Interaction of calcium with plasma membrane of epithelial (MDCK) cells during junction formation

Ouabain's Influence on TRPV4 Channels of Epithelial Cells: An Exploration of TRPV4 Activity, Expression, and Signaling Pathways

Ouabain, a substance originally obtained from plants, is now classified as a hormone because it is produced endogenously in certain animals, including humans. However, its precise effects on the body remain largely unknown. Previous studies have shown that ouabain can influence the phenotype of epithelial cells by affecting the expression of cell-cell molecular components and voltage-gated potassium channels. In this study, we conducted whole-cell clamp assays to determine whether ouabain affects the activity and/or expression of TRPV4 channels. Our findings indicate that ouabain has a statistically significant effect on the density of TRPV4 currents (dITRPV4), with an EC50 of 1.89 nM. Regarding treatment duration, dITRPV4 reaches its peak at around 1 h, followed by a subsequent decline and then a resurgence after 6 h, suggesting a short-term modulatory effect related to on TRPV4 channel activity and a long-term effect related to the promotion of synthesis of new TRPV4 channel units. The enhancement of dITRPV4 induced by ouabain was significantly lower in cells seeded at low density than in cells in a confluent monolayer, indicating that the action of ouabain depends on intercellular contacts. Furthermore, the fact that U73122 and neomycin suppress the effect caused by ouabain in the short term suggests that the short-term induced enhancement of dITRPV4 is due to the depletion of PIP2 stores. In contrast, the fact that the long-term effect is inhibited by PP2, wortmannin, PD, FR18, and IKK16 suggests that cSrc, PI3K, Erk1/2, and NF-kB are among the components included in the signaling pathways.

Half-Sandwich Arene Ruthenium(II) Thiosemicarbazone Complexes: Evaluation of Anticancer Effect on Primary and Metastatic Ovarian Cancer Cell Lines

In this study, we describe the synthesis, characterization and antiproliferative activity of three organo-ruthenium(II) half-sandwich complexes [RuCl(η6-p-cym)(N,S-L)]Cl (I, II, and III). To form these complexes, three thiosemicarbazone ligands (TSCs) were synthesized; L = 5-nitro-2-carboxyaldehyde-thiophen-N-methyl-thiosemicarbazone, (L1); 2-acetyl-5-bromo-thiophen-N-methyl-thiosemicarbazone, (L2) and 2-acetyl-5-bromo-thiophen-N,N-dimethyl-thiosemicarbazone, (L3). The isolated compounds were analyzed using spectroscopic techniques such as elemental analysis, conductance measurements, FT-IR, 1H NMR spectroscopy, MALDI-TOF mass spectrometry, and single-crystal XRD. Our results demonstrated that the synthesized thiosemicarbazone ligands (TSCs) are bound to the metal ion as a bidentate ligand that coordinates through the thiocarbonyl sulfur and azomethine nitrogen atoms in all complexes (I, II, and III). The X-ray crystal structures of L1 and L2 revealed that both compounds are crystallized in the triclinic crystal system with space group P-1. The biological potency of newly synthesized TSC ligands (L1, L2, and L3) and their corresponding ruthenium complexes (I, II, and III) were investigated on human primary ovarian (A2780) and human metastatic ovarian (OVCAR-3) cell lines. To get detailed information respecting antitumor properties, cytotoxicity, DNA/BSA binding affinity, cellular uptake, DNA binding competition, and trans-epithelial resistance measurement assays were performed. Our results demonstrate that newly synthesized ruthenium(II) complexes possess potential biological activity. Moreover, we observe that the ruthenium complexes reported here show anticancer activity on primary (A2780) and metastatic (OVCAR-3) ovarian cancer cells.

Zonula occludens 2 and Cell-Cell Contacts Are Required for Normal Nuclear Shape in Epithelia

MAGUK protein ZO-2 is present at tight junctions (TJs) and nuclei. In MDCK ZO-2 knockdown (KD) cells, nuclei exhibit an irregular shape with lobules and indentations. This condition correlates with an increase in DNA double strand breaks, however cells are not senescent and instead become resistant to UV-induced senescence. The irregular nuclear shape is also observed in isolated cells and in those without TJs, due to the lack of extracellular calcium. The aberrant nuclear shape of ZO-2 KD cells is not accompanied by a reduced expression of lamins A/C and B and lamin B receptors. Instead, it involves a decrease in constitutive and facultative heterochromatin, and microtubule instability that is restored with docetaxel. ZO-2 KD cells over-express SUN-1 that crosses the inner nuclear membrane and connects the nucleoskeleton of lamin A to nesprins, which traverse the outer nuclear membrane. Nesprins-3 and -4 that indirectly bind on their cytoplasmic face to vimentin and microtubules, respectively, are also over-expressed in ZO-2 KD cells, whereas vimentin is depleted. SUN-1 and lamin B1 co-immunoprecipitate with ZO-2, and SUN-1 associates to ZO-2 in a pull-down assay. Our results suggest that ZO-2 forms a complex with SUN-1 and lamin B1 at the inner nuclear membrane, and that ZO-2 and cell–cell contacts are required for a normal nuclear shape.

E7 oncoprotein from human papillomavirus 16 alters claudins expression and the sealing of epithelial tight junctions

Tight junctions (TJs) are cell-cell adhesion structures frequently altered by oncogenic transformation. In the present study the role of human papillomavirus (HPV) 16 E7 oncoprotein on the sealing of TJs was investigated and also the expression level of claudins in mouse cervix and in epithelial Madin-Darby Canine Kidney (MDCK) cells. It was found that there was reduced expression of claudins -1 and -10 in the cervix of 7-month-old transgenic K14E7 mice treated with 17β-estradiol (E₂), with invasive cancer. In addition, there was also a transient increase in claudin-1 expression in the cervix of 2-month-old K14E7 mice, and claudin-10 accumulated at the border of cells in the upper layer of the cervix in FvB mice treated with E₂, and in K14E7 mice treated with or without E₂. These changes were accompanied by an augmented paracellular permeability of the cervix in 2- and 7-monthold FvB mice treated with E₂, which became more pronounced in K14E7 mice treated with or without E₂. In MDCK cells the stable expression of E7 increased the space between adjacent cells and altered the architecture of the monolayers, induced the development of an acute peak of transepithelial electrical resistance accompanied by a reduced expression of claudins -1, -2 and -10, and an increase in claudin-4. Moreover, E7 enhances the ability of MDCK cells to migrate through a 3D matrix and induces cell stiffening and stress fiber formation. These observations revealed that cell transformation induced by HPV16 E7 oncoprotein was accompanied by changes in the pattern of expression of claudins and the degree of sealing of epithelial TJs.

Activation of the Ca2+ sensing receptor and the PKC/WNK4 downstream signaling cascade induces incorporation of ZO-2 to tight junctions and its separation from 14-3-3

Zonula occludens-2 (ZO-2) is a tight junction (TJ) cytoplasmic protein, whose localization varies according to cell density and Ca²⁺ in the media. In cells cultured in low calcium (LC), ZO-2 displays a diffuse cytoplasmic distribution, but activation of the Ca²⁺ sensing receptor (CaSR) with Gd³⁺ triggers the appearance of ZO-2 at the cell borders. CaSR downstream signaling involves activation of protein kinase C, which phosphorylates and activates with no lysine kinase-4 that phosphorylates ZO-2 inducing its concentration at TJs. In LC, ZO-2 is protected from degradation by association to 14-3-3 proteins. When monolayers are transferred to normal calcium, the complexes ZO-2/14-3-3ζ and ZO-2/14-3-3σ move to the cell borders and dissociate. The 14-3-3 proteins are then degraded in proteosomes, whereas ZO-2 integrates to TJs. From the plasma membrane residual ZO-2 is endocyted and degradaded in lysosomes. The unique region 2 of ZO-2, and S261 located within a nuclear localization signal, are critical for the interaction with 14-3-3 ζ and σ and for the efficient nuclear importation of ZO-2. These results explain the molecular mechanism through which extracellular Ca²⁺ triggers the appearance of ZO-2 at TJs in epithelial cells and reveal the novel interaction between ZO-2 and 14-3-3 proteins, which is critical for ZO-2 protection and intracellular traffic.

