Glycinergic N-methyl-D-aspartate receptors (NRs) are a unique type of excitatory channels. They differ from the traditional glutamatergic NRs in that that they are activated by glycine alone, are insensitive to glutamate, and have a lower unitary conductance, Ca2+ permeability, and sensitivity to voltage-dependent Mg2+ block. They are tetramers of GluN1 (N1) and GluN3 (N3A or N3B) subunits. The N1 subunit has eight splice variants (N1-1a through N1-4b), each with specific spatiotemporal expression patterns in the brain. The N3A subunit is specifically expressed neonatally and is critical for spine development and synaptic plasticity. Altered expression of N3A has been linked to the negative symptoms of schizophrenia. Contrary to what has been observed in glutamatergic NRs (N1/N2), strikingly distinct macroscopic current is produced dependent on the N1 splice variant with which N3A assembles. I propose that these differences may arise in part from distinct gating kinetics conferred onto N1/N3A receptors by the C-terminal cassettes of N1 splice variants. To test this hypothesis, I will pursue the following three aims: 1) Fully characterize the high-activity N1-4a/N3 isoform using kinetic analyses and state modeling. 2) Identify the kinetic contributions of each C-terminal cassette by systematically comparing reaction mechanisms of selected N1/N3A isoforms. 3) Identify the elements on each cassette responsible for gating modulation by combining sequence-based mutagenesis and kinetic analyses. The results from this proposal will provide insights into the mechanism by which differential slicing of N1 subunits controls the functional output of N1/N3A receptors. Despite having been cloned almost two decades ago, very little is known about the functional mechanism of N1/N3A receptors and how they contribute to both physiological and pathological states. Insights from this proposal will spur rational hypotheses to address these gaps in knowledge and will afford a more comprehensive understanding of the underlying molecular mechanism of the pathophysiology of schizophrenia.

Public Health Relevance

Glycinergic NMDA receptors are a novel class of excitatory receptors that respond to glycine alone. They are expressed at high levels in the brain during development and continued expression into adulthood is a hallmark of schizophrenia. The proposed work will provide insight about the functional mechanisms of receptors native to specific brain regions and lay a concrete foundation for the development of therapeutics to better ameliorate symptoms of schizophrenia.

National Institute of Health (NIH)
National Institute of Neurological Disorders and Stroke (NINDS)
Predoctoral Individual National Research Service Award (F31)
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Special Emphasis Panel (ZRG1)
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Silberberg, Shai D
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State University of New York at Buffalo
Schools of Medicine
United States
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Cummings, Kirstie A; Popescu, Gabriela K (2016) Protons Potentiate GluN1/GluN3A Currents by Attenuating Their Desensitisation. Sci Rep 6:23344
Borschel, William F; Cummings, Kirstie A; Tindell, LeeAnn K et al. (2015) Kinetic contributions to gating by interactions unique to N-methyl-D-aspartate (NMDA) receptors. J Biol Chem 290:26846-55
Cummings, Kirstie A; Popescu, Gabriela K (2015) Glycine-dependent activation of NMDA receptors. J Gen Physiol 145:513-27
Maki, Bruce A; Cummings, Kirstie A; Paganelli, Meaghan A et al. (2014) One-channel cell-attached patch-clamp recording. J Vis Exp :