Unravelling the role and mechanisms of SK4 channels in atrial fibrillation: the immune-cardiac crosstalk

Atrial fibrillation (AF) is the most common cardiac arrhythmia associated with increased risk of stroke and heart failure. Current therapies are largely symptomatic and fail to address the underlying molecular mechanisms driving AF progression1. The calcium-activated SK4 potassium (K+) channel has emerged as a promising druggable target, as preclinical studies demonstrated SK4 inhibition to reduce inflammation and acute AF susceptibility in the setting of heart failure2,3. However, the cellular mechanism linking SK4 activity to AF remains unclear. Here, we investigate whether SK4 contributes to immune-driven atrial remodeling and pro-arrhythmic electrophysiological alterations in AF. Public single-cell RNA-seq data (Heart Cell Atlas)4 showed that SK4 expression is predominantly detected in macrophages, with negligible expression in atrial and ventricular cardiomyocytes in healthy hearts. To access its relevance in AF, bulk RNA-seq data from human atrial appendix biopsies (AF and sinus rhythm patients) were analyzed. In the AF group, SK4 expression positively correlation with inflammation-associated markers, including CD11c and CXCL10, supporting its association with cardiac inflammatory activation. In vitro, pro-inflammatory M1-polarized macrophages exhibited increased SK4 gene expression compared to non-polarized controls in human THP-1 and mouse Raw 264.7 cells, indicating that SK4 is upregulated during inflammatory activation. Functionally, pharmacological SK4 inhibition (BAb69) prolonged action potential duration (APD) and reduced excitability in human iPSC-derived atrial cardiomyocytes (hiPSC-aCMs). Furthermore, in a co-culture system of mouse HL-1 cardiomyocytes and Raw 264.7 macrophages, the presence of M1-polarized macrophages shortened HL-1 APD, mimicking AF-like electrophysiological remodeling. BAb69 reversed this macrophage-induced APD shortening, suggesting that inhibition of the SK4 channel can ameliorate AF phenotypes. Collectively, our findings identify SK4 as a macrophage-enriched ion-channel that is upregulated during inflammation and associated with immune remodeling in AF. Pharmacological inhibition of SK4 conserves cardiomyocyte electrophysiology and mitigates pro-inflammatory macrophage-induced APD shortening, indicating the role of SK4 in immune-cardiac crosstalk and highlighting its potential as a therapeutic target in AF.