Rab-genome analysis reveals novel insights in Weibel-Palade body exocytosis

Advance Online Publication August 16, 2012 doi: 10.1242/?jcs.104174 Weibel-Palade bodies (WPBs) are endothelial-specific organelles, which, upon fusion with the plasma membrane, release cargo molecules that are essential in critical blood vessel functions such as thrombosis, inflammation and angiogenesis. Despite the importance of WPBs, the basic mechanisms that mediate their secretion are only poorly understood. Rab GTPases play fundamental role in the trafficking of intracellular organelles. Yet, the only known WPB-associated Rabs are Rab27a and Rab3d. Here, to determine the full spectrum of WPB-Rabs we performed a complete Rab-genome screening by analyzing the localization of all Rabs in WPBs and their involvement in the secretory process in endothelial cells. Apart from Rab3 and Rab27, we identified three additional Rabs, Rab15 (a previously reported endocytic Rab), Rab33 and Rab37, on the WPB limiting membrane. A knocking down approach using siRNAs showed that among these five WPB-Rabs only Rab3, Rab27 and Rab15 are required for exocytosis. Intriguingly, we found that Rab15 cooperates with Rab27a in WPB secretion. Furthermore, a specific effector of Rab27, Munc13-4, appears to be also an effector of Rab15 and is required for WPB exocytosis. These data indicate that WPB secretion requires the coordinated function of a specific group of Rabs and that, among them, Rab27a and Rab15, as well as their effector Munc13-4, cooperate to drive exocytosis.

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Rab11 regulates exocytosis of recycling vesicles at the plasma membrane

Advance Online Publication June 8, 2012 doi: 10.1242/?jcs.102913 Senye Takahashi, Keiji Kubo, Satoshi Waguri, Atsuko Yabashi, Hye-Won Shin, Yohei Katoh and Kazuhisa Nakayama*?*Corresponding author: Kazuhisa Nakayama, Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan; Phone: +81-75-753-4527; Fax: +81-75-753-4557; E-mail: kazunaka{at}pharm.kyoto-u.ac.jp. Rab11 is known to associate primarily with perinuclear recycling endosomes and regulate recycling of endocytosed proteins. However, the recycling step in which Rab11 participates remains unknown. We here show that, in addition to causing tubulation of recycling endosomes, Rab11 depletion gives rise to accumulation of recycling carriers containing endocytosed transferrin and transferrin receptor beneath the plasma membrane. We also show that the carriers are transported from perinuclear recycling endosomes to the cell periphery along microtubules. Total internal reflection fluorescence microscopy of cells expressing EGFP-tagged transferrin receptor revealed that Rab11 depletion inhibits tethering and fusion of recycling carriers to the plasma membrane. Depletion of a component of the exocyst tethering complex, Sec15 or Exo70, the former which interacts with Rab11, leads to essentially the same phenotypes as those of Rab11 depletion. Thus, in addition to its role in recycling processes at perinuclear recycling endosomes, Rab11 is transported along microtubules to the cell periphery through association with recycling carriers, and directly regulates vesicle exocytosis at the plasma membrane in concert with the exocyst.

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