GIRK4 Mutations R52H and E246K Impair Channel Gating but not Inward Rectification

G-protein inwardly rectifying potassium channels (GIRK) mediates inhibitory transmission via G-protein coupled receptors in heart, brain and adrenal cortex. Of four GIRK subunits (GIRK1-4), GIRK4 (KCNJ5) subunits are abundant in the heart and in adrenal cortex. Recent studies suggest that KCNJ5 mutations are involved in overproduction of aldosterone in primary aldosteronism (PA) and aldosterone-producing adenomas (APA). As of this point the consensus is that most or all GIRK4 mutations cause loss of GIRK channel selectivity, leading to increased permeability to Na+, depolarization of cell membrane, opening of voltage dependent Ca2+ channels and eventually Ca2+-dependent aldosterone secretion. This mechanism has been extended to mutations in cytosolic N and C terminal domains outside the pore region, GIRK4R52H and GIRK4E246K1. Using confocal microscopy, we found that YFP-tagged GIRK1 coexpressed with GIRK4R52H or GIRK4E246K in Xenopus laevis oocytes yield similar expression levels as YFP-GIRK1/GIRK4WT channels. Nevertheless, GIRK1/GIRK4R52H channels had significantly smaller whole-cell currents than GIRK1/GIRK4WT, suggesting impaired gating. Coexpression of Gβγ greatly increased the currents of both mutants, suggesting that Gβγ may “correct” a gating defect. Additionally, analysis of current-voltage curves revealed that both mutations did not affect the reversal potential or inward rectification of GIRK1/4. Our results suggest that previous interpretations of the effects of mutations GIRK4R52H and GIRK4E246K might have been incorrect, and that these mutations impair channel's gating but not selectivity or rectification. Overall, our results urge to reconsider the existing model of the mechanism responsible for aldosterone overproduction in patients with the out-of-pore mutations of KCNJ5, and prompt for looking for alternative explanations.1. Murthy M et al. Hypertension 63, 783-789 (2014).