Biology of the Sinus Node and its Disease

Differential effects of anesthetics and sex on supraventricular electrophysiology and atrial fibrillation substrate in rats

Rodents are increasingly used in atrial electrophysiology research, yet such studies are often performed under anesthesia owing to technical challenges. Here we developed an implantable device for comprehensive atrial studies in ambulatory rats and investigated the effects of commonly used anesthetics on supraventricular electrophysiology and arrhythmic substrate, comparing them with the unanesthetized state (UAS). Adult rats were evaluated 4 weeks after implantation. Studies were conducted in the UAS under 2% isoflurane (ISO) and under 40 mg/kg pentobarbital (PEN). Pacing protocols determined various parameters, including sinoatrial node recovery time, atrioventricular node effective refractory period and atrial effective refractory period. Arrhythmic substrate was assessed after 20 triggering bursts per condition, and arrhythmic tendency was analyzed manually and through the complexity ratio, an unbiased measure recently developed by our group. PEN mildly increased heart rate in both sexes, while ISO did not affect heart rate but prolonged the corrected sinus node recovery time in males. PEN increased atrioventricular node effective refractory period in both sexes, while ISO affected males only. Both ISO and PEN prolonged atrial effective refractory period compared with UAS in both sexes. Arrhythmic measures were higher in males and were attenuated by ISO and, to a lesser extent, by PEN in males only. The dominant frequency of arrhythmic events was reduced by both anesthetics in both sexes. These findings demonstrate a significant impact of commonly used anesthetics on rat supraventricular electrophysiology, with sex-based differences, highlighting the importance of methodologies that enable cardiac electrophysiology studies in unanesthetized rodents.

Does Long-Term Sport Practice Facilitate the Development of Idiopathic Bradycardia Requiring Early Pacemaker Implantation During the Course of Life?

Background: Sinus bradycardia and first-/second-degree atrioventricular (AV) block in athletes are traditionally considered secondary to increased vagal tone and therefore reversible. However, recent studies have suggested that they may persist even after the cessation of physical activity, and combined with the effects of aging, lead to the earlier onset of clinically significant bradyarrhythmias. Methods: We evaluated the correlation between lifetime sport practice and the age of the onset of premature (≤70 years old) idiopathic sinoatrial node or AV node dysfunction requiring pacemaker (PM) implantation. Results: Of the 1316 patients followed up with at our PM clinic in 2024, we included 79 (6%) who received a PM when they were ≤70 years old for bradyarrhythmias in the absence of secondary causes. Nineteen (24%) had engaged in at least 6 h of sports/week for ≥20 years and were classified as former athletes. For comparison, former athletes who received a PM for idiopathic bradycardia at >70 years old were 6% (p < 0.001). In the group ≤70 years old, the average age of PM implantation was 62.8 years in non-athletes versus 57.9 years in former athletes (p = 0.03). The main reason for PM implantation was AV block in both subgroups. Among former athletes, the correlation between the lifetime volume of sports activity and the age of PM implantation reached borderline statistical significance (p = 0.08). Echocardiography at the time of implant did not reveal significant differences between former athletes and non-athletes. Conclusions: In a cohort of patients who received a PM for bradyarrhythmia before the age of 70 years old in the absence of secondary causes, former athletes were implanted on average ≈5 years before non-athletes. This may suggest a contributing role of cumulative sports activity volume in the development of idiopathic sinus/AV node dysfunction.

Right coronary ostial atresia as a cause of arrhythmia and cardiogenic shock in a young woman: a case report

A 19-year-old woman presented to the emergency department with arrhythmia and signs of cardiogenic shock. After a 12-lead electrocardiogram ruled out acute myocardial infarction, and cardiac magnetic resonance showed no sign of cardiomyopathy, cardiac computed tomography angiography (CCTA) was performed, displaying ostial atresia of the right coronary artery. She was thus referred to a specialist centre for congenital cardiovascular disease, where an electrophysiological study observed an arrhythmogenic focus on the posteromedial papillary muscle, which was ablated, and she has been asymptomatic since. When dealing with patients presenting with arrhythmias or cardiogenic shock, and no signs of myocardial infarction or cardiomyopathy, performing CCTA to study the anatomy of the coronary arteries is vital.

Cardiac automaticity is modulated by IKACh in sinoatrial node during pregnancy

AIMS

Pregnant (P) women have a significantly elevated resting heart rate (HR), which makes cardiac arrhythmias more likely to occur. Although electrical remodelling of the sinoatrial node (SAN) has been documented, the underlying mechanism is not fully understood. The acetylcholine-activated potassium current (IKACh), one of the major repolarizing currents in the SAN, plays a critical role in HR control by hyperpolarizing the maximal diastolic potential (MDP) of the SAN action potential (AP), thereby reducing SAN automaticity and HR. Thus, considering its essential role in cardiac automaticity, this study aims to determine whether changes in IKACh are potentially involved in the increased HR associated with pregnancy.

METHODS AND RESULTS

Experiments were conducted on non-pregnant (NP) and pregnant (P; 17-18 days gestation) female CD-1 mice aged 2 to 4 months. IKACh was recorded on spontaneously beating SAN cells using the muscarinic agonist carbachol (CCh). Voltage-clamp data showed a reduction in IKACh density during pregnancy, which returned to control values shortly after delivery. The reduction in IKACh was explained by a decrease in protein expression of Kir3.1 channel subunit and the muscarinic type 2 receptor. In agreement with these findings, current-clamp data showed that the MDP of SAN cells from P mice were less hyperpolarized following CCh administration. Surface electrocardiograms (ECGs) recorded on anaesthetized mice revealed that the cholinergic antagonist atropine and the selective KACh channel blocker tertiapin-Q increased HR in NP mice and had only a minimal effect on P mice. AP and ECG data also showed that pregnancy is associated with a decrease in beating and HR variability, respectively.

CONCLUSION

IKACh function and expression are decreased in the mouse SAN during pregnancy, strongly suggesting that, in addition to other electrical remodelling of the SAN, reduced IKACh also plays an important role in the pregnancy-induced increased HR.

Decoding the molecular, cellular, and functional heterogeneity of zebrafish intracardiac nervous system

The proper functioning of the heart relies on the intricate interplay between the central nervous system and the local neuronal networks within the heart itself. While the central innervation of the heart has been extensively studied, the organization and functionality of the intracardiac nervous system (IcNS) remain largely unexplored. Here, we present a comprehensive taxonomy of the IcNS, utilizing single-cell RNA sequencing, anatomical studies, and electrophysiological techniques. Our findings reveal a diverse array of neuronal types within the IcNS, exceeding previous expectations. We identify a subset of neurons exhibiting characteristics akin to pacemaker/rhythmogenic neurons similar to those found in Central Pattern Generator networks of the central nervous system. Our results underscore the heterogeneity within the IcNS and its key role in regulating the heart's rhythmic functionality. The classification and characterization of the IcNS presented here serve as a valuable resource for further exploration into the mechanisms underlying heart functionality and the pathophysiology of associated cardiac disorders.

Infection with COVID-19 Complicated by Sinus Arrest

As a respiratory tract-transmitted disease, coronavirus disease 2019 (COVID-19) exerts a profound immune injury effect, leading not only to pulmonary impairment but also to cardiac complications. We present a case of a 79-year-old woman, who had previously contracted COVID-19 and subsequently developed sinus arrest (SA) following her second infection. The longest asystole time detected by Holter monitoring was 7.2 seconds. Although the patient met criteria for permanent pacemaker implantation, her family declined this intervention and conservative management was pursued instead. However, after a period of observation, the patient's SA resolved. The present case study describes a patient who experienced SA upon reinfection with COVID-19, which was not present during the initial infection. It emphasizes the impact of COVID-19 on cardiac health, particularly its potential to induce arrhythmias. In addition, it is worth noting that the arrhythmia induced by a COVID-19 infection may show reversibility, suggesting that a permanent pacemaker might not be the priority option if further pacing therapy is being considered.

Cardiac conduction diseases: understanding the molecular mechanisms to uncover targets for future treatments

INTRODUCTION

The cardiac conduction system (CCS) is crucial for maintaining adequate cardiac frequency at rest and modulation during exercise. Furthermore, the atrioventricular node and His-Purkinje system are essential for maintaining atrioventricular and interventricular synchrony and consequently maintaining an adequate cardiac output.