Mesenchymal Stromal Cells Accelerate Epithelial Tight Junction Assembly via the AMP-Activated Protein Kinase Pathway, Independently of Liver Kinase B1

BACKGROUND

Mesenchymal stromal cells (MSC) are fibroblast-like multipotent cells capable of tissue-repair properties. Given the essentiality of tight junctions (TJ) in epithelial integrity, we hypothesized that MSC modulate TJ formation, via the AMP-activated kinase (AMPK) pathway. Liver kinase-β1 (LKB1) and Ca²⁺-calmodulin-dependent protein kinase kinase (CaMKK) represent the main kinases that activate AMPK.

METHODS

The in vitro Ca²⁺ switch from 5 μM to 1.8 mM was performed using epithelial Madin-Darby canine kidney (MDCK) cells cultured alone or cocultured with rat bone marrow-derived MSC or preexposed to MSC-conditioned medium. TJ assembly was measured by assessing ZO-1 relocation to cell-cell contacts. Experiments were conducted using MDCK stably expressing short-hairpin-RNA (shRNA) against LKB1 or luciferase (LUC, as controls). Compound STO-609 (50 μM) was used as CaMKK inhibitor.

RESULTS

Following Ca²⁺ switch, ZO-1 relocation and phosphorylation/activation of AMPK were significantly higher in MDCK/MSC compared to MDCK. No difference in AMPK phosphorylation was observed between LKB1-shRNA and Luc-shRNA MDCK following Ca²⁺ switch. Conversely, incubation with STO-609 prior to Ca²⁺ switch prevented AMPK phosphorylation and ZO-1 relocation. MSC-conditioned medium slightly but significantly increased AMPK activation and accelerated TJ-associated distribution of ZO-1 post Ca²⁺ switch in comparison to regular medium.

CONCLUSIONS

MSC modulate the assembly of epithelial TJ, via the CaMKK/AMPK pathway independently of LKB1.

"You Shall Not Pass"-tight junctions of the blood brain barrier

The structure and function of the barrier layers restricting the free diffusion of substances between the central nervous system (brain and spinal cord) and the systemic circulation is of great medical interest as various pathological conditions often lead to their impairment. Excessive leakage of blood-borne molecules into the parenchyma and the concomitant fluctuations in the microenvironment following a transient breakdown of the blood-brain barrier (BBB) during ischemic/hypoxic conditions or because of an autoimmune disease are detrimental to the physiological functioning of nervous tissue. On the other hand, the treatment of neurological disorders is often hampered as only minimal amounts of therapeutic agents are able to penetrate a fully functional BBB or blood cerebrospinal fluid barrier. An in-depth understanding of the molecular machinery governing the establishment and maintenance of these barriers is necessary to develop rational strategies allowing a controlled delivery of appropriate drugs to the CNS. At the basis of such tissue barriers are intimate cell-cell contacts (zonulae occludentes, tight junctions) which are present in all polarized epithelia and endothelia. By creating a paracellular diffusion constraint TJs enable the vectorial transport across cell monolayers. More recent findings indicate that functional barriers are already established during development, protecting the fetal brain. As an understanding of the biogenesis of TJs might reveal the underlying mechanisms of barrier formation during ontogenic development numerous in vitro systems have been developed to study the assembly and disassembly of TJs. In addition, monitoring the stage-specific expression of TJ-associated proteins during development has brought much insight into the “developmental tightening” of tissue barriers. Over the last two decades a detailed molecular map of transmembrane and cytoplasmic TJ-proteins has been identified. These proteins not only form a cell-cell adhesion structure, but integrate various signaling pathways, thereby directly or indirectly impacting upon processes such as cell-cell adhesion, cytoskeletal rearrangement, and transcriptional control. This review will provide a brief overview on the establishment of the BBB during embryonic development in mammals and a detailed description of the ultrastructure, biogenesis, and molecular composition of epithelial and endothelial TJs will be given.

Effect of vitamin D receptor knockout on cornea epithelium wound healing and tight junctions

PURPOSE

Our laboratory previously determined that vitamin D3, the vitamin D receptor (VDR), and 1α hydroxylase are present and active in the eye. In this study, we examined the effects of VDR knockout on wound healing, the tight junction-associated proteins occludin and ZO-1, and tight junction numbers in mouse corneas.

METHODS

Epithelial wounds (2-mm) were made with an agar brush on 4-week-old and 10-week-old wild-type, heterozygous, and VDR knockout mouse corneas. Mice were on a normal or high lactose, Ca(2+), and PO₄(-) diet. Wound-healing area was measured over time. Real-time PCR was used to quantify occludin and ZO-1 message expression. Western blot was used for protein expression. Transmission electron microscopy was used to examine corneal epithelium and endothelium tight junctions. Immunofluorescence was used to examine epithelial ZO-1 distribution.

RESULTS

Results showed a decreased healing rate in 10-week-old VDR knockout mice compared with wild-types. Vitamin D receptor knockout mice on the special diet had no difference in healing rate compared with wild-types. Real-time PCR showed decreased expression of occludin and ZO-1 in 10-week-old VDR knockout mice compared with wild-types. Western blot of 10-week-old knockout mouse corneas showed decreased occludin expression compared with wild-types. Transmission electron microscopy showed a significant difference in tight junction numbers in VDR knockouts versus wild-types. Immunofluorescence showed a change in ZO-1 distribution among genotypes.

CONCLUSIONS

Vitamin D receptor knockout affects mouse corneal epithelium wound healing and tight junction integrity.

A dynamic cell adhesion surface regulates tissue architecture in growth plate cartilage

The architecture and morphogenetic properties of tissues are founded in the tissue-specific regulation of cell behaviors. In endochondral bones, the growth plate cartilage promotes bone elongation via regulated chondrocyte maturation within an ordered, three-dimensional cell array. A key event in the process that generates this cell array is the transformation of disordered resting chondrocytes into clonal columns of discoid proliferative cells aligned with the primary growth vector. Previous analysis showed that column-forming chondrocytes display planar cell divisions, and the resulting daughter cells rearrange by ∼90° to align with the lengthening column. However, these previous studies provided limited information about the mechanisms underlying this dynamic process. Here we present new mechanistic insights generated by application of a novel time-lapse confocal microscopy method along with immunofluorescence and electron microscopy. We show that, during cell division, daughter chondrocytes establish a cell-cell adhesion surface enriched in cadherins and β-catenin. Rearrangement into columns occurs concomitant with expansion of this adhesion surface in a process more similar to cell spreading than to migration. Column formation requires cell-cell adhesion, as reducing cadherin binding via chelation of extracellular calcium inhibits chondrocyte rearrangement. Importantly, physical indicators of cell polarity, such as cell body alignment, are not prerequisites for oriented cell behavior. Our results support a model in which regulation of adhesive surface dynamics and cortical tension by extrinsic signaling modifies the thermodynamic landscape to promote organization of daughter cells in the context of the three-dimensional growth plate tissue.

