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PubMed1981

The distribution of orthogonal arrays and their relationship to intercellular junctions in neuroglia of the freeze-fractured hypothalamo-neurohypophysial system.

Using freeze-fracture techniques, we have investigated membrane specializations of the glia associated with the hypothalamo-neurohypophysial system of the rat. In the paraventricular (PVN) and supraoptic (SON) nuclei, astrocytes in areas of high neuronal density (i.e., magnocellular regions) display orthogonal arrays of 6--7 nm particles solely near gap junctions, while astrocytes in areas of lower neuronal density (i.e., parvocellular regions), contain additional arrays in membranes not displaying gap junctions. Arrays are especially numerous on astrocytic perivascular end-feet in both nuclei and in the laminations of the pial-glial limitans ventral to the SON. Ependymal cells near the PVN show arrays both on their lateral surfaces (displaying gap junctions) and on their apical surfaces (facing the CSF). Tight junctions are not noted on astrocytes or ependymal cells, but are noted on both the somas and myelin lamellae of oligodendroglia. Both of these latter membranes occasionally contain gap junctions as well; however, orthogonal arrays are never noted on oligodendroglia. The plasma membranes of pituicytes in the neurohypophysis display gap junctions, complex junctions, and tight junctions. Orthogonal arrays are noted near the first two of these, but not near the last. Arrays in the neutral lobe appear most dense on membranes adjacent to subpial or perivascular spaces. Pituicyte membranes containing orthogonal arrays appear infrequently near the neural stalk, increasing towards the distal end of the neural lobe. The distribution of orthogonal arrays in this system, as well as in other systems in which they have been noted, suggests a polarization of membrane activity.

Animals↗
PubMed1983

Insect intercellular junctions: rapid freezing by jet propane.

The ventral nerve cord of the cockroach, Periplaneta americana, and that of the locust, Schistocerca gregaria, have been studied after rapid freezing by cryo-jet, using liquid propane. Such tissues, unfixed and uncryoprotected, have been compared with unfixed cryoprotected tissues, as well as with material fixed with glutaraldehyde and cryoprotected with glycerol or polyvinylpyrrolydone. The perineurial tight junctions in the cryo-jet-frozen tissues exhibit characteristic intramembranous P-face ridges, but frequently these are composed of smooth-surfaced strands, comparable to those seen with fast-freezing in mammalian tissues, rather than of bead-like fibres. The intramembranous PF ridges, characteristic of axonal and glial processes in the insect central nervous system, also display a smooth surface in rapidly frozen preparations. Prior fixation and/or cryoprotection produces a bead-like appearance in the ridges. The interglial gap junctions, after fast-freezing, exhibit both clustered connexon arrays in the E-face and loosely aggregated ones; hence the coupled state cannot be unequivocally associated with the latter configuration. The septate junctions between glial cells are unchanged after rapid freezing, exhibiting the typical rows of P-face intramembranous particles with complementary E-face pits that are found in replicas from fixed and cryoprotected tissues. The surfaces of the axons and glial processes exhibit pleiomorphic depressions and associated particles as well as PF pits with complementary EF mounds, both with associated IMPs. These structures are not usually seen after fixation or cryoprotection and may represent some kind of receptor structure, or axo-glial specialization.

Animals↗
PubMed1987

Protein processing in lens intercellular junctions: cleavage of MP70 to MP38.

Membrane protein MP70 is a component specifically of lens fiber 16-17 nm junctions. SDS-PAGE analysis of membrane preparations made separately from the sheep lens equatorial region (outer cortex), inner cortex and central region (nucleus) showed abundant MP70 in the young fibers in the outer cortex and drastically reduced levels of MP70 in the older fibers deeper in the lens. MP70 was cleaved to MP38 and thereby lost the epitope to monoclonal anti MP70 (6-4-B2-C6, Kistler et al J Cell Biol 101:28, 1985). This cleavage is fiber age-related and is effected by a calcium-dependent, lens-endogenous protease.

Animals↗
PubMed1980

The morphologic characteristics of intercellular junctions between normal human liver cells and cells from patients with extrahepatic cholestasis.

