Tight junctions and apical/basolateral polarity

Tight Junction Structure and Function Revisited

Tight junctions (TJs) are intercellular junctions critical for building the epithelial barrier and maintaining epithelial polarity. The claudin family of membrane proteins play central roles in TJ structure and function. However, recent findings have uncovered claudin-independent aspects of TJ structure and function, and additional players including junctional adhesion molecules (JAMs), membrane lipids, phase separation of the zonula occludens (ZO) family of scaffolding proteins, and mechanical force have been shown to play important roles in TJ structure and function. In this review, we discuss how these new findings have the potential to transform our understanding of TJ structure and function, and how the intricate network of TJ proteins and membrane lipids dynamically interact to drive TJ assembly.

Claudins and JAM-A coordinately regulate tight junction formation and epithelial polarity

Tight junctions (TJs) establish the epithelial barrier and are thought to form a membrane fence to regulate epithelial polarity, although the roles of TJs in epithelial polarity remain controversial. Claudins constitute TJ strands in conjunction with the cytoplasmic scaffolds ZO-1 and ZO-2 and play pivotal roles in epithelial barrier formation. However, how claudins and other TJ membrane proteins cooperate to organize TJs remains unclear. Here, we systematically knocked out TJ components by genome editing and show that while ZO-1/ZO-2-deficient cells lacked TJ structures and epithelial barriers, claudin-deficient cells lacked TJ strands and an electrolyte permeability barrier but formed membrane appositions and a macromolecule permeability barrier. Moreover, epithelial polarity was disorganized in ZO-1/ZO-2-deficient cells, but not in claudin-deficient cells. Simultaneous deletion of claudins and a TJ membrane protein JAM-A resulted in a loss of membrane appositions and a macromolecule permeability barrier and in sporadic epithelial polarity defects. These results demonstrate that claudins and JAM-A coordinately regulate TJ formation and epithelial polarity.

Entry of PIP3-containing polyplexes into MDCK epithelial cells by local apical-basal polarity reversal

The polarized architecture of epithelium presents a barrier to therapeutic drug/gene carriers, which is mainly due to a limited (apical) internalization of the carrier systems. The bacterium Pseudomonas aeruginosa invades epithelial cells by inducing production of apical phosphatidylinositol-3, 4, 5-triphosphate (PIP3), which results in the recruitment of basolateral receptors to the apical membrane. Since basolateral receptors are known receptors for gene delivery vectors, apical PIP3 may improve the internalization of such vectors into epithelial cells. PIP3 and nucleic acids were complexed by the cationic polymer polyethylenimine (PEI), forming PEI/PIP3 polyplexes. PEI/PIP3 polyplexes showed enhanced internalization compared to PEI polyplexes in polarized MDCK cells, while basolateral receptors were found to redistribute and colocalize with PEI/PIP3 polyplexes at the apical membrane. Following their uptake via endocytosis, PEI/PIP3 polyplexes showed efficient endosomal escape. The effectiveness of the PIP3-containing delivery system to generate a physiological effect was demonstrated by an essentially complete knock down of GFP expression in 30% of GFP-expressing MDCK cells following anti-GFP siRNA delivery. Here, we demonstrate that polyplexes can be successfully modified to mimic epithelial entry mechanisms used by Pseudomonas aeruginosa. These findings encourage the development of pathogen-inspired drug delivery systems to improve drug/gene delivery into and across tissue barriers.

Where polarity meets fusion: role of Par6 in trophoblast differentiation during placental development and preeclampsia

Trophoblast cell fusion is a prerequisite for proper human placental development. Herein we examined the contribution of Par6 (Partitioning defective protein 6), a key regulator of cell polarity, to trophoblast cell fusion in human placental development. During early placentation, Par6 localized to nuclei of cytotrophoblast cells but with advancing gestation Par6 shifted its localization to the cytoplasm and apical brush border of the syncytium. Exposure of primary isolated trophoblasts to 3% O(2) resulted in elevated Par6 expression, maintenance of tight junction marker ZO-1 at cell boundaries, and decreased fusogenic syncytin 1 expression compared with cells cultured at 20% O(2). Treatment of choriocarcinoma BeWo cells with forskolin, a known inducer of fusion, increased syncytin 1 expression but decreased that of Par6 and ZO-1. Par6 overexpression in the presence of forskolin maintained ZO-1 at cell boundaries while decreasing syncytin 1 levels. In contrast, silencing of Par6 disrupted ZO-1 localization at cell boundaries and altered the expression and distribution of acetylated α-tubulin. Par6 expression was elevated in preeclamptic placentas relative to normotensive preterm controls and Par6 located to trophoblast cells expressing ZO-1. Together, our data indicate that Par6 negatively regulates trophoblast fusion via its roles on tight junctions and cytoskeleton dynamics and provide novel insight into the contribution of this polarity marker in altered trophoblast cell fusion typical of preeclampsia.

Techniques of functional and motility test: how to perform and interpret intestinal permeability

Transport of molecules across the intestinal epithelium takes place through 2 major routes, ie, trans-cellular and paracellular. Assessment of intestinal permeability is performed to assess the overall function of transport through the intestinal epithelial paracellular route. Urinary excretion of disaccharides and monosaccharides and ratio of their excretion is a basis for measurement of intestinal permeability. Lactulose and mannitol ratio is the most commonly used test for assessment of small intestinal permeability and the most reliable method for measurement of concentration of lactulose and mannitol in the urine is high performance liquid chromatography. After the measurement of concentration of probes in the urine; the results are expressed as the ratio of percentage excretion of the ingested dose of lactulose and mannitol in the urine. Testing of intestinal permeability is not required for routine patient care, however it is an important tool to understand the function of the paracellular transport in the research setting. Increase in intestinal permeability has been implicated in the pathogenesis of many autoimmune diseases including celiac disease, Crohn's disease, type I diabetes and food allergy.

