Laboratory of Membrane Trafficking Mechanisms, Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Aoba-ku, Sendai, Miyagi, Japan.
Mol Biol Cell. 2012 Aug;23(16):3229-39. doi: 10.1091/mbc.E12-02-0104. Epub 2012 Jul 5.
Most cells in tissues are polarized and usually have two distinct plasma membrane domains-an apical membrane and a basolateral membrane, which are the result of polarized trafficking of proteins and lipids. However, the mechanism underlying the cell polarization is not fully understood. In this study, we investigated the involvement of synaptotagmin-like protein 2-a (Slp2-a), an effector molecule for the small GTPase Rab27, in polarized trafficking by using Madin-Darby canine kidney II cells as a model of polarized cells. The results show that the level of Slp2-a expression in MDCK II cells increases greatly as the cells become polarized and that its expression is specifically localized at the apical membrane. The results also reveal that Slp2-a is required for targeting of the signaling molecule podocalyxin to the apical membrane in a Rab27A-dependent manner. In addition, ezrin, a downstream target of podocalyxin, and ERK1/2 are activated in Slp2-a-knockdown cells, and their activation results in a dramatic reduction in the amount of the tight junction protein claudin-2. Because both Slp2-a and claudin-2 are highly expressed in mouse renal proximal tubules, Slp2-a is likely to regulate claudin-2 expression through trafficking of podocalyxin to the apical surface in mouse renal tubule epithelial cells.
组织中的大多数细胞都具有极性，通常具有两个不同的质膜区域——顶膜和基底外侧膜，这是蛋白质和脂质极化运输的结果。然而，细胞极化的机制尚未完全了解。在这项研究中，我们使用 Madin-Darby 犬肾 II 细胞作为极化细胞的模型，研究了突触融合相关蛋白样蛋白 2-a（Slp2-a）——一种小 GTPase Rab27 的效应分子——在极化运输中的作用。结果表明，随着细胞极化，MDCK II 细胞中 Slp2-a 的表达水平大大增加，并且其表达特异性定位于顶膜。结果还表明，Slp2-a 是信号分子足细胞蛋白聚糖靶向到顶膜的必需因子，其靶向作用依赖于 Rab27A。此外，足细胞蛋白聚糖的下游靶标埃兹蛋白和 ERK1/2 在 Slp2-a 敲低细胞中被激活，其激活导致紧密连接蛋白 Claudin-2 的量显著减少。因为 Slp2-a 和 Claudin-2 在小鼠肾近端小管中都高度表达，所以 Slp2-a 可能通过将足细胞蛋白聚糖运输到小鼠肾小管上皮细胞的顶表面来调节 Claudin-2 的表达。
