RyR2 modulates a Ca2+-activated K+ current in mouse cardiac myocytes

hiPSC-derived cardiomyocytes as a model to study the role of small-conductance Ca2+-activated K+ (SK) ion channel variants associated with atrial fibrillation

Atrial fibrillation (AF), the most common arrhythmia, has been associated with different electrophysiological, molecular, and structural alterations in atrial cardiomyocytes. Therefore, more studies are required to elucidate the genetic and molecular basis of AF. Various genome-wide association studies (GWAS) have strongly associated different single nucleotide polymorphisms (SNPs) with AF. One of these GWAS identified the rs13376333 risk SNP as the most significant one from the 1q21 chromosomal region. The rs13376333 risk SNP is intronic to the KCNN3 gene that encodes for small conductance calcium-activated potassium channels type 3 (SK3). However, the functional electrophysiological effects of this variant are not known. SK channels represent a unique family of K+ channels, primarily regulated by cytosolic Ca2+ concentration, and different studies support their critical role in the regulation of atrial excitability and consequently in the development of arrhythmias like AF. Since different studies have shown that both upregulation and downregulation of SK3 channels can lead to arrhythmias by different mechanisms, an important goal is to elucidate whether the rs13376333 risk SNP is a gain-of-function (GoF) or a loss-of-function (LoF) variant. A better understanding of the functional consequences associated with these SNPs could influence clinical practice guidelines by improving genotype-based risk stratification and personalized treatment. Although research using native human atrial cardiomyocytes and animal models has provided useful insights, each model has its limitations. Therefore, there is a critical need to develop a human-derived model that represents human physiology more accurately than existing animal models. In this context, research with human induced pluripotent stem cells (hiPSC) and subsequent generation of cardiomyocytes derived from hiPSC (hiPSC-CMs) has revealed the underlying causes of various cardiovascular diseases and identified treatment opportunities that were not possible using in vitro or in vivo studies with animal models. Thus, the ability to generate atrial cardiomyocytes and atrial tissue derived from hiPSCs from human/patients with specific genetic diseases, incorporating novel genetic editing tools to generate isogenic controls and organelle-specific reporters, and 3D bioprinting of atrial tissue could be essential to study AF pathophysiological mechanisms. In this review, we will first give an overview of SK-channel function, its role in atrial fibrillation and outline pathophysiological mechanisms of KCNN3 risk SNPs. We will then highlight the advantages of using the hiPSC-CM model to investigate SNPs associated with AF, while addressing limitations and best practices for rigorous hiPSC studies.

Regulation of Papillary Muscle Contractility by NAD and Ammonia Interplay: Contribution of Ion Channels and Exchangers

Various models, including stem cells derived and isolated cardiomyocytes with overexpressed channels, are utilized to analyze the functional interplay of diverse ion currents involved in cardiac automaticity and excitation-contraction coupling control. Here, we used β-NAD and ammonia, known hyperpolarizing and depolarizing agents, respectively, and applied inhibitory analysis to reveal the interplay of several ion channels implicated in rat papillary muscle contractility control. We demonstrated that: 4 mM β-NAD, having no strong impact on resting membrane potential (RMP) and action potential duration (APD90) of ventricular cardiomyocytes, evoked significant suppression of isometric force (F) of paced papillary muscle. Reactive blue 2 restored F to control values, suggesting the involvement of P2Y-receptor-dependent signaling in β-NAD effects. Meantime, 5 mM NH₄Cl did not show any effect on F of papillary muscle but resulted in significant RMP depolarization, APD90 shortening, and a rightward shift of I-V relationship for total steady state currents in cardiomyocytes. Paradoxically, NH₄Cl, being added after β-NAD and having no effect on RMP, APD, and I-V curve, recovered F to the control values, indicating β-NAD/ammonia antagonism. Blocking of HCN, Kir2.x, and L-type calcium channels, Ca²⁺-activated K⁺ channels (SK, IK, and BK), or NCX exchanger reverse mode prevented this effect, indicating consistent cooperation of all currents mediated by these channels and NCX. We suggest that the activation of Kir2.x and HCN channels by extracellular K⁺, that creates positive and negative feedback, and known ammonia and K⁺ resemblance, may provide conditions required for the activation of all the chain of channels involved in the interplay. Here, we present a mechanistic model describing an interplay of channels and second messengers, which may explain discovered antagonism of β-NAD and ammonia on rat papillary muscle contractile activity.

