs is automaticity, which is regulated by both Ca2+ and membrane clocks. To investigate the mechanisms coupling these clocks, we tested three hypotheses: (1) normal automaticity of spontaneously beating hiPSC-CMs is regulated by local Ca2+ releases (LCRs) and cAMP/PKA-dependent coupling of Ca2+ clock to M clock; (2) the LCR period indicates the level of crosstalk within the coupled-clock system; and (3) perturbing the activity of even one clock can lead to hiPSC-CM-altered automaticity due to diminished crosstalk within the coupled-clock system. By measuring the local and global Ca2+ transients, we found that the LCRs properties are correlated with the spontaneous beat interval. Changes in cAMP-dependent coupling of the Ca2+ and M clocks, caused by a pharmacological intervention that either activates the β-adrenergic or cholinergic receptor or upregulates/downregulates PKA signaling, affected LCR properties, which in turn altered hiPSC-CMs automaticity. Clocks' uncoupling by attenuating the pacemaker current If or the sarcoplasmic reticulum Ca2+ kinetics, decreased hiPSC-CMs beating rate, and prolonged the LCR period. Finally, LCR characteristics of spontaneously beating (at comparable rates) hiPSC-CMs and rabbit SAN are similar. In conclusion, hiPSC-CM automaticity is controlled by the coupled-clock system whose function is mediated by Ca2+-cAMP-PKA signaling.

Increase in Ca2+-Activated cAMP/PKA Signaling Prevents Hydroxychloroquine-Induced Bradycardia of the Cardiac Pacemaker

Bradycardia or tachycardia are known side effects of drugs that limit their clinical use. The heart pacemaker function which control the heart rate under normal conditions is determined by coupled clock system. Thus, interfering with specific clock mechanism will affect other clock mechanisms through changes in interconnected signaling and can lead to rhythm disturbance. However, upregulation of a different clock components can compensate for this change. We focus here on hydroxychloroquine (HCQ), which has been shown effective in treating COVID-19 patients, however its bradycardic side effect limits its clinical use. We aim to decipher the mechanisms underlying the effect of HCQ on pacemaker automaticity, to identify a potential drug that will eliminate the bradycardia. We used isolated rabbit sinoatrial node (SAN) cells, human-induced pluripotent stem cell–derived cardiomyocytes (hiPSC-CMs) and mouse SAN cells residing in SAN tissue. Further, we employed SAN cell computational model to suggest mechanistic insights of the effect of HCQ on pacemaker function. HCQ increased mean spontaneous beat interval and variability in all three models in parallel to slower intracellular kinetics. The computational model suggested that HCQ affects the pacemaker (funny) current (If), L-type Ca2+ current (ICa,L), transient outward potassium (Ito) and due to changes in Ca2+ kinetics, the sodium-calcium exchanger current (INCX). Co-application of 3’-isobutylmethylxanthine (IBMX) and HCQ prevented the increase in beat interval and variability in all three experimental models. The HCQ-induced increase in rabbit and mice SAN cell and hiPSC-CM spontaneous beat interval, can be prevented by a phosphodiester inhibitor that restores automaticity due to slower intracellular Ca2+ kinetics.

Deciphering cellular signals in adult mouse sinoatrial node cells

Sinoatrial node (SAN) cells are the pacemakers of the heart. This study describes a method for culturing and infection of adult mouse SAN cells with FRET-based biosensors that can be exploited to examine signaling events. SAN cells cultured in media with blebbistatin or (S)-nitro-blebbistatin retain their morphology, protein distribution, action potential (AP) waveform, and cAMP dynamics for at least 40 h. SAN cells expressing targeted cAMP sensors show distinct β-adrenergic-mediated cAMP pools. Cyclic GMP, protein kinase A, Ca2+/CaM kinase II, and protein kinase D in SAN cells also show unique dynamics to different stimuli. Heart failure SAN cells show a decrease in cAMP and cGMP levels. In summary, a reliable method for maintaining adult mouse SAN cells in culture is presented, which facilitates studies of signaling networks and regulatory mechanisms during physiological and pathological conditions.

Phosphodiesterase type 4 anchoring regulates cAMP signaling to Popeye domain-containing proteins

Cyclic AMP is a ubiquitous second messenger used to transduce intracellular signals from a variety of Gs-coupled receptors. Compartmentalisation of protein intermediates within the cAMP signaling pathway underpins receptor-specific responses. The cAMP effector proteins protein-kinase A and EPAC are found in complexes that also contain phosphodiesterases whose presence ensures a coordinated cellular response to receptor activation events. Popeye domain containing (POPDC) proteins are the most recent class of cAMP effectors to be identified and have crucial roles in cardiac pacemaking and conduction. We report the first observation that POPDC proteins exist in complexes with members of the PDE4 family in cardiac myocytes. We show that POPDC1 preferentially binds the PDE4A sub-family via a specificity motif in the PDE4 UCR1 region and that PDE4s bind to the Popeye domain of POPDC1 in a region known to be susceptible to a mutation that causes human disease. Using a cell-permeable disruptor peptide that displaces the POPDC1-PDE4 complex we show that PDE4 activity localized to POPDC1 modulates cycle length of spontaneous Ca²⁺ transients firing in intact mouse sinoatrial nodes.

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POPDC1 forms a complex with type 4 phosphodiesterases (PDE4s) in cardiac myocytes.

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POPDC1 binds PDE4 enzymes in the Upstream Conserved Region 1 (UCR1) domain.

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The PDE4 binding motif within the Popeye domain lies in a region that harbours a mutation, which underpins human disease.

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Disruption of the POPDC1-PDE4 complex modulates the cycle length of spontaneous Ca2+ transients in the sinoatrial node.

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Disruption of the POPDC1-PDE4 complex causes a significant prolongation of the action potential repolarization phase.

Mechanisms of Sinoatrial Node Dysfunction in Heart Failure With Preserved Ejection Fraction

BACKGROUND

The ability to increase heart rate during exercise and other stressors is a key homeostatic feature of the sinoatrial node (SAN). When the physiological heart rate response is blunted, chronotropic incompetence limits exercise capacity, a common problem in patients with heart failure with preserved ejection fraction (HFpEF). Despite its clinical relevance, the mechanisms of chronotropic incompetence remain unknown.

METHODS

Dahl salt-sensitive rats fed a high-salt diet and C57Bl6 mice fed a high-fat diet and an inhibitor of constitutive nitric oxide synthase (Nω-nitro-L-arginine methyl ester [L-NAME]; 2-hit) were used as models of HFpEF. Myocardial infarction was created to induce HF with reduced ejection fraction. Rats and mice fed with a normal diet or those that had a sham surgery served as respective controls. A comprehensive characterization of SAN function and chronotropic response was conducted by in vivo, ex vivo, and single-cell electrophysiologic studies. RNA sequencing of SAN was performed to identify transcriptomic changes. Computational modeling of biophysically-detailed human HFpEF SAN was created.

RESULTS

Rats with phenotypically-verified HFpEF exhibited limited chronotropic response associated with intrinsic SAN dysfunction, including impaired β-adrenergic responsiveness and an alternating leading pacemaker within the SAN. Prolonged SAN recovery time and reduced SAN sensitivity to isoproterenol were confirmed in the 2-hit mouse model. Adenosine challenge unmasked conduction blocks within the SAN, which were associated with structural remodeling. Chronotropic incompetence and SAN dysfunction were also found in rats with HF with reduced ejection fraction. Single-cell studies and transcriptomic profiling revealed HFpEF-related alterations in both the "membrane clock" (ion channels) and the "Ca2+ clock" (spontaneous Ca2+ release events). The physiologic impairments were reproduced in silico by empirically-constrained quantitative modeling of human SAN function.

