Characteristics of C6 glioma cells overexpressing a gap junction protein

Cx43 and the Actin Cytoskeleton: Novel Roles and Implications for Cell-Cell Junction-Based Barrier Function Regulation

Barrier function is a vital homeostatic mechanism employed by epithelial and endothelial tissue. Diseases across a wide range of tissue types involve dynamic changes in transcellular junctional complexes and the actin cytoskeleton in the regulation of substance exchange across tissue compartments. In this review, we focus on the contribution of the gap junction protein, Cx43, to the biophysical and biochemical regulation of barrier function. First, we introduce the structure and canonical channel-dependent functions of Cx43. Second, we define barrier function and examine the key molecular structures fundamental to its regulation. Third, we survey the literature on the channel-dependent roles of connexins in barrier function, with an emphasis on the role of Cx43 and the actin cytoskeleton. Lastly, we discuss findings on the channel-independent roles of Cx43 in its associations with the actin cytoskeleton and focal adhesion structures highlighted by PI3K signaling, in the potential modulation of cellular barriers. Mounting evidence of crosstalk between connexins, the cytoskeleton, focal adhesion complexes, and junctional structures has led to a growing appreciation of how barrier-modulating mechanisms may work together to effect solute and cellular flux across tissue boundaries. This new understanding could translate into improved therapeutic outcomes in the treatment of barrier-associated diseases.

Examination of sulfonamide-based inhibitors of MMP3 using the conditioned media of invasive glioma cells

Glioblastoma multiforme (GBM) is the deadliest and the most common primary malignant brain tumour. The median survival for patients with GBM is around one year due to the nature of glioma cells to diffusely invade that make the complete surgical resection of tumours difficult. Based upon the connexin43 (Cx43) model of glioma migration we have developed a computational framework to evaluate MMP inhibition in materials relevant to GBM. Using the ilomastat Leu-Trp backbone, we have synthesised novel sulphonamides and monitored the performance of these compounds in conditioned media expressing MMP3. From the results discussed herein we demonstrate the performance of sulfonamide based MMPIs included AP-3, AP-6, and AP-7.

Therapeutic strategies targeting connexins

The connexin family of channel-forming proteins is present in every tissue type in the human anatomy. Connexins are best known for forming clustered intercellular channels, structurally known as gap junctions, where they serve to exchange members of the metabolome between adjacent cells. In their single-membrane hemichannel form, connexins can act as conduits for the passage of small molecules in autocrine and paracrine signalling. Here, we review the roles of connexins in health and disease, focusing on the potential of connexins as therapeutic targets in acquired and inherited diseases as well as wound repair, while highlighting the associated clinical challenges.

Cell-cell communication in the kidney microcirculation

In the renal vasculature of humans, rats, and mice, at least four isoforms of Cx, Cxs 37, 40, 43, and 45 are expressed. In the ECs, Cx40 is the predominantly expressed Cx, whereas Cx45 is suggested to be expressed in the VSMCs. The preglomerular vasculature has a higher expression of Cxs than the postglomerular vasculature. Cxs form gap junctions between neighboring cells, and as in other organ systems, the major function of Cxs in the kidney appears to be mediation of intercellular communication. Cxs may also form hemichannels that allow cellular secretion of signaling molecules like ATP, and thereby mediate paracrine signaling. Renal Cxs facilitate vascular conduction, juxtaglomerlar apparatus calcium signaling, and enable ECs and VSMCs to communicate. Thus, current research suggests multiple roles for Cxs in important regulatory mechanisms within the kidney, including the renin-angiotensin system, TGF, and salt and water homeostasis. Interestingly, changes in the activity of the renin-angiotensin system or changes in blood pressure seem to affect the expression of the renal vascular Cxs. At the systemic level, renal Cxs may be involved in blood pressure regulation, and possibly in the pathogenesis of hypertension and diabetes.

