Claudin-11/OSP-based tight junctions of myelin sheaths in brain and Sertoli cells in testis.
(9/1902)
Members of the newly identified claudin gene family constitute tight junction (TJ) strands, which play a pivotal role in compartmentalization in multicellular organisms. We identified oligodendrocyte-specific protein (OSP) as claudin-11, a new claudin family member, due to its sequence similarity to claudins as well as its ability to form TJ strands in transfected fibroblasts. Claudin-11/OSP mRNA was expressed in the brain and testis. Immunofluorescence microscopy with anti-claudin-11/OSP polyclonal antibody (pAb) and anti-neurofilament mAb revealed that in the brain claudin-11/OSP-positive linear structures run in a gentle spiral around neurofilament-positive axons. At the electron microscopic level, these linear structures were identified as the so-called interlamellar strands in myelin sheaths of oligodendrocytes. In testis, well-developed TJ strands of Sertoli cells were specifically labeled with anti-claudin-11/OSP pAb both at immunofluorescence and electron microscopic levels. These findings indicated that the interlamellar strands of oligodendrocyte myelin sheaths can be regarded as a variant of TJ strands found in many other epithelial cells, and that these strands share a specific claudin species, claudin-11/OSP, with those in Sertoli cells to create and maintain the repeated compartments around axons by oligodendrocytes. (+info)
Interferon-gamma decreases barrier function in T84 cells by reducing ZO-1 levels and disrupting apical actin.
(10/1902)
The effects of interferon-gamma (IFN-gamma) on tight junctions in T84 human intestinal epithelial cells were investigated. Treatment of T84 cells with IFN-gamma caused a dose- and time-dependent increase in monolayer permeability as assessed by transepithelial electrical resistance measurements. Examination of specific proteins associated with tight junctions by immunoblotting and confocal microscopy revealed changes in the expression levels and localization of some of these proteins after exposure of the cells to IFN-gamma. Specifically, IFN-gamma treatment resulted in an almost total loss of zonula occludens (ZO)-1, whereas the levels of ZO-2 and occludin showed relatively modest decreases compared with untreated cells. Loss of ZO-1 was associated with the altered localization of ZO-2 and occludin. In IFN-gamma-treated cells, ZO-2 and occludin were diffusely distributed, whereas, in control cells, they, along with ZO-1, were predominantly localized to the tight junctions. Analysis of ZO-1 protein and RNA by pulse chase and RT-PCR, respectively, showed an increase in protein turnover, a decrease in protein synthesis, and a reduction in RNA levels following IFN-gamma treatment. In contrast to ZO-1, ZO-2 and occludin did not show any major changes in these parameters. In addition, the organization of actin in the apical and tight junction regions, but not in the mid- or basal regions, of the cells was also perturbed by IFN-gamma treatment of cells. Time-course analysis of IFN-gamma-induced alterations in ZO-1 expression and apical actin perturbation indicated that these two effects were intimately linked and could not be dissociated. These results suggest that IFN-gamma affects barrier function in T84 cells by decreasing the levels of ZO-1 and perturbing apical actin organization, which leads to a disorganization of the tight junction and an increase in paracellular permeability. (+info)
cAMP-dependent and -independent downregulation of type II Na-Pi cotransporters by PTH.
(11/1902)
Parathyroid hormone (PTH) leads to the inhibition of Na-Pi cotransport activity and to the downregulation of the number of type II Na-Pi cotransporters in proximal tubules, as well as in opossum kidney (OK) cells. PTH is known also to lead to an activation of adenylate cyclase and phospholipase C in proximal tubular preparations, as well as in OK cells. In the present study, we investigated the involvement of these two regulatory pathways in OK cells in the PTH-dependent downregulation of the number of type II Na-Pi cotransporters. We have addressed this issue by using pharmacological activators of protein kinase A (PKA) and protein kinase C (PKC), i.e., 8-bromo-cAMP (8-BrcAMP) and beta-12-O-tetradecanoylphorbol 13-acetate (beta-TPA), respectively, as well as by the use of synthetic peptide fragments of PTH that activate adenylate cyclase and/or phospholipase C, i.e., PTH-(1-34) and PTH-(3-34), respectively. Our results show that PTH signal transduction via cAMP-dependent, as well as cAMP-independent, pathways leads to a membrane retrieval and degradation of type II Na-Pi cotransporters and, thereby, to the inhibition of Na-Pi cotransport activity. Thereby, the cAMP-independent regulatory pathway leads only to partial effects (approximately 50%). (+info)
Role of tyrosine phosphorylation in the reassembly of occludin and other tight junction proteins.
(12/1902)
After the simulation of anoxia by ATP depletion of MDCK cell monolayers with metabolic inhibitors, the tight junction (TJ) is known to become structurally perturbed, leading to loss of the permeability barrier. Peripheral TJ proteins such as zonula occludens 1 (ZO-1), ZO-2, and cingulin become extremely insoluble and associate into large macromolecular complexes (T. Tsukamoto and S. K. Nigam. J. Biol. Chem. 272: 16133-16139, 1997). For up to 3 h, this process is reversible by ATP repletion. We now show that the reassembly process depends on tyrosine phosphorylation. Recovery of transepithelial electrical resistance in ATP-replete monolayers was markedly inhibited by the tyrosine kinase inhibitor, genistein. Indirect immunofluorescence revealed a decrease in staining of occludin, a membrane component of the TJ, in the region of the TJ after ATP depletion, which reversed after ATP repletion; this reversal process was inhibited by genistein. Examination of the Triton X-100 solubilities of occludin and several nonmembrane TJ proteins revealed a shift of occludin and nonmembrane TJ proteins into an insoluble pool following ATP depletion. These changes reversed after ATP repletion, and the movement of insoluble occludin, ZO-1, and ZO-2 back into the soluble pool was again via a genistein-sensitive mechanism. Rate-zonal centrifugation analyses of detergent-soluble TJ proteins showed a reversible increase in higher density fractions following ATP depletion-repletion, although this change was not affected by genistein. In 32P-labeled cells, dephosphorylation of all studied TJ proteins was observed during ATP depletion, followed by rephosphorylation during ATP repletion; rephosphorylation of occludin was inhibited by genistein. Furthermore, during the ATP repletion phase, tyrosine phosphorylation of Triton X-100-insoluble occludin, which is localized at the junction, as well as ZO-2, p130/ZO-3 (though not ZO-1), and other proteins was evident; this tyrosine phosphorylation was completely inhibited by genistein. This indicates that tyrosine kinase activity is necessary for TJ reassembly during ATP repletion and suggests an important role for the tyrosine phosphorylation of occludin, ZO-2, p130/ZO-3, and possibly other proteins in the processes involved in TJ (re)formation. (+info)
A dominant mutant of occludin disrupts tight junction structure and function.
