Emerging Concepts in Ion Channel Biophysics

Emerging Concepts in Ion Channel Biophysics

Poster Abstracts

68-POS Board 68 Regulation of Class Ib anti-Arrhythmic Drug Block by the Cardiac Na + Channel Voltage- Sensing Domains Wandi Zhu, Jonathan R. Silva . Washington University in St. Louis, St. Louis, MO, USA. Background: Clinical studies have shown that class I anti-arrhythmics are helpful in a subset of patients while being lethal in others. For example, patients with Long QT mutations respond differently to mexiletine, a class Ib anti-arrhythmic. We observed the conformational dynamics of different cardiac Na + channel (NaV1.5) domains to discover characteristics that facilitate or prevent effective mexiletine block. Methods: NaV1.5 contains four domains (DI-DIV), each with a voltage-sensing domain (VSD). We previously created four DNA constructs that contain a cysteine within a single VSD. Channels were expressed in Xenopus oocytes and cysteines were labeled with fluorophores. Ionic current and VSD-tracking fluorescence emission were simultaneously recorded. Results: Mexiletine binding to WT channels stabilizes the DIII-VSD in the activated conformation without affecting the other domains. LQT3 mutant channels show variable mexiletine sensitivity (R1626P>P1332L>WT=S941N>M1652R). These mutants also show varying DIII-VSD activation voltage-dependence, despite the distal locations of the mutations. The DIII-VSD activation shift strongly correlates with mexiletine sensitivity (QT-shortening). The highly-sensitive mutations stabilize the activated DIII-VSD, while the insensitive mutation destabilizes it. Thus, an activated DIII-VSD facilitates mexiletine blockade. To test this hypothesis, we assessed 13 additional mutations, and quantified gating parameters such as conductance voltage-dependence, inactivation, DIII-VSD activation and drug block. A partial least square regression (PLSR) model showed that DIII-VSD activation and inactivation represent components that regulate drug block. Conclusion: Traditionally, channel activation and inactivation were linked to Class-Ib drug block. We propose a novel mechanism where DIII-VSD conformation facilitates or impairs block. By assessing how mutations affect the DIII-VSD, we expect to predict whether a patient will respond to mexiletine. This mechanism could also be utilized to develop a new type of combination therapy by stabilizing the DIII-VSD activated state to increase the efficacy of class- Ib drugs.

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