Tight junction development between cultured hepatoma cells: possible stages in assembly and enhancement with dexamethasone

Minireview: Steroid/nuclear receptor-regulated dynamics of occluding and anchoring junctions

A diverse set of physiological signals control intercellular interactions by regulating the structure and function of occluding junctions (tight junctions) and anchoring junctions (adherens junctions and desmosomes). These plasma membrane junctions are comprised of multiprotein complexes of transmembrane and cytoplasmic peripheral plasma membrane proteins. Evidence from many hormone-responsive tissues has shown that expression, modification, molecular interactions, stability, and localization of junctional complex-associated proteins can be targeted by nuclear hormone receptors and their ligands through transcriptional and nontranscriptional mechanisms. The focus of this minireview is to discuss molecular, cellular, and physiological studies that directly link nuclear receptor- and ligand-triggered signaling pathways to the regulation of occluding and anchoring junction dynamics.

Tight junctions and the regulation of gene expression

Cell adhesion is a key regulator of cell differentiation. Cell interactions with neighboring cells and the extracellular matrix regulate gene expression, cell proliferation, polarity and apoptosis. Apical cell-cell junctions participate in these processes using different types of proteins, some of them exhibit nuclear and junctional localization and are called NACos for Nuclear Adhesion Complexes. Tight junctions are one type of such cell-cell junctions and several signaling complexes have been identified to associate with them. In general, expression of tight junction components suppresses proliferation to allow differentiation in a coordinated manner with adherens junctions and extracellular matrix adhesion. These tight junction components have been shown to affect several signaling and transcriptional pathways, and changes in the expression of tight junction proteins are associated with several disease conditions, such as cancer. Here, we will review how tight junction proteins participate in the regulation of gene expression and cell proliferation, as well as how they are regulated themselves by different mechanisms involved in gene expression and cell differentiation.

Reexpression of the TJ protein CLDN1 induces apoptosis in breast tumor spheroids

Members of the claudin family together with occludin are the major constituents of the tight junction (TJ) complex. The human homologue of the murine CLDN1, previously called SEMP1, was identified by differential expression analysis, and the CLDN1 mRNA was found to be downregulated or completely lost in human breast cancer cells in vitro. Retroviral-induced CLDN1 reexpression in breast cancer cells results in plasma membrane homing of the protein and reconstitution of paracellular flux inhibition, which is not dependent on the presence of occludin protein. In this report, we investigated the physiologic role of CLDN1 in CLDN1-transduced MDA-MB 361 breast tumor cells in adherent 2D and suspension 3D spheroid cell cultures. Retroviral-transduced bulk cultures were FACS-sorted to enrich for 100% CLDN1-positive clonal derivatives with similar expression levels of CLDN1 mRNA and protein. There was no difference in proliferation and cell death characteristics in 2D adherent cell cultures of CLDN1-positive compared to control CLDN1-negative and mock-transduced cell cultures. In contrast, the majority of the CLDN1-transduced derivatives displayed a significant elevation of apoptosis that became evident as early as 2 days after 3D spheroid culture onset. This elevated apoptosis was independent of the volume of established spheroids. The cellular immunofluorescence analysis of CLDN1 protein expression in transduced bulk cultures revealed a CLDN1-positive subfraction with a heterogeneous pattern of membrane and cytosolic immunostaining. In the clonal MDA-MB 361 CLDN1-positive cultures, we found that a more prominent cell membrane localization correlated with a pronounced increase of apoptosis in tumor spheroids. In parallel, inhibition of the paracellular flux rate was observed. These findings support a potential role of the TJ protein CLDN1 in restricting nutrient and growth factor supplies in breast cancer cells, and they indicate that the loss of the cell membrane localization of the TJ protein CLDN1 in carcinomas may be a crucial step during tumor progression.