AREAS COVERED

In this review article, we examine the anatomy, physiology, and pathophysiology of the CCS. We then discuss in detail the most common genetic mutations and the molecular mechanisms of cardiac conduction disease (CCD) and provide our perspectives on future research and therapeutic opportunities in this field.

EXPERT OPINION

Significant advancement has been made in understanding the molecular mechanisms of CCD, including the recognition of the heterogeneous signaling at the subcellular levels of sinoatrial node, the involvement of inflammatory and autoimmune mechanisms, and the potential impact of epigenetic regulations on CCD. However, the current treatment of CCD manifested as bradycardia still relies primarily on cardiovascular implantable electronic devices (CIEDs). On the other hand, an If specific inhibitor was developed to treat inappropriate sinus tachycardia and sinus tachycardia in heart failure patients with reduced ejection fraction. More work is needed to translate current knowledge into pharmacologic or genetic interventions for the management of CCDs.

Intrinsic Sinus Node/Atrioventricular Node Dysfunction Requiring Pacemaker Implantation: Role of Former Professional Sport Activity

BACKGROUND

Sinus bradycardia and first degree or second degree Mobitz type I atrioventricular (AV) block in an athlete are considered adaptive and reversible phenomena; however, some evidence suggests that they may persist after detraining and become pathological. The aim of the study was to investigate the characteristics of a group of former professional athletes who required pacemaker (PM) implantation for intrinsic (idiopathic) sinus node (SN) dysfunction or AV block in comparison to control groups of sedentary individuals.

METHODS

We included all patients who underwent PM implantation during 2022. Three groups were compared: group 1 including 18 former professional athletes who received a PM for SN dysfunction/AV block in the absence of heart disease; group 2 including the first 20 sedentary individuals without heart disease who underwent PM implantation; and group 3 including all other 323 patients who received PM, the majority with underlying heart diseases.

RESULTS

Compared to the non-athlete control group 2, the mean age at diagnosis and at the time of PM implantation of former professional athletes did not show statistically significant differences. However, subgroup analysis revealed significant differences depending on the type of sports discipline: the age at diagnosis and at PM implantation was significantly lower in former endurance athletes than former strength/mixed athletes, control non-athletes, and all other patients. Moreover, former endurance professional athletes exhibited a higher prevalence of second or third degree AV block (78%) as the reason for PM implantation compared to power/mixed athletes (44%). The other clinical characteristics, including echocardiographic parameters, did not differ between former athletes and non-athletes.

CONCLUSIONS

Former professional endurance athletes with idiopathic SN dysfunction/AV block manifested the disease earlier in the life course compared to former power/mixed athletes and non-athletes. This suggests that bradycardia/AV block caused by intense and prolonged endurance sports may not always be benign and adaptive phenomena.

High spinal cord injury precipitating syncope: a rare indication for pacemaker insertion

The current evidence for vasovagal syncope management is that cardiac pacing is only indicated in a highly select group of patients where symptoms can be linked to bradycardic episodes. High spinal cord injury can lead to autonomic dysfunction and sympathetic nervous system hypoactivity. A high spinal cord injury can theoretically precipitate profound bradycardia leading to haemodynamic instability and syncope. A patient in his 50s with a history of C2 spinal injury was admitted to our tertiary centre for management of what was initially thought to be septic shock causing hypotension and syncope. With evidence to suggest this patient's presentation may be profound reflex syncope in the context of unopposed parasympathetic signalling, consensus was reached to implant a permanent pacemaker. Remarkably, the patient's haemodynamics stabilised and there were no further episodes of syncope.

Sinus Node Dysfunction

Sinus node dysfunction (SND) is a multifaceted disorder most prevalent in older individuals, but may also occur at an earlier age. In most cases, the SND diagnosis is ultimately established by documenting its ECG manifestations. EPS has limited utility. The treatment strategy is largely dictated by symptoms and ECG manifestations. Not infrequently, both bradycardia and tachycardia coexist in the same patients, along with other diseases common in the elderly (e.g., hypertension, coronary artery disease), thereby complicating treatment strategy. Prevention of the adverse consequences of both bradyarrhythmia and tachyarrhythmia is important to reduce susceptibility to syncope, falls, and thromboembolic complications.

The role of P21-activated kinase (Pak1) in sinus node function

Sinoatrial node (SAN) dysfunction (SND) and atrial arrhythmia frequently occur simultaneously with a hazard ratio of 4.2 for new onset atrial fibrillation (AF) in SND patients. In the atrial muscle attenuated activity of p21-activated kinase 1 (Pak1) increases the risk for AF by enhancing NADPH oxidase 2 dependent production of reactive oxygen species (ROS). However, the role of Pak1 dependent ROS regulation in SAN function has not yet been determined. We hypothesize that Pak1 activity maintains SAN activity by regulating the expression of the hyperpolarization activated cyclic nucleotide gated cation channel (HCN). To determine Pak1 dependent changes in heart rate (HR) regulation we quantified the intrinsic sinus rhythm in wild type (WT) and Pak1 deficient (Pak1-/-) mice of both sexes in vivo and in isolated Langendorff perfused hearts. Pak1-/- hearts displayed an attenuated HR in vivo after autonomic blockage and in isolated hearts. The contribution of the Ca2+ clock to pacemaker activity remained unchanged, but Ivabradine (3 μM), a blocker of HCN channels that are a membrane clock component, eliminated the differences in SAN activity between WT and Pak1-/- hearts. Reduced HCN4 expression was confirmed in Pak1-/- right atria. The reduced HCN activity in Pak1-/- could be rescued by class II HDAC inhibition (LMK235), ROS scavenging (TEMPOL) or attenuation of Extracellular Signal-Regulated Kinase (ERK) 1/2 activity (SCH772984). No sex specific differences in Pak1 dependent SAN regulation were determined. Our results establish Pak1 as a class II HDAC regulator and a potential therapeutic target to attenuate SAN bradycardia and AF susceptibility.

Palmitoylation regulates the magnitude of HCN4-mediated currents in mammalian cells

The sinoatrial node (SAN) and subsidiary pacemakers in the cardiac conduction system generate spontaneous electrical activity which is indispensable for electrical and therefore contractile function of the heart. The hyperpolarisation-activated cyclic nucleotide-gated channel HCN4 is responsible for genesis of the pacemaker "funny" current during diastolic depolarisation. S-palmitoylation, the reversible conjugation of the fatty acid palmitate to protein cysteine sulfhydryls, regulates the activity of key cardiac Na+ and Ca2+ handling proteins, influencing their membrane microdomain localisation and function. We investigated HCN4 palmitoylation and its functional consequences in engineered human embryonic kidney 293T cells as well as endogenous HCN4 in neonatal rat ventricular myocytes. HCN4 was palmitoylated in all experimental systems investigated. We mapped the HCN4 palmitoylation sites to a pair of cysteines in the HCN4 intracellular amino terminus. A double cysteine-to-alanine mutation CC93A/179AA of full length HCN4 caused a ∼67% reduction in palmitoylation in comparison to wild type HCN4. We used whole-cell patch clamp to evaluate HCN4 current (IHCN4) in stably transfected 293T cells. Removal of the two N-terminal palmitoylation sites did not significantly alter half maximal activation voltage of IHCN4 or the activation slope factor. IHCN4 was significantly larger in cells expressing wild type compared to non-palmitoylated HCN4 across a range of voltages. Phylogenetic analysis revealed that although cysteine 93 is widely conserved across all classes of HCN4 vertebrate orthologs, conservation of cysteine 179 is restricted to placental mammals. Collectively, we provide evidence for functional regulation of HCN4 via palmitoylation of its amino terminus in vertebrates. We suggest that by recruiting the amino terminus to the bilayer, palmitoylation enhances the magnitude of HCN4-mediated currents, but does not significantly affect the kinetics.

More than a key-the pathological roles of SARS-CoV-2 spike protein in COVID-19 related cardiac injury

Cardiac injury is common in hospitalized coronavirus disease 2019 (COVID-19) patients and cardiac abnormalities have been observed in a significant number of recovered COVID-19 patients, portending long-term health issues for millions of infected individuals. To better understand how Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2, CoV-2 for short) damages the heart, it is critical to fully comprehend the biology of CoV-2 encoded proteins, each of which may play multiple pathological roles. For example, CoV-2 spike glycoprotein (CoV-2-S) not only engages angiotensin converting enzyme II (ACE2) to mediate virus infection but also directly activates immune responses. In this work, the goal is to review the known pathological roles of CoV-2-S in the cardiovascular system, thereby shedding lights on the pathogenesis of COVID-19 related cardiac injury.