A Ca2+-Dependent Mechanism Boosting Glycolysis and OXPHOS by Activating Aralar-Malate-Aspartate Shuttle, upon Neuronal Stimulation

Calcium is an important second messenger regulating a bioenergetic response to the workloads triggered by neuronal activation. In embryonic mouse cortical neurons using glucose as only fuel, activation by NMDA elicits a strong workload (ATP demand)-dependent on Na+ and Ca2+ entry, and stimulates glucose uptake, glycolysis, pyruvate and lactate production, and oxidative phosphorylation (OXPHOS) in a Ca2+-dependent way. We find that Ca2+ upregulation of glycolysis, pyruvate levels, and respiration, but not glucose uptake, all depend on Aralar/AGC1/Slc25a12, the mitochondrial aspartate-glutamate carrier, component of the malate-aspartate shuttle (MAS). MAS activation increases glycolysis, pyruvate production, and respiration, a process inhibited in the presence of BAPTA-AM, suggesting that the Ca2+ binding motifs in Aralar may be involved in the activation. Mitochondrial calcium uniporter (MCU) silencing had no effect, indicating that none of these processes required MCU-dependent mitochondrial Ca2+ uptake. The neuronal respiratory response to carbachol was also dependent on Aralar, but not on MCU. We find that mouse cortical neurons are endowed with a constitutive ER-to-mitochondria Ca2+ flow maintaining basal cell bioenergetics in which ryanodine receptors, RyR2, rather than InsP3R, are responsible for Ca2+ release, and in which MCU does not participate. The results reveal that, in neurons using glucose, MCU does not participate in OXPHOS regulation under basal or stimulated conditions, while Aralar-MAS appears as the major Ca2+-dependent pathway tuning simultaneously glycolysis and OXPHOS to neuronal activation.SIGNIFICANCE STATEMENT Neuronal activation increases cell workload to restore ion gradients altered by activation. Ca2+ is involved in matching increased workload with ATP production, but the mechanisms are still unknown. We find that glycolysis, pyruvate production, and neuronal respiration are stimulated on neuronal activation in a Ca2+-dependent way, independently of effects of Ca2+ as workload inducer. Mitochondrial calcium uniporter (MCU) does not play a relevant role in Ca2+ stimulated pyruvate production and oxygen consumption as both are unchanged in MCU silenced neurons. However, Ca2+ stimulation is blunt in the absence of Aralar, a Ca2+-binding mitochondrial carrier component of Malate-Aspartate Shuttle (MAS). The results suggest that Ca2+-regulated Aralar-MAS activation upregulates glycolysis and pyruvate production, which fuels mitochondrial respiration, through regulation of cytosolic NAD+/NADH ratio.

Cardiac small-conductance calcium-activated potassium channels in health and disease

Small-conductance Ca2+-activated K+ (SK, KCa2) channels are encoded by KCNN genes, including KCNN1, 2, and 3. The channels play critical roles in the regulation of cardiac excitability and are gated solely by beat-to-beat changes in intracellular Ca2+. The family of SK channels consists of three members with differential sensitivity to apamin. All three isoforms are expressed in human hearts. Studies over the past two decades have provided evidence to substantiate the pivotal roles of SK channels, not only in healthy heart but also with diseases including atrial fibrillation (AF), ventricular arrhythmia, and heart failure (HF). SK channels are prominently expressed in atrial myocytes and pacemaking cells, compared to ventricular cells. However, the channels are significantly upregulated in ventricular myocytes in HF and pulmonary veins in AF models. Interests in cardiac SK channels are further fueled by recent studies suggesting the possible roles of SK channels in human AF. Therefore, SK channel may represent a novel therapeutic target for atrial arrhythmias. Furthermore, SK channel function is significantly altered by human calmodulin (CaM) mutations, linked to life-threatening arrhythmia syndromes. The current review will summarize recent progress in our understanding of cardiac SK channels and the roles of SK channels in the heart in health and disease.

