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

Abstract

Calmodulin (CaM) is a ubiquitous Ca2+‐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 ISK 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 CaMWT or variant and whole‐cell patch‐clamp recordings were performed with a calculated free Ca2+ concentration of 400 nmol/L. MDCK cells were transiently transfected with SK3 and/or CaMWT or variant to address SK3 and CaM localization by immunocytochemistry. The LQTS‐associated variants CaMD96V, CaMD130G, and CaMF142L reduced ISK,Ca compared with CaMWT (P < 0.01, P < 0.001, and P < 0.05, respectively). The CPVT associated variant CaMN54I also reduced the ISK,Ca (P < 0.05), which was linked to an accumulation of SK3/CaMN54I channel complexes in intracellular compartments (P < 0.05). The CPVT associated variants, CaMA103V and CaMD132E only revealed a tendency toward reduced current, while the variants CaMF90L and CaMN98S, causing LQTS syndrome, did not have any impact on ISK,Ca. In conclusion, we found that the arrhythmogenic CaM variants CaMN54I, CaMD96V, CaMD130G, and CaMF142L significantly down‐regulate the SK3 channel current, but with distinct mechanism.

Arrhythmia mutations in calmodulin can disrupt cooperativity of Ca2+ binding and cause misfolding

“…2014), and because of the CPVT phenotype in several patients, suggest that any mechanism of disease is likely to involve RyR2 or other ion channels instead (Pancaroglu & Van Petegem, 2018; Saljic et al . 2019).…”

“…Of note, a recent report also found an effect of the N54I mutation on the Ca 2+ ‐activated potassium channel SK3, showing an additional ion channel to play a role in the pathophysiology of CaM arrhythmia mutations (Saljic et al . 2019).…”

Cardiac small-conductance calcium-activated potassium channels in health and disease

“…Since SK channel activation and membrane localization are critically dependent on CaM binding to the CaMBD in the C terminus of the channel, CaM mutations, associated with human arrhythmia syndrome, significantly alter SK channel function [ 38 , 81 , 104 ]. Five human CaM mutations (N54I, N98S, D96V, D130G, and F90L) significantly reduces SK2 current relative to wild-type CaM without a change in the membrane or intracellular protein expression [ 104 ].…”

“…Five human CaM mutations (N54I, N98S, D96V, D130G, and F90L) significantly reduces SK2 current relative to wild-type CaM without a change in the membrane or intracellular protein expression [ 104 ]. Similarly, long QT syndrome (LQTS)-associated CaM variants (D96V, D130G, F142L) and catecholaminergic polymorphic ventricular tachycardia (CPVT)-associated variant (N54I) reduces SK3 current [ 81 ]. To address the mechanistic underpinnings of the CaM mutations on SK channel function, we took advantage of human induced pluripotent stem cell-derived cardiomyocyte-like cells (hiPSC-CMs) as well as structural modeling and molecular dynamics simulation [ 38 ].…”

“…SK channels may represent novel therapeutic targets against AF [ 10 , 19 – 21 , 76 , 82 , 84 , 88 , 95 ]. Furthermore, SK channel function is significantly altered by human calmodulin (CaM) mutations, linked to life-threatening arrhythmia syndromes [ 38 , 81 , 104 ]. The current review will summarize recent progress in our understanding of cardiac SK channels and serve as a discussion platform for the roles of SK channels in the heart in health and disease.…”