CONCLUSIONS

Chronotropic incompetence and SAN dysfunction were seen in both models of HF. We identified that intrinsic abnormalities of SAN structure and function underlie the chronotropic response in HFpEF.

Phosphoprotein Phosphatase 1 but Not 2A Activity Modulates Coupled-Clock Mechanisms to Impact on Intrinsic Automaticity of Sinoatrial Nodal Pacemaker Cells

Spontaneous AP (action potential) firing of sinoatrial nodal cells (SANC) is critically dependent on protein kinase A (PKA) and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-dependent protein phosphorylation, which are required for the generation of spontaneous, diastolic local Ca²⁺ releases (LCRs). Although phosphoprotein phosphatases (PP) regulate protein phosphorylation, the expression level of PPs and phosphatase inhibitors in SANC and the impact of phosphatase inhibition on the spontaneous LCRs and other players of the oscillatory coupled-clock system is unknown. Here, we show that rabbit SANC express both PP1, PP2A, and endogenous PP inhibitors I-1 (PPI-1), dopamine and cyclic adenosine 3′,5′-monophosphate (cAMP)-regulated phosphoprotein (DARPP-32), kinase C-enhanced PP1 inhibitor (KEPI). Application of Calyculin A, (CyA), a PPs inhibitor, to intact, freshly isolated single SANC: (1) significantly increased phospholamban (PLB) phosphorylation (by 2–3-fold) at both CaMKII-dependent Thr¹⁷ and PKA-dependent Ser¹⁶ sites, in a time and concentration dependent manner; (2) increased ryanodine receptor (RyR) phosphorylation at the Ser²⁸⁰⁹ site; (3) substantially increased sarcoplasmic reticulum (SR) Ca²⁺ load; (4) augmented L-type Ca²⁺ current amplitude; (5) augmented LCR’s characteristics and decreased LCR period in intact and permeabilized SANC, and (6) increased the spontaneous basal AP firing rate. In contrast, the selective PP2A inhibitor okadaic acid (100 nmol/L) had no significant effect on spontaneous AP firing, LCR parameters, or PLB phosphorylation. Application of purified PP1 to permeabilized SANC suppressed LCR, whereas purified PP2A had no effect on LCR characteristics. Our numerical model simulations demonstrated that PP inhibition increases AP firing rate via a coupled-clock mechanism, including respective increases in the SR Ca²⁺ pumping rate, L-type Ca²⁺ current, and Na⁺/Ca²⁺-exchanger current. Thus, PP1 and its endogenous inhibitors modulate the basal spontaneous firing rate of cardiac pacemaker cells by suppressing SR Ca²⁺ cycling protein phosphorylation, the SR Ca²⁺ load and LCRs, and L-type Ca²⁺ current.

RyR2 and Calcium Release in Heart Failure

Heart Failure (HF) is defined as the inability of the heart to efficiently pump out enough blood to maintain the body's needs, first at exercise and then also at rest. Alterations in Ca2+ handling contributes to the diminished contraction and relaxation of the failing heart. While most Ca2+ handling protein expression and/or function has been shown to be altered in many models of experimental HF, in this review, we focus in the sarcoplasmic reticulum (SR) Ca2+ release channel, the type 2 ryanodine receptor (RyR2). Various modifications of this channel inducing alterations in its function have been reported. The first was the fact that RyR2 is less responsive to activation by Ca2+ entry through the L-Type calcium channel, which is the functional result of an ultrastructural remodeling of the ventricular cardiomyocyte, with fewer and disorganized transverse (T) tubules. HF is associated with an elevated sympathetic tone and in an oxidant environment. In this line, enhanced RyR2 phosphorylation and oxidation have been shown in human and experimental HF. After several controversies, it is now generally accepted that phosphorylation of RyR2 at the Calmodulin Kinase II site (S2814) is involved in both the depressed contractile function and the enhanced arrhythmic susceptibility of the failing heart. Diminished expression of the FK506 binding protein, FKBP12.6, may also contribute. While these alterations have been mostly studied in the left ventricle of HF with reduced ejection fraction, recent studies are looking at HF with preserved ejection fraction. Moreover, alterations in the RyR2 in HF may also contribute to supraventricular defects associated with HF such as sinus node dysfunction and atrial fibrillation.

Short-term exercise affects cardiac function ex vivo partially via changes in calcium channel levels, without influencing hypoxia sensitivity

Exercise is known to improve cardiac recovery following coronary occlusion. However, whether short-term exercise can improve cardiac function and hypoxia tolerance ex vivo independent of reperfusion injury and the possible role of calcium channels in improved hypoxia tolerance remains unknown. Therefore, in the current study, heart function was measured ex vivo using the Langendorff method at different oxygen levels after a 4-week voluntary wheel-running regimen in trained and untrained male mice (C57Bl/6NCrl). The levels of cardiac Ca²⁺-channels: L-type Ca²⁺-channel (CACNA1C), ryanodine receptor (RyR-2), sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA2), and sodium-calcium exchanger were measured using western blot. Trained mice displayed lower cardiac afterload pressure generation capacity (rate and amplitude), but unaltered hypoxia tolerance when compared to untrained mice with similar heart rates. The level of CACNA1C positively correlated with the pressure generation rate and amplitude. Furthermore, the CACNA1C-RYR-2 ratio also positively correlated with the pressure generation rate. While the 4-week training period was not enough to alter the intrinsic cardiac hypoxia tolerance, interestingly it decreased pressure generation capacity and slowed pressure decreasing capacity in the mouse hearts ex vivo. This reduction in pressure generation rate could be linked to the level of channel proteins in sarcolemmal Ca²⁺-cycling in trained mice. However, the Ca^2+-channel levels did not differ significantly between the groups, and thus, the level of calcium channels cannot fully explain all the functional alterations, despite the detected correlations. Therefore, additional studies are warranted to reveal further mechanisms that contribute to the reduced intrinsic capacity for pressure production in trained mouse hearts.

ESC working group on cardiac cellular electrophysiology position paper: relevance, opportunities, and limitations of experimental models for cardiac electrophysiology research

Cardiac arrhythmias are a major cause of death and disability. A large number of experimental cell and animal models have been developed to study arrhythmogenic diseases. These models have provided important insights into the underlying arrhythmia mechanisms and translational options for their therapeutic management. This position paper from the ESC Working Group on Cardiac Cellular Electrophysiology provides an overview of (i) currently available in vitro, ex vivo, and in vivo electrophysiological research methodologies, (ii) the most commonly used experimental (cellular and animal) models for cardiac arrhythmias including relevant species differences, (iii) the use of human cardiac tissue, induced pluripotent stem cell (hiPSC)-derived and in silico models to study cardiac arrhythmias, and (iv) the availability, relevance, limitations, and opportunities of these cellular and animal models to recapitulate specific acquired and inherited arrhythmogenic diseases, including atrial fibrillation, heart failure, cardiomyopathy, myocarditis, sinus node, and conduction disorders and channelopathies. By promoting a better understanding of these models and their limitations, this position paper aims to improve the quality of basic research in cardiac electrophysiology, with the ultimate goal to facilitate the clinical translation and application of basic electrophysiological research findings on arrhythmia mechanisms and therapies.

