Modulation of Ca2+ entry and plasma membrane potential by human TRPM4b

TRPM4b is a Ca2+-activated, voltage-dependent monovalent cation channel that has been shown to act as a negative regulator of Ca2+ entry and to be involved in the generation of oscillations of Ca2+ influx in Jurkat T-lymphocytes. Transient overexpression of TRPM4b as an enhanced green fluorescence fusion protein in human embryonic kidney (HEK) cells resulted in its localization in the plasma membrane, as demonstrated by confocal fluorescence microscopy. The functionality and plasma membrane localization of overexpressed TRPM4b was confirmed by induction of Ca2+-dependent inward and outward currents in whole cell patch clamp recordings. HEK-293 cells stably overexpressing TRPM4b showed higher ionomycin-activated Ca2+ influx than wild-type cells. In addition, analysis of the membrane potential using the potentiometric dye bis-(1,3-dibutylbarbituric acid)-trimethine oxonol and by current clamp experiments in the perforated patch configuration revealed a faster initial depolarization after activation of Ca2+ entry with ionomycin. Furthermore, TRPM4b expression facilitated repolarization and thereby enhanced sustained Ca2+ influx. In conclusion, in cells with a small negative membrane potential, such as HEK-293 cells, TRPM4b acts as a positive regulator of Ca2+ entry.