Inhibition of gap junction and adherens junction assembly by connexin and A-CAM antibodies

Gating properties of heterotypic gap junction channels formed of connexins 40, 43, and 45

Connexins (Cxs) 40, 43, and 45 are expressed in many different tissues, but most abundantly in the heart, blood vessels, and the nervous system. We examined formation and gating properties of heterotypic gap junction (GJ) channels assembled between cells expressing wild-type Cx40, Cx43, or Cx45 and their fusion forms tagged with color variants of green fluorescent protein. We show that these Cxs, with exception of Cxs 40 and 43, are compatible to form functional heterotypic GJ channels. Cx40 and Cx43 hemichannels are unable or effectively impaired in their ability to dock and/or assemble into junctional plaques. When cells expressing Cx45 contacted those expressing Cx40 or Cx43 they readily formed junctional plaques with cell-cell coupling characterized by asymmetric junctional conductance dependence on transjunctional voltage, V(j). Cx40/Cx45 heterotypic GJ channels preferentially exhibit V(j)-dependent gating transitions between open and residual states with a conductance of approximately 42 pS; transitions between fully open and closed states with conductance of approximately 52 pS in magnitude occur at substantially lower ( approximately 10-fold) frequency. Cx40/Cx45 junctions demonstrate electrical signal transfer asymmetry that can be modulated between unidirectional and bidirectional by small changes in the difference between holding potentials of the coupled cells. Furthermore, both fast and slow gating mechanisms of Cx40 exhibit a negative gating polarity.

N-cadherin signaling in synapse formation and neuronal physiology

Neural cadherin (N-cadherin) is an adhesion receptor that is localized in abundance at neuronto- neuron synapses. N-cadherin contains an extracellular domain that binds to other cadherins on juxtaposed cell membranes, a single-pass transmembrane region, and a cytoplasmic tail that interacts with various proteins, including catenins, kinases, phosphatases, and presenilin 1. N-cadherin contributes to the structural and functional organization of the synaptic complex by ensuring the adhesion between synaptic membranes and organizing the underlying actin cytoskeleton. Additionally, recent findings have shown that N-cadherin may participate in synaptic physiology by regulating calcium influx through voltage-activated calcium currents. The diverse activities of N-cadherin stem from its ability to operate as both an adhesion molecule that links cytoskeletons across cell membranes and a ligand-activated homophilic receptor capable of initiating intracellular signaling. An important mechanism of cadherin signaling is the regulation of small Rho guanosine triphosphatase activity that affects cytoskeleton dynamics and calcium influx. Because both the regulation of cadherin adhesive activity and cadherin-mediated signaling are affected by the binding of molecules to the intracellular domain, changes in the composition of the N-cadherin complex are central to the regulation of cadherin-mediated functions. This article focuses on the roles that N-cadherin might play at the level of the synapse through its effect on adhesion and signaling in the proximity of the synaptic junction.

Inhibition of gap junction communication at ectopic Eph/ephrin boundaries underlies craniofrontonasal syndrome

Mutations in X-linked ephrin-B1 in humans cause craniofrontonasal syndrome (CFNS), a disease that affects female patients more severely than males. Sorting of ephrin-B1–positive and –negative cells following X-inactivation has been observed in ephrin-B1^+/− mice; however, the mechanisms by which mosaic ephrin-B1 expression leads to cell sorting and phenotypic defects remain unknown. Here we show that ephrin-B1^+/− mice exhibit calvarial defects, a phenotype autonomous to neural crest cells that correlates with cell sorting. We have traced the causes of calvarial defects to impaired differentiation of osteogenic precursors. We show that gap junction communication (GJC) is inhibited at ectopic ephrin boundaries and that ephrin-B1 interacts with connexin43 and regulates its distribution. Moreover, we provide genetic evidence that GJC is implicated in the calvarial defects observed in ephrin-B1^+/− embryos. Our results uncover a novel role for Eph/ephrins in regulating GJC in vivo and suggest that the pleiotropic defects seen in CFNS patients are due to improper regulation of GJC in affected tissues.

Mutations inephrin-B1 associated with X-linked CFNS result in aberrant formation of facial bone structures.Ephrin-B1 interacts with gap junctions to influence the cell sorting events that underlie the CFNS deformations.

Life cycle of connexins in health and disease

Evaluation of the human genome suggests that all members of the connexin family of gap-junction proteins have now been successfully identified. This large and diverse family of proteins facilitates a number of vital cellular functions coupled with their roles, which range from the intercellular propagation of electrical signals to the selective intercellular passage of small regulatory molecules. Importantly, the extent of gap-junctional intercellular communication is under the direct control of regulatory events associated with channel assembly and turnover, as the vast majority of connexins have remarkably short half-lives of only a few hours. Since most cell types express multiple members of the connexin family, compensatory mechanisms exist to salvage tissue function in cases when one connexin is mutated or lost. However, numerous studies of the last decade have revealed that mutations in connexin genes can also lead to severe and debilitating diseases. In many cases, single point mutations lead to dramatic effects on connexin trafficking, assembly and channel function. This review will assess the current understanding of wild-type and selected disease-linked mutant connexin transport through the secretory pathway, gap-junction assembly at the cell surface, internalization and degradation.

Ochratoxin A alters cell adhesion and gap junction intercellular communication in MDCK cells

Ochratoxin A (OTA) is one of the most potent renal carcinogens studied to date, but the mechanism of tumor formation by ochratoxin A remains largely unknown. Cell adhesion and cell-cell communication participate in the regulation of signaling pathways involved in cell proliferation and growth control and it is therefore not surprising that modulation of cell-cell signaling has been implicated in cancer development. Several nephrotoxicants and renal carcinogens have been shown to alter cell-cell signaling by interference with gap junction intercell communication (GJIC) and/or cell adhesion, and the aim of this study was to determine if disruption of cell-cell interactions occurs in kidney epithelial cells in response to OTA treatment. MDCK cells were treated with OTA (0-50 microM) for up to 24h and gap junction function was analyzed using the scrape-load/dye transfer assay. In addition, expression and intracellular localization of C x 43, E-cadherin and beta-catenin were determined by immunoblot and immunofluorescence analysis. A clear decrease in the distance of dye transfer was evident following treatment with OTA at concentrations/incubation times which did not affect cell viability. Consistent with the functional inhibition of GJIC, treatment with OTA resulted in a dose-dependent decrease in C x 43 expression. In contrast to C x 43, OTA did not alter total amount of the adherens junction proteins E-cadherin and beta-catenin. Moreover, Western blot analysis of Triton X-100 soluble and insoluble protein fractions did not indicate translocation of cell adhesion molecules from the membrane to the cytoplasm. However, a approximately 78 kDa fragment of beta-catenin was detected in the detergent soluble fraction, indicating proteolytic cleavage of beta-catenin. Immunofluorescence analysis also revealed changes in the pattern of both beta-catenin and E-cadherin labeling, suggesting that OTA may alter cell-adhesion. Taken together, these data support the hypothesis that disruption of cell-cell signaling may contribute to OTA toxicity and carcinogenicity.