Activation of the Ca²+-sensing receptor induces deposition of tight junction components to the epithelial cell plasma membrane

The Ca(2+)-sensing receptor (CaSR) belongs to the G-protein-coupled receptor superfamily and plays essential roles in divalent ion homeostasis and cell differentiation. Because extracellular Ca(2+) is essential for the development of stable epithelial tight junctions (TJs), we hypothesized that the CaSR participates in regulating TJ assembly. We first assessed the expression of the CaSR in Madin-Darby canine kidney (MDCK) cells at steady state and following manipulations that modulate TJ assembly. Next, we examined the effects of CaSR agonists and antagonists on TJ assembly. Immunofluorescence studies indicate that endogenous CaSR is located at the basolateral pole of MDCK cells. Stable transfection of human CaSR in MDCK cells further reveals that this protein co-distributes with β-catenin on the basolateral membrane. Switching MDCK cells from low-Ca(2+) medium to medium containing a normal Ca(2+) concentration significantly increases CaSR expression at both the mRNA and protein levels. Exposure of MDCK cells maintained in low-Ca(2+) conditions to the CaSR agonists neomycin, Gd(3+) or R-568 causes the transient relocation of the tight junction components ZO-1 and occludin to sites of cell-cell contact, while inducing no significant changes in the expression of mRNAs encoding junction-associated proteins. Stimulation of CaSR also increases the interaction between ZO-1 and the F-actin-binding protein I-afadin. This effect does not involve activation of the AMP-activated protein kinase. By contrast, CaSR inhibition by NPS-2143 significantly decreases interaction of ZO-1 with I-afadin and reduces deposition of ZO-1 at the cell surface following a Ca(2+) switch from 5 µM to 200 µM [Ca(2+)]e. Pre-exposure of MDCK cells to the cell-permeant Ca(2+) chelator BAPTA-AM, similarly prevents TJ assembly caused by CaSR activation. Finally, stable transfection of MDCK cells with a cDNA encoding a human disease-associated gain-of-function mutant form of the CaSR increases the transepithelial electrical resistance of these cells in comparison to expression of the wild-type human CaSR. These observations suggest that the CaSR participates in regulating TJ assembly.

The Na+-K+-ATPase as self-adhesion molecule and hormone receptor

Thanks to the homeostasis of the internal milieu, metazoan cells can enormously simplify their housekeeping efforts and engage instead in differentiation and multiple forms of organization (tissues, organs, systems) that enable them to produce an astonishing diversity of mammals. The stability of the internal milieu despite drastic variations of the external environment (air, fresh or seawater, gastrointestinal fluids, glomerular filtrate, bile) is due to transporting epithelia that can adjust their specific permeability to H(2)O, H(+), Na(+), K(+), Ca(2+), and Cl(-) over several orders of magnitude and exchange substances with the outer milieu with exquisite precision. This exchange is due to the polarized expression of membrane proteins, among them Na(+)-K(+)-ATPase, an oligomeric enzyme that uses chemical energy from ATP molecules to translocate ions across the plasma membrane of epithelial cells. Na(+)-K(+)-ATPase presents two types of asymmetries: the arrangement of its subunits, and its expression in one pole of the epithelial cell ("polarity"). In most epithelia, polarity consists of the expression of Na(+)-K(+)-ATPase towards the intercellular space and arises in part from the interaction of the extracellular segment of the β-subunit with another β-subunit present in a Na(+)-K(+)-ATPase molecule expressed by a neighboring cell. In addition to enabling the Na(+)-K(+)-ATPase to transport ions and water vectorially, this position exposes its receptors to ouabain and analogous cardiotonic steroids, which are present in the internal milieu because these were secreted by endocrine cells.

Lymphocytes accelerate epithelial tight junction assembly: role of AMP-activated protein kinase (AMPK)

The tight junctions (TJs), characteristically located at the apicolateral borders of adjacent epithelial cells, are required for the proper formation of epithelial cell polarity as well as for sustaining the mucosal barrier to the external environment. The observation that lymphocytes are recruited by epithelial cells to the sites of infection [1] suggests that they may play a role in the modulation of epithelial barrier function and thus contribute to host defense. To test the ability of lymphocytes to modulate tight junction assembly in epithelial cells, we set up a lymphocyte-epithelial cell co-culture system, in which Madin-Darby canine kidney (MDCK) cells, a well-established model cell line for studying epithelial TJ assembly [2], were co-cultured with mouse lymphocytes to mimic an infection state. In a typical calcium switch experiment, the TJ assembly in co-culture was found to be accelerated compared to that in MDCK cells alone. This accelaration was found to be mediated by AMP-activated protein kinase (AMPK). AMPK activation was independent of changes in cellular ATP levels but it was found to be activated by the pro-inflammatory cytokine TNF-alpha. Forced suppression of AMPK, either with a chemical inhibitor or by knockdown, abrogated the accelerating effect of lymphocytes on TJ formation. Similar results were also observed in a co-culture with lymphocytes and Calu-3 human airway epithelial cells, suggesting that the activation of AMPK may be a general mechanism underlying lymphocyte-accelerated TJ assembly in different epithelia. These results suggest that signals from lymphocytes, such as cytokines, facilitate TJ assembly in epithelial cells via the activation of AMPK.

Vitamin D for cancer prevention: global perspective

PURPOSE

Higher serum levels of the main circulating form of vitamin D, 25-hydroxyvitamin D (25(OH)D), are associated with substantially lower incidence rates of colon, breast, ovarian, renal, pancreatic, aggressive prostate and other cancers.

METHODS

Epidemiological findings combined with newly discovered mechanisms suggest a new model of cancer etiology that accounts for these actions of 25(OH)D and calcium. Its seven phases are disjunction, initiation, natural selection, overgrowth, metastasis, involution, and transition (abbreviated DINOMIT). Vitamin D metabolites prevent disjunction of cells and are beneficial in other phases.

RESULTS/CONCLUSIONS

It is projected that raising the minimum year-around serum 25(OH)D level to 40 to 60 ng/mL (100-150 nmol/L) would prevent approximately 58,000 new cases of breast cancer and 49,000 new cases of colorectal cancer each year, and three fourths of deaths from these diseases in the United States and Canada, based on observational studies combined with a randomized trial. Such intakes also are expected to reduce case-fatality rates of patients who have breast, colorectal, or prostate cancer by half. There are no unreasonable risks from intake of 2000 IU per day of vitamin D(3), or from a population serum 25(OH)D level of 40 to 60 ng/mL. The time has arrived for nationally coordinated action to substantially increase intake of vitamin D and calcium.

Effects of formaldehyde inhalation on the junctional proteins of nasal respiratory mucosa of rats

Exposure to formaldehyde, which is an organic compound, disturbs the integrity of nasal mucosa. In this study, we aimed to clarify the protein changes in the junctional complex of nasal mucosa of Wistar rats exposed to formaldehyde inhalation. The study was performed in 20 female Wistar rats. Rats were divided into two groups randomly. Control rats were allowed free access to standard rat chaw and tap water (n:10). Experimental group was exposed to formaldehyde vapor at 15ppm, 6h/day, 5 days/week for 12 weeks (n:10). Histological evaluation of the experimental model was determined by hematoxylin-eosin (HE) and periodic acid Schiff (PAS) stainings of paraffin-embedded nasal mucosa tissues and by electron microscopy. The effects of formaldehyde inhalation on the distribution of occludin, E-cadherin, and gamma-catenin were assessed by immunohistochemistry. The nasal mucosa of the experimental group was correlated with hypertrophy in goblet cell, degeneration in basal lamina, stratification of epithelium, and proliferation. Thickness of basal lamina and also local degenerative regions, vacuole increase in cytoplasmic areas, irregular forms of kinocilium and loss of sharpness in the kinocilium membrane were the findings at the ultrastructural level. The expressions of E-cadherin, occludin, gamma-catenin proteins in intercellular junctional complexes of rat nasal mucosa were also decreased in experimental group compared to control group. The findings of the present study indicated that formaldehyde vapor inhalation in the concentrations and duration of exposure used in the present experiment significantly decreased the density of structural proteins of the junctional complex in the nasoepithelium. It was suggested that, the formaldehyde inhalation could cause complete impairment of intercellular junctional complexes and disturb the tissue integrity in nasal mucosa at higher concentrations.