Comparison of FcRn- and pIgR-mediated transport in MDCK cells by fluorescence confocal microscopy

Protein delivery across polarized epithelia is controlled by receptor-mediated transcytosis. Many studies have examined basolateral-to-apical trafficking of polymeric IgA (pIgA) by the polymeric immunoglobulin receptor (pIgR). Less is known about apical-to-basolateral transcytosis, the direction the neonatal Fc receptor (FcRn) transports maternal IgGs across intestinal epithelia. To compare apical-to-basolateral and basolateral-to-apical transcytosis, we co-expressed FcRn and pIgR in Madin-Darby canine kidney (MDCK) cells and used pulse-chase experiments with confocal microscopy to examine transport of apically applied IgG Fcgamma and basolaterally applied pIgA. Fcgamma and pIgA trafficking routes were initially separate but intermixed at later chase times. Fcgamma was first localized near the apical surface, but became more equally distributed across the cell, consistent with concomitant transcytosis and recycling. By contrast, pIgA transport was strongly unidirectional: pIgA shifted from near the basolateral surface to an apical location with increasing time. Some Fcgamma and pIgA fluorescence colocalized in early (EEA1-positive), recycling (Rab11a-positive), and transferrin (Tf)-positive common/basolateral recycling endosomes. Fcgamma became more enriched in Tf-positive endosomes with time, whereas pIgA was sorted from these compartments. Live-cell imaging revealed that vesicles containing Fcgamma or pIgA shared similar mobility characteristics and were equivalently affected by depolymerizing microtubules, indicating that both trafficking routes depended to roughly the same extent on intact microtubules.

Domain-swapped dimerization of the second PDZ domain of ZO2 may provide a structural basis for the polymerization of claudins

Zonula occludens proteins (ZOs), including ZO1/2/3, are tight junction-associated proteins. Each of them contains three PDZ domains. It has been demonstrated that ZO1 can form either homodimers or heterodimers with ZO2 or ZO3 through the second PDZ domain. However, the underlying structural basis is not well understood. In this study, the solution structure of the second PDZ domain of ZO2 (ZO2-PDZ2) was determined using NMR spectroscopy. The results revealed a novel dimerization mode for PDZ domains via three-dimensional domain swapping, which can be generalized to homodimers of ZO1-PDZ2 or ZO3-PDZ2 and heterodimers of ZO1-PDZ2/ZO2-PDZ2 or ZO1-PDZ2/ZO3-PDZ2 due to high conservation between PDZ2 domains in ZO proteins. Furthermore, GST pulldown experiments and immunoprecipitation studies demonstrated that interactions between ZO1-PDZ2 and ZO2-PDZ2 and their self-associations indeed exist both in vitro and in vivo. Chemical cross-linking and dynamic laser light scattering experiments revealed that both ZO1-PDZ2 and ZO2-PDZ2 can form oligomers in solution. This PDZ domain-mediated oligomerization of ZOs may provide a structural basis for the polymerization of claudins, namely the formation of tight junctions.

Differences in expression patterns of the tight junction proteins,claudin 1, 3, 4 and 5, in human ovarian surface epithelium as compared to epithelia in inclusion cysts and epithelial ovarian tumours

Human ovarian surface epithelium (OSE), regarded as the precursor cell of epithelial ovarian adenocarcinoma, is not a fully developed epithelium when situated on the ovarian surface. It lacks epithelial characteristics such as the cell-cell adhesion factor epithelial (E)-cadherin, but as we have shown earlier, this OSE can form functional tight junctions (TJs) in culture. Recent gene-expression data on ovarian adenocarcinoma has pointed out a family of TJ proteins, the claudins, to be highly expressed in malignant compared to benign ovarian tumours. The purpose of this study was to investigate the distribution of claudin 1-5 proteins in cultured OSE (n=4), normal ovarian (n=11), benign (n=8), borderline (n=7) and ovarian cancer (n=22) tissues. We found that a weak or absence of expression of claudin 3 and claudin 4 in surface OSE changed to typical cell-border localisation in OSE of inclusion cysts in the normal ovarian stroma. Semiquantitative estimations of immunoblots showed that claudin 3 was significantly increased in ovarian adenocarcinomas compared to benign and borderline-type tumours. Claudin 4 was significantly increased in both borderline-type and ovarian adenocarcinomas compared to benign tumours, whereas no changes were found for claudin 1 or 5. Concerning relation to Federation for International Gynaecology and Obstetrics (FIGO) grade, claudin 3, but not claudin 4, was significantly increased in moderately, poorly and undifferentiated adenocarcinomas compared to well-differentiated and borderline-type tumours. No significant changes were noticed for any claudin with regard to FIGO stages. We conclude that both claudin 3 and 4, even though they differ in expression during ovarian malignant transformation, might be used as novel markers for ovarian tumours. The observations of lack of claudin 4 and low expression of claudin 3 in OSE strengthen our current knowledge about the biological nature of these cells as an undeveloped epithelium.

Lanthanum Effect on the Dynamics of Tight Junction Opening and Closing

We present a comparative study in frog urinary bladders (FUB) and A6 cell monolayers (A6CM) on the effect of La3+ on tight junction (TJ) dynamics. These tissues react similarly to changes of basolateral Ca2+ (Ca(2+)bl), while responding differently to the action of La3+(bl). In FUB, La(3+)bl shows a Ca(2+)-antagonistic effect that promotes TJ opening in the presence of a normal Ca(2+)bl concentration. In A6CM, in contrast, La(3+)bl always shows a clear Ca(2+)-agonistic effect. The fact that a concentration of La(3+)bl one fifth of the normal Ca(2+)bl leads in FUB to TJ opening and in A6CM to a complete recovery of the TJ seal indicates a high affinity of La3+ for the Ca(2+)-binding sites in both tissues. In FUB, apical La3+ (La(3+)ap) exhibits, differently from its basolateral effect, an evident Ca(2+)-agonistic effect, suggesting a dual effect of La3+, depending on which side of the bladder La3+ is applied. In A6CM La(3+)ap has a Ca(2+)-agonistic effect similar to La(3+)bl. The effects of La(3+)bl in FUB and in A6CM are consistent, according to our previous publications, with La3+ acting antagonistically or agonistically, respectively, on the Ca2+ binding sites of zonula adhaerens. Despite the fact that the effect of La(3+)ap is clear in both tissues, its site of action is yet to be determined. Protonation of the Ca(2+)-binding sites causes a decrease of its agonistic effect on A6CM, consistent with a negatively charged binding site. In A6CM La3+ apparently replaces Ca2+, mimicking the effect of Ca2+ triggering the cascade of events leading to TJ closure. In FUB, La3+ interacts with the binding sites, dislodging Ca2+, with a high affinity, but this interaction is inadequate to initiate or sustain the process of junction closing. Possibly, the difference between the two preparations resides in subtle conformation differences of the outer segment of E-cadherin molecules.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Formation and Barrier Function of Tight Junctions in Human Ovarian Surface Epithelium1

