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

Golgi-derived vesicles from developing epithelial cells bind actin filaments and possess myosin-I as a cytoplasmically oriented peripheral membrane protein

In the intestinal brush border, the mechanoenzyme myosin-I links the microvillus core actin filaments with the plasma membrane. Previous immunolocalization shows that myosin-I is associated with vesicles in mature enterocytes (Drenckhahn, D., and R. Dermietzel. 1988. J. Cell Biol. 107:1037-1048) suggesting a potential role mediating vesicle motility. We now report that myosin-I is associated with Golgi-derived vesicles isolated from cells that are rapidly assembling brush borders in intestinal crypts. Crypt cells were isolated in hyperosmotic buffer, homogenized, and fractionated using differential- and equilibrium-density centrifugation. Fractions containing 50-100-nm vesicles, a similar size to those observed in situ, were identified by EM and were shown to contain myosin-I as demonstrated by immunoblotting and immunolabel negative staining. Galactosyltransferase, a marker enzyme for trans-Golgi membranes was present in these fractions, as was alkaline phosphatase, which is an apical membrane targeted enzyme. Galactosyltransferase was also present in vesicles immuno-purified with antibodies to myosin-I. Villin, a marker for potential contamination from fragmented microvilli, was absent. Myosin-I was found to reside on the vesicle "outer" or cytoplasmic surface for it was accessible to exogenous proteases and intact vesicles could be immunolabeled with myosin-I antibodies in solution. The bound myosin-I could be extracted from the vesicles using NaCl, KI and Na2CO3, suggesting that it is a vesicle peripheral membrane protein. These vesicles were shown to bundle actin filaments in an ATP-dependent manner. These results are consistent with a role for myosin-I as an apically targeted motor for vesicle translocation in epithelial cells.

Lysosome recruitment and fusion are early events required for trypanosome invasion of mammalian cells

Trypanosoma cruzi invades most nucleated cells by a mechanism distinct from classical phagocytosis. Although parasites enter at the lysosome-poor peripheral cell margins, lysosomal markers are immediately incorporated into the parasitophorous vacuole. No accumulation of polymerized actin was detected around recently internalized parasites, and disruption of microfilaments significantly facilitated invasion. Lysosomes were observed to aggregate at the sites of trypanosome attachment and to fuse with the vacuole at early stages of its formation. Experimentally induced, microtubule-dependent movement of lysosomes from the perinuclear area to the cell periphery enhanced entry. Conditions that deplete cells of peripheral lysosomes or interfere with lysosomal fusion capacity inhibited invasion. These observations reveal a novel mechanism for cell invasion:recruitment of lysosomes for fusion at the site of parasite internalization.

Antimicrotubule drugs inhibit the polarized insertion of an intracellular glycoprotein pool into the apical membrane of Madin-Darby canine kidney (MDCK) cells

Previous experiments on MDCK cells have demonstrated that the polarized appearance of a 135 kDa glycoprotein (gp135) on the apical plasma membrane can occur through the insertion of both newly synthesized gp135 as well as a pre-existing gp135 intracellular pool. In this study, anticytoskeletal drugs were utilized to determine the role of the cytoskeleton in the polarized delivery of gp135. Colchicine and nocodazole produced a 15–20% inhibition in the apical surface accumulation of newly synthesized gp135 and inhibited the appearance of the gp135 pool by approximately 33%, while cytochalasin D had no affect on the apical accumulation of either newly synthesized gp135 or the gp135 pool. These results indicate that microtubules, but not microfilaments, are involved in the intracellular targeting of gp135. Quantitative immunogold electron microscopy of nocodazole-treated cells demonstrated that gp135 was not mistargeted to the basolateral membrane, suggesting the possibility that some vesicles containing gp135 did not fuse with the apical membrane and remained in the cells. These experiments demonstrate that microtubules are an important component of gp135 insertion into the apical membrane. They also suggest that gp135 resides within vesicles which have an apical membrane recognition signal and cannot fuse with the basolateral membrane. The possibility that these data, and those of others, could support a hypothesis for the presence of two constitutive apical transport pathways is discussed.

Distribution of the surface antigen HAM-4 and cytoskeleton during reformation of bile-canalicular structures in rat primary cultured hepatocytes

The distribution of rat bile-canalicular surface antigen (HAM-4 antigen) and cytoskeletal elements (microtubules, actin filaments, and cytokeratin filaments) was examined during the reformation of bile-canalicular structures (BC-structures) in primary cultures of dissociated hepatocytes obtained following collagenase perfusion. HAM-4 antigen, which initially dispersed after cell dissociation, became focused into regions of cell-to-cell contact even before formation of BC-structures. Typical bile-canalicular microvilli also appeared in these regions before the intercellular spaces were completely closed. Finally, after in vitro reformation of BC, HAM-4 antigen was localized specifically at the BC-surface. The process of BC-reformation and the intracellular organization of actin and cytokeratin filaments were not significantly affected by microtubule inhibitors (nocodazole, colcemid, and colchicine). However, the localization of HAM-4 antigen molecules at the surface of BC was disrupted by these inhibitors, suggesting that the distribution of HAM-4 antigen, which represents a marker for the reconstruction of surface polarity, is dependent on microtubule function.

Distinct pathways for basolateral targeting of membrane and secretory proteins in polarized epithelial cells

Polarized epithelial cells target distinct sets of membrane and secretory proteins to their apical and basolateral domains. Here we examine whether constitutively secreted and membrane proteins that are bound for the same domain share the same carrier vesicles. To address the issue, differential effects of microtubule depolymerization on basolateral protein targeting in the polarized Madin-Darby canine kidney II cell line were studied. We find that the basolateral insertion of the active, ouabain-binding Na+,K(+)-ATPase and of a set of very late antigen integrins is little affected by microtubule disruption. Under equivalent conditions, the basolateral secretion of the basement membrane protein laminin is strongly suppressed. More specifically, it is demonstrated that microtubules are involved in targeting laminin, but not integrins, from the compartment related to the accumulation of newly synthesized proteins at 20 degrees C (trans-Golgi network) to the basolateral domain. Our study also reveals that laminin associated with basolateral binding sites interacts with those sites only secondarily to secretion. The data provide evidence for a branch in the basolateral targeting pathway, with secreted and membrane proteins loaded into distinct carrier vesicles.

