The Voltage Activation of Cortical KCNQ Channels Depends on Global PIP2 Levels

Abstract

The slow afterhyperpolarization (sAHP) is a calcium-activated potassium conductance with critical roles in multiple physiological processes. Pharmacological and genetic data suggest that KCNQ channels partly mediate the sAHP. However, these channels are not typically open within the observed voltage range of the sAHP. Recent work has shown that the sAHP is gated by increased PIP2 levels, which are generated downstream of calcium binding by neuronal calcium sensors such as hippocalcin. Here, we examined whether changes in PIP2 levels could shift the voltage-activation range of KCNQ channels. In HEK293T cells, expression of the PIP5 kinase PIPKIγ90, which increases global PIP2 levels, shifted the KCNQ voltage activation to within the operating range of the sAHP. Further, the sensitivity of this effect on KCNQ3 channels appeared to be higher than that on KCNQ2. Therefore, we predict that KCNQ3 plays an essential role in maintaining the sAHP under low PIP2 conditions. In support of this notion, we find that sAHP inhibition by muscarinic receptors that increase phosphoinositide turnover in neurons is enhanced in Kcnq3-knockout mice. Likewise, the presence of KCNQ3 is essential for maintaining the sAHP when hippocalcin is ablated, a condition that likely impairs PIP2 generation. Together, our results establish the relationship between PIP2 and the voltage dependence of cortical KCNQ channels (KCNQ2/3, KCNQ3/5, and KCNQ5), and suggest a possible mechanism for the involvement of KCNQ channels in the sAHP.

Early-onset epileptic encephalopathy caused by a reduced sensitivity of Kv7.2 potassium channels to phosphatidylinositol 4,5-bisphosphate

“…The present observation that increasing cellular PIP 2 levels with PIP5K negatively shifted the V ½ of Kv7.2 channels (as previously described for Kv7.159, Kv7.249, and other heteromeric Kv7 channels31) and decreased retigabine-induced responses in both in Kv7.2 and Kv7.2 R325G channels, indicates that retigabine and PIP 2 act via at least partially overlapping mechanisms to stabilize voltage-dependent pore opening; consistent with this view is the fact that PIP 2 -depleted Kv7.3 channels are insensitive to retigabine60.…”

“…In conclusion, the present results add severe forms of Kv7.2-related epilepsy to the growing list of channelopathies caused by changes in PIP 2 -dependent regulation159; the recent observation that Kv7 channels are critical determinants of the cortical excitability changes occurring upon dynamic regulation of PIP 2 levels31, lends further support to the pathogenetic role of the proposed mechanism in individuals carrying the Kv7.2 R325G variant. Future studies will further explore the structural implications of the functional results herein presented, and define whether such specific molecular defect is associated with distinct clinical features.…”

“…5a and b), and increased current voltage-sensitivity (Fig. 5c), as described2831. Notably, co-expression with PIP5K led to the appearance of measurable currents in cells transfected with Kv7.2 R325G cDNA (Fig.…”

Modulation of Kv7 channels and excitability in the brain

“…While it has been known that PIP 2 interaction with Kv7 is mandatory for channel function [34,35], more recently it was shown that PIP 2 has multiple sites of interaction within the channel, with varying effects [36]. One such PIP 2 interaction modulates coupling the voltage-sensing to the pore-gating domain [37–40]. Functionally, PIP 2 interaction within the voltage-sensing domain of the S4-S5 linker has been attributed to increasing open probability thus voltage-conductance [40].…”

Ion channel regulation by β-secretase BACE1 – enzymatic and non-enzymatic effects beyond Alzheimer's disease

“…However, BACE1 did not alter the decrease of KCNQ2/3 currents upon PIP 2 depletion, nor did PIP 2 displace BACE1. 96,103 Thus, BACE1 appears to act differently than KCNE1, which sensitizes KCNQ1 to PIP 2 . 104 Furthermore, the currentpromoting effect of BACE1 was different from that of the established M-current activator retigabine.…”