In freeze-fracture replicas the bile canaliculi of normal human livers showed a lumen of rather constant size with parallel margins. The zonula occludens consists of a complex anastomosing network of intramembranous ridges on the P face and complementary grooves of the E face of the plasmalemma of liver parenchymal cells. The zonula occludens is usually composed of three to five ridges running parallel to the lumen of the bile canaliculus that are surrounded by a looser meshwork of variable orientation. All tight junctions observed in control replicas appeared as continuous barriers without any disruptions. Extrahepatic cholestasis produced considerable morphologic alterations in the canaliculi and tight junctions. The lumen of the canliculi enlarged, and the microvilli disappeared. Side branches, irregularities, and outpouchings of the canalicular membrane extending into the cytoplasm of the hepatocytes were frequently observed. The complexity of the branching pattern and the number of strands in the zonulae occludentes changed extensively. Junctional strands away from their usual pericanalicular location were present on the lateral surface of the plasma membrane. The altered zonulae occludents contain regions in which the strands had a fragmented appearance or were completely absent. These discontinuities in the junctional meshwork provide a direct pathway between the lumen of the bile canaliculus and the intercellular space. They strongly suggest a leakage of the canaliculosinusoidal barrier. Of further interest is the diffuse aggregation of the usually randomly distributed intramembranous particles of the P face of the plasmalemma. The aggregates consist of 10-50 individual particles. Concomitantly, the desmosomes appeared to be more numerous than normally. The number and structure of gap junctions remained unaffected. The results of this investigation are discussed in relation to those obtained after experimental bile duct ligation in rats.

Adult↗
PubMed2000

Gap junctional intercellular communication and connexin 43 expression in ovarian carcinoma.

OBJECTIVES: Gap junctions, which are composed of subunits termed connexins, are plasma membrane channels that link the interior of adjacent cells and permit cells to directly exchange small molecules and ions. Loss or dysfunction of gap junctions appears to be important in allowing cancer cells to escape growth regulation. In a previous study we showed that human ovarian surface epithelial cells exhibited extensive gap junctions and expression of connexin 43. These were nearly absent in human ovarian adenocarcinoma cell lines. To ensure that this variation was not artificially produced by culturing techniques, this study evaluated gap junctions and connexin 43 expressions in normal ovaries and in ovarian adenocarcinomas. STUDY DESIGN: Specimens of normal ovaries and ovarian adenocarcinomas were obtained at the time of surgery and flash-frozen in liquid nitrogen. Connexin 43 immunostaining was performed on all specimens. RESULTS: Among the 11 normal ovaries an average of 59% of the surface epithelium stained positively for connexin 43. In contrast, among the 10 ovarian adenocarcinomas only 19% of each specimen stained positively for connexin 43 (P =.01). CONCLUSION: Similar to our studies on human ovarian surface epithelial cells and ovarian adenocarcinoma cell lines, surgical specimens of normal ovary exhibited extensive connexin 43 expression, whereas connexin 43 expression was nearly absent in ovarian adenocarcinomas. It thus appears that the previously reported loss of gap junctions and connexin 43 was actually associated with a neoplastic process, rather than being artificially induced in the laboratory.

Connexin 43↗
PubMed2001

Connexin mimetic peptides: specific inhibitors of gap-junctional intercellular communication.

Intercellular co-operation is a fundamental and widespread feature in tissues and organs. An important mechanism ensuring multicellular homoeostasis involves signalling between cells via gap junctions that directly connect the cytosolic contents of adjacent cells. Cell proliferation and intercellular communication across gap junctions are closely linked, and a number of pathologies in which communication is disrupted are known where connexins, the gap-junctional proteins, are modified. The proteins of gap junctions thus emerge as therapeutic targets inviting the development and exploitation of chemical tools and drugs that specifically influence intercellular communication. Connexin mimetic peptides that correspond to short specific sequences in the two extracellular loops of connexins are a class of benign, specific and reversible inhibitors of gap-junctional communication that have been studied recently in a broad range of cells, tissues and organs. This review summarizes the properties and uses of these short synthetic peptides, and compares their probable mechanism of action with those of a wide range of other less specific traditional gap-junction inhibitors.

Amino Acid Sequence↗
PubMedDate not supplied

Intercellular junctions, apical differentiation, and infiltrative features in colon cancer: an ultrastructural study. The Colon Cancer Team at IMAS.

