Patrick A. Schweizer

Upregulation of K(2P)3.1 K+ Current Causes Action Potential Shortening in Patients With Chronic Atrial Fibrillation

Background— Antiarrhythmic management of atrial fibrillation (AF) remains a major clinical challenge. Mechanism-based approaches to AF therapy are sought to increase effectiveness and to provide individualized patient care. K2P3.1 (TASK-1 [tandem of P domains in a weak inward-rectifying K+ channel–related acid-sensitive K+ channel-1]) 2-pore-domain K+ (K2P) channels have been implicated in action potential regulation in animal models. However, their role in the pathophysiology and treatment of paroxysmal and chronic patients with AF is unknown. Methods and Results— Right and left atrial tissue was obtained from patients with paroxysmal or chronic AF and from control subjects in sinus rhythm. Ion channel expression was analyzed by quantitative real-time polymerase chain reaction and Western blot. Membrane currents and action potentials were recorded using voltage- and current-clamp techniques. K2P3.1 subunits exhibited predominantly atrial expression, and atrial K2P3.1 transcript levels were highest among functional K2P channels. K2P3.1 mRNA and protein levels were increased in chronic AF. Enhancement of corresponding currents in the right atrium resulted in shortened action potential duration at 90% of repolarization (APD90) compared with patients in sinus rhythm. In contrast, K2P3.1 expression was not significantly affected in subjects with paroxysmal AF. Pharmacological K2P3.1 inhibition prolonged APD90 in atrial myocytes from patients with chronic AF to values observed among control subjects in sinus rhythm. Conclusions— Enhancement of atrium-selective K2P3.1 currents contributes to APD shortening in patients with chronic AF, and K2P3.1 channel inhibition reverses AF-related APD shortening. These results highlight the potential of K2P3.1 as a novel drug target for mechanism-based AF therapy.

cAMP Sensitivity of HCN Pacemaker Channels Determines Basal Heart Rate But Is Not Critical for Autonomic Rate Control

Background—HCN channels activate the pacemaker current If, which is thought to contribute significantly to generation and regulation of heart rhythm. HCN4 represents the dominant isotype in the sinoatrial node and binding of cAMP was suggested to be necessary for autonomic heart rate regulation. Methods and Results—In a candidate gene approach, a heterozygous insertion of 13 nucleotides in exon 6 of the HCN4 gene leading to a truncated cyclic nucleotide-binding domain was identified in a 45-year-old woman with sinus bradycardia. Biophysical properties determined by whole-cell patch-clamp recording of HEK293 cells demonstrated that mutant subunits (HCN4-695X) were insensitive to cAMP. Heteromeric channels composed of wild-type and mutant subunits failed to respond to cAMP-like homomeric mutant channels, indicating a dominant-negative suppression of cAMP-induced channel activation by mutant subunits. Pedigree analysis identified 7 additional living carriers showing similar clinical phenotypes, that is, sinus node dysfunction with mean resting heart rate of 45.9±4.6 bpm (n=8) compared with 66.5±9.1 bpm of unaffected relatives (n=6; P<0.01). Clinical evaluation revealed no ischemic or structural heart disease in any family member. Importantly, mutant carriers exhibited normal heart rate variance and full ability to accelerate heart rate under physical activity or pharmacological stimulation. Moreover, mutant carriers displayed distinctive sinus arrhythmias and premature beats linked to adrenergic stress. Conclusions—In humans, cAMP responsiveness of If determines basal heart rate but is not critical for maximum heart rate, heart rate variability, or chronotropic competence. Furthermore, cAMP-activated If may stabilize heart rhythm during chronotropic response.