The funny current in genetically modified mice

Since its first description in 1979, the hyperpolarization-activated funny current (I_f) has been the object of intensive research aimed at understanding its role in cardiac pacemaker activity and its modulation by the sympathetic and parasympathetic branches of the autonomic nervous system. I_f was described in isolated tissue strips of the rabbit sinoatrial node using the double-electrode voltage-clamp technique. Since then, the rabbit has been the principal animal model for studying pacemaker activity and I_f for more than 20 years. In 2001, the first study describing the electrophysiological properties of mouse sinoatrial pacemaker myocytes and those of I_f was published. It was soon followed by the description of murine myocytes of the atrioventricular node and the Purkinje fibres. The sinoatrial node of genetically modified mice has become a very popular model for studying the mechanisms of cardiac pacemaker activity. This field of research benefits from the impressive advancement of in-vivo exploration techniques of physiological parameters, imaging, genetics, and large-scale genomic approaches. The present review discusses the influence of mouse genetic on the most recent knowledge of the funny current's role in the physiology and pathophysiology of cardiac pacemaker activity. Genetically modified mice have provided important insights into the role of I_f in determining intrinsic automaticity in vivo and in myocytes of the conduction system. In addition, gene targeting of f-(HCN) channel isoforms have contributed to elucidating the current's role in the regulation of heart rate by the parasympathetic nervous system. This review is dedicated to Dario DiFrancesco on his retirement.

Impaired Binding to Junctophilin-2 and Nanostructural Alteration in CPVT Mutation

[Figure: see text].

Cellular and Molecular Mechanisms of Functional Hierarchy of Pacemaker Clusters in the Sinoatrial Node: New Insights into Sick Sinus Syndrome

The sinoatrial node (SAN), the primary pacemaker of the heart, consists of a heterogeneous population of specialized cardiac myocytes that can spontaneously produce action potentials, generating the rhythm of the heart and coordinating heart contractions. Spontaneous beating can be observed from very early embryonic stage and under a series of genetic programing, the complex heterogeneous SAN cells are formed with specific biomarker proteins and generate robust automaticity. The SAN is capable to adjust its pacemaking rate in response to environmental and autonomic changes to regulate the heart's performance and maintain physiological needs of the body. Importantly, the origin of the action potential in the SAN is not static, but rather dynamically changes according to the prevailing conditions. Changes in the heart rate are associated with a shift of the leading pacemaker location within the SAN and accompanied by alterations in P wave morphology and PQ interval on ECG. Pacemaker shift occurs in response to different interventions: neurohormonal modulation, cardiac glycosides, pharmacological agents, mechanical stretch, a change in temperature, and a change in extracellular electrolyte concentrations. It was linked with the presence of distinct anatomically and functionally defined intranodal pacemaker clusters that are responsible for the generation of the heart rhythm at different rates. Recent studies indicate that on the cellular level, different pacemaker clusters rely on a complex interplay between the calcium (referred to local subsarcolemmal Ca²⁺ releases generated by the sarcoplasmic reticulum via ryanodine receptors) and voltage (referred to sarcolemmal electrogenic proteins) components of so-called "coupled clock pacemaker system" that is used to describe a complex mechanism of SAN pacemaking. In this review, we examine the structural, functional, and molecular evidence for hierarchical pacemaker clustering within the SAN. We also demonstrate the unique molecular signatures of intranodal pacemaker clusters, highlighting their importance for physiological rhythm regulation as well as their role in the development of SAN dysfunction, also known as sick sinus syndrome.

Conserved Role of the Large Conductance Calcium-Activated Potassium Channel, KCa1.1, in Sinus Node Function and Arrhythmia Risk

BACKGROUND

KCNMA1 encodes the α-subunit of the large-conductance Ca²⁺-activated K⁺ channel, K_Ca1.1, and lies within a linkage interval for atrial fibrillation (AF). Insights into the cardiac functions of K_Ca1.1 are limited, and KCNMA1 has not been investigated as an AF candidate gene.

METHODS

The KCNMA1 gene was sequenced in 118 patients with familial AF. The role of K_Ca1.1 in normal cardiac structure and function was evaluated in humans, mice, zebrafish, and fly. A novel KCNMA1 variant was functionally characterized.

RESULTS

A complex KCNMA1 variant was identified in 1 kindred with AF. To evaluate potential disease mechanisms, we first evaluated the distribution of K_Ca1.1 in normal hearts using immunostaining and immunogold electron microscopy. K_Ca1.1 was seen throughout the atria and ventricles in humans and mice, with strong expression in the sinus node. In an ex vivo murine sinoatrial node preparation, addition of the K_Ca1.1 antagonist, paxilline, blunted the increase in beating rate induced by adrenergic receptor stimulation. Knockdown of the K_Ca1.1 ortholog, kcnma1b, in zebrafish embryos resulted in sinus bradycardia with dilatation and reduced contraction of the atrium and ventricle. Genetic inactivation of the Drosophila K_Ca1.1 ortholog, slo, systemically or in adult stages, also slowed the heartbeat and produced fibrillatory cardiac contractions. Electrophysiological characterization of slo-deficient flies revealed bursts of action potentials, reflecting increased events of fibrillatory arrhythmias. Flies with cardiac-specific overexpression of the human KCNMA1 mutant also showed increased heart period and bursts of action potentials, similar to the K_Ca1.1 loss-of-function models.

CONCLUSIONS

Our data point to a highly conserved role of K_Ca1.1 in sinus node function in humans, mice, zebrafish, and fly and suggest that K_Ca1.1 loss of function may predispose to AF.

Pharmacologic Approach to Sinoatrial Node Dysfunction

The spontaneous activity of the sinoatrial node initiates the heartbeat. Sino-atrial node dysfunction (SND) and sick sinoatrial (sick sinus) syndrome are caused by the heart's inability to generate a normal sinoatrial node action potential. In clinical practice, SND is generally considered an age-related pathology, secondary to degenerative fibrosis of the heart pacemaker tissue. However, other forms of SND exist, including idiopathic primary SND, which is genetic, and forms that are secondary to cardiovascular or systemic disease. The incidence of SND in the general population is expected to increase over the next half century, boosting the need to implant electronic pacemakers. During the last two decades, our knowledge of sino-atrial node physiology and of the pathophysiological mechanisms underlying SND has advanced considerably. This review summarizes the current knowledge about SND mechanisms and discusses the possibility of introducing new pharmacologic therapies for treating SND.

Small and Intermediate Calcium Activated Potassium Channels in the Heart: Role and Strategies in the Treatment of Cardiovascular Diseases

Calcium-activated potassium channels are a heterogeneous family of channels that, despite their different biophysical characteristics, structures, and pharmacological signatures, play a role of transducer between the ubiquitous intracellular calcium signaling and the electric variations of the membrane. Although this family of channels was extensively described in various excitable and non-excitable tissues, an increasing amount of evidences shows their functional role in the heart. This review aims to focus on the physiological role and the contribution of the small and intermediate calcium-activated potassium channels in cardiac pathologies.

Concomitant genetic ablation of L-type Cav1.3 (α1D) and T-type Cav3.1 (α1G) Ca2+ channels disrupts heart automaticity

Cardiac automaticity is set by pacemaker activity of the sinus node (SAN). In addition to the ubiquitously expressed cardiac voltage-gated L-type Ca_v1.2 Ca²⁺ channel isoform, pacemaker cells within the SAN and the atrioventricular node co-express voltage-gated L-type Ca_v1.3 and T-type Ca_v3.1 Ca²⁺ channels (SAN-VGCCs). The role of SAN-VGCCs in automaticity is incompletely understood. We used knockout mice carrying individual genetic ablation of Ca_v1.3 (Ca_v1.3^−/−) or Ca_v3.1 (Ca_v3.1^−/−) channels and double mutant Ca_v1.3^−/−/Ca_v3.1^−/− mice expressing only Ca_v1.2 channels. We show that concomitant loss of SAN-VGCCs prevents physiological SAN automaticity, blocks impulse conduction and compromises ventricular rhythmicity. Coexpression of SAN-VGCCs is necessary for impulse formation in the central SAN. In mice lacking SAN-VGCCs, residual pacemaker activity is predominantly generated in peripheral nodal and extranodal sites by f-channels and TTX-sensitive Na⁺ channels. In beating SAN cells, ablation of SAN-VGCCs disrupted late diastolic local intracellular Ca²⁺ release, which demonstrates an important role for these channels in supporting the sarcoplasmic reticulum based “Ca²⁺clock” mechanism during normal pacemaking. These data implicate an underappreciated role for co-expression of SAN-VGCCs in heart automaticity and define an integral role for these channels in mechanisms that control the heartbeat.