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.

The virtual sinoatrial node: What did computational models tell us about cardiac pacemaking?

Since its discovery, the sinoatrial node (SAN) has represented a fascinating and complex matter of research. Despite over a century of discoveries, a full comprehension of pacemaking has still to be achieved. Experiments often produced conflicting evidence that was used either in support or against alternative theories, originating intense debates. In this context, mathematical descriptions of the phenomena underlying the heartbeat have grown in importance in the last decades since they helped in gaining insights where experimental evaluation could not reach. This review presents the most updated SAN computational models and discusses their contribution to our understanding of cardiac pacemaking. Electrophysiological, structural and pathological aspects - as well as the autonomic control over the SAN - are taken into consideration to reach a holistic view of SAN activity.

Translational implications of bradyarrhythmia in hibernating brown bears

The brown bear Ursus arctos undergoes exceptional physiological adaptions during annual hibernation that minimize energy consumption, including profound decrease in heart rate, cardiac output, and respiratory rate. These changes are completely reversible after the bears reenter into the active state in spring. In this case report, we show episodes of sinus arrest in a hibernating Scandinavian brown bear and in humans, recorded by implantable loop recorders and discuss the possible underlying mechanisms. Lessons learned from cardiac adaptations in hibernating bears might prove useful in the treatment of patients with sinus node dysfunction.

Characterization of the pace-and-drive capacity of the human sinoatrial node: A 3D in silico study

The sinoatrial node (SAN) is a complex structure that spontaneously depolarizes rhythmically ("pacing") and excites the surrounding non-automatic cardiac cells ("drive") to initiate each heart beat. However, the mechanisms by which the SAN cells can activate the large and hyperpolarized surrounding cardiac tissue are incompletely understood. Experimental studies demonstrated the presence of an insulating border that separates the SAN from the hyperpolarizing influence of the surrounding myocardium, except at a discrete number of sinoatrial exit pathways (SEPs). We propose a highly detailed 3D model of the human SAN, including 3D SEPs to study the requirements for successful electrical activation of the primary pacemaking structure of the human heart. A total of 788 simulations investigate the ability of the SAN to pace and drive with different heterogeneous characteristics of the nodal tissue (gradient and mosaic models) and myocyte orientation. A sigmoidal distribution of the tissue conductivity combined with a mosaic model of SAN and atrial cells in the SEP was able to drive the right atrium (RA) at varying rates induced by gradual If block. Additionally, we investigated the influence of the SEPs by varying their number, length, and width. SEPs created a transition zone of transmembrane voltage and ionic currents to enable successful pace and drive. Unsuccessful simulations showed a hyperpolarized transmembrane voltage (-66 mV), which blocked the L-type channels and attenuated the sodium-calcium exchanger. The fiber direction influenced the SEPs that preferentially activated the crista terminalis (CT). The location of the leading pacemaker site (LPS) shifted toward the SEP-free areas. LPSs were located closer to the SEP-free areas (3.46 ± 1.42 mm), where the hyperpolarizing influence of the CT was reduced, compared with a larger distance from the LPS to the areas where SEPs were located (7.17± 0.98 mm). This study identified the geometrical and electrophysiological aspects of the 3D SAN-SEP-CT structure required for successful pace and drive in silico.

Coupling and heterogeneity modulate pacemaking capability in healthy and diseased two-dimensional sinoatrial node tissue models

Both experimental and modeling studies have attempted to determine mechanisms by which a small anatomical region, such as the sinoatrial node (SAN), can robustly drive electrical activity in the human heart. However, despite many advances from prior research, important questions remain unanswered. This study aimed to investigate, through mathematical modeling, the roles of intercellular coupling and cellular heterogeneity in synchronization and pacemaking within the healthy and diseased SAN. In a multicellular computational model of a monolayer of either human or rabbit SAN cells, simulations revealed that heterogenous cells synchronize their discharge frequency into a unique beating rhythm across a wide range of heterogeneity and intercellular coupling values. However, an unanticipated behavior appeared under pathological conditions where perturbation of ionic currents led to reduced excitability. Under these conditions, an intermediate range of intercellular coupling (900-4000 MΩ) was beneficial to SAN automaticity, enabling a very small portion of tissue (3.4%) to drive propagation, with propagation failure occurring at both lower and higher resistances. This protective effect of intercellular coupling and heterogeneity, seen in both human and rabbit tissues, highlights the remarkable resilience of the SAN. Overall, the model presented in this work allowed insight into how spontaneous beating of the SAN tissue may be preserved in the face of perturbations that can cause individual cells to lose automaticity. The simulations suggest that certain degrees of gap junctional coupling protect the SAN from ionic perturbations that can be caused by drugs or mutations.

A single cell transcriptional roadmap of human pacemaker cell differentiation

Each heartbeat is triggered by the sinoatrial node (SAN), the primary pacemaker of the heart. Studies in animal models have revealed that pacemaker cells share a common progenitor with the (pro)epicardium, and that the pacemaker cardiomyocytes further diversify into ‘transitional’, ‘tail’, and ‘head’ subtypes. However, the underlying molecular mechanisms, especially of human pacemaker cell development, are poorly understood. Here, we performed single cell RNA sequencing (scRNA-seq) and trajectory inference on human induced pluripotent stem cells (hiPSCs) differentiating to SAN-like cardiomyocytes (SANCMs) to construct a roadmap of transcriptional changes and lineage decisions. In differentiated SANCM, we identified distinct clusters that closely resemble different subpopulations of the in vivo SAN. Moreover, the presence of a side population of proepicardial cells suggested their shared ontogeny with SANCM, as also reported in vivo. Our results demonstrate that the divergence of SANCM and proepicardial lineages is determined by WNT signaling. Furthermore, we uncovered roles for TGFβ and WNT signaling in the branching of transitional and head SANCM subtypes, respectively. These findings provide new insights into the molecular processes involved in human pacemaker cell differentiation, opening new avenues for complex disease modeling in vitro and inform approaches for cell therapy-based regeneration of the SAN.

Targeted Treatment of Inappropriate Sinoatrial Node Tachycardia Based on Electrophysiological and Structural Mechanisms

The purpose of this review is to determine the causal mechanisms and treatment of inappropriate sinoatrial tachycardia (IST), defined as a non-physiological elevation in resting heart rate. IST is defined as a resting daytime sinus rate >100 beats/minute and an average 24-hour heart rate >90 beats/minute. Potential causal mechanisms include sympathetic receptor hypersensitivity, blunted parasympathetic tone, or enhanced intrinsic automaticity within the sinoatrial node (SAN) pacemaker-conduction complex. These anomalies may coexist in the same patient. Recent ex-vivo near-infrared transmural optical imaging of the SAN in human and animal hearts provides important insights into the functional and molecular features of this complex structure. In particular, it reveals the existence of preferential sinoatrial conduction pathways that ensure robust SAN activation with electrical conduction. The mechanism of IST is debated because even high-resolution electroanatomical mapping approaches cannot reveal intramural conduction in the 3-dimensional SAN complex. It may be secondary to enhanced automaticity, intranodal re-entry, or sinoatrial conduction pathway re-entry. Different pharmacological approaches can target these mechanisms. Long-acting β blockers in IST can act on both primarily increased automaticity and dysregulated autonomic system. Ivabradine targets sources of increased SAN automaticity. Conventional or hybrid ablation may target all the described abnormalities. This review provides a state-of-the-art overview of putative IST mechanisms. In conclusion, based on current knowledge, pharmacological and ablation approaches for IST, including the novel hybrid SAN sparing ablation, are discussed.

