Park C S, Elgersma Y, Grant S G N, Morrison J H
Department of Neuroscience, Mount Sinai School of Medicine, New York, NY 10029, USA.
Neuroscience. 2008 Jan 2;151(1):43-55. doi: 10.1016/j.neuroscience.2007.09.075. Epub 2007 Oct 12.
N-methyl-d-aspartate receptors (NMDARs) are critical determinants of bidirectional synaptic plasticity, however, studies of NMDAR function have been based primarily on pharmacological and electrophysiological manipulations, and it is still debated whether there are subunit-selective forms of long-term potentiation (LTP) and long-term depression (LTD). Here we provide ultrastructural analyses of axospinous synapses in cornu ammonis field 1 of hippocampus (CA1) stratum radiatum of transgenic mice with mutations to two key underlying postsynaptic density (PSD) proteins, postsynaptic density protein 95 (PSD-95) and the alpha-isoform of calcium-calmodulin-dependent protein kinase II (alphaCaMKII). Distribution profiles of synaptic proteins in these mice reveal very different patterns of subunit-specific NMDAR localization, which may be related to the divergent phenotypes of the two mutants. In PSD-95, Dlg, ZO-1/Dlg-homologous region (PDZ) 3-truncated mutant mice in which LTD could not be induced but LTP was found to be enhanced, we found a subtle, yet preferential displacement of synaptic N-methyl-d-aspartate receptor subunit 2B (NR2B) subunits in lateral regions of the synapse without affecting changes in the localization of N-methyl-d-aspartate receptor subunit 2A (NR2A) subunits. In persistent inhibitory alphaCaMKII Thr305 substituted with Asp in alpha-isoform of calcium-calmodulin kinase II (T305D) mutant mice with severely impaired LTP but stable LTD expression, we found a selective reduction of NR2A subunits at both the synapse and throughout the cytoplasm of the spine without any effect on the NR2B subunit. In an experiment of mutual exclusivity, neither PSD-95 nor alphaCaMKII localization was found to be affected by mutations to the corresponding PSD protein suggesting that they are functionally independent of the other in the regulation of NR2A- and NR2B-containing NMDARs preceding synaptic activity. Consequently, there may exist at least two distinct PSD-95 and alphaCaMKII-specific NMDAR complexes involved in mediating LTP and LTD through opposing signal transduction pathways in synapses of the hippocampus. The contrasting phenotypes of the PSD-95 and alphaCaMKII mutant mice further establish the prospect of an independent and, possibly, competing mechanism for the regulation of NMDAR-dependent bidirectional synaptic plasticity.
N-甲基-D-天冬氨酸受体（NMDARs）是双向突触可塑性的关键决定因素，然而，对NMDAR功能的研究主要基于药理学和电生理学操作，并且关于是否存在亚基选择性形式的长时程增强（LTP）和长时程抑制（LTD）仍存在争议。在此，我们对转基因小鼠海马体（CA1）辐射层角回1中轴棘突触进行了超微结构分析，这些小鼠的两个关键的突触后致密物（PSD）蛋白，即突触后致密物蛋白95（PSD-95）和钙调蛋白依赖性蛋白激酶II的α异构体（αCaMKII）发生了突变。这些小鼠中突触蛋白的分布概况揭示了亚基特异性NMDAR定位的非常不同的模式，这可能与两个突变体的不同表型有关。在PSD-95、Dlg、ZO-1/Dlg同源区域（PDZ）3截短的突变小鼠中，无法诱导LTD，但发现LTP增强，我们发现在突触外侧区域，突触N-甲基-D-天冬氨酸受体亚基2B（NR2B）亚基有细微但优先的移位，而不影响N-甲基-D-天冬氨酸受体亚基2A（NR2A）亚基的定位变化。在持续抑制性钙调蛋白激酶IIα异构体（T305D）突变小鼠中，LTP严重受损但LTD表达稳定，其中钙调蛋白激酶IIα异构体中的苏氨酸305被天冬氨酸取代，我们发现在突触和整个棘突细胞质中，NR2A亚基选择性减少，而对NR2B亚基没有任何影响。在一个相互排斥实验中，未发现PSD-95和αCaMKII的定位受相应PSD蛋白突变的影响，这表明它们在突触活动之前对含NR2A和NRB的NMDAR的调节中在功能上相互独立。因此，可能至少存在两种不同的PSD-95和αCaMKII特异性NMDAR复合物，它们通过海马体突触中的相反信号转导途径参与介导LTP和LTD。PSD-95和αCaMKII突变小鼠的对比表型进一步确立了一种独立且可能相互竞争的机制来调节NMDAR依赖性双向突触可塑性的前景。
