Occluding junctions in MDCK cells: modulation of transepithelial permeability by the cytoskeleton

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.

The mechanobiology of tight junctions

Tight junctions (TJ) play a central role in the homeostasis of epithelial and endothelial tissues, by providing a semipermeable barrier to ions and solutes, by contributing to the maintenance of cell polarity, and by functioning as signaling platforms. TJ are associated with the actomyosin and microtubule cytoskeletons, and the crosstalk with the cytoskeleton is fundamental for junction biogenesis and physiology. TJ are spatially and functionally connected to adherens junctions (AJ), which are essential for the maintenance of tissue integrity. Mechano-sensing and mechano-transduction properties of several AJ proteins have been characterized during the last decade. However, little is known about how mechanical forces act on TJ and their proteins, how TJ control the mechanical properties of cells and tissues, and what are the underlying molecular mechanisms. Here I review recent studies that have advanced our understanding of the relationships between mechanical force and TJ biology.

The cerebral cavernous malformation disease causing gene KRIT1 participates in intestinal epithelial barrier maintenance and regulation

Epithelial barrier maintenance and regulation requires an intact perijunctional actomyosin ring underneath the cell-cell junctions. By searching for known factors affecting the actin cytoskeleton, we identified Krev interaction trapped protein 1 (KRIT1) as a major regulator for epithelial barrier function through multiple mechanisms. KRIT1 is expressed in both small intestinal and colonic epithelium, and KRIT1 knockdown in differentiated Caco-2 intestinal epithelium decreases epithelial barrier function and increases cation selectivity. KRIT1 knockdown abolished Rho-associated protein kinase-induced and myosin II motor inhibitor-induced barrier loss by limiting both small and large molecule permeability but did not affect myosin light chain kinase-induced increases in epithelial barrier function. These data suggest that KRIT1 participates in Rho-associated protein kinase- and myosin II motor-dependent (but not myosin light chain kinase-dependent) epithelial barrier regulation. KRIT1 knockdown exacerbated low-dose TNF-induced barrier loss, along with increased cleaved caspase-3 production. Both events are blocked by pan-caspase inhibition, indicating that KRIT1 regulates TNF-induced barrier loss through limiting epithelial apoptosis. These data indicate that KRIT1 controls epithelial barrier maintenance and regulation through multiple pathways, suggesting that KRIT1 mutation in cerebral cavernous malformation disease may alter epithelial function and affect human health.-Wang, Y., Li, Y., Zou, J., Polster, S. P., Lightle, R., Moore, T., Dimaano, M., He, T.-C., Weber, C. R., Awad, I. A., Shen, L. The cerebral cavernous malformation disease causing gene KRIT1 participates in intestinal epithelial barrier maintenance and regulation.

Modulation of Intestinal Paracellular Transport by Bacterial Pathogens

The passive and regulated movement of ions, solutes, and water via spaces between cells of the epithelial monolayer plays a critical role in the normal intestinal functioning. This paracellular pathway displays a high level of structural and functional specialization, with the membrane-spanning complexes of the tight junctions, adherens junctions, and desmosomes ensuring its integrity. Tight junction proteins, like occludin, tricellulin, and the claudin family isoforms, play prominent roles as barriers to unrestricted paracellular transport. The past decade has witnessed major advances in our understanding of the architecture and function of epithelial tight junctions. While it has been long appreciated that microbes, notably bacterial and viral pathogens, target and disrupt junctional complexes and alter paracellular permeability, the precise mechanisms remain to be defined. Notably, renewed efforts will be required to interpret the available data on pathogen-mediated barrier disruption in the context of the most recent findings on tight junction structure and function. While much of the focus has been on pathogen-induced dysregulation of junctional complexes, commensal microbiota and their products may influence paracellular permeability and contribute to the normal physiology of the gut. Finally, microbes and their products have become important tools in exploring host systems, including the junctional properties of epithelial cells. © 2018 American Physiological Society. Compr Physiol 8:823-842, 2018.

Tension-Dependent Stretching Activates ZO-1 to Control the Junctional Localization of Its Interactors

Tensile forces regulate epithelial homeostasis, but the molecular mechanisms behind this regulation are poorly understood. Using structured illumination microscopy and proximity ligation assays, we show that the tight junction protein ZO-1 exists in stretched and folded conformations within epithelial cells, depending on actomyosin-generated force. We also show that ZO-1 and ZO-2 regulate the localization of the transcription factor DbpA and the tight junction membrane protein occludin in a manner that depends on the organization of the actin cytoskeleton, myosin-II activity, and substrate stiffness, resulting in modulation of gene expression, cell proliferation, barrier function, and cyst morphogenesis. Pull-down experiments show that interactions between N-terminal (ZPSG) and C-terminal domains of ZO-1 prevent binding of DbpA to the ZPSG, suggesting that force-dependent intra-molecular interactions regulate ZPSG binding to ligands within cells. In vivo and in vitro experiments also suggest that ZO-1 heterodimerization with ZO-2 promotes the stretched conformation and ZPSG interaction with ligands. Magnetic tweezers single-molecule experiments suggest that pN-scale tensions (∼2-4 pN) are sufficient to maintain the stretched conformation of ZO-1, while keeping its structured domains intact, and that 5-20 pN force is required to disrupt the interaction between the extreme C-terminal and the ZPSG domains of ZO-1. We propose that tensile forces regulate epithelial homeostasis by activating ZO proteins through stretching, to control the junctional recruitment and downstream signaling of their interactors.

Comparison of transepithelial resistance measurement techniques: Chopsticks vs. Endohm

Background

TER measurements across confluent cellular monolayers provide a useful indication of TJ strength between epithelial and endothelial cells in culture. Having a reliable and accurate method of measuring cell-to-cell adhesion is critical to studies in pathophysiology and cancer metastasis. However, the use of different technical approaches to measure TER has reportedly yielded inconsistent measurements within the same cell lines.

Methods

In the current study, we compared the peak TER values for the MDCK (canine kidney) and MCF-7 (human breast cancer) epithelial cell lines using two common approaches (Chopstick and Endohm) and two types of polymer inserts (PC and PET).

Results

Both cell lines demonstrated a statistically significant difference in the peak TERs obtained using the two different approaches. Further, the MDCK (but not the MCF-7) cells demonstrated a statistically significant difference between the peak TERs when using the same approach but different inserts.

Conclusion

Our study indicates the importance of using a single approach when seeking to measure and compare the TER values of cultured cell lines.

Extracts from Flammulina velutipes Inhibit the Adhesion of Pathogenic Fungi to Epithelial Cells

BACKGROUND

Recently, extracts from natural sources have been tested for their antifungal properties. In this aspect, Flammulina velutipes extracts possess a significant amount of branch-chained carbohydrates with mannose moieties that, hypothetically, can reduce the adhesion.

