1.A.10.1.3
GIC, NMDA-subtype, Grin C2 (highly permeable to Ca²⁺ and monovalent cations). A single residue in the GluN2 subunit controls NMDA receptor channel properties via intersubunit interactions (Retchless et al., 2012). Memantine (Namenda) is prescribed as a treatment for moderate to severe Alzheimer's Disease. Memantine functions by blocking the NMDA receptor, and the sites of interaction have been identified (Limapichat et al. 2013).  Genetic mutations in multiple NMDAR subunits cause various childhood epilepsy syndromes (Li et al. 2016). NMDA receptor gating is complex, exhibiting multiple closed, open, and desensitized states, but the structure-energy landscape of gating for the rat homologue has been mapped (Dolino et al. 2017). NMDARs are tetrameric complexes consisting of two glycine-binding GluN1 and two glutamate-binding GluN2 subunits. Four GluN2 subunits encoded by different genes can produce up to ten different di- and triheteromeric receptors.  These heteromeric systems have been modeled (Gibb et al. 2018). A conserved glycine associated with diseases permits NMDA receptors to acquire high Ca²⁺ permeability (Amin et al. 2018). The ND2 protein (see TC# 3.D.1.6.1), a component of the NMDAR complex, enables Src tyrosine protein kinase (TC# 8.A.23.1.12) regulation of NMDA receptors (Scanlon et al. 2017). Drug effects, regulatory protein modulators and positive allosteric modulators have been reviewed (Fu et al. 2019).
   NMDARs are heterotetramers composed of GluN1 and GluN2 subunits, which bind glycine and glutamate, respectively, to activate their ion channels. Chou et al. 2020 showed the detailed patterns of conformational changes and inter-subunit and -domain reorientation leading to agonist-gating and subunit-dependent competitive inhibition by providing multiple structures in distinct ligand states at 4 Å or better. The structures revealed that activation and competitive inhibition by both GluN1 and GluN2 antagonists occur by controlling the tension of the linker between the ligand-binding domain and the transmembrane ion channel of the GluN2 subunit (Chou et al. 2020). GRIN2D recurrent de novo dominant mutation causes a severe epileptic encephalopathy treatable with NMDA Receptor channel blockers (Li et al. 2016).