Sara I. Liin

KCNE1 divides the voltage sensor movement in KCNQ1/KCNE1 channels into two steps

The functional properties of KCNQ1 channels are highly dependent on associated KCNE β subunits. Mutations in KCNQ1 or KCNE subunits can cause congenital channelopathies, such as deafness, cardiac arrhythmias, and epilepsy. The mechanism by which KCNE1 beta subunits slow the kinetics of KCNQ1 channels is a matter of current controversy. Here we show that KCNQ1/KCNE1 channel activation occurs in two steps: first, mutually independent voltage sensor movements in the four KCNQ1 subunits generate the main gating charge movement and underlie the initial delay in the activation time course of KCNQ1/KCNE1 currents. Second, a slower and concerted conformational change of all four voltage sensors and the gate, which opens the KCNQ1/KCNE1 channel. Our data show that KCNE1 divides the voltage sensor movement into two steps with widely different voltage dependences and kinetics. The two voltage sensor steps in KCNQ1/KCNE1 channels can be pharmacologically isolated and further separated by a disease-causing mutation.

Polyunsaturated fatty acid analogs act antiarrhythmically on the cardiac IKs channel

Significance More than 300 mutations in the genes encoding the cardiac IKs channel have been identified in patients with cardiac arrhythmia. These mutations cause either loss of function or gain of function of the IKs channel. This study describes how polyunsaturated fatty acids and their analogues activate or inhibit the IKs channel. These modulators can restore rhythmic firing in arrhythmic firing cardiac myocytes and restore prolonged QT interval in guinea pig hearts. The study also describes a mechanism by which an auxiliary β-subunit alters the pharmacological sensitivity of the IKs channel. Our findings may form the basis for future design of antiarrhythmic compounds that target IKs channels for treating different cardiac arrhythmias caused by mutations in the IKs channel. Polyunsaturated fatty acids (PUFAs) affect cardiac excitability. Kv7.1 and the β-subunit KCNE1 form the cardiac IKs channel that is central for cardiac repolarization. In this study, we explore the prospects of PUFAs as IKs channel modulators. We report that PUFAs open Kv7.1 via an electrostatic mechanism. Both the polyunsaturated acyl tail and the negatively charged carboxyl head group are required for PUFAs to open Kv7.1. We further show that KCNE1 coexpression abolishes the PUFA effect on Kv7.1 by promoting PUFA protonation. PUFA analogs with a decreased pKa value, to preserve their negative charge at neutral pH, restore the sensitivity to open IKs channels. PUFA analogs with a positively charged head group inhibit IKs channels. These different PUFA analogs could be developed into drugs to treat cardiac arrhythmias. In support of this possibility, we show that PUFA analogs act antiarrhythmically in embryonic rat cardiomyocytes and in isolated perfused hearts from guinea pig.

Drug-induced ion channel opening tuned by the voltage sensor charge profile

Introduction of charged residues into the voltage sensor provides insight into the molecular mechanisms underlying potassium channel sensitivity to polyunsaturated fatty acids.

The KCNQ1 channel – remarkable flexibility in gating allows for functional versatility

The KCNQ1 channel (also called Kv7.1 or KvLQT1) belongs to the superfamily of voltage‐gated K+ (Kv) channels. KCNQ1 shares several general features with other Kv channels but also displays a fascinating flexibility in terms of the mechanism of channel gating, which allows KCNQ1 to play different physiological roles in different tissues. This flexibility allows KCNQ1 channels to function as voltage‐independent channels in epithelial tissues, whereas KCNQ1 function as voltage‐activated channels with very slow kinetics in cardiac tissues. This flexibility is in part provided by the association of KCNQ1 with different accessory KCNE β‐subunits and different modulators, but also seems like an integral part of KCNQ1 itself. The aim of this review is to describe the main mechanisms underlying KCNQ1 flexibility.

Actions and mechanisms of polyunsaturated fatty acids on voltage-gated ion channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (NaV), potassium (KV), calcium (CaV), and proton (HV) channels, as well as calcium-activated potassium (KCa), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Polyunsaturated fatty acids are potent openers of human M-channels expressed in Xenopus laevis oocytes

Polyunsaturated fatty acids have been reported to reduce neuronal excitability, in part by promoting inactivation of voltage‐gated sodium and calcium channels. Effects on neuronal potassium channels are less explored and experimental data ambiguous. The aim of this study was to investigate anti‐excitable effects of polyunsaturated fatty acids on the neuronal M‐channel, important for setting the resting membrane potential in hippocampal and dorsal root ganglion neurones.

