Non-sedating antihistamines block G-protein-gated inwardly rectifying K+ channels

Ethosuximide: Subunit- and Gβγ-dependent blocker and reporter of allosteric changes in GIRK channels

BACKGROUND AND PURPOSE

The antiepileptic drug ethosuximide (ETX) suppresses epileptiform activity in a mouse model of GNB1 syndrome, caused by mutations in Gβ1 protein, likely through the inhibition of G-protein gated K+ (GIRK) channels. Here, we investigated the mechanism of ETX inhibition (block) of different GIRKs.

EXPERIMENTAL APPROACH

We studied ETX inhibition of GIRK channels expressed in Xenopus oocytes with or without their physiological activator, the G protein subunit dimer Gβγ. ETX binding site and mode of action were analysed using molecular dynamic (MD) simulations and kinetic modelling, and the predictions were tested by mutagenesis and functional testing.

KEY RESULTS

We show that ETX is a subunit-selective, allosteric blocker of GIRKs. The potency of ETX block is increased by Gβγ, in parallel with channel activation. MD simulations and mutagenesis locate the ETX binding site in GIRK2 to a region associated with phosphatidylinositol-4,5-bisphosphate (PIP2) regulation, and suggest that ETX acts by closing the helix bundle crossing (HBC) gate and altering channel's interaction with PIP2. The apparent affinity of ETX block is highly sensitive to changes in channel gating caused by mutations in Gβ1 or GIRK subunits.

CONCLUSION AND IMPLICATIONS

ETX block of GIRKs is allosteric, subunit-specific, and enhanced by Gβγ through an intricate network of allosteric interactions within the channel molecule. Our findings pose GIRK as a potential therapeutic target for ETX and ETX as a potent allosteric GIRK blocker and a tool for probing gating-related conformational changes in GIRK.

Structural determinants of the direct inhibition of GIRK channels by Sigma-1 receptor antagonist

G-protein-gated inward rectifier K+ (GIRK) channels play a critical role in the regulation of the excitability of cardiomyocytes and neurons and include GIRK1, GIRK2, GIRK3 and GIRK4 subfamily members. BD1047 dihydrobromide (BD1047) is one of the representative antagonists of the multifunctional Sigma-1 receptor (S1R). In the analysis of the effect of BD1047 on the regulation of Gi-coupled receptors by S1R using GIRK channel as an effector, we observed that BD1047, as well as BD1063, directly inhibited GIRK currents even in the absence of S1R and in a voltage-independent manner. Thus, we aimed to clarify the effect of BD1047 on GIRK channels and identify the structural determinants. By electrophysiological recordings in Xenopus oocytes, we observed that BD1047 directly inhibited GIRK channel currents, producing a much stronger inhibition of GIRK4 compared to GIRK2. It also inhibited ACh-induced native GIRK current in isolated rat atrial myocytes. Chimeric and mutagenesis studies of GIRK2 and GIRK4 combined with molecular docking analysis demonstrated the importance of Leu77 and Leu84 within the cytoplasmic, proximal N-terminal region and Glu147 within the pore-forming region of GIRK4 for inhibition by BD1047. The activator of GIRK channels, ivermectin, competed with BD1047 at Leu77 on GIRK4. This study provides us with a novel inhibitor of GIRK channels and information for developing pharmacological treatments for GIRK4-associated diseases.

Licorice metabolite 18β-glycyrrhetinic acid activates G protein-gated inwardly rectifying K+ channels

BACKGROUND AND PURPOSE

Licorice (liquorice) is a common food additive and is used in Chinese medicine. Excess licorice intake can induce atrial fibrillation. Patients with atrial fibrillation possess constitutively activated G protein-gated inwardly rectifying K+ (GIRK) channels. Whether licorice affects GIRK channel activity is unknown. We aimed to clarify the effects of licorice ingredients on GIRK current and the mechanism of action.

EXPERIMENTAL APPROACH

A major component of licorice, glycyrrhizic acid (GA), and its metabolite, 18β-glycyrrhetinic acid (18β-GA), were tested. We performed electrophysiological recordings in Xenopus oocytes to examine the effects of GA and 18β-GA on various GIRK subunits (Kir 3.1-Kir 3.4), mutagenesis analyses to identify the crucial residues for drug action and motion analysis in cultured rat atrial myocytes to clarify effects of 18β-GA on atrial functions.

