Mechanisms of Calmodulin Regulation of Different Isoforms of Kv7.4 K+ Channels

Abstract

Calmodulin (CaM), a Ca2؉ -sensing protein, is constitutively bound to IQ domains of the C termini of human Kv7 (hKv7, KCNQ) channels to mediate Ca 2؉ -dependent reduction of Kv7 currents. However, the mechanism remains unclear. We report that CaM binds to two isoforms of the hKv7.4 channel in a Ca 2؉ -independent manner but that only the long isoform (hKv7.4a) is regulated by Ca 2؉ /CaM. Ca 2؉ /CaM mediate reduction of the hKv7.4a channel by decreasing the channel open probability and altering activation kinetics. We took advantage of a known missense mutation (G321S) that has been linked to progressive hearing loss to further examine the inhibitory effects of Ca 2؉ / CaM on the Kv7.4 channel. Using multidisciplinary techniques, we demonstrate that the G321S mutation may destabilize CaM binding, leading to a decrease in the inhibitory effects of Ca 2؉ on the channels. Our study utilizes an expression system to dissect the biophysical properties of the WT and mutant Kv7.4 channels. This report provides mechanistic insights into the critical roles of Ca 2؉ /CaM regulation of the Kv7.4 channel under physiological and pathological conditions.

Lack of correlation between surface expression and currents in epileptogenic AB-calmodulin binding domain Kv7.2 potassium channel mutants

“…Often, facilitation is manifested by a left shift of the voltage-current relationship, such as that caused by apo-CaM for Kv7.2 and Kv7.4 channels [7,9,42], or holo-CaM for Kv7.1 [10]. However, changes in these parameters were not observed (Kv7.2/Kv7.3 heteromers) or were in the opposite direction (Kv7.2 homomers) [27].…”

Modulation of Kv7 channels and excitability in the brain

“…Consequently, calcium-bound CaM decreases Kv7.2 channel efficacy for PIP 2 , thereby inhibiting the M-current [44,90,91]. On the other hand, a recent study showed that splicing variants of Kv7.4 are differentially modulated by CaM [92]. This mechanism may explain why bradykinin induced suppression of the M-current is usually smaller than that by muscarinic agonists.…”

Atomistic Insights of Calmodulin Gating of Complete Ion Channels

“…Protein Casein kinase 2 (CK2) and protein phosphatase 2A (PP2A) form a signaling complex with SK2 channels and regulate channel activity through phosphorylation/dephospohylation [63]. CK2 docks to an N-terminal binding site (125-RRALF-129 in SK2, equivalent to 15-RKRKL-20 in SK4) located at the base of S1.…”

“…CK2 requires positively charged compounds to phosphorylate CaM [64], but these compounds are not present in functional excised patch experiments. As an alternative, it has been suggested that the charged residues at the base of S1 serve this purpose [63]. It has been shown that the mechanism involves phosphorylation of Thr 79 at the CaM lobe linker, leading to an increased dependency on PIP 2 binding, which is a lipid co-factor required for the function of SK2 channels [51].…”