The polarized distribution of poly(A+)-mRNA-induced functional ion channels in the Xenopus oocyte plasma membrane is prevented by anticytoskeletal drugs

Foreign mRNA was expressed in Xenopus laevis oocytes. Newly expressed ion currents localized in defined plasma membrane areas were measured using the two-electrode voltage clamp technique in combination with a specially designed chamber, that exposed only part of the surface on the oocytes to channel agonists or inhibitors. Newly expressed currents were found to be unequally distributed in the surface membrane of the oocyte. This asymmetry was most pronounced during the early phase of expression, when channels could almost exclusively be detected in the animal hemisphere of the oocyte. 4 d after injection of the mRNA, or later, channels could be found at a threefold higher density at the animal than at the vegetal pole area. The pattern of distribution was observed to be similar with various ion channels expressed from crude tissue mRNA and from cRNAs coding for rat GABAA receptor channel subunits. Electron microscopical analysis revealed very similar microvilli patterns at both oocyte pole areas. Thus, the asymmetric current distribution is not due to asymmetric surface structure. Upon incubation during the expression period in either colchicine or cytochalasin D, the current density was found to be equal in both pole areas. The inactive control substance beta-lumicolchicine had no effect on the asymmetry of distribution. Colchicine was without effect on the amplitude of the expressed whole cell current. Our measurements reveal a pathway for plasma membrane protein expression endogenous to the Xenopus oocyte, that may contribute to the formation and maintenance of polarity of this highly organized cell.

Disruption of microtubules alters polarity of basement membrane proteoglycan secretion in epithelial cells

Basement membrane proteins such as the heparan sulfate proteoglycan (HSPG) are secreted in a polarized fashion from the basolateral membrane of epithelial cells. We have used the microtubule-disrupting drug colchicine to study the role of the microtubule network in directing constitutive secretion to the basolateral membrane of LLC-PK1 renal epithelial cells. Microtubule depolymerization induced by colchicine resulted in fragmentation and redistribution of fluorescently labeled trans-Golgi membranes. Increased immunofluorescent staining of HSPG was associated with these dispersed Golgi cisternae. The biosynthetic processing of HSPG was not significantly altered by the loss of microtubules or by the dispersal of the Golgi elements. The most striking effect of microtubule disruption was the loss of polarity of HSPG secretion. Immunoprecipitation studies showed that HSPG was secreted from both apical and basolateral surfaces of LLC-PK1 cells treated with colchicine, and a similar result was found for the delivery of laminin, another basement membrane protein. In contrast, there was no change in the distribution of an integral basolateral membrane protein, Na(+)-K(+)-ATPase, following colchicine treatment. Our results provide the first demonstration that microtubules are involved in the constitutive trafficking of basolateral secretory proteins. These data also suggest that there may be an inherent difference in the targeting or delivery of membrane and secretory proteins to the basolateral cell surface.

Microtubular organization and its involvement in the biogenetic pathways of plasma membrane proteins in Caco-2 intestinal epithelial cells

We characterized the three-dimensional organization of microtubules in the human intestinal epithelial cell line Caco-2 by laser scanning confocal microscopy. Microtubules formed a dense network approximately 4-microns thick parallel to the cell surface in the apical pole and a loose network 1-micron thick in the basal pole. Between the apical and the basal bundles, microtubules run parallel to the major cell axis, concentrated in the vicinity of the lateral membrane. Colchicine treatment for 4 h depolymerized 99.4% of microtubular tubulin. Metabolic pulse chase, in combination with domain-selective biotinylation, immune and streptavidin precipitation was used to study the role of microtubules in the sorting and targeting of four apical and one basolateral markers. Apical proteins have been recently shown to use both direct and transcytotic (via the basolateral membrane) routes to the apical surface of Caco-2 cells. Colchicine treatment slowed down the transport to the cell surface of apical and basolateral proteins, but the effect on the apical proteins was much more drastic and affected both direct and indirect pathways. The final effect of microtubular disruption on the distribution of apical proteins depended on the degree of steady-state polarization of the individual markers in control cells. Aminopeptidase N (APN) and sucrase-isomaltase (SI), which normally reach a highly polarized distribution (110 and 75 times higher on the apical than on the basolateral side) were still relatively polarized (9 times) after colchicine treatment. The decrease in the polarity of APN and SI was mostly due to an increase in the residual basolateral expression (10% of control total surface expression) since 80% of the newly synthesized APN was still transported, although at a slower rate, to the apical surface in the absence of microtubules. Alkaline phosphatase and dipeptidylpeptidase IV, which normally reach only low levels of apical polarity (four times and six times after 20 h chase, nine times and eight times at steady state) did not polarize at all in the presence of colchicine due to slower delivery to the apical surface and increased residence time in the basolateral surface. Colchicine-treated cells displayed an ectopic localization of microvilli or other apical markers in the basolateral surface and large intracellular vacuoles. Polarized secretion into apical and basolateral media was also affected by microtubular disruption. Thus, an intact microtubular network facilitates apical protein transport to the cell surface of Caco-2 cells via direct and indirect routes; this role appears to be crucial for the final polarity of some apical plasma membrane proteins but only an enhancement factor for others.

Disruption of microtubules alters polarity of basement membrane proteoglycan secretion in epithelial cells

Basement membrane proteins such as the heparan sulfate proteoglycan (HSPG) are secreted in a polarized fashion from the basolateral membrane of epithelial cells. We have used the microtubule-disrupting drug colchicine to study the role of the microtubule network in directing constitutive secretion to the basolateral membrane of LLC-PK1 renal epithelial cells. Microtubule depolymerization induced by colchicine resulted in fragmentation and redistribution of fluorescently labeled trans-Golgi membranes. Increased immunofluorescent staining of HSPG was associated with these dispersed Golgi cisternae. The biosynthetic processing of HSPG was not significantly altered by the loss of microtubules or by the dispersal of the Golgi elements. The most striking effect of microtubule disruption was the loss of polarity of HSPG secretion. Immunoprecipitation studies showed that HSPG was secreted from both apical and basolateral surfaces of LLC-PK1 cells treated with colchicine, and a similar result was found for the delivery of laminin, another basement membrane protein. In contrast, there was no change in the distribution of an integral basolateral membrane protein, Na+-K+-ATPase, following colchicine treatment. Our results provide the first demonstration that microtubules are involved in the constitutive trafficking of basolateral secretory proteins. These data also suggest that there may be an inherent difference in the targeting or delivery of membrane and secretory proteins to the basolateral cell surface.

polarized secretion; heparan sulfate proteoglycans; sorting; Golgi processing

Submitted on July 10, 1990

Accepted on November 12, 1990

Light and electron microscopic observations on ciliated vacuoles and cysts in the oviductal and endocervical epithelia of the rabbit