Potassium Channels Lost During Harvesting of Epithelial Cells are Restored with a Kinetics that Depends on Channel Species

The polarized distribution of K(+) channels in MDCK cells is lost upon harvesting and restored upon re-seeding. Using semi-quantitative PCR, in the present work we find that (i) Cells do not "wait" for the normal recycling of membrane proteins to restore their lost channels, but trigger their replacement, suggesting that the membrane has a way of engaging the nucleus. (ii) Replacement channels do not come from an internal reservoir, as it is the case with Na(+), K(+)-ATPase, but requires a de novo synthesis. (iii) Replacement is not an all-or-none response, since mRNA for MaxiK channels increases by 8-fold after re-seeding, but those for Kv1.6 and Kv1.7 are not affected by harvesting/re-seeding. (iv) TEA, charybdotoxin and iberiotoxin fail to trigger the replacement response in mature monolayers, suggesting that replacement is not due to suppression of channel function. (v) MDCK cells have a typical transporting epithelial phenotype (TEP) consisting of tight junctions (TJs) plus polarity. Although the polarized distribution of K-channels is a prominent attribute of TEP, blocking their function does not perturb the development of TEP, as gauged through the development of TJs, nor level of expression (Western blot) and distribution (confocal microscopy) of occludin, and claudins 1, 3 and 7.

Cell adhesion molecules in chemically-induced renal injury

Cell adhesion molecules are integral cell-membrane proteins that maintain cell-cell and cell-substrate adhesion and in some cases act as regulators of intracellular signaling cascades. In the kidney, cell adhesion molecules, such as the cadherins, the catenins, the zonula occludens protein-1 (ZO-1), occludin and the claudins are essential for maintaining the epithelial polarity and barrier integrity that are necessary for the normal absorption/excretion of fluid and solutes. A growing volume of evidence indicates that these cell adhesion molecules are important early targets for a variety of nephrotoxic substances including metals, drugs, and venom components. In addition, it is now widely appreciated that molecules, such as intracellular adhesion molecule-1 (ICAM-1), integrins, and selectins play important roles in the recruitment of leukocytes and inflammatory responses that are associated with nephrotoxic injury. This review summarizes the results of recent in vitro and in vivo studies indicating that these cell adhesion molecules may be primary molecular targets in many types of chemically-induced renal injury. Some of the specific agents that are discussed include cadmium (Cd), mercury (Hg), bismuth (Bi), cisplatin, aminoglycoside antibiotics, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), and various venom toxins. This review also includes a discussion of the various mechanisms, by which these substances can affect cell adhesion molecules in the kidney.

AMP-activated protein kinase regulates the assembly of epithelial tight junctions

AMP activated protein kinase (AMPK), a sensor of cellular energy status in all eukaryotic cells, is activated by LKB1-dependent phosphorylation. Recent studies indicate that activated LKB1 induces polarity in epithelial cells and that this polarization is accompanied by the formation of tight junction structures. We wished to determine whether AMPK also contributes to the assembly of tight junctions in the epithelial cell polarization process. We found that AMPK is activated during calcium-induced tight junction assembly. Activation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleoside facilitates tight junction assembly under conditions of normal extracellular Ca2+ concentrations and initiates tight junction assembly in the absence of Ca2+ as revealed by the relocation of zonula occludens 1, the establishment of transepithelial electrical resistance, and the paracellular flux assay. Expression of a dominant negative AMPK construct inhibits tight junction assembly in MDCK cells, and this defect in tight junction assembly can be partially ameliorated by rapamycin. These results suggest that AMPK plays a role in the regulation of tight junction assembly.

Changes in the distribution of ZO-1, occludin, and claudins in the rat uterine epithelium during the estrous cycle

During the estrous cycle, the endometrium epithelium experiences marked cellular structural changes. For fertilization to proceed, maintenance of an adequate uterine environment by ovarian hormones is essential. Epithelial cells lining the uterine lumen are associated with each other by tight junctions (TJs), which regulate the passage of ions and molecules through the paracellular pathway. The aim of the present study was to assess by confocal immunofluorescence the distribution pattern of the TJ proteins ZO-1, occludin, and claudins 1-7 in the rat uterus during the estrous cycle. Our results reveal that on proestrus, the day when mating takes place, ZO-1, occludin, and claudins 1 and 5 are located in the TJs, while claudins 3 and 7 display a basolateral distribution. In contrast, on metestrus day, when no sexual mating occurs and the uterine lumen is devoid of secretions, none of these proteins were detected in the TJ region, and only a diffuse cytosolic staining was observed for some of the proteins. On estrus and diestrus days, an intermediate situation was encountered, since ZO-1 localized in the TJs, whereas occludin was no longer detectable in the TJs. The distribution of claudins during these stages varied from the lowermost portion of the basolateral membrane to its apex. In conclusion, the results show that the protein composition of TJs present in the luminal epithelial cells of the uterus changes during the different days of the estrous cycle, and suggest that the expression of TJ proteins participates in providing an adequate environment for a successful fertilization.

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.

Pulses of cell Ca(2+) and the dynamics of tight junction opening and closing

A mathematical modeling of tight junction (TJ) dynamics was elaborated in a previous study to better understand the dynamics of TJ opening and closing, as well as oscillations of TJ permeability that are observed in response to changes of extracellular Ca(2+) levels. In this model, TJs were assumed to be specifically controlled by the Ca(2+) concentration levels at the extracellular Ca(2+) binding sites of zonula adhaerens. Despite the fact that the model predicts all aspects of TJ dynamics, we cannot rule out the likelihood that changes of intracellular Ca(2+) concentration (Ca(2+) (cell)), which might result from changes \ of extracellular Ca(2+) concentration (Ca(2+) (extl)), contribute to the observed results. In order to address this aspect of TJ regulation, fast Ca(2+)-switch experiments were performed in which changes of Ca(2+) (cell) were induced using the Ca(2+) ionophore A23187 or thapsigargin, a specific inhibitor of the sarco-endoplasmic reticulum Ca(2+)-ATPase. The results indicate that the ionophore or thapsigargin per se do not affect basal tissue electrical conductance ( G), showing that the sealing of TJs is not affected by a rise in Ca(2+) (cell). When TJs were kept in a dynamic state, as partially open structures or in oscillation, conditions in which the junctions are very sensitive to disturbances that affect their regulation, a rise of Ca(2+) (cell) never led to a decline of G, indicating that a rise of Ca(2+) (cell) does not trigger per se TJ closure. On the contrary, always the first response to a rise of Ca(2+) (cell) is an increase of G that, in most cases, is a transient response. Despite these observations we cannot assure that a rise of Ca(2+) (cell) is without effect on the TJs, since an increase of Ca(2+) (cell) not only causes a transient increase of G but, in addition, during oscillations a rise of Ca(2+) (cell) induced by the Ca(2+) ionophore transiently halted the oscillatory pattern of TJs. The main conclusion of this study is that TJ closure that is observed when basolateral Ca(2+) concentration (Ca(2+) (bl)) is increased after TJs were opened by Ca(2+) (bl) removal cannot be ascribed to a rise of Ca(2+) (cell) and might be a consequence of Ca(2+) binding to extracellular Ca(2+) sites.

Claudin-2 expression induces cation-selective channels in tight junctions of epithelial cells

Tight junctions seal the paracellular pathway of epithelia but, in leaky tissues, also exhibit specific permeability. In order to characterize the contribution of claudin-2 to barrier and permeability properties of the tight junction in detail, we studied two strains of Madin-Darby canine kidney cells (MDCK-C7 and MDCK-C11) with different tight junctional permeabilities. Monolayers of C7 cells exhibited a high transepithelial resistance (>1 kOhms cm(2)), compared with C11 cells (<100 Ohms cm(2)). Genuine expression of claudin-1 and claudin-2, but not of occludin or claudin-3, was reciprocal to transepithelial resistance. However, confocal microscopy revealed a marked subjunctional localization of claudin-1 in C11 cells, indicating that claudin-1 is not functionally related to the low tight junctional resistance of C11 cells. Strain MDCK-C7, which endogenously does not express junctional claudin-2, was transfected with claudin-2 cDNA. In transfected cells, but not in vector controls, the protein was detected in colocalization with junctional occludin by means of immunohistochemical analyses. Overexpression of claudin-2 in the originally tight epithelium with claudin-2 cDNA resulted in a 5.6-fold higher paracellular conductivity and relative ion permeabilities of Na(+) identical with 1, K(+)=1.02, NMDG(+)=0.79, choline(+)=0.71, Cl(-)=0.12, Br(-)=0.10 (vector control, 1:1.04:0.95:0.94:0.85:0.83). By contrast, fluxes of (radioactively labeled) mannitol and lactulose and (fluorescence labeled) 4 kDa dextran were not changed. Hence, with regular Ringer's, Na(+) conductivity was 0.2 mS cm(-2) in vector controls and 1.7 mS cm(-2) in claudin-2-transfected cells, while Cl(-) conductivity was 0.2 mS cm(-2) in both cells. Thus, presence of junctional claudin-2 causes the formation of cation-selective channels sufficient to transform a 'tight' tight junction into a leaky one.