Successful acute and long-term management of electrical storm in Brugada syndrome using orciprenaline and quinine/quinidine

Brugada syndrome (BrS) is an inherited electrical disorder characterized by ST segment elevation in the right precordial ECG leads and a widened QRS complex [1–5]. BrS accounts for approximately 20% of sudden cardiac deaths in the absence of obvious structural heart disease [1]. Typical ECG manifestations are often concealed but may be unmasked by sodium channel blockers during intravenous drug challenge [6]. Mutations in the gene encoding for the cardiac sodium channel a-subunit, SCN5A, were identified in 18–30% of Brugada syndrome cases [2]. Ventricular fibrillation (VF) and sudden death occur usually at rest and at night. While incessant ventricular tachycardia may be observed in patients with coronary artery disease, congestive heart failure, myocarditis, or drug-induced long QT syndrome [7–10], electrical storm (ES; defined as C3 VF episodes in 24 h) associated with BrS is a seldom reported, potentially lethal event [11–13]. Radiofrequency catheter ablation of VF-triggering premature ventricular complexes, extracorporeal membrane oxygenation, or even heart transplantation as last resort may be required in order to provide circulatory and respiratory support and to prevent a fatal outcome in these rare cases [14–17]. Clinical management of ES in Brugada syndrome primarily relies on reported cases owing to its low prevalence and to the lack of randomized clinical studies. Case report

hERG K+ channel-associated cardiac effects of the antidepressant drug desipramine

Cardiac side effects of antidepressant drugs are well recognized. Adverse effects precipitated by the tricyclic drug desipramine include prolonged QT intervals, torsade de pointes tachycardia, heart failure, and sudden cardiac death. QT prolongation has been primarily attributed to acute blockade of hERG/IKr currents. This study was designed to provide a more complete picture of cellular effects associated with desipramine. hERG channels were expressed in Xenopus laevis oocytes and human embryonic kidney (HEK 293) cells, and potassium currents were recorded using patch clamp and two-electrode voltage clamp electrophysiology. Ventricular action potentials were recorded from guinea pig cardiomyocytes. Protein trafficking and cell viability were evaluated in HEK 293 cells and in HL-1 mouse cardiomyocytes by immunocytochemistry, Western blot analysis, or colorimetric MTT assay, respectively. We found that desipramine reduced hERG currents by binding to a receptor site inside the channel pore. hERG protein surface expression was reduced after short-term treatment, revealing a previously unrecognized mechanism. When long-term effects were studied, forward trafficking was impaired and hERG currents were decreased. Action potential duration was prolonged upon acute and chronic desipramine exposure. Finally, desipramine triggered apoptosis in cells expressing hERG channels. Desipramine exerts at least four different cellular effects: (1) direct hERG channel block, (2) acute reduction of hERG surface expression, (3) chronic disruption of hERG trafficking, and (4) induction of apoptosis. These data highlight the complexity of hERG-associated drug effects.

PKC-dependent activation of human K2P18.1 K+ channels

BACKGROUND AND PURPOSE Two‐pore‐domain K+ channels (K2P) mediate K+ background currents that modulate the membrane potential of excitable cells. K2P18.1 (TWIK‐related spinal cord K+ channel) provides hyperpolarizing background currents in neurons. Recently, a dominant‐negative loss‐of‐function mutation in K2P18.1 has been implicated in migraine, and activation of K2P18.1 channels was proposed as a therapeutic strategy. Here we elucidated the molecular mechanisms underlying PKC‐dependent activation of K2P18.1 currents.

The pathology and treatment of cardiac arrhythmias: focus on atrial fibrillation

Atrial fibrillation (AF) is the most frequently encountered sustained cardiac arrhythmia in clinical practice and a major cause of morbidity and mortality. Effective treatment of AF still remains an unmet medical need. Treatment of AF is based on drug therapy and ablative strategies. Antiarrhythmic drug therapy is limited by a relatively high recurrence rate and proarrhythmic side effects. Catheter ablation suppresses paroxysmal AF in the majority of patients without structural heart disease but is more difficult to achieve in patients with persistent AF or with concomitant cardiac disease. Stroke is a potentially devastating complication of AF, requiring anticoagulation that harbors the risk of bleeding. In search of novel treatment modalities, targeted pharmacological treatment and gene therapy offer the potential for greater selectivity than conventional small-molecule or interventional approaches. This paper summarizes the current understanding of molecular mechanisms underlying AF. Established drug therapy and interventional treatment of AF is reviewed, and emerging clinical and experimental therapeutic approaches are highlighted.