Ciliated vacuoles and intraepithelial cysts have been observed in oviductal and endocervical epithelia of rabbits. In this study, rabbits under various hormonal conditions were studied by light and transmission electron microscopy and tissue culture in an attempt to determine their distribution and origin. Ciliated vacuoles most frequently lay in the basal cytoplasm, below or beside the nucleus, and very close to the basal lamina. A few were apically located. Their average diameter was 8.8 by 5.1 microns. Cilia and microvilli projected into the vacuolar lumen. These vacuoles were located intracellularly as evidenced first by the degeneration of both their cilia and microvilli and the moderately dense matrix that often filled the vacuolar lumen, as observed by electron microscopy. Secondly, phase microscopy of the living endocervical epithelium allowed us to observe the beating of the cilia within the vacuoles, not on the surface of such cells. Thirdly, ruthenium red stained the surface glycocalyx of ciliated and secretory cells, but not that of the cilia and microvilli within the vacuoles. The intraepithelial cysts were not observed in all tissue blocks. The largest numbers were found in ovariectomized animals treated for 3 and 5 days with estradiol. More were seen in the isthmus and cervix than in the fimbria and ampulla. The cysts were located most often within the epithelium along the sides of, and at the bases of, the mucosal folds. They were lined by flattened epithelium of various combinations of secretory and ciliated cells. An unusual cell type was associated with some of the cysts and ciliated vacuoles. Its cytoplasm contained aggregates of mitochondria and vesicles whose contents varied in density. Although the genesis of the ciliated vacuoles is not certain, our results indicate that they may arise from aberrant positioning of proliferating procentrioles or from a defect in targeting or transporting the centrioles to the apical plasma membrane to serve as basal bodies. Fusion of adjacent ciliated vacuoles with lumina lined by secretory cells having deep apical invaginations appeared to contribute to the formation of cysts.

Development of the Na(+)-dependent hexose carrier in LLC-PK1 cells is dependent on microtubules

The Na(+)-dependent hexose carrier, an endogenous apical marker, develops during differentiation of LLC-PK1, an established cell line with characteristics of the proximal tubule. This development was inhibited by the microtubule-disrupting drugs, colchicine and nocodazole, while it was insensitive to lumicolchicine. This strongly suggests that microtubules are involved in the plasma membrane expression of the Na(+)-dependent hexose carrier. We also analyzed the increase in activity of endogenous apical and basolateral membrane proteins during the polarization process. The development of three apical (Na(+)-dependent hexose carrier, gamma-glutamyltransferase and alkaline phosphatase) and one basolateral membrane protein (Na+/K(+)-ATPase) was studied during the reorganization of LLC-PK1 cells into a polarized epithelium. Colchicine inhibited the rapid, transient increase in the expression of the Na(+)-dependent hexose carrier during this polarization process. A similar result was observed for the development of the other apical proteins, while the development of Na+/K(+)-ATPase seemed to be largely insensitive to colchicine. Our results are in agreement with the model that the vesicles containing the apical membrane proteins use microtubules as tracks to reach the plasma membrane. The transport of vesicles containing basolateral membrane proteins clearly occurs by a different pathway which is independent on an intact microtubular network. Since the inhibition by the microtubule-disrupting drugs was complete, it can be concluded that after disruption of microtubules, the apical vesicles do not use the basolateral pathway by default.

Effect of nocodazole on vesicular traffic to the apical and basolateral surfaces of polarized MDCK cells

A polarized cell, to maintain distinct basolateral and apical membrane domains, must tightly regulate vesicular traffic terminating at either membrane domain. In this study we have examined the extent to which microtubules regulate such traffic in polarized cells. Using the polymeric immunoglobulin receptor expressed in polarized MDCK cells, we have examined the effects of nocodazole, a microtubule-disrupting agent, on three pathways that deliver proteins to the apical surface and two pathways that deliver proteins to the basolateral surface. The biosynthetic and transcytotic pathways to the apical surface are dramatically altered by nocodazole in that a portion of the protein traffic on each of these two pathways is misdirected to the basolateral surface. The apical recycling pathway is slowed in the presence of nocodazole but targeting is not disrupted. In contrast, the biosynthetic and recycling pathways to the basolateral surface are less affected by nocodazole and therefore appear to be more resistant to microtubule disruption.

Dissociation of a 110-kD peripheral membrane protein from the Golgi apparatus is an early event in brefeldin A action

Brefeldin A (BFA) has a profound effect on the structure of the Golgi apparatus, causing Golgi proteins to redistribute into the ER minutes after drug treatment. Here we describe the dissociation of a 110-kD cytoplasmically oriented peripheral membrane protein (Allan, V. J., and T. E. Kreis. 1986. J. Cell Biol. 103:2229-2239) from the Golgi apparatus as an early event in BFA action, preceding other morphologic changes. In contrast, other peripheral membrane proteins of the Golgi apparatus were not released but followed Golgi membrane into the ER during BFA treatment. The 110-kD protein remained widely dispersed throughout the cytoplasm during drug treatment, but upon removal of BFA it reassociated with membranes during reformation of the Golgi apparatus. Although a 30-s exposure to the drug was sufficient to cause the redistribution of the 110-kD protein, removal of the drug after this short exposure resulted in the reassociation of the 110-kD protein and no change in Golgi structure. If cells were exposed to BFA for 1 min or more, however, a portion of the Golgi membrane was committed to move into and out of the ER after removal of the drug. ATP depletion also caused the reversible release of the 110-kD protein, but without Golgi membrane redistribution into the ER. These findings suggest that the interaction between the 110-kD protein and the Golgi apparatus is dynamic and can be perturbed by metabolic changes or the drug BFA.

Differential microtubule requirements for transcytosis in MDCK cells

Given the role of microtubules in directing the transport of many intracellular organelles, we investigated whether intact microtubules were also required for transcytosis across epithelia. Using polarized MDCK cells expressing receptors for the Fc domain of IgG (FcRII-B2) or polymeric immunoglobulin (pIg-R), we examined the involvement of microtubules in apical to basolateral and basolateral to apical transcytosis, respectively. While depolymerization of microtubules with nocodozole had no effect on apical to basolateral transcytosis via FcR, basolateral to apical transcytosis of dimeric IgA via pIg-R was almost completely blocked. Inhibition due to nocodozole was selective for basolateral to apical transcytosis, since neither endocytosis nor receptor recycling was significantly affected at either plasma membrane domain. As shown by confocal microscopy, the block in transcytosis was due to the inability of MDCK cells to translocate IgA-containing vesicles from the basolateral to the apical cytoplasm in the absence of an intact microtubule network. The nocodazole sensitive step could be partially by-passed, however, by allowing cells to internalize IgA at 17 degrees C prior to nocodazole treatment. Although incubation at 17 degrees C blocked release of IgA into the apical medium, it did not prevent translocation of IgA-containing vesicles to the apical cytoplasm. Thus, receptor-mediated transcytosis in opposite directions exhibits distinct requirements for microtubules, a feature which reflects the spatial organization of MDCK cells.

