Akhtar N, Hotchin N A
School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.
Mol Biol Cell. 2001 Apr;12(4):847-62. doi: 10.1091/mbc.12.4.847.
The establishment of cadherin-dependent cell-cell contacts in human epidermal keratinocytes are known to be regulated by the Rac1 small GTP-binding protein, although the mechanisms by which Rac1 participates in the assembly or disruption of cell-cell adhesion are not well understood. In this study we utilized green fluorescent protein (GFP)-tagged Rac1 expression vectors to examine the subcellular distribution of Rac1 and its effects on E-cadherin-mediated cell-cell adhesion. Microinjection of keratinocytes with constitutively active Rac1 resulted in cell spreading and disruption of cell-cell contacts. The ability of Rac1 to disrupt cell-cell adhesion was dependent on colony size, with large established colonies being resistant to the effects of active Rac1. Disruption of cell-cell contacts in small preconfluent colonies was achieved through the selective recruitment of E-cadherin-catenin complexes to the perimeter of multiple large intracellular vesicles, which were bounded by GFP-tagged L61Rac1. Similar vesicles were observed in noninjected keratinocytes when cell-cell adhesion was disrupted by removal of extracellular calcium or with the use of an E-cadherin blocking antibody. Moreover, formation of these structures in noninjected keratinocytes was dependent on endogenous Rac1 activity. Expression of GFP-tagged effector mutants of Rac1 in keratinocytes demonstrated that reorganization of the actin cytoskeleton was important for vesicle formation. Characterization of these Rac1-induced vesicles revealed that they were endosomal in nature and tightly colocalized with the transferrin receptor, a marker for recycling endosomes. Expression of GFP-L61Rac1 inhibited uptake of transferrin-biotin, suggesting that the endocytosis of E-cadherin was a clathrin-independent mechanism. This was supported by the observation that caveolin, but not clathrin, localized around these structures. Furthermore, an inhibitory form of dynamin, known to inhibit internalization of caveolae, inhibited formation of cadherin vesicles. Our data suggest that Rac1 regulates adherens junctions via clathrin independent endocytosis of E-cadherin.
已知人表皮角质形成细胞中钙黏蛋白依赖性细胞间接触的建立受Rac1小GTP结合蛋白调控，尽管Rac1参与细胞间黏附组装或破坏的机制尚不清楚。在本研究中，我们利用绿色荧光蛋白（GFP）标记的Rac1表达载体来检测Rac1的亚细胞分布及其对E-钙黏蛋白介导的细胞间黏附的影响。用组成型激活的Rac1显微注射角质形成细胞导致细胞铺展和细胞间接触的破坏。Rac1破坏细胞间黏附的能力取决于集落大小，已形成的大集落对活性Rac1的作用具有抗性。通过将E-钙黏蛋白-连环蛋白复合物选择性募集到多个大型细胞内囊泡的周边，实现了小的预汇合集落中细胞间接触的破坏，这些囊泡由GFP标记的L61Rac1界定。当通过去除细胞外钙或使用E-钙黏蛋白阻断抗体破坏细胞间黏附时，在未注射的角质形成细胞中也观察到类似的囊泡。此外，这些结构在未注射的角质形成细胞中的形成依赖于内源性Rac1活性。在角质形成细胞中表达GFP标记的Rac1效应突变体表明，肌动蛋白细胞骨架的重组对囊泡形成很重要。对这些Rac1诱导的囊泡的表征显示，它们本质上是内体，并且与转铁蛋白受体紧密共定位，转铁蛋白受体是再循环内体的标志物。GFP-L61Rac1的表达抑制了转铁蛋白-生物素的摄取，表明E-钙黏蛋白的内吞作用是一种不依赖网格蛋白的机制。这一观点得到了如下观察结果的支持：小窝蛋白而非网格蛋白定位于这些结构周围。此外，一种已知可抑制小窝内吞作用的动力蛋白抑制形式，抑制了钙黏蛋白囊泡的形成。我们的数据表明，Rac1通过E-钙黏蛋白的不依赖网格蛋白的内吞作用来调节黏着连接。
