Whole-Exome Sequencing Implicates Neuronal Calcium Channel with Familial Atrial Fibrillation

Background: Atrial Fibrillation (AF) is the most prevalent sustained cardiac arrhythmia, responsible for considerable morbidity and mortality. The heterogenic and complex pathogenesis of AF remains poorly understood, which contributes to the current limitation in effective treatments. We aimed to identify rare genetic variants associated with AF in patients with familial AF.

Methods and results: We performed whole exome sequencing in a large family with familial AF and identified a rare variant in the gene CACNA1A c.5053G > A which co-segregated with AF. The gene encodes for the protein variants Ca_V2.1-V1686M, and is important in neuronal function. Functional characterization of the CACNA1A, using patch-clamp recordings on transiently transfected mammalian cells, revealed a modest loss-of-function of Ca_V2.1-V1686M.

Conclusion: We identified a rare loss-of-function variant associated with AF in a gene previously linked with neuronal function. The results allude to a novel link between dysfunction of an ion channel previously associated with neuronal functions and increased risk of developing AF.

Inhibition of KCa2 Channels Decreased the Risk of Ventricular Arrhythmia in the Guinea Pig Heart During Induced Hypokalemia

BACKGROUND

Hypokalemia reduces the cardiac repolarization reserve. This prolongs the QT-interval and increases the risk of ventricular arrhythmia; a risk that is exacerbated by administration of classical class 3 anti-arrhythmic agents.Small conductance Ca2+-activated K+-channels (KCa2) are a promising new atrial selective target for treatment of atrial fibrillation. Under physiological conditions KCa2 plays a minor role in ventricular repolarization. However, this might change under hypokalemia because of concomitant increases in ventriculay -60r intracellur Ca2+.

PURPOSE

To study the effects of pharmacological KCa2 channel inhibition by the compounds AP14145, ICA, or AP30663 under hypokalemic conditions as compared to dofetilide and hypokalemia alone time-matched controls (TMC).

METHODS

The current at +10 mV was compared in HEK293 cells stably expressing KCa2.3 perfused first with normo- and then hypokalemic solutions (4 mM K+ and 2.5 mM K+, respectively). Guinea pig hearts were isolated and perfused with normokalemic (4 mM K+) Krebs-Henseleit solution, followed by perfusion with drug or vehicle control. The perfusion was then changed to hypokalemic solution (2.5 mM K+) in presence of drug. 30 animals were randomly assigned to 5 groups: ICA, AP14145, AP30663, dofetilide, or TMC. QT-interval, the interval from the peak to the end of the T wave (Tp-Te), ventricular effective refractory period (VERP), arrhythmia score, and ventricular fibrillation (VF) incidence were recorded.

RESULTS

Hypokalemia slightly increased KCa2.3 current compared to normokalemia. Application of KCa2 channel inhibitors and dofetilide prolonged the QT interval corrected for heart rate. Dofetilide, but none of the KCa2 channel inhibitors increased Tp-Te during hypokalemia. During hypokalemia 4/6 hearts in the TMC group developed VF (two spontaneously, two by S1S2 stimulation) whereas 5/6 hearts developed VF in the dofetilide group (two spontaneously, three by S1S2 stimulation). In comparison, 0/6, 1/6, and 1/6 hearts developed VF when treated with the KCa2 channel inhibitors AP30663, ICA, or AP14145, respectively.

CONCLUSION

Hypokalemia was associated with an increased incidence of VF, an effect that also seen in the presence of dofetilide. In comparison, the structurally and functionally different KCa2 channel inhibitors, ICA, AP14145, and AP30663 protected the heart from hypokalemia induced VF. These results support that KCa2 inhibition may be associated with a better safety and tolerability profile than dofetilide.

