In KCNQ1 Channels, a Long QT Mutation Induces a Regulation by Cholesterol Instead of Phosphatidylinositol-4,5-Bisphosphate

Phosphatidylinositol-4,5-bisphosphate (PIP2) is a membrane cofactor necessary for the activity of KCNQ1 channels. Some Long QT mutations of KCNQ1, including R243H, R539W and R555C have been shown to decrease KCNQ1 interaction with PIP2. A previous study suggested that when compared to WT channel, R539W is paradoxically less sensitive to PIP2 screening by intracellular magnesium, despite a decreased interaction with PIP2. In the present study, we confirm this peculiar behavior of R539W and suggest a molecular mechanism underlying it. COS-7 cells were transfected with WT or mutated KCNQ1 channel, and patch-clamp recordings in giant-patch or whole-cell configuration were performed. Similar to other channels with a decreased PIP2 affinity, we observed that the R243H and R555C mutations lead to an accelerated current rundown when available PIP2 level is decreased. As opposed to R243H and R555C mutant channels, R539W is not more but rather less sensitive to PIP2 decrease than the WT channel. Consistent with that, the R539W channel is also insensitive to variations in extracellular osmolarity, known to modulate the channel activity via available PIP2. These results suggest that KCNQ1-R539W mutation shortcuts PIP2 in the channel open pore stabilization. Both structural model prediction and functional analysis implicate membrane cholesterol in this effect. This de novo interaction wanes the sensitivity to PIP2 variations, showing that a mutated channel with a decreased affinity to PIP2 could paradoxically present a slowed current rundown compared to the WT channel. This suggests that caution is required when using measurements of current rundown as an indicator to compare WT and mutant channel PIP2 sensitivity.