@article{f88f00b0106f4a2e8f0393954bbc98eb,
title = "Mutational analysis of block and facilitation of HERG current by A Class III anti-arrhythmic agent, nifekalant",
abstract = "Chemicals and toxins are useful tools to elucidate the structure-function relationship of various proteins including ion channels. The HERG channel is blocked by many compounds and this may cause life-threatening cardiac arrhythmia. Besides block, some chemicals such as the class III anti-arrhythmic agent nifekalant stimulate HERG at low potentials by shifting its activation curve towards hyperpolarizing voltages. This is called facilitation. Here, we report mutations and simulations analyzing the association between nifekalant and channel pore residues for block and facilitation. Alanine-scanning mutagenesis was performed in the pore region of HERG. The mutations at the base of the pore helix (T623A), the selectivity filter (V625A) and the S6 helix (G648A, Y652A and F656A) abolished and S624A attenuated both block and facilitation induced by the drug. On the other hand, the mutation of other residues caused either an increase or a decrease in nifekalant-induced facilitation without affecting block. An open-state homology model of the HERG pore suggested that T623, S624, Y652 and F656 faced the central cavity, and were positioned within geometrical range for the drug to be able to interact with all of them at the same time. Of these, S649 was the only polar residue located within possible interaction distance from the drug held in its blocking position. Further mutations and flexible-docking simulations suggest that the size, but not the polarity, of the side chain at S649 is critical for drug induced facilitation.",
keywords = "Anti-arrhythmic agent, Docking simulation, Facilitation, HERG, Nifekalant, Potassium channel",
author = "Yukio Hosaka and Miki Iwata and Narutoshi Kamiya and Mitsuhiko Yamada and Kengo Kinoshita and Yoshifumi Fukunishi and Kenji Tsujimae and Hiroshi Hibino and Yoshifusa Aizawa and Atsushi Inanobe and Haruki Nakamura and Yoshihisa Kurachi",
note = "Funding Information: Mechanistic and clinical significance. We confirm that nifekalant is associated with the facilitation of HERG current, and that pure IKr blockers E-4031 and dofetilide exhibit no such effect. It was reported that nifekalant effectively suppresses ventricular tachyarrhythmias not only in canine models,39-42 but also in patients with acute myocardial infarction or severe ventricular dysfunction.43The effectiveness of nifekalant against lethal ventricular arrhythmias might be related to its effect of facilitating HERG channels. In this context it is interesting to note that while the reduction of HERG channel current by E-4031 increased the magnitude of rate-dependent action potential duration (APD) alternans in a computer model of canine endocardial myocytes,44 increasing HERG channel current by shifting the activation curve to more negative voltages abolished APD alternans.39 Nifekalant not only blocks HERG current but also induces facilitation via a negative shift of the activation curve. This drug is therefore likely to have a lower risk for induction of alternans and may thus be more appropriate for the treatment of arrhythmias than pure IKr blockers. Further studies on the detailed mechanism of facilitation should be useful for the logical design of clinically effective IKr blockers with reduced proarrhythmic effects. Acknowledgements We are grateful to Dr. Ian Findlay (CNRS UMR 6542 Facult{\'e} des Sciences, Universit{\'e} de Tours, France) for critical reading of this manuscript. We also thank Ms. Miki Fukumoto, Ms. Chizuru Tsuzuki and Ms. Yukiko Nishida for technical assistance, and Ms. Rika Kitai, Ms. Yuko Nakaie, Ms. Mami Yokokawa, Ms. Junko Inoue and Ms. Satoko Kuwayama for secretarial assistance. This work was supported by the Leading Project for Biosimulation “Development of models for disease and drug action” from Ministry of Education, Science and Sports in Japan to Yoshihisa Kurachi.",
year = "2007",
doi = "10.4161/chan.4691",
language = "English",
volume = "1",
pages = "198--208",
journal = "Channels",
issn = "1933-6950",
publisher = "Landes Bioscience",
number = "3",
}