Maurocalcin and its analog MCaE12A facilitate Ca2+ mobilization in cardiomyocytes

Ryanodine receptors are responsible for the massive release of calcium from the sarcoplasmic reticulum that triggers heart muscle contraction. Maurocalcin (MCa) is a 33 amino acid peptide toxin known to target skeletal ryanodine receptor. We investigated the effect of MCa and its analog MCaE12A on isolated cardiac ryanodine receptor (RyR2), and showed that they increase RyR2 sensitivity to cytoplasmic calcium concentrations promoting channel opening and decreases its sensitivity to inhibiting calcium concentrations. By measuring intracellular Ca2+ transients, calcium sparks and contraction on cardiomyocytes isolated from adult rats or differentiated from human-induced pluripotent stem cells, we demonstrated that MCaE12A passively penetrates cardiomyocytes and promotes the abnormal opening of RyR2. We also investigated the effect of MCaE12A on the pacemaker activity of sinus node cells from different mice lines and showed that, MCaE12A improves pacemaker activity of sinus node cells obtained from mice lacking L-type Cav1.3 channel, or following selective pharmacologic inhibition of calcium influx via Cav1.3. Our results identify MCaE12A as a high-affinity modulator of RyR2 and make it an important tool for RyR2 structure-to-function studies as well as for manipulating Ca2+ homeostasis and dynamic of cardiac cells.

Aberrant sinus node firing during β-adrenergic stimulation leads to cardiac arrhythmias in diabetic mice

AIM

Cardiovascular complications, including cardiac arrhythmias, result in high morbidity and mortality in patients with type-2 diabetes mellitus (T2DM). Clinical and experimental data suggest electrophysiological impairment of the natural pacemaker of the diabetic heart. The present study examined sinoatrial node (SAN) arrhythmias in a mouse model of T2DM and physiologically probed their underlying cause.

METHODS

Electrocardiograms were obtained from conscious diabetic db/db and lean control db/+ mice. In vivo SAN function was probed through pharmacological autonomic modulation with isoprenaline, atropine and carbachol. Blood pressure stability and heart rate variability (HRV) were evaluated. Intrinsic SAN function was evaluated through ex vivo imaging of spontaneous Ca²⁺ transients in isolated SAN preparations.

RESULTS

While lean control mice showed constant RR intervals during isoprenaline challenge, the diabetic mice experienced SAN arrhythmias with large RR fluctuations in a dose-dependent manner. These arrhythmias were completely abolished by atropine pre-treatment, while carbachol pretreatment significantly increased SAN arrhythmia frequency in the diabetic mice. Blood pressure and HRV were comparable in db/db and db/+ mice, suggesting that neither augmented baroreceptor feedback nor autonomic neuropathy is a likely arrhythmia mechanism. Cycle length response to isoprenaline was comparable in isolated SAN preparations from db/db and db/+ mice; however, Ca²⁺ spark frequency was significantly increased in db/db mice compared to db/+ at baseline and after isoprenaline.

CONCLUSION

Our results demonstrate a dysfunction of cardiac pacemaking in an animal model of T2DM upon challenge with a β-adrenergic agonist. Ex vivo, higher Ca²⁺ spark frequency is present in diabetic mice, which may be directly linked to in vivo arrhythmias.

Distinct mechanisms mediate pacemaker dysfunction associated with catecholaminergic polymorphic ventricular tachycardia mutations: Insights from computational modeling

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a stress-induced ventricular arrhythmia associated with rhythm disturbance and impaired sinoatrial node cell (SANC) automaticity (pauses). Mutations associated with dysfunction of Ca²⁺-related mechanisms have been shown to be present in CPVT. These dysfunctions include impaired Ca²⁺ release from the ryanodine receptor (i.e., RyR2^R4496C mutation) or binding to calsequestrin 2 (CASQ2). In SANC, Ca²⁺ signaling directly and indirectly mediates pacemaker function. We address here the following research questions: (i) what coupled-clock mechanisms and pathways mediate pacemaker mutations associated with CPVT in basal and in response to β-adrenergic stimulation? (ii) Can different mechanisms lead to the same CPVT-related pacemaker pauses? (iii) Can the mutation-induced deteriorations in SANC function be reversed by drug intervention or gene manipulation? We used a numerical model of mice SANC that includes membrane and intracellular mechanisms and their interconnected signaling pathways. In the basal state of RyR2^R4496C SANC, the model predicted that the Na⁺-Ca²⁺ exchanger current (I_NCX) and T-type Ca²⁺ current (I_CaT) mediate between changes in Ca²⁺ signaling and SANC dysfunction. Under β-adrenergic stimulation, changes in cAMP-PKA signaling and the sodium currents (I_Na), in addition to I_NCX and I_CaT, mediate between changes in Ca²⁺ signaling and SANC automaticity pauses. Under basal conditions in Casq2^-/-, the same mechanisms drove changes in Ca²⁺ signaling and subsequent pacemaker dysfunction. However, SANC automaticity pauses in response to β-AR stimulation were mediated by I_CaT and I_Na. Taken together, distinct mechanisms can lead to CPVT-associated SANC automaticity pauses. In addition, we predict that specifically increasing SANC cAMP-PKA activity by either a pharmacological agent (IBMX, a phosphodiesterase (PDE) inhibitor), gene manipulation (overexpression of adenylyl cyclase 1/8) or direct manipulation of the SERCA phosphorylation target through changes in gene expression, compensate for the impairment in SANC automaticity. These findings suggest new insights for understanding CPVT and its therapeutic approach.

Conditional ablation and conditional rescue models for Casq2 elucidate the role of development and of cell-type specific expression of Casq2 in the CPVT2 phenotype

Cardiac calsequestrin (Casq2) associates with the ryanodine receptor 2 channel in the junctional sarcoplasmic reticulum to regulate Ca2+ release into the cytoplasm. Patients carrying mutations in CASQ2 display low resting heart rates under basal conditions and stress-induced polymorphic ventricular tachycardia (CPVT). In this study, we generate and characterize novel conditional deletion and conditional rescue mouse models to test the influence of developmental programs on the heart rate and CPVT phenotypes. We also compare the requirements for Casq2 function in the cardiac conduction system (CCS) and in working cardiomyocytes. Our study shows that the CPVT phenotype is dependent upon concurrent loss of Casq2 function in both the CCS and in working cardiomyocytes. Accordingly, restoration of Casq2 in only the CCS prevents CPVT. In addition, occurrence of CPVT is independent of the developmental history of Casq2-deficiency. In contrast, resting heart rate depends upon Casq2 gene activity only in the CCS and upon developmental history. Finally, our data support a model where low basal heart rate is a significant risk factor for CPVT.