OBJECTIVE

In this study, we assessed the capacity of extracts from F. velutipes (wild-type AQF-1 and ATCC 34574 as the reference strain) to inhibit the adhesion of S. schenkii and C. albicans to epithelial cells.

MATERIALS AND METHODS

The aqueous extracts from F. velutipes strains were obtained by sonication, total carbohydrate and protein was analyzed by Dubois and Lowry methods respectively. Effect of the extracts (50, 100 and 150 μg/mL) on the fungi adhesion to host cells was evaluated after 1 h interaction, and the percentage of inhibition of adhesion was measured. After of interaction the cytoskeleton from cell was analyzed with phalloidin-FITC.

RESULTS

The extract from strain AQF-1 (50, 100 and 150 μg/mL) inhibited the adhesion of: S. schenkii in a dose-dependent manner (4.9, 7.5 and 12.7%, respectively) and C. albicans in a dose-independent manner (5.2%). The percentage of inhibition by extracts from the strain ATCC34574 at the same concentrations, shown that are dose independent for both fungi: 3.9% for S. schenkii and 2.6% for C. albicans.

CONCLUSION

The extracts from F. velutipes inhibit the adhesion of pathogenic fungi to host cells. The mechanism molecular is unknown; however, is probably an interaction between the polysaccharides from extracts with the fungi receptors. This aspect is currently analyzed.

SUMMARY

The yields of mycelium from two strains of F. velutipes and the extract from it were similar.Extracts from both strains have inhibited adhesion of S. schenkii and C. albicans to epithelial cells in vitro, but the extract from strain AQF-1 was more effective.The extracts have not prevented damage to epithelial cells caused by pathogenic fungi. Abbreviation Used: YPG: Yeast peptone glucose, DMEM: Dulbecco's Modified Eagle's medium, FITC: Fluorescein isothiocyanate.

The cytokine response of U937-derived macrophages infected through antibody-dependent enhancement of dengue virus disrupts cell apical-junction complexes and increases vascular permeability

Severe dengue (SD) is a life-threatening complication of dengue that includes vascular permeability syndrome (VPS) and respiratory distress. Secondary infections are considered a risk factor for developing SD, presumably through a mechanism called antibody-dependent enhancement (ADE). Despite extensive studies, the molecular bases of how ADE contributes to SD and VPS are largely unknown. This work compares the cytokine responses of differentiated U937 human monocytic cells infected directly with dengue virus (DENV) or in the presence of enhancing concentrations of a humanized monoclonal antibody recognizing protein E (ADE-DENV infection). Using a cytometric bead assay, ADE-DENV-infected cells were found to produce significantly higher levels of the proinflammatory cytokines interleukin 6 (IL-6), IL-12p70, and tumor necrosis factor alpha (TNF-α), as well as prostaglandin E2 (PGE2), than cells directly infected. The capacity of conditioned supernatants (conditioned medium [CM]) to disrupt tight junctions (TJs) in MDCK cell cultures was evaluated. Exposure of MDCK cell monolayers to CM collected from ADE-DENV-infected cells (ADE-CM) but not from cells infected directly led to a rapid loss of transepithelial electrical resistance (TER) and to delocalization and degradation of apical-junction complex proteins. Depletion of either TNF-α, IL-6, or IL-12p70 from CM from ADE-DENV-infected cells fully reverted the disrupting effect on TJs. Remarkably, mice injected intraperitoneally with ADE-CM showed increased vascular permeability in sera and lungs, as indicated by an Evans blue quantification assay. These results indicate that the cytokine response of U937-derived macrophages to ADE-DENV infection shows an increased capacity to disturb TJs, while results obtained with the mouse model suggest that such a response may be related to the vascular plasma leakage characteristic of SD.

CRTAM: A molecule involved in epithelial cell adhesion

Class I-restricted T cell associated molecule (CRTAM) is a member of the immunoglobulin superfamily that complies with the structural characteristics of the JAM family of proteins and is phylogenetically more closely related to nectin-like proteins. Here we demonstrate for the first time, that CRTAM is expressed in epithelial cells along the lateral membrane and is important for early cell-cell contacts and cell-substrate interactions. CRTAM is sensitive to intermediate filament disruption and treatment of monolayers with soluble CRTAM enhances cell-cell dissociation and lowers transepithelial electrical resistance. Incubation of newly plated cells with anti-CRTAM antibody decreases the formation of cell aggregates and promotes cell detachment. Co-cultures of epithelial cells and fibroblasts that lack CRTAM expression and in vitro binding assays, demonstrate the participation of CRTAM in homotypic and heterotypic trans-interactions. Hence we conclude that CRTAM is a molecule involved in epithelial cell adhesion.

Protective effect of Coenzyme Q(10) against oxidative damage in human lens epithelial cells by novel ocular drug carriers

The evaluation of N-trimethyl chitosan (TMC)-coated liposomes containing Coenzyme Q(10) as potential ophthalmic drug delivery system was carried out. Firstly, transcorneal permeation studies were conducted at 34°C using a side-by-side diffusion apparatus. The transport process of the fluorescent marker, rhodamine B, across the corneal epithelium was visualized with confocal laser scanning microscopy. Secondly, the human lens epithelial cells (HLECs) were cultured without or with Coenzyme Q(10) followed by addition of H(2)O(2). The cell viability and apoptosis were evaluated. The permeability coefficient for rhodamine B with TMC-coated liposomes increased more than two times in comparison with the value obtained for solution as control, from (0.42±0.018)×10(5)cms(-1) to (1.31±0.030)×10(5)cms(-1). Confocal laser scanning microscopy revealed that a TMC coating enhanced the transepithelial transport, dependent on the TMC concentration and contacting time. Coenzyme Q(10) elevated the cell viability and reduced the oxidative damage with the decreased percentage of apoptotic cells in a positive concentration-dependent manner. The ATP content of liposome-treated cells was increased about 2-fold compared with that of H(2)O(2)-treated cells. Together, our findings demonstrate that with the enhanced permeation effect of the TMC coating, Coenzyme Q(10)-loaded TMC-coated liposomes appear to be a promising ophthalmic drug delivery carrier with an efficacy in protecting HLECs against H(2)O(2)-induced oxidative damage.

