Cardiac GR Mediates the Diurnal Rhythm in Ventricular Arrhythmia Susceptibility.

BACKGROUND: Ventricular arrhythmias (VAs) demonstrate a prominent day-night rhythm, commonly presenting in the morning. Transcriptional rhythms in cardiac ion channels accompany this phenomenon, but their role in the morning vulnerability to VAs and the underlying mechanisms are not understood. We investigated the recruitment of transcription factors that underpins transcriptional rhythms in ion channels and assessed whether this mechanism was pertinent to the heart's intrinsic diurnal susceptibility to VA. METHODS AND RESULTS: Assay for transposase-accessible chromatin with sequencing performed in mouse ventricular myocyte nuclei at the beginning of the animals' inactive (ZT0) and active (ZT12) periods revealed differentially accessible chromatin sites annotating to rhythmically transcribed ion channels and distinct transcription factor binding motifs in these regions. Notably, motif enrichment for the glucocorticoid receptor (GR; transcriptional effector of corticosteroid signaling) in open chromatin profiles at ZT12 was observed, in line with the well-recognized ZT12 peak in circulating corticosteroids. Molecular, electrophysiological, and in silico biophysically-detailed modeling approaches demonstrated GR-mediated transcriptional control of ion channels (including Scn5a underlying the cardiac Na+ current, Kcnh2 underlying the rapid delayed rectifier K+ current, and Gja1 responsible for electrical coupling) and their contribution to the day-night rhythm in the vulnerability to VA. Strikingly, both pharmacological block of GR and cardiomyocyte-specific genetic knockout of GR blunted or abolished ion channel expression rhythms and abolished the ZT12 susceptibility to pacing-induced VA in isolated hearts. CONCLUSIONS: Our study registers a day-night rhythm in chromatin accessibility that accompanies diurnal cycles in ventricular myocytes. Our approaches directly implicate the cardiac GR in the myocyte excitability rhythm and mechanistically link the ZT12 surge in glucocorticoids to intrinsic VA propensity at this time.

Intrinsic Electrical Remodeling Underlies Atrioventricular Block in Athletes.

[Figure: see text].

In vivo knockdown of SK3 channels using antisense oligonucleotides protects against atrial fibrillation in rats.

INTRODUCTION: GapmeRs are oligonucleotides that bind to a specific RNA sequence and thereby affecting posttranscriptional gene regulation. They therefore hold the potential to manipulate targets where current pharmacological modulators are inefficient or exhibit adverse side effects. Here, we show that a treatment with a GapmeR, mediating knockdown of small conductance Ca2+-activated K+ channels (SK3), has an in vivo protective effect against atrial fibrillation (AF) in rats. MATERIAL AND METHODS: A unique SK3-GapmeR design was selected after thorough in vitro evaluation. 22 rats were randomly assigned to receive either 50 mg/kg SK3-GapmeR or vehicle subcutaneously once a week for two weeks. Langendorff experiments were performed seven days after the last injection, where action potential duration (APD90), effective refractory period (ERP) and AF propensity were investigated. SK3 channel activity was evaluated using the SK channel blocker, ICA (N-(pyridin-2-yl)-4-(pyridine-2-yl)thiazol-2-amine). SK3 protein expression was assessed by Western Blot. RESULTS: The designed GapmeR effectively down-regulate the SK3 protein expression in the heart (48% downregulation, p = 0.0095) and did indeed protect against AF. Duration of AF episodes elicited by burst pacing in the rats treated with SK3-GapmeR was reduced 78% compared to controls (3.7 s vs. 16.8 s, p = 0.0353). The number of spontaneous AF episodes were decreased by 68% in the SK3-GapmeR group (39 episodes versus 123 in the control group, respectively) and were also significantly shorter in duration (7.2 s versus 29.7 s in the control group, p = 0.0327). Refractoriness was not altered at sinus rhythm, but ERP prolongation following ICA application was blunted in the SK3-GapmeR group. CONCLUSION: The selected GapmeR silenced the cardiac SK3 channels, thereby preventing AF in rats. Thus, GapmeR technology can be applied as an experimental tool of downregulation of cardiac proteins and could potentially offer a novel modality for treatment of cardiac diseases.

