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

Rab家族小G蛋白在哺乳动物细胞中由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。Rab 3A已被证明是调节Ca^2+依赖性突触囊泡胞吐的调节组分。Rab 3A在GDP结合的非活性形式和GTP结合的活性形式之间循环，并在突触前神经末梢的胞质溶胶与突触囊泡的膜和突触前质膜之间易位。其激活和转运受三个调节因子（RabGDI、Rab 3 GAP、Rab 3 GEP）的调节。我们先前对Rab GDIα缺陷和Rab 3 GEP缺陷小鼠的研究表明，这些调节因子参与调节短时程突触可塑性。在这项研究中，我们已经成功地产生Rab 3 GAP缺陷小鼠，我们正在研究Rab 3 GAP在突触可塑性中的作用。Rab 13定位于极化上皮细胞的紧密连接（TJ），但其功能知之甚少。在本研究中，我们研究了Rab 13在调节TJ跨膜蛋白的囊泡转运中的作用。在胞吐途径中，我们发现Rab 13指导claudin-1的细胞表面转运，但不是基底外侧跨膜蛋白，低密度脂蛋白受体。我们还发现闭合蛋白池被持续内吞并再循环回到上皮细胞的细胞表面。我们的研究结果表明，Rab 13特异性调节occludin的内吞再循环。此外，我们发现Rab 13不影响转铁蛋白受体的再循环，其再循环到基底外侧质膜结构域。我们分离出一种蛋白质，它与Rab 13的GTP结合形式特异性相互作用，但不与其GDP结合形式相互作用。我们的研究结果表明，Rab 13是一个至关重要的调节occludin和claudins运输到TJ和上皮可塑性。

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.