Single-cell epithelial defects close rapidly by an actinomyosin purse string mechanism with functional tight junctions

Restitution of single-cell defects, a frequent event in epithelia with high turnover, is poorly understood. Morphological and functional changes were recorded, using intravital time-lapse video microscopy, confocal fluorescence microscopy, and conductance scanning techniques. After artificial single-cell loss from an HT-29/B6 colonic cell monolayer, the basal ends of adjacent cells extended. Concurrently, the local conductive leak associated with the defect sealed with an exponential time course (from 0.48 +/- 0.05 microS 2 min post lesion to 0.17 +/- 0.02 microS 8 min post lesion, n = 17). Between 3 and 10 min post lesion, a band of actin arose around the gap, which colocalized with a ring of ZO-1 and occludin. Hence, tight junction proteins bound to the actin band facing the gap, and competent tight junctions assembled in the adjoining cell membranes. Closure and sealing were inhibited when actin polymerization was blocked by cytochalasin D, delayed following decrease of myosin-ATPase activity by butanedione monoxime, and blocked after myosin light chain kinase inhibition by ML-7. The Rho-associated protein kinase inhibitor Y-27632 did not affect restitution. After loosening of intercellular contacts in low Ca(2+) Ringer solution, the time course of restitution was not significantly altered. Albeit epithelial conductivity was 12-fold higher in low Ca(2+) Ringer solution than in controls, under both conditions the repaired epithelium assumed the same conductivity as distant intact epithelium. In conclusion, epithelial restitution of single-cell defects comprises rapid closure by an actinomyosin 'purse-string' mechanism and simultaneous formation of a functional barrier from tight junction proteins also associated with the purse string.

Molecular physiology and pathophysiology of tight junctions. I. Biogenesis of tight junctions and epithelial polarity

The tight junction (TJ) was first noticed through its ability to control permeation across the paracellular route, but the homologies of its molecular components with peptides that participate in tumor suppression, nuclear addressing, and cell proliferation indicate that it may be involved in many other fundamental functions. TJs are formed by a dozen molecular species that assemble through PDZ and other protein-protein clustering promoting sequences, in response to the activation of E-cadherin. The TJ occupies a highly specific position between the apical and the basolateral domains. Its first molecular components seem to be delivered to such a position by addressing signals in their molecule and, once anchored, serve as a clustering nucleus for further TJ-associated molecules. Although in mature epithelial cells TJs and E-cadherin do not colocalize, a complex chain of reactions goes from one to the other that involves alpha-, beta-, and gamma-catenins, two different G proteins, phospholipase C, protein kinase C, calmodulin, mitogen-activated protein kinase, and molecules pertaining to the cytoskeleton, which keep the TJ sensitive to physiological requirements and local conditions (notably to Ca(2+)-dependent cell-cell contacts) throughout the life of the epithelium.

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.

H(2)O(2)-mediated permeability: role of MAPK and occludin

H(2)O(2)-mediated elevation in endothelial solute permeability is associated with pathological events such as ischemia-reperfusion and inflammation. To understand how H(2)O(2) mediates increased permeability, we investigated the effects of H(2)O(2) administration on vascular endothelial barrier properties and tight junction organization and function. We report that H(2)O(2) exposure caused an increase in endothelial solute permeability in a time-dependent manner through extracellularly regulated kinase 1 and 2 (ERK1/ERK2) signal pathways. H(2)O(2) exposure caused the tight junctional protein occludin to be rearranged from endothelial cell-cell junctions. Occludin rearrangement involved redistribution of occludin on the cell surface and dissociation of occludin from ZO-1. Occludin also was heavily phosphorylated on serine residues upon H(2)O(2) administration. H(2)O(2) mediates changes in ERK1/ERK2 phosphorylation, increases endothelial solute permeability, and alters occludin localization and phosphorylation were all blocked by PD-98059, a specific mitogen-activated protein (MAP) or ERK kinase 1 inhibitor. These data strongly suggest that H(2)O(2)-mediated increased endothelial solute permeability involves the loss of endothelial tight junction integrity through increased ERK1/ERK2 activation.

Tight junction proteins ZO-1, ZO-2, and occludin along isolated renal tubules

BACKGROUND

Tight junctions play a critical role in tubular function. In mammalian kidney, the transepithelial electrical resistance and the complexity of the tight junction increase from the proximal to the collecting tubule. The differential expression of three tight junction proteins, ZO-1, ZO-2, and occludin, along isolated rabbit renal tubules is examined in this article.

METHODS

Microdissected rabbit renal tubules were processed for immunofluorescence detection of ZO-1, ZO-2, and occludin. The quantitation of these proteins was done by Western blot determinations in Percoll isolated tubules.

RESULTS

ZO-1 stained cell boundaries independently of the identity of the tubule. However, the amount found in distal segments was significantly higher than that expressed in proximal regions. ZO-2 in the proximal region was found diffusely distributed in the cytoplasm, with faint staining at cell borders, while a clear signal at cell perimeters was detectable from the Henle's loop to collecting tubules. Nuclear staining of ZO-2 was found along the whole nephron. The presence of occludin at the proximal region was faint and discontinuous, while its expression in the more distant portions was conspicuous. The quantity of ZO-2 and occludin present at the distal region was significantly higher compared with the proximal segment.

CONCLUSIONS

The distribution of ZO-1, ZO-2, and occludin follows the increase in junction complexity encountered in renal tubules. The amount of the three proteins found in proximal and distal segments is significantly higher in the latter.

Evidence that E-cadherin may be a target for cadmium toxicity in epithelial cells

E-cadherin is a Ca(2+)-dependent cell adhesion molecule that plays an important role in the development and maintenance of epithelial polarity and barrier function. This commentary describes the results of recent studies showing that the environmental pollutant Cd(2+) can damage the E-cadherin-dependent junctions between many types of epithelial cells and reviews the evidence indicating that this effect results from the direct interaction of Cd(2+) with the E-cadherin molecule. In addition, the implications of these findings with respect to the mechanisms of Cd(2+) toxicity in specific target organs such as lung, kidney, bone, and the vascular endothelium are discussed.

Tight junctions of the blood-brain barrier

1. The blood-brain barrier is essential for the maintenance and regulation of the neural microenvironment. The blood-brain barrier endothelial cells comprise an extremely low rate of transcytotic vesicles and a restrictive paracellular diffusion barrier. The latter is realized by the tight junctions between the endothelial cells of the brain microvasculature, which are subject of this review. Morphologically, blood-brain barrier-tight junctions are more similar to epithelial tight junctions than to endothelial tight junctions in peripheral blood vessels. 2. Although blood-brain barrier-tight junctions share many characteristics with epithelial tight junctions, there are also essential differences. However, in contrast to tight junctions in epithelial systems, structural and functional characteristics of tight junctions in endothelial cells are highly sensitive to ambient factors. 3. Many ubiquitous molecular constituents of tight junctions have been identified and characterized including claudins, occludin, ZO-1, ZO-2, ZO-3, cingulin, and 7H6. Signaling pathways involved in tight junction regulation comprise, among others, G-proteins, serine, threonine, and tyrosine kinases, extra- and intracellular calcium levels, cAMP levels, proteases, and TNF alpha. Common to most of these pathways is the modulation of cytoskeletal elements which may define blood-brain barrier characteristics. Additionally, cross-talk between components of the tight junction- and the cadherin-catenin system suggests a close functional interdependence of the two cell-cell contact systems. 4. Recent studies were able to elucidate crucial aspects of the molecular basis of tight junction regulation. An integration of new results into previous morphological work is the central intention of this review.

