Exocytosis of IgG as mediated by the receptor, FcRn: An analysis at the single-molecule level
Exocytosis of IgG as mediated by the receptor, FcRn: An analysis at the single-molecule level
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DOI：
10.1073/pnas.0402970101
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发表时间：
2004-07-27
影响因子：
11.1
通讯作者：
Ward, ES
中科院分区：
文献类型：
--
作者：
Ober, RJ;Martinez, C;Ward, ES
IgG在细胞内和细胞间的转运对于有效的体液免疫是必不可少的。通过生物化学和体内分析的组合，已知MHC I类相关的新生儿Fc受体（FcRn）在细胞内和跨细胞递送IgG中发挥核心作用。然而，对于从表达FcRn的细胞中胞吐IgG所涉及的分子和细胞机制知之甚少。在这里，我们使用单分子荧光显微镜来分析在FcRN-GFP转染的人内皮细胞的胞吐过程。我们表明，胞吐作用可以通过多种模式发生，范围从胞吐囊泡与质膜的完全融合到仅涉及膜内容物的部分混合的缓慢释放模式（“延长释放”）。即使对于延长的释放，也可能发生FcRn扩散到质膜中，表明FcRn直接参与IgG胞吐。缓慢释放模式的特征在于IgG的周期性逐步释放，而不是完全融合事件中观察到的快速爆发。单分子轨迹分析表明，IgG可能在胞吐后与FcRn结合几秒钟。出乎意料的是，在扩散出胞吐位点后，IgG和FcRn分子也可以迁移回释放位点的中心。这种逆行运动可能代表FcRn回收的机制。我们的研究提供了深入了解导致IgG胞吐的事件。
IgG transport within and across cells is essential for effective humoral immunity. Through a combination of biochemical and in vivo analyses, the MHC class I-related neonatal Fc receptor (FcRn) is known to play a central role in delivering IgGs within and across cells. However, little is known about the molecular and cellular mechanisms that are involved in the exocytosis of IgG from cells that express FcRn. Here, we use single-molecule fluorescence microscopy to analyze exocytic processes in FcRn-GFP-transfected human endothelial cells. We show that exocytosis can occur by means of multiple modes that range from complete fusion of the exocytic vesicle with the plasma membrane to a slower-release mode ("prolonged release") that only involves partial mixing of membrane contents. Even for prolonged release, diffusion of FcRn into the plasma membrane can occur, indicating that FcRn is directly involved in IgG exocytosis. The slower-release mode is characterized by periodic, stepwise release of IgG, rather than the rapid burst that is observed for complete-fusion events. Analyses of single-molecule tracks suggest that IgG may be bound to FcRn for several seconds after exocytosis. Unexpectedly, after diffusion out of the exocytic site, IgG and FcRn molecules can also migrate back into the epicenter of the release site. Such retrograde movement may represent a mechanism for FcRn retrieval. Our studies provide insight into the events that lead to IgG exocytosis.