The regulation of the small-conductance calcium-activated potassium current and the mechanisms of sex dimorphism in J wave syndrome

Apamin-sensitive small-conductance calcium-activated potassium (SK) current (I_KAS) plays an important role in cardiac repolarization under a variety of physiological and pathological conditions. The regulation of cardiac I_KAS relies on SK channel expression, intracellular Ca²⁺, and interaction between SK channel and intracellular Ca²⁺. I_KAS activation participates in multiple types of arrhythmias, including atrial fibrillation, ventricular tachyarrhythmias, and automaticity and conduction abnormality. Recently, sex dimorphisms in autonomic control have been noticed in I_KAS activation, resulting in sex-differentiated action potential morphology and arrhythmogenesis. This review provides an update on the Ca²⁺-dependent regulation of cardiac I_KAS and the role of I_KAS on arrhythmias, with a special focus on sex differences in I_KAS activation. We propose that sex dimorphism in autonomic control of I_KAS may play a role in J wave syndrome.

Small-conductance Ca2+-activated K+ channels: insights into their roles in cardiovascular disease

Life-threatening malignant arrhythmias in pathophysiological conditions can increase the mortality and morbidity of patients with cardiovascular diseases. Cardiac electrical activity depends on the coordinated propagation of excitatory stimuli and the generation of action potentials in cardiomyocytes. Action potential formation results from the opening and closing of ion channels. Recent studies have indicated that small-conductance calcium-activated potassium (SK) channels play a critical role in cardiac repolarization in pathophysiological but not normal physiological conditions. The aim of this review is to describe the role of SK channels in healthy and diseased hearts, to suggest cardiovascular pathophysiologic targets for intervention, and to discuss studies of agents that target SK channels for the treatment of cardiovascular diseases.

Coupling of SK channels, L-type Ca2+ channels, and ryanodine receptors in cardiomyocytes

Small-conductance Ca²⁺-activated K⁺ (SK) channels regulate the excitability of cardiomyocytes by integrating intracellular Ca²⁺ and membrane potentials on a beat-to-beat basis. The inextricable interplay between activation of SK channels and Ca²⁺ dynamics suggests the pathology of one begets another. Yet, the exact mechanistic underpinning for the activation of cardiac SK channels remains unaddressed. Here, we investigated the intracellular Ca²⁺ microdomains necessary for SK channel activation. SK currents coupled with Ca²⁺ influx via L-type Ca²⁺ channels (LTCCs) continued to be elicited after application of caffeine, ryanodine or thapsigargin to deplete SR Ca²⁺ store, suggesting that LTCCs provide the immediate Ca²⁺ microdomain for the activation of SK channels in cardiomyocytes. Super-resolution imaging of SK2, Ca_v1.2 Ca²⁺ channel, and ryanodine receptor 2 (RyR2) was performed to quantify the nearest neighbor distances (NND) and localized the three molecules within hundreds of nanometers. The distribution of NND between SK2 and RyR2 as well as SK2 and Ca_v1.2 was bimodal, suggesting a spatial relationship between the channels. The activation mechanism revealed by our study paved the way for the understanding of the roles of SK channels on the feedback mechanism to regulate the activities of LTCCs and RyR2 to influence local and global Ca²⁺ signaling.