N-cadherin signals through Rac1 determine the localization of connexin 43 in cardiac myocytes

It has been proposed that the formation of gap junction is influenced by adherens junction in cardiac myocytes. To examine whether signals through N-cadherin are involved in the distribution of connexin 43 (Cx43), the distribution of cell-cell adhesion molecules, N-cadherin and Cx43, was analyzed in aligned cardiac myocytes. To induce cell orientation running in parallel to tension direction, neonatal rat cardiac myocytes were plated for 3 hours and exposed to 20% cyclic stretch for 24 hours on silicone dishes. The aligned cells cultured for 0-5 days were immunostained with anti- N-cadherin or anti-Cx43 antibody. After cultivation for 3-5 days, following the accumulation of N-cadherin, Cx43 was localized at the longitudinal cell termini. Adenoviral gene transfer of dominant negative N-cadherin significantly attenuated the localization of Cx43 at the longitudinal cell termini, suggesting that Cx43 localization is regulated downstream of N-cadherin. In the process of Cx43 localization, Rho family proteins, RhoA and Rac1, were activated, but not Cdc42. RhoA and Rac1 activation was inhibited by the transfection of dominant negative N-cadherin, indicating that RhoA and Rac1 were activated by N-cadherin in the oriented cardiac myocytes. The inhibition of Rho family proteins by Rho GDI significantly attenuated the accumulation of Cx43, but not that of N-cadherin. Furthermore, the translocation of Cx43 to longitudinal cell termini was prevented by the inhibition of Rac1, but not RhoA. Collectively, these findings suggest that the localization of Cx43 was determined through the Rac1 pathway downstream of N-cadherin in cardiac myocytes.

Dysregulation of cell adhesion proteins and cardiac arrhythmogenesis

Proper mechanical and electrical coupling of cardiomyocytes is crucial for normal propagation of the electrical impulse throughout the working myocardium. Various proteins on the surface of cardiomyocytes are responsible for the integration of structural information and cell-cell communication. Increasing evidence from diseased myocardium and animal models indicates that alteration in electrical coupling via gap junctions is a critical determinant in the development of an arrhythmogenic substrate. What is less clear is how gap junctions are maintained and regulated in the working myocardium. In this review, we present data from human disease and animal models that support the idea that cell adhesion proteins regulate the stability of the gap junction protein, connexin.

Heteromerization of innexin gap junction proteins regulates epithelial tissue organization in Drosophila

Gap junctions consist of clusters of intercellular channels, which enable direct cell-to-cell communication and adhesion in animals. Whereas deuterostomes, including all vertebrates, use members of the connexin and pannexin multiprotein families to assemble gap junction channels, protostomes such as Drosophila and Caenorhabditis elegans use members of the innexin protein family. The molecular composition of innexin-containing gap junctions and the functional significance of innexin oligomerization for development are largely unknown. Here, we report that heteromerization of Drosophila innexins 2 and 3 is crucial for epithelial organization and polarity of the embryonic epidermis. Both innexins colocalize in epithelial cell membranes. Innexin3 is mislocalized to the cytoplasm in innexin2 mutants and is recruited into ectopic expression domains defined by innexin2 misexpression. Conversely, RNA interference (RNAi) knockdown of innexin3 causes mislocalization of innexin2 and of DE-cadherin, causing cell polarity defects in the epidermis. Biochemical interaction studies, surface plasmon resonance analysis, transgenesis, and biochemical fractionation experiments demonstrate that both innexins interact via their C-terminal cytoplasmic domains during the assembly of heteromeric channels. Our data provide the first molecular and functional demonstration that innexin heteromerization occurs in vivo and reveal insight into a molecular mechanism by which innexins may oligomerize into heteromeric gap junction channels.

Hydroxy apatite microspheres enhance gap junctional intercellular communication of human osteoblasts composed of connexin 43 and 45

The aseptic loosening of artificial joints with associated periprosthetic bone resorption may be partly due to the suppression of osteoblast function to form new bone by wear debris from the joint. To assess the effect of wear debris on osteoblasts, effects of model wear debris on gap junctional intercellular communication (GJIC) of normal human osteoblasts were estimated. The GJIC activity of the osteoblasts after a 1-day incubation with the microspheres was similar to that of normal osteoblasts. However, hydroxy apatite particles, which have been reported to enhance the differentiation of osteoblasts in contact with them, enhanced the GJIC function of the osteoblasts. From RT-PCR studies, not only connexin 43 but also connexin 45 is suggested to play a role in the GJIC of the osteoblasts in an early stage of coculture with the microspheres, although it is still unclear how these connexins work and are regulated in the GJIC and differentiation. However, this study suggests that there is a relationship between the early levels of GJIC and the differentiation of the cells. Therefore, estimating the effect of biomaterials, even in the microsphere form, on the GJIC of model cells, with which the biomaterials may be in contact in vivo, can provide important information about their biocompatibility.

Gap junctional communication in the male reproductive system

Male fertility is a highly controlled process that allows proliferation, meiosis and differentiation of male germ cells in the testis, final maturation in the epididymis and also requires functional male accessory glands: seminal vesicles, prostate and corpus cavernosum. In addition to classical endocrine and paracrine controls, mainly by gonadotropins LH and FSH and steroids, there is now strong evidence that all these processes are dependent upon the presence of homocellular or heterocellular junctions, including gap junctions and their specific connexins (Cxs), between the different cell types that structure the male reproductive tract. The present review is focused on the identification of Cxs, their distribution in the testis and in different structures of the male genital tract (epididymis, seminal vesicle, prostate, corpus cavernosum), their crucial role in the control of spermatogenesis and their implication in the function of the male accessory glands, including functional smooth muscle tone. Their potential dysfunctions in some testis (spermatogenic arrest, seminoma) and prostate (benign hyperplasia, adenocarcinoma) diseases and in the physiopathology of the human erectile function are also discussed.

Intercellular communication: the Drosophila innexin multiprotein family of gap junction proteins

Gap junctions belong to the most conserved cellular structures in multicellular organisms, from Hydra to man. They contain tightly packed clusters of hydrophilic membrane channels connecting the cytoplasms of adjacent cells, thus allowing direct communication of cells and tissues through the diffusion of ions, metabolites, and cyclic nucleotides. Recent evidence suggests that gap junctions are constructed by three different families of four transmembrane proteins: the Connexins and the Innexins found in vertebrates and in invertebrates, respectively, and the Innexin-like Pannexins, which were recently discovered in humans. This article focuses on the Drosophila Innexin multiprotein family, which is comprised of eight members. We highlight common structural features and discuss recent findings that suggest close similarities in cellular distribution, function, and regulation of Drosophila Innexins and vertebrate gap junction proteins.

Zonula occludens-1 alters connexin43 gap junction size and organization by influencing channel accretion

Regulation of gap junction (GJ) organization is critical for proper function of excitable tissues such as heart and brain, yet mechanisms that govern the dynamic patterning of GJs remain poorly defined. Here, we show that zonula occludens (ZO)-1 localizes preferentially to the periphery of connexin43 (Cx43) GJ plaques. Blockade of the PDS95/dlg/ZO-1 (PDZ)-mediated interaction between ZO-1 and Cx43, by genetic tagging of Cx43 or by a membrane-permeable peptide inhibitor that contains the Cx43 PDZ-binding domain, led to a reduction of peripherally associated ZO-1 accompanied by a significant increase in plaque size. Biochemical data indicate that the size increase was due to unregulated accumulation of gap junctional channels from nonjunctional pools, rather than to increased protein expression or decreased turnover. Coexpression of native Cx43 fully rescued the aberrant tagged-connexin phenotype, but only if channels were composed predominately of untagged connexin. Confocal image analysis revealed that, subsequent to GJ nucleation, ZO-1 association with Cx43 GJs is independent of plaque size. We propose that ZO-1 controls the rate of Cx43 channel accretion at GJ peripheries, which, in conjunction with the rate of GJ turnover, regulates GJ size and distribution.

