Ivermectin (IVM) is an antiparasitic drug that is used worldwide and rescues hundreds of millions of people from onchocerciasis and lymphatic filariasis. It was discovered by Satoshi Ōmura and William C. Campbell, to whom the 2015 Nobel Prize in Physiology or Medicine was awarded. It kills parasites by activating glutamate-gated Cl channels, and it also targets several ligand-gated ion channels and receptors, including Cys-loop receptors, P2X receptors and fernesoid X receptors. Recently, we found that IVM also activates a novel target, the G-protein-gated inwardly rectifying K channel, and also identified the structural determinant for the activation. In this review, we aim to provide an update and summary of recent progress in the identification of IVM targets, as well as their modulation mechanisms, through molecular structures, chimeras and site-directed mutagenesis, and molecular docking and modelling studies.

Ciliary opsins were classically thought to function only in vertebrates for vision, but they have also been identified recently in invertebrates for non-visual photoreception. Larvae of the annelid are used as a zooplankton model, and this zooplankton species possesses a "vertebrate-type" ciliary opsin (named c-opsin) in the brain. c-opsin is suggested to relay light signals for melatonin production and circadian behaviors. Thus, the spectral and biochemical characteristics of this c-opsin would be directly related to non-visual photoreception in this zooplankton model. Here we demonstrate that the c-opsin can sense UV to activate intracellular signaling cascades and that it can directly bind exogenous all--retinal. These results suggest that this c-opsin regulates circadian signaling in a UV-dependent manner and that it does not require a supply of 11--retinal for photoreception. Avoidance of damaging UV irradiation is a major cause of large-scale daily zooplankton movement, and the observed capability of the c-opsin to transmit UV signals and bind all-retinal is ideally suited for sensing UV radiation in the brain, which presumably lacks enzymes producing 11--retinal. Mutagenesis analyses indicated that a unique amino acid residue (Lys-94) is responsible for c-opsin-mediated UV sensing in the brain. We therefore propose that acquisition of the lysine residue in the c-opsin would be a critical event in the evolution of to enable detection of ambient UV light. In summary, our findings indicate that the c-opsin possesses spectral and biochemical properties suitable for UV sensing by the zooplankton model.

Ivermectin (IVM) is a widely used antiparasitic drug in humans and pets which activates glutamate-gated Cl channel in parasites. It is also known that IVM binds to the transmembrane domains (TMs) of several ligand-gated channels, such as Cys-loop receptors and P2X receptors. In this study, we found that the G-protein-gated inwardly rectifying K (GIRK) channel is activated by IVM directly. Electrophysiological recordings in Xenopus oocytes revealed that IVM activates GIRK channel in a phosphatidylinositol-4,5-biphosphate (PIP )-dependent manner, and that the IVM-mediated GIRK activation is independent of G subunits. We found that IVM activates GIRK2 more efficiently than GIRK4. In cultured hippocampal neurons, we also observed that IVM activates native GIRK current. Chimeric and mutagenesis analyses identified an amino acid residue unique to GIRK2 among the GIRK family, Ile82, located in the slide helix between the TM1 and the N-terminal cytoplasmic tail domain (CTD), which is critical for the activation. The results demonstrate that the TM-CTD interface in GIRK channels, rather than the TMs, governs IVM-mediated activation. These findings provide us with novel insights on the mode of action of IVM in ion channels that could lead to identification of new pharmacophores which activate the GIRK channel.