Eliminating contraction during culture maintains global and local Ca2+ dynamics in cultured rabbit pacemaker cells

Pacemaker cells residing in the sinoatrial node generate the regular heartbeat. Ca²⁺ signaling controls the heartbeat rate-directly, through the effect on membrane molecules (NCX exchange, K⁺ channel), and indirectly, through activation of calmodulin-AC-cAMP-PKA signaling. Thus, the physiological role of signaling in pacemaker cells can only be assessed if the Ca²⁺ dynamics are in the physiological range. Cultured cells that can be genetically manipulated and/or virally infected with probes are required for this purpose. Because rabbit pacemaker cells in culture experience a decrease in their spontaneous action potential (AP) firing rate below the physiological range, Ca²⁺ dynamics are expected to be affected. However, Ca²⁺ dynamics in cultured pacemaker cells have not been reported before. We aim to a develop a modified culture method that sustains the global and local Ca²⁺ kinetics along with the AP firing rate of rabbit pacemaker cells. We used experimental and computational tools to test the viability of rabbit pacemaker cells in culture under various conditions. We tested the effect of culture dish coating, pH, phosphorylation, and energy balance on cultured rabbit pacemaker cells function. The cells were maintained in culture for 48 h in two types of culture media: one without the addition of a contraction uncoupler and one enriched with either 10 mM BDM (2,3-Butanedione 2-monoxime) or 25 μM blebbistatin. The uncoupler was washed out from the medium prior to the experiments. Cells were successfully infected with a GFP adenovirus cultured with either BDM or blebbistatin. Using either uncoupler during culture led to the cell surface area being maintained at the same level as fresh cells. Moreover, the phospholamban and ryanodine receptor densities and their phosphorylation level remained intact in culture when either blebbistatin or BDM were present. Spontaneous AP firing rate, spontaneous Ca²⁺ kinetics, and spontaneous local Ca²⁺ release parameters were similar in the cultured cells with blebbistatin as in fresh cells. However, BDM affects these parameters. Using experimental and a computational model, we showed that by eliminating contraction, phosphorylation activity is preserved and energy is reduced. However, the side-effects of BDM render it less effective than blebbistatin.

Post-Translational Modifications and Diastolic Calcium Leak Associated to the Novel RyR2-D3638A Mutation Lead to CPVT in Patient-Specific hiPSC-Derived Cardiomyocytes

Background: Sarcoplasmic reticulum Ca²⁺ leak and post-translational modifications under stress have been implicated in catecholaminergic polymorphic ventricular tachycardia (CPVT), a highly lethal inherited arrhythmogenic disorder. Human induced pluripotent stem cells (hiPSCs) offer a unique opportunity for disease modeling. Objective: The aims were to obtain functional hiPSC-derived cardiomyocytes from a CPVT patient harboring a novel ryanodine receptor (RyR2) mutation and model the syndrome, drug responses and investigate the molecular mechanisms associated to the CPVT syndrome. Methods: Patient-specific cardiomyocytes were generated from a young athletic female diagnosed with CPVT. The contractile, intracellular Ca²⁺ handling and electrophysiological properties as well as the RyR2 macromolecular remodeling were studied. Results: Exercise stress electrocardiography revealed polymorphic ventricular tachycardia when treated with metoprolol and marked improvement with flecainide alone. We found abnormal stress-induced contractile and electrophysiological properties associated with sarcoplasmic reticulum Ca²⁺ leak in CPVT hiPSC-derived cardiomyocytes. We found inadequate response to metoprolol and a potent response of flecainide. Stabilizing RyR2 with a Rycal compound prevents those abnormalities specifically in CPVT hiPSC-derived cardiomyocytes. The RyR2-D3638A mutation is located in the conformational change inducing-central core domain and leads to RyR2 macromolecular remodeling including depletion of PP2A and Calstabin2. Conclusion: We identified a novel RyR2-D3638A mutation causing 3D conformational defects and aberrant biophysical properties associated to RyR2 macromolecular complex post-translational remodeling. The molecular remodeling is for the first time revealed using patient-specific hiPSC-derived cardiomyocytes which may explain the CPVT proband’s resistance. Our study promotes hiPSC-derived cardiomyocytes as a suitable model for disease modeling, testing new therapeutic compounds, personalized medicine and deciphering underlying molecular mechanisms.

Chromosome 4q25 Variant rs6817105 Bring Sinus Node Dysfunction and Left Atrial Enlargement

Genome-wide association studies have reported a strong association of the single nucleotide polymorphism (SNP) rs6817105 (T > C) on chromosome 4q25 with atrial fibrillation (AF), but phenotype alterations conferred by this SNP have not been described. We genotyped SNP rs6817105 and examined the relationships among rs6817105 genotype, clinical characteristics, echocardiographic parameters, and electrophysiological parameters in 574 AF patients and 1,554 non-AF controls. Further, multiple microRNAs (miRNAs) are reported to be involved in atrial remodeling and AF pathogenesis, so we investigated relationships between rs6817105 genotype and serum concentrations of 2555 miRNAs. The rs6817105 minor allele frequency was significantly higher in AF patients than non-AF controls (66% vs. 47%, odds ratio 2.12, p = 4.9 × 10⁻²⁶). Corrected sinus node recovery time (CSRT) was longer and left atrial volume index (LAVI) was larger in AF patients with the rs6817105 minor allele than patient non-carriers (CSRT: CC 557 ± 315 ms, CT 486 ± 273 ms, TT 447 ± 234 ms, p = 0.001; LAVI: CC 43.6 ± 12.1, CT 42.4 ± 13.6, TT 39.8 ± 11.6, p = 0.030). There were no significant differences between rs6817105 genotype and the serum concentrations of miRNAs. These findings strongly implicate rs6817105 minor allele in sinus node dysfunction and left atrial enlargement.

A delayed diagnosis of catecholaminergic polymorphic ventricular tachycardia with a mutant of RYR2 at c.7580T>G for 6 years in a 9-year-old child

RATIONALE

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare but potentially lethal inherited arrhythmia syndrome induced by adrenergic stress. Due to the atypical clinical manifestations in early age, limited recognition and experience of pediatric cardiologists, and low awareness of the significance of genetic diagnosis in some underdeveloped areas in China, a delayed or missed diagnosis of CPVT in children is common and concerning.

PATIENT CONCERNS

A 9-year and 3-month male child with recurrent exercise-induced syncope accompanied by convulsion was initially misdiagnosed as epilepsy since the first manifestation at the age of 3 years. Due to the identification of polymorphic ventricular premature beats, nonsustained ventricular tachycardia (VT), and supraventricular tachycardia, a cardiogenic etiology was established. The patient received a successive treatment by propafenone, amiodarone, a combination of amiodarone with metoprolol, and metoprolol alone for up to 6 years.

DIAGNOSES

Given the poor response to conventional antiarrhythmics, excise-induced syncope, QRS morphology and a structurally normal heart, the diagnosis of CPVT was suspected, and ultimately confirmed by detection of polymorphic and bidirectional VT with degeneration into ventricular fibrillation during exercise testing. In addition, a heterozygous mutant of RYR2 at c.7580T > G was identified by genetic testing.

INTERVENTIONS

Due to the unavailability of flecainide in China and the refusal of implantable cardioverter defibrillator implantation by his parents, this patient continued to be treated with oral metoprolol.

OUTCOMES

Unfortunately, the effect was unfavorable during 4 months outpatient follow-up.

LESSONS

CPVT should be suspected in young patients with a normal baseline electrocardiogram (EKG), a structurally normal heart and polymorphic and/or bidirectional ventricular tachycardia induced by exercise or emotional stress. Exercise and genetic testing is essential and significant for a timely and accurate diagnosis of CPVT. The current study firstly reported a case with CPVT associated with a mutant of RYR2 at c.7580T > G in children.

