MOLECULAR ANALYSIS OF NMDA RECEPTOR MODULATORY SITES

N-甲基-D-天冬氨酸受体（NMDAR）对于神经系统的正常功能是重要的，但是它们的过度激活导致许多发育障碍和神经系统疾病。这种双重作用对旨在改善精神发育迟滞和神经退行性疾病的可能治疗策略施加了重要限制。必须在不干扰正常功能的情况下抑制NMDAR过度活动。我们的研究小组是第一个表明，无竞争力的，开放通道阻滞剂，美金刚，可以减少 NMDAR的过度活性，同时使正常的神经传递基本上不减弱。部分基于我们的工作，美金刚胺最近在美国被批准用于临床。在这里，我们开发了双功能衍生物NO-美金刚胺，其使用美金刚胺部分将NO靶向NMDAR上的S-亚硝基化位点，以比单独的美金刚胺更好地进一步下调过度活性。在我们的初步结果中，我们已经表明：（i）NR 1和NR 2亚基的第二跨膜（M2）结构域中的谷氨酰胺/精氨酸/天冬酰胺（Q/R/N）位点形成美金刚胺的特异性结合位点，美金刚胺与该位点的结合可用于将NO靶向NMDAR亚硝基化位点;（ii）缺氧通过S-亚硝基化增强NMDAR活性的下调;脑中存在一个新的NMDAR亚基家族，NR 3A和NR 3B（我们已经克隆并表征了这些亚基）。 NR 1/NR 2A/NR 3A（3B）在重组系统中的共表达降低了NMDA/甘氨酸激活电流的单通道记录中的单位电导、Ca 2+渗透性和Mg 2+敏感性。NR 3引起的NMDAR活性降低可能在正常发育过程中起保护作用;（iv）NR 3和NR 1亚基的共表达 （无NR 2）在非洲爪蟾卵母细胞和哺乳动物HEK 293细胞中形成兴奋性赖氨酸激活的阳离子通道（在无谷氨酸的情况下）;（v）晶体学实验中NR 1结构的S-亚硝基化。这一发现使我们开始将结构与功能联系起来。我们提出以下具体目的：[1]证明NO-美金刚胺靶向NMDAR的特异性，并阐明缺氧条件下S-亚硝基化增强NMDAR活性下调的机制。重要的是，这里研究的药物 项目I将用于P01资助项目中其他项目的神经保护实验; [2]表征含NR 3通道的通道孔区域; [3]表征含NR 3受体的配体结合位点，并使用[2]和[3]中的信息， 区分NR 1/NR 3“双联体”受体与NR 1/NR 2/NR 3“三联体”受体的表达。 [4]利用晶体学进一步研究NMDAR亚基配体结合和亚硝基化结构域的结构。

N-methyl-D-aspartate receptors (NMDARs) are important for the normal function of the nervous system, but their excessive activation contributes to a number of developmental disorders and neurological diseases. This dual role imposes important constraints on possible therapeutic strategies aimed at ameliorating mental retardation and neurodegenerative disorders. Inhibition of NMDAR overactivity must be accomplished without interference of normal function. Our group was the first to show that the uncompetitive, open-channel blocker, Memantine, can curtail excessive activity of the NMDAR while leaving normal neurotransmission essentially unabated. Based in part on our work, Memantine was recently approved for clinical use in the USA. Here we develop dual-functional derivatives, the NO-Memantines, that use the Memantine moiety to target NO to S-nitrosylation site(s) on the NMDAR to further downregulate excessive activity better than Memantine alone. In our preliminary results, we have shown: (i) the glutamine/arginine/asparagine (Q/R/N) sites in the second transmembrane (M2) domains of NR1 and NR2 subunits form a specific binding site for Memantine; the binding of Memantine to this site can be used for targeting NO to the NMDAR nitrosylation site; (ii) hypoxia enhances downregulation of NMDAR activity via S-nitrosylation; (in) a novel family of NMDAR subunits, NR3A and NR3B, exist in the brain (we have cloned and characterized these subunits). Co-expression of NR1/NR2A/NR3A(3B) in recombinant systems decreases unitary conductance, Ca2+ permeability, and Mg2+ sensitivity in single-channel recordings of NMDA/glycine-activated currents. Decreased NMDAR activity engendered by NR3 may be protective during normal development; (iv) co-expression of NR3 and NR1 subunits (without NR2) in Xenopus oocytes and mammalian HEK 293 cells form excitatory lycine-activated cation channels (in the absence of glutamate); (v) S-nitrosylation of the NR1 structure in crystallography experiments. This finding allows us to begin to relate structure to function. We propose the following Specific Aims: [1] To prove the specificity of NO-Memantine targeting to the NMDAR, and to elucidate the mechanism of enhanced downregulation of NMDAR activity by S-nitrosylation under hypoxic conditions. Importantly, drugs studied here in Project I will be used for neuroprotection experiments in the other Projects of this P01 grant; [2] To characterize the channel pore region of NR3-containing channels; [3] To characterize the ligand-binding site of NR3-containing receptors, and use information from [2] and [3] to distinguish expression of NR1/NR3 "doublet" receptors from NR1/NR2/NR3 "triplet" receptors. [4] To further study the structure of NMDAR subunit ligand-binding and nitrosylation domains using crystallography.