Influence of the ligand binding domain heterodimer interface on NMDA receptor kinetics
Borschel, William Francis
MetadataShow full item record
Glutamatergic neurotransmission is the major contributor to excitatory synaptic communication in the mammalian brain. Activation of post-synaptic ionotropic glutamate receptors (iGluRs) causes an inward flux of positively charged ions, depolarizing the post-synaptic membrane, which can initiate an action potential. A member of this family, the NMDA receptor, plays a vital role in numerous physiological processes including neuronal development and synaptic plasticity, while dysfunction of this receptor is implicated in several neuropathological conditions ranging from neurodegenerative diseases to psychiatric disorders. iGluRs are large tetrameric membrane protein complexes composed of distinct modular domains. Interactions between the extracellular ligand binding domains (LBDs) of adjacent subunits are believed to be an essential determinant of receptor function. The current working model of activation and desensitization has been assumed to be similar for all iGluRs even though NMDA receptors have distinctively different LBD dimer contacts and functional properties than non-NMDA iGluRs. I hypothesized that interactions between LBD dimers of NMDA and non-NMDA iGluRs have distinct functional roles. To test this hypothesis, I examined the effects of several perturbations at the NMDA receptor LBD heterodimer interface at both unique and conserved contact sites across iGluRs. A combination of macroscopic and microscopic recordings from receptors with either stabilized or destabilized interactions allowed us to determine the influence of the LBD dimer interface on NMDA receptor gating kinetics. Engineered disulfide bonds between the LBD dimers at conserved iGluR contact sites impairs NMDA receptor activity through both increased activation energy barriers and destabilized open states. Both stabilizing and destabilizing mutations at these contact sites had no significant effect on the duration of microscopic desensitization, suggesting that activation but not desensitization requires rearrangements at the LBD heterodimer interface. Additionally, unique dimer interactions were determined to contribute to the distinctively slow and prolonged activation kinetics of NMDA receptors. I propose that interactions between LBD dimers have fundamentally different roles in receptor function between NMDA and non-NMDA iGluRs. These studies have illustrated how interactions at the LBD heterodimer interface influence NMDA receptor kinetics. This insight will allow further determination of the molecular mechanisms of receptor function that are specific to NMDA receptors and will help identify precise drug targets against NMDA receptor mediated neuropathologies.