Presenilin-1 forms complexes with the cadherin/catenin cell-cell adhesion system and is recruited to intercellular and synaptic contacts

In MDCK cells, presenilin-1 (PS1) accumulates at intercellular contacts where it colocalizes with components of the cadherin-based adherens junctions. PS1 fragments form complexes with E-cadherin, beta-catenin, and alpha-catenin, all components of adherens junctions. In confluent MDCK cells, PS1 forms complexes with cell surface E-cadherin; disruption of Ca(2+)-dependent cell-cell contacts reduces surface PS1 and the levels of PS1-E-cadherin complexes. PS1 overexpression in human kidney cells enhances cell-cell adhesion. Together, these data show that PS1 incorporates into the cadherin/catenin adhesion system and regulates cell-cell adhesion. PS1 concentrates at intercellular contacts in epithelial tissue; in brain, it forms complexes with both E- and N-cadherin and concentrates at synaptic adhesions. That PS1 is a constituent of the cadherin/catenin complex makes that complex a potential target for PS1 FAD mutations.

Relationship between Na(+),K(+)-ATPase and cell attachment

A prolonged ouabain blockade of the Na(+),K(+)-ATPase detaches cells from each other and from the substrate. This suggests the existence of a link between pump (P) and attachment (A). In the present work, we report that MDCK-W cells treated with ouabain increase tyrosine phosphorylation and content of active MAP kinase, redistribute molecules involved in cell attachment (occludin, ZO-1, desmoplakin, cytokeratin, alpha-actinin, vinculin and actin), and detach. Genistein and UO126, inhibitors of protein tyrosine kinase and of MAP kinase kinase, respectively, block this detachment. The content of P190(Rho-GAP), a GTPase activating protein of the Rho small G-protein subfamily, is increased by ouabain, suggesting that both the Rho/Rac and MAPK pathways are involved. Another clone of MDCK cells whose Na(+),K(+)-ATPase has a negligible affinity for the drug, show none of the effects described for MDCK-W and remain attached. Ma104 cells, a line that has a high affinity for ouabain and stops pumping, fail to modify phosphorylation, as well as the pattern of distribution of attaching molecules, and remain in the monolayer. Taken together, these results suggest that there is a mechanism (P--&gt;A) that transduces a blockade of the pump in a detachment of the cell from neighbors and substrate, in which Ma104 cells are faulty.

A theory for the mechanism of homocysteine-induced vascular pathogenesis

An attempt is made to reveal the mechanism of homocysteine-induced vascular pathogenesis and a theory is developed on the basis of an analysis of available data. In the blood, homocysteine molecules, which possess the capacity of forming chelate complexes with metallic cations including calcium ions, interact with the calcium ions of the calcium-dependent cell adhesions/junctions of the vascular endothelium. Such an action results in the departure of calcium ions from some cell adhesions/junctions, causing the latter structures to dissociate and the vascular endothelium to be injured. While such factors as high levels of plasma calcium ion tend to restore the disrupted cell adhesions/junctions, other factors including blood pressure, bloodstream shearing force and high cholesterol/phospholipid ratio in the membranes are unfavorable to the repair. The injuries to the vascular endothelium may initiate inflammatory reactions. Over a long time, as such endothelial injuries occur repeatedly and, additionally, a small number of unrepaired cell adhesion/junction disruptions exacerbate slowly but progressively, inflammation may be constantly present. As the inflammation becomes excessive or uncontrollable, irreversible vascular pathogenesis takes place. Added to the condition is thrombosis caused by excessive coagulation activities triggered by the vascular endothelium injuries. In addition to the interference due to decreased membrane fluidity to the restoration of interrupted cell adhesions/junctions, cholesterol may further promote vascular pathogenesis (induced by either homocysteine or other factors) by impeding the detachment and departure of the foam cells from the vascular lesions. Such impediment is presumably related to an inhibitory effect on cell shape change and cell movement by the formation of a stable and rigid cytoskeleton-membrane network characterized by reduced membrane malleability and enhanced cytoskeleton-membrane association.

A theory for the mechanism of action of the alpha-hydroxy acids applied to the skin

A theory for the mechanism of action of alpha-hydroxy acids topically applied to the skin is proposed on the basis of an analysis with the experimental and clinical data. The alpha-hydroxy acids reduce the calcium ion concentration in the epidermis and remove calcium ions from the cell adhesions by chelation. This causes a loss of calcium ions from the cadherins of the desmosomes and adherens junctions, from the tight junctions, and possibly also from other divalent metallic cation-dependent cell adhesion molecules. The cell adhesions are thereby disrupted, resulting in desquamation. Desquamation is enhanced by cleavage of the endogenous stratum corneum chymotryptic enzyme on the cadherins, which are otherwise protected from proteolysis by conjugation with calcium ions. The decrease of calcium ion level so brought about in the epidermis also tends to promote cell growth and retard cell differentiation, giving rise to a younger-looking skin. This property of alpha-hydroxy acids suggests that caution should be taken with excessive and chronic use of these compounds and studies in this regard are warranted. Alpha-hydroxy acids may also possess anti-inflammatory capacities.

Influence of temperature and membrane lipid composition on the osmotic water permeability of teleost gills

Osmotic water uptake was measured gravimetrically in isolated, ligated gill arches from trout (acclimated to and incubated at 5 degrees and 20 degrees C) and tilapia (21.5 degrees and 33 degrees C). For both species, incubation of arches at the higher temperature led to 1.5- to 3-fold greater measures of water weight gain. However, gills from warmer-acclimated trout and tilapia had 1- to >3-fold lower the initial rate and 1.5- to >2.5-fold lower the extent of water uptake seen in colder-acclimated conspecifics. Both the incubation temperature sensitivity and the acclimation effects are consistent with transmembrane water permeation. Calcium-free incubations (permitting paracellular water movement) also indicated that interfacial cell membranes contribute to gill permeability characteristics; without calcium, trout gill osmotic water uptake values increased 1.5- to 2-fold, and the temperature dependence of water uptake decreased (initial rate) or was eliminated (extent). The specific contribution of cholesterol to restricting barrier membrane water permeability was indicated by concentration-dependent increases in water uptake in the presence of either nystatin (a cholesterol-complexing, pore-forming agent) or methyl-beta-cyclodextrin (which selectively depletes membrane cholesterol). In addition, a cholesterol-specific cytochemical probe (filipin) intensely labeled the apical surface membranes of trout and tilapia gill epithelium. In summary, these studies implicate membrane cholesterol in determining water permeability in fish gills.

Calcium-dependent actin filament-severing protein scinderin levels and localization in bovine testis, epididymis, and spermatozoa

We assessed the levels and localization of the actin filament-severing protein scinderin, in fetal and adult bovine testes, and in spermatozoa during and following the epididymal transit. We performed immunoblots on seminiferous tubules and interstitial cells isolated by enzymatic digestion, and on bovine chromaffin cells, spermatozoa, aorta, and vena cava. Immunoperoxidase labeling was done on Bouin's perfusion-fixed testes and epididymis tissue sections, and on spermatozoa. In addition, immunofluorescence labeling was done on spermatozoa. Immunoblots showed one 80-kDa band in chromaffin cells, fetal and adult tubules, interstitial cells, spermatozoa, aorta, and vena cava. Scinderin levels were higher in fetal than in adult seminiferous tubules but showed no difference between fetal and adult interstitial cells. Scinderin levels were higher in epididymal than in ejaculated spermatozoa. Scinderin was detected in a region corresponding with the subacrosomal space in the round spermatids and with the acrosome in the elongated spermatids. In epididymal spermatozoa, scinderin was localized to the anterior acrosome and the equatorial segment, but in ejaculated spermatozoa, the protein appeared in the acrosome and the post-equatorial segment of the head. In Sertoli cells, scinderin was detected near the cell surface and within the cytoplasm, where it accumulated near the base in a stage-specific manner. In the epididymis, scinderin was localized next to the surface of the cells; in the tail, it collected near the base of the principal cells. In Sertoli cells and epididymal cells, scinderin may contribute to the regulation of tight junctional permeability and to the release of the elongated spermatids by controlling the state of perijunctional actin. In germ cells, scinderin may assist in the shaping of the developing acrosome and influence the fertility of the spermatozoa.