Disruption of mitochondria-sarcoplasmic reticulum microdomain connectomics contributes to sinus node dysfunction in heart failure

The sinoatrial node (SAN), the leading pacemaker region, generates electrical impulses that propagate throughout the heart. SAN dysfunction with bradyarrhythmia is well documented in heart failure (HF). However, the underlying mechanisms are not completely understood. Mitochondria are critical to cellular processes that determine the life or death of the cell. The release of Ca2+ from the ryanodine receptors 2 (RyR2) on the sarcoplasmic reticulum (SR) at mitochondria-SR microdomains serves as the critical communication to match energy production to meet metabolic demands. Therefore, we tested the hypothesis that alterations in the mitochondria-SR connectomics contribute to SAN dysfunction in HF. We took advantage of a mouse model of chronic pressure overload-induced HF by transverse aortic constriction (TAC) and a SAN-specific CRISPR-Cas9-mediated knockdown of mitofusin-2 (Mfn2), the mitochondria-SR tethering GTPase protein. TAC mice exhibited impaired cardiac function with HF, cardiac fibrosis, and profound SAN dysfunction. Ultrastructural imaging using electron microscope (EM) tomography revealed abnormal mitochondrial structure with increased mitochondria-SR distance. The expression of Mfn2 was significantly down-regulated and showed reduced colocalization with RyR2 in HF SAN cells. Indeed, SAN-specific Mfn2 knockdown led to alterations in the mitochondria-SR microdomains and SAN dysfunction. Finally, disruptions in the mitochondria-SR microdomains resulted in abnormal mitochondrial Ca2+ handling, alterations in localized protein kinase A (PKA) activity, and impaired mitochondrial function in HF SAN cells. The current study provides insights into the role of mitochondria-SR microdomains in SAN automaticity and possible therapeutic targets for SAN dysfunction in HF patients.

Biological noise is a key determinant of the reproducibility and adaptability of cardiac pacemaking and EC coupling

Each heartbeat begins with the generation of an action potential in pacemaking cells in the sinoatrial node. This signal triggers contraction of cardiac muscle through a process termed excitation-contraction (EC) coupling. EC coupling is initiated in dyadic structures of cardiac myocytes, where ryanodine receptors in the junctional sarcoplasmic reticulum come into close apposition with clusters of CaV1.2 channels in invaginations of the sarcolemma. Cooperative activation of CaV1.2 channels within these clusters causes a local increase in intracellular Ca2+ that activates the juxtaposed ryanodine receptors. A salient feature of healthy cardiac function is the reliable and precise beat-to-beat pacemaking and amplitude of Ca2+ transients during EC coupling. In this review, we discuss recent discoveries suggesting that the exquisite reproducibility of this system emerges, paradoxically, from high variability at subcellular, cellular, and network levels. This variability is attributable to stochastic fluctuations in ion channel trafficking, clustering, and gating, as well as dyadic structure, which increase intracellular Ca2+ variance during EC coupling. Although the effects of these large, local fluctuations in function and organization are sometimes negligible at the macroscopic level owing to spatial-temporal summation within and across cells in the tissue, recent work suggests that the "noisiness" of these intracellular Ca2+ events may either enhance or counterintuitively reduce variability in a context-dependent manner. Indeed, these noisy events may represent distinct regulatory features in the tuning of cardiac contractility. Collectively, these observations support the importance of incorporating experimentally determined values of Ca2+ variance in all EC coupling models. The high reproducibility of cardiac contraction is a paradoxical outcome of high Ca2+ signaling variability at subcellular, cellular, and network levels caused by stochastic fluctuations in multiple processes in time and space. This underlying stochasticity, which counterintuitively manifests as reliable, consistent Ca2+ transients during EC coupling, also allows for rapid changes in cardiac rhythmicity and contractility in health and disease.

Virus-induced inhibition of cardiac pacemaker channel HCN4 triggers bradycardia in human-induced stem cell system

The enterovirus Coxsackievirus B3 (CVB3) is known to be a major source for the development of cardiac dysfunctions like viral myocarditis (VMC) and dilatative cardiomyopathy (DCM), but also results in bradycardia and fatal cardiac arrest. Besides clinical reports on bradycardia and sudden cardiac death, very little is known about the influence of CVB3 on the activity of human cardiac pacemaker cells. Here, we address this issue using the first human induced pluripotent stem cell (hiPSC)-derived pacemaker-like cells, in which the expression of a transgenic non-infectious variant of CVB3 can be controlled dose- and time-dependently. We found that CVB3 drastically changed hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) distribution and function in hiPSC-derived pacemaker-like tissue. In addition, using HCN4 cell expression systems, we found that HCN4 currents were decreased with altered voltage dependency of activation when CVB3 was expressed. Increased autophagosome formation and autophagosomal HCN4 insertion was observed in hiPSC-derived pacemaker-like cells under CVB3 expression as well. Individual effects of single, non-structural CVB3 proteins were analyzed and demonstrated that CVB3 proteins 2C and 3A had the most robust effect on HCN4 activity. Treatment of cells with the Rab7 inhibitor CID 106770 or the CVB3-3A inhibitor GW5074 led to the recovery of the cytoplasmatic HCN4 accumulation into a healthy appearing phenotype, indicating that malfunctioning Rab7-directed autophagosome transport is involved in the disturbed, cytoplasmatic HCN4 accumulation in CVB3-expressing human pacemaker-like cells. Summarizing, the enterovirus CVB3 inhibits human cardiac pacemaker function by reducing the pacemaker channel plasma membrane density, an effect that can be corrected by pharmacological intervention of endocytic vesicle trafficking.

Frequency-Dependent Properties of the Hyperpolarization-Activated Cation Current, If, in Adult Mouse Heart Primary Pacemaker Myocytes

A number of distinct electrophysiological mechanisms that modulate the myogenic spontaneous pacemaker activity in the sinoatrial node (SAN) of the mammalian heart have been investigated extensively. There is agreement that several (3 or 4) different transmembrane ionic current changes (referred to as the voltage clock) are involved; and that the resulting net current interacts with direct and indirect effects of changes in intracellular Ca2+ (the calcium clock). However, significant uncertainties, and important knowledge gaps, remain concerning the functional roles in SAN spontaneous pacing of many of the individual ion channel- or exchanger-mediated transmembrane current changes. We report results from patch clamp studies and mathematical modeling of the hyperpolarization-activated current, If, in the generation/modulation of the diastolic depolarization, or pacemaker potential, produced by individual myocytes that were enzymatically isolated from the adult mouse sinoatrial node (SAN). Amphotericin-mediated patch microelectrode recordings at 35 °C were made under control conditions and in the presence of 5 or 10 nM isoproterenol (ISO). These sets of results were complemented and integrated with mathematical modeling of the current changes that take place in the range of membrane potentials (-70 to -50 mV), which corresponds to the 'pacemaker depolarization' in the adult mouse SAN. Our results reveal a very small, but functionally important, approximately steady-state or time-independent current generated by residual activation of If channels that are expressed in these pacemaker myocytes. Recordings of the pacemaker depolarization and action potential, combined with measurements of changes in If, and the well-known increases in the L-type Ca2+ current, ICaL, demonstrated that ICaL activation, is essential for myogenic pacing. Moreover, after being enhanced (approximately 3-fold) by 5 or 10 nM ISO, ICaL contributes significantly to the positive chronotropic effect. Our mathematical model has been developed in an attempt to better understand the underlying mechanisms for the pacemaker depolarization and action potential in adult mouse SAN myocytes. After being updated with our new experimental data describing If, our simulations reveal a novel functional component of If in adult mouse SAN. Computational work carried out with this model also confirms that in the presence of ISO the residual activation of If and opening of ICaL channels combine to generate a net current change during the slow diastolic depolarization phase that is essential for the observed accelerated pacemaking rate of these SAN myocytes.

Cancer Chemotherapy-Induced Sinus Bradycardia: A Narrative Review of a Forgotten Adverse Effect of Cardiotoxicity

Cardiotoxicity is a common adverse effect of anticancer drugs (ACDs), including the so-called targeted drugs, and increases morbidity and mortality in patients with cancer. Attention has focused mainly on ACD-induced heart failure, myocardial ischemia, hypertension, thromboembolism, QT prolongation, and tachyarrhythmias. Yet, although an increasing number of ACDs can produce sinus bradycardia (SB), this proarrhythmic effect remains an underappreciated complication, probably because of its low incidence and severity since most patients are asymptomatic. However, SB merits our interest because its incidence increases with the aging of the population and cancer is an age-related disease and because SB represents a risk factor for QT prolongation. Indeed, several ACDs that produce SB also prolong the QT interval. We reviewed published reports on ACD-induced SB from January 1971 to November 2020 using the PubMed and EMBASE databases. Published reports from clinical trials, case reports, and recent reviews were considered. This review describes the associations between ACDs and SB, their clinical relevance, risk factors, and possible mechanisms of onset and treatment.