Cardiac-specific loss of N-cadherin leads to alteration in connexins with conduction slowing and arrhythmogenesis

The remodeling of ventricular gap junctions, as defined by changes in size, distribution, or function, is a prominent feature of diseased myocardium. However, the regulation of assembly and maintenance of gap junctions remains poorly understood. To investigate N-cadherin function in the adult myocardium, we used a floxed N-cadherin gene in conjunction with a cardiac-specific tamoxifen-inducible Cre transgene. The mutant animals appeared active and healthy until their sudden death approximately 2 months after deleting N-cadherin from the heart. Electrophysiologic analysis revealed abnormal conduction in the ventricles of mutant animals, including diminished QRS complex amplitude consistent with loss of electrical coupling in the myocardium. A significant decrease in the gap junction proteins, connexin-43 and connexin-40, was observed in N-cadherin-depleted myocytes. Perturbation of connexin function resulted in decreased ventricular conduction velocity, as determined by optical mapping. Our data suggest that perturbation of the N-cadherin/catenin complex in heart disease may be an underlying cause, leading to the establishment of the arrythmogenic substrate by destabilizing gap junctions at the cell surface.

Cell-cell interactions in regulating osteogenesis and osteoblast function

Endochondral bone formation requires an elaborate interplay among autocrine, paracrine, and endocrine signals, positional cues, and cell-cell contacts to mediate the complex three-dimensional architecture and function of the skeleton. Embryonic bone development occurs by migration, aggregation, and condensation of immature mesenchymal progenitor cells to form the cartilaginous anlage. Upon vascular invasion, the cartilaginous scaffold is colonized and subsequently mineralized by osteoblasts. Likewise, bone remodeling in the adult skeleton is a dynamic process that requires coordinated cellular activities among osteoblasts, osteocytes, and osteoclasts to maintain bone homeostasis. This review examines the role of cell-cell interactions mediated by adherens junctions formed by cadherins and communicative gap junctions formed by connexins in regulating bone development and osteogenic function.

Connexin phosphorylation as a regulatory event linked to gap junction internalization and degradation

Gap junction proteins, connexins, are dynamic polytopic membrane proteins that exhibit unprecedented short half-lives of only a few hours. Consequently, it is well accepted that in addition to channel gating, gap junctional intercellular communication is regulated by connexin biosynthesis, transport and assembly as well as the formation and removal of gap junctions from the cell surface. At least nine members of the 20-member connexin family are known to be phosphorylated en route or during their assembly into gap junctions. For some connexins, notably Cx43, evidence exists that phosphorylation may trigger its internalization and degradation. In recent years it has become apparent that the mechanisms underlying the regulation of connexin turnover are quite complex with the identification of many connexin binding molecules, a multiplicity of protein kinases that phosphorylate connexins and the involvement of both lysosomal and proteasomal pathways in degrading connexins. This paper will review the evidence that connexin phosphorylation regulates, stimulates or triggers gap junction disassembly, internalization and degradation.

Connexin43 Associated with an N-cadherin-containing Multiprotein Complex Is Required for Gap Junction Formation in NIH3T3 Cells

Previous studies have indicated an intimate linkage between gap junction and adherens junction formation. It was suggested this could reflect the close membrane-membrane apposition required for junction formation. In NIH3T3 cells, we observed the colocalization of connexin43 (Cx43alpha1) gap junction protein with N-cadherin, p120, and other N-cadherin-associated proteins at regions of cell-cell contact. We also found that Cx43alpha1, N-cadherin, and N-cadherin-associated proteins were coimmunoprecipitated by antibodies to either Cx43alpha1, N-cadherin, or various N-cadherin-associated proteins. These findings suggest that Cx43alpha1 and N-cadherin are coassembled in a multiprotein complex containing various N-cadherin-associated proteins. Studies using siRNA knockdown indicated that cell surface expression of Cx43alpha1 required N-cadherin, and conversely, N-cadherin cell surface expression required Cx43alpha1. Pulse-chase labeling and cell surface biotinylation experiments indicated that in the absence of N-cadherin, Cx43alpha1 cell surface trafficking is blocked. Surprisingly, siRNA knockdown of p120, an N-cadherin-associated protein known to modulate cell surface turnover of N-cadherin, reduced N-cadherin cell surface expression without altering Cx43alpha1 expression. These observations suggest that in contrast to the coregulated cell surface trafficking of Cx43alpha1 and N-cadherin, N-cadherin turnover at the cell surface may be regulated independently of Cx43alpha1. Functional studies showed gap junctional communication is reduced and cell motility inhibited with N-cadherin or Cx43alpha1 knockdown, consistent with the observed loss of both gap junction and cadherin contacts with either knockdown. Overall, these studies indicate that the intracellular coassembly of connexin and cadherin is required for gap junction and adherens junction formation, a process that likely underlies the intimate association between gap junction and adherens junction formation.

ZO-1 alters the plasma membrane localization and function of Cx43 in osteoblastic cells

ZO-1 is the major connexin-interacting protein in ROS 17/2.8 (ROS) osteoblastic cells. We examined the role of ZO-1 in Cx43-mediated gap junction formation and function in ROS cells that expressed the connexin-interacting fragment of ZO-1 (ROS/ZO-1dn) cells. Expression of this ZO-1(7-444) fusion protein in ROS cells disrupted the Cx43/ZO-1 interaction and decreased dye transfer by 85%, although Cx43 was retained on the plasma membrane as assessed by surface biotinylation. Fractionation of lysates derived from ROS/ZO-1dn cells on a 5-30% sucrose flotation gradient showed that 40% of the Cx43 floated into these sucrose gradients, whereas none of the Cx43 in ROS cell lysates entered the gradients, suggesting that more Cx43 is associated with lipid rafts in the transfected ROS cells than in lysates derived from untransfected ROS cells. In contrast to the ROS/ZO-1dn cells, ROS cells that over-expressed ZO-1 protein (ROS/ZO-1myc cells) exhibited increased gap junctional permeability and appositional membrane staining for Cx43. These data demonstrate that ZO-1 regulates Cx43-mediated gap junctional communication in osteoblastic cells and alters the membrane localization of Cx43. They suggest that ZO-1-mediated delivery of Cx43 from a lipid raft domain to gap junctional plaques may be an important regulatory step in gap junction formation.

Dexamethasone inhibits the HSV-tk/ ganciclovir bystander effect in malignant glioma cells

BACKGROUND

HSV-tk/ ganciclovir (GCV) gene therapy has been extensively studied in the setting of brain tumors and largely relies on the bystander effect. Large studies have however failed to demonstrate any significant benefit of this strategy in the treatment of human brain tumors. Since dexamethasone is a frequently used symptomatic treatment for malignant gliomas, its interaction with the bystander effect and the overall efficacy of HSV-TK gene therapy ought to be assessed.

METHODS

Stable clones of TK-expressing U87, C6 and LN18 cells were generated and their bystander effect on wild type cells was assessed. The effects of dexamethasone on cell proliferation and sensitivity to ganciclovir were assessed with a thymidine incorporation assay and a MTT test. Gap junction mediated intercellular communication was assessed with microinjections and FACS analysis of calcein transfer. The effect of dexamethasone treatment on the sensitivity of TK-expressing to FAS-dependent apoptosis in the presence or absence of ganciclovir was assessed with an MTT test. Western blot was used to evidence the effect of dexamethasone on the expression of Cx43, CD95, CIAP2 and BclXL.

RESULTS

Dexamethasone significantly reduced the bystander effect in TK-expressing C6, LN18 and U87 cells. This inhibition results from a reduction of the gap junction mediated intercellular communication of these cells (GJIC), from an inhibition of their growth and thymidine incorporation and from a modulation of the apoptotic cascade.

