Cytosolic calcium accumulation and delayed repolarization associated with ventricular arrhythmias in a guinea pig model of Andersen-Tawil syndrome

Cardiogenetics: the role of genetic testing for inherited arrhythmia syndromes and sudden death

There have been remarkable advances in our knowledge of the underlying heritability of cardiac arrhythmias. Long QT syndrome, Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia, progressive cardiac conduction disease and the short QT syndrome comprise the inherited arrhythmia syndromes (IASs). Pathogenic variants in cardiac ion channel and calcium handling protein genes lead to these conditions, usually in the absence of overt structural cardiac disease. Diagnosis is contingent on the ECG phenotype but genetic testing may help to confirm the diagnosis and provide information on the mechanism of arrhythmogenesis that may guide treatment and provide prognostic information in relation to the risk of sudden arrhythmic death. Clinical genetic testing uses 'panels' of genes that are the likely culprits for the IASs being investigated. An International Consortium (Clinical Genome Resource) has curated gene panels based on genetic and experimental evidence of causation of inherited conditions and that have a role in clinical genetic testing. A 'single gene' or monogenic basis for IASs exists but in future, missing heritability and incomplete penetrance will be uncovered by association of common variants through genome-wide association studies. Novel rare variants will also be detected through whole-genome sequencing. The formulation of polygenic risk scores will likely help to predict phenotypic expression and response to treatments/risk stratification and move genetic testing very much to the fore of the diagnostic process.

Vascular endothelial growth factor promotes atrial arrhythmias by inducing acute intercalated disk remodeling

Atrial fibrillation (AF) is the most common arrhythmia and is associated with inflammation. AF patients have elevated levels of inflammatory cytokines known to promote vascular leak, such as vascular endothelial growth factor A (VEGF). However, the contribution of vascular leak and consequent cardiac edema to the genesis of atrial arrhythmias remains unknown. Previous work suggests that interstitial edema in the heart can acutely promote ventricular arrhythmias by disrupting ventricular myocyte intercalated disk (ID) nanodomains rich in cardiac sodium channels (Na_V1.5) and slowing cardiac conduction. Interestingly, similar disruption of ID nanodomains has been identified in atrial samples from AF patients. Therefore, we tested the hypothesis that VEGF-induced vascular leak can acutely increase atrial arrhythmia susceptibility by disrupting ID nanodomains and slowing atrial conduction. Treatment of murine hearts with VEGF (30–60 min, at clinically relevant levels) prolonged the electrocardiographic P wave and increased susceptibility to burst pacing-induced atrial arrhythmias. Optical voltage mapping revealed slower atrial conduction following VEGF treatment (10 ± 0.4 cm/s vs. 21 ± 1 cm/s at baseline, p < 0.05). Transmission electron microscopy revealed increased intermembrane spacing at ID sites adjacent to gap junctions (GJs; 64 ± 9 nm versus 17 ± 1 nm in controls, p < 0.05), as well as sites next to mechanical junctions (MJs; 63 ± 4 nm versus 27 ± 2 nm in controls, p < 0.05) in VEGF–treated hearts relative to controls. Importantly, super-resolution microscopy and quantitative image analysis revealed reorganization of Na_V1.5 away from dense clusters localized near GJs and MJs to a more diffuse distribution throughout the ID. Taken together, these data suggest that VEGF can acutely predispose otherwise normal hearts to atrial arrhythmias by dynamically disrupting Na_V1.5-rich ID nanodomains and slowing atrial conduction. These data highlight inflammation-induced vascular leak as a potential factor in the development and progression of AF.

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

Background

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

Methods and Results

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

Conclusions

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

Intercellular Sodium Regulates Repolarization in Cardiac Tissue with Sodium Channel Gain of Function