Mechanisms of Action of the KCa2-Negative Modulator AP30663, a Novel Compound in Development for Treatment of Atrial Fibrillation in Man

Aims

Small conductance Ca²⁺-activated K⁺ channels (SK channels, K_Ca2) are a new target for treatment of atrial fibrillation (AF). AP30663 is a small molecule inhibitor of K_Ca2 channels that is currently in clinical development for treatment of AF. The aim of this study is to present the electrophysiological profile and mechanism of action of AP30663 and its efficacy in prolonging atrial refractoriness in rodents, and by bioinformatic analysis investigate if genetic variants in KCNN2 or KCNN3 influence the expression level of these in human heart tissue.

Methods and Results

Whole-cell and inside-out patch-clamp recordings of heterologously expressed K_Ca2 channels revealed that AP30663 inhibits K_Ca2 channels with minor effects on other relevant cardiac ion channels. AP30663 modulates the K_Ca2.3 channel by right-shifting the Ca²⁺-activation curve. In isolated guinea pig hearts AP30663 significantly prolonged the atrial effective refractory period (AERP) with minor effects on the QT-interval corrected for heart rate. Similarly, in anaesthetized rats 5 and 10 mg/kg of AP30663 changed the AERP to 130.7±5.4% and 189.9±18.6 of baseline values. The expression quantitative trait loci analyses revealed that the genome wide association studies for AF SNP rs13376333 in KCNN3 is associated with increased mRNA expression of KCNN3 in human atrial appendage tissue.

Conclusions

AP30663 is a novel negative allosteric modulator of K_Ca2 channels that concentration-dependently prolonged rodent atrial refractoriness with minor effects on the QT-interval. Moreover, AF associated SNPs in KCNN3 influence KCNN3 mRNA expression in human atrial tissue. These properties support continued development of AP30663 for treatment of AF in man.

P699Mechanisms of action of the small conductance Ca2+-activated K+-channel modulator AP30663, a novel compound being developed for treatment of atrial fibrillation in man

Background

Small conductance Ca2+-activated K+-channels (SK-channels) are a promising new atrial selective target for treatment of atrial fibrillation (AF). AP30663 is a small molecule inhibitor of SK-channels that is effective in converting vernakalant-resistant AF in tachy-paced pigs. Detailed understanding of the molecular mechanism of AP30663 is important for the development of SK channel inhibition for use in man.

Purpose

To establish the electrophysiological profile, mechanism of action and efficacy in prolonging atrial refractoriness ex vivo of AP30663.

Methods

AP30663 potency and mechanism of action were established by whole cell and inside-out patch clamp recordings of expressed SK channels. The ion channel selectivity profile of AP30663 was investigated on heterologous expressed channels. Effects of AP30663 or vehicle (DMSO) on atrial refractoriness (AERP) and ventricular repolarization (QTcB) were investigated on isolated perfused guinea pig hearts.

Results

AP30663 was found to be a selective negative allosteric modulator of SK channels (IC50=0.77±x0.13 μM) with no or minor effects on a panel of other cardiac ion channels, including hERG/KV11.1, (IKr), KV7.1/KCNE1 (IKs), KV4.3/KChiP2 (Ito), Kir2.1 (IK1), Kir3.1/Kir3.4 (IKACh), KV1.5 (IKur), NaV1.5 (INa) and CaV1.2 (ICa). AP30663 inhibited the SK-channel by right-shifting the calcium activation curve of the SK-channel (the EC50 of Ca2+ increased from 0.43±0.02 μM (control, n=6) to 1.37±0.05 μM (in the presence of 7μM AP30663, n=6).

In isolated guinea pig hearts, administration of vehicle had no effect on AERP or QTcB. AP30663 significantly prolonged the AERP in 3 μM (to 131±6% of baseline) and 10 μM (to 165±3% of baseline) without any effects on the QTcB.

Conclusion

AP30663 is a selective negative allosteric modulator of SK channels, acting by means of shifting the calcium dependence of SK-channel activation. AP30663 prolonged atrial refractoriness without affecting the QT-interval in isolated perfused heart preparations. These properties support continued development of AP30663 for treatment of AF in man.

