Role of the Rab family small G proteins in synaptic plasticity

长期增强（LTP）被认为是理解记忆形成和储存的细胞和分子机制的重要关键。大量研究强烈表明LTP的重要机制涉及突触前细胞谷氨酸释放的改变和突触后谷氨酸受体的反应性。包括谷氨酸在内的神经递质通过突触囊泡胞吐释放，胞吐受多种成分调节。它们被分为神经递质释放的必需和调节性两类。Rab3A小G蛋白已被证明是一种调节Ca^<2+>依赖性突触囊泡胞外分泌的调节成分。Rab3A在gdp结合的无活性形式（GDP-Rab3A）和gtp结合的活性形式（GTP-Rab3A）之间循环，并在突触前神经末梢的细胞质、突触囊泡膜和突触前质膜之间易位。Rab3 GDI、Rab3 GAP、Rab3 GEP三种调控因子共同调控其活化和易位。我们之前对Rab GDIα缺乏小鼠的研究表明，Rab GDIα不是基础神经递质释放所必需的，而是通过调节突触可塑性来抑制高兴奋性。在我们的研究中，我们发现Rab3 gep缺陷小鼠表现出更严重的表型，并在出生后因呼吸功能不全而立即死亡。神经肌肉连接处的电镜分析显示突触囊泡和活跃区总数明显减少。我们从大鼠脑突触囊泡粗片段中分离出一种与Rab3、GEP和GAP共免疫沉淀的新蛋白。这种蛋白被命名为rabconnectin-3，至少结合了Rab3 GEP和GAP。大鼠组织和亚细胞分布分析表明，rabcoithectin-3在大脑中大量表达，并在突触囊泡部分富集。免疫荧光和免疫电镜显示rabconnectin-3集中在突触的突触囊泡上。少

Long-term potentiation (LTP) is believed to provide an important key to understanding the cellular and molecular mechanisms by winch memories are formed and stored. Numerous studies strongly suggest that important mechanisms underlying LTP involves alternations in the release of glutamate from the presynaptic cells and the responsiveness of the postsynaptic glutamate receptors. Neurotransmitters including glutamate are released by synaptic vesicle exocytosis which is regulated by many components. They are classified into essential and modulatory ones for neurotransmitter release. Rab3A small G protein has been shown to be a modulatory component which regulates Ca^<2+>-dependent synaptic vesicle exocytosis. Rab3A cycles between the GDP-bound inactive form (GDP-Rab3A) and the GTP-bound active form (GTP-Rab3A) and translocates between the cytosol of the presynaptic nerve terminal and the membranes of synaptic vesicles and the presynaptic plasma membrane. The activation and the translocati … More on are regulated by three regulators (Rab GDI, Rab3 GAP, Rab3 GEP). Our previous studies on Rab GDIα-deficient mice had revealed that Rab GDIα is not essential for basal neurotransmitter release but suppress hyperexcitability via modulation of synaptic plasticity. In tins study, we found that Rab3 GEP-deficient mice show much more severe phenotypes and die immediately after birth due to respiratory insufficiency. Electron microscopic analysis of the neuromuscular junction revealed that total numbers of synaptic vesicles and active zones are markedly reduced. We isolated a novel protein that was co-immunoprecipitated with Rab3 GEP and GAP by their respective antibodies from the crude synaptic vesicle fraction of rat brain. This protein, named rabconnectin-3, bound at least both Rab3 GEP and GAP. Tissue and subcellular distribution analyses in rat indicated that rabcoithectin-3 was abundantly expressed in the brain where it was enriched in the synaptic vesicle fraction. Immunofluorescence and immunoelectron microscopy revealed that rabconnectin-3 was concentrated on synaptic vesicles at synapses. Less