CONCLUSION

The overall efficacy of HSV-TK gene therapy is adversely affected by dexamethasone co-treatment in vitro. Future HSV-tk/ GCV gene therapy clinical protocols for gliomas should address this interference of corticosteroid treatment.

Gap junction channels reconstituted in two closely apposed lipid bilayers

Intercellular communication mediated by gap junction channels plays an important role in many cellular processes. In contrast to other channels, gap junction channels span two plasma membranes resulting in an intracellular location for both ends of the junctional pore and the regulatory sites for channel gating. This configuration presents unique challenges for detailed experimental studies of junctional channel physiology and ligand-activation in situ. Availability of an appropriate model system would significantly facilitate future studies of gap junction channel function and structure. Here we show that the double-membrane channel can be reconstituted in pairs of closely apposed lipid bilayers, as experienced in cells. We have trapped the calcium-sensitive dye, arsenazo III (AIII), partially calcium-saturated (AIII-Ca), in one population of connexin32 reconstituted-liposomes, and EGTA in a second one. In such mixtures, the interaction of EGTA with AIII-Ca was measured by a large color shift from blue to red (decreased absorbance at 652 nm). The exchange of these compounds through gap junctions was proportional to these decrements. Results indicate that these connexon-mediated interliposomal channels are functional and are inhibited by the addition of alpha-glycyrrhetinic acid and by flufenamic acid, two gap junction communication inhibitors. Future use of this model system has the potential to improve our understanding of the permeability and modulation of junctional channels in its native intercellular assembly.

Calcium-dependent Open/Closed Conformations and Interfacial Energy Maps of Reconstituted Hemichannels

Using an atomic force microscope, we have studied three-dimensional molecular topography and calcium-sensitive conformational changes of individual hemichannels. Full-length (non-truncated) Cx43 hemichannels (connexons), when reconstituted in lipid bilayer, appear as randomly distributed individual particles and clusters. They show a lack of preferential orientation of insertion into lipid membrane; in a single bilayer, connexons with protrusion of either the extracellular face or the large non-truncated cytoplasmic face are observed. Extracellular domains of these undocked hemichannels are structurally different from hemichannels in the docked gap junctional plaques examined after their exposure by force dissection or chemical dissection. Calcium induced a reversible change in the extracellular pore diameter. Hemichannels imaged in a physiological buffer with 1.8 mm Ca(+2) had the pore diameter of approximately 1.8 nm, consistent with the closed channel conformation. Reducing Ca(+2) concentration to approximately 1.4, 1, and 0 mm, which changes hemichannels from the closed to open conformation, increased the pore diameter to approximately 2.5 nm for approximately 27, 74, and 100% of hemichannels, respectively. Thus, open/close probability of the hemichannel appears to be [Ca(2+)]-dependent. Computational analysis of the atomic force microscopy phase mode imaging reveals a significantly higher interfacial energy for open hemichannels that results from the interactions between the atomic force microscope probe and the hydrophobic domains. Thus, hydrophobic extracellular domains of connexins regulate calcium-dependent conformational changes.

CONNEXINS AND CELL SIGNALING IN DEVELOPMENT AND DISEASE

Gap junctions contain hydrophilic membrane channels that allow direct communication between neighboring cells through the diffusion of ions, metabolites, and small cell signaling molecules. They are made up of a hexameric array of polypeptides encoded by the connexin multi-gene family. Cell-cell communication mediated by connexins is crucial to various cellular functions, including the regulation of cell growth, differentiation, and development. Mutations in connexin genes have been linked to a variety of human diseases, including cardiovascular anomalies, peripheral neuropathy, deafness, skin disorders, and cataracts. In addition to their coupling function, recent studies suggest that connexin proteins may also mediate signaling. This could involve interactions with other protein partners that may play a role not only in connexin assembly, trafficking, gating and turnover, but also in the coordinate regulation of cell-cell communication with cell adhesion and cell motility. The integration of these cell functions is likely to be important in the role of gap junctions in development and disease.

Opening hemichannels in nonjunctional membrane stimulates gap junction formation

We studied gap junction formation in pairs of Xenopus laevis oocytes expressing connexins that form functional hemichannels and found no correlation between junctional conductance (G(j)) and whole-cell hemichannel conductances (G(hemi)) within the first few hours of pairing. However, opening hemichannels to a threshold current stimulated a rapid G(j) increase. Moreover, cx46 hemichannel current stimulated cx40 G(j) even though cx40 and cx46 do not form heteromeric or heterotypic gap junctions. Initial growth rate and final steady-state level of stimulated G(j) were proportional to the product of hemichannel conductances. External calcium affected the growth rate of stimulated G(j) but not the final steady-state value. Time constants of formation were short in low [Ca(2+)](out) (3 min in 200 micro M Ca(2+)) and long in high [Ca(2+)](out) (15 min in 1 mM Ca(2+)), but in oocyte pairs pretreated with lectins to reduce steric hindrance imposed by large membrane glycoproteins the time constant was short and Ca(2+)-independent. We suggest that hemichannel activity stimulates G(j) by collapsing the extracellular volume between membranes to allow the end-to-end binding between hemichannels. These studies suggest the possibility that functional hemichannels could trigger or enhance junctional formation in vivo in response to appropriate stimuli.

Modulation of intercellular communication in macrophages: possible interactions between GAP junctions and P2 receptors

Gap junctions are connexin-formed channels that play an important role in intercellular communication in most cell types. In the immune system, specifically in macrophages, the expression of connexins and the establishment of functional gap junctions are still controversial issues. Macrophages express P2X(7) receptors that, once activated by the binding of extracellular ATP, lead to the opening of transmembrane pores permeable to molecules of up to 900 Da. There is evidence suggesting an interplay between gap junctions and P2 receptors in different cell systems. Thus, we used ATP-sensitive and -insensitive J774.G8 macrophage cell lines to investigate this interplay. To study junctional communication in J774-macrophage-like cells, we assessed cell-to-cell communication by microinjecting Lucifer Yellow. Confluent cultures of ATP-sensitive J774 cells (ATP-s cells) are coupled, whereas ATP-insensitive J774 cells (ATP-i cells), derived by overexposing J774 cells to extracellular ATP until they do not display the phenomenon of ATP-induced permeabilization, are essentially uncoupled. Western-blot and reverse-transcription polymerase chain reaction assays revealed that ATP-s and ATP-i cells express connexin43 (Cx43), whereas only ATP-s cells express the P2X(7) receptor. Accordingly, ATP-i cells did not display any detectable ATP-induced current under whole-cell patch-clamp recordings. Using immunofluorescence microscopy, Cx43 reactivity was found at the cell surface and in regions of cell-cell contact of ATP-s cells, whereas, in ATP-i cells, Cx43 immunoreactivity was only present in cytosolic compartments. Using confocal microscopy, it is shown here that, in ATP-s cells as well as in peritoneal macrophages, Cx43 and P2X(7) receptors are co-localized to the membrane of ATP-s cells and peritoneal macrophages.

Regulation of connexin biosynthesis, assembly, gap junction formation, and removal

Gap junctions (GJs) are the only known cellular structures that allow a direct transfer of signaling molecules from cell-to-cell by forming hydrophilic channels that bridge the opposing membranes of neighboring cells. The crucial role of GJ-mediated intercellular communication (GJIC) for coordination of development, tissue function, and cell homeostasis is now well documented. In addition, recent findings have fueled the novel concepts that connexins, although redundant, have unique and specific functions, that GJIC may play a significant role in unstable, transient cell-cell contacts, and that GJ hemi-channels by themselves may function in intra-/extracellular signaling. Assembly of these channels is a complicated, highly regulated process that includes biosynthesis of the connexin subunit proteins on endoplasmic reticulum membranes, oligomerization of compatible subunits into hexameric hemi-channels (connexons), delivery of the connexons to the plasma membrane, head-on docking of compatible connexons in the extracellular space at distinct locations, arrangement of channels into dynamic, spatially and temporally organized GJ channel aggregates (so-called plaques), and coordinated removal of channels into the cytoplasm followed by their degradation. Here we review the current knowledge of the processes that lead to GJ biosynthesis and degradation, draw comparisons to other membrane proteins, highlight novel findings, point out contradictory observations, and provide some provocative suggestive solutions.

