Study of membrane electric property in N1E-115 neuroblastoma cells: Can voltage-gated sodium channels sense intramembrane dipole potential?

The intramembrane electric field has at least three potential components, namely, transmembrane potential, surface potential, and dipole potential. While the transmembrane potential and surface potential are known to be modulators of voltage-gated ion channels, it is not known if the dipole potential can be sensed by the voltage sensor residing in the channel proteins. Previous studies have revealed that the intramembrane electric field associated with the dipole potential can be 50 times greater than that associated with the transmembrane potential, and the gating charges of the sodium and potassium channels move across the whole cell membrane upon depolarization of the cell membrane. This suggests that the gating charges of the voltage-gated channels can sense the dipole electric fields of the cell membrane. We test this hypothesis by combining fluorescence ratio imaging of a potentiometric dye with the patch clamp technique in cultured N1E-115 neuroblastoma cells. It has been reported that the dipole electric field is unevenly distributed along the cell surface in the differentiated N1E-l15 neuroblastoma cells: the outer leaflet of the growth cone membrane has a higher dipole electric field than that of the soma membrane. Correspondingly, our cell-attached single channel recordings reveal that the growth cone sodium channels have a faster activation rate than the soma sodium channels. This is consistent with the idea that the voltage sensor of the sodium channel is biased by the higher dipole electric field in the growth cone membrane. The dipole electric field can be chemically modulated by addition of 6-ketocholestanol, a reagent known to increase the dipole potential of the cell membrane. Indeed, this causes a shift in dual wavelength ratio, a sensitive indicator of the intramembrane electric field. 6-ketocholestanol also shifts the voltage-dependent activation and inactivation curves to more negative transmembrane voltages. A correlation between the change of the fluorescence ratio and the shift of the steady-state activation/inactivation curves is found in the present study. These results suggest that the gating charges residing the channel proteins may sense the intramembrane dipole potential which, in turn, can affect the kinetics of the voltage-gated ion channels.