Sertoli cell junctional complexes in gossypol-treated neonatal and adult guinea pigs

The effect of gossypol, an experimental male contraceptive agent, on the development and maintenance of the blood-testis barrier was determined by feeding gossypol daily to prepubertal and adult guinea pigs, and then examining their testes by electron microscopy of thin sections and freeze-fracture replicas. In guinea pigs of 10 to 30 and 10 to 40 days of age that were fed gossypol, impermeable continuous junctional zones did not develop between adjacent Sertoli cells. Compartmentalization of germ cells in the seminiferous epithelium, therefore, was nonexistent. These findings were obtained by use of the sterol-binding polyene, filipin, used as a low molecular weight tracer in combination with freeze-fracture. In general, the seminiferous tubules lacked lumina and spermatogenesis did not progress beyond the pachytene spermatocyte stage. In adult guinea pigs fed gossypol daily for five weeks, continuous zonules at the base of the seminiferous epithelium appeared intact and were impermeable to filipin. Discontinuous zonules found higher in the epithelium showed distensions between interrupted junctional strands and were permeated by filipin. In addition, vacuolated spaces between Sertoli cells and clumps of heterochromatin were conspicuous in some of the Sertoli cell nuclei. Spermatogenesis was disturbed in about 10% of the seminiferous tubules examined. These perturbations included exfoliation of round and elongated spermatids with concomitant formation of multi-nucleated giant cells. Spermatozoa from these adult male guinea pigs were immotile. These findings suggest that, in neonatal animals, gossypol appears to prevent the maturation of Sertoli cells and this effect is expressed as the failure of focal Sertoli cell tight junctional strands to assemble into continuous zonules. In adult animals, gossypol appears to have no effect on the maintenance of the blood-testis barrier.

Tight junction formation in cultured epithelial cells (MDCK)

Synthesis and assembly of tight junctions are studied in monolayers of MDCK cells plated at a density sufficient for confluence, allowed to attach for 1 hr, and transferred to fresh media without cells containing or not Ca2+. 20 hr later, while monolayers with Ca2+ have fully developed junctions that confer an electrical resistance across of 346 +/- 51 omega cm2, those without Ca2+ have a negligible resistance. If at this time Ca2+ is added, junctions assemble and seal with a fast kinetics, that can be followed through the development of electrical resistance, penetration of ruthenium red, and electron microscopy. Drugs that impair synthesis, maturation and transport of proteins (cycloheximide, tunicamycin, monensin) indicate that protein components are synthesized early upon plating, do not seem to require N-glycosylation, and are stored in the Golgi compartment. Upon addition of Ca2+ they are transferred to the membrane with the participation of microfilaments but not of microtubules. These components seem to insert directly in the position they occupy in the strands, and the cell circles its perimeter with one strand as early as 15 min, even if in some segments it only consists of a row of particles. New strands develop in association with previous ones, and the pattern completes in 4 to 6 hr. Ca2+ is required for the maintenance of the assembly and also for the sealing with neighboring cells. These processes cannot occur below 25 degrees C. Serum is not required. Polarized distribution of intramembrane particles (IMP) in apical and basolateral regions follows the same time course as junction formation, in spite of the fence constituted by those strands that are already assembled. This suggests that IMP do not redistribute by lateral displacements in the plane of the membrane, but by removal and insertion in the apical and basolateral domains.

Changes in intercellular junctions. II. Modulation in embryonic chick liver in vitro by cytosine arabinoside and dexamethasone