In cardiac myocytes, action potentials are initiated by an influx of sodium (Na⁺) ions via voltage-gated Na⁺ channels. Na⁺ channel gain of function (GOF), arising in both inherited conditions associated with mutation in the gene encoding the Na⁺ channel and acquired conditions associated with heart failure, ischemia, and atrial fibrillation, enhance Na⁺ influx, generating a late Na⁺ current that prolongs action potential duration (APD) and triggering proarrhythmic early afterdepolarizations (EADs). Recent studies have shown that Na⁺ channels are highly clustered at the myocyte intercalated disk, facilitating formation of Na⁺ nanodomains in the intercellular cleft between cells. Simulations from our group have recently predicted that narrowing the width of the intercellular cleft can suppress APD prolongation and EADs in the presence of Na⁺ channel mutations because of increased intercellular cleft Na⁺ ion depletion. In this study, we investigate the effects of modulating multiple extracellular spaces, specifically the intercellular cleft and bulk interstitial space, in a novel computational model and experimentally via osmotic agents albumin, dextran 70, and mannitol. We perform optical mapping and transmission electron microscopy in a drug-induced (sea anemone toxin, ATXII) Na⁺ channel GOF isolated heart model and modulate extracellular spaces via osmotic agents. Single-cell patch-clamp experiments confirmed that the osmotic agents individually do not enhance late Na⁺ current. Both experiments and simulations are consistent with the conclusion that intercellular cleft narrowing or expansion regulates APD prolongation; in contrast, modulating the bulk interstitial space has negligible effects on repolarization. Thus, we predict that intercellular cleft Na⁺ nanodomain formation and collapse critically regulates cardiac repolarization in the setting of Na⁺ channel GOF.

Super-Resolution Imaging Using a Novel High-Fidelity Antibody Reveals Close Association of the Neuronal Sodium Channel NaV1.6 with Ryanodine Receptors in Cardiac Muscle

The voltage-gated sodium channel [pore-forming subunit of the neuronal voltage-gated sodium channel (NaV1.6)] has recently been found in cardiac myocytes. Emerging studies indicate a role for NaV1.6 in ionic homeostasis as well as arrhythmogenesis. Little is known about the spatial organization of these channels in cardiac muscle, mainly due to the lack of high-fidelity antibodies. Therefore, we developed and rigorously validated a novel rabbit polyclonal NaV1.6 antibody and undertook super-resolution microscopy studies of NaV1.6 localization in cardiac muscle. We developed and validated a novel rabbit polyclonal antibody against a C-terminal epitope on the neuronal sodium channel 1.6 (NaV1.6). Raw sera showed high affinity in immuno-fluorescence studies, which was improved with affinity purification. The antibody was rigorously validated for specificity via multiple approaches. Lastly, we used this antibody in proximity ligation assay (PLA) and super-resolution STochastic Optical Reconstruction Microscopy (STORM) studies, which revealed enrichment of NaV1.6 in close proximity to ryanodine receptor (RyR2), a key calcium (Ca2+) cycling protein, in cardiac myocytes. In summary, our novel NaV1.6 antibody demonstrates high degrees of specificity and fidelity in multiple preparations. It enabled multimodal microscopic studies and revealed that over half of the NaV1.6 channels in cardiac myocytes are located within 100 nm of ryanodine receptor Ca2+ release channels.

Cardiac Arrhythmias as Manifestations of Nanopathies: An Emerging View

A nanodomain is a collection of proteins localized within a specialized, nanoscale structural environment, which can serve as the functional unit of macroscopic physiologic processes. We are beginning to recognize the key roles of cardiomyocyte nanodomains in essential processes of cardiac physiology such as electrical impulse propagation and excitation–contraction coupling (ECC). There is growing appreciation of nanodomain dysfunction, i.e., nanopathy, as a mechanistic driver of life-threatening arrhythmias in a variety of pathologies. Here, we offer an overview of current research on the role of nanodomains in cardiac physiology with particular emphasis on: (1) sodium channel-rich nanodomains within the intercalated disk that participate in cell-to-cell electrical coupling and (2) dyadic nanodomains located along transverse tubules that participate in ECC. The beat to beat function of cardiomyocytes involves three phases: the action potential, the calcium transient, and mechanical contraction/relaxation. In all these phases, cell-wide function results from the aggregation of the stochastic function of individual proteins. While it has long been known that proteins that exist in close proximity influence each other’s function, it is increasingly appreciated that there exist nanoscale structures that act as functional units of cardiac biophysical phenomena. Termed nanodomains, these structures are collections of proteins, localized within specialized nanoscale structural environments. The nano-environments enable the generation of localized electrical and/or chemical gradients, thereby conferring unique functional properties to these units. Thus, the function of a nanodomain is determined by its protein constituents as well as their local structural environment, adding an additional layer of complexity to cardiac biology and biophysics. However, with the emergence of experimental techniques that allow direct investigation of structure and function at the nanoscale, our understanding of cardiac physiology and pathophysiology at these scales is rapidly advancing. Here, we will discuss the structure and functions of multiple cardiomyocyte nanodomains, and novel strategies that target them for the treatment of cardiac arrhythmias.