SK4 K+ channels are therapeutic targets for the treatment of cardiac arrhythmias

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a stress‐provoked ventricular arrhythmia, which also manifests sinoatrial node (SAN) dysfunction. We recently showed that SK4 calcium‐activated potassium channels are important for automaticity of cardiomyocytes derived from human embryonic stem cells. Here SK4 channels were identified in human induced pluripotent stem cell‐derived cardiomyocytes (hiPSC‐CMs) from healthy and CPVT2 patients bearing a mutation in calsequestrin 2 (CASQ2‐D307H) and in SAN cells from WT and CASQ2‐D307H knock‐in (KI) mice. TRAM‐34, a selective blocker of SK4 channels, prominently reduced delayed afterdepolarizations and arrhythmic Ca²⁺ transients observed following application of the β‐adrenergic agonist isoproterenol in CPVT2‐derived hiPSC‐CMs and in SAN cells from KI mice. Strikingly, in vivo ECG recording showed that intraperitoneal injection of the SK4 channel blockers, TRAM‐34 or clotrimazole, greatly reduced the arrhythmic features of CASQ2‐D307H KI and CASQ2 knockout mice at rest and following exercise. This work demonstrates the critical role of SK4 Ca²⁺‐activated K⁺ channels in adult pacemaker function, making them promising therapeutic targets for the treatment of cardiac ventricular arrhythmias such as CPVT.

Contribution of small conductance K+ channels to sinoatrial node pacemaker activity: insights from atrial-specific Na+ /Ca2+ exchange knockout mice

KEY POINTS

Repolarizing currents through K⁺ channels are essential for proper sinoatrial node (SAN) pacemaking, but the influence of intracellular Ca²⁺ on repolarization in the SAN is uncertain. We identified all three isoforms of Ca²⁺ -activated small conductance K⁺ (SK) channels in the murine SAN. SK channel blockade slows repolarization and subsequent depolarization of SAN cells. In the atrial-specific Na⁺ /Ca²⁺ exchanger (NCX) knockout mouse, cellular Ca²⁺ accumulation during spontaneous SAN pacemaker activity produces intermittent hyperactivation of SK channels, leading to arrhythmic pauses alternating with bursts of pacing. These findings suggest that Ca²⁺ -sensitive SK channels can translate changes in cellular Ca²⁺ into a repolarizing current capable of modulating pacemaking. SK channels are a potential pharmacological target for modulating SAN rate or treating SAN dysfunction, particularly under conditions characterized by abnormal increases in diastolic Ca²⁺ .

ABSTRACT

Small conductance K⁺ (SK) channels have been implicated as modulators of spontaneous depolarization and electrical conduction that may be involved in cardiac arrhythmia. However, neither their presence nor their contribution to sinoatrial node (SAN) pacemaker activity has been investigated. Using quantitative PCR (q-PCR), immunostaining and patch clamp recordings of membrane current and voltage, we identified all three SK isoforms (SK1, SK2 and SK3) in mouse SAN. Inhibition of SK channels with the specific blocker apamin prolonged action potentials (APs) in isolated SAN cells. Apamin also slowed diastolic depolarization and reduced pacemaker rate in isolated SAN cells and intact tissue. We investigated whether the Ca²⁺ -sensitive nature of SK channels could explain arrhythmic SAN pacemaker activity in the atrial-specific Na⁺ /Ca²⁺ exchange (NCX) knockout (KO) mouse, a model of cellular Ca²⁺ overload. SAN cells isolated from the NCX KO exhibited higher SK current than wildtype (WT) and apamin prolonged their APs. SK blockade partially suppressed the arrhythmic burst pacing pattern of intact NCX KO SAN tissue. We conclude that SK channels have demonstrable effects on SAN pacemaking in the mouse. Their Ca²⁺ -dependent activation translates changes in cellular Ca²⁺ into a repolarizing current capable of modulating regular pacemaking. This Ca²⁺ dependence also promotes abnormal automaticity when these channels are hyperactivated by elevated Ca²⁺ . We propose SK channels as a potential target for modulating SAN rate, and for treating patients affected by SAN dysfunction, particularly in the setting of Ca²⁺ overload.

RyR2R420Q catecholaminergic polymorphic ventricular tachycardia mutation induces bradycardia by disturbing the coupled clock pacemaker mechanism

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a lethal genetic arrhythmia that manifests syncope or sudden death in children and young adults under stress conditions. CPVT patients often present bradycardia and sino-atrial node (SAN) dysfunction. However, the mechanism remains unclear. We analyzed SAN function in two CPVT families and in a novel knock-in (KI) mouse model carrying the RyR2R420Q mutation. Humans and KI mice presented slower resting heart rate. Accordingly, the rate of spontaneous intracellular Ca2+ ([Ca2+]i) transients was slower in KI mouse SAN preparations than in WT, without any significant alteration in the "funny" current (If ). The L-type Ca2+ current was reduced in KI SAN cells in a [Ca2+]i-dependent way, suggesting that bradycardia was due to disrupted crosstalk between the "voltage" and "Ca2+" clock, and the mechanisms of pacemaking was induced by aberrant spontaneous RyR2- dependent Ca2+ release. This finding was consistent with a higher Ca2+ leak during diastolic periods produced by long-lasting Ca2+ sparks in KI SAN cells. Our results uncover a mechanism for the CPVT-causing RyR2 N-terminal mutation R420Q, and they highlight the fact that enhancing the Ca2+ clock may slow the heart rhythm by disturbing the coupling between Ca2+ and voltage clocks.

Genetic Determinants of Hereditary Bradyarrhythmias: A Contemporary Review of a Diverse Group of Disorders

Bradyarrhythmia is a common clinical presentation. Although the majority of cases are acquired, genetic screening of families with bradyarrhythmia has led to the discovery of a growing number of causative hereditary mutations. These mutations can interfere with any of the steps required for the occurrence of each cardiac cycle, including generation of an action potential in the sinoatrial node, successful exit of the action potential from the node, propagation of the action potential throughout the atria until the depolarization waves reach the atrioventricular node, and finally transmission of the action potential to the ventricles through the His-Purkinje system. As expected, channelopathies are the predominant culprit for hereditary bradyarrhythmias, because they play a crucial role in action potential generation and propagation. Interestingly, there are an increasing number of genes that encode for various regulatory or structural cellular components that have been linked to hereditary bradyarrhythmias. Furthermore, population-based genetic screening has revealed that age-related conduction defects may in fact be caused by genetic predispositions rather than the simple process of aging. With recent advances in genetic testing and the creation of animal models, not only have we discovered new culprit genes but it has also has become evident that there are still significant gaps in our knowledge of cardiac pathophysiology. In this review, we discuss the clinical presentations of known hereditary bradyarrhythmias and their associated conditions in addition to detailing our current molecular understanding of the mechanisms by which they are manifested.

Ablation of the cardiac ryanodine receptor phospho-site Ser2808 does not alter the adrenergic response or the progression to heart failure in mice. Elimination of the genetic background as critical variable

BACKGROUND

Phosphorylation of the cardiac ryanodine receptor (RyR2) phospho-site S2808 has been touted by the Marks group as a hallmark of heart failure (HF) and a critical mediator of the physiological fight-or-flight response of the heart. In support of this hypothesis, mice unable to undergo phosphorylation at RyR2-S2808 (S2808A) were significantly protected against HF and displayed a blunted response to adrenergic stimulation. However, the issue remains highly controversial because several groups have been unable to reproduce these findings. An important variable not considered before is the genetic background of the mice used to obtain these divergent results.

METHODS AND RESULTS

We backcrossed a RyR2-S2808A mouse into a congenic C57Bl/6 strain, the same strain used by the Marks group to conduct their experiments. We then performed several key experiments to confirm or discard the genetic background of the mouse as a relevant variable, including induction of HF by myocardial infarction and tests of integrity of adrenergic response. Congenic C57Bl/6 harboring the S2808A mutation showed similar echocardiographic parameters that indicated identical progression towards HF compared to wild type controls, and had a normal response to adrenergic stimulation in whole animal and cellular experiments.