The normal ovarian surface epithelium (OSE) is a primitive epithelium made up by a single layer of mesothelial-type epithelial cells. When these cells get trapped in the ovarian stroma, expression of epithelial specific markers, such as E-cadherin, are induced. Most epithelial cells are also characterized by the ability to form tight junctions (TJ). Incomplete TJ have earlier been demonstrated in the OSE by electron microscopy studies. We have investigated expression and localization of the TJ proteins ZO-1, occludin, and claudin-1 in tissue biopsies from normal human ovaries and OSE in culture. The dynamics of TJ formation were studied in human OSE cultured on porous filters in culture inserts by measuring trans epithelial resistance (TER) including Ca(2+) switch experiments. Confluent OSE cells were also analyzed by electron microscopy. The results show that normal human OSE has expression of all three TJ proteins investigated. These proteins, ZO-1, occludin, and claudin-1, were localized to OSE cell borders both in ovarian biopsies and in cultured OSE. There was no difference in this regard between fertile and postmenopausal women. Cells in culture were polarized and presented junctional complexes seen by electron microscopy. In the Ca(2+) switch experiments, removing free Ca(2+) transiently, TER decreased significantly (P < 0.05) in the Ca(2+)-free group compared with nontreated OSE. TER was fully restored after 24 h. N-cadherin but not E-cadherin was expressed in the OSE and localized to the cell borders. We conclude that normal human OSE express and form functional TJ both in vivo and vitro. This report also describes a method to study the influence of ovarian-derived mediators on TJ in cultured OSE.

Targeted delivery of antiprotease to the epithelial surface of human tracheal xenografts

The cystic fibrosis (CF) lung is uniquely susceptible to Pseudomonas aeruginosa, and infection with this organism incites an intense, compartmentalized inflammatory response that leads to chronic airway obstruction and bronchiectasis. Neutrophils migrate into the airway, and released neutrophil elastase contributes to the progression of the lung disease characteristic of CF. We have developed a strategy that permits the delivery of antiproteases to the inaccessible CF airways by targeting the respiratory epithelium via the human polymeric immunoglobulin receptor (hpIgR). A fusion protein consisting of a single-chain Fv directed against secretory component, the extracellular portion of the pIgR, linked to human alpha1-antitrypsin is effectively ferried across human tracheal xenografts and delivers the antiprotease to the apical surface to a much greater extent than occurs by passive diffusion of human alpha1-antitrypsin alone. Targeted antiprotease delivery paralleled hpIgR expression in the respiratory epithelium in vivo and was not increased by escalating dose, so airway penetration was receptor-dependent, not dose-dependent. Thus, this approach provides us with the ability to deliver therapeutics, like antiproteases, specifically to the lumenal surface of the respiratory epithelium, within the airway surface fluid, where it will be in highest concentration at this site.

MAGUK proteins: structure and role in the tight junction

ZO-1, ZO-2 and ZO-3 are tight junction (TJ)-associated proteins that belong to the MAGUK family. In addition to the presence of the characteristic MAGUK modules (PDZ, SH3 and GK), ZOs have a distinctive carboxyl terminal with splicing domains, acidic- and proline-rich regions. The modular organization of these proteins allows them to function as scaffolds, which associate to transmembrane TJ proteins, the cytoskeleton and signal transduction molecules. ZOs shuttle between the TJ and the nucleus, where they may regulate gene expression.

Structural correlates of the transepithelial water transport

Transepithelial permeability is one of the fundamental problems in cell biology. Epithelial cell layers protect the organism from its environment and form a selective barrier to the exchange of molecules between the lumen of an organ and an underlying tissue. This chapter discusses some problems and analyzes the participation of intercellular junctions in the paracellular transport of water, migration of intramembrane particles in the apical membrane during its permeability changes for isotonic fluid in cells of leaky epithelia, insertion of water channels into the apical membrane and their cytoplasmic sources in cells of tight epithelia under ADH (antidiuretic hormone)-induced water flows, the osmoregulating function of giant vacuoles in the transcellular fluxes of hypotonic fluid across tight epithelia, and the role of actin filaments and microtubules in the transcellular transport of water across epithelia.

Transport of bifunctional proteins across respiratory epithelial cells via the polymeric immunoglobulin receptor

Neutrophil elastase (NE) contributes to progression of the lung disease characteristic of cystic fibrosis (CF). We developed a strategy that permits the delivery of alpha(1)-antitrypsin (alpha(1)-AT) to inaccessible CF airways by targeting the respiratory epithelium via the polymeric immunoglobulin receptor (pIgR). A fusion protein consisting of a single-chain Fv directed against human secretory component (SC) and linked to human alpha(1)-AT was effectively transported in a basolateral-to-apical direction across in vitro model systems of polarized respiratory epithelium consisting of 16HBEo cells transfected with human pIgR complementary DNA, which overexpress the receptor, and human respiratory epithelial cells grown in primary culture at an air-liquid interface. When applied to the basolateral surface, the anti-SC Fv/alpha(1)-AT fusion protein penetrated the respiratory epithelia, with transcytosis of the fusion protein being related to the amount of SC detected at the apical surface. Significantly less fusion protein crossed the cells in the opposite direction. In addition, because the antihuman SC Fv/alpha(1)-AT fusion protein was transported vectorially and deposited into the small volume of apical surface fluid, the antiprotease component of this protein was concentrated atop the epithelium. Thus, in cell models, this system is capable of concentrating the antiprotease of the fusion protein, in the thin film of epithelial surface fluid to a level expected to be therapeutic in the airways of many patients with CF.