Tetrodotoxin-sensitive Navs contribute to early and delayed afterdepolarizations in long QT arrhythmia models

Neuronal Na⁺ channels contribute to catecholaminergic polymorphic ventricular tachycardia in the heart, but their role in other types of arrhythmias is unknown. Koleske et al. show that they contribute to early and delayed afterdepolarizations common to long QT, catecholaminergic polymorphic ventricular tachycardia, and overlap phenotypes.

Recent evidence suggests that neuronal Na⁺ channels (nNa_vs) contribute to catecholamine-promoted delayed afterdepolarizations (DADs) and catecholaminergic polymorphic ventricular tachycardia (CPVT). The newly identified overlap between CPVT and long QT (LQT) phenotypes has stoked interest in the cross-talk between aberrant Na⁺ and Ca²⁺ handling and its contribution to early afterdepolarizations (EADs) and DADs. Here, we used Ca²⁺ imaging and electrophysiology to investigate the role of Na⁺ and Ca²⁺ handling in DADs and EADs in wild-type and cardiac calsequestrin (CASQ2)-null mice. In experiments, repolarization was impaired using 4-aminopyridine (4AP), whereas the L-type Ca²⁺ and late Na⁺ currents were augmented using Bay K 8644 (BayK) and anemone toxin II (ATX-II), respectively. The combination of 4AP and isoproterenol prolonged action potential duration (APD) and promoted aberrant Ca²⁺ release, EADs, and DADs in wild-type cardiomyocytes. Similarly, BayK in the absence of isoproterenol induced the same effects in CASQ2-null cardiomyocytes. In vivo, it prolonged the QT interval and, upon catecholamine challenge, precipitated wide QRS polymorphic ventricular tachycardia that resembled human torsades de pointes. Treatment with ATX-II produced similar effects at both the cellular level and in vivo. Importantly, nNa_v inhibition with riluzole or 4,9-anhydro-tetrodotoxin reduced the incidence of ATX-II–, BayK-, or 4AP-induced EADs, DADs, aberrant Ca²⁺ release, and VT despite only modestly mitigating APD prolongation. These data reveal the contribution of nNa_Vs to triggered arrhythmias in murine models of LQT and CPVT-LQT overlap phenotypes. We also demonstrate the antiarrhythmic impact of nNa_V inhibition, independent of action potential and QT interval duration, and provide a basis for a mechanistically driven antiarrhythmic strategy.

Sub-cellular Electrical Heterogeneity Revealed by Loose Patch Recording Reflects Differential Localization of Sarcolemmal Ion Channels in Intact Rat Hearts

The cardiac action potential (AP) is commonly recoded as an integral signal from isolated myocytes or ensembles of myocytes (with intracellular microelectrodes and extracellular macroelectrodes, respectively). These signals, however, do not provide a direct measure of activity of ion channels and transporters located in two major compartments of a cardiac myocyte: surface sarcolemma and the T-tubule system, which differentially contribute to impulse propagation and excitation-contraction (EC) coupling. In the present study we investigated electrical properties of myocytes within perfused intact rat heart employing loose patch recording with narrow-tip (2 μm diameter) extracellular electrodes. Using this approach, we demonstrated two distinct types of electric signals with distinct waveforms (single peak and multi-peak AP; AP1 and AP2, respectively) during intrinsic pacemaker activity. These two types of waveforms depend on the position of the electrode tip on the myocyte surface. Such heterogeneity of electrical signals was lost when electrodes of larger pipette diameter were used (5 or 10 μm), which indicates that the electric signal was assessed from a region of <5 μm. Importantly, both pharmacological and mathematical simulation based on transverse (T)-tubular distribution suggested that while the AP1 and the initial peak of AP2 are predominantly attributable to the fast, inward Na+ current in myocyte's surface sarcolemma, the late components of AP2 are likely representative of currents associated with L-type Ca2+ channel and Na+/Ca2+ exchanger (NCX) currents which are predominantly located in T-tubules. Thus, loose patch recording with narrow-tip pipette provides a valuable tool for studying cardiac electric activity on the subcellular level in the intact heart.

