Role of the Rab family small G proteins in synaptic and epithelial plasticity

Rab家族小G蛋白由哺乳动物细胞中的60多个家族成员组成，在决定囊泡转运途径的特异性方面发挥着至关重要的作用。Rab3A已被证明是一种调控钙依赖的突触小泡胞吐的调节成分。Rab3A在GDP结合的非活动型和GTP结合的活动型之间循环，并在突触前神经末梢的胞浆、突触小囊膜和突触前质膜之间移位。Rab GDI、Rab3 GAP、Rab3 GEP三种调控因子对其激活和易位进行调控。我们之前对Rab GDIα缺陷小鼠和Rab3GEP缺陷小鼠的研究表明，这些调节因子参与了短期突触可塑性的调节。在本研究中，我们成功地建立了Rab3GAP缺陷小鼠，并正在研究Rab3GAP在突触可塑性中的作用。Rab13定位于极化上皮细胞的紧密连接(TJ)，但其功能尚不清楚。在本研究中，我们研究了Rab13在调节TJ跨膜蛋白的囊泡运输中的作用。在胞吐途径中，我们发现Rab13引导Claudin-1的细胞表面转运，而不是基侧跨膜蛋白低密度脂蛋白受体。我们还发现，在上皮细胞中，封闭素池不断地被内吞并循环回到细胞表面。我们的结果表明，Rab13特异性地调控了occludin的内吞循环。此外，我们发现Rab13不影响转铁蛋白受体的循环，转铁蛋白受体被循环到基侧的质膜区。我们分离出一种与GTP结合形式的Rab13特异相互作用的蛋白质，但不能与其GDP结合形式相互作用。我们的结果表明，Rab13是阻滞素和克拉丁转运到TJ和上皮可塑性的重要调节因子。

The Rab family small G proteins, which consist of more than 60 family members in mammalian cells, play a crucial role in determining the specificity of vesicular transport pathways. Rab3A has been shown to be a modulatory component which regulates Ca^<2+>-dependent synaptic vesicle exocytosis. Rab3A cycles between the GDP-bound inactive form and the GTP-bound active form 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 translocation are regulated by three regulators (Rab GDI, Rab3 GAP, Rab3 GEP). Our previous studies on Rab GDIα-deficient and Rab3 GEP-deficient mice revealed that these regulators are involved in modulating short-term synaptic plasticity. In this study, we have succeeded in generating Rab3 GAP-deficient mice and we are investigating the role of Rab3 GAP in synaptic plasticity. Rab13 localizes to tight junctions (TJs) in polarized epithelial cells, but its function is poorly understood. In the present study, we have examined the role of Rab13 in regulating the vesicular transport of TJ transmembrane proteins. In the exocytotic pathways, we found that Rab13 directs the cell-surface transport of claudin-1, but not a basolateral transmembrane protein, low-density lipoprotein receptor. We have also found that a pool of occludin was continuously endocytosed and recycled back to the cell-surface in epithelial cells. Our results indicated that Rab13 specifically regulated the endocytic recycling of occludin. Furthermore, we found that Rab13 did not affect the recycling of transferrin receptor, that was recycled to the basolateral plasma membrane domain. We isolated a protein that specifically interacted with the GTP-bound form of Rab13, but not its GDP-bound form. Our results suggest that Rab13 is a crucial regulator for the transport of occludin and claudins to TJs and for the epithelial plasticity.