Intracellular sorting and basolateral appearance of the G protein of vesicular stomatitis virus in Madin-Darby canine kidney cells

Perforated MDCK cells support intracellular transport

We have developed a method for perforating the plasma membrane of MDCK cells while retaining cellular functions. A nitrocellulose acetate filter was applied to the apical side of cells, grown on a glass coverslip, and allowed to dry. Segments of the apical plasma membrane adhered to the filter and were detached from the cell layer by shearing when the filter was peeled off. This allowed macromolecules such as antibodies and enzymes to diffuse into the cells. The cells were otherwise intact as judged by light and electron microscopy. The perforated cells maintained their capacity to support vesicular transport of proteins and lipids. Vesicular stomatitis virus infected cells readily incorporated [35S]methionine into G protein following permeabilization. This G protein was core-glycosylated during assembly in the endoplasmic reticulum, and was further transported to the trans Golgi with high efficiency. Experiments using lipid probes demonstrated that newly synthesized fluorescent sphingolipids were transported from the Golgi complex to the basolateral cell surface in perforated cells. Our results show that perforated cells provide a convenient and efficient alternative to cell-free assays for studying the molecular mechanism of intracellular transport.

Modulation of fodrin (membrane skeleton) stability by cell-cell contact in Madin-Darby canine kidney epithelial cells

During growth of Madin-Darby canine kidney (MDCK) epithelial cells, there is a dramatic change in the stability, biophysical properties, and distribution of the membrane skeleton (fodrin) which coincides temporally and spatially with the development of the polarized distribution of the Na+, K+-ATPase, a marker protein of the basolateral domain of the plasma membrane. These changes occur maximally upon the formation of a continuous monolayer of cells, indicating that extensive cell-cell contact may play an important role in the organization of polarized MDCK cells (Nelson, W. J., and P. J. Veshnock, 1986, J. Cell Biol., 103:1751-1766). To directly analyze the role of cell-cell contact in these events, we have used an assay in which the organization of fodrin and membrane proteins is analyzed in confluent monolayers of MDCK cells in the absence or presence of cell-cell contact by adjusting the concentration Ca++ in the growth medium. Our results on the stability and solubility properties of fodrin reported here show directly that there is a positive correlation between cell-cell contact and increased stability and insolubility of fodrin. Furthermore, we show that fodrin can be recruited from an unstable pool of protein to a stable pool during induction of cell-cell contact; significantly, the stabilization of fodrin is not affected by the addition of cyclohexamide, indicating that proteins normally synthesized during the induction of cell-cell contact are not required. Together these results indicate that cell-cell contact may play an important role in the development of polarity in MDCK cells by initiating the formation of a stable, insoluble matrix of fodrin with preexisting (membrane) proteins at the cell periphery. This matrix may function subsequently to trap proteins targeted to the membrane, resulting in the maintenance of membrane domains.

Membranes of sorting organelles display lateral heterogeneity in receptor distribution

This study describes the distribution of an intrinsic membrane protein, the asialoglycoprotein receptor (ASGP-R) in the trans-Golgi reticulum and compartment of uncoupling receptor and ligand (CURL) of rat liver cells. Using quantitative immunogold electron microscopy and membrane length measurements, we showed lateral nonhomogeneity of receptors in the membranes of trans-Golgi reticulum and CURL, in particular in the membranes of secretory vesicles (identified by their content of albumin and very low density lipoprotein particles) and of CURL vesicles (endosomes), including multivesicular bodies. The characteristic tubulovesicular morphology of both sorting organelles defines the transition of receptor-rich tubular membrane and the receptor-poor limiting membrane of the attached vesicles. There was a direct relationship between the size of the secretory and CURL vesicles and the density of ASGP-Rs in their membranes. Receptor density in the smallest vesicles was similar to that found in adjacent continuous tubules. The larger the vesicles, the less receptor was detectable in their membranes. We propose that the receptor molecules are excluded from the vesicle membranes by dynamic lateral redistribution. Nonrandom receptor distribution in the CURL vesicle membranes was present even at the multivesicular body stage. These observations strongly suggest the existence of barriers to ASGP-R diffusion at the junctions of tubules and vesicles. In addition, our observations suggest that ASGP-Rs are transported to the plasma membrane via a mechanism other than the normal secretory pathway.