CONCLUSIONS

The genetic background of the different mouse models is unlikely to be the source of the divergent results obtained by the Marks group in comparison to several other groups. Cardiac adrenergic response and progression towards HF proceed unaltered in mice harboring the RyR2-S2808A mutation. Preventing RyR2-S2808 phosphorylation does not preclude a normal sympathetic response nor mitigates the pathophysiological consequences of MI.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited cardiac arrhythmia disorder that is characterized by emotion- and exercise-induced polymorphic ventricular arrhythmias and may lead to sudden cardiac death (SCD). CPVT plays an important role in SCD in the young and therefore recognition and adequate treatment of the disease are of vital importance. In the past years tremendous improvements have been made in the diagnostic methods and treatment of the disease. In this review, we summarize the clinical characteristics, genetics, and diagnostic and therapeutic strategies of CPVT and describe the most recent advances and some of the current challenges. (Circ J 2016; 80: 1285-1291).

Clinical Challenges in Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inheritable cardiac disorder associated with exercise- and stress-induced sudden death in young individuals. Although important steps forward have been made in the comprehension and treatment of this disease, several aspects remain unclear. Firstly, from an epidemiological standpoint the actual prevalence of CPVT is still unknown and possibly underestimated. In addition, the diagnostic process remains very challenging and can be supported by genetic analysis in only about half of the cases. Finally, up to one third of CPVT patients continue to present complex arrhythmias despite beta blocker treatment; the role of newer therapeutic options, such as flecainide and left cardiac sympathetic denervation, needs to be further elucidated. All these points constitute challenges for the cardiologist in the management of CPVT patients and fuel research into new diagnostic, prognostic and therapeutic approaches.

L-type Cav1.3 channels regulate ryanodine receptor-dependent Ca2+ release during sino-atrial node pacemaker activity

AIMS

Sino-atrial node (SAN) automaticity is an essential mechanism of heart rate generation that is still not completely understood. Recent studies highlighted the importance of intracellular Ca(2+) ([Ca(2+)]i) dynamics during SAN pacemaker activity. Nevertheless, the functional role of voltage-dependent L-type Ca(2+) channels in controlling SAN [Ca(2+)]i release is largely unexplored. Since Cav1.3 is the predominant L-type Ca(2+) channel isoform in SAN cells, we studied [Ca(2+)]i dynamics in isolated cells and ex vivo SAN preparations explanted from wild-type (WT) and Cav1.3 knockout (KO) mice (Cav1.3(-/-)).

METHODS AND RESULTS

We found that Cav1.3 deficiency strongly impaired [Ca(2+)]i dynamics, reducing the frequency of local [Ca(2+)]i release events and preventing their synchronization. This impairment inhibited the generation of Ca(2+) transients and delayed spontaneous activity. We also used action potentials recorded in WT SAN cells as voltage-clamp commands for Cav1.3(-/-) cells. Although these experiments showed abolished Ca(2+) entry through L-type Ca(2+) channels in the diastolic depolarization range of KO SAN cells, their sarcoplasmic reticulum Ca(2+) load remained normal. β-Adrenergic stimulation enhanced pacemaking of both genotypes, though, Cav1.3(-/-) SAN cells remained slower than WT. Conversely, we rescued pacemaker activity in Cav1.3(-/-) SAN cells and intact tissues through caffeine-mediated stimulation of Ca(2+)-induced Ca(2+) release.

CONCLUSIONS

Cav1.3 channels play a critical role in the regulation of [Ca(2+)]i dynamics, providing an unanticipated mechanism for triggering local [Ca(2+)]i releases and thereby controlling pacemaker activity. Our study also provides an additional pathophysiological mechanism for congenital SAN dysfunction and heart block linked to Cav1.3 loss of function in humans.

Modified sympathetic nerve regulation in AKAP5-null mice

Genetic analyses have revealed an important association between A-kinase anchoring proteins (AKAPs) and the intracellular calcium modulating system. AKAP5, also known as AKAP79/150, is an anchoring protein between PKA and voltage-dependent calcium channels, ryanodine receptor-2, phospholamban and other molecules. The aim of the present study was to elucidate the physiological importance of AKAP5 in the creation of cardiac rhythm using AKAP5-null mice. ECG analysis showed a normal sinus rhythm and a decreased responsiveness to isoproterenol in AKAP5-null mice compared with wild-type mice. Analysis of heart rate variability revealed that the R-R interval was unstable in AKAP5-null mutants and that the low-frequency components had decreased, indicating that the tonus of the sympathetic nervous system was affected. Furthermore, the atrium of the AKAP5-null mice showed a decreased positive inotropic response to isoproterenol, indicating the involvement of AKAP5 in a PKA-dependent pathway. Thus, our present study revealed that AKAP5 plays a significant role in the regulation of sympathetic nerve activities.

R4496C RyR2 mutation impairs atrial and ventricular contractility

A ryanodine receptor 2 mutation associated with catecholaminergic polymorphic ventricular tachycardia renders cardiomyocytes incapable of mediating a positive inotropic response.

Ryanodine receptor (RyR2) is the major Ca²⁺ channel of the cardiac sarcoplasmic reticulum (SR) and plays a crucial role in the generation of myocardial force. Changes in RyR2 gating properties and resulting increases in its open probability (P_o) are associated with Ca²⁺ leakage from the SR and arrhythmias; however, the effects of RyR2 dysfunction on myocardial contractility are unknown. Here, we investigated the possibility that a RyR2 mutation associated with catecholaminergic polymorphic ventricular tachycardia, R4496C, affects the contractile function of atrial and ventricular myocardium. We measured isometric twitch tension in left ventricular and atrial trabeculae from wild-type mice and heterozygous transgenic mice carrying the R4496C RyR2 mutation and found that twitch force was comparable under baseline conditions (30°C, 2 mM [Ca²⁺]_o, 1 Hz). However, the positive inotropic responses to high stimulation frequency, 0.1 µM isoproterenol, and 5 mM [Ca²⁺]_o were decreased in R4496C trabeculae, as was post-rest potentiation. We investigated the mechanisms underlying inotropic insufficiency in R4496C muscles in single ventricular myocytes. Under baseline conditions, the amplitude of the Ca²⁺ transient was normal, despite the reduced SR Ca²⁺ content. Under inotropic challenge, however, R4496C myocytes were unable to boost the amplitude of Ca²⁺ transients because they are incapable of properly increasing the amount of Ca²⁺ stored in the SR because of a larger SR Ca²⁺ leakage. Recovery of force in response to premature stimuli was faster in R4496C myocardium, despite the unchanged rates of recovery of L-type Ca²⁺ channel current (I_Ca-L) and SR Ca²⁺ content in single myocytes. A faster recovery from inactivation of the mutant R4496C channels could explain this behavior. In conclusion, changes in RyR2 channel gating associated with the R4496C mutation could be directly responsible for the alterations in both ventricular and atrial contractility. The increased RyR2 P_o and fractional Ca²⁺ release from the SR induced by the R4496C mutation preserves baseline contractility despite a slight decrease in SR Ca²⁺ content, but cannot compensate for the inability to increase SR Ca²⁺ content during inotropic challenge.

