Mari Muriel, Bujny Miriam V, Zeuschner Dagmar, Geerts Willie J C, Griffith Janice, Petersen Claus M, Cullen Pete J, Klumperman Judith, Geuze Hans J
Cell Microscopy Center, Department of Cell Biology, Institute of Biomembranes, University Medical Centre (UMC) Utrecht, AZU Rm G02.525, 3584 CX Utrecht, The Netherlands.
Traffic. 2008 Mar;9(3):380-93. doi: 10.1111/j.1600-0854.2007.00686.x. Epub 2007 Dec 9.
Mannose-6-phosphate receptors (MPRs) transport lysosomal hydrolases from the trans Golgi network (TGN) to endosomes. Recently, the multi-ligand receptor sortilin has also been implicated in this transport, but the transport carriers involved herein have not been identified. By quantitative immuno-electron microscopy, we localized endogenous sortilin of HepG2 cells predominantly to the TGN and endosomes. In the TGN, sortilin colocalized with MPRs in the same clathrin-coated vesicles. In endosomes, sortilin and MPRs concentrated in sorting nexin 1 (SNX1)-positive buds and vesicles. SNX1 depletion by small interfering RNA resulted in decreased pools of sortilin in the TGN and an increase in lysosomal degradation. These data indicate that sortilin and MPRs recycle to the TGN in SNX1-dependent carriers, which we named endosome-to-TGN transport carriers (ETCs). Notably, ETCs emerge from early endosomes (EE), lack recycling plasma membrane proteins and by three-dimensional electron tomography exhibit unique structural features. Hence, ETCs are distinct from hitherto described EE-derived membranes involved in recycling. Our data emphasize an important role of EEs in recycling to the TGN and indicate that different, specialized exit events occur on the same EE vacuole.
甘露糖-6-磷酸受体（MPRs）将溶酶体水解酶从反式高尔基体网络（TGN）转运至内体。最近，多配体受体sortilin也被认为参与了这一转运过程，但其中涉及的转运载体尚未明确。通过定量免疫电子显微镜技术，我们将HepG2细胞内源性sortilin主要定位到TGN和内体。在TGN中，sortilin与MPRs共定位于同一网格蛋白包被小泡中。在内体中，sortilin和MPRs集中在分选连接蛋白1（SNX1）阳性的芽和小泡中。用小干扰RNA敲低SNX1导致TGN中sortilin池减少，溶酶体降解增加。这些数据表明，sortilin和MPRs在依赖SNX1的载体中循环至TGN，我们将其命名为内体到TGN转运载体（ETCs）。值得注意的是，ETCs源自早期内体（EE），缺乏循环质膜蛋白，并且通过三维电子断层扫描显示出独特的结构特征。因此，ETCs不同于迄今所描述的参与循环的源自EE的膜。我们的数据强调了EE在循环至TGN中的重要作用，并表明在同一个EE液泡上发生了不同的、专门的出芽事件。