Modulation of the expression of an apical plasma membrane protein of Madin-Darby canine kidney epithelial cells: cell-cell interactions control the appearance of a novel intracellular storage compartment

Experimental conditions that abolish or reduce to a minimum intercellular contacts between Madin-Darby canine kidney epithelial cells result in the appearance of an intracellular storage compartment for apical membrane proteins. Subconfluent culture, incubation in 1-5 microM Ca++, or inclusion of dissociated cells within agarose or collagen gels all caused the intracellular accumulation of a 184-kD apical membrane protein within large (0.5-5 micron) vacuoles, rich in microvilli. Influenza virus hemagglutinin, an apically targeted viral glycoprotein, is concentrated within these structures but the basolateral glycoprotein G of vesicular stomatitis virus and a cellular basolateral 63-kD membrane protein of Madin-Darby canine kidney cells were excluded. This novel epithelial organelle (VAC), which we designate the vacuolar apical compartment, may play an as yet unrecognized role in the biogenesis of the apical plasma membrane during the differentiation of normal epithelia.

Formation of the apical pole of epithelial (Madin-Darby canine kidney) cells: polarity of an apical protein is independent of tight junctions while segregation of a basolateral marker requires cell-cell interactions

The time course of development of polarity of an apical (184-kD) and a basolateral (63-kD) plasma membrane protein of Madin-Darby canine kidney cells was followed using semiquantitative immunofluorescence on semithin (approximately 0.5-micron) frozen sections and monoclonal antibody probes. The 184-kD protein became highly polarized to the apical pole within the initial 24 h both in normal medium and in 1-5 microM Ca2+, which results in well-spread, dome-shaped cells, lacking tight junctions and other lateral membrane interactions. In contrast, the basolateral 63-kD membrane protein developed full polarity only after incubation in normal Ca2+ concentrations for greater than 72 h, a time much longer than that required for the formation of tight junctions (approximately 18 h) and failed to polarize in 1-5 microM Ca2+. These results demonstrate that intradomain restriction mechanisms independent of tight junctions, such as self-aggregation or specific interactions with the submembrane cytoskeleton, participate in the regionalization of at least some epithelial plasma membrane proteins. The full operation of these mechanisms depends on the presence of normal cell-cell interactions in the case of the basolateral 63-kD antigen but not in the case of the apical 184-kD protein.

The large external domain is sufficient for the correct sorting of secreted or chimeric influenza virus hemagglutinins in polarized monkey kidney cells

MA104.11 rhesus kidney cells express several characteristics of polarized epithelial cells, including the formation of "domes" on impermeable substrates, the establishment of a transmonolayer electrical resistance when grown on collagen gels, the polarized maturation of influenza and vesicular stomatitis viruses, and the expression of the glycoproteins of those viruses at a single surface domain. The polarized expression of the influenza virus hemagglutinin (HA) is maintained in MA104.11 cells infected with SV40-derived vectors carrying a cDNA gene for either the wild-type influenza virus HA, a truncated HA gene encoding a secreted form of HA (HAsec), or a chimeric gene encoding a hybrid protein with the external domain of the HA and the transmembrane and cytoplasmic domains of the vesicular stomatitis virus G protein (HAG). Thus, the recognition event separating glycoproteins, such as HA, destined for the apical surface from proteins, such as G, destined for the basolateral membranes involves features of the external domains of the proteins. The transmembrane and cytoplasmic domains of HA have no role in this process.