Acknowledgement/Funding

Innovation Fund Denmark, Wellcome Trust

The KCa2 Channel Inhibitor AP14145, But Not Dofetilide or Ondansetron, Provides Functional Atrial Selectivity in Guinea Pig Hearts

Background and Purpose: Prolongation of cardiac action potentials is considered antiarrhythmic in the atria but can be proarrhythmic in ventricles if the current carried by Kv11.1-channels (I_Kr) is inhibited. The current mediated by K_Ca2-channels, I_KCa, is considered a promising new target for treatment of atrial fibrillation (AF). Selective inhibitors of I_Kr (dofetilide) and I_KCa (AP14145) were used to compare the effects on ventricular and atrial repolarization. Ondansetron, which has been reported to be a potent blocker of both I_Kr and I_KCa, was included to examine its potential atrial antiarrhythmic properties. Experimental Approach: The expression of K_Ca2- and K_v11.1-channels in the guinea pig heart was investigated using quantitative polymerase chain reaction (qPCR). Whole-cell patch clamp technique was used to investigate the effects of dofetilide, AP14145, and ondansetron on I_KCa and/or I_Kr. The effect of dofetilide, AP14145, and ondansetron on atrial and ventricular repolarization was investigated in isolated hearts. A novel atrial paced in vivo guinea pig model was further validated using AP14145 and dofetilide. Key Results: AP14145 increased the atrial effective refractory period (AERP) without prolonging the QT interval with Bazett's correction for heart rate (QTcB) both ex vivo and in vivo. In contrast, dofetilide increased QTcB and, to a lesser extent, AERP in isolated hearts and prolonged QTcB with no effects on AERP in the in vivo guinea pig model. Ondansetron did not inhibit I_KCa, but did inhibit I_Kr in vitro. Ondansetron prolonged ventricular, but not atrial repolarization ex vivo. Conclusion and Implications: I_KCa inhibition by AP14145 selectively increases atrial repolarization, whereas I_Kr inhibition by dofetilide and ondansetron increases ventricular repolarization to a larger extent than atrial repolarization.

Profiling of GABAA and GABAB receptor expression in the myometrium of the human uterus

AIMS

γ-aminobutyric acid (GABA) mediates its physiological effects through the GABA_A and GABA_B receptors. In this study the putative expression of GABA_AR and GABA_BR subunits in human myometrium tissue was investigated.

MAIN METHODS

The expression levels of the 19 GABA_AR subunits (α₁-α₆, β₁-β₃, γ₁-γ₃, δ, ε, π, θ, ρ₁-ρ₃) and the three GABA_BR subunits (GABA_B1a, GABA_B1b, GABA_B2) were characterized by RT-qPCR analysis on two commercial samples and six samples derived from surgically removed myometrial tissues from different women. We probed for functional GABA_AR expression in primary human myometrial smooth muscle cells (HMSMCs) by whole-cell patch-clamp electrophysiology.

KEY FINDINGS

The absolute mRNA levels of the 22 GABA_AR and GABA_BR genes varied considerably across the eight samples, but a pronounced overlap existed between the specific subunits detected in the samples, with α₂, β₂, β₃, ε, π, θ, GABA_B1a and GABA_B1b mRNAs being detected in most samples. The expression profile of GABA_AR and GABA_BR subunit mRNAs in HMSMCs correlated with that observed in the eight tissue samples, albeit the subunit transcripts were detected at lower relative levels. Neither muscimol nor GABA evoked significant currents in these cells in the patch-clamp recordings.

SIGNIFICANCE

While the expression of the GABA_B1 subunits on their own is unlikely to give rise to functional GABA_BR expression, the GABA_AR subunits identified at mRNA level would be able to form functional receptors in the human myometrial tissue. Although GABA_AR-mediated currents could not be recorded from HMSMCs in this study, this suggests a role for GABAergic transmission in the human myometrium.

PKD Phosphorylation as Novel Pathway of KV11.1 Regulation

BACKGROUND/AIMS

The voltage-gated potassium channel KV11.1 has been originally cloned from the brain and is expressed in a variety of tissues. The role of phosphorylation for channel function is a matter of debate. In this study, we aimed to elucidate the extent and role of protein kinase D mediated phosphorylation.