Microtubule perturbation inhibits intracellular transport of an apical membrane glycoprotein in a substrate-dependent manner in polarized Madin-Darby canine kidney epithelial cells

The effects of microtubule perturbation on the transport of two different viral glycoproteins were examined in infected Madin-Darby canine kidney (MDCK) cells grown on both permeable and solid substrata. Quantitative biochemical analysis showed that the microtubule-depolymerizing drug nocodazole inhibited arrival of influenza hemagglutinin on the apical plasma membrane in MDCK cells grown on both substrata. In contrast, the microtubule-stabilizing drug taxol inhibited apical appearance of hemagglutinin only when MDCK cells were grown on permeable substrata. On the basis of hemagglutinin mobility on sodium dodecyl sulfate gels and its sensitivity to endo H, it was evident that nocodazole and taxol arrested hemagglutinin at different intracellular sites. Neither drug caused a significant increase in the amount of hemagglutinin detected on the basolateral plasma membrane domain. In addition, neither drug had any noticeable effect on the transport of the vesicular stomatitis virus (VSV)-G protein to the basolateral surface. These results shed light on previous conflicting reports using this model system and support the hypothesis that microtubules play a role in the delivery of membrane glycoproteins to the apical, but not the basolateral, domain of epithelial cells.

Segregation of viral structural proteins in cultured neurons of rat spinal ganglia and cord

Cultured spinal ganglion and spinal cord neurons were used to examine the intraneuronal distribution of five structural proteins of Sendai virus by immunohistochemistry. In spinal ganglion cells the internal, cytosolic viral proteins (the nucleocapsid, polymerase and matrix proteins) were confined to the perikarya, while the envelope glycoproteins (the haemagglutinin-neuraminidase and fusion proteins) also appeared in the axon-like processes. All five proteins occurred in the dendrite-like processes of spinal cord neurons. In both types of neuron the cytosolic viral proteins showed a pattern of distribution similar to that observed for the microtubule-associated protein MAP2. The segregated occurrence of the viral envelope and cytosolic proteins in axons may prevent virus assembly in axons and limit long-distance spread of paramyxoviruses in the nervous system.

Determination of apical membrane polarity in mammary epithelial cell cultures: the role of cell-cell, cell-substratum, and membrane-cytoskeleton interactions

The membrane glycoprotein, PAS-O, is a major differentiation antigen on mammary epithelial cells and is located exclusively in the apical domain of the plasma membrane. We have used 734B cultured human mammary carcinoma cells as a model system to study the role of tight junctions, cell-substratum contacts, and submembraneous cytoskeletal elements in restricting PAS-O to the apical membrane. Immunofluorescence and immunoelectronmicroscopy experiments demonstrated that while tight junctions demarcate PAS-O distribution in confluent cultures, apical polarity could be established at low culture densities when cells could not form tight junctions with neighboring cells. In such cultures the boundary between apical and basal domains was observed at the point of cell contact with the substratum. Immunocytochemical analysis of these cell-substratum contacts revealed the absence of a characteristic basement membrane containing laminin, collagen (IV), and heparan sulfate proteoglycan. However, serum-derived vitronectin was associated with the basal cell surface and the cells were shown to express the vitronectin receptor on their basolateral membranes. Additionally, treatment of cultures with antibodies against the vitronectin receptor caused cell detachment. We suggest, then, that interactions between vitronectin and its receptor, are responsible for establishment of membrane domains in the absence of tight junctions. The role of cytoskeletal elements in restricting PAS-O distribution was examined by treating cultures with cytochalasin D, colchicine, or acrylamide. Cytochalasin D led to a redistribution of PAS-O while colchicine and acrylamide did not. We hypothesize that PAS-O is restricted to the apical membrane by interactions with a microfilament network and that the cytoskeletal organization is dependent upon cell-cell and cell-substratum interactions.

Cytoskeletal control of centrioles movement during the establishment of polarity in Madin-Darby canine kidney cells

The two centrioles that are localized close to each other and to the nucleus in single Madin-Darby Canine kidney cells (MDCK) move apart by distances as large as 13 microns after the establishment of extensive cellular junctions. Microfilaments, and possibly microtubules appear to be responsible for this separation. In fully polarized cells, the centrioles are localized just beneath the apical membrane. After disruption of intercellular junctions in low calcium medium, the centrioles move back towards the cell center. This process requires intact microtubules but happens even in the absence of microfilaments. These results indicate that the position of centrioles is determined by opposing forces produced by microtubules and microfilaments and suggest that the balance between these forces is modulated by the assembly of cellular junctions. Centriole separation appears to be an early event in the process that precedes their final positioning in the apical-most region of the polarized cell.

Microtubule-dependent retrograde transport of proteins into the ER in the presence of brefeldin A suggests an ER recycling pathway

Characteristics of brefeldin A (BFA)-induced redistribution of Golgi proteins into the endoplasmic reticulum (ER) and its relationship to an ER retrieval pathway were investigated. Retrograde movement of Golgi proteins into the ER occurred via long, tubulovesicular processes extending out of the Golgi along microtubules. Microtubule-disrupting agents (i.e., nocodazole), energy poisons, and reduced temperatures inhibited this pathway. In BFA-treated cells Golgi proteins appeared to cycle between the ER and an intermediate compartment marked by a 53 kd protein. Addition of nocodazole disrupted this dynamic cycle by preferentially inhibiting retrograde movement, causing Golgi proteins to accumulate in the intermediate compartment. In the absence of BFA, such an ER cycling pathway appeared to be followed normally by the 53 kd protein but not by Golgi proteins, as revealed by temperature shift experiments. We propose that BFA induces the interaction of the Golgi with an intermediate "recycling" compartment that utilizes a microtubule-dependent pathway into the ER.

Morphology of the differentiation and maturation of LLC-PK1 epithelia

In the present study, a stereologic approach was utilized to quantitatively assess morphological changes during the differentiation of LLC-PK1 cells into an epithelial membrane. This renal epithelial cell line has been described to undergo morphological changes during differentiation and maturation from subconfluent culture to a confluent epithelial layer. An increase in the number of apical microvilli, interpreted as an areal increase in this membrane domain was reported. This morphological differentiation was found to be accompanied by an increase in the expression of apical Na(+)-dependent hexose transport and the activities of certain brush border enzymes. Since no data are available that quantify the morphologic changes during LLC-PK1 differentiation, a quantitative morphologic-stereologic-investigation was performed for an early (6 days) and a late (12 days) state of confluence of LLC-PK1 monolayer cultures. The following morphological parameters were determined by light and electron microscopic morphometry: volume fractions (Vv) of nuclei, mitochondria, and lysosomes, and surface densities (Sv) of the apical and basolateral cell membrane domains. For the apical membrane surface, the microvillous fraction has been measured separately. Since the stereologic approach used in the present study allows the determination of absolute cell volumes, the absolute measures of organelle volumes (V) and membrane surfaces (S) per average cell can be calculated from volume and surface densities. Although no changes in cell density were found for 6 and 12 day old LLC-PK1 monolayers, indicating ceased cell proliferation due to contact inhibition, remarkable changes were found concerning the absolute cell volume and apical membrane surface. The observed increase in the apical cell surface was exclusively due to the enlarged microvillous surface fraction. This finding is in good agreement with the increased number of Na(+)-dependent hexose transporters as well as with the increased expression of apical membrane marker enzymes observed during the differentiation of LLC-PK1 monolayers.