Low density lipoprotein binding induces asymmetric redistribution of the low density lipoprotein receptors in endothelial cells

The uptake and transport of cholesterol-carrying low density lipoprotein (LDL) by the arterial wall is a continuous dynamic process, contributing to the cholesterol homeostasis in the plasma and in the cellular components of the vessel wall. Upon exposure to endothelial cells (EC), LDL interacts in part, with specific surface receptors (LDL-R). In this study we questioned: (i) the distribution of LDL receptors on the apical and basal cell membranes in endothelial cells; (ii) the role of LDL receptors in the control of cholesterol homeostasis and (iii) the translocation of LDL receptor across the EC. To this purpose bovine aortic EC were cultured on filters in a double-chamber system, in Dulbecco's medium supplemented either with 10% fetal calf serum (FCS) or with 10% lipoprotein-deficient serum (LPDS). The cells were exposed for 3h to 13H]acetate (40 microCi) added to both compartments of the cell culture inserts. The newly synthesized [3H]cholesterol was detected by thin layer chromatography and quantified by liquid scintillation counting. The LDL-R were detected in EC protein homogenates by immunoblotting using a monoclonal antibody against LDL-R (IgG-C7); the intracellular pathway of LDL-R was examined by electron microscopy using a complex made of protein A 5 nm or 20 nm colloidal gold particles and an anti-LDL receptor antibody (Au-PA-C7). To evaluate the distribution and the transport of LDL-R from one cell surface to the other, EC grown in LPDS were radioiodinated either on the apical or on the basolateral surface, incubated on the same surface with LDL, and subsequently biotinylated on the opposite non-radiolabeled surface. The EC were further solubilized and the protein extract immunoprecipitated with anti-LDL-R antibody or with mouse IgG (as control). The eluted antigen-antibody complexes were precipitated with streptavidin-agarose beads, solubilized, and subjected to SDS-PAGE. The results showed that: (a) the LDL-R were present on both endothelial cell fronts; (b) using the complex Au-PA-C7, the LDL-R were localized in endothelial plasmalemmal vesicles as well as coated pits and coated vesicles in multivesicular bodies and lysosomes, irrespective of the cell surface exposed to the complex; (c) biochemical assays indicated that upon ligand binding, the LDL-R were translocated preferentially from the apical to the basal plasma membrane.

Vinculin but not alpha-actinin is a target of PKC phosphorylation during junctional assembly induced by calcium

The establishment of the junctional complex in epithelial cells requires the presence of extracellular calcium, and is controlled by a network of reactions involving G-proteins, phospholipase C and protein kinase C. Since potential candidates for phosphorylation are the tight junction associated proteins ZO1, ZO2 and ZO3, in a previous work we specifically explored these molecules but found no alteration in their phosphorylation pattern. To continue the search for the target of protein kinase C, in the present work we have studied the subcellular distribution and phosphorylation of vinculin and alpha-actinin, two actin binding proteins of the adherent junctions. We found that during the junctional sealing induced by Ca2+, both proteins move towards the cell periphery and, while there is a significant increase in the phosphorylation of vinculin, alpha-actinin remains unchanged. The increased phosphorylation of vinculin is due to changes in phosphoserine and phosphothreonine content and seems to be regulated by protein kinase C, since: (1) DiC8 (a kinase C stimulator) added to monolayers cultured without calcium significantly increases the vinculin phosphorylation level; (2) H7 and calphostin C (both protein kinase C inhibitors) completely abolish this increase during a calcium switch; (3) inhibition of phosphorylation during a calcium switch blocks the subcellular redistribution of vinculin and alpha-actinin. These results therefore suggest that vinculin phosphorylation by protein kinase C is a crucial step in the correct assembly of the epithelial junctional complex.

Role of tight junctions in establishing and maintaining cell polarity

The tight junction (TJ) is not randomly located on the cell membrane, but occupies a precise position at the outermost edge of the intercellular space and, therefore, is itself considered a polarized structure. This article reviews the most common experimental approaches for studying this relationship. We then discuss three main topics. (a) The mechanisms of polarization that operate regardless of the presence of TJs: We explore a variety of polarization mechanisms that operate at stages of the cell cycle in which TJs may be already established. (b) TJs and polarity as partners in highly dynamic processes: Polarity and TJs are steady state situations that may be drastically changed by a variety of signaling events. (c) Polarized distribution of membrane molecules that depend on TJs: This refers to molecules (mainly lipids) whose polarized distribution, although not the direct result of TJs, depends on these structures to maintain such distribution.

Cytokines and thyroid epithelial integrity: interleukin-1alpha induces dissociation of the junctional complex and paracellular leakage in filter-cultured human thyrocytes

Locally produced proinflammatory cytokines are likely to play a pathophysiological role in autoimmune thyroid disease. An important feature of the thyroid, not previously considered in cytokine actions, is the barrier created by the follicular epithelium, which secludes two lumenal autoantigens [thyroglobulin (Tg) and thyroperoxidase] from the extrafollicular space. We examined the influence of recombinant cytokines on the barrier function of human thyrocytes cultured as a tight and polarized monolayer in bicameral chambers. Whereas interleukin (IL)-6 (100 U/mL), interferon-gamma (100 U/mL), tumor necrosis factor-alpha (10 ng/mL), and transforming growth factor-beta1 (10 ng/mL) had no effects, exposure to IL-1alpha for 24-48 h reduced the transepithelial resistance from >1000 to <50 omega x cm2 and increased the paracellular flux of [3H]inulin and exogenous 125I-Tg. This response to IL-1alpha, which was dose dependent (1-1000 U/mL) and reversible, was accompanied by dramatic morphological changes of the epithelial junction complex, including aberrant localization of the tight junction protein zonula occludens-1. At the same time, IL-1alpha decreased the apical secretion of endogenous Tg and stimulated the basolateral release of a novel high-molecular-mass protein. We conclude that IL-1alpha reduces the thyroid epithelial barrier without signs of general cytotoxicity. The observation suggests a mechanism by which IL-1alpha may promote the exposure of hidden autoantigens to the immune system in thyroid autoimmunity.

Distribution of Na+,K(+)-ATPase in photoreceptor cells of insects

Light stimulation of insect photoreceptors causes opening of cation channels and an inward current that is partially carried by Na+ ions. There is also an efflux of K+ ions upon photostimulation. Na+ and K+ gradients across the photoreceptor membrane are reestablished by the activity of the enzyme Na+,K(+)-ATPase. About two-thirds of the total amount of ATP consumed in response to a light stimulus is attributed to the activity of this ion pump, demonstrating the importance of this enzyme for photoreceptor function. Insect photoreceptor cells are polarized epithelial cells; their plasma membrane is organized into two domains having a distinct morphology, molecular composition, and function. The visual pigment rhodopsin and the molecular components of the transduction machinery are localized in the rhabdomere, an array of densely packed microvilli, whereas Na+,K(+)-ATPase resides in the nonrhabdomeric membrane. Comparative immunolocalization studies on compound eyes of diverse insect species have demonstrated subtle variations in the distribution patterns of Na+,K(+)-ATPase. These may be accounted for by differences in the mechanisms responsible for Na+,K(+)-ATPase positioning.