Changes in the structure and number of cell junctions have been related to the infiltrative and metastatic potential of tumor cells. Apparently, the loss of cell adhesion should be coordinated with significant changes in the apical and basal cell domains. The authors have performed a sequential ultrastructural study of cells in the superficial, middle, and deep regions of well- and moderately differentiated colon adenocarcinomas. This was to investigate the differences in the organization of different membrane domains among tumor cells in the in situ areas, the advancing, infiltrative edge of the tumors, and the infiltrating zones between these two extreme zones. The results of the study suggest that the organization of these domains is not strictly coordinated, and that, for each infiltration level, both a settling and an infiltrating cell population can be found. These findings could explain the fact that apparently well-differentiated tumors are able to seed distant tissues with individual cells, rather than with well-differentiated glandular aggregates that would hardly be able to reach the vessel lumina without significantly modifying their organization.

Actin Cytoskeleton↗
PubMed2000

PECAM-1/CD31 trans-homophilic binding at the intercellular junctions is independent of its cytoplasmic domain; evidence for heterophilic interaction with integrin alphavbeta3 in Cis.

PECAM-1/CD31 is a cell adhesion and signaling molecule that is enriched at the endothelial cell junctions. Alternative splicing generates multiple PECAM-1 splice variants, which differ in their cytoplasmic domains. It has been suggested that the extracellular ligand-binding property, homophilic versus heterophilic, of these isoforms is controlled by their cytoplasmic tails. To determine whether the cytoplasmic domains also regulate the cell surface distribution of PECAM-1 splice variants, we examined the distribution of CD31-EGFPs (PECAM-1 isoforms tagged with the enhanced green fluorescent protein) in living Chinese hamster ovary cells and in PECAM-1-deficient endothelial cells. Our results indicate that the extracellular, rather than the cytoplasmic domain, directs PECAM-1 to the cell-cell borders. Furthermore, coculturing PECAM-1 expressing and deficient cells along with transfection of CD31-EGFP cDNAs into PECAM-1 deficient cells reveal that this PECAM-1 localization is mediated by homophilic interactions. Although the integrin alphavbeta3 has been shown to interact with PECAM-1, this trans-heterophilic interaction was not detected at the borders of endothelial cells. However, based on cocapping experiments performed on proT cells, we provide evidence that the integrin alphavbeta3 associates with PECAM-1 on the same cell surface as in a cis manner.

Alternative Splicing↗
PubMed2003

TPA-induced signal transduction: a link between PKC and EGFR signaling modulates the assembly of intercellular junctions in Caco-2 cells.

Recent studies suggest that signal transduction may have an important role in the development and regulation of the metastatic phenotype. Here, we investigated the role of the epidermal growth factor receptor (EGFR), and protein kinase C (PKC), in the process of reassembly of cadherin-dependent cell-cell adhesion of Caco-2 cells. We used chemical activation of PKC and EGFR with 12- O-tetradecanoylphorbol-13-acetate (TPA), a tumor-promoting agent, pretreatment with protein kinase inhibitors and subcellular fractionation to analyze the effect of the phorbol ester on the redistribution of junctional proteins. Transepithelial resistance (TER), electron microscopy and immunofluorescence analyses were also carried out. Activation with TPA resulted in disassembly of adherens junctions (AJs), but the tight junction (TJ) structure and function remained unaltered. TPA affected E-cadherin levels. In Caco-2 cells at day 2 of culture, when most E-cadherin is not associated with the cytoskeleton, a decrease in the level of this protein was observed as soon as 6 h after TPA addition. However, at day 5 of culture, the major effect observed after 6 h of treatment was a translocation of the protein from the Triton-insoluble to the -soluble fraction. On the other hand, TPA did not significantly affect the E-cadherin-associated proteins alpha and beta-catenins. Potent specific EGFR inhibitors, such as PD153035 and Tyrphostin 25, as well as Calphostin C, an inhibitor of PKC, significantly blocked the effect of TPA on AJs. Furthermore, inhibition of the TPA effect by the PD98059 MAPK inhibitor suggests that activation of this kinase was the final event in the modulation of cadherin-dependent cell-cell adhesion. Pretreatment of cell monolayers with Calphostin C before EGF treatment, one of the ligands of EGFR, blocked the redistribution of E-cadherin caused by EGF. Based on these results, we conclude that both EGFR and PKC activation are involved in TPA-induced cell signaling for modulation of cadherin-dependent cell-cell adhesion and cell shape in Caco-2 cells.

Caco-2 Cells↗
PubMed1979

Freeze-fracture and tracer studies on the intercellular junctions of insect rectal tissues.