Nonpolarized expression of a secreted murine leukemia virus glycoprotein in polarized epithelial cells

Vaccinia virus recombinants were generated which express the intact gp70/p15E of Friend mink cell focus inducing virus (F-MCFV) or truncated forms of the glycoprotein that lack the transmembrane and cytoplasmic domains. The transport of the intact and truncated envelope glycoproteins to apical or basolateral surfaces was studied in the polarized epithelial MDCK cell line. Infection of MDCK cells with the recombinant expressing the intact F-MCFV envelope glycoprotein resulted in transport exclusively to the basolateral surfaces, whereas the recombinant expressing the truncated glycoprotein was found to be secreted from both the apical and basolateral surfaces. Thus removal of the transmembrane and cytoplasmic domains of the p15E protein results in a loss of directional transport to the basolateral membrane of polarized epithelial cells.

Alteration of the cytoplasmic domain of the membrane-spanning glycoprotein p62 of Semliki Forest virus does not affect its polar distribution in established lines of Madin-Darby canine kidney cells

Expression of the Semliki Forest virus p62/E2 protein was studied in the polarized epithelial cell line Madin-Darby canine kidney (MDCK). After infection this transmembrane protein, together with the other spike subunit E1, accumulates at the basolateral surface of MDCK cells (Fuller, S. D., C.-H. von Bonsdorff, and K. Simons, 1985, EMBO (Eur. Mol. Biol. Organ.) J., 4:2475-2485). The cDNAs encoding truncated forms of the protein were used to stably transform MDCK cells to examine the role of subunit oligomerization (E1-E2) and the cytoplasmic domain of p62/E2 in directed transport to the basolateral surface. The biochemical characteristics and polarity of the expressed proteins were studied using cell monolayers grown on nitrocellulose filters. A wild-type form of p62/E2, in the absence of E1, and two forms having either 15 or 3 of the wild-type 31-amino acid carboxyl cytoplasmic domain were all localized to the basolateral surface. These results indicate that the cytoplasmic domain of E2 does not contain the information essential for directed transport to the plasma membrane, and imply that this information resides in either the lumenal and/or membrane-spanning segments of this transmembrane protein.

Deletion of the cytoplasmic domain of the polymeric immunoglobulin receptor prevents basolateral localization and endocytosis

We deleted the cytoplasmic domain of the polymeric immunoglobulin receptor. When expressed in fibroblasts, the truncated receptor, like the wild-type, reaches the cell surface, can bind ligand, and is cleaved to secretory component. Unlike the wild-type, it is not endocytosed. When expressed in polarized Madin-Darby canine kidney cells, the mutant receptor is transported from the Golgi apparatus directly to the apical surface and cleaved to secretory component. In contrast, the wild-type receptor travels from the Golgi to the basolateral surface and is then endocytosed and sent to the apical surface. These results suggest that the cytoplasmic domain of the receptor is necessary for both basolateral localization and endocytosis.

Dynamics of membrane-skeleton (fodrin) organization during development of polarity in Madin-Darby canine kidney epithelial cells

Madin-Darby canine kidney (MDCK) epithelial cells exhibit a polarized distribution of membrane proteins between the apical and basolateral domains of the plasma membrane. We have initiated studies to investigate whether the spectrin-based membrane skeleton plays a role in the establishment and maintenance of these membrane domains. MDCK cells express an isoform of spectrin composed of two subunits, Mr 240,000 (alpha-subunit) and Mr 235,000 (gamma-subunit). This isoform is immunologically and structurally related to fodrin in lens and brain cells, which is a functional and structural analog of alpha beta-spectrin, the major component of the erythrocyte membrane skeleton. Analysis of fodrin in MDCK cells by immunoblotting, immunofluorescence, and metabolic labeling revealed significant changes in the biophysical properties, subcellular distribution, steady-state levels, and turnover of the protein during development of a continuous monolayer of cells. The changes in the cellular organization of fodrin did not appear to coincide with the distributions of microfilaments, microtubules, or intermediate filaments. These changes result in the formation of a highly insoluble, relatively dense and stable layer of fodrin which appears to be localized to the cell periphery and predominantly in the region of the basolateral plasma membrane of MDCK cells in continuous monolayers. The formation of this structure coincides temporally and spatially with extensive cell-cell contact, and with the development of the polarized distribution of the Na+, K+-ATPase, a marker protein of the basolateral plasma membrane.