Neuroepithelial cells downregulate their plasma membrane polarity prior to neural tube closure and neurogenesis

Cell differentiation often involves changes in cell polarity. In this study we show that neuroepithelial cells, the progenitors of all neurons and macroglial cells of the vertebrate central nervous system, downregulate the polarized delivery to the apical and basolateral plasma membrane domains during development. Upon infection of the neuroepithelium of mouse embryos with fowl plague virus (FPV), polarized delivery of the viral envelope hemagglutinin, an apical marker, occurred at the neural plate stage (E8), but was downregulated at the open neural tube stage (E9). Upon infection with vesicular stomatitis virus, the viral envelope G protein, a basolateral marker, showed an unpolarized delivery not only at the open neural tube stage, but already at the neural plate stage. These results show that a progressive downregulation of plasma membrane polarity of neuroepithelial cells precedes neural tube closure and the onset of neurogenesis.

COOH terminus of occludin is required for tight junction barrier function in early Xenopus embryos

Occludin is the only known integral membrane protein localized at the points of membrane- membrane interaction of the tight junction. We have used the Xenopus embryo as an assay system to examine: (a) whether the expression of mutant occludin in embryos will disrupt the barrier function of tight junctions, and (b) whether there are signals within the occludin structure that are required for targeting to the sites of junctional interaction. mRNAs transcribed from a series of COOH-terminally truncated occludin mutants were microinjected into the antero-dorsal blastomere of eight-cell embryos. 8 h after injection, the full-length and the five COOH-terminally truncated proteins were all detected at tight junctions as defined by colocalization with both endogenous occludin and zonula occludens-1 demonstrating that exogenous occludin correctly targeted to the tight junction. Importantly, our data show that tight junctions containing four of the COOH-terminally truncated occludin proteins were leaky; the intercellular spaces between the apical cells were penetrated by sulfosuccinimidyl-6-(biotinamido) Hexanoate (NHS-LC-biotin). In contrast, embryos injected with mRNAs coding for the full-length, the least truncated, or the soluble COOH terminus remained impermeable to the NHS-LC-biotin tracer. The leakage induced by the mutant occludins could be rescued by coinjection with full-length occludin mRNA. Immunoprecipitation analysis of detergent-solubilized embryo membranes revealed that the exogenous occludin was bound to endogenous Xenopus occludin in vivo, indicating that occludin oligomerized during tight junction assembly. Our data demonstrate that the COOH terminus of occludin is required for the correct assembly of tight junction barrier function. We also provide evidence for the first time that occludin forms oligomers during the normal process of tight junction assembly. Our data suggest that mutant occludins target to the tight junction by virtue of their ability to oligomerize with full-length endogenous molecules.

Development of tight junctions between odontoblasts in early dentinogenesis as revealed by freeze-fracture

BACKGROUND

Mature odontoblasts possess junctional structures constituted by adherens, gap, and tight junctions. Although adherens and gap junctions appear early between odontoblasts, there is no information on the appearance and development of tight junctions between odontoblasts. In this study, we have examined freeze-fracture replicas of early dentinogenesis to study the development of tight junctions between odontoblasts and to determine whether these junctions are of zonular or macular type.

METHODS

Upper first molar tooth germs of Wistar rats between 1 and 3 days old were fixed in buffered 4% glutaraldehyde/4% formaldehyde and subsequently cryoprotected with cacodylate-buffered glycerol. Freeze-fracture replicas were obtained in a Balzers 301 apparatus, and early stages of dentinogenesis were examined in a Jeol 100 CX II electron microscope.

RESULTS

In the stage of early dentine matrix prior to mineralization, odontoblasts exhibit only gap junctions. With the progression of development, the distal plasma membranes of odontoblasts show numerous short tight junctions formed by fused particles and grooves. In the stage of advanced mineralization, branched and continuous rows of fused particles or grooves constitute tight junctions of the focal or macular type.

CONCLUSIONS

The present study shows that tight junctions of focal or macular type appear on distal plasma membrane of early odontoblasts during differentiation. Formation of tight junctions indicates the establishment of a distal membrane domain and maturation of odontoblasts. These events occur as mantle dentine formation ceases and circumpulpar dentine formation begins.

The SH3 domain of the tight junction protein ZO-1 binds to a serine protein kinase that phosphorylates a region C-terminal to this domain

ZO-1 is a tight junction phosphoprotein partially homologous to a tumor suppressor in Drosophila. The homologous region contains an SH3 domain with an unidentified function. Using fusion proteins containing the SH3 domain and various N- and C-terminal sequences, we tested for association of a kinase with this protein domain in extracts of MDCK cells. We show that the SH3 domain of ZO-1 binds a serine protein kinase that phosphorylates a region immediately C-terminal to the SH3 domain. This kinase associates specifically with the SH3 domain of ZO-1 and appears to be also associated with junctional complexes extracted from MDCK cells.

Tight junction dynamics in the frog urinary bladder

In a previous study in frog skin (Castro et al., J. Memb. Biol. 134:15-29, 1993), it was shown that TJs experimentally disrupted by a selective deposition of BaSO4 could be resealed upon addition of Ca2+ to the apical solution; in the absence of apical Ca2+, the normal Ca2+ activity of the Na2SO4-Ringer's bathing the basolateral side was not able to induce TJ resealing. We now show that apical Ca2+ also activates the TJ sealing mechanism in frog urinary bladders. Three known procedures were utilized to increase TJ permeability, all in the absence of apical Ca2+: (i) exposure to high positive transepithelial clamping potentials; (ii) exposure of the apical surface to hypertonic solutions; and (iii) selective deposition of BaSO4 in the TJs. The resealing of the TJs was promoted by raising the concentration of Ca2+ in the apical solution. This effect of Ca2+ is not impaired by the presence of Ca2+ channel blockers (nifedipine, verapamil, Mn2+ or Cd2+) in the apical solution, indicating that junction resealing does not depend on Ca2+ entering the cells through the apical membrane. TJ resealing that occurs in response to raised apical Ca2+ most likely results from a direct effect of Ca2+, entering the disrupted TJs from the apical solution and reaching the zonula adhaerens Ca2+ receptors (E-cadherins). Protein kinase C (PKC) must play a significant role in the control of TJ assembly in this tight epithelia since the PKC inhibitor (H7) and the activator (diC8) markedly affect TJ recovery after disruption by apical hypertonicity. H7 treated tissues show marked recuperation of conductance even in the absence of apical Ca2+. In contrast, diC8 prevents tissue recuperation which normally occurs after addition of Ca2+ to the apical solution.