Influence of antimicrotubular drugs on the Golgi apparatus of stomach secretory mucoid cells and small intestine absorptive cells

The effects of vinblastine and colchicine on the Golgi apparatus of stomach surface mucoid and absorptive intestinal cells were compared by cytochemical analysis. The two epithelial cells were chosen because of their different specific functions in the formation of secretory granules, the production of lysosomes and the intensity of membrane traffic in the cytoplasm. For the analysis, adult mice were injected with 1 mg/100 g b.w. of vinblastine and 1 mg/100 g b.w. of colchicine. For the demonstration of cis and trans cisternae of the Golgi apparatus, prolonged osmification, thiamine pyrophosphatase and acid phosphatase activity identification were applied. After treatment with vinblastine or colchicine, polarity of stacks in the Golgi apparatus of surface mucoid cells is preserved although the number of cisternae with thiamine pyrophosphatase or acid phosphatase activity decreases. However, the Golgi apparatus of intestinal absorptive cells completely disintegrates and only a few separated cis or trans cisternae can be identified. The main effect seems to be a reduction of vesicles which can be cytochemically identified as parts of the Golgi apparatus and an accumulation of vesicles which probably originate from budding ER. Communication between the ER and the Golgi apparatus seems to be interrupted.

The subcellular organization of Madin-Darby canine kidney cells during the formation of a polarized epithelium

Studies of the developing trophectoderm in the mouse embryo have shown that extensive cellular remodeling occurs during epithelial formation. In this investigation, confocal immunofluorescence microscopy is used to examine the three-dimensional changes in cellular architecture that take place during the polarization of a terminally differentiated epithelial cell line. Madin-Darby canine kidney cells were plated at a low density on permeable filter supports. Antibodies that specifically recognize components of the tight junction, adherens junction, microtubules, centrosomes, and the Golgi complex were used to study the spatial remodeling of the cytoarchitecture during the formation of the polarized cell layer. The immunofluorescence data were correlated with establishment of functional tight junctions as measured by transepithelial resistance and back-exchange of the cell surface, labeled with metabolites of the fluorescent lipid analogue N-(7-[4-nitrobenzo-2-oxa-1,3-diazole]) aminocaproyl sphingosine. 1 d after plating, single cells had microtubules, radiating from a broad region, that contained the centrosomes and the Golgi complex. 2 d after plating, the cells had grown to confluence and had formed functional tight junctions close to the substratum. The centrioles had split and no longer organized the microtubules which were running above and below the nucleus. The Golgi complex had spread around the nucleus. By the fifth day after plating, the final polarized state had been achieved. The junctional complex had moved greater than 10 microns upward from its basal location. The centrioles were together below the apical membrane, and the Golgi complex formed a ribbon-like convoluted structure located in the apical region above the nucleus. The microtubules were organized in an apical web and in longitudinal microtubule bundles in the apical-basal axis of the columnar cell. The longitudinal microtubules were arranged with their minus ends spread over the apical region of the cell and their plus ends toward the basal region. These findings show that there is an extensive remodeling of epithelial cytoarchitecture after formation of cell-cell contacts. Reorganization of the microtubule network results in functional polarization of the cytoplasm.

The response of the Golgi complex to microtubule alterations: the roles of metabolic energy and membrane traffic in Golgi complex organization

A striking example of the interrelation between the Golgi complex (GC) and microtubules is the reversible fragmentation and dispersal of the GC which occurs upon microtubule depolymerization. We have characterized dispersal of the GC after nocodazole treatment as well as its recovery from the dispersed state by immunofluorescent localization of beta 1, 4-galactosyltransferase in Madin-Darby bovine kidney cells. Immunofluorescent anti-tubulin staining allowed simultaneous examination of the microtubule array. Based on our results, dispersal can be divided into a three-step process: microtubule depolymerization, GC fragmentation, and fragment dispersal. In cells treated with metabolic inhibitors after microtubule depolymerization, neither fragmentation nor dispersal occur, despite the absence of assembled microtubules. Thus, fragmentation is energy dependent and not tightly linked to microtubule depolymerization. The slowing of fragmentation and dispersal by monensin or ammonium chloride, as well as progressive inhibition at less than 34 degrees C, suggest that ongoing membrane traffic is required for these processes. Similarly, recovery may be separated into four steps: microtubule depolymerization, GC fragment centralization, fragment coalescence, and polarization of the reticular GC network. Fragment centralization and coalescence were arrested by metabolic inhibitors, despite the presence of microtubules. Neither monensin nor ammonium choride inhibited GC recovery. Partial inhibition of recovery at reduced temperatures paralleled the extent of microtubule assembly. These data demonstrate that dispersal and recovery are multi-step operations, and that the individual steps differ in temperature dependence, energy dependence, and sensitivity to ionic perturbation. GC distribution and microtubule status have also been clearly dissociate, thereby proving that organization of the GC is an active process that is not simply determined by microtubule binding. Furthermore, the results indicate that ongoing intra-GC membrane traffic may participate in fragmentation and dispersal.

Steady-state distribution and biogenesis of endogenous Madin-Darby canine kidney glycoproteins: evidence for intracellular sorting and polarized cell surface delivery

We used domain-selective biotinylation/125I-streptavidin blotting (Sargiacomo, M., M. P. Lisanti, L. Graeve, A. Le Bivic, and E. Rodriguez-Boulan. 1989 J. Membr. Biol. 107:277-286), in combination with lectin precipitation, to analyze the apical and basolateral glycoprotein composition of Madin-Darby canine kidney (MDCK) cells and to explore the role of glycosylation in the targeting of membrane glycoproteins. All six lectins used recognized both apical and basolateral glycoproteins, indicating that none of the sugar moieties detected were characteristic of the particular epithelial cell surface. Pulse-chase experiments coupled with domain-selective glycoprotein recovery were designed to detect the initial appearance of newly synthesized glycoproteins at the apical or basolateral cell surface. After a short pulse with a radioactive precursor, glycoproteins reaching each surface were biotinylated, extracted, and recovered via precipitation with immobilized streptavidin. Several basolateral glycoproteins (including two sulfated proteins) and at least two apical glycoproteins (one of them the major sulfated protein of MDCK cells) appeared at the corresponding surface after 20-40 min of chase, but were not detected in the opposite surface, suggesting that they were sorted intracellularly and vectorially delivered to their target membrane. Several "peripheral" apical proteins were detected at maximal levels on the apical surface immediately after the 15-min pulse, suggesting a very fast intracellular transit. Finally, domain-selective labeling of surface carbohydrates with biotin hydrazide (after periodate oxidation) revealed strikingly different integral and peripheral glycoprotein patterns, resembling the Con A pattern, after labeling with sulfo-N-hydroxy-succinimido-biotin. The approaches described here should be useful in characterizing the steady-state distribution and biogenesis of endogenous cell surface components in a variety of epithelial cell lines.