Transferrin receptor polarity and recycling accuracy in "tight" and "leaky" strains of Madin-Darby canine kidney cells

We have characterized the polarity of the transferrin receptor in the epithelial Madin-Darby canine kidney (MDCK) cell line. The receptor is present in approximately 165,000 copies per cell, migrates as a diffuse band upon SDS gel electrophoresis with Mr 90,000, displays a dissociation constant for diferritransferrin at neutral pH of approximately 2 nM, and is active in essentially all of the cells of the population. Transferrin-mediated 55Fe uptake was used to measure the polarity of active transferrin receptors in filter-grown MDCK cells. The ratio of basolateral to apical receptors was approximately 800:1 for the high resistance strain I MDCK cells (typically greater than 2,000 ohm X cm2) and approximately 300:1 for the lower resistance strain II cells (less than 350 ohm X cm2). In combination with morphometric data this shows that a difference in resistance between these two strains is not reflected in a significant difference in cell surface polarity. We used the recycling of transferrin receptor in filter-grown MDCK cells to evaluate the accuracy of the sorting of a basolateral protein during endocytosis. Monitoring the amount of apically released 125I-labeled transferrin after application of 55Fe- and 125I-labeled transferrin to the basolateral surface provided a sensitive assay of the accuracy of sorting during recycling of the receptor from endosomes to the plasma membrane. The accuracy of transferrin receptor sorting (greater than 99.88%) during a single cycle of transit between the endosome and the plasma membrane is sufficient to maintain the high level of polarity of the cell.

The trans Golgi network: sorting at the exit site of the Golgi complex

The Golgi complex is a series of membrane compartments through which proteins destined for the plasma membrane, secretory vesicles, and lysosomes move sequentially. A model is proposed whereby these three different classes of proteins are sorted into different vesicles in the last Golgi compartment, the trans Golgi network. This compartment corresponds to a tubular reticulum on the trans side of the Golgi stack, previously called Golgi endoplasmic reticulum lysosomes (GERL).

Intracellular sorting and polarized cell surface delivery of (Na+,K+)ATPase, an endogenous component of MDCK cell basolateral plasma membranes

Madin Darby Canine Kidney (MDCK) cells grown on polycarbonate filters in a two-chamber culture system were used to study the postsynthetic sorting of the alpha-subunit of the (Na+,K+)ATPase, an important native protein of the MDCK cell basolateral plasmalemmal domains. The N-azidobenzoyl derivative of ouabain (NAB-ouabain) and anti-ouabain antibodies were used in pulse labeling experiments to monitor the arrival of newly synthesized molecules of (Na+,K+)ATPase at the apical and basolateral cell surfaces. The results show that newly synthesized alpha-subunits bind NAB-ouabain and become substrates for immunoprecipitation only when this compound is present in the basolateral chamber. No more than 10% of the (Na+,K+)ATPase synthesized during the pulse period could appear at the apical surface without being detected by our assay. Thus, sorting of this native protein is effected intracellularly prior to its direct insertion into the basolateral plasmalemmal domain. Passage through an acidic compartment is not required for proper sorting.

Polymeric immunoglobulin receptor expressed in MDCK cells transcytoses IgA

We expressed cDNA for the rabbit polymeric immunoglobulin receptor in polarized Madin-Darby Canine Kidney epithelial cells, which normally do not produce this receptor. The receptor appeared to function as in vivo; dimeric IgA was transported from the basolateral to the apical surface and released into the apical medium, together with the cleaved fragment of the receptor, known as secretory component. This system enabled us, for the first time, to study quantitatively IgA transcytosis in vitro and thus make the following observations. First, greater than 90% of the newly made receptor that is ultimately cleaved to secretory component and released into the apical medium goes first to the basolateral surface. Second, transport of the receptor does not depend on ligand binding. Third, transcytosis of bound ligand has a t 1/2 of 30 min.

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.