Keratinocyte growth factor enhances barrier function without altering claudin expression in primary alveolar epithelial cells

Keratinocyte growth factor (KGF) has efficacy in several experimental models of lung injury; however, the mechanisms underlying KGF's protective effect remain incompletely understood. This study was undertaken to determine whether KGF augments barrier function in primary rat alveolar epithelial cells grown in culture, specifically whether KGF alters tight junction function via claudin expression. KGF significantly increased alveolar epithelial barrier function in culture as assessed by transepithelial electrical resistance (TER) and paracellular permeability. Fluorescence-activated cell sorting of freshly isolated type 1 (AT1) and type 2 (AT2) cells followed by quantitative real-time RT-PCR revealed that more than 97% of claudin mRNA transcripts in these cells were for claudins-3, -4, and -18. Using cultured AT2 cells, we then examined the effect of KGF on the protein levels of the claudins with the highest mRNA levels: -3, -4, -5, -7, -12, -15, and -18. KGF did not alter the levels of any of the claudins tested, nor of zona occludens-1 (ZO-1) or occludin. Moreover, localization of claudins-3, -4, -18, and ZO-1 was unchanged. KGF did induce a marked increase in the apical perijunctional F-actin ring. Actin depolymerization with cytochalasin D blocked the KGF-mediated increase in TER without significantly changing TER in control cells. Together, these data support a novel mechanism by which KGF enhances alveolar barrier function, modulation of the actin cytoskeleton. In addition, these data demonstrate the complete claudin expression profile for AT1 and AT2 cells and indicate that claudins-3, -4, and -18 are the primary claudins expressed in these cell types.

Rho signaling and tight junction functions

Tight junctions are heteromeric protein complexes that act as signaling centers by mediating the bidirectional transmission of information between the environment and the cell interior to control paracellular permeability and differentiation. Insight into tight junction-associated signaling mechanisms is of fundamental importance for our understanding of the physiology of epithelia and endothelia in health and disease.

Stimulus-induced reorganization of tight junction structure: the role of membrane traffic

The tight junction forms a barrier that limits paracellular movement of water, ions, and macromolecules. The permeability properties of this barrier are regulated in response to both physiological and pathophysiological stimuli, and this regulation has been modeled by pharmacological agents. Although it is now known that vesicular traffic plays important roles in tight junction assembly, the molecular mechanisms by which vesicular traffic contributes to tight junction regulation remain to be defined. This review summarizes recent progress in understanding mechanisms and pathways of tight junction protein internalization and the relevance of these to tight junction regulation.

Altered Morphology and Distribution of Cellular Junction Proteins in Non-Lesional Psoriatic Epidermis: An Insight into Disease Severity

BACKGROUND

Psoriasis affects 2.7% of the world's population. Keratinocyte proliferation outside the basal layer suggests alterations in cell-cell interactions in affected epidermis. Anomalous expression of proteins forming intercellular junctions has been reported in lesional skin of psoriatic patients. In contrast, little is known about possible alterations in psoriatic non-lesional skin.

METHODS

Ten clinically diagnosed psoriasis vulgaris patients and ten controls were studied. All patients were diagnosed with active but controlled psoriatic plates (PASI 3 to 5) and had not received any systemic treatment. The mean age was 43 years for patients and 43.5 years for controls. Four-mm2 skin samples were taken from lesional and non-lesional zones in patients and from abdomen in controls. Five-mum sections were examined for integrity and structural organization by fluorescent labeling of actin filaments and nuclei. Specific antibodies were utilized to localize occludin, E-cadherin, beta-catenin, and proliferation-specific keratins in sections and epidermal sheets. Samples were also processed for immunoblotting with occludin antibody.

RESULTS

Lesional and non-lesional psoriatic epidermis from all patients showed keratinocyte hyperproliferation, lack of rete ridges and dermal papillae in the dermal-epidermal junction in some areas. Proteins forming tight and adherens junctions in non-lesional skin keratinocytes from two patients who during the course of the study evolved to uncontrolled disease, showed similar alterations to those observed in lesional skin of all the patients. However, the occludin isoforms expressed were apparently the same in all samples.

CONCLUSIONS

Analysis of non-lesional skin in psoriatic patients diagnosed with controlled disease may provide clues about incipient structural abnormalities in the pathogenesis of psoriasis, providing an early diagnostic indicator for evolution to a generalized form of the disease.

Melatonin as a cytoskeletal modulator: implications for cell physiology and disease

The cytoskeleton is a phylogenetically well-preserved structure that plays a key role in cell physiology. Dynamic and differential changes in cytoskeletal organization occur in cellular processes according to the cell type and the specific function. In neurons, microtubules, microfilaments and intermediate filament (IF) rearrangements occur during axogenesis, and neurite formation which eventually differentiate into axons and dendrites to constitute synaptic patterns of connectivity. In epithelial cells, dynamic modifications occur in the three main cytoskeletal components and phosphorylation of cytoskeletal associated proteins takes place during the formation of the epithelial cell monolayer that eventually will transport water. In pathological processes such as neurodegenerative and psychiatric diseases an abnormal cytoskeletal organization occurs. Melatonin, the main product secreted by pineal gland during dark phase of the photoperiod, is capable of influencing microfilament, microtubule and IF organization by acting as a cytoskeletal modulator. In this paper we will summarize the evidence which provides the data that melatonin regulates cytoskeletal organization and we describe recent findings, which indicate that melatonin effects on microfilament rearrangements in stress fibers are involved in the mechanism by which the indole synchronizes water transport in kidney-derived epithelial cells. In addition, we review recent data, which indicates that melatonin protects the neuro-cytoskeletal organization from damage caused by free radicals contributing to cell survival, in addition to the already described mechanism elicited by the indole to prevent apoptosis and to scavenge free radicals. Moreover, we discuss the implications of an altered cytoskeletal organization for neurodegenerative and psychiatric illnesses and its re-establishment by melatonin.

Actin depolymerization disrupts tight junctions via caveolae-mediated endocytosis

The tight junction (TJ) determines epithelial barrier function. Actin depolymerization disrupts TJ structure and barrier function, but the mechanisms of this effect remain poorly understood. The goal of this study was to define these mechanisms. Madin-Darby canine kidney (MDCK) cells expressing enhanced green fluorescent protein-, enhanced yellow fluorescent protein-, or monomeric red fluorescent protein 1-fusion proteins of beta-actin, occludin, claudin-1, ZO-1, clathrin light chain A1, and caveolin-1 were imaged by time-lapse multidimensional fluorescence microscopy with simultaneous measurement of transepithelial electrical resistance (TER). Actin depolymerization was induced with latrunculin A (LatA). Within minutes of LatA addition TER began to fall. This coincided with occludin redistribution and internalization. In contrast, ZO-1 and claudin-1 redistribution occurred well after maximal TER loss. Occludin internalization and TER loss, but not actin depolymerization, were blocked at 14 degrees C, suggesting that membrane traffic is required for both events. Inhibition of membrane traffic with 0.4 M sucrose also blocked occludin internalization and TER loss. Internalized occludin colocalized with caveolin-1 and dynamin II, but not with clathrin, and internalization was blocked by dominant negative dynamin II (K44A), but not by Eps15Delta95-295 expression. Inhibition of caveolae-mediated endocytosis by cholesterol extraction prevented both LatA-induced TER loss and occludin internalization. Thus, LatA-induced actin depolymerization causes TJ structural and functional disruption by mechanisms that include caveolae-mediated endocytosis of TJ components.

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.