Morphogenesis of the polarized epithelial cell phenotype

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.

Colchicine-induced redistribution of an apical membrane glycoprotein (gp330) in proximal tubules

Factors governing the selective, polarized insertion of membrane proteins are poorly understood, but some studies have suggested that microtubules are involved in the generation and maintenance of cell polarity. We have examined by immunocytochemistry the effect of the microtubule-disrupting agent, colchicine, on the cellular distribution of an endogenous glycoprotein, gp330, which is normally inserted only into the apical plasma membrane of proximal tubule epithelial cells. In control rats, gp330 was localized in the brush border and in apical invaginations and vesicles. Six hours after injection of colchicine, however, vesicles containing gp330 were dispersed throughout the entire cytoplasm of the cell. Many vesicles were packed into basolateral infoldings, close to the plasma membrane, but there was no significant insertion of gp330 into the basolateral membrane. When rabbit anti-gp330 antiserum was injected intravenously into colchicine-treated rats, immune complexes appeared in the glomerular basement membrane but could not be detected in peritubular basement membranes. This supports the conclusion that colchicine treatment does not result in the insertion of gp330 into the basolateral plasma membrane of proximal tubule cells. Our results indicate that although microtubules are involved in the accumulation of gp330-containing vesicles at the apical pole of the cell, other factors must be required for fusion with the plasma membrane to occur.

Studies of terminal differentiation of electrolyte transport in the renal proximal tubule using short-term primary cultures

There are several lines of indirect evidence suggesting that the renal tubule cells have not yet reached terminal differentiation at birth. Methods used in cell biology can now be applied to study renal ontogeny. This review describes how primary cultures of proximal tubule cells from rats can be used to investigate developmental changes in Na permeability and Na-K-ATPase-mediated transport.

Role of microtubules in polarized delivery of apical membrane proteins to the brush border of the intestinal epithelium

Colchicine- and vinblastine-induced depolymerization of microtubules (MTs) in the intestinal epithelium of rats and mice resulted in significant delivery of three apical membrane proteins (alkaline phosphatase, sucrase-isomaltase, and aminopeptidase N) to the basolateral membrane domain. In addition, typical brush borders (BBs) occurred at the basolateral cell surface, consisting of numerous microvilli that contained the four major components of the cytoskeleton of apical microvilli (actin, villin, fimbrin, and the 110-kD protein). Formation of basolateral microvilli required polymerization of actin and proceeded at glycocalyx-studded plaques that resembled the dense plaques located at the tips of apical microvilli. BBs from the basolateral membrane became internalized into BB-containing vacuoles which served as recipient organelles for newly synthesized apical membrane proteins. The BB vacuoles fused with each other and finally were inserted into the apical BB. Polarized distribution of Na+,K+-ATPase, a basolateral membrane protein, was not affected by drug-induced depolymerization of MTs. These observations indicate that Golgi-derived carrier vesicles (CVs) containing apical membrane proteins are vectorially guided to the apical cell surface by a retrograde transport along MTs. MTs are uniformly oriented towards a narrow space underneath the apical terminal web (termed subterminal space) that contains MT-organizing properties and controls polarized alignment of MTs. In contrast to apical CVs, targeting of basolateral CVs appears to be independent of MTs but demands a barrier at the apical membrane domain that prevents basolateral CVs from apical fusion (transport barrier hypothesis).

Differing polarity of the constitutive and regulated secretory pathways for von Willebrand factor in endothelial cells

von Willebrand factor (vWf) is secreted from endothelial cells by one of two pathways-a constitutive pathway and a regulated pathway originating from the Weibel-Palade bodies. The molecular form of vWf from each of these pathways differs, with the most biologically potent molecules being released from Weibel-Palade bodies (Loesberg, C., M. D. Gonsalves, J. Zandbergen, C. Willems, W. G. Van Aken, H. V. Stel, J. A. Van Mourik, and P. G. DeGroot. 1983. Biochim. Biophys. Acta. 763:160-168; Sporn, L. A., V. J. Marder, and D. D. Wagner. 1987. Cell. 46:185-190). We investigated the polarity of the two secretory pathways using human umbilical vein endothelial cells cultured on polycarbonate membrane filters which allowed sampling of media from both the apical and basolateral compartments. After metabolic labeling of cells, vWf (constitutively secreted during a 10-min period or released during a 10-min treatment with a secretagogue) was purified from the apical and basolateral chambers and subjected to gel analysis. Approximately equal amounts of vWf were constitutively secreted into both chambers, and therefore this secretory pathway appeared to be nonpolarized. On the contrary, an average of 90% of vWf released from Weibel-Palade bodies after treatment with the calcium ionophore A23187 or PMA appeared in the basolateral chamber, indicating that the regulated pathway of secretion is highly polarized. Thrombin, a secretagogue which promotes disruption of the endothelial monolayer, led to release of vWf from cells with no apparent polarity. The presence of microtubule-depolymerizing agents nocodazol and colchicine inhibited the polarized release of vWf. Ammonium chloride treatment did not disrupt the polarity of the regulated secretory pathway, indicating that maintenance of low pH in intracellular compartments was not required for the polarized delivery of preformed Weibel-Palade bodies to the plasma membrane.

Involvement of the Golgi apparatus in sorting of materials to opposite ends of frog rod retinal photoreceptors

We have studied the rod cells of retinas of Rana pipiens by phosphatase cytochemistry and immunocytochemistry. We find that the Golgi apparatus of these cells, although different in its intracellular distribution from that of other neurons, has a cis-trans organization like that of other neurons as regards morphological features and the distribution of phosphatase activities. Antibodies against opsin bind to several sacs of the rod Golgi apparatus, especially those at the trans side of the Golgi stack. This suggests that Golgi involvement in the packaging of opsin for eventual delivery to the photoreceptive outer segments of the cell involves passage through trans Golgi systems. Proteins destined for the opposite end of the cell--the presynaptic terminal--also seem to pass through trans Golgi systems, as is indicated both by immunocytochemical localization of the synaptic vesicle protein p38 (synaptophysin) and by the presence of thiamine pyrophosphatase activity in some of the synaptic vesicles. Our findings suggest that sorting of membrane proteins destined for opposite ends of the photoreceptor takes place in systems at or near the trans Golgi face.