Gap junction proteins on the move: connexins, the cytoskeleton and migration

Connexin43 (Cx43) has roles in cell-cell communication as well as channel independent roles in regulating motility and migration. Loss of function approaches to decrease Cx43 protein levels in neural cells result in reduced migration of neurons during cortical development in mice and impaired glioma tumor cell migration. In other cell types, correlations between Cx43 expression and cell morphology, adhesion, motility and migration have been noted. In this review we will discuss the common themes that have been revealed by a detailed comparison of the published results of neuronal cells with that of other cell types. In brief, these comparisons clearly show differences in the stability and directionality of protrusions, polarity of movement, and migration, depending on whether a) residual Cx43 levels remain after siRNA or shRNA knockdown, b) Cx43 protein levels are not detectable as in cells from Cx43(-/-) knockout mice or in cells that normally have no endogenous Cx43 expression, c) gain-of-function approaches are used to express Cx43 in cells that have no endogenous Cx43 and, d) Cx43 is over-expressed in cells that already have low endogenous Cx43 protein levels. What is clear from our comparisons is that Cx43 expression influences the adhesiveness of cells and the directionality of cellular processes. These observations are discussed in light of the ability of cells to rearrange their cytoskeleton and move in an organized manner. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Regulatory effect of connexin 43 on basal Ca2+ signaling in rat ventricular myocytes

Background

It has been found that gap junction-associated intracellular Ca²⁺ [Ca²⁺]_i disturbance contributes to the arrhythmogenesis and hyperconstriction in diseased heart. However, whether functional gaps are also involved in the regulation of normal Ca²⁺ signaling, in particular the basal [Ca²⁺]_i activities, is unclear.

Methods and Results

Global and local Ca²⁺ signaling and gap permeability were monitored in cultured neonatal rat ventricular myocytes (NRVMs) and freshly isolated mouse ventricular myocytes by Fluo4/AM and Lucifer yellow (LY), respectively. The results showed that inhibition of gap communication by heptanol, Gap 27 and flufenamic acid or interference of connexin 43 (Cx43) with siRNA led to a significant suppression of LY uptake and, importantly, attenuations of global Ca²⁺ transients and local Ca²⁺ sparks in monolayer NRVMs and Ca²⁺ sparks in adult ventricular myocytes. In contrast, overexpression of rat-Cx43 in NRVMs induced enhancements in the above measurements, and so did in HEK293 cells expressing rat Cx43. Additionally, membrane-permeable inositol 1,4,5-trisphosphate (IP₃ butyryloxymethyl ester) and phenylephrine, an agonist of adrenergic receptor, could relieve the inhibited Ca²⁺ signal and LY uptake by gap uncouplers, whereas blockade of IP₃ receptor with xestospongin C or 2-aminoethoxydiphenylborate mimicked the effects of gap inhibitors. More importantly, all these gap-associated effects on Ca²⁺ signaling were also found in single NRVMs that only have hemichannels instead of gap junctions. Further immunostaining/immunoblotting single myocytes with antibody against Cx43 demonstrated apparent increases in membrane labeling of Cx43 and non-junctional Cx43 in overexpressed cells, suggesting functional hemichannels exist and also contribute to the Ca²⁺ signaling regulation in cardiomyocytes.

Conclusions

These data demonstrate that Cx43-associated gap coupling plays a role in the regulation of resting Ca²⁺ signaling in normal ventricular myocytes, in which IP₃/IP₃ receptor coupling is involved. This finding may provide a novel regulatory pathway for mediation of spontaneous global and local Ca²⁺ activities in cardiomyocytes.

Connexins, cell motility, and the cytoskeleton

Connexins (Cx) comprise a family of transmembrane proteins, which form intercellular channels between plasma membranes of two adjoining cells, commonly known as gap junctions. Recent reports revealed that Cx proteins interact with diverse cellular components to form a multiprotein complex, which has been termed "Nexus". Potential interaction partners include proteins such as cytoskeletal proteins, scaffolding proteins, protein kinases and phosphatases. These interactions allow correct subcellular localization of Cxs and functional regulation of gap junction-mediated intercellular communication. Evidence is accruing that Cxs might have channel-independent functions, which potentially include regulation of cell migration, cell polarization and growth control. In the current review, we summarize recent knowledge on Cx interactions with cytoskeletal proteins and highlight some aspects of their role in cellular motility.