Pharmacological Modulation and (Patho)Physiological Roles of TRPM4 Channel-Part 2: TRPM4 in Health and Disease

Transient receptor potential melastatin 4 (TRPM4) is a unique member of the TRPM protein family and, similarly to TRPM5, is Ca²⁺ sensitive and permeable for monovalent but not divalent cations. It is widely expressed in many organs and is involved in several functions; it regulates membrane potential and Ca²⁺ homeostasis in both excitable and non-excitable cells. This part of the review discusses the currently available knowledge about the physiological and pathophysiological roles of TRPM4 in various tissues. These include the physiological functions of TRPM4 in the cells of the Langerhans islets of the pancreas, in various immune functions, in the regulation of vascular tone, in respiratory and other neuronal activities, in chemosensation, and in renal and cardiac physiology. TRPM4 contributes to pathological conditions such as overactive bladder, endothelial dysfunction, various types of malignant diseases and central nervous system conditions including stroke and injuries as well as in cardiac conditions such as arrhythmias, hypertrophy, and ischemia-reperfusion injuries. TRPM4 claims more and more attention and is likely to be the topic of research in the future.

The Role of POPDC Proteins in Cardiac Pacemaking and Conduction

The Popeye domain-containing (POPDC) gene family, consisting of Popdc1 (also known as Bves), Popdc2, and Popdc3, encodes transmembrane proteins abundantly expressed in striated muscle. POPDC proteins have recently been identified as cAMP effector proteins and have been proposed to be part of the protein network involved in cAMP signaling. However, their exact biochemical activity is presently poorly understood. Loss-of-function mutations in animal models causes abnormalities in skeletal muscle regeneration, conduction, and heart rate adaptation after stress. Likewise, patients carrying missense or nonsense mutations in POPDC genes have been associated with cardiac arrhythmias and limb-girdle muscular dystrophy. In this review, we introduce the POPDC protein family, and describe their structure function, and role in cAMP signaling. Furthermore, the pathological phenotypes observed in zebrafish and mouse models and the clinical and molecular pathologies in patients carrying POPDC mutations are described.

Electrophysiological Consequences of Cardiac Fibrosis

For both the atria and ventricles, fibrosis is generally recognized as one of the key determinants of conduction disturbances. By definition, fibrosis refers to an increased amount of fibrous tissue. However, fibrosis is not a singular entity. Various forms can be distinguished, that differ in distribution: replacement fibrosis, endomysial and perimysial fibrosis, and perivascular, endocardial, and epicardial fibrosis. These different forms typically result from diverging pathophysiological mechanisms and can have different consequences for conduction. The impact of fibrosis on propagation depends on exactly how the patterns of electrical connections between myocytes are altered. We will therefore first consider the normal patterns of electrical connections and their regional diversity as determinants of propagation. Subsequently, we will summarize current knowledge on how different forms of fibrosis lead to a loss of electrical connectivity in order to explain their effects on propagation and mechanisms of arrhythmogenesis, including ectopy, reentry, and alternans. Finally, we will discuss a histological quantification of fibrosis. Because of the different forms of fibrosis and their diverging effects on electrical propagation, the total amount of fibrosis is a poor indicator for the effect on conduction. Ideally, an assessment of cardiac fibrosis should exclude fibrous tissue that does not affect conduction and differentiate between the various types that do; in this article, we highlight practical solutions for histological analysis that meet these requirements.

Further insights into the molecular complexity of the human sinus node - The role of 'novel' transcription factors and microRNAs

RESEARCH PURPOSE

The sinus node (SN) is the heart's primary pacemaker. Key ion channels (mainly the funny channel, HCN4) and Ca2+-handling proteins in the SN are responsible for its function. Transcription factors (TFs) regulate gene expression through inhibition or activation and microRNAs (miRs) do this through inhibition. There is high expression of macrophages and mast cells within the SN connective tissue. 'Novel'/unexplored TFs and miRs in the regulation of ion channels and immune cells in the SN are not well understood. Using RNAseq and bioinformatics, the expression profile and predicted interaction of key TFs and cell markers with key miRs in the adult human SN vs. right atrial tissue (RA) were determined.

PRINCIPAL RESULTS

68 and 60 TFs significantly more or less expressed in the SN vs. RA respectively. Among those more expressed were ISL1 and TBX3 (involved in embryonic development of the SN) and 'novel' RUNX1-2, CEBPA, GLI1-2 and SOX2. These TFs were predicted to regulate HCN4 expression in the SN. Markers for different cells: fibroblasts (COL1A1), fat (FABP4), macrophages (CSF1R and CD209), natural killer (GZMA) and mast (TPSAB1) were significantly more expressed in the SN vs. RA. Interestingly, RUNX1-3, CEBPA and GLI1 also regulate expression of these cells. MiR-486-3p inhibits HCN4 and markers involved in immune response.

MAJOR CONCLUSIONS

In conclusion, RUNX1-2, CSF1R, TPSAB1, COL1A1 and HCN4 are highly expressed in the SN but not miR-486-3p. Their complex interactions can be used to treat SN dysfunction such as bradycardia. Interestingly, another research group recently reported miR-486-3p is upregulated in blood samples from severe COVID-19 patients who suffer from bradycardia.

Sinus Node Dysfunction

Sinus node dysfunction (SND) is a multifaceted disorder most prevalent in older individuals, but may also occur at an earlier age. In most cases, the SND diagnosis is ultimately established by documenting its ECG manifestations. EPS has limited utility. The treatment strategy is largely dictated by symptoms and ECG manifestations. Not infrequently, both bradycardia and tachycardia coexist in the same patients, along with other diseases common in the elderly (e.g., hypertension, coronary artery disease), thereby complicating treatment strategy. Prevention of the adverse consequences of both bradyarrhythmia and tachyarrhythmia is important to reduce susceptibility to syncope, falls, and thromboembolic complications.

Zebrafish Models in Therapeutic Research of Cardiac Conduction Disease

Malfunction in the cardiac conduction system (CCS) due to congenital anomalies or diseases can cause cardiac conduction disease (CCD), which results in disturbances in cardiac rhythm, leading to syncope and even sudden cardiac death. Insights into development of the CCS components, including pacemaker cardiomyocytes (CMs), atrioventricular node (AVN) and the ventricular conduction system (VCS), can shed light on the pathological and molecular mechanisms underlying CCD, provide approaches for generating human pluripotent stem cell (hPSC)-derived CCS cells, and thus improve therapeutic treatment for such a potentially life-threatening disorder of the heart. However, the cellular and molecular mechanisms controlling CCS development remain elusive. The zebrafish has become a valuable vertebrate model to investigate early development of CCS components because of its unique features such as external fertilization, embryonic optical transparency and the ability to survive even with severe cardiovascular defects during development. In this review, we highlight how the zebrafish has been utilized to dissect the cellular and molecular mechanisms of CCS development, and how the evolutionarily conserved developmental mechanisms discovered in zebrafish could be applied to directing the creation of hPSC-derived CCS cells, therefore providing potential therapeutic strategies that may contribute to better treatment for CCD patients.

Permanent pacemaker implantation late after transcatheter aortic valve implantation

BACKGROUND

Impairment of atrioventricular (AV) conduction may occur late after transcatheter aortic valve implantation (TAVI), and progression to complete AV block is a matter of concern.

OBJECTIVE

The purpose of this study was to describe the incidence of permanent pacemaker (PPM) implantation late after TAVI.

METHODS

In a prospective TAVI registry, we retrospectively identified patients with PPM implantation after hospital discharge for TAVI and analyzed serial electrocardiograms for AV conduction impairment before PPM implantation.

RESULTS

Among 1059 patients discharged after TAVI without PPM between January 2012 and December 2017, 62 patients (5.9%) underwent PPM implantation at a median of 305 days after discharge for TAVI. Indications for PPM implantation late after TAVI were AV conduction impairment in 46 patients (74.2%); sick sinus syndrome in 10 (16.1%); cardiac resynchronization or implantable cardioverter-defibrillator indication in 2 (3.2%); and a pace and ablate strategy in 4 (6.5%). Clinical symptoms leading to PPM implantation late after TAVI included syncope in 19 patients (30.7%), presyncope in 7 (11.3%), and dyspnea in 8 (12.9%). First-degree AV block and new left bundle branch block (LBBB) after TAVI as well as valve-in-valve procedure during follow-up were independent predictors of PPM implantation late after TAVI due to AV conduction impairment.

