Structural and functional studies of the TRPM2 channel

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Body temperature is strictly maintained in a narrow range to protect the delicate nerves in the brain and other body tissues, because improper body temperature gives rise to fever, brain injury, and stroke. TRPM2 is the major warmth-sensing receptor in the brain regulating core body temperature and preventing overheating as fever occurs. TRPM2 is a Ca2+-permeable, nonselective ion channel that is highly expressed in brain but is also found in the heart, vascular and smooth muscle, and immune cells. It is uniquely activated by Ca2+ and ADP ribose (ADPR), a product of the metabolism of NAD+ and a secondary messenger released upon oxidative stress. The activation of TRPM2 results in both Ca2+ entry across the plasma membrane and Ca2+ release from lysosomes. Therefore, TRPM2 plays fundamental role in Ca2+-dependent array of physiological processes and cellular functions from insulin secretion to immune response to cell death. It has been implicated in Alzheimer disease, stroke, and other neurodegenerative diseases. TRPM2 belongs to the TRPM (melastatin-like transient receptor potential) subfamily of the TRP superfamily. Despite sharing the characteristic TRPM N-terminal homology regions (MHRs) and C-terminal coiled-coil domains, TRPM2 is uniquely assembled with a C-terminal NHDT9-H domain, a homolog to the human mitochondrial ADP-ribose pyrophosphatase NUDT9. Functional studies, including binding assays, electrophysiology, and molecular simulations, provided a consensus view that ADPR binds to the NUDT9-H domain, but proof of the ADPR binding site is lacking, and the molecular basis for the action of the agonist ADPR on TRPM2 in the presence of calcium remains unknown. The gating of TRPM2 is further modulated by many molecules and ions that range from protons to nucleotides (cyclic ADPR, AMP, 8-Br-cADPR) to curcumin (which is isolated from rhizomes of Curcuma longa), acting by way of multiple mechanisms. At present, we don't know where these molecules and ions bind to TRPM2 or how they activate the channel or modulate its function. We have obtained two cryo-EM structures of zebrafish TRPM2 in the apo/closed and ADPR/Ca2+-bound open state, with the latter representing the first active state of TRPM family members. We identified a novel ADPR binding site that is located outside the NUDT9-H domain and was completely unknown before. Building on this preliminary data, we propose to continue the structural studies of TRPM2 combined with complementary electrophysiology experiments, binding assays, and X-ray crystallography, which will define the molecular basis for a comprehensive gating mechanism and pharmacology. These advances will provide a solid foundation for developing new drugs against neurodegenerative diseases and for a deeper understanding the function of the entire TRPM family.