Matricellular protein CCN3 (NOV) regulates actin cytoskeleton reorganization

CCN3 (NOV), a putative ligand for integrin receptors, is tightly associated with the extracellular matrix and mediates diverse cellular functions, including cell adhesion and proliferation. CCN3 has been shown to negatively regulate growth although it promotes migration in a cell type-specific manner. In this study, overexpression of CCN3 reduces growth and increases intercellular adhesion of breast cancer cells. Interestingly, CCN3 overexpression also led to the formation of multiple pseudopodia that are enriched in actin, CCN3, and vinculin. Breast cancer cells preincubated with exogenous CCN3 protein also induced the same phenotype, indicating that secreted CCN3 is sufficient to induce changes in cell morphology. Surprisingly, extracellular CCN3 is internalized to the early endosomes but not to the membrane protrusions, suggesting pseudopodia-enriched CCN3 may derive from a different source. The presence of an intracellular variant of CCN3 will be consistent with our finding that the cytoplasmic tail of the gap junction protein connexin43 (Cx43) associates with CCN3. Cx43 is a channel protein permitting intercellular communication to occur. However, neither the channel properties nor the protein levels of Cx43 are affected by the CCN3 protein. In contrast, CCN3 proteins are down-regulated in the absence of Cx43. Finally, we showed that overexpression of CCN3 increases the activity of the small GTPase Rac1, thereby revealing a pathway that links Cx43 directly to actin reorganization.

Conformational maturation and post-ER multisubunit assembly of gap junction proteins

For all previously well-characterized oligomeric integral membrane proteins, folding, multisubunit assembly, and recognition of conformationally immature molecules for degradation occurs at their organelle of synthesis. This cannot, however, be the case for the gap junction-forming protein connexin43 (Cx43), which when endogenously expressed undergoes multisubunit assembly into connexons only after its transport to the trans-Golgi network. We have developed two novel assays to assess Cx43 folding and assembly: acquisition of resistance of disulfide bonds to reduction by extracellularly added DTT and Triton X-114 detergent phase partitioning. We show that Cx43 synthesized at physiologically relevant levels undergoes a multistep conformational maturation process in which folding of connexin monomers within the ER is a prerequisite for multisubunit assembly in the TGN. Similar results were obtained with Cx32, disproving the widely reported contention that the site of endogenous beta connexin assembly is the ER. Exogenous overexpression of Cx43, Cx32, or Cx26 allows these events to take place within the ER, the first example of the TGN and ER as alternative sites for oligomeric assembly. Our findings also constitute the first biochemical evidence that defective connexin folding is a cause of the human disorder X-linked Charcot-Marie-Tooth disease.

Tumor-suppressive effects of pannexin 1 in C6 glioma cells

Mammalian gap junction proteins, connexins, have long been implicated in tumor suppression. Recently, a novel family of proteins named pannexins has been identified as the mammalian counterpart of the invertebrate gap junction proteins, innexins. To date, pannexin 1 (Panx1) and pannexin 2 (Panx2) mRNAs are reported to be expressed in the brain. Most neoplastic cells, including rat C6 gliomas, exhibit reduced connexin expression, aberrant gap junctional intercellular communication (GJIC), and an increased proliferation rate. When gap junctions are up-regulated by transfecting C6 cells with connexin43, GJIC is restored and the proliferation is reduced. In this study, we examined the tumor-suppressive effects of Panx1 expression in C6 cells. Reverse transcription-PCR analysis revealed that C6 cells do not express any of the pannexin transcripts, whereas its nontumorigenic counterpart, rat primary astrocytes, exhibited mRNAs for all three pannexins. On generation of stable C6 transfectants with tagged Panx1 [myc or enhanced green fluorescent protein (EGFP)], a localization of Panx1 expression to the Golgi apparatus and plasma membrane was observed. In addition, Panx1 transfectants exhibited a flattened morphology, which differs greatly from the spindle-shaped control cells (EGFP only). Moreover, Panx1 expression increased gap junctional coupling as shown by the passage of sulforhodamine 101. Finally, we showed that stable expression of Panx1 in C6 cells significantly reduced cell proliferation in monolayers, cell motility, anchorage-independent growth, and in vivo tumor growth in athymic nude mice. Altogether, we conclude that the loss of pannexin expression may participate in the development of C6 gliomas, whereas restoration of Panx1 plays a tumor-suppressive role.