Sinoatrial node function and the role of sinoatrial conduction in the typical atrial flutter substrate.

BACKGROUND: Sinoatrial node (SAN) activation and sinoatrial conduction pathways (SACPs) have been assessed in animals but not in humans. OBJECTIVES: We used ultrahigh-density mapping and simulated models to characterize the SAN and to investigate whether slowed SAN conduction may contribute to the atrial flutter (AFL) substrate. METHODS: Twenty-seven patients undergoing electrophysiologic procedures had right atrial mapping. SAN activation patterns and conduction block were analyzed. The interaction between the SAN and the intercaval line of block (LOB) was analyzed, and right atrial simulations with different degrees of block were created to investigate arrhythmia mechanisms. RESULTS: Fifteen AFL patients and 12 reference patients were enrolled. SACPs were identified in all patients with sinus rhythm maps. An SAN-adjacent LOB was observed in AFL patients. SAN conduction velocity was slower in AFL vs reference (0.60 m/s [0.56-0.78 m/s] vs 1.13 m/s [1.00-1.21 m/s]; P = .0021). Coronary sinus paced maps displayed an intercaval LOB in AFL patients but not in reference patients, which was completed superiorly by the SAN-adjacent LOB. Corrected sinus node recovery time was longer in AFL patients (552.3 ± 182.9 ms vs 325.4 ± 138.3 ms; P < .006) and correlated with degree of intercaval block (r = 0.7236; P = .0003). Computer modeling supported an important role of SAN-associated block in the flutter substrate. CONCLUSION: Ultrahigh-density mapping accurately identifies SAN activation and SACPs. The LOB important for typical AFL was longer in AFL patients, and when partial, it was always present inferiorly and completed superiorly because of slowed conduction across the SAN. Corrected sinus node recovery time correlated with intercaval block, suggesting a role for SAN disease in the genesis of the typical AFL substrate.

Glucagon-like peptide-1 increases heart rate by a direct action on the sinus node.

AIMS: Glucagon-like peptide-1 receptor agonists (GLP-1 RAs) are increasingly used to treat type 2 diabetes and obesity. Albeit cardiovascular outcomes generally improve, treatment with GLP-1 RAs is associated with increased heart rate, the mechanism of which is unclear. METHODS AND RESULTS: We employed a large animal model, the female landrace pig, and used multiple in vivo and ex vivo approaches including pharmacological challenges, electrophysiology, and high-resolution mass spectrometry to explore how GLP-1 elicits an increase in heart rate. In anaesthetized pigs, neither cervical vagotomy, adrenergic blockers (alpha, beta, or combined alpha-beta blockade), ganglionic blockade (hexamethonium), nor inhibition of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (ivabradine) abolished the marked chronotropic effect of GLP-1. GLP-1 administration to isolated perfused pig hearts also increased heart rate, which was abolished by GLP-1 receptor blockade. Electrophysiological characterization of GLP-1 effects in vivo and in isolated perfused hearts localized electrical modulation to the atria and conduction system. In isolated sinus nodes, GLP-1 administration shortened the action potential cycle length of pacemaker cells and shifted the site of earliest activation. The effect was independent of HCN blockade. Collectively, these data support a direct effect of GLP-1 on GLP-1 receptors within the heart. Consistently, single nucleus RNA sequencing showed GLP-1 receptor expression in porcine pacemaker cells. Quantitative phosphoproteomics analyses of sinus node samples revealed that GLP-1 administration leads to phosphorylation changes of calcium cycling proteins of the sarcoplasmic reticulum, known to regulate heart rate. CONCLUSION: GLP-1 has direct chronotropic effects on the heart mediated by GLP-1 receptors in pacemaker cells of the sinus node, inducing changes in action potential morphology and the leading pacemaker site through a calcium signalling response characterized by PKA-dependent phosphorylation of Ca2+ cycling proteins involved in pacemaking. Targeting the pacemaker calcium clock may be a strategy to lower heart rate in people treated with GLP-1 RAs.

Proteomics couples electrical remodelling to inflammation in a murine model of heart failure with sinus node dysfunction.

