Mechanism of external K+ sensitivity of KCNQ1 channels

Abstract

KCNQ1 voltage-gated K+ channels are involved in a wide variety of fundamental physiological processes and exhibit the unique feature of being markedly inhibited by external K+. Despite the potential role of this regulatory mechanism in distinct physiological and pathological processes, its exact underpinnings are not well understood. In this study, using extensive mutagenesis, molecular dynamics simulations, and single-channel recordings, we delineate the molecular mechanism of KCNQ1 modulation by external K+. First, we demonstrate the involvement of the selectivity filter in the external K+ sensitivity of the channel. Then, we show that external K+ binds to the vacant outermost ion coordination site of the selectivity filter inducing a diminution in the unitary conductance of the channel. The larger reduction in the unitary conductance compared to whole-cell currents suggests an additional modulatory effect of external K+ on the channel. Further, we show that the external K+ sensitivity of the heteromeric KCNQ1/KCNE complexes depends on the type of associated KCNE subunits.

Sensing its own permeant ion: KCNQ1 channel inhibition by external K+

“…Likewise, in basolateral membranes of intestinal and tracheal epithelial cells ( Preston et al, 2010 ), where KCNQ1 co-assembles with KCNE3 to form voltage-independent KCNQ1/KCNE3 potassium channels, extracellular K + inhibition might impact apical water and Cl − secretion with pathophysiological consequences such as promoting inflammatory bowel disease. Nevertheless, as highlighted by the current study of Abrahamyan and co-workers ( Abrahamyan et al, 2023 ), the degree of K + o -dependent channel inhibition not only depends on the type of β subunit with which KCNQ1 associates, but also on the number of accessory subunits assembled. Thus, while heteromeric KCNQ1/KCNE1 channels with saturating KCNE1 content (4:4) are insensitive to external K + , KCNQ1/KCNE1 complexes at lower stoichiometric rates (and heteromeric KCNQ1/KCNE3 channels irrespective of the amount of KCNE3 subunits) would be more significantly impacted by external K + .…”

“…However, a detailed understanding of the mechanistic basis underlying Kv7 channel inhibition, including those by the permeant ion, K + , has not yet been achieved. In this issue of the Journal of General Physiology , Abrahamyan et al (2023) used systematic alanine scanning mutagenesis, whole cell and single channel electrophysiology, and molecular dynamic (MD) simulation approaches to systematically investigate the molecular mechanism and structural components of Kv7.1 inhibition by external K + and unraveled the role of the selectivity filter (SF; at the most extracellular K + site, S0, in particular) in this process.…”

“…A comprehensive review of the abovementioned modulation of KCNQ1 is beyond the scope of this commentary and can be found elsewhere ( Liin et al, 2015 ; Jones et al, 2021 ). Instead, this commentary focuses on the less-well understood mechanisms by which extracellular potassium ions (K + o ) inhibit KCNQ1 channel conductance, investigated in a comprehensive study by Abrahamyan and co-workers ( Abrahamyan et al, 2023 ). It was previously shown that extracellular potassium can inhibit KCNQ1 channels by a mechanism that differed from the classical C-type inactivation seen in other Kv channels ( Larsen et al, 2011 ).…”

“…In the current article, Abrahamyan and coworkers ( Abrahamyan et al, 2023 ) tackle this question directly by combining systematic alanine scanning mutagenesis with a method to estimate the fractional fast inactivation in KCNQ1 channels ( Tristani-Firouzi and Sanguinetti, 1998 ). To address whether the electronegative potential produced by the negatively charged residues surrounding the extracellular surface of the pore acts as the K + o regulatory site, the authors first mutated to alanine all these residues, which were identified based on the published high-resolution structure of KCNQ1 ( Sun and MacKinnon, 2017 ).…”

“…High resolution structural work under conditions of low and high external potassium will be needed to investigate whether K + o -mediated inhibition of KCNQ1 channels would involve neither dilation nor constriction of the external pore as predicted for a C-type inactivated filter. In any case, the model proposed by Abrahamyan et al (2023) suggests that the ionic current inhibition in KCNQ1 channels results from an increased occupancy of the filter S0 site by external potassium. This is an important finding with implications toward our understanding of the modulation of KCNQ1 or other related channels.…”