Molecular characterization of the tight junction protein ZO-1 in MDCK cells

Most of the information on the structure and function of the tight junction (TJ) has been obtained in MDCK cells. Accordingly, we have sequenced ZO-1 in this cell type, because this protein is involved in the response of the TJ to changes in Ca2+, phosphorylation, and the cytoskeleton. ZO-1 of MDCK cells comprises 6805 bp with a predicted open reading frame of 1769 amino acids. This sequence is 92 and 87% homologous to human and mouse ZO-1, respectively. Two nuclear sorting signals located at the PDZ1 and GK domains and 17 SH3 putative binding sites at the proline-rich domain were detected. We found two new splicing regions at the proline-rich region: beta had not been reported in human and mouse counterparts, and gamma, which was previously sequenced in human and mouse ZO-1, is now identified as a splicing region. The expression of different beta and gamma isoforms varies according to the tissue tested. With the information provided by the sequence, Southern blot, and PCR experiments we can predict a single genomic copy of MDCK-ZO-1 that is at least 13.16 kb long. MDCK-ZO-1 mRNA is 7.4 kb long. Its expression is regulated by calcium, while the expression of MDCK-ZO-1 protein is not.

Purinergic receptor-induced changes in paracellular resistance across cultures of human cervical cells are mediated by two distinct cytosolic calcium-related mechanisms

In human cervical (CaSki) cells, extracellular adenosine triphosphate (ATP) induces an acute decrease in the resistance of the lateral intercellular space (RLIS), phase I response, followed by an increase in tight junctional resistance (RTJ), phase II response. ATP also stimulates release of calcium from intracellular stores, followed by augmented calcium influx, and both effects have similar sensitivities to ATP (EC50 of 6 microM). The objective of the study was to determine the degree to which the changes in [Ca2+]i mediate the responses to ATP. 1,2-bis (2-aminophenoxy) ethane-N,N,N1,N1-tetraacetic acid (BAPTA) abrogated calcium mobilization and phase I response; in contrast, nifedipine and verapamil inhibited calcium influx and attenuated phase II response. Barium, La3+, and Mn2+ attenuated phase I response and attenuated and shortened the ionomycin-induced phase I-like decrease in RLIS, suggesting that store depletion-activated calcium entry was inhibited. Barium and La3+ also inhibited the ATP-induced phase II response, but Mn2+ had no effect on phase II response, and in the presence of low extracellular calcium it partly restored the increase in RTJ. KCl-induced membrane depolarization stimulated an acute decrease in RLIS and a late increase in RTJ similar to ATP, but only the latter was inhibited by nifedipine. KCl also induced a nifedipine-sensitive calcium influx, suggesting that acute increases in [Ca2+]i, regardless of mobilization or influx, mediate phase I response. Phase II-like increases in RTJ could be induced by treatment with diC8, and were not affected by nifedipine. Biphasic, ATP-like changes in RTE could be induced by treating the cells with ionomycin plus diC8. We conclude that calcium mobilization mediates the early decrease in RLIS, and calcium influx via calcium channels activates protein kinase C and mediates the late increase in RTJ.

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.

Aspects of the structure and assembly of desmosomes

Desmosomes are found principally in epithelial cells and consist of disc-like plaques, the extracellular face of which is paired with that of a neighbouring cell. There is increasing evidence that desmosomes are adhesive structures, and that two types of desmosomal glycoproteins, the desmogleins (Dsg) and desmocollins (Dsc) both Ca(2+)-binding cadherin-like molecules, perform this role in adhesion through interaction of their extracellular domains. A number of isoforms of Dsg and Dsc are present in specific tissues. The cytoplasmic side of the plaque is attached to intermediate filaments through desmoplakin, a major plaque protein. Also associated with desmosomes are plakoglobin and beta-catenin, suggesting that the adhesive function of desmosomes might be mediated by signal transduction. Formation of desmosomes can be studied by growing epithelial cells in low-Ca2+ medium (LCM, < 0.1 mM), where desmosomal proteins are either synthesized but not assembled, or form partially assembled but unstable half-desmosomes. Addition of Ca2+ (to about 2mM) initiates cell contact and, in the case of half-desmosomes, leads to stabilization by incorporation into membranes and formation of typical paired structures. In cases where such pre-assembled structures are not formed, recruitment of desmosomal proteins appears to occur by vesicular transport of desmocollins and desmogleins to the cell surface, where association is made with plakoglobin and later, with desmoplakin. Although much remains to be learned of the assembly process, specific interacting domains of the molecular components are being recognized. Desmosome assembly is part of a coordinated pattern of junction formation which accompanies the establishment of cell polarity, resulting in differentiation of apical and basolateral cell surfaces. Desmosomes are now being regarded, not as static and inert structures, but as membrane specializations linked to systems involved in cell-cell communication as well as adhesion.

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.

Aspirin-like drugs cause gastrointestinal injuries by metallic cation chelation

Aspirin-like (nonsteroidal anti-inflammatory) drugs may cause injuries including ulcers to the gastrointestinal tract by chelation of the divalent and/or multivalent metallic cations in the gastrointestinal mucus and mucosa, as suggested by the chemical properties of these drugs and supported by experimental and clinical data.

Calcium site specificity. Early Ca2+-related tight junction events

The molecular mechanisms by which Ca2+ and metal ions interact with the binding sites that modulate the tight junctions (TJs) have not been fully described. Metal ions were used as probes of these sites in the frog urinary bladder. Basolateral Ca2+ withdrawal induces the opening of the TJs, a process that is abruptly terminated when Ca2+ is readmitted, and is followed by a complete recovery of the TJ seal. Mg2+ and Ba2+ were incapable of keeping the TJ sealed or of inducing TJ recovery. In addition, Mg2+ causes a reversible concentration-dependent inhibition of the Ca2+-induced TJ recovery. The effects of extracellular Ca2+ manipulation on the TJs apparently is not mediated by changes of cytosolic Ca2+ concentration. The transition elements, Mn2+ and Cd2+, act as Ca2+ agonists. In the absence of Ca2+, they prevent TJ opening and almost immediately halt the process of TJ opening caused by Ca2+ withdrawal. In addition, Mn2+ promotes an almost complete recovery of the TJ seal. Cd2+, in spite of stabilizing the TJs in the closed state and halting TJ opening, does not promote TJ recovery, an effect that apparently results from a superimposed toxic effect that is markedly attenuated by the presence of Ca2+. The interruption of TJ opening caused by Ca2+, Cd2+, or Mn2+, and the stability they confer to the closed TJs, might result from the interaction of these ions with E-cadherin. Addition of La3+ (2 microM) to the basolateral Ca2+-containing solution causes an increase of TJ permeability that fully reverses when La3+ is removed. This effect of La3+, observed in the presence of Ca2+ (1 mM), indicates a high La3+ affinity for the Ca2+-binding sites. This ability of La3+ to open TJs in the presence of Ca2+ is a relevant aspect that must be considered when using La3+ in the evaluation of TJ permeability of epithelial and endothelial membranes, particularly when used during in vivo perfusion or in the absence of fixatives.