KEY RESULTS

GA inhibited Kir 3.1-containing channels, while 18β-GA activated all Kir 3.x subunits. A pore helix residue Phe137 in Kir 3.1 was critical for GA-mediated inhibition, and the corresponding Ser148 in Kir 3.2 was critical for 18β-GA-mediated activation. 18β-GA activated GIRK channel in a Gβγ -independent manner, whereas phosphatidylinositol 4,5-bisphosphate (PIP2 ) was essential for activation. Glu236 located at the cytoplasmic pore of Kir 3.2 appeared to be important to interactions with 18β-GA. In rat atrial myocytes, 18β-GA suppressed spontaneous beating via activation of GIRK channels.

CONCLUSION AND IMPLICATIONS

GA acts as a novel GIRK inhibitor, and 18β-GA acts as a novel GIRK activator. 18β-GA alters atrial function via activation of GIRK channels. This study elucidates the pharmacological activity of licorice ingredients and provides information for drug design.

A novel ion conducting route besides the central pore in an inherited mutant of G-protein-gated inwardly rectifying K+ channel

G-protein-gated inwardly rectifying K⁺ (GIRK; Kir3.x) channels play important physiological roles in various organs. Some of the disease-associated mutations of GIRK channels are known to induce loss of K⁺ selectivity but their structural changes remain unclear. In this study, we investigated the mechanisms underlying the abnormal ion selectivity of inherited GIRK mutants. By the two-electrode voltage-clamp analysis of GIRK mutants heterologously expressed in Xenopus oocytes, we observed that Kir3.2 G156S permeates Li⁺ better than Rb⁺ , while T154del or L173R of Kir3.2 and T158A of Kir3.4 permeate Rb⁺ better than Li⁺ , suggesting a unique conformational change in the G156S mutant. Applications of blockers of the selectivity filter (SF) pathway, Ba²⁺ or Tertiapin-Q (TPN-Q), remarkably increased the Li⁺ -selectivity of Kir3.2 G156S but did not alter those of the other mutants. In single-channel recordings of Kir3.2 G156S expressed in mouse fibroblasts, two types of events were observed, one attributable to a TPN-Q-sensitive K⁺ current and the second a TPN-Q-resistant Li⁺ current. The results show that a novel Li⁺ -permeable and blocker-resistant pathway exists in G156S in addition to the SF pathway. Mutations in the pore helix, S148F and T151A also induced high Li⁺ permeation. Our results demonstrate that the mechanism underlying the loss of K⁺ selectivity of Kir3.2 G156S involves formation of a novel ion permeation pathway besides the SF pathway, which allows permeation of various species of cations. KEY POINTS: G-protein-gated inwardly rectifying K⁺ (GIRK; Kir3.x) channels play important roles in controlling excitation of cells in various organs, such as the brain and the heart. Some of the disease-associated mutations of GIRK channels are known to induce loss of K⁺ selectivity but their structural changes remain unclear. In this study, we investigated the mechanisms underlying the abnormal ion selectivity of inherited mutants of Kir3.2 and Kir3.4. Here we show that a novel Na⁺ , Li⁺ -permeable and blocker-resistant pathway exists in an inherited mutant, Kir3.2 G156S, in addition to the conventional ion conducting pathway formed by the selectivity filter (SF). Our results demonstrate that the mechanism underlying the loss of K⁺ selectivity of Kir3.2 G156S involves formation of a novel ion permeation pathway besides the SF pathway, which allows permeation of various species of cations.

Inhibitory effect of terfenadine on Kir2.1 and Kir2.3 channels

Terfenadine is a second-generation H1-antihistamine that despite potentially can produce severe side effects it has recently gained attention due to its anticancer properties. Lately, the subfamily 2 of inward rectifier potassium channels (Kir2) has been implicated in the progression of some tumoral processes. Hence, we characterized the effects of terfenadine on Kir2.x channels expressed in HEK-293 cells. Terfenadine inhibited Kir2.3 channels with a strikingly greater potency (IC50 = 1.06 ± 0.11 μmol L-1) compared to Kir2.1 channels (IC50 = 27.8 ± 4.8 μmol L-1). The Kir2.3(I213L) mutant, possessing a larger affinity for phosphatidylinositol 4,5-bisphosphate (PIP2) than the wild-type Kir2.3, was less sensitive to terfenadine inhibition (IC50 = 13.0 ± 2.9 μmol L-1). Additionally, the PIP2 intracellular application had largely reduced the inhibition of Kir2.1 channels by terfenadine. Our data support that Kir2.x channels are targets of terfena-dine by affecting their interaction with PIP2, which could be regarded as a mechanism of the antitumor properties of terfenadine.