Down-regulation of Cx43 by Retroviral Delivery of Small Interfering RNA Promotes an Aggressive Breast Cancer Cell Phenotype

Connexins are gap junction proteins that assemble into channels that mediate direct intercellular communication. Connexins are well-documented tumor suppressors and are thought to regulate both cell growth and differentiation. As previously reported, most human breast tumors and cell lines down-regulate gap junctions or have defective gap junctional intercellular communication. Furthermore, overexpression of connexins in breast cancer cells inhibits tumor growth in vivo. In this study, we hypothesize that controlled Cx43 down-regulation would induce breast tumor cells to acquire a more aggressive phenotype. Here we report that Cx43 was down-regulated in both normal rat kidney (NRK) cells and human breast cancer cell lines (MDA-MB-231 and Hs578T) by transfection with chemically synthesized small interfering RNA (siRNA) or short hairpin RNA generated from a retroviral infection. Furthermore, we show that retroviral delivery and expression of siRNA directed to different coding regions of Cx43 resulted in differential levels of Cx43 silencing and impaired gap junctional intercellular communication. Cx43-silenced Hs578T cells grew faster and were more migratory. Finally, Western blot analysis revealed that down-regulation of Cx43 resulted in decreased expression of thrombospondin-1, an antiangiogenesis molecule, and increased expression of vascular endothelial growth factor. Taken together, these results suggest that Cx43 is required for maintaining cell differentiation and the regulation of molecules important in angiogenesis.

CCN3 (NOV) interacts with connexin43 in C6 glioma cells: possible mechanism of connexin-mediated growth suppression

Many tumor cells exhibit aberrant gap junctional intercellular communication, which can be restored by transfection with connexin genes. We have previously discovered that overexpression of connexin43 (Cx43) in C6 glioma cells not only reduces proliferation but also leads to production of soluble growth-inhibitory factors. We identified that several members of the CCN (Cyr61/connective tissue growth factor/nephroblastoma-overexpressed) family are up-regulated following Cx43 expression, including CCN3 (NOV). We now report evidence for an association between CCN3 and Cx43. Western blot analysis demonstrated that the 48-kDa full-length CCN3 protein was present in the lysate and conditioned medium of growth-suppressed C6-Cx43 cells, as well as primary astrocytes, but not in C6 parental and human glioma cells. Immunocytochemical examination of CCN3 revealed diffuse localization in parental C6 cells, whereas transfection of C6 cells with Cx43 (C6-Cx43) or with a modified Cx43 tagged to green fluorescent protein on its C terminus (Cx43-GFP) resulted in punctate staining, suggesting that CCN3 co-localizes with Cx43 in plaques at the plasma membrane. In cells expressing a C-terminal truncation of Cx43 (Cx43Delta244-382), this co-localization was lost. Glutathione S-transferase pull-down assay and co-immunoprecipitation demonstrated that CCN3 was able to physically interact with Cx43. In contrast, CCN3 was not found to associate with Cx43Delta244-382. Similar experiments revealed that CCN3 did not co-localize or associate with other connexins, including Cx40 or Cx32. Taken together, these data support an interaction of CCN3 with the C terminus of Cx43, which could play an important role in mediating growth control induced by specific gap junction proteins.

Increased invasive capacity of connexin43-overexpressing malignant glioma cells

OBJECT

Malignant glioma cells, similar to astrocytes, express connexin43 (Cx43) universally but at widely varied levels. Data from previous studies have demonstrated that malignant glioma cells form functional gap junction channels among themselves as well as with astrocytes and that such a communication has the potential to modulate the phenotypic characteristics of astrocytes. Recently, gap junctions have been demonstrated to play a role in the invasive phenotype of malignant gliomas. In this study, the authors have further investigated the motility and invasion ability of Cx43-overexpressing and Cx43-deficient malignant glioma cells.

