Impact of arrhythmogenic calmodulin variants on small conductance Ca(2+)‐activated K(+) (SK3) channels

Calmodulin (CaM) is a ubiquitous Ca(2+)‐sensing protein regulating many important cellular processes. Several CaM‐associated variants have been identified in a small group of patients with cardiac arrhythmias. The mechanism remains largely unknown, even though a number of ion channels, including the ryanodine receptors and the L‐type calcium channels have been shown to be functionally affected by the presence of mutant CaM. CaM is constitutively bound to the SK channel, which underlies the calcium‐gated I (SK) contributing to cardiac repolarization. The CaM binding to SK channels is essential for gating, correct assembly, and membrane expression. To elucidate the effect of nine different arrhythmogenic CaM variants on SK3 channel function, HEK293 cells stably expressing SK3 were transiently co‐transfected with CaM(WT or variant) and whole‐cell patch‐clamp recordings were performed with a calculated free Ca(2+) concentration of 400 nmol/L. MDCK cells were transiently transfected with SK3 and/or CaM(WT or variant) to address SK3 and CaM localization by immunocytochemistry. The LQTS‐associated variants CaM(D96V), CaM(D130G), and CaM(F142L) reduced I (SK,Ca) compared with CaM(WT) (P < 0.01, P < 0.001, and P < 0.05, respectively). The CPVT associated variant CaM(N54I) also reduced the I (SK,Ca) (P < 0.05), which was linked to an accumulation of SK3/CaM(N54I) channel complexes in intracellular compartments (P < 0.05). The CPVT associated variants, CaM(A103V) and CaM(D132E) only revealed a tendency toward reduced current, while the variants CaM(F90L) and CaM(N98S), causing LQTS syndrome, did not have any impact on I (SK,Ca). In conclusion, we found that the arrhythmogenic CaM variants CaM(N54I), CaM(D96V), CaM(D130G), and CaM(F142L) significantly down‐regulate the SK3 channel current, but with distinct mechanism.