The transient receptor potential (TRP) ion channel family recently emerged as harboring crucial channel proteins involved in processes such as sensory perception, magnesium homeostasis or apoptosis. We identified TRPM2, a member of the TRP family, as a calcium-permeable cation channel gated by the novel second messenger ADP-ribose (ADPR) through the channel's intrinsic pyrophosphatase domain. TRPM2 is expressed in a variety of cell types, including pancreatic beta cells and neutrophils. Recently, the role of the molecules in the metabolic network surrounding ADPR on the functional properties of TRPM2 have emerged. Here, the novel calcium-release compound cADPR and other adenine dinucleotides materialize as crucial factors in TRPM2 physiology and hence processes involving cellular calcium homeostasis. Based on its primary activation by ADPR and reactive oxygen species, TRPM2 may have a function in cellular stress and remains a prime candidate for being involved in the apoptotic process. To further our understanding of TRPM2 in cell physiology, we propose experiments in Specific Aim 1 that will focus on the function of TRPM2 in events initiated by adenine-nucleotides. We hypothesize that TRPM2 may function as dual calcium influx and calcium release channel by taking advantage of a TRPM2 overexpressing cell system and a TRPM2 knock-out mouse model. We will explore agonist-receptor interactions engaging the main candidate in generating adenine dinucleotides, namely CD38. We will investigate the involvement of TRPM2 in cell death using a streptozotocin-based mouse model for diabetes type-1. These experiments will be performed in an expression system and mouse neutrophils and beta cells isolated from wild-type, CD38, PARP and TRPM2 knock-out mice models using a combination of patch-clamp, imaging and apoptotic studies.
In Specific Aim 2 we will investigate the molecular determinants of TRPM2 agonist binding and address the question whether calmodulin is the mediator of the calcium-dependent facilitation of TRPM2. Understanding TRPM2 and adenine-dinucleotide function in calcium signaling will refine our knowledge of calcium regulation in tissues expressing this ion channel and may provide new targets for therapeutic interference in processes involving TRPM2 function, including diabetes-causing cell death of pancreatic beta cells.
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