Wilson J M, Colton T L
Department of Cell Biology and Anatomy, Steele Memorial Children's Research Center, University of Arizona, Tucson 85724, USA.
J Cell Biol. 1997 Jan 27;136(2):319-30. doi: 10.1083/jcb.136.2.319.
Polarized cells such as epithelial cells and neurons have distinct endosomal compartments associated with different plasma membrane domains. The endosomes of the neuronal cell body and the basolateral cytoplasm of epithelial cells are thought to perform cellular "housekeeping" functions such as the uptake of nutrients and metabolites, while the endosomes in the apical cytoplasm or axons are thought to be specialized for the sorting and transcytosis of cell type-specific ligands and receptors. However, it is not known if nonpolarized cells such as fibroblasts contain a specialized endosomal compartment analogous to the specialized endosomes found in neurons and epithelia. We have expressed a protein that is normally found in the apical early endosomes of developing intestinal epithelial cells in normal rat kidney fibroblasts. This apical endosomal marker, called endotubin, is targeted to early endosomes in transfected fibroblasts, and is present in peripheral as well as perinuclear endosomes. The peripheral endosomes that contain endotubin appear to exclude transferrin, fluid phase markers, and the mannose-6-phosphate receptor, although in the perinuclear region colocalization of endotubin and these markers is present. In addition, endotubin positive structures do not tubulate in response to brefeldin A and instead redistribute to a diffuse perinuclear location. Since this endosomal compartment has many of the characteristics of an apical or axonal endosomal compartment, our results indicate that nonpolarized cells also contain a specialized early endosomal compartment.
极化细胞，如上皮细胞和神经元，具有与不同质膜结构域相关的独特内体区室。神经元细胞体的内体以及上皮细胞的基底外侧细胞质被认为执行细胞的“内务管理”功能，如营养物质和代谢产物的摄取，而顶端细胞质或轴突中的内体则被认为专门用于细胞类型特异性配体和受体的分选及转胞吞作用。然而，尚不清楚诸如成纤维细胞等非极化细胞是否含有类似于在神经元和上皮细胞中发现的特化内体的特化内体区室。我们在正常大鼠肾成纤维细胞中表达了一种通常存在于发育中的肠上皮细胞顶端早期内体中的蛋白质。这种顶端内体标记物称为内管蛋白，在转染的成纤维细胞中靶向早期内体，并且存在于外周以及核周内体中。含有内管蛋白的外周内体似乎排除了转铁蛋白、液相标记物和甘露糖-6-磷酸受体，尽管在内核周区域内管蛋白与这些标记物存在共定位。此外，内管蛋白阳性结构不会因布雷菲德菌素A而形成管状，而是重新分布到弥漫性的核周位置。由于这个内体区室具有许多顶端或轴突内体区室的特征，我们的结果表明非极化细胞也含有一个特化的早期内体区室。