Mechanical responses of single non-confluent epithelial cells to low extracellular calcium

Single non-confluent MDCK cells respond immediately to a sharp decrease in extracellular Ca2+ (< or = 5 microM) with an intense reversible retraction, along with an increase in cell height, correlating in overall rate and extent with initial cell size. Optical sectioning of individual cells by confocal microscopy showed that this structural response, observed in about 50% of the population, involves narrowing and even furrowing near the base of the cell by a thickened peripheral belt of actin filaments, which remains associated with the cortex instead of being internalized in the cytoplasm. Single cells retracted significantly in response to low Ca2+ under conditions that have been found largely inhibitory for retraction of confluent cells, such as Ca2+ replacement with Ba2+ and the substitution of Na+ with choline, a non-permeant cation. Conversely, the Na(+)-ionophore monensin applied in the normal-Ca2+ medium elicited by itself an earlier and much greater retraction in single cells than in confluent cultures. These observations indicate that single cells can retract more readily than those forming confluent cultures, suggesting that retraction in typical monolayers is resisted in part by the cell junctions. According to this view, inward actin-myosin mediated tension around the periphery of individual cells precedes and probably helps dissociation of E-cadherins in confluent cultures exposed to low Ca2+.

Actin re-distribution in response to hydrogen peroxide in airway epithelial cells

Reactive oxygen species (ROS) disrupt the barrier function of airway epithelial cells through a mechanism that appears to involve remodeling of the actin cytoskeleton. Similarly, keratinocyte growth factor (KGF) has been shown to protect against ROS-induced loss of barrier function through a mechanism that may also involve the actin cytoskeleton. To further determine the role of the actin cytoskeleton in ROS-induced barrier injury, we quantified the relative amount of total actin associated with the cytoskeleton following exposure to hydrogen peroxide (H(2)O(2)) and pretreatment with KGF. We also determined the role of the actin-myosin contractile mechanism in the process by quantifying the relative amount of myosin heavy chain (MHC) associated with the cytoskeleton. While the transepithelial resistance (TER) of a monolayer of airway epithelial cells (Calu-3) decreased after 2 h of continuous exposure to 0.5 mM H(2)O(2), actin and MHC, both dissociated from the cytoskeleton within 15 min of H(2)O(2) exposure. The TER of the monolayers remained depressed although both actin and myosin returned to the cytoskeleton by 4 h after the initiation of H(2)O(2) exposure. Filamentous actin (f-actin) staining suggested that the re-associating actin took the form of short fibers associated with cortical actin rather than long stress fibers. Furthermore, pretreatment with KGF prevented the loss of actin and MHC from the actin cytoskeleton but did not prevent the decrease in TER. These studies suggest that actin disassembly from the cytoskeleton is important in the loss of barrier function, but that it is not the overall amount of actin that is associated with the cytoskeleton that is important, rather it is the contribution this actin makes to the architectural cohesiveness of the cell that contributes to the barrier function.

BFA-sensitive and insensitive exocytic pathways in Entamoeba histolytica trophozoites: their relationship to pathogenesis

Entamoeba histolytica manifests its pathogenicity through several cellular processes triggered by external stimuli that activate signal transduction pathways. The intense secretory activity resulting from stimulation is not correlated with a typical endoplasmic reticulum (ER) or Golgi organization, and little is known in this parasite about endocytic/exocytic pathways. The interactions of trophozoites with fibronectin (FN) and cultured mammalian cells, which elicit secretory activities, were chosen to study mechanisms that regulate cytoplamic traffic. Results showed that Brefeldin A (BFA) induced redistribution of the vesicular network recognized by antibodies against amoebic proteins PDI and ERD2. Furthermore, BFA diminished traffic to the plasma membrane of the beta1 integrin-like FN receptor and the heavy subunit of the Gal/GalNAc lectin, required for adhesion to FN and target cells, respectively. However, BFA did not prevent thiol-proteinase secretion or inhibit the traffic of de novo synthesized proteinases. These data suggest that two distinct transport systems occur in E. histolytica, one similar to classical membrane protein transport and another independent of BFA and inducible by external stimuli. Actin-myosin contractility of the cortical cytoskeleton seems necessary for the final release of exported proteinases and the proper function of the surface proteins involved in adhesion.

2,3-butanedione monoxime (BDM), a potent inhibitor of actin-myosin interaction, induces ion and fluid transport in MDCK monolayers

Membrane-cytoskeleton interactions have been shown to be crucial to modulate polarity, cell shape and the paracellular pathway in epithelial MDCK cell monolayers. In particular, actin organization and myosin-dependent contractility play an important role in the regulation of these functions. Participation of myosin in vectorial transport, expressed as formation of domes, was investigated in confluent monolayers of high transepithelial electrical resistance (TER) plated on non-permeable supports. Cells exposed to 2,3-butanedione monoxime, a selective inhibitor of myosin ATPase, showed a remarkable increase in the number of domes. Replacement of extracellular Na+ and Cl- and inhibition of Na+-K+-ATPase blocked the induction of domes. The monoxime also caused a reduction of the TER leading to an increase in the paracellular flux of small molecular weight dextran. However, immunofluorescence microscopy of drug-treated cells showed that the localization and staining pattern of tight junction proteins ZO-1, occludin, and claudin 1, or the actin-myosin ring at the zonula adherens, were not modified. Treatment with the drug produced striking re-arrangements of actin filaments at the microvilli and at the basal level of the cells. Our data show that disruption of actin-myosin interaction at several cellular sites contributed importantly to the increased transport activity and the formation of the domes. These results point to the relevant role or actin-myosin dynamics and actin organization in the regulation of ion and water channel activity in these cells.

Inhibitors of glycosphingolipid biosynthesis reduce transepithelial electrical resistance in MDCK I and FRT cells

Madin-Darby canine kidney (MDCK) I and Fisher rat thyroid (FRT) cells exhibit transepithelial electrical resistance (TER) values in excess of 5,000 Omega. cm(2). When these cells were incubated in the presence of various inhibitors of sphingolipid biosynthesis, a >5-fold reduction of TER was observed without changes in the gate function for uncharged solutes or the fence function for apically applied fluorescent lipids. The localization of ZO-1 and occludin was not altered between control and inhibitor-treated cells, indicating that the tight junction was still intact. Furthermore, the complexity of tight junction strands, analyzed by freeze-fracture microscopy, was not reduced. Once the inhibitor was removed and the cells were allowed to synthesize sphingolipids, a gradual recovery of the TER was observed. Interestingly, these inhibitors did not attenuate the TER of MDCK II cells, a cell line that typically exhibits values below 800 omega x cm(2.) These results suggest that glycosphingolipids play a role in regulating the electrical properties of epithelial cells.

Melatonin induced cyclic modulation of vectorial water transport in kidney-derived MDCK cells

BACKGROUND

Melatonin, newly synthesized by the pineal gland, is rapidly released to general circulation reaching a nanomolar concentration. Cyclic production of melatonin synchronizes body rhythms with the photoperiod. Moreover, changes in urine production and osmolarity have been observed in the kidney during the night. However, the precise mechanisms by which plasma-circulating melatonin modifies renal physiology are not clearly understood.

