Identification of Direct and Indirect Effectors of the Transient Receptor Potential Melastatin 2 (TRPM2) Cation Channel

Transient receptor potential melastatin 2 (TRPM2) is a Ca2+ permeable cation channel involved in physiological and pathophysiological processes linked to oxidative stress. TRPM2 channels are co-activated by intracellular Ca2+ and ADP-ribose (ADPR) but also modulated in intact cells by several additional factors. Superfusion of TRPM2-expressing cells with H2O2 or intracellular dialysis of cyclic ADPR (cADPR) or nicotinic acid adenine dinucleotide phosphate (NAADP) activates, whereas dialysis ofAMPinhibits, TRPM2 whole-cell currents. Additionally, H2O2, cADPR, and NAADP enhance ADPR sensitivity of TRPM2 currents in intact cells. Because in whole-cell recordings the entire cellular machinery for nucleotide and Ca2+ homeostasis is intact, modulators might affect TRPM2 activity either directly, by binding to TRPM2, or indirectly, by altering the local concentrations of the primary ligands ADPR and Ca2+. To identify direct modulators of TRPM2, we have studied the effects of H2O2, AMP, cADPR, NAADP, and nicotinic acid adenine dinucleotide in inside-out patches from Xenopus oocytes expressing human TRPM2, by directly exposing the cytosolic faces of the patches to these compounds. H2O2 (1mM) and enzymatically purified cADPR (10 mu M) failed to activate, whereas AMP (200 mu M) failed to inhibit TRPM2 currents. NAADP was a partial agonist (maximal efficacy, similar to 50%), and nicotinic acid adenine dinucleotide was a full agonist, but both had very low affinities (K-0.5 = 104 and 35 mu M). H2O2, cADPR, and NAADP did not enhance activation by ADPR. Considering intracellular concentrations of these compounds, none of them are likely to directly affect the TRPM2 channel protein in a physiological context.