We have used an in vitro model to investigate the relationship between cell proliferation, cell maturation, and intercellular junctional changes that are seen during normal liver development. Chick liver fragments of embryonic stages 21, 28, and 39 were grown in organ culture in Medium 199 with 10% bovine serum albumin for 24 hr. Concurrent cultures were treated with cytosine arabinoside (ara-c, 20 micrograms/ml) to inhibit cell proliferation or with dexamethasone (2 microns/ml) to promote cell maturation. Control and treated liver cultures were analyzed using autoradiography to determine labeling index and quantitative computer graphic analysis of freeze-fracture micrographs to evaluate junctional changes. Explants treated with ara-c had a near zero labeling index. Those from embryonic stage 21 showed a 4-fold increase in tight junctional area, while those from embryonic stage 28 showed a 2.5-fold increase in tight junctional area and a 5-fold increase in gap junctional area. In liver of embryonic stage 39, junctional area increased, and conformation of junctions took on the typical form seen in the liver of the hatched chick. Thus high mitotic activity appears to disrupt cell junctions, reducing the amount of junctions present and causing pleomorphic configurations. Junctional area could also be modified with no change in mitotic activity by treatment with dexamethasone. Stage 28 embryo livers in culture with dexamethasone showed a 4-fold increase in the amount of tight junctions with no change in the amount of gap junctions. Some major findings based on these age-dependent responses are (1) if mitosis is blocked in young embryonic stages when proliferation is high, intercellular junctions tend to increase in amount, and to be more mature in configuration; (2) if maturation is stimulated with no change in proliferation, those junctions which normally increase with differentiation will increase in amount; and (3) it is possible to produce a change in the amount of one junctional type without affecting the other. Therefore the amount of cell surface occupied by junctions at any given time seems to depend on both the pattern of synthetic activity in the cell and the degree to which cell to cell contacts are stable and undisrupted by mitotic activity.

Further observations on the Sertoli cell junctions of the mouse testis after metal contract freeze-fracture, and comparisons with cellular junctions of other epithelial cells

Using a metal contact freezing methods, the junctional complexes of Sertoli cells and other epithelial cells (intestine, liver, and epididymis) in the mouse were studied by freeze-fracture in unfixed and in aldehyde-fixed specimens. The tight junctions of the Sertoli cells without fixation consists of discrete rows of particles located preferentially on the E face. Continuous furrows with or without tight-junctional particles are found on both fractured faces. The majority of the tight-junctional particles of th Sertoli cells are situated at one side of the furrow rather than in the center. The "aisle" configuration of the gap junction is seen in developing Sertoli cells prepared without fixation. Tight junctions of epithelial cells other than Sertoli cells consists of discrete rows of particles on the E face and furrows on the P face when prepared without fixation. The tight and gap junctions of the Sertoli cells are not affected by fixation, but the tight junctions of other types of epithelial cells are effected as previously suggested. A distribution pattern of the gap-junctional particles of the Sertoli cells and of other cells shows a pleomorphism. Changes in gap junctions produced by fixation could not be detected clearly. Characteristics of the Sertoli-cell junctions are discussed. Desmosomes between the intestinal epithelial cells without fixation are presented.

Cell junctions in explanted tissues from early chick embryos

Hypoblast and definitive endoblast derived from young chick embryos were explanted and grown for 24 h in culture. The junctional complexes which characterise these tissues were studied on freeze-fracture replicas and thin sections. Cell membranes of the hypoblast displayed tight junctions only, disposed in randomly arranged strands or narrow belts which included many discontinuous strands. The definitive endoblast showed tight and gap junctions as well as desmosomes in close association with the tight junctions. It is suggested that the differences between the two types of tissue may be related to cell cohesiveness, which appears to be relatively low in the hypoblast and high in the definitive endoblast.

Rapid massive assembly of tight junction strands

Incubation at 37 degrees C of excised rat prostate tissue results in massive proliferative assembly of new tight junction strands along the entire lengths of the lateral plasma membranes of the columnar epithelial cells. The new tight junction elements are assembled within 5 minutes and have an average length six times that of those present in the apical tight junction band. Massive assembly occurs in the presence of protein synthesis inhibitors (cycloheximide) or of metabolic uncouplers (dinitrophenol). Thus, proliferative assembly of tight junction strands involves molecular reorganization from a pool of preexisting, probably membrane-associated, components. The fascia occludens and some examples of experimentally induced tight junction proliferation may reflect the massive emergence of tight junction strands when tissue is subjected to diverse stressful conditions.