STIM1-Ca2+ signaling modulates automaticity of the mouse sinoatrial node

Cardiac pacemaking is governed by specialized cardiomyocytes located in the sinoatrial node (SAN). SAN cells (SANCs) integrate voltage-gated currents from channels on the membrane surface (membrane clock) with rhythmic Ca(2+) release from internal Ca(2+) stores (Ca(2+) clock) to adjust heart rate to meet hemodynamic demand. Here, we report that stromal interaction molecule 1 (STIM1) and Orai1 channels, key components of store-operated Ca(2+) entry, are selectively expressed in SANCs. Cardiac-specific deletion of STIM1 in mice resulted in depletion of sarcoplasmic reticulum (SR) Ca(2+) stores of SANCs and led to SAN dysfunction, as was evident by a reduction in heart rate, sinus arrest, and an exaggerated autonomic response to cholinergic signaling. Moreover, STIM1 influenced SAN function by regulating ionic fluxes in SANCs, including activation of a store-operated Ca(2+) current, a reduction in L-type Ca(2+) current, and enhancing the activities of Na(+)/Ca(2+) exchanger. In conclusion, these studies reveal that STIM1 is a multifunctional regulator of Ca(2+) dynamics in SANCs that links SR Ca(2+) store content with electrical events occurring in the plasma membrane, thereby contributing to automaticity of the SAN.

Burst pacemaker activity of the sinoatrial node in sodium-calcium exchanger knockout mice

In sinoatrial node (SAN) cells, electrogenic sodium-calcium exchange (NCX) is the dominant calcium (Ca) efflux mechanism. However, the role of NCX in the generation of SAN automaticity is controversial. To investigate the contribution of NCX to pacemaking in the SAN, we performed optical voltage mapping and high-speed 2D laser scanning confocal microscopy (LSCM) of Ca dynamics in an ex vivo intact SAN/atrial tissue preparation from atrial-specific NCX knockout (KO) mice. These mice lack P waves on electrocardiograms, and isolated NCX KO SAN cells are quiescent. Voltage mapping revealed disorganized and arrhythmic depolarizations within the NCX KO SAN that failed to propagate into the atria. LSCM revealed intermittent bursts of Ca transients. Bursts were accompanied by rising diastolic Ca, culminating in long pauses dominated by Ca waves. The L-type Ca channel agonist BayK8644 reduced the rate of Ca transients and inhibited burst generation in the NCX KO SAN whereas the Ca buffer 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethyl ester) (BAPTA AM) did the opposite. These results suggest that cellular Ca accumulation hinders spontaneous depolarization in the NCX KO SAN, possibly by inhibiting L-type Ca currents. The funny current (If) blocker ivabradine also suppressed NCX KO SAN automaticity. We conclude that pacemaker activity is present in the NCX KO SAN, generated by a mechanism that depends upon If. However, the absence of NCX-mediated depolarization in combination with impaired Ca efflux results in intermittent bursts of pacemaker activity, reminiscent of human sinus node dysfunction and "tachy-brady" syndrome.

The importance of Ca(2+)-dependent mechanisms for the initiation of the heartbeat

Mechanisms underlying pacemaker activity in the sinus node remain controversial, with some ascribing a dominant role to timing events in the surface membrane (“membrane clock”) and others to uptake and release of calcium from the sarcoplasmic reticulum (SR) (“calcium clock”). Here we discuss recent evidence on mechanisms underlying pacemaker activity with a particular emphasis on the many roles of calcium. There are particular areas of controversy concerning the contribution of calcium spark-like events and the importance of I(f) to spontaneous diastolic depolarisation, though it will be suggested that neither of these is essential for pacemaking. Sodium-calcium exchange (NCX) is most often considered in the context of mediating membrane depolarisation after spark-like events. We present evidence for a broader role of this electrogenic exchanger which need not always depend upon these spark-like events. Short (milliseconds or seconds) and long (minutes) term influences of calcium are discussed including direct regulation of ion channels and NCX, and control of the activity of calcium-dependent enzymes (including CaMKII, AC1, and AC8). The balance between the many contributory factors to pacemaker activity may well alter with experimental and clinical conditions, and potentially redundant mechanisms are desirable to ensure the regular spontaneous heart rate that is essential for life. This review presents evidence that calcium is central to the normal control of pacemaking across a range of temporal scales and seeks to broaden the accepted description of the “calcium clock” to cover these important influences.

Potential effects of intrinsic heart pacemaker cell mechanisms on dysrhythmic cardiac action potential firing

The heart's regular electrical activity is initiated by specialized cardiac pacemaker cells residing in the sinoatrial node. The rate and rhythm of spontaneous action potential firing of sinoatrial node cells are regulated by stochastic mechanisms that determine the level of coupling of chemical to electrical clocks within cardiac pacemaker cells. This coupled-clock system is modulated by autonomic signaling from the brain via neurotransmitter release from the vagus and sympathetic nerves. Abnormalities in brain-heart clock connections or in any molecular clock activity within pacemaker cells lead to abnormalities in the beating rate and rhythm of the pacemaker tissue that initiates the cardiac impulse. Dysfunction of pacemaker tissue can lead to tachy-brady heart rate alternation or exit block that leads to long atrial pauses and increases susceptibility to other cardiac arrhythmia. Here we review evidence for the idea that disturbances in the intrinsic components of pacemaker cells may be implemented in arrhythmia induction in the heart.

Sick sinus syndrome and atrial fibrillation in older persons - A view from the sinoatrial nodal myocyte

Sick sinus syndrome remains a highly relevant clinical entity, being responsible for the implantation of the majority of electronic pacemakers worldwide. It is an infinitely more complex disease than it was believed when first described in the mid part of the 20th century. It not only involves the innate leading pacemaker region of the heart, the sinoatrial node, but also the atrial myocardium, predisposing to atrial tachydysrhythmias. It remains controversial as to whether the dysfunction of the sinoatrial node directly causes the dysfunction of the atrial myocardium, or vice versa, or indeed whether these two aspects of the condition arise through some related underlying pathological mechanism, such as extracellular matrix remodeling, i.e., fibrosis. This review aims to shed new light on the myriad possible contributing factors in the development of sick sinus syndrome, with a particular focus on the sinoatrial nodal myocyte. This article is part of a Special Issue entitled CV Aging.

Functional role of voltage gated Ca(2+) channels in heart automaticity

Pacemaker activity of automatic cardiac myocytes controls the heartbeat in everyday life. Cardiac automaticity is under the control of several neurotransmitters and hormones and is constantly regulated by the autonomic nervous system to match the physiological needs of the organism. Several classes of ion channels and proteins involved in intracellular Ca(2+) dynamics contribute to pacemaker activity. The functional role of voltage-gated calcium channels (VGCCs) in heart automaticity and impulse conduction has been matter of debate for 30 years. However, growing evidence shows that VGCCs are important regulators of the pacemaker mechanisms and play also a major role in atrio-ventricular impulse conduction. Incidentally, studies performed in genetically modified mice lacking L-type Cav1.3 (Cav1.3(-/-)) or T-type Cav3.1 (Cav3.1(-/-)) channels show that genetic inactivation of these channels strongly impacts pacemaking. In cardiac pacemaker cells, VGCCs activate at negative voltages at the beginning of the diastolic depolarization and importantly contribute to this phase by supplying inward current. Loss-of-function of these channels also impairs atrio-ventricular conduction. Furthermore, inactivation of Cav1.3 channels promotes also atrial fibrillation and flutter in knockout mice suggesting that these channels can play a role in stabilizing atrial rhythm. Genomic analysis demonstrated that Cav1.3 and Cav3.1 channels are widely expressed in pacemaker tissue of mice, rabbits and humans. Importantly, human diseases of pacemaker activity such as congenital bradycardia and heart block have been attributed to loss-of-function of Cav1.3 and Cav3.1 channels. In this article, we will review the current knowledge on the role of VGCCs in the generation and regulation of heart rate and rhythm. We will discuss also how loss of Ca(2+) entry through VGCCs could influence intracellular Ca(2+) handling and promote atrial arrhythmias.