AIMS: In patients with heart failure (HF), concomitant sinus node dysfunction (SND) is an important predictor of mortality, yet its molecular underpinnings are poorly understood. Using proteomics, this study aimed to dissect the protein and phosphorylation remodelling within the sinus node in an animal model of HF with concurrent SND. METHODS AND RESULTS: We acquired deep sinus node proteomes and phosphoproteomes in mice with heart failure and SND and report extensive remodelling. Intersecting the measured (phospho)proteome changes with human genomics pharmacovigilance data, highlighted downregulated proteins involved in electrical activity such as the pacemaker ion channel, Hcn4. We confirmed the importance of ion channel downregulation for sinus node physiology using computer modelling. Guided by the proteomics data, we hypothesized that an inflammatory response may drive the electrophysiological remodeling underlying SND in heart failure. In support of this, experimentally induced inflammation downregulated Hcn4 and slowed pacemaking in the isolated sinus node. From the proteomics data we identified proinflammatory cytokine-like protein galectin-3 as a potential target to mitigate the effect. Indeed, in vivo suppression of galectin-3 in the animal model of heart failure prevented SND. CONCLUSION: Collectively, we outline the protein and phosphorylation remodeling of SND in heart failure, we highlight a role for inflammation in electrophysiological remodelling of the sinus node, and we present galectin-3 signalling as a target to ameliorate SND in heart failure.

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.

Inhibition of Adenosine Pathway Alters Atrial Electrophysiology and Prevents Atrial Fibrillation.

BACKGROUND: Adenosine leads to atrial action potential (AP) shortening through activation of adenosine 1 receptors (A1-R) and subsequent opening of G-protein-coupled inwardly rectifying K+ channels. Extracellular production of adenosine is drastically increased during stress and ischemia. OBJECTIVE: The aim of this study was to address whether the pharmacological blockade of endogenous production of adenosine and of its signaling prevents atrial fibrillation (AF). METHODS: The role of A1-R activation on atrial action potential duration, refractoriness, and AF vulnerability was investigated in rat isolated beating heart preparations (Langendorff) with an A1-R agonist [2-chloro-N 6-cyclopentyladenosine (CCPA), 50 nM] and antagonist [1-butyl-3-(3-hydroxypropyl)-8-(3-noradamantyl)xanthine (PSB36), 40 nM]. Furthermore, to interfere with the endogenous adenosine release, the ecto-5'-nucleotidase (CD73) inhibitor was applied [5'-(α,ß-methylene) diphosphate sodium salt (AMPCP), 500 µM]. Isolated trabeculae from human right atrial appendages (hRAAs) were used for comparison. RESULTS: As expected, CCPA shortened AP duration at 90% of repolarization (APD90) and effective refractory period (ERP) in rat atria. PSB36 prolonged APD90 and ERP in rat atria, and CD73 inhibition with AMPCP prolonged ERP in rats, confirming that endogenously produced amount of adenosine is sufficiently high to alter atrial electrophysiology. In human atrial appendages, CCPA shortened APD90, while PSB36 prolonged it. Rat hearts treated with CCPA are prone to AF. In contrast, PSB36 and AMPCP prevented AF events and reduced AF duration (vehicle, 11.5 ± 2.6 s; CCPA, 40.6 ± 16.1 s; PSB36, 6.5 ± 3.7 s; AMPCP, 3.0 ± 1.4 s; P < 0.0001). CONCLUSION: A1-R activation by intrinsic adenosine release alters atrial electrophysiology and promotes AF. Inhibition of adenosine pathway protects atria from arrhythmic events.

The biophysics of piezo1 and piezo2 mechanosensitive channels.

The ability to sense mechanical stimuli and elaborate a response to them is a fundamental process in all organisms, driving crucial mechanisms ranging from cell volume regulation up to organ development or regeneration. Nevertheless, only in few cases the underlying molecular players are known. In particular, mammals possess a large variety of mechanoreceptors, providing highly specialized functions in sensory cells, but also several housekeeping molecular systems are involved in the complex mechanism of mechanotransduction. Recently, a new class of almost ubiquitous membrane channels has been identified in mammalians, namely piezo1 and piezo2, that is thought to play a crucial role in the mechanobiology of mammals. This review focuses on recent findings on these novel channels, and highlights open biophysical questions that largely remain to be addressed.