METHODS

Madin-Darby canine kidney (MDCK) cell monolayers transport water vectorially from the apical to the basolateral side forming blisters or domes. Transport in epithelial cells is regulated by tight junction sealing, ion pumps and channels, and cytoskeleton organization, among other processes. MDCK cells were used to study vectorial water transport to determine the role of microfilament organization and protein kinase C (PKC) in dome formation in culture conditions that mimic the cyclic pattern of melatonin circulation in plasma.

RESULTS

Melatonin cyclically increased dome formation by 50% and caused enlargement and thickening of stress fibers in cells surrounding the domes. Optimal increase in dome formation was observed at nanomolar concentrations of melatonin after 6 hours, concomitantly with a 28% decrease in the transepithelial electrical resistance, which remained low for up to 12 hours, without apparent change in fluorescein isothiocyanate (FITC)-dextran flux. A blockage in dome formation elicited by melatonin was observed in monolayers preincubated with the Na+-K+-ATPase or PKC inhibitors.

CONCLUSION

The results obtained indicate that melatonin cyclically modifies the transepithelial permeability in kidney-derived cells through PKC activation and microfilament reorganization, and supports the hypothesis that melatonin may synchronize daily body rhythms through cyclic cytoskeletal rearrangements.

Glucocorticoid down-regulation of RhoA is required for the steroid-induced organization of the junctional complex and tight junction formation in rat mammary epithelial tumor cells

In Con8 mammary epithelial tumor cells, we have documented previously that the synthetic glucocorticoid dexamethasone induces the reorganization of the tight junction and adherens junction (apical junction) and stimulates the monolayer transepithelial electrical resistance (TER), which is a reliable in vitro measurement of tight junction sealing. Western blots demonstrated that dexamethasone treatment down-regulated the level of the RhoA small GTPase prior to the stimulation of the monolayer TER. To test the role of RhoA in the steroid regulation of apical junction dynamics functionally, RhoA levels were altered in Con8 cells by transfection of either constitutively active (RhoA.V14) or dominant negative (RhoA.DN19) forms of RhoA. Ectopic expression of constitutively active RhoA disrupted the dexamethasone-stimulated localization of zonula occludens-1 and beta-catenin to sites of cell-cell contact, inhibited tight junction sealing, and prevented the complete formation of the F-actin ring structure at the apical side of the cell monolayer. In a complementary manner, dominant negative RhoA caused a precocious organization of the tight junction, adherens junction, and the F-actin rings in the absence of steroid, whereas the monolayer TER remained glucocorticoid-responsive. Taken together, our results demonstrate that the glucocorticoid down-regulation of RhoA is a required step in the steroid signaling pathway which controls the organization of the apical junctional complex and the actin cytoskeleton in mammary epithelial cells.

Identification of a tight junction-associated guanine nucleotide exchange factor that activates Rho and regulates paracellular permeability

Rho family GTPases are important regulators of epithelial tight junctions (TJs); however, little is known about how the GTPases themselves are controlled during TJ assembly and function. We have identified and cloned a canine guanine nucleotide exchange factor (GEF) of the Dbl family of proto-oncogenes that activates Rho and associates with TJs. Based on sequence similarity searches and immunological and functional data, this protein is the canine homologue of human GEF-H1 and mouse Lfc, two previously identified Rho-specific exchange factors known to associate with microtubules in nonpolarized cells. In agreement with these observations, immunofluorescence of proliferating MDCK cells revealed that the endogenous canine GEF-H1/Lfc associates with mitotic spindles. Functional analysis based on overexpression and RNA interference in polarized MDCK cells revealed that this exchange factor for Rho regulates paracellular permeability of small hydrophilic tracers. Although overexpression resulted in increased size-selective paracellular permeability, such cell lines exhibited a normal overall morphology and formed fully assembled TJs as determined by measuring transepithelial resistance and by immunofluorescence and freeze-fracture analysis. These data indicate that GEF-H1/Lfc is a component of TJs and functions in the regulation of epithelial permeability.

Effects of cadmium on E-cadherin and VE-cadherin in mouse lung

Exposure to Cd(2+) via inhalation or intratracheal instillation results in pulmonary edema, which is followed by the influx of leukocytes, the proliferation of type II pneumocytes and eventual scarring and fibrotic changes. While the general toxic effects of Cd(2+) in the lung have been well characterized, the specific molecular mechanisms underlying these effects have yet to be elucidated. Previously we have shown that Cd(2+) can disrupt the adhering junctions between various types of epithelial and endothelial cells in culture, most likely by perturbing the function of the Ca(2+) dependent cell adhesion molecules E-cadherin and VE-cadherin respectively. The objectives of this study were to determine whether respiratory exposure to Cd(2+) can alter the localization of E-cadherin and VE-cadherin in the lung, and to determine whether this effect may play a role in the acute pneumotoxic response to Cd(2+). Male CF-1 mice were exposed to CdCl(2) (0, 16.25, 32.5, 65 or 130 nmoles in 50 microl saline) via intratracheal instillation. After 24 hours, the lungs were removed and either subjected to bronchoalveolar lavage or analyzed for histopathologic changes. The results showed that Cd(2+) caused an increase in lung weight and in the protein content of the lavage fluid. These effects were accompanied by a pronounced decrease in the amount of E-cadherin in epithelial cells of the alveoli and small bronchioles and of VE-cadherin in vascular endothelial cells. Assessment of cell membrane integrity with ethidium homodimer-1 showed no evidence of severe injury or death in alveolar epithelial cells. These findings suggest that E-cadherin and VE-cadherin may be important early targets of Cd(2+) toxicity in the lung.

Actin cytoskeleton role in the structural response of epithelial (MDCK) cells to low extracellular Ca2+

Kinetic and stereometric assessment of the mechanical responses of epithelial cells to variations in the concentration of extracellular Ca2+ was carried out in vivo at the single cell level. Continuous monitoring of individual MDCK cells in subconfluent cultures attested to an intense, immediately relaxable, and cytochalasin D-sensitive contraction, equivalent to that seen in confluent monolayers following depletion of external Ca2+ (<0.1 mM). Increasingly greater and less readily reversible contractions were performed upon repeated stimulation with short-term cycles of alternating normal (30 min) and low Ca2+ (30 min) media. Constriction of a narrow horizontal girdle corresponding in position to the major ring-like bundle of actin filaments eventually develops into a deep lateral furrow in intensely contracted cells. Substantial membrane infolding in the contracted state is indicated also by stereometric estimates of apparent bounding surface area. Irrespective of the contracted or relaxed cell condition, rhodamine-phalloidin labeling showed a marginal position of the ring-like bundle of microfilaments and other components of the actin cytoskeleton. These results suggest, contrary to prevalent views, that the actin-myosin system stays associated to the cortex and retains contractile capability in epithelial cells deprived of external Ca2+. Hence, the mechanical responses to variations of Ca2+ may be an overstrained expression of a physiological mechanism.

