Cummings Kirstie A, Popescu Gabriela K
Department of Biochemistry, University at Buffalo, State University of New York, Buffalo, NY 14214.
Department of Biochemistry, University at Buffalo, State University of New York, Buffalo, NY 14214
J Gen Physiol. 2015 Jun;145(6):513-27. doi: 10.1085/jgp.201411302. Epub 2015 May 11.
N-methyl-d-aspartate (NMDA) receptors are the only neurotransmitter receptors whose activation requires two distinct agonists. Heterotetramers of two GluN1 and two GluN2 subunits, NMDA receptors are broadly distributed in the central nervous system, where they mediate excitatory currents in response to synaptic glutamate release. Pore opening depends on the concurrent presence of glycine, which modulates the amplitude and time course of the glutamate-elicited response. Gating schemes for fully glutamate- and glycine-bound NMDA receptors have been described in sufficient detail to bridge the gap between microscopic and macroscopic receptor behaviors; for several receptor isoforms, these schemes include glutamate-binding steps. We examined currents recorded from cell-attached patches containing one GluN1/GluN2A receptor in the presence of several glycine-site agonists and used kinetic modeling of these data to develop reaction schemes that include explicit glycine-binding steps. Based on the ability to match a series of experimentally observed macroscopic behaviors, we propose a model for activation of the glutamate-bound NMDA receptor by glycine that predicts apparent negative agonist cooperativity and glycine-dependent desensitization in the absence of changes in microscopic binding or desensitization rate constants. These results complete the basic steps of an NMDA receptor reaction scheme for the GluN1/GluN2A isoform and prompt a reevaluation of how glycine controls NMDA receptor activation. We anticipate that our model will provide a useful quantitative instrument to further probe mechanisms and structure-function relationships of NMDA receptors and to better understand the physiological and pathological implications of endogenous fluctuations in extracellular glycine concentrations.
N-甲基-D-天冬氨酸（NMDA）受体是唯一一类其激活需要两种不同激动剂的神经递质受体。NMDA受体由两个GluN1亚基和两个GluN2亚基组成异源四聚体，广泛分布于中枢神经系统，在那里它们介导对突触谷氨酸释放的兴奋性电流。孔的开放取决于甘氨酸的同时存在，甘氨酸可调节谷氨酸引发反应的幅度和时间进程。对于完全结合谷氨酸和甘氨酸的NMDA受体，其门控机制已得到足够详细的描述，以弥合微观和宏观受体行为之间的差距；对于几种受体亚型，这些机制包括谷氨酸结合步骤。我们在存在几种甘氨酸位点激动剂的情况下，检测了从含有一个GluN1/GluN2A受体的细胞贴附膜片钳记录的电流，并对这些数据进行动力学建模，以开发包括明确甘氨酸结合步骤的反应机制。基于匹配一系列实验观察到的宏观行为的能力，我们提出了一个甘氨酸激活结合谷氨酸的NMDA受体的模型，该模型预测在微观结合或脱敏速率常数不变的情况下，存在明显的负激动剂协同作用和甘氨酸依赖性脱敏。这些结果完善了GluN1/GluN2A亚型NMDA受体反应机制的基本步骤，并促使人们重新评估甘氨酸如何控制NMDA受体的激活。我们预计，我们的模型将提供一个有用的定量工具，以进一步探究NMDA受体的机制和结构-功能关系，并更好地理解细胞外甘氨酸浓度内源性波动的生理和病理意义。