Decreased RyR2 refractoriness determines myocardial synchronization of aberrant Ca2+ release in a genetic model of arrhythmia

Dysregulated intracellular Ca(2+) signaling is implicated in a variety of cardiac arrhythmias, including catecholaminergic polymorphic ventricular tachycardia. Spontaneous diastolic Ca(2+) release (DCR) can induce arrhythmogenic plasma membrane depolarizations, although the mechanism responsible for DCR synchronization among adjacent myocytes required for ectopic activity remains unclear. We investigated the synchronization mechanism(s) of DCR underlying untimely action potentials and diastolic contractions (DCs) in a catecholaminergic polymorphic ventricular tachycardia mouse model with a mutation in cardiac calsequestrin. We used a combination of different approaches including single ryanodine receptor channel recording, optical imaging (Ca(2+) and membrane potential), and contractile force measurements in ventricular myocytes and intact cardiac muscles. We demonstrate that DCR occurs in a temporally and spatially uniform manner in both myocytes and intact myocardial tissue isolated from cardiac calsequestrin mutation mice. Such synchronized DCR events give rise to triggered electrical activity that results in synchronous DCs in the myocardium. Importantly, we establish that synchronization of DCR is a result of a combination of abbreviated ryanodine receptor channel refractoriness and the preceding synchronous stimulated Ca(2+) release/reuptake dynamics. Our study reveals how aberrant DCR events can become synchronized in the intact myocardium, leading to triggered activity and the resultant DCs in the settings of a cardiac rhythm disorder.

Potassium channel activators differentially modulate the effect of sodium channel blockade on cardiac conduction

AIMS

Diminished repolarization reserve contributes to the arrhythmogenic substrate in many disease states. Pharmacological activation of K(+) channels has been suggested as a potential antiarrhythmic therapy in such conditions. Having previously demonstrated that I(K1) and I(Kr) can modulate cardiac conduction, we tested here the effects of pharmacological I(KATP) and I(Ks) activation on cardiac conduction and its dependence on the sodium current (I(Na)).

METHODS AND RESULTS

Bath electrocardiograms (ECGs) recorded from Langendorff-perfused guinea pig ventricles revealed QRS prolongation during I(KATP) activation by pinacidil but not during I(Ks) activation by R-L3 relative to control. In contrast, when I(Na) was partially blocked by flecainide, R-L3 but not pinacidil prolonged the QRS relative to flecainide alone. Conduction velocity (θ) was quantified by optical mapping during epicardial pacing. Both longitudinal (θ(L)) and transverse (θ(T)) θ were reduced by pinacidil (by 10 ± 1 and 9 ± 3%, respectively) and R-L3 (by 11 ± 2% and 15 ± 4%, respectively). Flecainide decreased θ(L) by 33 ± 4% and θ(T) by 36 ± 5%. Whereas pinacidil did not further slow θ relative to flecainide alone, R-L3 decreased both θ(L) and θ(T).

CONCLUSION

Pharmacological activation of I(KATP) and I(Ks) slows cardiac conduction; however, they demonstrate diverse effects on θ dependence on I(Na) blockade. These findings may have significant implications for the use of K(+) channel activators as antiarrhythmic drugs and for patients with Na(+) channel abnormalities or being treated with Na(+) channel blockers.

Store-dependent deactivation: cooling the chain-reaction of myocardial calcium signaling

In heart cells, Ca(2+) released from the internal storage unit, the sarcoplasmic reticulum (SR) through ryanodine receptor (RyR2) channels is the predominant determinant of cardiac contractility. Evidence obtained in recent years suggests that SR Ca(2+) release is tightly regulated not only by cytosolic Ca(2+) but also by intra-store Ca(2+) concentration. Specifically, Ca(2+)-induced Ca(2+) release (CICR) that relies on auto-catalytic action of Ca(2+) at the cytosolic side of RyR2s is precisely balanced and counteracted by RyR2 deactivation dependent on a reciprocal decrease of Ca(2+) at the luminal side of RyR2s. Dysregulation of this inherently unstable Ca(2+) signaling is considered to be an underlying cause of triggered arrhythmias, and is associated with genetic and acquired forms of sudden cardiac death. In this article, we present an overview of recent advances in our understanding of the regulatory role luminal Ca(2+) plays in Ca(2+) handling, with a particular emphasis on the role of Ca(2+)release refractoriness in aberrant Ca(2+) release.

Inhibition of Na+ channels ameliorates arrhythmias in a drug-induced model of Andersen-Tawil syndrome

BACKGROUND

Andersen-Tawil syndrome (ATS1)-associated ventricular tachycardias (VTs) are initiated by frequent, hypokalemia-exacerbated, premature ventricular activity (PVA). We previously demonstrated that a guinea pig model of drug-induced ATS1 (DI-ATS1) evidenced increased arrhythmias from regions with high Na(+)/Ca(2+)-exchange expression.