Gap junctions and neurological disorders of the central nervous system

Gap junctions are intercellular channels which directly connect the cytoplasm between neighboring cells. In the central nervous system (CNS) various kinds of cells are coupled by gap junctions, which play an important role in maintaining normal function. Neuronal gap junctions are involved in electrical coupling and may also contribute to the recovery of function after cell injury. Astrocytes are involved in the pathology of most neuronal disorders, including brain ischemia, Alzheimer's disease and epilepsy. In the pathology of brain tumors, gap junctions may be related to the degree of malignancy and metastasis. However, the role of connexins, gap junctions and hemichannels in the pathology of the diseases in the CNS is still ambiguous. Of increasing importance is the unraveling of the function of gap junctions in the neural cell network, involving neurons, astrocytes, microglia and oligodendrocytes. A better understanding of the role of gap junctions may contribute to the development of new therapeutic approaches to treating diseases of the CNS.

Differentiating the effects of Cx36 and E-cadherin for proper insulin secretion of MIN6 cells

Connexins have been implicated in many cell functions, even though in most cases it is still unclear whether these functions may actually be mediated by other proteins that are secondarily affected by connexin changes. Secretory systems provide useful models in which to tackle this central question. Primary pancreatic beta-cells and insulin-producing lines are connected by gap junction channels made of Cx36. Using stable transfection of an antisense Cx36 cDNA, we have previously obtained clones of MIN6 cells that featured a markedly reduced expression of Cx36 and impaired insulin secretion. Here, we first show that this change also resulted in loss of E-cadherin and occludin expression, thus preventing the attribution of the secretory defects to a specific type of membrane protein. To investigate this question, we have now restored the expression of either Cx36 or E-cadherin in the Cx36 antisense-transfected cells. We show that a lentivirus-mediated transduction efficiently restored Cx36 expression in MIN6 cells and allowed for expression of variable levels of this protein. We further document that adequate but not excessive levels of Cx36 allowed for recover of normal insulin secretion in response to various secretagogues. Finally, we demonstrate that restoration of normal E-cadherin expression was not able to achieve the same secretory correction. The data demonstrate that Cx36, but not E-cadherin, is necessary to control specific steps of beta-cell secretion.

Gap junction channel protein innexin 2 is essential for epithelial morphogenesis in the Drosophila embryo

Direct communication of neighboring cells by gap junction channels is essential for the development of tissues and organs in the body. Whereas vertebrate gap junctions are composed of members of the connexin family of transmembrane proteins, in invertebrates gap junctions consist of Innexin channel proteins. Innexins display very low sequence homology to connexins. In addition, very little is known about their cellular role during developmental processes. In this report, we examined the function and the distribution of Drosophila Innexin 2 protein in embryonic epithelia. Both loss-of-function and gain-of-function innexin 2 mutants display severe developmental defects due to cell death and a failure of proper epithelial morphogenesis. Furthermore, immunohistochemical analyses using antibodies against the Innexins 1 and 2 indicate that the distribution of Innexin gap junction proteins to specific membrane domains is regulated by tissue specific factors. Finally, biochemical interaction studies together with genetic loss- and gain-of-function experiments provide evidence that Innexin 2 interacts with core proteins of adherens and septate junctions. This is the first study, to our knowledge, of cellular distribution and protein-protein interactions of an Innexin gap junctional channel protein in the developing epithelia of Drosophila.

Diversity in protein–protein interactions of connexins: emerging roles

Gap junctions, specialised membrane structures that mediate cell-to-cell communication in almost all tissues, are composed of channel-forming integral membrane proteins termed connexins. The activity of these intercellular channels is closely regulated, particularly by intramolecular modifications as phosphorylations of proteins by protein kinases, which appear to regulate the gap junction at several levels, including assembly of channels in the plasma membrane, connexin turnover as well as directly affecting the opening and closure ("gating") of channels. The regulation of membrane channels by protein phosphorylation/dephosphorylation processes commonly requires the formation of a multiprotein complex, where pore-forming subunits bind to auxiliary proteins (e.g. scaffolding proteins, catalytic and regulatory subunits), that play essential roles in channel localisation and activity, linking signalling enzymes, substrates and effectors into a structure frequently anchored to the cytoskeleton. The present review summarises the up-to-date progress regarding the proteins capable of interacting or at least of co-localising with connexins and their functional importance.

Gap junctions and connexin-mediated communication in the immune system

Gap junctions and connexins are present in the immune system. In haematopoiesis, connexin 43, the most widely distributed gap junction protein, appears to be a key player in the development of progenitor cells and their communication with stromal cells. Connexin 43 is expressed by macrophages, neutrophils and mast cells. Lymphocytes also express connexin 43, and inhibition of gap junction channels in these cells by using highly specific connexin mimetic reagents has profound effects on immunoglobulin secretion and synthesis of cytokines. Lymphocytes and leukocytes also communicate directly in vitro with endothelial cells via gap junctions. Connexins are implicated in inflammatory reactions in a range of tissues. Their involvement in atherosclerotic plaque formation in the vascular system is also a current growth point in research, and could lead to the development of therapeutic interventions.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Effects of 18beta-glycyrrhetinic acid on the junctional complex and steroidogenesis in rat adrenocortical cells

Cellular junctions play important roles in cell differentiation, signal transduction, and cell function. This study investigated their function in steroid secretion by adrenal cells. Immunofluorescence staining revealed the presence of gap junctions and adherens junctions between adrenal cells. The major gap junction protein, connexin43, was seen as a linear dotted pattern of the typical gap junction plaques, in contrast to alpha-, beta-, and gamma-catenin, which were seen as continuous, linear staining of cell-cell adherens junction. Treatment with 18beta-glycyrrhetinic acid, a gap junction inhibitor, reduced the immunoreactivity of these proteins in a time- and dose-dependent manner, and caused the gap junction and adherens junction to separate longitudinally from the cell-cell contact sites, indicating the structural interdependency of these two junctions. Interestingly, 18beta-glycyrrhetinic acid stimulated a two- to three-fold increase in steroid production in these adrenal cells lacking intact cell junctions. These data raise the question of the necessity for cell communication for the endocrine function of adrenal cells. Pharmacological analyses indicated that the steroidogenic effect of 18beta-glycyrrhetinic acid was partially mediated by extracellular signal-related kinase and calcium/calmodulin-dependent kinase, a pathway distinct from the protein kinase A signaling pathway already known to mediate steroidogenesis in adrenal cells.

