ORGANIZATION OF THE ENDOPLASMIC-RETICULUM IN RENAL-CELL LINES MDCK AND LLC-PK1

The spatial organization of the endoplasmic reticulum has been studied in two renal cell lines, MDCK and LLC-PK1, which originate from the d istal and proximal portions of the mammalian nephron, respectively, an d which form a polarized epithelium when they reach confluence in tiss ue culture. The two renal cell lines, grown to confluence on either so lid or permeable supports, were investigated by fluorescence microscop y, confocal microscopy, and transmission electron microscopy. Fluoresc ence labeling of the endoplasmic reticulum was achieved using the cati onic fluorescent dye DIOC6 (3). In order to differentiate fluorescent labeling of the endoplasmic reticulum from that of the mitochondria, c ells were also labeled with rhodamine 123. For electron microscopy, th e spatial organization of the endoplasmic reticulum was examined in th ick sections using the long-duration osmium impregnation technique or the ferrocyanide/osmium technique. In both cell lines, the endoplasmic reticulum formed an abundant tubular network of canaliculi that frequ ently abutted the basolateral domain of the plasma membrane and occasi onally the apical membrane. Elements of the endoplasmic reticulum were also found in close proximity to mitochondria that, as in the nephron , formed branched structures. Canaliculi appeared circular or flattene d and had an inner diameter of 10-70 nm for MDCK cells and 20-90 nm fo r LLC-PK1 cells. Such a three-dimensional organization might facilitat e the translocation of defined lipid species between the endoplasmic r eticulum and the plasma membrane, and between the endoplasmic reticulu m and mitochondria.