OBJECTIVE

Therefore, we hypothesize that reduced cytosolic Na(+) entry through either cardiac isoform of or tetrodotoxin (TTX)-sensitive Na(+) channels during DI-ATS1 can ameliorate arrhythmia burden.

METHODS

DI-ATS1 was induced with 10 μM BaCl(2) and 2 mM extracellular K(+). Ca(2+) transients and conduction velocity (CV) were optically mapped with indo-1 and di-4-ANEPPS, respectively, from Langendorff-perfused guinea pig ventricles.

RESULTS

Nonselective Na(+) channel blockade with 1 μM flecainide reduced amplitude (Ca(A)), slowed left ventricular CV, reduced tissue excitability, and abolished the incidence of VT while decreasing the incidence of PVA relative to DI-ATS1. Selective, TTX-sensitive Na(+) channel blockade with TTX (100 nM) during DI-ATS1 decreased Ca(A) and decreased the inducibility of VTs and PVA relative to DI-ATS1 without slowing CV. Ranolazine altered Ca(A), left ventricular CV, tissue excitability, and reduced inducibility of VT and PVA in a concentration-dependent manner. None of the aforementioned interventions altered diastolic Ca(2+) levels or Ca(2+) transient decay time constant.

CONCLUSIONS

These data suggest that cytosolic Na(+) entry and its modulation of Ca(2+) handling are necessary for arrhythmogenesis. During the loss of inward-rectifier K(+) current function, not only Na(+)/Ca(2+)-exchange dominance but Na(+) flux may determine arrhythmia burden. Therefore, selective inhibition of TTX-sensitive Na(+) channels may offer a potential therapeutic target to alleviate arrhythmias during states of Ca(2+) overload secondary to loss of inward-rectifier K(+) current function without compromising the excitability reserve.

NCX is an important determinant for premature ventricular activity in a drug-induced model of Andersen-Tawil syndrome

AIMS

Andersen-Tawil syndrome (ATS1)-associated ventricular arrhythmias are initiated by premature ventricular activity (PVA) resulting from diastolic Ca(2+) (Ca(D)) accumulation. We hypothesized that relatively high Na(+)-Ca(2+) exchanger (NCX) expression coupled with slower Ca(2+) uptake may constitute an arrhythmogenic substrate during drug-induced ATS1 (DI-ATS1).

METHODS AND RESULTS

DI-ATS1 was induced with 10 µmol/L BaCl(2) and 2 mmol/L [K(+)](o). Ca(2+) transients and action potentials were optically mapped from Langendorff-perfused guinea pig ventricles. Intracellular Ca(2+) handling was modulated by either direct NCX inhibition with 5 µmol/L KB-R7943 or by sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) inhibition with cyclopiazonic acid (CPA). During DI-ATS1, PVA was more frequent in left ventricular (LV)-base (LVB) vs. LV-apex (LVA) (2.2 ± 0.8 vs. 0.6 ± 0.3 PVA/10 min), consistent with greater Ca(D) (1.65 ± 0.13 vs. 1.42 ± 0.09 normalized-Ca(D) units) and western blot-assessed NCX protein expression (81.2 ± 30.9%) in LVB relative to LVA. Further, regions of high NCX (LVB) evidenced a shorter PVA coupling interval relative to regions of low NCX expression (LVA, 67.7 ± 3.5 vs. 78.5 ± 3.6%). Inhibiting NCX during DI-ATS1 lowered the incidence of ventricular tachycardias (VTs, 0 vs. 25%) and PVA (1.5 ± 0.4 vs. 4.3 ± 1.4 PVA/10 min), but it did not affect PVA coupling intervals in LVB nor LVA (70.8 ± 4.3 vs. 73.8 ± 2.5%). Conversely, inhibition of SERCA2a with CPA, thereby increasing the role of NCX in Ca(2+) handling, significantly increased the incidence of VTs and PVA relative to DI-ATS1 alone, while decreasing the PVA coupling interval in all regions.

CONCLUSION

PVA preferentially occurs in regions of enhanced NCX expression with relatively slower Ca(2+) uptake and during perfusion of CPA which further reduces sarcoplasmic reticular Ca(2+) uptake.