Biogenesis of epithelial cell plasma membranes

Microtubules and actin filaments are not critically involved in the biogenesis of epithelial cell surface polarity

We have studied the role of microtubules and actin filaments in the biogenesis of epithelial cell surface polarity, using influenza hemagglutinin and vesicular stomatitis G protein as model apical and basolateral proteins in infected Madin-Darby canine kidney cells. Addition of colchicine or nocodazole to confluent monolayers at concentrations sufficient to completely disassemble microtubules did not affect the asymmetric budding of influenza or vesicular stomatitis virus and only slightly reduced the typical asymmetric surface distribution of their envelope proteins, despite extensive cytoplasmic redistribution of the Golgi apparatus. Alteration of microtubular function by taxol or dissociation of actin filaments by cytochalasin D also failed to have a significant effect. Furthermore, neither colchicine nor cytochalasin D pretreatment blocked the ability of subconfluent Madin-Darby canine kidney cells to sustain polarized budding of influenza virus a few hours after attachment to the substrate. Our results indicate that domain-specific microtubule or actin filament "tracks" are not responsible for the vectorial delivery of apically or basolaterally directed transport vesicles. In conjunction with currently available evidence, they are compatible with a model in which receptors in the cytoplasmic aspect of apical or basolateral regions provide vectoriality to the transport of vesicles carrying plasma membrane proteins to their final surface localization.

Kinin effects on ion transport in monolayers of HCA-7 cells, a line from a human colonic adenocarcinoma

Using epithelial monolayers of HCA-7 cells, derived from a primary human colonic adenocarcinoma and grown on pervious supports, it is shown that responses to lysylbradykinin can be elicited from either side. It is proposed that kinin receptors are inserted into both apical and basolateral membrane domains.

An enzymatic assay reveals that proteins destined for the apical or basolateral domains of an epithelial cell line share the same late Golgi compartments

The expression of viral envelope proteins on the plasma membrane domains of the epithelial cell line, MDCK, is polar. Influenza virus infection of these cells leads to expression of the viral haemagglutinin and neuraminidase glycoproteins on the apical domain of the plasma membrane while vesicular stomatitis virus (VSV) infection yields basolateral expression of the sialic acid-bearing G protein. We have exploited the ability of the influenza neuraminidase to desialate the G protein of VSV to test for contact between these proteins during their intracellular transport to separate plasma membrane domains. We were able to select for VSV-G protein expression in doubly-infected cells because VSV protein production was accelerated in cells pre-infected with influenza virus. During double infection the envelope proteins of both viruses displayed the same polar localization as during single infection but the VSG-G protein was undersialated due to the action of the influenza neuraminidase. Incubation of singly-infected cells at 20 degrees C blocked the transport of VSV-G protein to the cell surface and resulted in increased sialation of the protein over that seen at 37 degrees C. This suggests that G protein is held in contact with the sialyl transferase at this temperature. 20 degrees C incubations of doubly-infected cells also produced the undersialated G protein characteristic of interaction with the neuraminidase. We conclude that most of the newly synthesised basolaterally-directed G protein is in physical contact with the majority of the neuraminidase through the terminal steps of Golgi processing.

Studies on the development and maintenance of epithelial cell surface polarity with monoclonal antibodies

We examined epithelial cell surface polarity in subconfluent and confluent Madin-Darby canine kidney (MDCK) cells with monoclonal antibodies directed against plasma membrane glycoproteins of 35,000, 50,000, and 60,000 mol wt. The cell surface distribution of these glycoproteins was studied by immunofluorescence and immunoelectron microscopy. At the ultrastructural level, the electron-dense reaction product localizing all three glycoproteins was determined to be uniformly distributed over the apical and basal cell surfaces of subconfluent MDCK cells as well as on the lateral surfaces between contacted cells; however, after formation of a confluent monolayer, these glycoproteins could only be localized on the basal-lateral plasma membrane. The development of cell surface polarity was followed by assessing glycoprotein distribution with immunofluorescence microscopy at selected time intervals during growth of MDCK cells to form a confluent monolayer. These results were correlated with transepithelial electrical resistance measurements of tight junction permeability and it was determined by immunofluorescence that polarized distributions of cell surface glycoproteins were established just after electrical resistance could be detected, but before the development of maximal resistance. Our observations provide evidence that intact tight junctions are required for the establishment of the apical and basal-lateral plasma membrane domains and that development of epithelial cell surface polarity is a continuous process.

Viral glycoproteins destined for apical or basolateral plasma membrane domains traverse the same Golgi apparatus during their intracellular transport in doubly infected Madin-Darby canine kidney cells

Madin-Darby canine kidney (MDCK) cells can sustain double infection with pairs of viruses of opposite budding polarity (simian virus 5 [SV5] and vesicular stomatitis virus [VSV] or influenza and VSV), and we observed that in such cells the envelope glycoproteins of the two viruses are synthesized simultaneously and assembled into virions at their characteristic sites. Influenza and SV5 budded exclusively from the apical plasma membrane of the cells, while VSV emerged only from the basolateral surfaces. Immunoelectron microscopic examination of doubly infected MDCK cells showed that the influenza hemagglutinin (HA) and the VSV G glycoproteins traverse the same Golgi apparatus and even the same Golgi cisternae. This indicates that the pathways of the two proteins towards the plasma membrane do not diverge before passage through the Golgi apparatus and therefore that critical sorting steps must take place during or after passage of the glycoproteins through this organelle. After its passage through the Golgi, the HA accumulated primarily at the apical membrane, where influenza virion assembly occurred. A small fraction of HA did, however, appear on the lateral surface and was incorporated into the envelope of budding VSV virions. Although predominantly found on the basolateral surface, significant amounts of G protein were observed on the apical plasma membrane well before disruption of the tight junctions was detectable. Nevertheless, assembly of VSV virions was restricted to the basolateral domain and in doubly infected cells the G protein was only infrequently incorporated into the envelope of budding influenza virions. These observations indicate that the site of VSV budding is not determined exclusively by the presence of G polypeptides. Therefore, it is likely that, at least for VSV, other cellular or viral components are responsible for the selection of the appropriate budding domain.

Intracellular transport of influenza virus hemagglutinin to the apical surface of Madin-Darby canine kidney cells

The intracellular pathway followed by the influenza virus hemagglutinin (HA) to the apical surface of Madin-Darby canine kidney cells was studied by radioimmunoassay, immunofluorescence, and immunoelectron microscopy. To synchronize the migration, we used a temperature-sensitive mutant of influenza WSN, ts61, which, at the nonpermissive temperature, 39.5 degrees C, exhibits a defect in the HA that prevents its exit from the endoplasmic reticulum. Upon transfer to permissive temperature, 32 degrees C, the HA appeared in the Golgi apparatus after 10 min, and on the apical surface after 30-40 min. In the presence of cycloheximide, the expression was not inhibited, indicating that the ts defect is reversible; a wave of HA migrated to the cell surface, where it accumulated with a half time of 60 min. After passage through the Golgi apparatus the HA was detected in a population of smooth vesicles, about twice the size of coated vesicles, located in the apical half of the cytoplasm. These HA-containing vesicles did not react with anti-clathrin antibodies. Monensin (10 microM) delayed the surface appearance of HA by 2 h, but not the transport to the Golgi apparatus. Incubation at 20 degrees C retarded the migration to the Golgi apparatus by approximately 30 min and blocked the surface appearance by acting at a late stage in the intracellular pathway, presumably at the level of the post-Golgi vesicles. The initial appearance of HA on the apical surface was in the center; no preference was observed for the tight-junctional regions.