Bonzelius F, Herman G A, Cardone M H, Mostov K E, Kelly R B
Department of Biochemistry and Biophysics and Hormone Research Institute, University of California, San Francisco, California 94143.
J Cell Biol. 1994 Dec;127(6 Pt 1):1603-16. doi: 10.1083/jcb.127.6.1603.
We have expressed in neuroendocrine PC12 cells the polymeric immunoglobulin receptor (pIgR), which is normally targeted from the basolateral to the apical surface of epithelial cells. In the presence of nerve growth factor, PC12 cells extend neurites which contain synaptic vesicle-like structures and regulated secretory granules. By immunofluorescence microscopy, pIgR, like the synaptic vesicle protein synaptophysin, accumulates in both the cell body and the neurites. On the other hand, the transferrin receptor, which normally recycles at the basolateral surface in epithelial cells, and the cation-independent mannose 6-phosphate receptor, a marker of late endosomes, are largely restricted to the cell body. pIgR internalizes ligand into endosomes within the cell body and the neurites, while uptake of ligand by the low density lipoprotein receptor occurs primarily into endosomes within the cell body. We conclude that transport of membrane proteins to PC12 neurites as well as to specialized endosomes within these processes is selective and appears to be governed by similar mechanisms that dictate sorting in epithelial cells. Additionally, two types of endosomes can be identified in polarized PC12 cells by the differential uptake of ligand, a housekeeping type in the cell bodies and a specialized endosome in the neurites. Recent findings suggest that specialized axonal endosomes in neurons are likely to give rise to synaptic vesicles (Mundigl, O., M. Matteoli, L. Daniell, A. Thomas-Reetz, A. Metcalf, R. Jahn, and P. De Camilli. 1993. J. Cell Biol. 122:1207-1221). Although pIgR reaches the specialized endosomes in the neurites of PC12 cells, we find by subcellular fractionation that under a variety of conditions it is efficiently excluded from synaptic vesicle-like structures as well as from secretory granules.
我们已在神经内分泌PC12细胞中表达了聚合免疫球蛋白受体（pIgR），该受体通常从上皮细胞的基底外侧靶向至顶端表面。在神经生长因子存在的情况下，PC12细胞伸出含有突触小泡样结构和调节性分泌颗粒的神经突。通过免疫荧光显微镜观察，pIgR与突触小泡蛋白突触素一样，在细胞体和神经突中均有积累。另一方面，通常在上皮细胞基底外侧表面循环的转铁蛋白受体以及晚期内体标志物阳离子非依赖性甘露糖6-磷酸受体，主要局限于细胞体。pIgR将配体内化到细胞体和神经突内的内体中，而低密度脂蛋白受体对配体的摄取主要发生在细胞体内的内体中。我们得出结论，膜蛋白向PC12神经突以及这些过程中的特化内体的转运是选择性的，并且似乎受与上皮细胞中分拣过程相同的机制支配。此外，通过配体的差异摄取，可以在极化的PC12细胞中鉴定出两种类型的内体，一种是细胞体中的管家型，另一种是神经突中的特化内体。最近的研究结果表明，神经元中的特化轴突内体可能会产生突触小泡（Mundigl, O., M. Matteoli, L. Daniell, A. Thomas-Reetz, A. Metcalf, R. Jahn, and P. De Camilli. 1993. J. Cell Biol. 122:1207 - 1221）。尽管pIgR到达了PC12细胞神经突中的特化内体，但我们通过亚细胞分级分离发现，在各种条件下，它都能有效地被排除在突触小泡样结构以及分泌颗粒之外。