METHODS

Using a standard invasion system of a Matrigel transwell invasion chamber, the authors found that the number of Cx43-transfected C6 glioma cells (C6-Cx43 cells) migrating through the Matrigel-coated membrane was similar to that of mock-transfected control cells (C6-mock cells) during the first 24 hours, but increased significantly thereafter. When these cells were cocultured with astrocytes, the number of invading C6-Cx43 cells was more than threefold greater than the number of invading C6-mock cells. Results of an in vitro cell motility assay also demonstrated that C6-Cx43 cells were more motile and scatter-active than C6-mock cells. Furthermore, zymographic analysis of MMPs, an important determinant in glioma invasion, demonstrated that the amounts of MMP-2 and MMP-9 in culture medium collected from C6-Cx43 cells were orders of magnitude higher than those from C6-mock cells. In addition, BB-94, a synthetic MMP inhibitor, significantly inhibited C6-Cx43 cell invasion.

CONCLUSIONS

The overexpression of gap junction proteins in glioma cells and the intercellular communication between tumor and nontumor glia cells may play important roles in the facilitation of glioma cell invasion.

Determination of a potential role of the CCN family of growth regulators in connexin43 transfected C6 glioma cells

Tumour cells often exhibit erratic cell growth, as well as decreased gap junctional intercellular communication (GJIC). C6 glioma cells are characterized by low levels of gap junction mRNA and protein, and decreased amounts of GJIC when compared with astrocytes. Previous work has shown that C6 glioma cells transfected with connexin43 (C6-Cx43) exhibit decreased proliferation in vivo and in vitro, as well as genes that are differentially expressed between these cells. In this study, RNA levels of two CCN (connective tissue growth factor [CTGF], Cyr61/Cef-10, nephroblastoma overexpressed [NOV]) gene family members are shown to be upregulated in C6-Cx43 cells: Cyr61 and Nov. Cyr61 has previously been shown to increase adhesion, migration and growth in many cell types, whereas NOV has growth suppressive capacities. Cyr61 RNA expression is shown here to respond to serum in quiescent cells in an immediate early gene fashion, independent of Cx43 expression. In contrast, Nov RNA levels remain constant, reflective of transfected Cx43 expression. Furthermore, confocal microscopy indicates that NOV colocalizes with Cx43 plaques at the cell membrane. These findings provide insight into the possible role of Nov and Cyr61 in tumour cells.

ELF magnetic fields induce internalization of gap junction protein connexin 43 in Chinese hamster lung cells

We have previously demonstrated that exposure of Chinese hamster lung (CHL) cells to 50 Hz magnetic fields (MFs) and/or 12-O-tetradecanoylphorbol-3-acetate (TPA)-inhibited gap junctional intercellular communication (GJIC). To explore and compare the mechanisms of GJIC inhibition induced by extremely low frequency (ELF) MF and TPA, the number and localization of connexin 43 (C x 43) were studied. The localization of C x 43 was determined with indirect immunofluorescence histochemical analysis and detected by confocal microscopy after exposing CHL cells to 50 Hz sinusoidal magnetic field at 0.8 mT for 24 h without or with TPA (5 ng/ml) for the last 1 h. The C x 43 levels in nuclei and in cytoplasm were examined by Western blotting analysis. The results showed that the cells exposed to MF and/or TPA displayed individual plaques at regions of intercellular contact, which were fewer than the normal cells in number, while the number of C x 43 in cytoplasm increased and congregated near the nuclei. Western blot analysis further demonstrated the quantity of changes in location of Cx43. These results suggest that reduction of C x 43 at regions of intercellular contact may be one of the mechanisms of GJIC inhibition induced by ELF MF.

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.

