The glutamate receptor called the N-methyl-D-aspartate (NMDA) receptor is a crucial mediator of many forms of excitotoxicity, a pathophysiological mechanism of neurologic disease. The NMDA receptor is constructed from combinations of NR1 and NR2 subunits, with each subunit conveying distinct properties. Different subtypes may play distinct roles in excitotoxicity, possibly based on the variable features of their intracellular domains. These domains link the receptor to intracellular signaling processes, and are potential sites for proteolytic modification by the enzyme calpain. Such modifications could play a crucial role in control of the subtype specific properties of NMDA receptors. Previously we have shown that the NR2A subunit is a selective substrate for calpain while the NR1 subunit is not cleaved by calpain. This cleavage may create receptors with novel properties, which could be regulated in a subunit selective manner during excitotoxicity and hypoxia. In this proposal, we will assess the effect of calpain cleavage on the physiological properties and localization of different NMDA receptor subtypes and the role of these events in excitotoxicity. We will initially determine the sites of cleavage by calpain in each NR2 subunit in vitro, in heterologous systems and in neurons. Following these experiments, we will define the effect of calpain cleavage on physiological properties of the NMDA receptor, and determine whether cleavage by calpain alters NMDA receptor localization in heterologous expression systems and in neurons. In the final Aim, we will investigate whether calpain cleavage of NMDA receptors occurs during models of excitotoxicity and hypoxia, and whether calpain cleavage of NMDA receptors alters the severity of cellular damage. In addition, we will ascertain whether calpain cleavage plays a role in a specific model of epilepsy, the kindling model. Overall, the present I proposal will define how calpain proteolysis of the NMDA receptor modulates receptor properties, allowing us to determine the role of this process in neuronal function and disease processes. ? ?
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