'Putting the squeeze' on the tight junction: understanding cytoskeletal regulation

The apical perijunctional actomyosin ring of epithelia is structurally associated with the tight junction. The functional association between the tight junction and the perijunctional actomyosin ring was initially described in studies using pharmacological agents that disrupt microfilaments. More recently, this interaction has been studied in physiological, pathophysiological, and molecular models of tight junction regulation. These studies have demonstrated the central role of actomyosin contraction in tight junction regulation. With the identification of novel tight junction proteins and characterization of their protein:protein interactions comes the promise of detailed understanding of the molecular interactions that mediate tight junction regulation.

MAGUK proteins: structure and role in the tight junction

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

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.

Contraction of epithelial (MDCK) cells in response to low extracellular calcium is dependent on extracellular sodium

Like other cells of epithelial origin, MDCK cells respond with a reversible structural transformation to a diminution in the concentration of extracellular Ca2+. Upon deprivation of Ca2+ in the medium the cells undergo an active contraction mediated by the actin-myosin cytoskeleton, in parallel to detachment of the intercellular contacts and appearance of free spaces in the epithelium or monolayer (Castillo et al., 1998). We now present results indicating that the decrease of external Ca2+ plays an indirect and non-specific role in activating contraction, probably by allowing an influx of Na+. The omission of external Ca2+ had no effect when it was replaced by Mg2+, Ba2+ or Hg2+, and the addition of any of these divalent cations induced relaxation of cells previously contracted by exposure to low Ca2+. A null or weak response was observed also when Ca2+ was lowered in a solution where Na+ was replaced by choline or in the presence of amiloride (30 microM), which reduces the permeability of the plasma membrane to Na+. Restitution of Na+ or removal of amiloride were followed by contraction in the same cultures. Li+ proved an able substitute of Na+ as requisite for cell contraction in response to Ca2+ depletion. Monensin (0.1 mM) -an ionophore selective for Na+- and to a lesser extent ouabain (0.1 mM) -an inhibitor of Na+ extrusion across the plasma membrane- , both stimulated contraction in the presence of the normal level of external Ca2+. Decreasing by half the normal concentration of external K+ facilitated cell contraction, but typical responses were observed when K+ was increased to 40 mM by partial substitution for Na+. These findings attest that cell contraction in response to low Ca2+ is likely due to an increase in the permeability of the plasma membrane to Na+, though not to membrane depolarization as such. Evidences from other motile systems suggest that Na+ influx might in turn cause an elevation of cytoplasmic Ca2+, which activates the actin-myosin cytoskeleton.

The C terminus of E1A regulates tumor progression and epithelial cell differentiation

The E1A gene of adenovirus has been considered both a dominant oncogene and a tumor suppressor. It has been reported to induce epithelial cell but to prevent myoblast differentiation. E1A enables oncogenes that are unable to transform primary cells on their own to do so, yet suppresses tumor progression toward invasion and metastasis. To try to reconcile the seemingly, conflicting E1A phenotypes, we examined the expression of epithelial cell specific and characterizing proteins in immortalized or tumorigenically transformed primary epithelial cells expressing wild-type E1A or a C-terminal mutant that has lost tumor suppressive abilities. All the cell types continued to express cytokeratin. Epithelial cell morphology, social behavior, and growth characteristics were retained by cells expressing wild-type E1A, even in the presence of an activated ras oncogene. Mutant E1A-expressing cells were less well differentiated even in the absence of ras. They were specifically defective in cell-cell junctional complexes, such as tight and adherens junctions and desmosomes. There was also a preference for those actin structures prominent in fibroblasts: stress fibers and filopodia, while in the wild-type E1A expressing cells, cortical actin and circumferential actin filaments were dominant. Thus the E1A-mutant-expressing cells were already predisposed to a more advanced tumor stage even when they were only immortalized and not transformed. The results suggest the possibility that the C terminus of E1A may be involved in regulating epithelial mesenchymal transitions, which have previously been linked to tumor progression.

Effects of beta-lactoglobulin on the tight-junctional stability of Caco-2-SF monolayer

The mechanisms for tight-junction (TJ) stabilization by beta-lactoglobulin (beta-Lg) were studied. Treatment of Caco-2-SF cells with inhibitors for some enzymes in the intracellular signal transduction pathways and a cytoskeleton-disturbing agent (cytochalasin D) reduced the TJ-stabilizing activity of beta-Lg. So beta-Lg is suggested to modulate the cytoskeletal structure through the activation of phospholipase C and protein kinase C, resulting in the TJ stabilization.

Disruption of endothelial microfilaments selectively reduces the transendothelial migration of monocytes

The transendothelial migration of leukocytes (diapedesis) is a central event in inflammatory and immunological processes. Although leukocyte-endothelium interactions occurring during diapedesis have been investigated intensively, little is known about the actual transmigration and the molecular mechanisms involved. Toward this end we analyzed whether the endothelial cytoskeleton plays a direct role during the transendothelial migration of monocytes. Filter-grown monolayers of human microvascular endothelial cells (HMEC-1) were treated with cytoskeleton stabilizing or destabilizing drugs and the effect of this treatment on the transmigration of peripheral blood monocytes was analyzed in a two-chamber assay. Our results show that taxol-induced stabilization of microtubules causes a reduction of leukocyte transmigration through HMEC-1, while the opposite effect is induced by the destabilization of microtubules with colchicine or nocodazol. Disruption of microfilaments with cytochalasin B or latrunculin A, on the other hand, significantly reduces the transendothelial migration although monocyte adhesion and endothelial permeability for macromolecules are slightly increased. An active participation of the endothelial microfilament system with a direct role of unconventional, calmodulin-regulated myosins is suggested by the finding that monocyte transmigration is decreased upon treatment of the endothelial cells with the Ca2+/CaM antagonist triflouperazine.

Myosin II-actin interaction in MDCK cells: role in cell shape changes in response to Ca2+ variations

Cultured MDCK cell monolayers respond to a low level of extracellular calcium ([Ca2+]e < or = 5 microM) with a loss of transepithelial electrical resistance and transport function, and changes in position of a circumferential ring of actin filaments tethered to the plasma membrane at the zonula adhaerens. Keeping this cytoskeletal structure in place seems necessary to preserve the architecture of the tight junctions and therefore their sealing capacity. All three effects are reversible upon restituting normal [Ca2+]e. Recent work provided evidence of actin-myosin interactions at the filament ring, thus suggesting a contraction process involved in the alteration of the actin cytoskeleton. We now report that active contraction does occur and causes an extensive morphological transformation of MDCK cells. A marked increase in cell height simultaneous with a decrease in width and area of contact to the substratum was seen within 10 min of removal of [Ca2+]e; recovery began immediately after replacing calcium, although it took longer for completion. Conventional and confocal epifluorescence studies showed actin colocalized with myosin II at various planes of resting or contracted cells, in particular at the ring level. Electron-micrographs revealed the circumferential actin ring associated with the plasma membrane in a waist-like constriction where Ca2+ was removed from the cultures. Contraction, as well as relaxation, in response to [Ca2+]e variations were inhibited by cytochalasin-D (an actin-filament disrupting drug), by okadaic acid( an inhibitor of myosin light-chain dephosphorylation), and by 2,3-butanedione monoxime (a blocker of myosin II ATPase activity). Similarly, no response was observed in cells previously depleted of metabolic energy by 2,4-dinitrophenol and 2-deoxy-D-glucose preincubation. The actin-myosin mediated reversible structural transformation of MDCK cells in response to [Ca2+]3 poses new questions for the interpretation of in vitro experiments, as well as for the understanding of epithelial function.

