PA-6 effectively inhibits gain-of-function KIR2.1 channels and induces their intracellular accumulation, highlighting its potential as a preclinical candidate for congenital short QT syndrome type 3 and atrial fibrillation.
The inward rectifier potassium current I K1 contributes to a stable resting membrane potential and phase 3 repolarization of the cardiac action potential. KCNJ2 gain-of-function mutations V93I and D172N associate with increased I K1 , short QT syndrome type 3 and congenital atrial fibrillation. Pentamidine-Analogue 6 (PA-6) is an efficient (IC 50 = 14 nM with inside-out patch clamp methodology) and specific I K1 inhibitor that interacts with the cytoplasmic pore region of the K IR 2.1 ion channel, encoded by KCNJ2 . At 10 μM, PA-6 increases wild-type (WT) K IR 2.1 expression in HEK293T cells upon chronic treatment. We hypothesized that PA-6 will interact with and inhibit V93I and D172N K IR 2.1 channels, whereas impact on channel expression at the plasma membrane requires higher concentrations. Molecular modelling was performed with the human K IR 2.1 closed state homology model using FlexX. WT and mutant K IR 2.1 channels were expressed in HEK293 cells. Patch-clamp single cell electrophysiology measurements were performed in the whole cell and inside-out mode of the patch clamp method. K IR 2.1 expression level and localization were determined by western blot analysis and immunofluorescence microscopy, respectively. PA-6 docking in the V93I/D172N double mutant homology model of K IR 2.1 demonstrated that mutations and drug-binding site are >30 Å apart. PA-6 inhibited WT and V93I outward currents with similar potency (IC 50 = 35.5 and 43.6 nM at +50 mV for WT and V93I), whereas D172N currents were less sensitive (IC 50 = 128.9 nM at +50 mV) using inside-out patch-clamp electrophysiology. In whole cell mode, 1 μM of PA-6 inhibited outward I K1 at −50 mV by 28 ± 36%, 18 ± 20% and 10 ± 6%, for WT, V93I and D172N channels respectively. Western blot analysis demonstrated that PA-6 (5 μM, 24 h) increased K IR 2.1 expression levels of WT (6.3 ± 1.5 fold), and V93I (3.9 ± 0.9) and D172N (4.8 ± 2.0) mutants. Immunofluorescent microscopy demonstrated dose-dependent intracellular K IR 2.1 accumulation following chronic PA-6 application (24 h, 1 and 5 μM). 1) KCNJ2 gain-of-function mutations V93I and D172N in the K IR 2.1 ion channel do not impair PA-6 mediated inhibition of I K1 , 2) PA-6 elevates K IR 2.1 protein expression and induces intracellular K IR 2.1 accumulation, 3) PA-6 is a strong candidate for further preclinical evaluation in treatment of congenital SQT3 and AF.
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Ji et al. (2017) studied this question.
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