Role of Gprotein and SNAP-25 in NMDA vs mGluR presynaptic long-term plasticity

描述（由申请人提供）：在海马体和其他大脑区域，突触的可塑性允许它根据过去的激活历史改变反应方式，这是细胞记忆的一种形式。突触的动态特性使得在长期增强（LTP）中持续增加突触的反应性，或在长期抑制（LTD）中持续减少突触的强度，从而有可能改变情景和空间记忆的形成。当记忆形成时，特定的神经元兴奋模式会在突触强度上产生长期的变化，当这些变化被重新激活时，就会作为记忆被回忆起来。在细胞水平上，LTP/LTD的诱导涉及多种谷氨酸受体亚型的激活，这些受体介导或调节局部神经元内流/钙和其他第二信使的释放。n -甲基- d -天冬氨酸受体（NMDAR）对于诱导LTP和LTD的突触前和突触后形式至关重要，而突触前II组代谢型谷氨酸受体（mGluRIIs）有助于诱导递质释放的LTD。NMDAR-LTD和LTP的突触前成分改变了神经递质释放的可能性，这种作用被认为与突触后神经元释放逆行信号有关。NMDAR和mGluRII都激活完全不同的途径，各自导致释放有限，或者两者由最终的共同途径介导。在囊泡融合过程中，SNARE蛋白协同作用介导囊泡神经递质释放。我假设G¿？由mGluRII在突触前末端激活释放，结合突触体相关蛋白25kD （SNAP-25）的c端，并与nmdar激活的逆行信使一氧化氮的释放结合，引起LTD。我们的实验室发现肉毒毒素A从SNAP-25的c端切割9个氨基酸，以及突触前注射SNAP-25的c端，都阻断了突触前LTD的表达。通过测量LTD的大小，使用脑切片现场和全细胞电生理记录技术，并评估LTD对NMDAR和mGluRII拮抗剂的敏感性，我计划阐明每种受体如何促进突触前LTD的诱导和表达。利用NMDAR和mGluRII拮抗剂在表达SNAP-25的未成熟同种异构体（SNAP-25a）的转基因小鼠的脑切片中，我提出验证这样一个假设，即在年轻小鼠中，LTD的主要表达形式存在发育转变，这与SNAP-25a向SNAP-25b表达的转变相一致。最后，利用双光子激光扫描显微镜技术，我将对Schaffer侧侧突触前释放位点的不同囊泡池中的囊泡释放进行成像，以测试由NMDAR和mgluri激活的生化级联在诱导突触前LTD中的作用。通过确定活化的NMDARs与mgluri和SNAP-25之间相互作用的机制，我希望进一步了解递质释放的长期变化如何促进记忆，并揭示改善阿尔茨海默氏症和癫痫等疾病的机制，这些疾病的突触可塑性被改变。

DESCRIPTION (provided by applicant): In the hippocampus, and other brain areas, plasticity of a synapse allows it to change the way it responds based on its past history of activation, a form of cellular memory. The dynamic nature of a synapse makes it possible to alter episodic and spatial memory formation by persistently increasing responsiveness of synapses, in long-term potentiation (LTP), or decreasing their strength, in long-term depression (LTD). When memories form, particular patterns of neuronal excitation produce long-term changes in synaptic strengths, patterns that, when reactivated, are recalled as memories. At the cellular level, induction of LTP/LTD involve activation of multiple subtypes of glutamate receptors that mediate or regulate local neuronal influx/release of calcium and other second messengers. The N-Methyl-D-aspartic acid receptor (NMDAR) is crucial for the induction of presynaptic and postsynaptic forms of both LTP and LTD, while presynaptic group II metabotropic glutamate receptors (mGluRIIs) contribute to induction of LTD of transmitter release. Presynaptic components of NMDAR-LTD and LTP alter probability of neurotransmitter release, an effect believed to be associated with release of a retrograde signal from the postsynaptic neuron. Both NMDAR and mGluRII activate completely different pathways that each result in LTD of release or the two are mediated by a final common pathway. During vesicular fusion, SNARE proteins work together to mediate vesicular neurotransmitter release. I hypothesize that G¿?, liberated by mGluRII activation in the presynaptic terminal, binds the C-terminus of the synaptosomal associated protein 25kD (SNAP-25), and that this combines with NMDAR-activated release of the retrograde messenger nitric oxide to evoke LTD. Our laboratory discovered that botulinum toxin A, which cleaves 9 amino acids from the C-terminus of SNAP-25, and presynaptic infusion of the C-terminus of SNAP- 25, each occlude the expression of presynaptic LTD. By measuring the magnitude of LTD, using field and whole cell electrophysiology recording techniques in brain slices, and evaluating the sensitivity of LTD to NMDAR and mGluRII antagonists, I plan to elucidate how each receptor contributes to induction and expression of presynaptic LTD. Using NMDAR and mGluRII antagonists in brain slices from transgenic mice expressing an immature isoform of SNAP-25 (SNAP-25a), I propose to test the hypothesis that there is a developmental shift in the dominant form of LTD expressed in young mice that coincides with the shift from SNAP-25a to SNAP-25b expression. Finally, with two-photon laser scanning microscopy techniques, I will image vesicular release from distinct vesicle pools of Schaffer collateral presynaptic release sites, to test the role of biochemical cascades activated by NMDAR and mGluRIIs in induction of presynaptic LTD. Through identifying the mechanisms of interaction between activated NMDARs and mGluRIIs, and SNAP-25, I hope to further understanding of how long-term changes in transmitter release contribute to memory, and uncover mechanisms to ameliorate diseases such as Alzheimer's and epilepsy, where synaptic plasticity is altered.