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

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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.