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Inter- and intrasyncytiotrophoblastic junctions within the human full term placenta were electronmicroscopically investigated using thin sections and freeze-fracturing. Narrow clefts were occasionally situated between surface areas where adjacent chorionic villi exhibited close contact. Within these clefts, extensive zonulae and maculae occludentes and numerous maculae adherentes were found. The zonulae occludentes showed a continuous and irregular course on the membrane surface, and the maculae occludentes were irregularly distributed over extended membrane areas. Besides these areas, maculae occludentes and maculae adherentes were observed on infoldings and invaginations of the synctiotrophoblastic surface membrane. Investigations of the inner surface of the syncytiotrophoblastic layer, that is, the layer facing the villous stroma, also revealed invaginations joined by maculae adherentes. The functional significance of the inter- and intrasyncytiotrophoblastic junctions is discussed with respect to the differentiation of the trophoblast.
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The characteristic feature of replicas obtained from the freeze-fractures of B. pertussis unfixed cultures developing on casein charcoal agar for 1-7 days is the associative growth of highly polymorphic cells, ensured by the ramified system of intercellular connections (IC) formed by the derivatives of the outer layers of the cell wall. This proves that the associative location of bacterial cells, linked by numerous IC, in the preparation is not the artefact appearing in the process of their chemical fixation. In replicas obtained from the freeze-fractures of B. pertussis cultures, previously fixed with glutaraldehyde, osmic acid and uranyl acetate, oval cells with the cytoplasm having a relatively homogeneous structure and with the smoothed-out three-layer cell wall prevail. As a rule, IC are limited to the sites of direct contacts between individual cells.
The junctions between endothelial cells in lymph capillaries may have different degrees of complexity and have been classified as three types: "end to end" (EE), "overlapping" (OVL), and "interdigitating" (ID). Under normal conditions, the more complex junction types (ID and OVL) are more frequent than the simple type (EE). The authors' hypothesis was that not only the folds and digitations of the endothelial profile but also the prevalence of the more complex junction types under normal conditions impart plasticity and distensibility to the endothelial wall. To verify this hypothesis they performed a transmission electron microscope study of lymph capillaries of rat liver under normal conditions and after treatment with histamine, which induces natural filling of lymph vessels. In lymph capillaries of histamine-treated animals they found a significant prevalence of the simplest (EE) junctions (50 +/- 5% versus 20 +/- 4% in controls, p less than 0.001) over the more complex OVL (28 +/- 4% versus 57 +/- 4% in controls, p less than 0.001) and ID junctions (12 +/- 3% versus 19 +/- 4% in controls, NS). The overall trend in junction types showed a highly significant shift toward the simpler types after histamine treatment (p less than .0001). There was also a significant reduction in the mean length of adjoining cell borders from 1.5 +/- 0.1 m in histamine-treated animals (p less than 0.001). These findings indicate the participation of the junctions in variations in capacity of lymph vessels in rat liver. This mobilization of cell junctions may also favor the drainage of interstitial fluids into the absorbing lymph vessels.
The glomeruli of rats rendered nephrotic by administration of aminonucleoside of puromycin were studied with the freeze-fracture technique. The alterations of the podocytes consisted of a progressive blunting of the foot processes and of the disappearance of the extracellular space normally present between the foot processes at the level of the slit pore. In parallel with the closure of the extracellular space, fracture faces of the plasma membrane of the latered podocytes revealed the presence of peculiar membrane differentiations represented mainly by irregular chains of membrane-associated particles in the fracture face A and by variegated grooves in the fracture face B. The presence of grooves in fracture face B indicates that the chains of particles in fracture face A are likely to be tight junctions rather than linear gap junctions. However, since the presence of continuous ridges is exceptional, it is suggested that the tight junctions are the "leaky" -type and that the podocytes many have only limited capacities for assembling junctions of the "tight" -type.
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To analyze the implication of PTEN in the control of tumor cell invasiveness, the canine kidney epithelial cell lines MDCKras-f and MDCKts-src, expressing activated Ras and a temperature-sensitive v-Src tyrosine kinase, respectively, were transfected with PTEN expression vectors. Likewise, the human PTEN-defective glioblastoma cell lines U87MG and U373MG, the melanoma cell line FM-45, and the prostate carcinoma cell line PC-3 were transfected. We demonstrate that ectopic expression of wild-type PTEN in MDCKts-src cells, but not expression of PTEN mutants deficient in either the lipid or both the lipid and protein phosphatase activities, reverted the morphological transformation, induced cell-cell aggregation, and suppressed the invasive phenotype in an E-cadherin-dependent manner. In contrast, overexpression of wild-type PTEN did not counteract Ras-induced invasiveness of MDCKras-f cells expressing low levels of E-cadherin. PTEN effects were not associated with marked changes in accumulation or phosphorylation levels of E-cadherin and associated catenins. Wild-type, but not mutant, PTEN also reverted the invasive phenotype of U87MG, U373MG, PC-3, and FM-45 cells. Interestingly, PTEN effects were mimicked by N-cadherin-neutralizing antibody in the glioblastoma cell lines. Our data confirm the differential activities of E- and N-cadherin on invasiveness and suggest that the lipid phosphatase activity of PTEN exerts a critical role in stabilizing junctional complexes and restraining invasiveness.
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Cell-to-cell-junctions of endothelial cells are specialized and differentiated areas of the plasma membrane. The main functions include the separation of the intravascular and extravascular compartments, the mechanical connection of the cells, and the maintenance of the cell polarity. Although a wide heterogeneity of endothelial cell-to-cell junctions exists in situ, they should be considered in general as adherens type junctions in which gap and tight junctions are morphologically inserted. Under certain pathological conditions, such as wound healing, angiogenesis and many types of inflammation, the interendothelial junctions have to be dissociated and reorganized in which proteins of the junctions are crucially involved. These important mechanisms predict a sophisticated regulation of junctional proteins. The present paper describes the organization and functional aspects of the occludin/ZO-1 complex typically found in tight junctions, the cadherin/catenin complex of the adherens junctions and the connection of these protein complexes to the dense peripheral band via actin filaments. In addition, special attention has been drawn on the function of junction-associated proteins with respect to their role under fluid shear stress and interendothelial gap formation during inflammation.
The phenomenon of the "bystander effect" (BE) observed in suicide gene therapy studies leads to the intriguing possibility that cytotoxicity can be achieved even in tumor cells that have not themselves been targeted with novel genetic material. There is considerable data suggesting the role of gap junction-mediated intercellular communication (GJIC) in the BE. Transfer of connexin (Cx)-encoding genes, the building blocks of GJIC, has been shown both in vitro and in vivo to increase the BE. Since the loss of GJIC is a common feature of cancer cells, we examined the consequence of GJIC up-regulation on the BE in suicide gene therapy. We used 8-bromo-cyclic-AMP to induce Cx43 and GJIC. In mixing assays, using various proportions of cells containing viral thymidine kinase delivered by an adenoviral delivery system or stably transduced by a retrovirus vector, 8-bromo-cyclic-AMP enhanced the BE of cell killing using ganciclovir. The induction in cell killing was more significant when a low percentage of the cell population was infected, which is the relevant clinical situation. We have demonstrated that this is not due to an effect on infectivity or suicide gene expression. Since decreased GJIC is part of the transformed phenotype, induction of Cxs provides an element of selectivity to suicide gene therapy. Our study adds strength to the rationale to develop clinically tolerable GJ inducers to potentiate the effect of suicide gene therapy via the BE.