Modulation of NMDA Receptor Current by Protein Kinase C: Molecular and Synaptic Studies

谷氨酸是最丰富的神经递质，脑细胞用它来触发与之接触的其他脑细胞的电脉冲。在这些被称为突触的接触部位，发生了许多重要的过程。谷氨酸受体由不同的基因家族编码，在突触上具有不同的特异性作用。谷氨酸受体的NMDA家族是特别重要的，因为它们可以触发脑细胞之间突触连接的敏感性水平的变化。被称为激酶的酶的调节将磷酸盐化学基团添加到NMDA受体上，导致通过这些受体的电流显著增强，并增强脑细胞之间的信号传导。这种调节作用在脑细胞的学习、发育和退化过程中起着重要作用。确定特定激酶靶向的NMDA受体亚基上的位点将为研究NMDA受体磷酸化在脑细胞中的生理重要性提供研究工具。在c型蛋白激酶（PKC）调节的情况下，这些位点的存在也可能取决于特定的NMDA受体家族成员。关于NMDA受体亚型的重要性，一个特别令人兴奋的例子是，在年轻大脑中更为普遍的NMDA受体亚基NR2B，已被证明可以产生更容易改变的突触连接。含有NR2B亚基的受体比其他NMDA受体通过更大的钙离子电流。目前的建议假设这种差异是由于对PKC的敏感性增加。在我的实验室中，选择性突变的NMDA受体的控制表达最近证明，蛋白激酶C对2个特定丝氨酸氨基酸位点（S1303和S1323）的直接磷酸化控制了NR2B受体的电流扩增。这一结果激发了该提案的目的，并且：1)挑战了先前的观点，即电流的激酶调节必须完全间接通过与突触中NMDA受体相关的其他蛋白质进行，2)为阐明直接激酶作用的机制提供了一个起点。这些研究独特地关注于NMDA电流的调节，而NMDA电流本身会引发脑细胞中长期的突触变化。

Glutamate is the most abundant neurotransmitter that brain cells use to trigger electrical impulses in other brain cells they contact. At these sites of contact, called synapses, many vital processes take place. There are many different types of receptors for glutamate encoded by different gene families, which have different specific actions at synapses. The NMDA family of glutamate receptors is of particular importance because they can trigger changes in the level of sensitivity of synaptic connections between brain cells. Modulation by enzymes called kinases adds phosphate chemical groups to NMDA receptors causing marked enhancement of current passing through these receptors and enhancing signaling between brain cells. This modulatory effect has an important role in processes related to learning, development, and degeneration of brain cells. The identification of the sites on NMDA receptor subunits targeted by specific kinases will provide research tools for studying the physiological importance of NMDA receptor phosphorylation in brain cells. In the case of modulation by protein kinases of the C-type (PKC), the presence of these sites may also depend on the particular NMDA receptor family member present. In a particularly exciting example of the importance of NMDA receptor subtype, the NMDA receptor subunit that is more prevalent in young brains, NR2B, has been shown to give rise to more easily modifiable synaptic connections. Receptors that contain NR2B subunits pass larger calcium ion currents than other NMDA receptors. The present proposal hypothesizes that this difference is due to an increased sensitivity to PKC. Controlled expression of selectively mutated NMDA receptors in my laboratory has recently demonstrated that direct phosphorylation of 2 specific serine amino acid sites (S1303 and S1323) by protein kinase C controls current amplification in NR2B receptors. This result motivates the aims of the proposal and: 1) challenges a previous viewpoint that kinase modulation of current must be entirely indirect via other proteins associated with NMDA receptors at the synapse, and 2) provides a starting point for elucidating a mechanism of direct kinase action. These studies uniquely focus on modulation of the NMDA current that itself triggers long-term synaptic changes in brain cells.