Experimental modulation of occluding junctions in a cultured transporting epithelium

The experimental opening and resealing of occluding junctions in monolayers of cultured MDCK cells (epithelioid of renal origin) was explored by measuring changes in the electrical resistance across the monolayer and by freeze-fracture electron microscopy. As in natural epithelia, the function of occluding junctions as permeability barriers specifically depends on extracellular Ca++ concentration and fails if this ion is replaced by Mg++ or Ba++. The removal of Ca++ and the addition of EGTA to the bathing medium opened the junctions and reduced the transepithelial resistance. Resealing was achieved within 10-15 min by restoring Ca++. Quantitative freeze-fracture electron microscopy showed that junctional opening, caused by lack of Ca++, was accompanied by simplification of the pattern of the membrane strands of the occluding junction without disassembly or displacement of the junctional components. Resealing of the cellular contacts involved the gradual return to a normal junctional pattern estimated as the average number of strands constituting the junction. The occluding junctions were also opened by the addition of the ionophore A23187, suggesting that the sealing of the contacts requires high Ca++ on the extracellular side and low Ca++ concentration of the cytoplasmic compartment. The opening process could be blocked by low temperature (7.5 degrees C). Resealing did not depend on serum factors and did not require protein synthesis; therefore, it seems to be caused by reassembly of preexisting membrane junctional components. The restoration of the junctions occurred simultaneously with the establishment of ion-selective channels; the Na+/Cl- and the cation/cation selectivity were recovered with the same time-course as the electrical resistance. The role of the cytoskeleton in the process of junctional reassembly is reported in the companion article.

Intramembrane events accompanying junction formation in a liver cell line

To study intramembrane events leading to the establishment of intercellular junctions between epithelial cells in vitro, we examined monolayer cultures of a rat liver cell line by an in situ freeze-fracturing technique (Pauli et al., J. Cell Biol., 72:763, 1977). Our observations indicate that an early step of junction formation between liver cells consists of the differentiation of a particle-poor membrane stretch showing a honeycomb pattern of shallow P-face depressions or E-face bulges ("formation band"). This change in membrane organization precedes and accompanies the subsequent aggregation of junctional particles. The latter process results in the formation of irregular particle islands with peripheral branchings which tend to encompass the depressions in the membrane. The linear branchings grow and interconnect in a network of beaded strands, which gradually transform into smooth tight junctional fibrils, as previously described in fetal liver in vivo (Montesano et al., J. Cell Biol., 67:310, 1975), while the particle islands assume the typical configuration of mature gap junctions. Formation bands are particularly prominent between liver cells growth in the presence of hydrocortisone (5 microgram/ml) in the culture medium.

Cytoplasmic regulation of tight-junction permeability: effect of plant cytokinins

The significance of the "leaky" tight junction might be understood better if cells of the epithelial monolayer possessed mechanisms to regulate molecular flow through the junction. To test this possibility, Necturus gallbladder, a representative leaky epithelium, was studied before, during, and after mucosal exposure to plant cytokinins and two other microfilament-active drugs, cytochalasin B and phalloidin. Concomitant with morphological changes in microfilaments, cytokinins induced rapid reversible increases in transepithelial resistance and potential difference (PD) and decreases in NaCl dilution potentials, with no change in the ratio of relative cell membrane resistances. Cytochalasin B (0.2-1.2 microM) and phalloidin (0.6-12.7 microM) caused similar changes in transepithelial resistance and PD. When the intramembranous structure of tight junctions was studied by freeze fracture, peak cytokinin-induced increments in transepithelial resistance were associated with more disorder in the strand meshwork resulting in a small increase in tight junction depth, but there was no evidence of de novo strand assembly. These studies suggest that permeability of the tight junction of Necturus gallbladder is subject to rapid reversible modulation, possibly under cytoskeletal control.

Gap junctions between Sertoli and germ cells of rat seminiferous tubules

Ultrastructural observations of rat seminiferous tubules show clearly the presence of plasma membrane junctions between Sertoli and germ cells in the basal and adluminal compartments. Results obtained from the freeze fracture and thin section techniques were correlated in order to elucidate the nature of these intercellular junctions. We suggest that these intercellular membrane specializations are gap junctions which occur within regions of plasma membrane that also exhibit adherens-like modifications.