Fatty acid analogue N-arachidonoyl taurine restores function of IKs channels with diverse long QT mutations

About 300 loss-of-function mutations in the IKs channel have been identified in patients with Long QT syndrome and cardiac arrhythmia. How specific mutations cause arrhythmia is largely unknown and there are no approved IKs channel activators for treatment of these arrhythmias. We find that several Long QT syndrome-associated IKs channel mutations shift channel voltage dependence and accelerate channel closing. Voltage-clamp fluorometry experiments and kinetic modeling suggest that similar mutation-induced alterations in IKs channel currents may be caused by different molecular mechanisms. Finally, we find that the fatty acid analogue N-arachidonoyl taurine restores channel gating of many different mutant channels, even though the mutations are in different domains of the IKs channel and affect the channel by different molecular mechanisms. N-arachidonoyl taurine is therefore an interesting prototype compound that may inspire development of future IKs channel activators to treat Long QT syndrome caused by diverse IKs channel mutations. DOI: http://dx.doi.org/10.7554/eLife.20272.001

Electronic polymers in lipid membranes

Electrical interfaces between biological cells and man-made electrical devices exist in many forms, but it remains a challenge to bridge the different mechanical and chemical environments of electronic conductors (metals, semiconductors) and biosystems. Here we demonstrate soft electrical interfaces, by integrating the metallic polymer PEDOT-S into lipid membranes. By preparing complexes between alkyl-ammonium salts and PEDOT-S we were able to integrate PEDOT-S into both liposomes and in lipid bilayers on solid surfaces. This is a step towards efficient electronic conduction within lipid membranes. We also demonstrate that the PEDOT-S@alkyl-ammonium:lipid hybrid structures created in this work affect ion channels in the membrane of Xenopus oocytes, which shows the possibility to access and control cell membrane structures with conductive polyelectrolytes.

Reciprocal voltage sensor-to-pore coupling leads to potassium channel C-type inactivation

Voltage-gated potassium channels open at depolarized membrane voltages. A prolonged depolarization causes a rearrangement of the selectivity filter which terminates the conduction of ions – a process called slow or C-type inactivation. How structural rearrangements in the voltage-sensor domain (VSD) cause alteration in the selectivity filter, and vice versa, are not fully understood. We show that pulling the pore domain of the Shaker potassium channel towards the VSD by a Cd2+ bridge accelerates C-type inactivation. Molecular dynamics simulations show that such pulling widens the selectivity filter and disrupts the K+ coordination, a hallmark for C-type inactivation. An engineered Cd2+ bridge within the VSD also affect C-type inactivation. Conversely, a pore domain mutation affects VSD gating-charge movement. Finally, C-type inactivation is caused by the concerted action of distant amino acid residues in the pore domain. All together, these data suggest a reciprocal communication between the pore domain and the VSD in the extracellular portion of the channel.

Specificity of M-channel activators: binding or effect?

The neuronal M-current is primarily generated by voltage-dependent heteromeric potassium channels formed by KCNQ2 and KCNQ3 subunits (Grunnet et al. 2014). These KCNQ2/KCNQ3 channels are expressed in the central nervous system where they protect neurons from potentially harmful hyperexcitability by stabilizing the neuronal resting membrane potential. Individuals with inherited mutations that reduce function of KCNQ2/KCNQ3 channels are at higher risk of suffering from epilepsies such as benign familial neonatal seizures. KCNQ2/KCNQ3 channel activators, which would boost the M-current, are therefore an attractive anti-epileptic therapy for epilepsy. Retigabine, which was approved for clinical use in 2011, is the first KCNQ2/KCNQ3 channel activator used to treat epilepsy. Unfortunately, because different KCNQ subunits are expressed in different tissues and retigabine shows poor specificity between channels formed by various KCNQ subunits (cardiac KCNQ1 being the exception), patients taking retigabine suffer from side-effects such as bladder dysfunction (Grunnet et al. 2014). To minimize side-effects and improve life quality for those treated with KCNQ2/KCNQ3 channel activators there is therefore a need to develop KCNQ channel activators with better subtype specificity. The study published in this issue of The Journal of Physiology by Wang and coworkers (Wang et al. 2017) brings us one important step closer to understanding how subtype specificity may be