Epithelial cells require apical–basal plasma membrane polarity to perform crucial vectorial transport functions and cytoplasmic polarity to generate different cell progenies for tissue morphogenesis. The establishment and maintenance of a polarized epithelial cell with apical, basolateral and ciliary surface domains is guided by an epithelial polarity programme (EPP) that is controlled by a network of protein and lipid regulators. The EPP is organized in response to extracellular cues and is executed through the establishment of an apical-basal axis, intercellular junctions, epithelial–specific cytoskeletal rearrangements and a polarized trafficking machinery. Recent studies have provided insight on the interactions of the EPP with the polarized trafficking machinery and how they regulate epithelial polarization and depolarization.

Polarized epithelial cells play fundamental roles in the ontogeny and function of a variety of tissues and organs in mammals. The morphogenesis of a sheet of polarized epithelial cells (the trophectoderm) is the first overt sign of cellular differentiation in early embryonic development. In the adult, polarized epithelial cells line all body cavities and occur in tissues that carry out specialized vectorial transport functions of absorption and secretion. The generation of this phenotype is a multistage process requiring extracellular cues and the reorganization of proteins in the cytoplasm and on the plasma membrane; once established, the phenotype is maintained by the segregation and retention of specific proteins and lipids in distinct apical and basal-lateral plasma membrane domains.

Experiments using mammalian epithelial cell lines have elucidated biosynthetic and recycling pathways for apical and basolateral plasma-membrane proteins, and have identified components that guide apical and basolateral proteins along these pathways. These components include apical and basolateral sorting signals, adaptors for basolateral signals, and docking and fusion proteins for vesicular trafficking. Recent live-cell-imaging studies provide a real-time view of sorting processes in epithelial cells, including key roles for actin, microtubules and motors in the organization of post-Golgi trafficking.

In budding yeast, the Sec6/8p complex is essential for generating cell polarity by specifying vesicle delivery to the bud tip. We show that Sec6/8 homologs are components of a cytosolic, approximately 17S complex in nonpolarized MDCK epithelial cells. Upon initiation of calcium-dependent cell-cell adhesion, approximately 70% of Sec6/8 is rapidly (t(1/2) approximately 3-6 hr) recruited to sites of cell-cell contact. In streptolysin-O-permeabilized MDCK cells, Sec8 antibodies inhibit delivery of LDL receptor to the basal-lateral membrane, but not p75NTR to the apical membrane. These results indicate that lateral membrane recruitment of the Sec6/8 complex is a consequence of cell-cell adhesion and is essential for the biogenesis of epithelial cell surface polarity.

Phagocytosis of shed photoreceptor rod outer segments (ROS) by the retinal pigment epithelium (RPE) is essential for retinal function. Here, we demonstrate that this process requires ␣v␤5 integrin, rather than ␣v␤3 integrin utilized by systemic macrophages. Although adult rat RPE expressed both ␣v␤3 and ␣v␤5 integrins, only ␣v␤3 was expressed at birth, when the retina is immature and phagocytosis is absent. Expression of ␣v␤5 was first detected in RPE at PN7 and reached adult levels at PN11, just before onset of phagocytic activity. Interestingly, ␣v␤5 localized in vivo to the apical plasma membrane, facing the photoreceptors, and to intracellular vesicles, whereas ␣v␤3 was expressed basolaterally. Using quantitative f luorimaging to assess in vitro uptake of f luorescent particles by human (ARPE-19) and rat (RPE-J) cell lines, ␣v␤5 function-blocking antibodies were shown to reduce phagocytosis by drastically decreasing (85%) binding of ROS but not of latex beads. In agreement with a role for ␣v␤5 in phagocytosis, immunof luorescence experiments demonstrated codistribution of ␣v␤5 integrin with internalized ROS. Control experiments showed that blocking ␣v␤3 function with antibodies did not inhibit ROS phagocytosis and that ␣v␤3 did not colocalize with phagocytosed ROS. Taken together, our results indicate that the RPE requires the integrin receptor ␣v␤5 specifically for the binding of ROS and that phagocytosis involves internalization of a ROS-␣v␤5 complex. ␣v␤5 integrin does not participate in phagocytosis by other phagocytic cells and is the first of the RPE receptors involved in ROS phagocytosis that may be specific for this process.Among the vital functions performed by the retinal pigment epithelium (RPE) (1) is the phagocytosis of rod outer segments (ROS) fragments (2). At birth, rat RPE cells lack phagocytic ability (3, 4). During postnatal retinal maturation, the RPE forms long, apical microvilli that ensheath developing photoreceptor outer segments. From about PN12, stacks of ROS membranes are shed daily from the distal end of photoreceptors and become efficiently phagocytosed by RPE cells (5). The essential role of RPE phagocytosis is highlighted by the rapid degeneration of photoreceptor neurons in Royal College of Surgeons rats. Royal College of Surgeons rats carry an autosomal recessive mutation that impairs RPE phagocytosis, resulting in subretinal accumulation of ROS (3, 6, 7). Photoreceptor death is irreversible and inevitably results in blindness (8, 9). RPE phagocytosis is poorly understood, compared with the well characterized phagocytosis by monocyte macrophages. RPE and systemic phagocytosis differ in that the former follows a circadian rhythm in many species (10). Furthermore, although RPE cells express Fc receptors, they highly favor ROS binding and uptake over internalization of opsonized bacteria, yeast or inert particles (11). Of special relevance to RPE phagocytosis is the phagocytosis of apoptotic cells by circulating macrophages. Clearance of senescent cells by monocyte macro...