METHODS

We employed mass spectrometry, whole-cell patch clamp electrophysiology, confocal microscopy, site-directed mutagenesis, and western blotting.

RESULTS

Using brain tissue from rat and mouse, we mapped several phosphorylated KV11.1 residues by LC-MS mass spectrometry and identified protein kinase D (PKD1) as possible regulatory kinase. Co-expression of KV11.1 with PKD1 reduced current amplitudes without altering protein levels or surface expression of the channel. Based on LC-MS results from in vivo and HEK293 cell experiments we chose four KV11.1 mutant candidates for further functional analysis. Ablation of the putative PKD phosphorylation site in the mutant S284A increased the maximal current indicating S284 as a main PKD target in KV11.1.

CONCLUSIONS

Our data might help mitigating a long-standing controversy in the field regarding PKC regulation of KV11.1. We propose that PKD1 mediates the PKC effects on KV11.1 and we found that PKD targets S284 in the N-terminus of the channel.

Termination of Vernakalant-Resistant Atrial Fibrillation by Inhibition of Small-Conductance Ca2+-Activated K+ Channels in Pigs

Supplemental Digital Content is available in the text.

Background

Evidence has emerged that small-conductance Ca²⁺-activated K⁺ (SK) channels constitute a new target for treatment of atrial fibrillation (AF). SK channels are predominantly expressed in the atria as compared with the ventricles. Various marketed antiarrhythmic drugs are limited by ventricular adverse effects and efficacy loss as AF progresses.

Methods and Results

A total of 43 pigs were used for the studies. AF reversion in conscious long-term tachypaced pigs: Pigs were subjected to atrial tachypacing (7 Hz) until they developed sustained AF that could not be reverted by vernakalant 4 mg/kg (18.8±3.3 days of atrial tachypacing). When the SK channel inhibitor AP14145 was tested in these animals, vernakalant-resistant AF was reverted to sinus rhythm, and reinduction of AF by burst pacing (50 Hz) was prevented in 8 of 8 pigs. Effects on refractory period and AF duration in open chest pigs: The effects of AP14145 and vernakalant on the effective refractory periods and acute burst pacing-induced AF were examined in anaesthetized open chest pigs. Both vernakalant and AP14145 significantly prolonged atrial refractoriness and reduced AF duration without affecting the ventricular refractoriness or blood pressure in pigs subjected to 7 days atrial tachypacing, as well as in sham-operated control pigs.

Conclusions

SK currents play a role in porcine atrial repolarization, and pharmacological inhibition of these with AP14145 demonstrates antiarrhythmic effects in a vernakalant-resistant porcine model of AF. These results suggest SK channel blockers as potentially interesting anti-AF drugs.

A phosphoinositide 3-kinase (PI3K)-serum- and glucocorticoid-inducible kinase 1 (SGK1) pathway promotes Kv7.1 channel surface expression by inhibiting Nedd4-2 protein

Epithelial cell polarization involves several kinase signaling cascades that eventually divide the surface membrane into an apical and a basolateral part. One kinase, which is activated during the polarization process, is phosphoinositide 3-kinase (PI3K). In MDCK cells, the basolateral potassium channel Kv7.1 requires PI3K activity for surface-expression during the polarization process. Here, we demonstrate that Kv7.1 surface expression requires tonic PI3K activity as PI3K inhibition triggers endocytosis of these channels in polarized MDCK. Pharmacological inhibition of SGK1 gave similar results as PI3K inhibition, whereas overexpression of constitutively active SGK1 overruled it, suggesting that SGK1 is the primary downstream target of PI3K in this process. Furthermore, knockdown of the ubiquitin ligase Nedd4-2 overruled PI3K inhibition, whereas a Nedd4-2 interaction-deficient Kv7.1 mutant was resistant to both PI3K and SGK1 inhibition. Altogether, these data suggest that a PI3K-SGK1 pathway stabilizes Kv7.1 surface expression by inhibiting Nedd4-2-dependent endocytosis and thereby demonstrates that Nedd4-2 is a key regulator of Kv7.1 localization and turnover in epithelial cells.