Functional interaction of Junctophilin 2 with small- conductance Ca2+ -activated potassium channel subtype 2(SK2) in mouse cardiac myocytes

Aim

Junctophilins (JPs), a protein family of the junctional membrane complex, maintain the close conjunction between cell surface and intracellular membranes in striate muscle cells mediating the crosstalk between extracellular Ca²⁺ entry and intracellular Ca²⁺ release. The small‐conductance Ca²⁺‐activated K⁺ channels are activated by the intracellular calcium and play an essential role in the cardiac action potential profile. Molecular mechanisms of regulation of the SK channels are still uncertain. Here, we sought to determine whether there is a functional interaction of junctophilin type 2 (JP2) with the SK channels and whether JP2 gene silencing might modulate the function of SK channels in cardiac myocytes.

Methods

Association of JP2 with SK2 channel in mouse heart tissue as well as HEK293 cells was studied using in vivo and in vitro approaches. siRNA knockdown of JP2 gene was assessed by real‐time PCR. The expression of proteins was analysed by Western blotting. Ca²⁺‐activated K⁺ current (I_K,Ca) in infected adult mouse cardiac myocytes was recorded using whole‐cell voltage‐clamp technique. The intracellular Ca²⁺ transient was measured using an IonOptix photometry system.

Results

We showed for the first time that JP2 associates with the SK2 channel in native cardiac tissue. JP2, via the membrane occupation and recognition nexus (MORN motifs) in its N‐terminus, directly interacted with SK2 channels. A colocalization of the SK2 channel with its interaction protein of JP2 was found in the cardiac myocytes. Moreover, we demonstrated that JP2 is necessary for the proper cell surface expression of the SK2 channel in HEK293. Functional experiments indicated that knockdown of JP2 caused a significant decrease in the density of I_K,Ca and reduced the amplitude of the Ca²⁺ transient in infected cardiomyocytes.

Conclusion

The present data provide evidence that the functional interaction between JP2 and SK2 channels is present in the native mouse heart tissue. Junctophilin 2, as junctional membrane complex (JMC) protein, is an important regulator of the cardiac SK channels.

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.

Calcium-Activated Potassium Channels: Potential Target for Cardiovascular Diseases

Ca(2+)-activated K(+) channels (KCa) are classified into three subtypes: big conductance (BKCa), intermediate conductance (IKCa), and small conductance (SKCa) KCa channels. The three types of KCa channels have distinct physiological or pathological functions in cardiovascular system. BKCa channels are mainly expressed in vascular smooth muscle cells (VSMCs) and inner mitochondrial membrane of cardiomyocytes, activation of BKCa channels in these locations results in vasodilation and cardioprotection against cardiac ischemia. IKCa channels are expressed in VSMCs, endothelial cells, and cardiac fibroblasts and involved in vascular smooth muscle proliferation, migration, vessel dilation, and cardiac fibrosis. SKCa channels are widely expressed in nervous and cardiovascular system, and activation of SKCa channels mainly contributes membrane hyperpolarization. In this chapter, we summarize the physiological and pathological roles of the three types of KCa channels in cardiovascular system and put forward the possibility of KCa channels as potential target for cardiovascular diseases.

Small-conductance Ca2+ -activated K+ channels and cardiac arrhythmias

Small-conductance Ca2+ -activated K+ (SK, KCa2) channels are unique in that they are gated solely by changes in intracellular Ca2+ and, hence, function to integrate intracellular Ca2+ and membrane potentials on a beat-to-beat basis. Recent studies have provided evidence for the existence and functional significance of SK channels in the heart. Indeed, our knowledge of cardiac SK channels has been greatly expanded over the past decade. Interests in cardiac SK channels are further driven by recent studies suggesting the critical roles of SK channels in human atrial fibrillation, the SK channel as a possible novel therapeutic target in atrial arrhythmias, and upregulation of SK channels in heart failure in animal models and in human heart failure. However, there remain critical gaps in our knowledge. Specifically, blockade of SK channels in cardiac arrhythmias has been shown to be both antiarrhythmic and proarrhythmic. This contemporary review provides an overview of the literature on the role of cardiac SK channels in cardiac arrhythmias and serves as a discussion platform for the current clinical perspectives. At the translational level, development of SK channel blockers as a new therapeutic strategy in the treatment of atrial fibrillation and the possible proarrhythmic effects merit further considerations and investigations.