The calcium-permeable non-selective cation channel TRPM2 is modulated by cellular acidification

TRPM2 is a calcium-permeable non-selective cation channel expressed in the plasma membrane and in lysosomes that is critically involved in aggravating reactive oxygen species (ROS)-induced inflammatory processes and has been implicated in cell death. TRPM2 is gated by ADP-ribose (ADPR) and modulated by physiological processes that produce peroxide, cyclic ADP-ribose (cADPR), nicotinamide adenine dinucleotide phosphate (NAADP) and Ca2+. We investigated the role of extra- and intracellular acidification on heterologously expressed TRPM2 in HEK293 cells. Our results show that TRPM2 is inhibited by external acidification with an IC50 of pH 6.5 and is completely suppressed by internal pH of 6. Current inhibition requires channel opening and is strongly voltage dependent, being most effective at negative potentials. In addition, increased cytosolic pH buffering capacity or elevated [Ca2+](i) reduces the rate of current inactivation elicited by extracellular acidification, and Na+ and Ca2+ influence the efficacy of proton-induced inactivation. Together, these results suggest that external protons permeate TRPM2 channels to gain access to an intracellular site that regulates channel activity. Consistent with this notion, single-channel measurements in HEK293 cells reveal that internal protons induce channel closure without affecting single-channel conductance, whereas external protons affect channel open probability as well as single-channel conductance of native TRPM2 in neutrophils. We conclude that protons compete with Na+ and Ca2+ for channel permeation and channel closure results from a competitive antagonism of protons at an intracellular Ca2+ binding site.