Integral and peripheral protein composition of the apical and basolateral membrane domains in MDCK cells

Selective biotinylation of the apical or basolateral domains of confluent MDCK monolayers grown on polycarbonate filters with a water soluble biotin analog, sulfo-NHS-biotin, was employed to reveal strikingly distinct patterns of endogenous "peripheral" and "integral" membrane proteins. "Peripheral" proteins were found to be approximately fivefold more abundant with this procedure than "integral" membrane proteins, both on the apical and on the basolateral surface. The distinct apical and basal patterns were shown to depend upon the integrity of the monolayer; when the tight junctions were disrupted by preincubation in calcium-depleted medium, the patterns appeared practically indistinguishable. Two-dimensional gel electrophoresis demonstrated that only a very small percentage of the biotinylated proteins were found in similar amounts on both apical and basolateral domains. These results indicate that the sorting mechanisms that segregate apical and basolateral epithelial proteins are very strict. The simple procedure described here has clear advantages over other methods available to label apical and basal epithelial surface domains, namely, higher accessibility of the biotin probe to the basolateral membrane, possibility of purifying biotinylated proteins via immobilized streptavidin and minimal exposure of the researcher to isotopes. It should be very useful in characterizing the apical and basolateral protein compositions of other epithelial cells and in studies on the development of epithelial cell polarity.

Differential extractability of influenza virus hemagglutinin during intracellular transport in polarized epithelial cells and nonpolar fibroblasts

Biochemical changes in the influenza virus hemagglutinin during intracellular transport to the apical plasma membrane of epithelial cells were investigated in Madin-Darby canine kidney (MDCK) cells and in LLC-PK1 cells stably transfected with a hemagglutinin gene. After pulse-labeling a substantial fraction of hemagglutinin was observed to become insoluble in isotonic solutions of Triton X-100. Insolubility of hemagglutinin was detected late in the transport pathway after addition of complex sugars in the Golgi complex but before insertion of the protein in the plasma membrane. Insolubility was not dependent on oligosaccharide modification since deoxymannojirimycin (dMM), which inhibits mannose trimming, failed to prevent its onset. Insolubility was not due to assembly of virus particles at the plasma membrane because insoluble hemagglutinin was also observed in transfected cells. Hemagglutinin insolubility was also seen in MDCK cells cultured in suspension and in chick embryo fibroblasts, indicating that insolubility and plasma membrane polarity are not simply correlated. In addition to insolubility, an apparent transport-dependent reduction of the disulfide bond linking HA1 and HA2 in hemagglutinin was detected. Because of the timing of both insolubility and the loss of the disulfide bond, these modifications may be important in the delivery of the hemagglutinin to the cell surface.

Type I collagen gel induces Madin-Darby canine kidney cells to become fusiform in shape and lose apical-basal polarity

In the embryo, epithelia give rise to mesenchyme at specific times and places. Recently, it has been reported (Greenburg, G., and E. D. Hay. 1986. Dev. Biol. 115:363-379; Greenberg, G., and E. D. Hay. 1988. Development (Camb.). 102:605-622) that definitive epithelia can give rise to fibroblast-like cells when suspended within type I collagen gels. We wanted to know whether Madin-Darby canine kidney (MDCK) cells, an epithelial line, can form mesenchyme under similar conditions. Small explants of MDCK cells on basement membrane were suspended within or placed on top of extracellular matrix gels. MDCK cells on basement membrane gel are tall, columnar in shape, and ultrastructurally resemble epithelia transporting fluid and ions. MDCK explants cultured on type I collagen gel give rise to isolated fusiform-shaped cells that migrate over the gel surface. The fusiform cells extend pseudopodia and filopodia, lose cell membrane specializations, and develop an actin cortex around the entire cell. Unlike true mesenchymal cells, which express vimentin and type I collagen, fusiform cells produce both keratin and vimentin, continue to express laminin, and do not turn on type I collagen. Fusiform cells are not apically-basally polarized, but show mesenchymal cell polarity. Influenza hemagglutinin and virus budding localize to the front end or entire cell surface. Na,K-ATPase occurs intracellularly and also symmetrically distributes on the cell surface. Fodrin becomes diffusely distributed along the plasma membrane, ZO-1 cannot be detected, and desmoplakins distribute randomly in the cytoplasm. The loss of epithelial polarity and acquisition of mesenchymal cell polarity and shape by fusiform MDCK cells on type I collagen gel was previously unsuspected. The phenomenon may offer new opportunities for studying cytoplasmic and nuclear mechanisms regulating cell shape and polarity.

Nocodazole, a microtubule-active drug, interferes with apical protein delivery in cultured intestinal epithelial cells (Caco-2)

The polarized delivery of membrane proteins to the cell surface and the initial secretion of lysosomal proteins into the culture medium were studied in the polarized human intestinal adenocarcinoma cell line Caco-2 in the presence or absence of the microtubule-active drug nocodazole. The appearance of newly synthesized proteins at the plasma membrane was measured by their sensitivity to proteases added either to the apical or the basolateral surface of cells grown on nitrocellulose filters. Nocodazole was found to reduce the delivery to the cell surface of an apical membrane protein, aminopeptidase N, and to lead to its partial missorting to the basolateral surface, whereas the drug had no influence on the delivery of a basolateral 120-kD membrane protein defined by a monoclonal antibody. Furthermore, nocodazole selectively blocked the apical secretion of two lysosomal proteins, cathepsin D and acid alpha-glucosidase, whereas the drug had no influence on their basolateral secretion. These results suggest that in Caco-2 cells an intact microtubular network is important for the transport of newly synthesized proteins to the apical cell surface.

Opposing microtubule- and actin-dependent forces in the development and maintenance of structural polarity in retinal photoreceptors

We have used embryonic cells grown in vitro to study the roles of microtubules and microfilaments in the development and maintenance of the polarized shape of retinal photoreceptors. After several days in culture, isolated cone photoreceptors displayed a highly elongated, compartmentalized morphology similar to that of photoreceptors in vivo. When treated with the microtubule-depolymerizing agent nocodazole, these elongated photoreceptors became progressively shorter, eventually losing their compartmentalized structure and becoming round. Conversely, treatment with the actin-depolymerizing agent cytochalasin D caused the elongated photoreceptors to lengthen even further. Computer-assisted, quantitative analysis showed that responses of individual cells to both nocodazole and Cytochalasin D were concentration-dependent, graded, and reversible. Immunocytochemical studies suggested the presence of longitudinally oriented actin filaments and microtubules in these photoreceptors, prominent in the region that undergoes the most pronounced length changes in response to cytoskeletal inhibitors. Prior to becoming elongated, photoreceptor precursors could be accurately identified in early retinal cultures. These round cells undergo a stereotyped sequence of morphogenetic transformations during in vitro development, including elongation and compartmentalization of the cell body as well as extension of a single neurite. Treatment with either cytochalasin D or nocodazole completely blocked morphogenesis. In addition, cytochalasin D caused the development of an abnormal, elongated cell process, which formed by a microtubule-dependent mechanism. These nocodazole and cytochalasin D effects also were reversible. Taken together, these data indicate that the complex developmental transformations leading to photoreceptor polarization occur in the absence of intercellular contacts, and are predominantly controlled by intracellular cytoskeletal forces. They suggest the existence of continuously active, oppositely directed, microtubule- and actin-dependent forces, the balance of which is a determining factor in the development as well as the maintenance of the elongated, compartmentalized organization of photoreceptor cells.