Coxsackievirus and adenovirus receptor, a tight junction membrane protein, is expressed in glomerular podocytes in the kidney

In nephrosis, filtration slits of podocytes are greatly narrowed, and slit diaphragms are displaced by junctions with close contact. Freeze-fracture studies have shown that the newly formed junctions consist of tight junctions and gap junctions. Several tight-junction proteins are known as integral membrane components, including occludin and claudins; but none of them have been found in podocytes. Coxsackievirus and adenovirus receptor (CAR) has recently been identified as a virus receptor that is a 46-kDa integral membrane protein with two Ig-like domains in the extracellular region. In polarized epithelial cells, CAR is expressed at the tight junction, where it associates with ZO-1 and plays a role in the barrier to the movement of macromolecules and ions. In the present study, we investigated the expression and localization of CAR in rat kidneys treated with puromycin aminonucleoside (PAN) and in rat kidneys perfused for 15 minutes with protamine sulfate (PS). Both the experimental models have been used to induce tight junctions in podocytes. Ribonuclease protection assay and Western blot analysis revealed a distinct increase of CAR transcript and protein in glomeruli during PAN nephrosis but no increase in glomeruli by PS perfusion. Immunohistochemistry revealed a significant increase in CAR staining intensity along the glomerular capillary wall in PAN nephrosis and after PS perfusion. Immunoelectron microscopy demonstrated in both the models that the immunogold particles for CAR along the capillary wall were found predominantly at close cell-cell contact sites of podocytes but were rarely found at slit diaphragms. In cultured podocytes, CAR was localized at cell-cell contact sites. CAR distribution was identical to that of ZO-1 and different from that of a gap junction protein, connexin43. These findings indicate that CAR is an integral membrane component of tight junction in podocytes and that CAR expression in podocytes is regulated at the transcriptional level and in the redistribution of protein.

Loss of function and impaired degradation of a cataract-associated mutant connexin50

A mutant human connexin50 (hCx50), hCx50P88S, has been linked to cataracts inherited as an autosomal dominant trait. The functional, biochemical and cellular behavior of wild-type and mutant hCx50 were examined in transfected cells. hCx50P88S was unable to induce gap junctional currents by itself, and it abolished gap junctional currents when co-expressed with wild-type (wt) hCx50. Cells transfected with hCx50P88S showed cytoplasmic accumulations of Cx50 immunoreactivity in addition to staining at appositional membranes; these accumulations did not significantly co-localize with markers for the endoplasmic reticulum, Golgi apparatus, lysosomes, endosomes or vimentin filaments. Immunoelectron microscopy studies localized hCx50P88S to cytoplasmic membrane stacks in close vicinity to the endoplasmic reticulum. In contrast, aggresome-like accumulations were induced by treatment of wt hCx50-transfected cells with proteasomal inhibitors. The formation of hCx50P88S accumulations in transiently transfected cells was not blocked by treatment with Brefeldin A suggesting that they form before Cx50 transits through the Golgi apparatus to the plasma membrane. Treatment of HeLa-hCx50P88S cells with cycloheximide demonstrated the presence of a very stable pool of hCx50P88S. Taken together, these results suggest that the P to S mutation at amino acid residue 88 causes a defect that leads to decreased degradation and subsequent accumulation of hCx50P88S in a cellular structure different from aggresomes.

The Caenorhabditis elegans innexin INX-3 is localized to gap junctions and is essential for embryonic development

Innexins are the proposed structural components of gap junctions in invertebrates. Antibodies that specifically recognize the Caenorhabditis elegans innexin protein INX-3 were generated and used to examine the patterns of inx-3 gene expression and the subcellular sites of INX-3 localization. INX-3 is first detected in two-cell embryos, concentrated at the intercellular interface, and is expressed ubiquitously throughout the cellular proliferation phase of embryogenesis. During embryonic morphogenesis, INX-3 expression becomes more restricted. Postembryonically, INX-3 is expressed transiently in several cell types, while expression in the posterior pharynx persists throughout development. Through immuno-EM techniques, INX-3 was observed at gap junctions in the adult pharynx, providing supporting evidence that innexins are components of gap junctions. An inx-3 mutant was isolated through a combined genetic and immunocytochemical screen. Homozygous inx-3 mutants exhibit defects during embryonic morphogenesis. At the comma stage of early morphogenesis, variable numbers of cells are lost from the anterior of inx-3(lw68) mutants. A range of terminal defects is seen later in embryogenesis, including localized rupture of the hypodermis, failure of the midbody to elongate properly, abnormal contacts between hypodermal cells, and failure of the pharynx to attach to the anterior of the animal.

Cadherin-mediated adhesion is essential for myofibril continuity across the plasma membrane but not for assembly of the contractile apparatus

The strong coordinated contraction of heart muscle is dependent on the correct alignment and connection of the myofibrils across the plasma membrane. Previous studies indicate that N-cadherin is involved in cardiac myocyte adhesion and myofibrillogenesis. To investigate whether N-cadherin is specifically required for normal myocyte structure and function, we cultured myocytes from wild-type, N-cadherin-null and mutant embryos expressing the epithelial cadherin E-cadherin. In contrast to previous studies in chicken using N-cadherin-perturbing antibodies, our in vitro studies with mouse cells demonstrate that N-cadherin is not required for myofibrillogenesis, but is critical for myofibril organization. That is, N-cadherin-deficient myocytes beat and myofibrils were well formed; however, alignment of the myofibrils through regions of cell-cell contact was lost, resulting in their random orientation. Gap junctions were perturbed in the N-cadherin-null myocytes. By contrast, focal contacts appeared normal in the mutant cells. Furthermore, E-cadherin restored normal cell morphology and behavior to the N-cadherin-deficient myocytes, including proper alignment of the myofibrils. We conclude that a different adhesive system, most probably integrin, is responsible for myofibrillogenesis in the N-cadherin-null myocytes.

Beyond the gap: functions of unpaired connexon channels

Gap junctions consist of intercellular channels that connect the cytoplasm of adjacent cells directly and allow the exchange of small molecules. These channels are unique in that they span two plasma membranes--the more orthodox ion or ligand-gated channels span only one. Each cell contributes half of the intercellular channel, and each half is known as a connexon or hemichannel. Recent studies indicate that connexons are also active in single plasma membranes and that they might be essential in intercellular signalling beyond their incorporation into gap junctions.

Neonatal hypothyroidism alters the localization of gap junctional protein connexin 43 in the testis and messenger RNA levels in the epididymis of the rat

The objectives of this study were to determine the effects of propylthiouracil (PTU)-induced neonatal hypothyroidism on the gap junctional protein Cx43 in rat testis and epididymis. PTU (0.02%) was administered via lactation from birth to Day 30, and the rats were sampled at 14, 18, 22, 26, 30, and 91 days of age. Testicular Cx43 was localized along the plasma membranes and cytoplasm of Sertoli cells until Day 22. At Day 30, the immunostaining was localized exclusively along the plasma membrane of Sertoli cells. In PTU-treated rats, Cx43 did not localize to the plasma membrane and was still cytoplasmic at 30 days of age. Occludin was present in tubules of treated rats, but was not localized to the blood-testis barrier in 30-day-old rats, as in controls. There were no differences in Cx43 immunostaining in the adult testis. In the proximal epididymis (initial segment, caput, corpus), Cx43 mRNA levels were lower in PTU-treated rats at 14, 18, and 22 days of age, but no differences were observed in the distal (cauda) epididymis at these ages. In 22- and 30-day-old rats, Cx43 was localized along the plasma membrane between principal and basal cells throughout the epididymis. In PTU-treated rats, Cx43 was not detectable in initial segment, caput, or corpus epididymidis. In the cauda epididymidis, however, Cx43 immunostaining in PTU-treated rats was similar to controls. These data suggest that thyroid hormones regulate Cx43-dependent gap junctional communication in the testis and epididymis.

