Regulation of NMDAR-Mediated Synaptic Signaling

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PROJECT SUMMARY/ABSTRACT We propose to study the NMDA-subtype (NMDARs) of ionotropic glutamate receptors (iGluRs) and their regulation by NRAP-1, the first identified NMDAR-specific auxiliary protein, which we recently discovered in a genetic screen for modifiers of NMDAR-mediated behavior in C. elegans. NMDARs are evolutionarily conserved and well known for their role in synaptic plasticity, i.e., long-term potentiation (LTP); their importance for cellular models of learning and memory; and their direct or indirect involvement in many neurological and psychiatric disorders. Although NRAP-1 modifies the function of postsynaptic NMDARs, we showed that it was released by presynaptic glutamatergic neurons. This discovery provided a major conceptual advance in our understanding of the regulation of NMDAR-mediated synaptic signaling, with implications for both the control of synaptic strength and for certain clinical disorders involving NMDARs. In preliminary experiments, we successfully obtained crystals of recombinantly produced NRAP-1 and determined the crystal structure at 1.9 Å resolution. Elucidating the structure of NRAP-1 has provided important new insight into how NRAP-1 modifies NMDAR function. By studying vertebrate and C. elegans NMDARs, we have also demonstrated a fundamental importance for the NMDAR amino-terminal domain (ATD) with respect to both receptor gating and to the mechanism of action of NRAP-1. We now plan to build on this foundational work and ask how NRAP-1 functions to modulate NMDAR function, i.e., what are the interactions between NRAP-1 and NMDARs, and how do these interactions change receptor kinetics? In contrast to overexpression of NMDARs, we discovered that overexpression of NRAP-1 in vivo significantly increased NMDAR-mediated currents and behavior. This has important implications for the control of synaptic plasticity. Furthermore, we found that NRAP-1 is actively transported along neural processes. Together, these findings suggest that modulating NRAP-1 secretion might be a mechanism used to regulate activity dependent changes in synaptic strength. Therefore, we will address the molecular requirements for the transport and secretion of NRAP-1. The relevance of our proposed studies is high because disorders of NMDAR-mediated signaling are implicated in synaptopathies associated with neurodegenerative disorders as well as for mental health illnesses such as schizophrenia and depression. Synaptic molecules are evolutionarily conserved, and our understanding of the mechanisms that regulate synaptic signaling has greatly benefited from genetics-based studies in invertebrates such as Drosophila and C. elegans. Notably, NMDARs and NRAP-1-like proteins appear to have co-evolved suggesting that vertebrate NMDARs are likely regulated by auxiliary proteins. We therefore anticipate that our planned studies will help provide a framework for a new mechanistic understanding of NMDARs centered on protein-protein interactions.