Mechanism and physiology of NMDA receptor desensitization

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PROJECT SUMMARY NMDA receptor hypofunction has been implicated in neuropsychiatric disorders such as schizophrenia, a disabling mental disorder that affects over 2.2 million patients in the US. One modality for NMDA receptor hypofunction is aberrant gating, the process where the receptor converts glutamate binding into opening of the associated ion channel. One pathway to regulate hypofunction is NMDA receptor desensitization, a gating configuration where the receptor is ligand-bound, but the ion channel is non-conducting. However, its physiological and mechanistic basis is poorly defined in part because of the lack of tools to study it. In this proposal I will take advantage of new tools – single site mutations that selectively alter specific features of NMDA receptor desensitization recently discovered in the Wollmuth lab – as well as cutting edge technologies to address the mechanism and physiology of NMDA receptor desensitization. Whereas desensitization in non-NMDA receptors depends almost exclusively on the rearrangement of the ligand-binding domain (LBD) dimer interface, such a mechanism is less significant in NMDA receptors and all domains, most notably the transmembrane domain (TMD), have been implicated in NMDA receptor desensitization. In Aim 1, I will test the general hypothesis that the mechanism of NMDA receptor desensitization is fundamentally different from that of non-NMDA receptors, depending strongly on the conformation of the ion channel. To test this hypothesis, I will take advantage of newly identified single-site mutations, patch clamp electrophysiology as well as techniques to regulate the subunit composition of NMDA receptors and light-activated unnatural amino acids. These experiments will help define the structural mechanisms that govern NMDA receptor desensitization, aiding in the development of novel therapeutics that can selectively modulate NMDA receptor activity by specifically targeting desensitization. In Aim 2, I will test the hypothesis that NMDA receptor desensitization leads to decreased excitatory signaling at synapses during high activity. Indeed, my preliminary data show that fast applications of glutamate caused a higher degree of current decay in these mutants, suggesting a potential role of desensitization in NMDA hypofunction. To test this hypothesis more rigorously, I will express mutant NMDA receptors that have altered desensitization properties in organotypic hippocampal slice cultures to address how they change synaptic dynamics including Ca2+ influx and synaptic plasticity. These experiments will address how NMDA receptor desensitization contributes to synaptic physiology. The information gained by the experiments in this proposal will provide insight into the mechanism of NMDA receptor desensitization and its role in synaptic dynamics. My experiments will aid in the development of novel therapeutics that selectively modulates NMDA receptor activity by specifically targeting desensitization.