Role of catenins in the development of gap junctions in rat cardiomyocytes

Gap junctions are intercellular communicating channels responsible for the synchronized activity of cardiomyocytes. Recent studies have shown that the membrane-associated guanylate kinase protein, zonula occludens-1 (ZO-1) can bind to catenins in epithelial cells and act as an adapter for the transport of the connexin isotype, Cx43 during gap junction formation. The significance of catenins in the development of gap junctions and whether complexes between catenins and ZO-1 are formed in cardiomyocytes are not clear. In this study, immunofluorescence and confocal microscopy showed sequential redistribution of alpha-catenin, beta-catenin, ZO-1, and Cx43 to the plasma membrane when rat cardiomyocytes were cultured in low Ca(2+) (<5 microM) medium, then shifted to 1.8 mM Ca(2+) medium (Ca(2+) switch). Diffuse cytoplasmic staining of alpha-catenin, beta-catenin, ZO-1, and Cx43 was seen in the cytoplasm when cardiomyocytes were cultured in low Ca(2+) medium. Staining of alpha-catenin, beta-catenin, and ZO-1 was detected at the plasma membrane of cell-cell contact sites 10 min after Ca(2+) switch, whereas Cx43 staining was first detected, colocalized with ZO-1 at the plasma membrane, 30 min after Ca(2+) switch. Distinct junctional and extensive cytoplasmic staining of alpha-catenin, beta-catenin, ZO-1, and Cx43 was seen 2 h after Ca(2+) switch. Immunoprecipitation of Triton X-100 cardiomyocyte extracts using anti-beta-catenin antibodies showed that beta-catenin was associated with alpha-catenin, ZO-1, and Cx43 at 2 h after Ca(2+) switch. Intracellular application of antisera against alpha-catenin, beta-catenin, or ZO-1 by electroporation of cardiomyocytes cultured in low Ca(2+) medium inhibited the redistribution of Cx43 to the plasma membrane following Ca(2+) switch. These results suggest the formation of a catenin-ZO-1-Cx43 complex in rat cardiomyocytes and that binding of catenins to ZO-1 is required for Cx43 transport to the plasma membrane during the assembly of gap junctions.

Impaired trafficking of connexins in androgen-independent human prostate cancer cell lines and its mitigation by alpha-catenin

Gap junctions, composed of connexins, provide a pathway of direct intercellular communication for the diffusion of small molecules between cells. Evidence suggests that connexins act as tumor suppressors. We showed previously that expression of connexin-43 and connexin-32 in an indolent prostate cancer cell line, LNCaP, resulted in gap junction formation and growth inhibition. To elucidate the role of connexins in the progression of prostate cancer from a hormone-dependent to -independent state, we introduced connexin-43 and connexin-32 into an invasive, androgen-independent cell line, PC-3. Expression of these proteins in PC-3 cells resulted in intracellular accumulation. Western blot analysis revealed a lack of Triton-insoluble, plaque-assembled connexins. In contrast to LNCaP cells, connexins could not be cell surface-biotinylated and did not reside in the cell surface derived endocytic vesicles, in PC-3 cells, suggesting impaired trafficking to the cell surface. Intracellular accumulation of connexins was observed in several androgen-independent prostate cancer cell lines. Transient expression of alpha-catenin facilitated the trafficking of both connexins to the cell surface and induced gap junction assembly. Our results suggest that impaired trafficking, and not the inability to form gap junctions, is the major cause of communication deficiency in human prostate cancer cell lines.

GAP junctional channel inhibition alters actin organization and calcium propagation in rat cultured astrocytes

Astrocytes are connected by gap junctions, which provide intercellular pathways that allow a direct exchange of ions and small metabolites including second messengers and the propagation of electric currents. The roles of gap junctional communication on whole-cell morphology, cytoskeletal organization, and intercellular communication in astrocytes are not yet clear even in vitro, though there are many studies that have examined the active relation between gap junctions and actin filaments in astrocytes. Here we examined the effects of gap junction inhibitors, which do not interrupt the formation but rather the function of gap junctions, on whole-cell morphology, cytoskeletal organization, and intercellular communication in rat cultured astrocytes. Functional blockade of gap junctions during the formation of an astrocytic monolayer resulted in discordance of actin stress fibers between neighboring cells, even though whole-cell morphology of these cells did not change by such treatment. Mechanical stimulation-induced calcium wave propagation was significantly reduced in these actin-discordance cells even after thorough wash out. Differentiation of astrocytes in the presence of gap junction inhibitors was associated with morphological disarrangement among neighboring cells due to disordered alignment of actin stress fibers between cells.Our results indicate that gap junctional communication enables cell-to-cell coordination of actin stress fibers in astrocytes, thus enhancing intercellular communication through calcium spread.

Peptides acting at gap junctions

Gap junction channels are low resistance pathways allowing an action potential to propagate from one cell to the neighboring. Moreover, small molecules (<1000 Da) may pass the channel providing a possibility for metabolic coupling, growth and differentiation control of a cell by its surrounding. Antiarrhythmic peptides can enhance the conductivity of the channels while other peptides, angiotensin or extracellular loop peptides, reduce intercellular communication. On the other hand, peptides like angiotensin II or endothelin-1 can increase expression of certain gap junction channel proteins and, thereby, may affect intercellular coupling chronically. Thus, intercellular communication can be controlled using peptide drugs.

Connexin 43 enhances the adhesivity and mediates the invasion of malignant glioma cells

A hallmark of astrocytic tumors is their infiltrative nature. Although their aggressive and typically widespread dispersal in the adult brain differs fundamentally from that of other brain tumors, little is known about their cellular basis. Astrocytic tumors express the gap junction protein connexin 43 (Cx43), and we show here that Cx43 expression induced the morphological transformation of glioma cells into an epithelial phenotype. In a short-term aggregation assay, Cx43 expression was associated with a several-fold increase in the competence of glioma cells to aggregate. Antibodies directed against the extracellular domain of Cx43 restored the connexin-deficient phenotype, as manifested by a dose-dependent reduction in aggregation. Apart from their role in gap junction formation, connexins may therefore be considered a distinct class of membrane proteins with adhesive properties. Moreover, implanted Cx43-expressing glioma cells established functional gap junction channels with host astrocytes and dispersed through a substantially greater volume of brain parenchyma than mock- and mutant Cx43-transfected sister cells. Cx43 expression therefore may modulate not only the adhesion of astrocytes to one another, but the spread of glial tumor cells throughout astrocytic syncytia. These observations widen our concept of the potential interactions between tumor cells and their surroundings and suggest that both connexin proteins and their derived gap junctions are critical determinants of the invasiveness of central gliomas.

Endothelial barriers: from hypothetical pores to membrane proteins

The anatomical counterpart of the physiologically defined small pore system of capillary endothelia has proved difficult to establish. In non-brain continuous capillaries, the contributions of caveolar and transmembrane pathways are likely to be small and paracellular clefts are probably the dominant routes. Analogy with epithelial paracellular pathways suggests that tight junctions may be the most restrictive elements. However, structural features of tight junction-based models are incompatible with physiological data; it is more likely that the tight junction acts as a shutter limiting the available cleft area. Proposed molecular sieves elsewhere in the paracellular pathway include the glycocalyx and the cadherin-based complexes of the adherens junctions. The molecular architecture of tight junctions and adherens junctions is moderately well defined in terms of molecular species, and there are differences at both sites between the endothelial and epithelial spectra of protein expression. However, definition of the size-restricting pore remains elusive and may require structural biology approaches to the spatial arrangements and interactions of the membrane molecular complexes surrounding the endothelial paracellular clefts.

