Enhanced Ca ²⁺ -Dependent SK-Channel Gating and Membrane Trafficking in Human Atrial Fibrillation

Abstract

Background: Small-conductance Ca 2+ -activated K + (SK)–channel inhibitors have antiarrhythmic effects in animal models of atrial fibrillation (AF), presenting a potential novel antiarrhythmic option. However, the regulation of SK-channels in human atrial cardiomyocytes and its modification in patients with AF are poorly understood and were the object of this study. Methods and Results: Apamin-sensitive SK-channel current (I SK ) and action potentials were recorded in human right-atrial cardiomyocytes from sinus rhythm control (Ctl) patients or patients with (long-term persistent) chronic AF (cAF). I SK was significantly higher, and apamin caused larger action potential prolongation in cAF- versus Ctl- cardiomyocytes. Sensitivity analyses in an in silico human atrial cardiomyocyte model identified I K1 and I SK as major regulators of repolarization. Increased I SK in cAF was not associated with increases in mRNA/protein levels of SK-channel subunits in either right- or left-atrial tissue homogenates or right-atrial cardiomyocytes, but the abundance of SK2 at the sarcolemma was larger in cAF versus Ctl in both tissue-slices and cardiomyocytes. Latrunculin-A and primaquine (anterograde and retrograde protein-trafficking inhibitors) eliminated the differences in SK2 membrane levels and I SK between Ctl- and cAF-cardiomyocytes. In addition, the phosphatase-inhibitor okadaic acid reduced I SK amplitude and abolished the difference between Ctl- and cAF-cardiomyocytes, indicating that reduced calmodulin-Thr80 phosphorylation due to increased protein phosphatase-2A levels in the SK-channel complex likely contribute to the greater I SK in cAF-cardiomyocytes. Finally, rapid electrical activation (5 Hz, 10 minutes) of Ctl-cardiomyocytes promoted SK2 membrane-localization, increased I SK and reduced action potential duration, effects greatly attenuated by apamin. Latrunculin-A or primaquine prevented the 5-Hz-induced I SK -upregulation. Conclusions: I SK is upregulated in patients with cAF due to enhanced channel function, mediated by phosphatase-2A-dependent calmodulin-Thr80 dephosphorylation and tachycardia-dependent enhanced trafficking and targeting of SK-channel subunits to the sarcolemma. The observed AF-associated increases in I SK , which promote reentry-stabilizing action potential duration shortening, suggest an important role for SK-channels in AF auto-promotion and provide a rationale for pursuing the antiarrhythmic effects of SK-channel inhibition in humans.

Dual effects of the small-conductance Ca2+-activated K+current on human atrial electrophysiology and Ca2+-driven arrhythmogenesis: anin silicostudy

“…To investigate the impact of SK channel modulation on atrial electrophysiology and arrhythmogenesis, we updated our established model of the human atrial myocyte in nSR and cAF conditions (41)(42)(43). We included into this cellular framework the ISK formulation that we have recently developed based on recordings in human right-atrial cardiomyocytes from nSR and cAF patients (29). To simulate INa, we used the formulation developed by Courtemanche et al (44), and adjusted its maximal conductance to maintain the physiologic AP upstroke velocity.…”

“…Our default cAF model does not account for any AF-associated SK channel remodeling. However, we performed a separate set of simulations in which both SK channel conductance and Ca 2+sensitivity are altered as seen in AF (29,47).…”

“…Despite the lack of a voltage sensor in the channel, inwardly rectifying ISK-voltage relationships have been observed in many studies in both native and cloned channels from various species (22)(23)(24)(25), and attributed to a voltage-dependent modulation of SK channel pore conductance by intracellular divalent (Ca 2+ and Mg 2+ ) cations (26,27). Recent data revealed that SK channel trafficking also increases in a Ca 2+ -dependent manner (28,29). These properties make SK channels of particular interest during fast pacing or tachy-arrhythmias (like AF), when Ca 2+ accumulation is likely to enhance their activation.…”

“…Indeed, both SK channels hyper-activity (24,29,(31)(32)(33)(34)(35)(36)(37)(38) and suppression (22,25,39) have been implicated in AF, likely through distinct mechanisms, and depending on species, heart rates, and disease etiology. Here, we aim to investigate the impact of modulating ISK on the electrophysiology of human atrial myocytes and their vulnerability to Ca 2+ -driven arrhythmias using mathematical modeling and simulation, which have proven useful tool for gaining mechanistic insight into AF mechanisms, and developing novel antiarrhythmic pharmacological approaches (4,40).…”

Targeting Small-Conductance Calcium-Activated Potassium Channels in Atrial Fibrillation: Therapeutic Opportunities

“…The elegant article by Heijman et al 10 published in this issue of Circulation Research addressed the mechanisms of SK channel gating and membrane targeting in atrial myocytes from patients with AF, as illustrated in the Figure. Using human atrial samples from patients without a history of AF (control group) or with a clinical diagnosis of persistent AF (chronic AF [cAF] group), the authors identified the upregulation of apamin-sensitive SK currents (I SK ) in cAF atrial myocytes.…”