The polarized distribution of an apical cell surface glycoprotein is maintained by interactions with the cytoskeleton of Madin-Darby canine kidney cells

A monoclonal antibody made against a 135-kD glycoprotein (gp135) on the plasma membrane of Madin-Darby canine kidney (MDCK) cells was used to study the development and maintenance of epithelial cell surface polarity. Immunofluorescence microscopy and immunogold electron microscopy of confluent monolayers demonstrated that gp135 had a polarized cell surface distribution and was only localized on the apical surface. The role of membrane contacts in establishing gp135 polarity was determined by plating cells in low Ca++-medium to prevent the formation of intercellular junctions. Quantitative immunogold electron microscopy demonstrated that gp135 had a polarized distribution on cells lacking membrane contacts and was observed on the apical surface at a density 24 times that of the basal membrane contacting the substratum. The possibility that gp135 was associated with components of the apical cytoskeleton was investigated using cytoskeleton-disrupting drugs. Incubation in cytochalasin D produced a clustering of both actin and gp135, and double-label fluorescence microscopy demonstrated that these proteins were colocalized. Experiments using nocodazole had no effect, suggesting that gp135 could be interacting with actin microfilaments, but not microtubules. Treatment with Triton X-100 extracted approximately 50% of the gp135 and immunofluorescence microscopy indicated that the gp135 which remained associated with the detergent-insoluble cytoskeleton had a distribution identical to that of control cells. Experiments demonstrating that gp23, a nonpolarized glycoprotein, was preferentially extracted from the apical membrane suggested that the improperly sorted apical gp23 did not interact with the cytoskeleton. These results provided evidence that the polarized cell surface distribution of gp135 was maintained through its interaction with actin in the apical cytoskeleton.

Selective anchoring in the specific plasma membrane domain: a role in epithelial cell polarity

We have studied the role of restrictions to lateral mobility in the segregation of proteins to apical and basolateral domains of MDCK epithelial cells. Radioimmunoassay and semiquantitative video analysis of immunofluorescence on frozen sections showed that one apical and three basolateral glycoproteins, defined by monoclonal antibodies and binding of beta-2-microglobulin, were incompletely extracted with 0.5% Triton X-100 in a buffer that preserves the cortical cytoskeleton (Fey, E. G., K. M. Wan, and S. Penman. 1984. J. Cell Biol. 98:1973-1984; Nelson, W. T. and P. J. Veshnock. 1986. J. Cell Biol. 103:1751-1766). The marker proteins were preferentially extracted from the "incorrect" domain (i.e., the apical domain for a basolateral marker), indicating that the cytoskeletal anchoring was most effective on the "correct" domain. The two basolateral markers were unpolarized and almost completely extractable in cells prevented from establishing cell-cell contacts by incubation in low Ca++ medium, while an apical marker was only extracted from the basal surface under the same conditions. Procedures were developed to apply fluorescent probes to either the apical or the basolateral surface of live cells grown on native collagen gels. Fluorescence recovery after photobleaching of predominantly basolateral antigens showed a large percent of cells (28-52%) with no recoverable fluorescence on the basal domain but normal fluorescence recovery on the apical surface of most cells (92-100%). Diffusion coefficients in cells with normal fluorescence recovery were in the order of 1.1 x 10(-9) cm2/s in the apical domain and 0.6-0.9 x 10(-9) cm2/s in the basal surface, but the difference was not significant. The data from both techniques indicate (a) the existence of mobile and immobile protein fractions in both plasma membrane domains, and (b) that linkage to a domain specific submembrane cytoskeleton plays an important role in the maintenance of epithelial cell surface polarity.

Junctional complexes and cell polarity in the urinary tubule

In this review, we demonstrate how differentiated membrane domains can be detected in epithelial cells using conventional light and electron microscopy, freeze-fracture electron microscopy and the immuno- and cytochemical detection of membrane components. Using specific examples from the kidney, we show how the polarized insertion of these components into either apical or basolateral plasma membrane regions on either side of the tight junction barrier is related to specific functions of principal and intercalated cells in the collecting duct. In addition, distinct basal and lateral membrane domains have been revealed in some cells that are maintained in the absence of a tight junctional barrier in the plane of the membrane. This suggests that other factors, possibly related to cytoskeletal elements, may be involved in the functional segregation of these membrane areas. We propose that epithelial cell plasma membranes should be subdivided into apical, lateral and basal regions, and that the term "basolateral" may be an oversimplification.

Cell polarity in sea urchin embryos: reorientation of cells occurs quickly in aggregates

Four apical components were used as markers for the apical end of the cell in studies centering on cell polarity in the early blastula stage of sea urchin embryos and in aggregates of cleavage stage cells. Cells were observed to maintain their polarity for several hours if dissociated and cultured in suspension. Orientation of cells in aggregates initially is random; however, within 3 hr the cells have reoriented so that their apical-basal axis corresponds to the correct inside-outside position in the aggregate. This reorientation occurs before formation of a basal lamina or a new hyalin layer in the aggregate, and appears to take place by a rotation or other movement of individual cells. The polarity within each cell is maintained during reorientation. An apical surface antigen is colocalized with concentrations of filamentous actin. Treatment of isolated cells with cytochalasin B causes the antigen to lose its apical position and eventually become distributed around the outside of the cell. Microtubules are visible radiating from two foci closely associated with the nucleus in untreated cells. Treatment of isolated cells with nocodazole leaves the apical cell surface marker and its associated actin undisturbed, but causes the nucleus to lose its apical position. Cytochalasin B and colchicine both prevent reorientation of cells in aggregates. Thus polarity appears to be a constant for the cells, and their reorientation in aggregates occurs prior to the polarized release of extraembryonic matrix and basal lamina.

Collagen receptors mediate early events in the attachment of epithelial (MDCK) cells

Madin-Darby canine kidney (MDCK) cells kept in suspension culture for 12-15 hr displayed high-affinity binding sites for 125I-lathyritic (soluble) collagen (120,000/cell, KD = 30 nM) and preferred collagens types I and IV over laminin or fibronectin as substrates during the first hour of attachment. On the other hand, after 4 hr, attachment to all four substrates was equally efficient. Upon challenge with a collagen substrate, the high-affinity sites were rapidly recruited on it (T1/2 = 6 min). Their occupancy by soluble collagen triggered the exocytosis of a second large population of low-affinity collagen binding sites that included laminin and seems to be involved in a second cell-attachment mechanism. These results are compatible with a two-step model of MDCK cell attachment to the substrate: first, via high-affinity collagen binding sites, and second, via laminin of cellular origin.