Contributions of amino-terminal domains to NMDA receptor functions
Ruvio, Stacy Ann
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Glutamate is the major excitatory neurotransmitter in mammalian brain, and it controls excitatory synaptic transmission by binding to post-synaptic ion channels like NMDA receptors. NMDA receptor activity is important for synapse development, maintenance and plasticity, processes that underlie the cellular mechanisms involved in learning and memory. Improper regulation of NMDA receptor activity has harmful effects and is implicated in several neuropathologies. NMDA receptors are obligate heterotetramers made of two glutamate-binding GluN2 subunits which have four different isoforms (A-D) and two glycine-binding GluN1 subunits. Each subunit contains a large, extracellular amino-terminal domain (NTD) that although has been implicated in several receptor properties, the exact role is not clearly understood. To increase our understanding of the contributions of the NTD in NMDA receptor function, we investigated the kinetic effects of perturbations at the NTD. First, we truncated the NTDs from NMDA receptors to determine the kinetic role of the NTD in NMDA receptor function. Second, we examined the kinetic effects of two channel inhibitors, zinc and ifenprodil, which bind with high-affinity within the NTD of GluN1/GluN2A and GluN1/GluN2B isoforms, respectively. Single-channel recordings and kinetic analyses were used to examine the effects of these NTD-modifications. NTD truncation decreased single-channel open probability and increased both microscopic and macroscopic desensitization, indicating a kinetic role of the NTD in channel desensitization. Zinc and ifenprodil exerted their inhibitory actions through generally similar mechanisms that decreased activation transition energy barriers and destabilized open states. Zinc- and ifenprodil-bound receptors accumulated in desensitized states, further indicating a role for the NTD in receptor desensitization. We propose that these NTD-binding modulators regulate NMDA receptors by opposing the intrinsic role of the NTD. These studies have increased our knowledge of the contributions of the NTD in NMDA receptor function by identifying kinetic parameters and transitions that are influenced by the NTD. The insight gained from these studies help fulfill an essential need to better comprehend NMDA receptor reaction mechanisms. These studies serve as a step towards more successful development of NTD-targeted drugs to combat NMDA receptor-related neuropathologies.