Increased tyrosine phosphorylation causes redistribution of adherens junction and tight junction proteins and perturbs paracellular barrier function in MDCK epithelia

Polarized monolayers of strain II Madin-Darby canine kidney cells (MDCK II) were treated with vanadate/H2O2, known inhibitors of protein tyrosine phosphatase activity. Vanadate/H2O2 treatment resulted in a rapid increase in paracellular permeability as revealed by decreased transepithelial resistance and increased permeability to inulin. These alterations in epithelial barrier function coincided with increased phosphotyrosine immunofluorescence in the vicinity of intercellular junctions and with redistribution of F-actin, the adherens junction protein E-cadherin and the tight junction protein ZO-1. The effects of vanadate/H2O2 on intercellular junction permeability and protein distribution were completely blocked by the specific protein tyrosine kinase (PTK) inhibitor tyrphostin 25 and partially inhibited by the alternative PTK inhibitor genistein. The relative potency of these two inhibitors in blocking the effects of vanadate/H2O2 on intercellular junctions correlated with their abilities to inhibit tyrosine phosphorylation. The potent ser/thr protein kinase inhibitor staurosporine had only a small influence on the vanadate/H2O2-induced increase in paracellular permeability and did not affect the observed redistribution of intercellular junction proteins or phosphotyrosine immunofluorescence. The relative potencies of these distinct protein kinase inhibitors in reversing the effects of vanadate/H2O2 indicate that these effects are directly related to tyrosine phosphorylation. In conclusion, our data provide evidence that enhanced tyrosine phosphorylation of intercellular junction proteins in MDCK epithelia increases paracellular permeability and can also induce prominent reorganization of the junctional complex.

Comparison of the effect of different chitosan salts and N-trimethyl chitosan chloride on the permeability of intestinal epithelial cells (Caco-2)

A partially quaternized chitosan derivative, N-trimethyl chitosan chloride (TMC) (degree of quaternization 12.28%), was synthesized and the effects of this novel polymer on the permeability of intestinal epithelial cells, using Caco-2 cell monolayers, were investigated and compared with those of chitosan hydrochloride and chitosan glutamate. Transepithelial electrical resistance (TEER) measurements at pH 6.20 revealed that all these polymers (0.25-1.5% w/v) caused an immediate and pronounced lowering in TEER values in the order chitosan hydrochloride (84% reduction after 2 h incubation) > chitosan glutamate (60% reduction) > TMC (24% reduction) at 0.25% w/v concentrations. At higher concentrations (up to 2.5% w/v), TMC was able to decrease the TEER further. Similar results were obtained in transport studies, using the hydrophilic radioactive markers, [14C]-mannitol (MW 182.2) and [14C]-polyethylene glycol 4000 (PEG-4000, MW 4000). Large increases in the permeation of these markers were found. The transport of [14C]-mannitol was increased 34-fold (chitosan hydrochloride), 25-fold (chitosan glutamate) and 11-fold (TMC) at 0.25% w/v concentrations. Further increases in the permeation of both markers were found at higher concentrations of TMC. Due to its quaternary structure, TMC is better soluble than the other chitosan salts, and its higher solubility may compensate for its lesser effectivity at similar concentrations. It is also soluble at pH 7.40, where the chitosan salts are insoluble and therefore ineffective. No deleterious effects to the cells could be demonstrated with trypan blue exclusion studies and confocal laser scanning microscopy (CLSM). CLSM confirmed that these polymers increase the transport of large hydrophilic compounds (using the fluorescent markers FD-4, MW 4400 and FD-20, MW 19,600) through opening of tight junctions to allow for paracellular transport. It is concluded from this study that the charge, charge density and the structural features of chitosans and chitosan derivatives are important factors determining their potential use as absorption enhancers.

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.

Effect of capsianoside, a diterpene glycoside, on tight-junctional permeability

Previous work (Hashimoto et al., (1994) Biosci. Biotech. Biochem. 58, 1345) revealed that a sweet pepper extract enhanced the tight-junctional (TJ) permeability of a human intestinal Caco-2 cell monolayer. In the present study, the substance which modulated the TJ permeability was chromatographically purified from the extract. The active substances were identified as capsianosides A-F, diterpene glycosides. Treatment of the cells with capsianoside F, the most active compound, decreased the cellular G-actin content by 40% and increased the F-actin content by 16%. The effect of capsianoside F was significantly suppressed by disturbing the cytoskeletal structure with cytochalasin D at a low dose (50 ng/ml). These results suggest that capsianosides affected the cytoskeletal function by modulating the reorganization of actin filaments, by which the TJ structure and permeability were changed. The possible involvement of a PKC inhibition in the mechanism of an increase in TJ permeability is also suggested.

Expression of the catalytic domain of myosin light chain kinase increases paracellular permeability

Contractile events resulting from phosphorylation of the 20-kDa myosin light chain (MLC20) have been implicated in the regulation of epithelial tight junction permeability. To address this question, Madin-Darby canine kidney cells were transfected with a murine leukemia retroviral vector containing DNA encoding either the catalytic domain of myosin light chain kinase (tMK) or the beta-galactosidase gene (beta-gal). Autoradiograms of sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of myosin immunoprecipitated from 32Pi-labeled transfected cells demonstrated that MLC20 phosphorylation was increased 3.1 +/- 0.9-fold in cells expressing tMK compared with cells expressing beta-gal. Phosphopeptide mapping confirmed that myosin light chain kinase was responsible for the increased MLC20 phosphorylation. Transepithelial electrical resistance, a measurement of barrier function, of tMK cell monolayers was consistently < 10% (123 +/- 20 omega.cm2) of that of monolayers comprised of wild-type cells (1,456 +/- 178 omega.cm2) or cells expressing beta-gal (1,452 +/- 174 omega.cm2). Dual 22Na+ and [3H]mannitol flux studies indicated that the decrease in resistance in tMK cells was attributable to increased paracellular flow. These data support the idea that MLC20 phosphorylation by myosin light chain kinase is involved in regulating epithelial tight junction permeability.