Relationships between intermediate filaments and cell-specific functions in renal cell lines derived from transgenic mice harboring the temperature-sensitive T antigen

Four renal cell lines were derived from glomeruli, proximal, distal, and cortical collecting tubules microdissected from the kidneys of transgenic mice carrying the temperature-sensitive mutant of the simian virus 40 large T antigen under the control of the vimentin promoter. All four cell lines contained large T antigen in their nuclei, grew rapidly, and contained vimentin filaments when grown in serum-enriched medium at the permissive temperature of 33 degrees C. The glomerular cell line formed multiple layers of cells and contained smooth muscle actin and desmin filaments, features of mesangial cells. The three tubule cell lines formed monolayers of polarized cuboid cells separated by tight junctions and having a patchy distribution of cytokeratins K8-K18. A shift from 33 degrees C to the restrictive temperature (39.5 degrees C) stopped cell growth in all cell lines and caused profound changes in the content of intermediate filaments. Vimentin was still present in mesangial-like cells, but the proximal, distal, and collecting tubule cells contained uniform networks of cytokeratins K8-K18 and desmoplakin I and II around the cell peripheries. Potassium transport, mediated by Na+-K+ ATPase pumps and specific cAMP hormonal sensitivities, significantly increased in proximal, distal, and collecting tubule cells when shifted from 33 degrees C to 39.5 degrees C. Thus, the temperature-dependent inactivation of large T antigen, responsible for the arrest of cell growth, did not affect the phenotype of mesangial-like glomerular cells but induced some changes in the expression of intermediate filaments and restored, at least partially, the main parental cell-specific functions in proximal, distal, and collecting tubule cultured cells.

Absorption enhancement of hydrophilic compounds by verapamil in Caco-2 cell monolayers

Caco-2 monolayers were used to determine whether verapamil enhanced the transport of hydrophilic compounds across epithelial cells. Transepithelial electrical resistance (TEER) measurements, as an indicator of the opening of tight junctions, and transport experiments with fluorescein-Na (Flu) and FITC-dextran Mw 4000 (FD-4) were used to assess the effect. (+/-) Verapamil concentrations up to 3 x 10(-4) M increased TEER dose-dependently, whereas from concentrations of 7 x 10(-4) M onwards a dose-dependent drop was found. After removal of verapamil (< 10(-3) M) the effects on TEER were reversible within 30 min. A second administration of verapamil after different time intervals produced a much larger effect on TEER than the first administration. The separate R- and S-enantiomers did not reveal a difference in enantiomer effect. (+/-) Verapamil at 7 x 10(-4) M increased Flu transport about 13-fold and 26-fold after the first and second treatment in the same monolayers, respectively. Transport of FD-4 increased approximately 4-fold and 6-fold after the first and second treatment, respectively. Potential damaging effects were assessed by trypan blue exclusion (cell death) and cell detachment. No cell death occurred at verapamil concentrations of 8.5 x 10(-4) M or lower, whereas cell detachment did not occur within 1 hr at all concentrations used in these experiments. At later times detachment was observed at concentrations of 7 x 10(-4) M and higher. Confocal laser scanning microscopy showed that verapamil opens the paracellular route, thereby enhancing the permeability of hydrophilic compounds. However, relatively high concentrations are needed to achieve this effect and only a narrow concentration range can be used without cytotoxic effects, which limits the potential application of verapamil as an absorption enhancing agent.

The effect of acetylcholine and serotonin on calcium transients and calcium currents in identified Helix pomatia L. neurons

The results presented demonstrate that in D neurons of the snail Helix pomatia L., acetylcholine (ACh) (10 divided by 100 microM) and serotonin (5-HT) (0.1 divided by 1000 microM) applications reduce both the basal intracellular concentration level ([Ca2+]in) and the amplitudes of calcium transients induced by membrane depolarization. It is likely that the mechanism of [Ca2+]in changes in the suppression of calcium inward currents (ICa). Influences of Ach and 5-HT on ICa were studied. Both effects were dose-dependent (ACh--0.01 divided by 100 microM and 5-HT--0.1 divided by 1000 microM). The half-maximal effects (IC50) were evoked by ACh concentration of 0.15 microM and 5-HT--15 microM. Furthermore we have also shown that in some cells 5-HT could evoke a transient increase in ICa (IC50 = 2 microM). The effects of Ach and 5-HT were nonadditive--the subsequent application of ACh after 5-HT, and vice versa, produced no inhibitory effects. This may indicate that both substances act through a common intermediate (possibly, G-protein).

Uncoupling of the molecular 'fence' and paracellular 'gate' functions in epithelial tight junctions

During epithelial morphogenesis, the establishment of tight junctions precedes the development of both the asymmetry in protein and lipid composition between apical and basolateral cell surfaces (the 'fence' function) and the restriction in the transport of ions and nonelectrolytes through the extracellular clefts between cells (the 'gate' function). Molecular models that explain both functions envision strands of particles extending as rings in the cell's perimeter that interact with similar strands located at the apposing cell. This model accounts for the 'fence' function, because the strands prevent diffusion of protein and lipids, and also for the 'gate' function, because the interaction between strands minimizes the width of the extracellular clefts, increasing transepithelial resistance to ions and decreasing non-electrolyte permeability. Here we describe the results of energy depletion, which for the first time separates both functions: it abolishes the gate function, as determined by the dramatic decrease in transepithelial resistance, but it leaves the fence function intact, as determined by the maintenance of lipid polarity.