Regulation of cell-cell adhesion by rac and rho small G proteins in MDCK cells

The Rho small G protein family, consisting of the Rho, Rac, and Cdc42 subfamilies, regulates various cell functions, such as cell shape change, cell motility, and cytokinesis, through reorganization of the actin cytoskeleton. We show here that the Rac and Rho subfamilies furthermore regulate cell-cell adhesion. We prepared MDCK cell lines stably expressing each of dominant active mutants of RhoA (sMDCK-RhoDA), Rac1 (sMDCK-RacDA), and Cdc42 (sMDCK-Cdc42DA) and dominant negative mutants of Rac1 (sMDCK-RacDN) and Cdc42 (sMDCK-Cdc42DN) and analyzed cell adhesion in these cell lines. The actin filaments at the cell-cell adhesion sites markedly increased in sMDCK-RacDA cells, whereas they apparently decreased in sMDCK-RacDN cells, compared with those in wild-type MDCK cells. Both E-cadherin and beta-catenin, adherens junctional proteins, at the cell-cell adhesion sites also increased in sMDCK-RacDA cells, whereas both of them decreased in sMDCK-RacDN cells. The detergent solubility assay indicated that the amount of detergent-insoluble E-cadherin increased in sMDCK-RacDA cells, whereas it slightly decreased in sMDCK-RacDN cells, compared with that in wild-type MDCK cells. In sMDCK-RhoDA, -Cdc42DA, and -Cdc42DN cells, neither of these proteins at the cell-cell adhesion sites was apparently affected. ZO-1, a tight junctional protein, was not apparently affected in any of the transformant cell lines. Electron microscopic analysis revealed that sMDCK-RacDA cells tightly made contact with each other throughout the lateral membranes, whereas wild-type MDCK and sMDCK-RacDN cells tightly and linearly made contact at the apical area of the lateral membranes. These results suggest that the Rac subfamily regulates the formation of the cadherin-based cell- cell adhesion. Microinjection of C3 into wild-type MDCK cells inhibited the formation of both the cadherin-based cell-cell adhesion and the tight junction, but microinjection of C3 into sMDCK-RacDA cells showed little effect on the localization of the actin filaments and E-cadherin at the cell-cell adhesion sites. These results suggest that the Rho subfamily is necessary for the formation of both the cadherin-based cell- cell adhesion and the tight junction, but not essential for the Rac subfamily-regulated, cadherin-based cell- cell adhesion.

Cadmium (Cd2+) disrupts E-cadherin-dependent cell-cell junctions in MDCK cells

Previous studies from our laboratory have shown that Cd2+ can selectively disrupt E-cadherin-dependent cell-cell junctions in the porcine renal epithelial cell line, LLC-PK1. The objective of the present studies was to determine whether or not Cd2+ could produce similar effects in Madin-Darby canine kidney (MDCK) cells, an immortal epithelial cell line derived from dog kidney. This is an important issue because MDCK cells have been used extensively as a model system to study the basic mechanisms of E-cadherin-dependent cell-cell adhesion. MDCK cells on permeable membrane supports were exposed to Cd2+ by adding CdCl2 to either the apical or the basolateral compartment. The integrity of cell-cell junctions was assessed by morphologic observation of the cells and by monitoring the transepithelial electrical resistance. The results showed that exposure to 10-40 microM Cd2+ for 15 min-4 h caused the cells to separate from each other without detaching from the growing surface. The separation of the cells was accompanied by a marked drop in the transepithelial electrical resistance, a loss of E-cadherin from the cell-cell contacts, and a reorganization of the actin cytoskeleton. These effects were much more pronounced when Cd2+ was added basolaterally than when it was added apically. Moreover, the effects of Cd2+ were qualitatively similar to those observed when the cells were incubated in Ca(2+)-free medium. These results show that Cd2+ can disrupt E-cadherin-dependent cell-cell junctions in MDCK cells, and they indicate that this cell line would be an appropriate model for further mechanistic studies in this area.

Comparison of tight junction permeability for albumin in iris pigment epithelium and retinal pigment epithelium in vitro

BACKGROUND

The degenerative retinal diseases are one of the major causes of visual loss in the western world. Although heterologous RPE transplants rescue the photoreceptors in the dystrophic rat model, rejection remains a major limiting factor. Given the common embryonic origin, iris pigment epithelial (IPE) cells might be able to take over the functions of retinal pigment epithelial (RPE) cells, serving as an autologous graft for transplantation and thereby preventing rejection. One of the main functions of RPE cells is the generation of tight junctions which form the outer blood-retinal barrier. In this study we compared the tight junction permeabilities of IPE and RPE cells isolated from Long Evans rats by measuring their albumin clearances.

METHODS

IPE and RPE cells were cultured on semipermeable filter supports with and without the addition of 0.02% ethylenediaminetetraacetic acid (EDTA). At selected intervals, the albumin clearances of the IPE and RPE cells were measured spectrophotometrically and compared. The morphology of the cells was compared using electron microscopy and fluorescent labeling.

RESULTS

IPE and RPE cells both restricted the passage of albumin in vitro. After the modulation of tight junctions with 0.02% EDTA, the clearance increased in both types of cells in a similar fashion. The morphology of tight junctions was visualized with electron microscopy.

CONCLUSION

These results indicate that the functional barrier for macromolecules is similar in IPE and RPE cells in vitro. This raises the possibility that IPE cells would form tight junctions in the subretinal space, thereby substituting for the blood-retinal barrier normally formed by RPE cells.

Dlg protein is required for junction structure, cell polarity, and proliferation control in Drosophila epithelia

The Discs large (Dlg) protein of Drosophila is the prototypic member of a growing family of proteins termed membrane-associated guanylate kinase homologs (MAGUKs). The MAGUKs are composed of a series of peptide domains that include one or three DHR/PDZs, an SH3, and a region homologous to guanylate kinase (GUK). We have previously shown that the product of this gene, the Dlg protein, is localized at the septate junctions between epithelial cells, and that mutations in the gene cause neoplastic overgrowth of the imaginal discs. The dlg locus is therefore defined as a tumor suppressor gene. In this paper, we show that the Dlg protein is localized on the cytoplasmic face of the septate junction and is required for the maintenance of this structure. It is also required for proper organization of the cytoskeleton, for the differential localization of membrane proteins, and for apicobasal polarity of epithelial cells. However, these other functions can be uncoupled from Dlg's role as a tumor suppressor since mutations in two domains of the protein, the SH3 and GUK, cause loss of normal cell proliferation control without affecting the other functions of the protein. These results suggest that, besides regulating cellular proliferation, the Dlg protein is a critical component of the septate junctions and is required for maintaining apicobasal polarity in Drosophila epithelium.

Alterations in cell cholesterol content modulate Ca(2+)-induced tight junction assembly by MDCK cells

Transepithelial electrical resistance (TER), a measure of tight junction (TJ) barrier function, develops more rapidly and reaches higher values after preincubation of MDCK cells for 24 h with 2 microM Lovastatin (lova), an inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase. While this effect was attributed to a 30% fall in cholesterol (CH), possible effects of lova on the supply of prenyl group precursors could not be excluded. In the current study, strategies were devised to examine effects on TER of agents that simultaneously lower CH and increase the flux of intermediates through the CH biosynthetic pathway. Zaragozic acid, 20 microM, an inhibitor of squalene synthase known to increase the synthesis of isoprenoids and levels of prenylated proteins, lowered cell CH by 30% after 24 h, while accelerating development of TER in the same manner as lova. TER was also enhanced, despite a 23% increase in the rate of [3H]acetate incorporation into CH, when total CH was reduced by 45% during a 2-h incubation with 2 mM methyl beta-cyclodextrin (MBCD), an agent that stimulates CH efflux from cells. The fact that the rate of TER development was diminished when cell CH content was elevated by incubation with a complex of CH and MBCD is further evidence that this sterol modulates development of the epithelial barrier. Cell associated CH derived from the complex was similar to endogenous CH with respect to its accessibility to cholesterol oxidase. Lova's effect on TER was diminished when 5 micrograms/mL of CH was added to the medium during the last 11 h of incubation with lova.

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.