Membrane-Volume Sorting, Polarized Sorting and Apical Recycling

Previous studies of fibroblasts have demonstrated that recycling of endocytic receptors occurs through a default mechanism of membrane-volume sorting. Epithelial cells require an additional level of polar membrane sorting, but there are conflicting models of polar sorting, some suggesting that it occurs in early endosomes, others suggesting it occurs in a specialized apical recycling endosome (ARE). The relationship between endocytic sorting to the lysosomal, recycling and transcytotic path-ways in polarized cells was addressed by characterizing the endocytic itineraries of LDL, transferrin (Tf) and IgA, respectively, in polarized Madin–Darby canine kidney (MDCK) cells. Quantitative analyses of 3-dimensional im-ages of living and fixed polarized cells demonstrate that endocytic sorting occurs sequentially. Initially internal