(13/1902)
The tight junction is the most apical intercellular junction of epithelial cells and forms a diffusion barrier between individual cells. Occludin is an integral membrane protein specifically associated with the tight junction which may contribute to the function or regulation of this intercellular seal. In order to elucidate the role of occludin at the tight junction, a full length and an N-terminally truncated murine occludin construct, both FLAG-tagged at the N terminus, were stably introduced into the murine epithelial cell line CSG 120/7. Both constructs were correctly targeted to the tight junction, as defined by colocalization with another tight junction protein, ZO-1. The construct lacking the N terminus and extracellular domains of occludin was found to exert a dramatic effect on tight junction integrity. Cell monolayers failed to develop an efficient permeability barrier, as demonstrated by low transcellular electrical resistance values and an increased paracellular flux to small molecular mass tracers. Furthermore, gaps were found to have been induced in the P-face associated tight junction strands, as visualized by freeze-fracture electron microscopy. These findings demonstrate an important role for the N-terminal half of occludin in tight junction assembly and maintaining the barrier function of the tight junction. (+info)
Effects of interferon alpha-2b on barrier function and junctional complexes of renal proximal tubular LLC-PK1 cells.
(14/1902)
BACKGROUND: Interferon alpha-2b (IFNalpha) treatment of diseases can be accompanied by impaired renal function and capillary leak syndrome. To explore potential mechanisms of IFNalpha-induced renal dysfunction, an in vitro cell culture model system was established to investigate the effects of IFNalpha on barrier function and junctional complexes. METHODS: LLC-PK1 cells were cultured on microporous membranes. Transepithelial resistance (TER) was measured, and the dose- and time-dependent effects of IFNalpha were assessed. The expression patterns of junctional proteins were examined by Western blot analysis and by confocal immunofluorescence microscopy. RESULTS: IFNalpha produced a dose- and time-dependent decrease in TER. The effect was reversible on removal of IFNalpha at doses up to 5 x 103 U/ml. Tyrphostin, an inhibitor of phosphotyrosine kinases, ameliorated the IFNalpha-induced decrease in TER. Increased expression of occludin and E-cadherin was detected by Western blot analysis after IFNalpha treatment. Immunofluorescence confocal microscopy revealed a broader staining of occludin and E-cadherin following IFNalpha treatment, with prominent staining at the basal cell pole in addition to localization at the junctional region. A marked increase in phosphotyrosine staining along the apico-lateral cell border was detected after IFNalpha treatment. CONCLUSIONS: These findings provide evidence that IFNalpha can directly affect barrier function in renal epithelial cells. The mechanisms involve enhanced tyrosine phosphorylation and overexpression and possibly displacement or missorting of the junctional proteins occludin and E-cadherin. (+info)
Disruption of circumferential actin filament causes disappearance of occludin from the cell borders of rat hepatocytes in primary culture without distinct changes of tight junction strands.
(15/1902)
We investigated the relationship of actin filament organization to occludin and tight junction strands in primary cultured rat hepatocytes using an actin depolymerizing agent, mycalolide B. In control cultures, well-developed circumferential actin filaments and occludin immunoreactivity were observed on the most subapical plasma membrane of the cells, and tight junction strands formed well-developed networks in freeze-fracture replicas. In hepatocytes treated with 3 microM mycalolide B for 6 h, circumferential actin filaments and occludin immunoreactivity disappeared from the cell borders. However, there were no marked abnormalities of tight junction strands in freeze fracture replicas. Similar results were obtained from cells cultured in medium with 0.05 mM Ca2+ for 6 h. The close association of occludin with actin and the existence of intact tight junction strands that are virtually free of both occludin and actin suggest a physiological role of occludin, but not the other proteins forming the tight junction strands, in the linkage between actin cytoskeleton and tight junction. (+info)
Transmembrane proteins in the tight junction barrier.
(16/1902)
Three types of transmembrane proteins have been identified within the tight junction, but it remains to be determined how they provide the molecular basis for regulating the paracellular permeability for water, solutes, and immune cells. Several of these proteins localize specifically within the continuous cell-to-cell contacts of the tight junction. One of these, occludin, is a cell adhesion molecule that has been demonstrated to influence ion and solute permeability. The claudins are a family of four-membrane spanning proteins; unexpectedly, other members of this family have already been characterized without recognizing their relationship to tight junctions. Junction adhesion molecule, the most recently identified tight junction component, is a member of the Ig superfamily and influences the paracellular transmigration of immune cells. A plaque of cytoplasmic proteins under the junction may be responsible for scaffolding the transmembrane proteins, creating a link to the perijunctional actin cytoskeleton and transducing regulatory signals that control the paracellular barrier. (+info)