Effects of acute vs. chronic phorbol ester exposure on transepithelial permeability and epithelial morphology

In previous experiments we have shown that acute (30 minutes) exposure to phorbol esters or other protein kinase C activators causes increased transepithelial permeability, specifically by the increased paracellular permeability through tight junctions. However, the role of protein kinase C activators in carcinogenesis is predicted upon a chronic exposure of an effective dose at frequent intervals for a prolonged period of time. We therefore sought to determine the effect of chronic phorbol ester exposure on transepithelial permeability by exposing cells of the polar renal epithelial cell line, LLC-PK1, to phorbol esters for time periods as long as 16 weeks. The following changes ensued: (1) after the initial drop in transepithelial resistance due to phorbol ester exposure, i.e., an increase in transepithelial permeability (in the acute phase of exposure), an adaptive response occurs as transepithelial resistances in chronically exposed cultures recover to approximately 50% of control values, (2) the cell sheets in chronically exposed cultures lose their acute responsiveness of transepithelial permeability to phorbol ester exposure, (3) cell sheet architecture changes as cells occasionally multilayer and actual polyp-like cell masses appear at high frequency, and (4) cytosolic protein kinase C activity decreases to 50% of control level with acute exposure and then is further decreased to less than 1% of control level in chronically treated cells; membrane-associated PKC activity is not as sharply decreased. The possible role of transepithelial permeability in carcinogenesis and the value of chronically treated epithelial cell cultures as a model for two-stage carcinogenesis are discussed.

Low intracellular pH induces redistribution of fodrin and instabilization of lateral walls in MDCK cells

We have studied the effect of intracellular pH on the establishment and maintenance of the cellular polarity in MDCK cells by utilizing nigericin which causes lowering of the cytoplasmic pH. At pH below 6.5, MDCK cells lost their polarized morphology and became roundish, with an increased apical area and shortened and unstable lateral walls. The lateral wall marker proteins uvomorulin and Na,K-ATPase remained segregated to the lateral walls in the acidified cells, as shown by immunofluorescence microscopy. Fodrin, on the other hand, was released from its normal basolateral residence and was found in the cytoplasm. Actin, which normally co-localizes with fodrin along the basolateral walls, showed a dotty distribution in the cytoplasm of acidified cells, while stress fibers remained intact. Microtubular network appeared flattened, but the Golgi complex retained its apical position. The pH change-induced alterations were readily reversible, as the normal basal-apical polarity (columnar shape, distinct apical and lateral domains with apposing and stiff lateral membranes) was reformed within 10 minutes after restoring the normal pH gradient across the cell membrane. This coincided with the translocation of fodrin from the cytoplasm to the lateral walls. The results show that lowering of intracellular pH leads to temporary segregation of fodrin from the other components of the membrane skeleton assembly, and that association of fodrin with the lateral walls seems to be a prerequisite for their close apposition and for the maintenance of normal basal-axial polarity.

A freeze-fracture study of the perineurium in galactose neuropathy: morphological changes associated with endoneurial oedema

Feeding rats with galactose as 40% of their diet results in peripheral nerve oedema related to the intrafascicular accumulation of galactitol and sodium. In this study, associated changes in the perineurium were examined by the freeze-fracture replication technique. Perineurial cells are linked by tight junctions (zonulae occludentes). In normal animals these are made up of anastomosing strands organized in a belt-like arrangement along the margins of continuous cells. The majority of the tight junctions in the galactose-fed animals displayed structural abnormalities. These ranged from slight separation of the strands to fragmentation and dispersal, with looping of isolated strands. Some of the tight junctions contained large dilated compartments within the junctional network. Short lengths of intramembranous particles, probably representing assembly or disassembly of tight junctional strands, were also observed. The membranes of perineurial cells normally possess numerous openings of caveolae. A quantitative assessment showed that the mean density of these caveolae openings was increased in the galactose-fed rats as compared with controls. The alterations in the tight junctions resemble those that have been produced experimentally in epithelia by subjecting them to abnormal osmotic gradients. They also resemble those seen in human diabetic neuropathy in which osmotic disturbances involving the perineurium have been considered to occur. If the alterations involve the inner layers of the perineurium, they are likely to impair its barrier function. The increased number of caveolae openings in galactose neuropathy may represent a reaction to the endoneurial oedema and indicate that the pinocytotic-like vesicles have a transport function.

Cell polarity regulates the release of secretory component, the epithelial receptor for polymeric immunoglobulins, from the surface of HT-29 colon carcinoma cells

The HT-29 human colon carcinoma cell line differentiates in glucose-free medium to an enterocytic phenotype. We previously isolated a series of HT-29 subclones selected for high levels of expression of secretory component (SC), the epithelial receptor for polymeric immunoglobulins. To develop a model system for studying effects of cell polarity on SC expression and release from the cell surface, the HT-29.74 subclone was induced to differentiate in glucose-free medium. Expression of SC was induced by glucose deprivation in both the parental HT-29 cell line and, to an even greater extent, in the HT-29.74 subclone. Prolonged glucose deprivation of HT-29.74 cells resulted in morphological changes consistent with enterocytic differentiation. Metabolic radiolabeling of SC in differentiated HT-29.74 cells indicated that proteolytic cleavage of membrane-bound to free SC occurred both on the cell surface and intracellularly, possibly in a vacuolar apical compartment or intrapeithelial lumen. To study effects of cell polarity on SC release, differentiated HT-29.74 cells were depolarized by culturing in low calcium medium. Within 2 hours after transfer of the cells into low calcium medium, a burst of SC release was observed concomitant with cell depolarization. Subsequently, release of SC declined significantly and remained low as long as cells were maintained in a depolarized state. The extent of cell depolarization could be controlled by varying the extracellular calcium concentration or by substituting the divalent cation Sr++, which partially prevents depolarization, for Ca++. In either case, the magnitude of the initial burst and subsequent decline in release of SC was proportional to the extent of cell depolarization. We conclude that cell polarity plays an important role in controlling the release of SC in intestinal epithelial cells, most likely by regulating the distribution of membrane-bound SC and SC protease, which are on the basolateral and apical cell surfaces, respectively, in differentiated cells.

Chapter 2 Biogenesis and Sorting of Plasma Membrane Proteins

The cell surface membrane is the boundary between a cell and its environment. In case of polarized epithelial cells, the apical plasma membrane is frequently the boundary between an organism and its environment. The plasmalemma possesses the elements that endow a cell with the capacity to converse with its environment. Plasmalemmal receptor and transducer proteins allow the cell to recognize and respond to various external influences. Membrane-associated proteins anchor cells to their substrata and mediate their integration into tissues. Many properties of a given cell type may be attributed to the protein composition of its plasma membrane. Most cells go to large lengths to control the nature and distribution of polypeptides that populate their plasmalemmas. Cells regulate the expression of genes encoding plasma membrane proteins. Proteins destined for the insertion into the plasma membrane pass through a complex system of processing organelles prior to arriving at their site of ultimate functional residence. Each of these organelles makes a unique contribution to the maturation of these proteins as they transit through them. This chapter discusses the postsynthetic steps involved in the biogenesis of plasma membrane proteins. The chapter discusses some of the events common to all plasmalemmal polypeptides, with special emphasis on those that contribute directly to the character of the cell surface. The chapter then discusses the specializations, associated with cell types, possessing differentiated cell surface sub-domains. The chapter highlights some of the important and fascinating questions confronting investigators interested in the cell biology of the plasma membrane.