Gap junctions assemble in the presence of cytoskeletal inhibitors, but enhanced assembly requires microtubules

The role of cytoskeletal elements in gap junction (GJ) assembly has been studied using Novikoff hepatoma cells treated with cytochalasin B (CB) to disrupt actin filaments or with colchicine or nocodazole to disrupt microtubules. After 60 min of cell reaggregation, freeze-fracture was used to evaluate quantitatively the "initiation," "maturation," and "growth" phases of GJ assembly. The development of junctional permeability to fluorescent dyes was also analyzed. The only effects of CB on the structure or permeability of the developing junctions involved an elongation of GJ aggregates and a small decrease in formation plaque areas. Colchicine (but not the inactive form, lumicolchicine) prevented the enhancement of GJ growth by cholesterol, but its effect on basal growth was equivocal. Nocodazole inhibited the growth of GJ, even under basal conditions, without an effect on initiation. Nocodazole also blocked the forskolin-enhanced increase in the growth of GJs and, in living MDCK cells, reduced the movement of transport intermediates containing green fluorescent protein-tagged connexin43. Thus, neither actin filaments nor microtubules appear to restrict GJ assembly by anchoring intramembrane GJ proteins, nor are they absolutely required for functional GJs to form. However, microtubules are necessary for enhanced GJ growth and likely for facilitating connexin trafficking under basal conditions.

Functional analysis of discoidin domain receptor 1: effect of adhesion on DDR1 phosphorylation

Discoidin domain receptor 1 (DDR1), a receptor tyrosine kinase (RTK), has been shown to be activated mainly by soluble fibrillar collagen. Unusually, the kinetics of phosphorylation of the receptor is slow, with maximal phosphorylation observed after 90 min. To understand the reasons for slow phosphorylation of the receptor, we examined several cell lines under different conditions. We confirm that endogenous DDR1 is phosphorylated slowly by collagen in adherent T47D and HCT116 cells. In detached and resuspended cells, collagen induced rapid phosphorylation of DDR1. This was further confirmed with a semiadherent cell line (COLO201) and one that grows as a suspension (K562), both of which express endogenous DDR1. Replating K562 on fibronectin to mimic adherent conditions altered the kinetics of phosphorylation from rapid to slow, similar to those of adherent cells. The slow kinetics of phosphorylation in the adherent state was probably not due to cell-cell contacts because EDTA had no major effect. However, pervanadate in the absence of collagen was able to induce strong DDR1 phosphorylation, indicating that a phosphatase may inhibit or delay the phosphorylation of DDR1. Further, downstream signals after phosphorylation of DDR1 by collagen were not transmitted through the classical mitogen-activated protein kinase pathway. In addition, a chimeric TrkA-DDR1 receptor failed to become phosphorylated on stimulation with nerve growth factor (NGF), although it dimerized normally. This is the first RTK whose kinetics of phosphorylation is dependent on cellular context. The interaction of the cells with the matrix, rather than cell-cell contact, is probably responsible for the inhibition of phosphorylation.

Control of intracellular movement of connexins by E-cadherin in murine skin papilloma cells

The gap junctional intercellular communication-deficient mouse skin papilloma cell line P3/22 expresses Cx43 but not E-cadherin. The E-cadherin gene-transfected cells (P3E1) communicate in a calcium-dependent manner and they were used to study how E-cadherin restores the function of connexins. At low calcium, Cx43 molecules remain in the cytoplasm of P3E1 cells and appear at cell-cell contact areas only in high-calcium medium. While Cx43 is unphosphorylated in P3E1 cells in low-calcium medium, two phosphorylated bands appeared at high calcium. However, when Cx26, which has no C-terminal tail that can undergo phosphorylation, was expressed in P3E1 cells, this connexin also moved to the plasma membrane after the calcium shift and partly colocalized with Cx43, suggesting that C-terminal phosphorylation is not essential for E-cadherin-mediated intracellular transport of connexins. In low calcium, both Cx26 and Cx43 remained and colocalized in the endoplasmic reticulum. As early as 30 min after the shift to high-calcium medium, both Cx43 and Cx26 began to accumulate in the Golgi apparatus. Intracellular movement of connexins to the cytoplasmic membrane at high calcium was effectively blocked by cytochalasin D and brefeldin A. These results suggest that E-cadherin junction formation at high calcium leads to formation of actin cables, which directly or indirectly transport connexins from the cytoplasm to the cell-cell contact membranes via the Golgi apparatus.

Gap junction assembly: PTX-sensitive G proteins regulate the distribution of connexin43 within cells

Cells expressing connexin43 are able to upregulate gap junction (GJ) communication by enhancing the assembly of new GJs, apparently through increased connexin trafficking. Because G proteins are known to regulate different aspects of protein trafficking, we examined the effects of pertussis toxin (PTX; a specific inhibitor of certain G proteins) on GJ assembly. Dissociated Novikoff hepatoma cells were reaggregated for 60 min to form nascent junctions. PTX inhibited GJ assembly, as indicated by a reduction in dye transfer. Electron microscopy also revealed a 60% decrease in the number of GJ channels per cell interface. Importantly, PTX blocked the twofold enhancement in GJ assembly found in the presence of low-density lipoprotein. Two G(i alpha) proteins (G(i alpha 2) and G(i alpha 3)), which have been implicated in the control of membrane trafficking, reacted with PTX in ADP-ribosylation studies. PTX and/or the trafficking inhibitors, brefeldin A and monensin, inhibited GJ assembly to comparable degrees. In addition, assays for GJ hemichannels demonstrated reduced plasma membrane levels of connexin43 following PTX treatment. These results suggest that PTX-sensitive G proteins regulate connexin43 trafficking, and, as a result of inhibition with PTX, the number of plasma membrane hemichannels available for GJ assembly is reduced.

The role of connexin-mediated cell-cell communication in breast cancer metastasis

Gap junctional intercellular communication (GJIC) is a form of cell-cell communication mediating the exchange of small molecules between neighboring cells. Gap junctions (GJs) are formed by connexins (Cxs), and are subject to tight and dynamic regulation. They are involved in the cell cycle, differentiation, and cell signaling. The loss of Cxs and GJs is a hallmark of carcinogenesis, while their induction in cancer cells leads to a reversal of the cancer phenotype, induction of differentiation, and regulation of cell growth. On the basis of the observations about Cx loss in breast cancer, this review examines Cxs' involvement in breast cancer metastasis. Previous work indicates that Cx expression is inversely correlated to metastatic potential. This is probably because of the loss of cooperation between neighboring cells, leading to cell heterogeneity and cell dissociation in the tumor. The possible involvement of Cx activity during metastasis will be discussed.

Modulation of mouse neural crest cell motility by N-cadherin and connexin 43 gap junctions

Connexin 43 (Cx43alpha1) gap junction has been shown to have an essential role in mediating functional coupling of neural crest cells and in modulating neural crest cell migration. Here, we showed that N-cadherin and wnt1 are required for efficient dye coupling but not for the expression of Cx43alpha1 gap junctions in neural crest cells. Cell motility was found to be altered in the N-cadherin-deficient neural crest cells, but the alterations were different from that elicited by Cx43alpha1 deficiency. In contrast, wnt1-deficient neural crest cells showed no discernible change in cell motility. These observations suggest that dye coupling may not be a good measure of gap junction communication relevant to motility. Alternatively, Cx43alpha1 may serve a novel function in motility. We observed that p120 catenin (p120ctn), an Armadillo protein known to modulate cell motility, is colocalized not only with N-cadherin but also with Cx43alpha1. Moreover, the subcellular distribution of p120ctn was altered with N-cadherin or Cx43alpha1 deficiency. Based on these findings, we propose a model in which Cx43alpha1 and N-cadherin may modulate neural crest cell motility by engaging in a dynamic cross-talk with the cell's locomotory apparatus through p120ctn signaling.