Potentiation of TRPM7 inward currents by protons

TRPM7 is unique in being both an ion channel and a protein kinase. It conducts a large outward current at +100 mV but a small inward current at voltages ranging from -100 to -40 mV under physiological ionic conditions. Here we show that the small inward current of TRPM7 was dramatically enhanced by a decrease in extracellular pH, with an approximately 10-fold increase at pH 4.0 and 1-2-fold increase at pH 6.0. Several lines of evidence suggest that protons enhance TRPM7 inward currents by competing with Ca(2+) and Mg(2+) for binding sites, thereby releasing blockade of divalent cations on inward monovalent currents. First, extracellular protons significantly increased monovalent cation permeability. Second, higher proton concentrations were required to induce 50% of maximal increase in TRPM7 currents when the external Ca(2+) and Mg(2+) concentrations were increased. Third, the apparent affinity for Ca(2+) and Mg(2+) was significantly diminished at elevated external H(+) concentrations. Fourth, the anomalous-mole fraction behavior of H(+) permeation further suggests that protons compete with divalent cations for binding sites in the TRPM7 pore. Taken together, it appears that at physiological pH (7.4), Ca(2+) and Mg(2+) bind to TRPM7 and inhibit the monovalent cationic currents; whereas at high H(+) concentrations, the affinity of TRPM7 for Ca(2+) and Mg(2+) is decreased, thereby allowing monovalent cations to pass through TRPM7. Furthermore, we showed that the endogenous TRPM7-like current, which is known as Mg(2+)-inhibitable cation current (MIC) or Mg nucleotide-regulated metal ion current (MagNuM) in rat basophilic leukemia (RBL) cells was also significantly potentiated by acidic pH, suggesting that MIC/MagNuM is encoded by TRPM7. The pH sensitivity represents a novel feature of TRPM7 and implies that TRPM7 may play a role under acidic pathological conditions.