Transepithelial resistance of ciliary epithelial cells in culture: functional modification by protamine and extracellular calcium

1. Bovine pigmented and human non-pigmented ciliary epithelial cells were cultured on porous filter supports to obtain measurements of transepithelial electrical parameters. 2. The non-pigmented cells showed maximal transepithelial resistance of 15-30 omega cm2 from the third to seventh day in culture. 3. The pigmented ciliary cells reached maximal resistances of 9-20 omega cm2 after the fourth day in culture. 4. The transepithelial resistances of the cultured epithelia were functionally increased by protamine. This effect could be reversed by heparin. 5. We conclude that the range of resistances in cultured ciliary epithelial cells is the same as in whole ciliary preparations. Thus, cultured ciliary epithelial cells can be used for studies on transepithelial transport.

Differentiation of an epithelium: factors affecting the polarized distribution of Na+,K(+)-ATPase in mouse trophectoderm

Na+,K(+)-ATPase is a marker of the basolateral plasma membrane domain of polarized epithelial cells, including the mural trophectoderm of the mammalian blastocyst (Watson and Kidder (1988). Dev. Biol. 126, 80-90). We have used this marker to explore the factors governing the establishment and maintenance of apical/basolateral polarity during differentiation of trophectoderm. A polyclonal antiserum (anti-GP80) against human cell-CAM 120/80, a homolog of the mouse cell-cell adhesion protein, uvomorulin, was used to prevent cell flattening (compaction) and formation of the epithelial junctional complex. The majority of treated embryos failed to develop a blastocoel; instead their blastomeres developed fluid-filled cavities that expanded while untreated control embryos were cavitating. Immunocytochemistry revealed that the catalytic subunit of Na+,K(+)-ATPase was contained within the membranes lining these cavities, as well as within numerous punctate foci in the cytoplasm. The down-regulation of expression of the enzyme that normally occurs in the ICM and polar trophectoderm did not take place, since the immunoreactivity remained equally strong in all blastomeres. The enzyme could not be detected in plasma membranes. We conclude that uvomorulin-mediated cell adhesion is involved in spatially restricting the expression of the catalytic subunit and is a prerequisite for the insertion of enzyme-laden vesicles into plasma membranes, but not for expression of the catalytic subunit gene. When fully developed blastocysts were treated with cytochalasins to disrupt the epithelial junctional complex, the catalytic subunit shifted from the basolateral to the apical plasma membrane. This finding suggests a primary role for the apical plasma membrane in the process of polarization, and implies that tight junctions are a manifestation of polarity that serve to maintain the separation between apical and basolateral markers.

Mechanisms of chemosensory transduction in taste cells

The application of new techniques to the study of taste cells has revealed much about both the basic physiology of these cells and also about the mechanisms of taste transduction. The taste cells are electrically excitable cells with a variety of voltage-dependent ion currents. These ionic currents have an important role in the transduction of salt taste in mammals and frogs. In mudpuppies different ion channels are involved in the transduction of acidic-sour stimuli. The role of ion currents in the transduction of sweet taste is less clear. Some proposed mechanisms suggest an important role for ion currents and others suggest that the transduction process may be a biochemical event involving cell surface receptors and intracellular second messengers, possibly cAMP. The transduction of bitter taste seems to be a biochemical event involving cell surface receptors and intracellular second messengers in the inositol trisphosphate pathway. Thus, one cannot talk about "the mechanism" of taste transduction. Different taste modalities are transduced by different mechanisms. A corollary to this is that taste cells are not a homogeneous population of cells. In order to provide animals with the ability to discriminate between different taste modalities the taste cells consist of distinct subpopulations of cells based on their primary taste modality. The primary taste modality in a given cell is determined by the receptors and transduction mechanism(s) expressed in that cell. Evidence suggests that modality-specific receptors are expressed in a segregated manner in distinct subpopulations of taste cells. Secondary responses observed in gustatory axons may arise due to a lack of absolute specificity in the transduction processes and nonspecific effects of low pH and high ionic strength and osmolarity on the taste cells. An interesting area for future work will be to elucidate the mechanism(s) by which basal cells become committed to a given taste modality and how the gustatory neurons influence this process of differentiation. The involvement of the gustatory neurons is critical as they must synapse with taste cells of the correct taste modality to preserve the integrity of the information transferred to the CNS. This process of synaptogenesis is presumably mediated by the expression of taste-modality-specific, cell surface antigens on the basolateral domain of a taste cell and receptors on the appropriate neurons, but much work will be necessary to elucidate this process.(ABSTRACT TRUNCATED AT 400 WORDS)

Transformation of airway epithelial cells with persistence of cystic fibrosis or normal ion transport phenotypes

These studies demonstrate the feasibility of transforming human airway epithelial cells while inducing only modest changes in function. Features central to the pathophysiology of cystic fibrosis, i.e., abnormal regulation of a chloride permeability in the apical cell membrane, appear to be preserved in the CF/T43 transformed cell line. This work indicates that additional cystic fibrosis and normal cell lines may be developed, as well as epithelial cell lines from other diseases of interest. In addition to SV40T gene, temperature-sensitive viral genes, or genes driven by inducible promoters (e.g., glucocorticoids, heavy metals) may produce cell lines in which proliferation or differentiation can be controlled. For example, the temperature-selective SV40A gene is expressed in cells cultured at the permissive (33 degrees) temperature but is degraded at the nonpermissive (40 degrees) temperature. Thus, the transfected gene may induce proliferation to increase cell number, and then be suppressed to permit expression of a differentiated phenotype. Out strategy of initially selecting clones by G418 resistance and then selecting clones that develop functional tight junctions (and a transepithelial resistance) was useful in identifying a cell line with highly differentiated phenotypic properties. Cell lines that do not form transepithelial resistances may be valuable for studies that do not depend on cell polarization. Initial evidence suggests that some of the differentiated properties of CF/T43, i.e., formation of functional tight junctions and a transepithelial resistance, are lost at late passages.(ABSTRACT TRUNCATED AT 250 WORDS)