CONCLUSION

PPM implantation late after TAVI is infrequent and is associated with clinical symptoms in half of patients. Impairment of AV conduction was the indication in three-quarters of patients. First-degree AV block and new LBBB after TAVI as well as valve-in-valve procedure during follow-up emerged as independent predictors.

The Organization of the Sinoatrial Node Microvasculature Varies Regionally to Match Local Myocyte Excitability

The cardiac cycle starts when an action potential is produced by pacemaking cells in the sinoatrial node. This cycle is repeated approximately 100 000 times in humans and 1 million times in mice per day, imposing a monumental metabolic demand on the heart, requiring efficient blood supply via the coronary vasculature to maintain cardiac function. Although the ventricular coronary circulation has been extensively studied, the relationship between vascularization and cellular pacemaking modalities in the sinoatrial node is poorly understood. Here, we tested the hypothesis that the organization of the sinoatrial node microvasculature varies regionally, reflecting local myocyte firing properties. We show that vessel densities are higher in the superior versus inferior sinoatrial node. Accordingly, sinoatrial node myocytes are closer to vessels in the superior versus inferior regions. Superior and inferior sinoatrial node myocytes produce stochastic subthreshold voltage fluctuations and action potentials. However, the intrinsic action potential firing rate of sinoatrial node myocytes is higher in the superior versus inferior node. Our data support a model in which the microvascular densities vary regionally within the sinoatrial node to match the electrical and Ca²⁺ dynamics of nearby myocytes, effectively determining the dominant pacemaking site within the node. In this model, the high vascular density in the superior sinoatrial node places myocytes with metabolically demanding, high-frequency action potentials near vessels. The lower vascularization and electrical activity of inferior sinoatrial node myocytes could limit these cells to function to support sinoatrial node periodicity with sporadic voltage fluctuations via a stochastic resonance mechanism.

Sudden Death without a Clear Cause after Comprehensive Investigation: An Example of Forensic Approach to Atypical/Uncertain Findings

Sudden death (SD) is defined as the unexpected natural death occurred within an hour after the onset of symptoms or from the last moment the subject has been seen in a healthy condition. Brugada syndrome (BrS) is one of the most remarkable cardiac causes of SD among young people. We report the case of a 20-year-old man who suddenly died after reportedly having smoked cannabis. Autopsy, toxicology, and genetic testing were performed. Autopsy found a long and thick myocardial bridging (MB) at 2 cm from the beginning of the left anterior descending coronary artery. Furthermore, at the histopathological examination, fibrosis and disarray in myocardial area above the MB, fatty tissue in the right ventricle and fibrosis of the sino-atrial node area were found. Toxicology testing was inconclusive, while genetic testing found a rare missense variant of the TTN gene, classified as likely benign, and a variant of unknown significance in the SLMAP gene (a gene that can be associated with BrS). Hence, despite several atypical features were found, no inference on the cause of the death could be made under current evidence.

LncRNA Nuclear-Enriched Abundant Transcript 1 Regulates Atrial Fibrosis via the miR-320/NPAS2 Axis in Atrial Fibrillation

Atrial fibrosis is a key contributor to atrial fibrillation (AF). Long non-coding ribonucleic acids (lncRNAs) were demonstrated to exhibit a key role in fibrotic remodeling; however, the function of nuclear-enriched abundant transcript 1 (NEAT1) in atrial fibrosis remains unclear. In the present study, we showed that NEAT1 was upregulated in atrial tissues of AF patients and was positively related to collagen I (coll I) and collagen III (coll III) expressions. Furthermore, the deletion of NEAT1 attenuated angiotensin II (Ang II)-caused atrial fibroblast proliferation, migration, and collagen production. We further observed that NEAT1 knockdown improved Ang II caused mouse atrial fibrosis in in vivo experiments. Moreover, we demonstrated that NEAT1 could negatively regulate miR-320 expression by acting as a competitive endogenous RNA (ceRNA). miR-320 directly targeted neuronal per arnt sim domain protein 2 (NPAS2) and suppressed its expression. We observed that NEAT1 exerted its function via the miR-320–NPAS2 axis in cardiac fibroblasts. These findings indicate that NEAT1 exerts a significant effect on atrial fibrosis and that this lncRNA is a new potential molecular target for AF treatment.

Reduced cardiac response to the adrenergic system is a key limiting factor for physical capacity in old age

Ageing is associated with a progressive reduction in physical capacity reducing quality of life. One key physiological limitation of physical capacity that deteriorates in a progressive age-dependent manner is cardiac reserve. Peak cardiac output falls progressively with advancing age such that in extreme old age there is limited ability to enhance cardiac output beyond basal function as is required to support the increased metabolic needs of physical activity. This loss of dynamic range in cardiac output associates with a progressive reduction in the heart's response to adrenergic stimulation. A combination of decreases in the expression and functioning of beta1 adrenergic receptors partially underlies this change. Changes in end effector proteins also have a role to play in this decline. Alterations in the efficiency of excitation-contraction coupling contribute to the reduced chronotropic, inotropic and lusitropic responses of the aged heart. Moderate to vigorous endurance exercise training however has some potential to counter elements of these changes. Further studies are required to fully elucidate the key pivotal mechanisms involved in the age-related loss of response to adrenergic signalling to allow targeted therapeutic strategies to be developed with the aim of preserving physical capacity in advanced old age.

Structural and Functional Properties of Subsidiary Atrial Pacemakers in a Goat Model of Sinus Node Disease

Background

The sinoatrial/sinus node (SAN) is the primary pacemaker of the heart. In humans, SAN is surrounded by the paranodal area (PNA). Although the PNA function remains debated, it is thought to act as a subsidiary atrial pacemaker (SAP) tissue and become the dominant pacemaker in the setting of sinus node disease (SND). Large animal models of SND allow characterization of SAP, which might be a target for novel treatment strategies for SAN diseases.

Methods

A goat model of SND was developed (n = 10) by epicardially ablating the SAN and validated by mapping of emergent SAP locations through an ablation catheter and surface electrocardiogram (ECG). Structural characterization of the goat SAN and SAP was assessed by histology and immunofluorescence techniques.

Results

When the SAN was ablated, SAPs featured a shortened atrioventricular conduction, consistent with the location in proximity of atrioventricular junction. SAP recovery time showed significant prolongation compared to the SAN recovery time, followed by a decrease over a follow-up of 4 weeks. Like the SAN tissue, the SAP expressed the main isoform of pacemaker hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) and Na⁺/Ca²⁺ exchanger 1 (NCX1) and no high conductance connexin 43 (Cx43). Structural characterization of the right atrium (RA) revealed that the SAN was located at the earliest activation [i.e., at the junction of the superior vena cava (SVC) with the RA] and was surrounded by the paranodal-like tissue, extending down to the inferior vena cava (IVC). Emerged SAPs were localized close to the IVC and within the thick band of the atrial muscle known as the crista terminalis (CT).

Conclusions

SAN ablation resulted in the generation of chronic SAP activity in 60% of treated animals. SAP displayed development over time and was located within the previously discovered PNA in humans, suggesting its role as dominant pacemaker in SND. Therefore, SAP in goat constitutes a promising stable target for electrophysiological modification to construct a fully functioning pacemaker.

Atrial and Sinoatrial Node Development in the Zebrafish Heart

Proper development and function of the vertebrate heart is vital for embryonic and postnatal life. Many congenital heart defects in humans are associated with disruption of genes that direct the formation or maintenance of atrial and pacemaker cardiomyocytes at the venous pole of the heart. Zebrafish are an outstanding model for studying vertebrate cardiogenesis, due to the conservation of molecular mechanisms underlying early heart development, external development, and ease of genetic manipulation. Here, we discuss early developmental mechanisms that instruct appropriate formation of the venous pole in zebrafish embryos. We primarily focus on signals that determine atrial chamber size and the specialized pacemaker cells of the sinoatrial node through directing proper specification and differentiation, as well as contemporary insights into the plasticity and maintenance of cardiomyocyte identity in embryonic zebrafish hearts. Finally, we integrate how these insights into zebrafish cardiogenesis can serve as models for human atrial defects and arrhythmias.