The lateral mobility of the (Na+,K+)-dependent ATPase in Madin-Darby canine kidney cells

Fluorescence microphotolysis (recovery after photobleaching) was used to determine the lateral mobility of the (Na+,K+)ATPase and a fluorescent lipid analogue in the plasma membrane of Madin-Darby canine kidney (MDCK) cells at different stages of development. Fluorescein-conjugated Fab' fragments prepared from rabbit anti-dog (Na+,K+)ATPase antibodies (IgG) and 5-(N-hexadecanoyl)aminofluorescein (HEDAF) were used to label the plasma membrane of confluent and subconfluent cultures of MDCK cells. Fractional fluorescence recovery was 50% and 80-90% for the protein and lipid probes, respectively, and was independent of developmental stage. The estimated diffusion constants of the mobile fraction were approximately 5 X 10(-10) cm2/s for the (Na+,K+)ATPase and approximately 2 X 10(-9) cm2/s for HEDAF. Only HEDAF diffusion showed dependency on developmental stage in that D for confluent cells was approximately twice that for subconfluent cells. These results indicate that (Na+,K+)ATPase is 50% immobilized in all developmental stages, whereas lipids in confluent MDCK cells are more mobile than in subconfluent cells. They suggest, furthermore, that the degree of immobilization of the (Na+,K+)ATPase is insufficient to explain its polar distribution, and they support restricted mobility of the ATPase through the tight junctions as the likely mechanism for preventing the diffusion of this protein into the apical domain of the plasma membrane in confluent cell cultures.

Secretion of endogenous and exogenous proteins from polarized MDCK cell monolayers

Confluent monolayers of MDCK (Madin-Darby canine kidney) cells provide a widely used system to study the biogenesis of epithelial cell polarity. We now report that these cells are also capable of the vectorial constitutive secretion of a major endogenous product, a glycoprotein of 81 kDa, which is released into the medium from the apical surface within 30 min of its synthesis. This release represents a bona fide exocytotic secretory process and is not the result of proteolytic cleavage of a plasma membrane-associated precursor since, in cells treated with chloroquine, a protein indistinguishable from the mature secretory product accumulated intracellularly. In contrast to the vectorial secretion of the endogenous product, a variety of exogenous exocrine and endocrine proteins synthesized in MDCK cells transfected with the corresponding genes were secreted from both the apical and basolateral surfaces. These included proteins such as rat growth hormone, chicken oviduct lysozyme, bovine gastric prochymosin, and rat salivary gland alpha 2u-globulin, which in their cells of origin are secreted via a regulated pathway, as well as the liver form of the alpha 2u-globulin and the immunoglobulin kappa chain, which are normally released constitutively. These results demonstrate the existence of secretory pathways that lead to both surfaces of MDCK cells and are accessible to the foreign secretory products. They are consistent with the operation of a sorting mechanism in which the polarized secretion of the endogenous product is effected through the recognition of signals that prevent its random distribution within the fluid phase in the cellular endomembrane system.

Exit of newly synthesized membrane proteins from the trans cisterna of the Golgi complex to the plasma membrane

The intracellular location at which the G protein of vesicular stomatitis virus accumulated when transport was blocked at 20 degrees C has been studied by biochemical, cytochemical, and immunocytochemical methods. Our results indicated that the viral G protein was blocked in that cisterna of the Golgi stack which stained for acid phosphatase. At 20 degrees C this trans cisterna became structurally altered by the accumulation of G protein. This alteration was characterized by extensive areas of membrane buds which were covered by a cytoplasmic coat. These coated structures were of two kinds--those that labeled with anti-clathrin antibodies and those that did not. The clathrin-coated pits consistently did not label with anti-G antibodies. Upon warming infected cells to 32 degrees C, G protein appeared on the surface within minutes. Concomitantly, the trans cisterna lost its characteristic structural organization. Double-labeling experiments were performed in which G protein localization was combined with staining for horseradish peroxidase, which had been taken up from the extracellular medium by endocytosis. The results suggest that the trans cisterna was distinct from the endosome compartment and that the latter was not an obligatory station in the route taken by G protein to the cell surface.