Carvedilol targets human K2P3.1 (TASK1) K+ leak channels

BACKGROUND AND PURPOSE Human K2P3.1 (TASK1) channels represent potential targets for pharmacological management of atrial fibrillation. K2P channels control excitability by stabilizing membrane potential and by expediting repolarization. In the heart, inhibition of K2P currents by class III antiarrhythmic drugs results in action potential prolongation and suppression of electrical automaticity. Carvedilol exerts antiarrhythmic activity and suppresses atrial fibrillation following cardiac surgery or cardioversion. The objective of this study was to investigate acute effects of carvedilol on human K2P3.1 (hK2P3.1) channels.

Alternative splicing determines mRNA translation initiation and function of human K2P10.1 K+ channels

Non‐technical summary The resting membrane potential of excitable cells such as neurones and cardiac myocytes depends on the distribution of potassium ions across the cell membrane. Specialized membrane proteins called K2P10.1 ion channels pass potassium ions and stabilize membranes of excitable cells at hyperpolarizing potentials below the threshold for action potential firing. Alternative mRNA translation initiation (ATI) contributes to K2P10.1 protein diversity: Ribosomal synthesis of K2P10.1 channel proteins harbouring different N‐terminal domains initiated from two downstream mRNA start codons regulates K2P10.1 function. We now demonstrate that splicing determines translation start sites of human K2P10.1 mRNA via recombination of short nucleotide signalling sequences preceding the first start mRNA codon, revealing a novel biological mechanism. Our study suggests that tissue‐specific K2P10.1 ion channel mRNA splicing and translation initiation determines the resting membrane potential and contributes to electrophysiological plasticity of neuronal and cardiac cells.

Inhibition of cardiac two-pore-domain K+ (K2P) channels--an emerging antiarrhythmic concept.

Effective and safe pharmacological management of cardiac arrhythmia still constitutes a major clinical challenge. Outward potassium currents mediated by two-pore-domain potassium (K2P) channels promote repolarization of excitable cells. In the heart, inhibition or genetic inactivation of K2P currents results in action potential prolongation. Human K2P3.1 (TASK-1) channels are predominantly expressed in the atria and represent targets for the treatment of atrial fibrillation. In addition, stretch-sensitive K2P2.1 (TREK-1) channels are implicated in mechanoelectrical feedback and arrhythmogenesis in atrial and ventricular tissue. K2P current inhibition by clinically used antiarrhythmic drugs indicates a role of the channels as potential drug targets. This work summarizes the current knowledge on function, pharmacology, and significance of cardiac K2P channels. Therapeutic implications with emphasis on atrial fibrillation are highlighted.

HERG K+ channel-dependent apoptosis and cell cycle arrest in human glioblastoma cells.

Glioblastoma (GB) is associated with poor patient survival owing to uncontrolled tumor proliferation and resistance to apoptosis. Human ether-a-go-go-related gene K+ channels (hERG; Kv11.1, KCNH2) are expressed in multiple cancer cells including GB and control cell proliferation and death. We hypothesized that pharmacological targeting of hERG protein would inhibit tumor growth by inducing apoptosis of GB cells. The small molecule hERG ligand doxazosin induced concentration-dependent apoptosis of human LNT-229 (EC50 = 35 µM) and U87MG (EC50 = 29 µM) GB cells, accompanied by cell cycle arrest in the G0/G1 phase. Apoptosis was associated with 64% reduction of hERG protein. HERG suppression via siRNA-mediated knock down mimicked pro-apoptotic effects of doxazosin. Antagonism of doxazosin binding by the non-apoptotic hERG ligand terazosin resulted in rescue of protein expression and in increased survival of GB cells. At the molecular level doxazosin-dependent apoptosis was characterized by activation of pro-apoptotic factors (phospho-erythropoietin-producing human hepatocellular carcinoma receptor tyrosine kinase A2, phospho-p38 mitogen-activated protein kinase, growth arrest and DNA damage inducible gene 153, cleaved caspases 9, 7, and 3), and by inactivation of anti-apoptotic poly-ADP-ribose-polymerase, respectively. In summary, this work identifies doxazosin as small molecule compound that promotes apoptosis and exerts anti-proliferative effects in human GB cells. Suppression of hERG protein is a crucial molecular event in GB cell apoptosis. Doxazosin and future derivatives are proposed as novel options for more effective GB treatment.