Cardiac Pacemaker Dysfunction Arising From Different Studies of Ion Channel Remodeling in the Aging Rat Heart

The function of the sinoatrial node (SAN), the pacemaker of the heart, declines with age, resulting in increased incidence of sinoatrial node dysfunction (SND) in older adults. The present study assesses potential ionic mechanisms underlying age associated SND. Two group studies have identified complex and various changes in some of membrane ion channels in aged rat SAN, the first group (Aging Study-1) indicates a considerable changes of gene expression with up-regulation of mRNA in ion channels of Cav1.2, Cav1.3 and KvLQT1, Kv4.2, and the Ca²⁺ handling proteins of SERCA2a, and down-regulation of Cav3.1, NCX, and HCN1 and the Ca²⁺-clock proteins of RYR2. The second group (Aging Study-2) suggests a different pattern of changes, including down regulation of Cav1.2, Cav1.3 and HCN4, and RYR2, and an increase of NCX and SERCA densities and proteins. Although both data sets shared a similar finding for some specific ion channels, such as down regulation of HCN4, NCX, and RYR2, there are contradictory changes for some other membrane ion channels, such as either up-regulation or down-regulation of Cav1.2, NCX and SERCA2a in aged rat SAN. The present study aims to test a hypothesis that age-related SND may arise from different ionic and molecular remodeling patterns. To test this hypothesis, a mathematical model of the electrical action potential of rat SAN myocytes was modified to simulate the functional impact of age-induced changes on membrane ion channels and intracellular Ca²⁺ handling as observed in Aging Study-1 and Aging Study-2. The role and relative importance of each individually remodeled ion channels and Ca²⁺-handling in the two datasets were evaluated. It was shown that the age-induced changes in ion channels and Ca²⁺-handling, based on either Aging Study-1 or Aging Study-2, produced similar bradycardic effects as manifested by a marked reduction in the heart rate (HR) that matched experimental observations. Further analysis showed that although the SND arose from an integrated action of all remodeling of ion channels and Ca²⁺-handling in both studies, it was the change to I _CaL that played the most important influence.

Ischemic Stroke in Patients With Sinus Node Disease, Atrial Fibrillation, and Other Cardiac Conditions

Background and Purpose- Atrial fibrillation (AF) is known to increase risk of ischemic stroke (IS), but the risk of IS in isolated sinus node disease (SND) is unclear. We compared the incidence of IS in patients with SND, patients with AF, and in a control population with other cardiac diseases (disease of the circulatory system using the International Classification of Diseases, Tenth Revision). Methods- This French longitudinal cohort study was based on the national database covering hospital care for the entire population from 2008 to 2015. Results- Of 1 692 157 patients included in the cohort, 100 366 had isolated SND, 1 564 270 had isolated AF, and 27 521 had AF associated with SND. Incidence of IS during follow-up was higher in isolated patients with AF than in AF associated with SND (yearly rate 2.22% versus 2.06%) and in isolated patients with AF than in isolated patients with SND (yearly rate 2.22% versus 1.59%). The incidence of IS was lower in a control population with other cardiac conditions (n=479 108) compared with SND and patients with AF (0.96%/y, 1.59%/y, and 2.22%/y, respectively). After 1:1 propensity score matching, SND was associated with lower incidence of IS compared to AF (hazard ratio, 0.77 [95% CI, 0.73-0.82]) but higher incidence of IS compared to control population (hazard ratio, 1.27 [95%CI, 1.19-1.35]). Conclusions- Patients with SND had a lower risk of thromboembolic events than patients with AF but a higher risk than a control population with other cardiac diseases. Randomized clinical trial in a selected SND population, with, for example, a high CHA₂DS₂-VASc score, would be required to determine the value of IS prevention by anticoagulation.

Pathological bradyarrhythmia in horses

Pathological bradyarrhythmia is rare in horses but should be especially considered when presented with a horse that has signs consistent with episodic weakness or collapse. This paper reviews the literature describing our current knowledge of, and possible mechanisms causing, clinically significant bradyarrhythmia in horses.

A study of the outward background current conductance gK1, the pacemaker current conductance gf, and the gap junction conductance gj as determinants of biological pacing in single cells and in a two-cell syncytium using the dynamic clamp

We previously demonstrated that a two-cell syncytium, composed of a ventricular myocyte and an mHCN2 expressing cell, recapitulated most properties of in vivo biological pacing induced by mHCN2-transfected hMSCs in the canine ventricle. Here, we use the two-cell syncytium, employing dynamic clamp, to study the roles of g_f (pacemaker conductance), g_K1 (background K⁺ conductance), and g_j (intercellular coupling conductance) in biological pacing. We studied g_f and g_K1 in single HEK293 cells expressing cardiac sodium current channel Na_v1.5 (SCN5A). At fixed g_f, increasing g_K1 hyperpolarized the cell and initiated pacing. As g_K1 increased, rate increased, then decreased, finally ceasing at membrane potentials near E_K. At fixed g_K1, increasing g_f depolarized the cell and initiated pacing. With increasing g_f, rate increased reaching a plateau, then decreased, ceasing at a depolarized membrane potential. We studied g_j via virtual coupling with two non-adjacent cells, a driver (HEK293 cell) in which g_K1 and g_f were injected without SCN5A and a follower (HEK293 cell), expressing SCN5A. At the chosen values of g_K1 and g_f oscillations initiated in the driver, when g_j was increased synchronized pacing began, which then decreased by about 35% as g_j approached 20 nS. Virtual uncoupling yielded similar insights into g_j. We also studied subthreshold oscillations in physically and virtually coupled cells. When coupling was insufficient to induce pacing, passive spread of the oscillations occurred in the follower. These results show a non-monotonic relationship between g_K1, g_f, g_j, and pacing. Further, oscillations can be generated by g_K1 and g_f in the absence of SCN5A.

Cold-Inducible RNA-Binding Protein Prevents an Excessive Heart Rate Response to Stress by Targeting Phosphodiesterase

RATIONALE

The stress response of heart rate, which is determined by the plasticity of the sinoatrial node (SAN), is essential for cardiac function and survival in mammals. As an RNA-binding protein, CIRP (cold-inducible RNA-binding protein) can act as a stress regulator. Previously, we have documented that CIRP regulates cardiac electrophysiology at posttranscriptional level, suggesting its role in SAN plasticity, especially upon stress conditions.

OBJECTIVE

Our aim was to clarify the role of CIRP in SAN plasticity and heart rate regulation under stress conditions.

METHODS AND RESULTS

Telemetric ECG monitoring demonstrated an excessive acceleration of heart rate under isoprenaline stimulation in conscious CIRP-KO (knockout) rats. Patch-clamp analysis and confocal microscopic Ca²⁺ imaging of isolated SAN cells demonstrated that isoprenaline stimulation induced a faster spontaneous firing rate in CIRP-KO SAN cells than that in WT (wild type) SAN cells. A higher concentration of cAMP-the key mediator of pacemaker activity-was detected in CIRP-KO SAN tissues than in WT SAN tissues. RNA sequencing and quantitative real-time polymerase chain reaction analyses of single cells revealed that the 4B and 4D subtypes of PDE (phosphodiesterase), which controls cAMP degradation, were significantly decreased in CIRP-KO SAN cells. A PDE4 inhibitor (rolipram) abolished the difference in beating rate resulting from CIRP deficiency. The mechanistic study showed that CIRP stabilized the mRNA of Pde4b and Pde4d by direct mRNA binding, thereby regulating the protein expression of PDE4B and PDE4D at posttranscriptional level.

CONCLUSIONS

CIRP acts as an mRNA stabilizer of specific PDEs to control the cAMP concentration in SAN, maintaining the appropriate heart rate stress response.

Risk factors and a 3-month risk score for predicting pacemaker implantation in patients with atrial fibrillations

Objectives

To identify risk factors and to develop a predictive risk score for pacemaker implantation in patients with atrial fibrillation (AF).

Methods

Using Danish nationwide registries, patients with newly diagnosed AF from 2000 to 2014 were identified. Cox proportional-hazards regression computed HRs for risk factors of pacemaker implantation. A logistic regression was used to fit a prediction model for 3-month risk of pacemaker implantation and derived a risk score using 80% of the data and its predictive accuracy estimated using the remaining 20%.