Drug-induced block of the cardiac rapid delayed rectifying potassium current (IKr), carried by the human ether-a-go-go-related gene (hERG) channel, is the most common cause of acquired long QT syndrome. Indeed, some, but not all, drugs that block hERG channels cause fatal cardiac arrhythmias. However, there is no clear method to distinguish between drugs that cause deadly arrhythmias and those that are clinically safe. Here we propose a mechanism that could explain why certain clinically used hERG blockers are less proarrhythmic than others. We demonstrate that several drugs that block hERG channels, but have favorable cardiac safety profiles, also evoke another effect; they facilitate the hERG current amplitude in response to low-voltage depolarization. To investigate how hERG facilitation impacts cardiac safety, we develop computational models of IKr block with and without this facilitation. We constrain the models using data from voltage clamp recordings of hERG block and facilitation by nifekalant, a safe class III antiarrhythmic agent. Human ventricular action potential simulations demonstrate the ability of nifekalant to suppress ectopic excitations, with or without facilitation. Without facilitation, excessive IKr block evokes early afterdepolarizations, which cause lethal arrhythmias. When facilitation is introduced, early afterdepolarizations are prevented at the same degree of block. Facilitation appears to prevent early afterdepolarizations by increasing IKr during the repolarization phase of action potentials. We empirically test this prediction in isolated rabbit ventricular myocytes and find that action potential prolongation with nifekalant is less likely to induce early afterdepolarization than action potential prolongation with dofetilide, a hERG channel blocker that does not induce facilitation. Our data suggest that hERG channel blockers that induce facilitation increase the repolarization reserve of cardiac myocytes, rendering them less likely to trigger lethal ventricular arrhythmias.

No abstract

Iodide transport and storage in the thyroid follicles is crucial for thyroid hormone synthesis. Pendrin, the iodide exporter that transports iodide to thyroid follicles, is responsible for Pendred syndrome, a disorder characterized by congenital hypothyroidism and hearing loss. However, thyroid hormone levels are basically normal in patients with Pendred syndrome, indicating the presence of another unknown iodide transporter. Here, we show that SLC26A7 is a novel iodide transporter in the thyroid. We observe that SLC26A7 is specifically expressed in normal thyroid tissues and demonstrate its function in iodide transport. Using whole-exome sequencing, we also find a homozygous nonsense mutation in SLC26A7 (c.1498 C > T; p.Gln500Ter) in two siblings with congenital goitrous hypothyroidism. The mutated SLC26A7 protein shows an abnormal cytoplasmic localisation and lacks the iodide transport function. These results reveal that SLC26A7 functions as a novel iodide transporter in the thyroid and its dysfunction affects thyroid hormonogenesis in humans and causes congenital goitrous hypothyroidism.

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 (Kir3.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 Kir3.1 subunits heterologously expressed in oocytes and native GIRK channels in atrial myocytes were inhibited by terfenadine and other non‐sedating antihistamines. In Kir3.1 subunits, mutation of Phe137, located in the centre of the pore helix, to the corresponding Ser in Kir3.2 subunits reduced the inhibition by terfenadine. Introduction of an amino acid with a large side chain in Kir3.2 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 Kir3.1 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.

Partial agonists are used clinically to avoid overstimulation of receptor-mediated signalling, as they produce a submaximal response even at 100% receptor occupancy. The submaximal efficacy of partial agonists is due to conformational change of the agonist–receptor complex, which reduces effector activation. In addition to signalling activators, several regulators help control intracellular signal transductions. However, it remains unclear whether these signalling regulators contribute to partial agonism. Here we show that regulator of G-protein signalling (RGS) 4 is a determinant for partial agonism of the M2 muscarinic receptor (M2R). In rat atrial myocytes, pilocarpine evoked smaller G-protein-gated K+ inwardly rectifying (KG) currents than those evoked by ACh. In a Xenopus oocyte expression system, pilocarpine acted as a partial agonist in the presence of RGS4 as it did in atrial myocytes, while it acted like a full agonist in the absence of RGS4. Functional couplings within the agonist–receptor complex/G-protein/RGS4 system controlled the efficacy of pilocarpine relative to ACh. The pilocarpine–M2R complex suppressed G-protein-mediated activation of KG currents via RGS4. Our results demonstrate that partial agonism of M2R is regulated by the RGS4-mediated inhibition of G-protein signalling. This finding helps us to understand the molecular components and mechanism underlying the partial agonism of M2R-mediated physiological responses.