Targeting small-conductance calcium-activated potassium channels in atrial fibrillation: therapeutic opportunities

Small-conductance Ca²⁺-activated K⁺ (SK, K_Ca2, encoded by KCNN genes) channels are unique in that they are gated solely by intracellular Ca²⁺. Therefore, SK channels in cardiomyocytes serve to integrate the beat-to-beat changes in intracellular Ca²⁺ and the membrane potentials and are tightly coupled with intracellular Ca²⁺ signaling. The expression of SK channels was found to be more abundant in atrial and pacemaking cells compared to ventricular myocytes^1–3. Studies over the past two decades have provided evidence to substantiate the pivotal roles of SK channels, not only in healthy heart but also in diseases including atrial fibrillation (AF) and heart failure (HF). Moreover, genome-wide association analysis (GWAS) reveals the association between AF and genetic variants in KCNN2 and KCNN3^4–6, supporting the roles of SK channels in human AF. SK channels may represent new therapeutic targets against AF, and a clinical study is currently ongoing⁷. However, the regulation and remodeling of SK channels in human AF remain incompletely understood^{8, 9}.

Cardiac SK channels form multi-protein complexes with calmodulin (CaM), α-actinin2, filamin A, myosin light chain 2 (MLC2), protein casein kinase II (CKII), protein phosphatase 2A (PP2A), and Ca²⁺/CaM-dependent protein kinase II (CaMKII)^{4, 5}. SK channel trafficking and gating are known to be regulated by intracellular Ca²⁺, phosphorylation and dephosphorylation of CaM by CKII and PP2A, and phosphorylation of SK channels by CaMKII and protein kinase A (PKA)^{4, 5}.

The elegant paper by Heijmen et al published in this issue of Circulation Research addressed the mechanisms of SK channel gating and membrane targeting in atrial myocytes from AF patients¹⁰, as illustrated in Figure 1. Using human atrial samples from patients without a history of AF (control group) or with a clinical diagnosis of persistent AF (chronic AF group, cAF), the authors identified the upregulation of apamin-sensitive SK currents (I_SK) in cAF atrial myocytes. Mechanistically, there were no changes in SK1 and SK2 subunits at mRNA or protein levels in whole-tissue homogenates, however, there was a significant increase in the sarcolemmal/cytosolic SK2 channel expression ratio in cAF-cardiomyocytes, suggesting increased SK2 channel membrane targeting. The authors tested the role of Ca²⁺ and α-actinin2 in the forward trafficking of SK2 channels, and found an increase in Ca²⁺- and α-actinin2-dependent SK2 channel trafficking in cAF. The findings further strengthened the conclusion from a previous study¹¹. Moreover, there was elevated expression of CaM in cAF atrial myocytes with decreased Thr89 phosphorylation, likely due to the increased PP2A expression levels, while CKII expression did not change significantly. In vitro tachypacing significantly increased SK current, which can be attenuated by BAPTA-AM, okadaic acid, or nifedipine, strongly supporting the importance of trans-sarcolemmal Ca²⁺ influx in SK channel activation¹². To further validate the findings, the authors used in silico human atrial myocyte model and demonstrated significant contributions of SK currents to action potential durations, with a switch from background inward rectifier K⁺ current (I_K1) to SK current dominance in cAF.

Figure 1. SK channels trafficking, gating, and interactome in AF.

SK channels interactome includes α-actinin2 (Actn2), filamin A (FLNA), myosin light chain 2 (MLC2), protein casein kinase II (CK2), protein phosphatase 2A (PP2A). Cardiac SK channels have been shown to couple to L-type Ca²⁺ channels through a physical bridge, α-actinin2. SK2 channels do not physically interact with the Ca²⁺ channels, instead the two channels co-localize via their interaction with α-actinin2 along the Z-line in atrial myocytes. AF or tachypacing, may not alter the expression of SK channels in atrial myocytes, but increase calmodulin (CaM) expression, enhance association of α-actinin2 with SK channels, increase dephosphorylation of CaM by PP2A, and stimulate SK channel forward trafficking and membrane targeting. This results in increased SK currents, shortening of the atrial action potentials, and maintenance of arrhythmias. Schematic representation was generated using BioRender.

The current study provides critical insights and rationales for the use of SK channel inhibitors in human AF. However, challenges remain. SK channels are known to be upregulated in HF and may serve to increase cardiac repolarization reserve by counterbalancing the downregulation of other K⁺ channels. SK channel inhibition has been shown to be proarrhythmic in HF models⁶. Indeed, one major limitation of current antiarrhythmic drugs is the risk for proarrhythmia, which can be life-threatening. Therefore, elucidating the role of SK channels’ intricate mechanisms in pathological conditions, as in the current study, will have profound clinical implications.

A recent cryo-electron microscopy (EM) study provides atomistic structures of human SK4 channels in closed, intermediate, and activated states, supporting the previous functional studies as well as providing detailed structural information for SK channel gating, laying a solid foundation for the understanding of the physiological and pharmacological properties of SK channels in the heart¹³. Moreover, the protein structure prediction strategy based on artificial intelligence (AI) has been established, introduced by AlphaFold (DeepMind, Google) and other researchers¹⁴. These emerging techniques may be used to predict the structure of proteins and generate protein-protein complexes. Thus, these innovative tools enable the investigation into the function, interactomes, and regulation of cardiac SK channels at the atomistic levels, making it possible for structure-guided designs of pharmacological agents for the activation or inhibition of cardiac SK channels, targeting cardiac diseases including AF.

Identification of cardiac SK channels has opened new avenues for therapeutic opportunities in cardiac arrhythmias^1–3. The differential expression of cardiac SK channels in pacemaking and contractile myocytes within each tissue type creates a remarkable prospect for cardiac regional-specific therapy. Indeed, SK channels may play critical roles beyond AF. Furthermore, SK channel function is significantly altered by human CaM mutations, linked to life-threatening arrhythmia syndromes. However, critical knowledge gaps remain. The possible heterogeneity of SK channels in atria and ventricles remains incompletely understood, and the functional roles of SK channels in pacemaking cells and cardiac conduction systems are only beginning to be realized. SK channels expression and function in intracellular organelles, including cardiac mitochondria, have not been extensively studied¹⁵. Additionally, the interactomes of SK channels within the microdomains are not well understood and may be identified with advanced proteomics approaches. At the translational level, new insights into the regulation of SK channel isoform-specific expression, trafficking, post-translational modifications, and compartmentalization in diseases will have profound clinical implications.