Both rectal pads of the cockroach and rectal papillae of the blowfly possess highly infolded lateral borders; these are associated by desmosomes and septate junctions that maintain the physical integrity of the cell layer at the luminal and basal intercellular regions. Adjacent cells are coupled by gap junctions that allow for cell-to-cell communication and which occur at intervals along the undulating lateral clefts. In rectal pads, occluding basal tight junctions are found as well as extensive scalariform junctions. The latter, like the stacked membrane infoldings of rectal papillae, exhibit intercellular columns and numerous intramembranous P face particles; these are undoubtedly involved in ion transport. In the inter-stack clefts of papillae, reticular septate junctions are encountered which, after freeze-fracture, possess a striking network of PF ridges and EF grooves that are discontinuous and not always complementary. These may serve to regulate the speed and extent of distension of the clefts during solute movement to allow for even and effective fluid flow in this transporting epithelium.

Animals↗
PubMed1986

Intercellular junctional coupling in preimplantation mouse embryos: effect of blocking transcription or translation.

Gap junction formation in cleavage stage mouse embryos was examined by testing for ionic coupling or by observing the intercellular movement of the fluorescent dye, Lucifer yellow CH. Our results confirm that embryo-wide cell coupling, mediated by cell-to-cell membrane channels (gap junctions), is acquired in the 8-cell stage after compaction has begun. However, not all partially compacted embryos were found to be ionically or dye coupled, suggesting that the initiation of gap junction assembly is not necessarily triggered by the onset of cell flattening. The rate of fluorescent dye movement throughout the embryo was found to increase as embryos proceed through compaction and beyond, indicating that the number of gap junctional channels between blastomeres increases as development progresses. The inhibitors alpha-amanitin and cycloheximide were used to assess the requirement of new transcription and protein synthesis, respectively, for the onset of intercellular coupling and its progressive increase during compaction. Treatment conditions were chosen to bring about suppression of mRNA and protein synthesis within 2 hr. Ionic coupling was detected in almost all compacted 8-cell embryos treated with either inhibitor from the 4-cell stage. On the other hand, dye coupling was weak or undetectable in such embryos. We propose that a limited supply of junctional components is present by the 4-cell stage to serve as a pool of precursors for the first gap junctions to be assembled in the 8-cell stage. However, it is apparent that continued embryonic gene expression is required for the full extent of junctional coupling to be established.

Amanitins↗
PubMed1982

The use of lanthanum chloride as a marker for intercellular junctions in rat exocrine pancreas.

A simple technique of perfusion and immersion of tissue in fixative containing lanthanum chloride as an extracellular tracer is described. In addition to functioning as a tracer, the lanthanum chloride appears to enhance electron staining. In rat exocrine pancreas, intercellular spaces between exocrine and centroductular cells were outlined clearly be electron dense material and, at cellular interfaces, spot desmosomes, gap junctions, and tight junctions were demonstrated. The technique proved simple and effective and should prove useful in studies of epithelial and other tissues.

Animals↗
PubMed1992

The crystalline lens. A system networked by gap junctional intercellular communication.

The vertebrate eye lens is a solid cyst of cells which grows throughout life by addition of new cells at the surface. The older cells, buried by the newer generations, differentiate into long, prismatic fibers, losing their cellular organelles and filling their cytoplasms with high concentrations of soluble proteins, the crystallins. The long-lived lens fibers are interconnected by gap junctions, both with themselves and with an anterior layer of simple cuboidal epithelial cells at the lens surface. This network of gap junctions joins the lens cells into a syncytium with respect to small molecules, permitting metabolic co-operation: intercellular diffusion of ions, metabolites, and water. In contact with nutrients at the lens surface, the epithelial cells retain their cellular organelles, and are able to provide the metabolic energy to maintain correct ion and metabolite concentrations within the lens fiber cytoplasms, such that the crystallins remain in solution and do not aggregate (cataract). Gap junctions are formed by a family of integral membrane channel-forming proteins called connexins. Gap junctions between lens epithelial cells are composed of a connexin which is common between many different cell types, notably myocardial cells and connective tissue fibroblasts. The gap junctions between epithelial cells and lens fibers have not yet been biochemically characterized. The gap junctions formed between lens fibers are composed of at least two different connexins, one of which has not been detected between other cell types. The unusual physiology and longevity of the lens fibers may require the special set of connexins which are found joining these cells.

Animals↗
PubMed1999

Gap junctional intercellular communication in the mouse ovarian follicle.