Results

Among 155 934 AF patients included, the median age (IQR) was 75 (65–83) and 51.3% were men. During a median follow-up time of 3.4 (1.2–5.0) years, 8348 (5.4%) patients received a pacemaker implantation. Risk factors of pacemaker implantation were (in order of highest risk first) age above 60 years, congenital heart disease, heart failure at age under 60 years, prior syncope, valvular AF, hypertension, ischaemic heart disease, male sex and diabetes mellitus. The derived risk score assigns points ranging from 1 to 14 to each of these risk factors. The 3-month risk of pacemaker implantation increased from 0.4% (95% CI: 0.2 to 0.8) at 1 point to 2.6% (95% CI: 1.9 to 3.6) at 18 points. Area under the receiver operator characteristics curve was 62.9 (95% CI: 60.3 to 65.5).

Conclusion

We highlighted risk factors of pacemaker implantation in newly diagnosed AF patients and created a risk score. The clinical utility of the risk score needs further investigation.

Retrospective study of allometric relationship between heart rate, electrocardiographic parameters and bodyweight in dogs

ABSTRACT The allometric relationship between bodyweight (BW) and heart rate (HR) has been described as inversely proportional in domestic species, but that has been refuted. The relationship between HR and electrocardiographic variables is described in literature. However, studies about the variation and influence of factors on the hemodynamic and electrocardiographic parameters in dogs are not abundant. As the metabolic rate is defined as the production and dissipation of heat by the body surface area (BSA) in m², it is essential to define that relationship. A retrospective study was conducted to analyze the correlation between HR, ECG parameters and BW in dogs. One thousand electrocardiographic tracings were analyzed in addition to the ECG parameters and clinical data such as gender, age and bodyweight. The determination of BSA was performed as follows: BSA (m2) = (10.1 x bodyweight 0.67) X 10-4. When the unified groups were analyzed, there was a negative but weak correlation (r= -0.14, P< 0.0001) between bodyweight and HR. There were differences between weight groups regarding electrocardiographic variables. There is no allometric relationship between BW and HR in dogs. Weight was associated with changes in ECG variables.

Embryology of the Cardiac Conduction System Relevant to Arrhythmias

Embryogenesis of the heart involves the complex cellular differentiation of slow-conducting primary myocardium into the rapidly conducting chamber myocardium of the adult. However, small areas of relatively undifferentiated cells remain to form components of the adult cardiac conduction system (CCS) and nodal tissues. Further investigation has revealed additional areas of nodal-like tissues outside of the established CCS. The embryologic origins of these areas are similar to those of the adult CCS. Under pathologic conditions, these areas can give rise to important clinical arrhythmias. Here, we review the embryologic basis for these proarrhythmic structures within the heart.

Functional Characterization of Rare Variants in the SHOX2 Gene Identified in Sinus Node Dysfunction and Atrial Fibrillation

Sinus node dysfunction (SND) and atrial fibrillation (AF) often coexist; however, the molecular mechanisms linking both conditions remain elusive. Mutations in the homeobox-containing SHOX2 gene have been recently associated with early-onset and familial AF. Shox2 is a key regulator of sinus node development, and its deficiency leads to bradycardia, as demonstrated in animal models. To provide an extended SHOX2 gene analysis in patients with distinct arrhythmias, we investigated SHOX2 as a susceptibility gene for SND and AF by screening 98 SND patients and 450 individuals with AF. The functional relevance of the novel mutations was investigated in vivo and in vitro, together with the previously reported p.H283Q variant. A heterozygous missense mutation (p.P33R) was identified in the SND cohort and four heterozygous variants (p.G77D, p.L129=, p.L130F, p.A293=) in the AF cohort. Overexpression of the pathogenic predicted mutations in zebrafish revealed pericardial edema for p.G77D and the positive control p.H283Q, whereas the p.P33R and p.A293= variants showed no effect. In addition, a dominant-negative effect with reduced heart rates was detected for p.G77D and p.H283Q. In vitro reporter assays demonstrated for both missense variants p.P33R and p.G77D significantly impaired transactivation activity, similar to the described p.H283Q variant. Also, a reduced Bmp4 target gene expression was revealed in zebrafish hearts upon overexpression of the p.P33R mutant. This study associates additional rare variants in the SHOX2 gene implicated in the susceptibility to distinct arrhythmias and allows frequency estimations in the AF cohort (3/990). We also demonstrate for the first time a genetic link between SND and AF involving SHOX2. Moreover, our data highlight the importance of functional investigations of rare variants.

Transcriptomic Profiling of the Developing Cardiac Conduction System at Single-Cell Resolution

RATIONALE

The cardiac conduction system (CCS) consists of distinct components including the sinoatrial node, atrioventricular node, His bundle, bundle branches, and Purkinje fibers. Despite an essential role for the CCS in heart development and function, the CCS has remained challenging to interrogate because of inherent obstacles including small cell numbers, large cell-type heterogeneity, complex anatomy, and difficulty in isolation. Single-cell RNA-sequencing allows for genome-wide analysis of gene expression at single-cell resolution.

OBJECTIVE

Assess the transcriptional landscape of the entire CCS at single-cell resolution by single-cell RNA-sequencing within the developing mouse heart.

METHODS AND RESULTS

Wild-type, embryonic day 16.5 mouse hearts (n=6 per zone) were harvested and 3 zones of microdissection were isolated, including: Zone I-sinoatrial node region; Zone II-atrioventricular node/His region; and Zone III-bundle branch/Purkinje fiber region. Tissue was digested into single-cell suspensions, cells isolated, mRNA reverse transcribed, and barcoded before high-throughput sequencing and bioinformatics analyses. Single-cell RNA-sequencing was performed on over 22 000 cells, and all major cell types of the murine heart were successfully captured including bona fide clusters of cells consistent with each major component of the CCS. Unsupervised weighted gene coexpression network analysis led to the discovery of a host of novel CCS genes, a subset of which were validated using fluorescent in situ hybridization as well as whole-mount immunolabeling with volume imaging (iDISCO+) in 3 dimensions on intact mouse hearts. Further, subcluster analysis unveiled isolation of distinct CCS cell subtypes, including the clinically relevant but poorly characterized transitional cells that bridge the CCS and surrounding myocardium.

CONCLUSIONS

Our study represents the first comprehensive assessment of the transcriptional profiles from the entire CCS at single-cell resolution and provides a characterization in the context of development and disease.

Sinoatrial node remodels in chronic sleep-restricted rats

Chronic Sleep Restriction (CSR) is known as a risk factor for cardiovascular diseases. However, the structural changes of Sinoatrial (SA) node cells have received less attention. This study aimed to evaluate the effects of CSR on SA node in an animal model using stereological methods. Adult male Sprague-Dawley rats were randomly divided into CSR, grid-floor, and control groups. The CSR procedure was designed such a way that the animals had a full cycle of sleep (6 hours) per day, while they were unable to have a Rapid Eye Movement (REM) sleep during the remaining 18 hours. This was induced by a multiplatform box containing water. The grid-floor animals were placed in the same multiplatform box with a grid-floor covering to prevent falling in water. After 21 days, the right atria were dissected out. Then, the location of the SA node was determined and evaluated by stereological techniques. The total volume of the SA node, the total volume of the main node cells, the volume of the connective tissue, and mean volume of the node cells were respectively enlarged by 60%, 47%, 68%, and 51% in the CSR animals compared to the grid-floor rats (p < 0.05). However, no significant changes were detected in these parameters in the control and grid-floor animals. The population of the main node cells remained constant in all animal groups. In addition, the three-dimensional reconstruction of the SA node in the CSR group showed a hypertrophied appearance. In conclusion, CSR induced hypertrophic changes in the rats' SA node structures without alteration in the number of main node cells.

Sick Sinus Syndrome

The sick sinus syndrome includes symptoms and signs related to sinus node dysfunction. This can be caused by intrinsic abnormal impulse formation and/or propagation from the sinus node or, in some cases, by extrinsic reversible causes. Careful evaluation of symptoms and of the electrocardiogram is of crucial importance, because diagnosis is mainly based on these 2 elements. In some cases, the pathophysiologic mechanism that induces sinus node dysfunction also favors the onset of atrial arrhythmias, which results in a more complex clinical condition, known as "bradycardia-tachycardia syndrome."