Advances in Targeting GIRK Channels in Disease

G protein-gated inwardly rectifying potassium (GIRK) channels are essential regulators of cell excitability in the brain. While they are implicated in a variety of neurological diseases in both human and animal model studies, their therapeutic potential has been largely untapped. Here, we review recent advances in the development of small molecule compounds that specifically modulate GIRK channels and compare them with first-generation compounds that exhibit off-target activity. We describe the method of discovery of these small molecule modulators, their chemical features, and their effects in vivo. These studies provide a promising outlook on the future development of subunit-specific GIRK modulators to regulate neuronal excitability in a brain region-specific manner.

The anesthetic bupivacaine induces cardiotoxicity by targeting L-type voltage-dependent calcium channels

Objective

Bupivacaine is an amide local anesthetic with possible side effects that include an irregular heart rate. However, the mechanism of bupivacaine-induced cardiotoxicity has not been fully elucidated, thus we aimed to examine this mechanism.

Methods

We performed electrocardiogram recordings to detect action potential waveforms in Sprague Dawley rats after application of bupivacaine, while calcium (Ca²⁺) currents in neonatal rat ventricular cells were examined by patch clamp recording. Western blot and quantitative real-time polymerase chain reaction assays were used to detect the expression levels of targets of interest.

Results

In the present study, after application of bupivacaine, abnormal action potential waveforms were detected in Sprague Dawley rats by electrocardiogram recordings, while decreased Ca²⁺ currents were confirmed in neonatal rat ventricular cells by patch clamp recording. These alterations may be attributed to a deficiency of Ca_V1.3 (L-type) Ca²⁺ channels, which may be regulated by the multifunctional protein calreticulin.

Conclusions

The present study identifies a possible role of the calreticulin–Ca_V1.3 axis in bupivacaine-induced abnormal action potentials and Ca²⁺ currents, which may lead to a better understanding anesthetic drug-induced cardiotoxicity.

Biophysical research in Okazaki, Japan

This commentary summarizes the recent biophysical research conducted at the National Institute for Basic Biology, the National Institute for Physiological Sciences, and the Institute for Molecular Science in Okazaki, Japan.

Non-sedating antihistamines block G-protein-gated inwardly rectifying K+ channels

BACKGROUND AND PURPOSE

A second-generation antihistamine, terfenadine, is known to induce arrhythmia by blocking hERG channels. In this study, we have shown that terfenadine also inhibits the activity of G-protein-gated inwardly rectifying K⁺ (GIRK) channels, which regulate the excitability of neurons and cardiomyocytes. To clarify the underlying mechanism(s), we examined the effects of several antihistamines on GIRK channels and identified the structural determinant for the inhibition.

EXPERIMENTAL APPROACH

Electrophysiological recordings were made in Xenopus oocytes and rat atrial myocytes to analyse the effects of antihistamines on various GIRK subunits (K_ir 3.x). Mutagenesis analyses identified the residues critical for inhibition by terfenadine and the regulation of ion selectivity. The potential docking site of terfenadine was analysed by molecular docking.

KEY RESULTS

GIRK channels containing K_ir 3.1 subunits heterologously expressed in oocytes and native GIRK channels in atrial myocytes were inhibited by terfenadine and other non-sedating antihistamines. In K_ir 3.1 subunits, mutation of Phe137, located in the centre of the pore helix, to the corresponding Ser in K_ir 3.2 subunits reduced the inhibition by terfenadine. Introduction of an amino acid with a large side chain in K_ir 3.2 subunits at Ser148 increased the inhibition. When this residue was mutated to a non-polar amino acid, the channel became permeable to Na⁺ . Phosphoinositide-mediated activity was also decreased by terfenadine.

CONCLUSION AND IMPLICATIONS

The Phe137 residue in K_ir 3.1 subunits is critical for inhibition by terfenadine. This study provides novel insights into the regulation of GIRK channels by the pore helix and information for drug design.