A targeted disruption of the gene encoding the gap junction protein connexin37 (Cx37; alpha 4) results in female infertility. Mutant follicles are not observed to develop beyond early antral stages, and there is a lack of both observable mature Graafian follicles and ovulation. The oocytes are unable to acquire meiotic competence. Following oocyte failure, the residual follicular cells do not undergo atresia but rather transdifferentiate into luteal cells, resulting in a mutant ovary populated with numerous, inappropriate corpora lutea. These results indicate that the Cx37-containing gap junctions formed between oocyte and follicular cells permit bidirectional signalling between the two cell types. These junctions are required for oocyte growth and development during preantral stages of the follicle, and for the inhibition of follicle cell luteinization. An additional role for these junctions may be to permit transfer of cytoplasmic signals required to hold oocytes in meiotic arrest. Since the mutant follicles never acquire meiotic competence, this latter role for gap junctional communication cannot be tested in this model.

Animals↗
PubMed2003

Differential connexin expression, gap junction intercellular coupling, and hemichannel formation in NT2/D1 human neural progenitors and terminally differentiated hNT neurons.

Connexin-mediated gap junctions and open hemichannels in nonjunctional membranes represent two biologically relevant mechanisms by which neural progenitors can coordinate their response to changes in the extracellular environment. NT2/D1 cells are a teratocarcinoma progenitor line that can be induced to differentiate terminally into functional hNT neurons and NT-G nonneuronal cells. Clinical transplants of hNT neurons and experimental grafts of NT2/D1 progenitors or hNT neurons have been used in cell-replacement therapy in vivo. Previous studies have shown that NT2/D1 cells express connexin 43 (Cx43) and that NT2/D1 progenitors are capable of dye transfer. To determine whether NT2/D1 progenitors and differentiated hNT cultures express other connexins, Cx26, Cx30, Cx32, Cx36, Cx37, Cx43, and Cx46.6 mRNA and protein were analyzed. NT2/D1 progenitors express Cx30, Cx36, Cx37, and Cx43. hNT/NT-G cultures express Cx36, Cx37, and de novo Cx46.6. Cx26 and Cx32 were not expressed in NT2/D1 or hNT/NT-G cells. NT2/D1 progenitors formed functional gap junctions as assessed by dye coupling as well as open hemichannels in nonjunctional membranes as assessed by dye-uptake studies. Dye coupling was inhibited by the gap junction blocker 18alpha-glycyrrhetinic acid. Hemichannel activity was inhibited by the dual-specificity chloride channel/connexin hemichannel inhibitor flufenamic acid but not by the chloride channel inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. Both dye coupling and dye uptake were substantially reduced following differentiation of NT2/D1 progenitors. We conclude that the pattern of connexin expression in NT2/D1 cells changes over the course of differentiation corresponding with a reduction in biochemical coupling and hemichannel activity in differentiated cells.

Antineoplastic Agents↗
PubMed1996

Inhibition of gap junction intercellular communications of cultured rat hepatocytes by ethanol: role of ethanol metabolism.

BACKGROUND/AIMS: In a previous study, we reported that in cultured rat hepatocytes, ethanol inhibits intercellular communication which is known to play a central role in the regulation of cell growth and differentiation. This work was designed to find out if ethanol exerts a direct action on cell membranes, comparable to other long-chain (C6-C9) alcohols, or an indirect action. METHODS: Intercellular communication was measured on short-term cultured rat hepatocytes by the fluorescent Lucifer-Yellow CH transfer method. Intracellular pH was measured by spectrofluorimetry and membrane expression of connexin 32 by indirect immunofluorescence. RESULTS: Under our conditions, ethanol (20 mM) inhibited intercellular communication of hepatocytes to the same extent as did octanol and 1 mM. Immunofluorescence semi-quantitative studies of connexin 32 suggested that the observed inhibition was not related to a decrease in the number of gap junction plaques. In contrast with those of octanol, the inhibitory effects of ethanol appeared to be indirect because the inhibition of ethanol metabolism by 4-methyl pyrazole abolished its effects on intercellular communication, while 4-methyl pyrazole did not influence the effects of octanol. Acetaldehyde, the main metabolite of ethanol was without effect on gap junctions. CONCLUSIONS: This suggests that the inhibition of intercellular communication induced by ethanol may be included among the consequences of intermediary cell metabolism disturbances indirectly due to ethanol oxidation. This may be one of the mechanisms by which ethanol metabolism exerts a hepatotoxic possibly carcinogenic action.

1-Octanol↗