Modulation of the junctional integrity by low or high concentrations of cytochalasin B and dihydrocytochalasin B is associated with distinct changes in F-actin and ZO-1

In a study of Necturus gallbladder epithelium Benzel et al. (Benzel et al., 1980) found that low (0.2-1.2 microM) and higher concentrations (1.5 microM and more) of cytochalasin B (CB) caused an increase and decrease in the transepithelial electrical resistance (TER), respectively. Moreover, there were slight changes in the height and complexicity of tight junction (TJ) strands, as visualized by freeze-fracture and freeze-etching. To elucidate the mechanisms of these findings, we first demonstrated that the effect is also present in monolayers of Madin-Darby Canine Kidney strain 1 (MDCK-1) cells. Thus, a low concentration (0.1 ng/ml) cytochalasin B (CB) strengthened the permeability barrier, as evidenced quantitatively by increases in TER on transepithelial electrical measurements. Furthermore, indirect immunofluorescence and confocal microscopy demonstrated that this effect was paralleled with an accumulation of F-actin and the tight junction marker protein, ZO-1, at the level of TJ. Equimolar concentrations of dihydrocytochalasin B (dhCB), on the other hand, did not lead to a tightening of the epithelium. Confirming previous studies, there was a general decrease in epithelial resistance after treatment with high concentrations (1 microgram/ml) of CB and dhCB, which was accompanied by distinct changes in the F-actin network and distribution of ZO-1. We speculate that the divergent effects of CB and dhCB on the F-actin and ZO-1 organization might be due to specific effects on the transport of monosaccharides across the plasma membrane, or that CB and dhCB in distinct ways involve the turnover of phosphatidylinositols in the membrane, thereby modulating junctional permeability and F-actin structure.

Regulated assembly of tight junctions by protein kinase C

We have previously shown that protein phosphorylation plays an important role in the sorting and assembly of tight junctions. We have now examined in detail the role of protein kinases in intercellular junction biogenesis by using a combination of highly specific and broad-spectrum inhibitors that act by independent mechanisms. Our data indicate that protein kinase C (PKC) is required for the proper assembly of tight junctions. Low concentrations of the specific inhibitor of PKC, calphostin C, markedly inhibited development of transepithelial electrical resistance, a functional measure of tight-junction biogenesis. The effect of PKC inhibitors on the development of tight junctions, as measured by resistance, was paralleled by a delay in the sorting of the tight-junction protein, zona occludens 1 (ZO-1), to the tight junction. The assembly of desmosomes and the adherens junction were not detectably affected, as determined by immunocytochemical analysis. In addition, ZO-1 was phosphorylated subsequent to the initiation of cell-cell contact, and treatment with calphostin C prevented approximately 85% of the phosphorylation increase. Furthermore, in vitro measurements indicate that ZO-1 may be a direct target of PKC. Moreover, membrane-associated PKC activity more than doubled during junction assembly, and immunocytochemical analysis revealed a pool of PKC zeta that appeared to colocalize with ZO-1 at the tight junction. A preformed complex containing ZO-1, ZO-2, p130, as well as 330- and 65-kDa phosphoproteins was detected by coimmunoprecipitation in both the presence and absence of cell-cell contact. Identity of the 330- and 65-kDa phosphoproteins remains to be determined, but the 65-kDa protein may be occludin. The mass of this complex and the incorporation of ZO-1 into the Triton X-100-insoluble cytoskeleton were not PKC dependent.

ATP depletion: a novel method to study junctional properties in epithelial tissues. I. Rearrangement of the actin cytoskeleton

The effect of cellular injury caused by depletion of intracellular ATP stores was studied in the Madin-Darby canine kidney (MDCK) and JTC cell lines. In prior studies, it was shown that ATP depletion uncouples the gate and fence functions of the tight junction. This paper extends these observations by studying the changes in the actin cytoskeleton and tight junction using electron microscopy and confocal fluorescence microscopy in combination with computer-aided three-dimensional reconstruction. Marked regional differences in the sensitivity to the effects of ATP depletion were observed in the actin cytoskeleton. Actin depolymerization appears to first affect the cortical actin network running along the apical basal axis of the cell. The next actin network that is disrupted is the stress fibers found at the basal surface of the cell. Finally, the actin ring at the level of the zonulae occludens and adherens is compromised. The breakup of the actin ring correlates with ultrastructural changes in tight junction strands and the loss of the tight junction's role as a molecular fence. During the process of actin network dissolution, polymerized actin aggregates form in the cytoplasm. The changes in the junctional complexes and the potential to reverse the ATP depletion suggest that this may be a useful method to study junctional complex formation and its relationship to the actin cytoskeletal network.

Quantitative approaches to delineate paracellular diffusion in cultured epithelial cell monolayers

When using cultured cell monolayers to determine the mechanism of transcellular diffusion of molecules, it may be important to identify the fraction that moves through the paracellular route or passively diffuses through tight junctions. We characterized the apparent diameter of the junctional pore in a variety of epithelial cell monolayers (Caco-2, MDCK, alveolar). Using hydrophilic extracellular permeants varying in molecular radii and charge (neutral, anionic, cationic, zwitterionic), rate-determining steps and factors of the paracellular route were quantitatively delineated by the model for molecular size-restricted diffusion within a negative electrostatic field of force. Protonated amines permeated the pores faster than their neutral images while organic anions were slower. With increasing molecular size the influence of charge diminished. This approach was used to quantify the relationship between permeant radius and transepithelial electrical resistance and to analyze changes in junctional pore size as a function of pharmacological perturbation, such as in the use of absorption promoters or adjuvants.

Cytoskeletal involvement in the modulation of cell-cell junctions by the protein kinase inhibitor H-7

The protein kinase inhibitor H-7 has been shown to block junction dissociation induced by low extracellular calcium in Madin Darby canine kidney epithelial cells (S. Citi, J. Cell Biol. (1992) 117, 169–178). To understand the basis of this effect, we have examined how H-7 affects the organization of junctions and the actin cytoskeleton in different types of epithelial cells in culture. Immunofluorescence microscopy showed that H-7 confers Ca2+ independence on cultured epithelial lens cells, which lack tight junctions and desmosomes but have microfilament-associated adherens junctions. In these cells, H-7 did not protect N-cadherin from trypsin digestion at low extracellular calcium, suggesting that H-7 does not stabilize the ‘active’ cadherin conformation. In cultured Madin Darby canine kidney cells, H-7 partially prevented the fall in transepithelial resistance induced by cytochalasin D, either alone or in conjunction with calcium chelators. Double-immunofluorescence microscopy showed that H-7 inhibits both the fragmentation of labeling for the tight junction protein cingulin and the condensation of actin into cytoplasmic foci induced by cytochalasin D. Taken together, these observations indicate that H-7 inhibits junction dissociation by affecting the contractility of the adherens junction-associated microfilaments following treatment with calcium chelators or cytochalasin D.