Investigation of the ADP-Ribose-Dependent Gating Mechanism of the TRPM2 Channel

Abstract

TRPM2 is a non-selective Ca2+ permeable cation channel, expressed abundantly in brain, immune cells, and pancreatic β-cells. Under physiological conditions it might play a role in glucose-induced insulin secretion, and in the immune response. Besides, it enhances the sensitivity of cells toward oxidative stress induced damage. Thus, under pathological conditions, TRPM2 is responsible for neuronal cell death during ischemia/reperfusion in the brain. The TRPM2 channel is co-activated by intracellular Ca2+ and ADP-ribose (ADPR). ADPR binds to the C-terminal NUDT9-H domain, while the Ca2+ binding sites have not yet been identified. Intriguingly, when expressed in isolation, the NUDT9-H domain functions as an active ADPR hydrolase which breaks ADPR into AMP and ribose-5-phosphate. In a previous study we showed that Ca2+ dependent gating is well described by the Monod-Wyman-Changeux model. However, little is known about the mechanism by which binding, and possibly hydrolysis, of ADPR is coupled to channel gating. Steady-state single-channel kinetic analysis could be very useful for addressing such details. However, wild-type TRPM2 channels are not amenable to such studies, because they inactivate irreversibly shortly after patch excision, precluding collection of sufficient numbers of gating events. Recently, we have identified that inactivation is caused by a conformational change of the selectivity filter, and can be completely prevented by a triple mutation in this region. This pore mutant TRPM2 channel (T5L), which mimics the filter sequence of TRPM5, retains unabated maximal activity for over 1 hour. Although the mutations result in altered permeation properties, ADPR- and Ca2+-dependent gating remains intact. Presently, we are investigating the ADPR-dependent gating mechanism of this mutant channel by steady-state single-channel kinetic analysis. Measurements are underway.