Intercellular calcium signaling in astrocytes via ATP release through connexin hemichannels

Astrocytes are capable of widespread intercellular communication via propagated increases in intracellular Ca(2+) concentration. We have used patch clamp, dye flux, ATP assay, and Ca(2+) imaging techniques to show that one mechanism for this intercellular Ca(2+) signaling in astrocytes is the release of ATP through connexin channels ("hemichannels") in individual cells. Astrocytes showed low Ca(2+)-activated whole-cell currents consistent with connexin hemichannel currents that were inhibited by the connexin channel inhibitor flufenamic acid (FFA). Astrocytes also showed molecular weight-specific influx and release of dyes, consistent with flux through connexin hemichannels. Transmembrane dye flux evoked by mechanical stimulation was potentiated by low Ca(2+) and was inhibited by FFA and Gd(3+). Mechanical stimulation also evoked release of ATP that was potentiated by low Ca(2+) and inhibited by FFA and Gd(3+). Similar whole-cell currents, transmembrane dye flux, and ATP release were observed in C6 glioma cells expressing connexin43 but were not observed in parent C6 cells. The connexin hemichannel activator quinine evoked ATP release and Ca(2+) signaling in astrocytes and in C6 cells expressing connexin43. The propagation of intercellular Ca(2+) waves in astrocytes was also potentiated by quinine and inhibited by FFA and Gd(3+). Release of ATP through connexin hemichannels represents a novel signaling pathway for intercellular communication in astrocytes and other non-excitable cells.

Propagation of intercellular calcium waves in C6 glioma cells transfected with connexins 43 or 32

In this study, gap junction-deficient C6 glioma cells, transfected with either connexin 43 (Cx43) or 32 (Cx32), have been used to evaluate the ability of these connexins to pass intercellular Ca2+ waves. Ca2+ waves, observed with fluorescence imaging using fura-2 or fluo-3, were initiated by mechanical stimulation in the presence of a supra-perfusion of the extracellular fluid or by the non-contact technique of flash photolysis of intracellular caged-IP3. Following manual mechanical stimulation, the parental C6 glioma cells and cells expressing Cx43 and Cx32 gap junctions all propagated intercellular Ca2+ waves. Ca2+ waves in cells expressing Cx43 traveled approximately twice the distance as compared to waves in cells expressing Cx32 or parental cells. The cells expressing Cx43 were also about twice as sensitive to ATP as cells expressing Cx32. In the presence of a supra-perfusion of extracellular fluid, the Ca2+ waves in parental cells were almost abolished while the mechanically induced Ca2+ waves in the cells expressing Cx43 and Cx32 propagate similar but limited distances of several cells in a direction opposite to the fluid flow. The photolytic release of IP3, but not Ca2+, in cells expressing Cx43 or Cx32 resulted in the propagation of Ca2+ waves that traveled distances similar to those observed in the presence of supra-perfusion. Parental C6 glioma cells did not initiate intercellular Ca2+ waves when stimulated by photolysis. From these studies we conclude that (1) both Cx43 and Cx32 based gap junctions are permeable to IP3 and can serve to communicate Ca2+ waves, (2) that Ca2+ wave propagation via gap junctions was dependent on the diffusion of IP3 but not Ca2+, (3) that an extracellular messenger capable of communicating waves is released from only the stimulated cell, and (4) that simultaneous intracellular and extracellular signaling can occur to enhance the propagation of intercellular Ca2+ waves.

Neuronal differentiation and growth control of neuro-2a cells after retroviral gene delivery of connexin43

Given the roles proposed for gap junctional intercellular communication in neuronal differentiation and growth control, we examined the effects of connexin43 (Cx43) expression in a neuroblastoma cell line. A vesicular stomatitis virus G protein (VSVG)-pseudotyped retrovector was engineered to co-express the green fluorescent protein (GFP) and Cx43 in the communication-deficient neuro-2a (N2a) cell line. The 293 GPG packaging cell line was used to produce VSVG-pseudotyped retrovectors coding for GFP, Cx43, or chimeric Cx43.GFP fusion protein. The titer of viral supernatant, as measured by flow cytometry for GFP fluorescence, was approximately 2.0 x 10(7) colony form units (CFU)/ml and was free of replication-competent retroviruses. After a 7-day treatment with retinoic acid (20 microm), N2a transformants (N2a-Cx43 and N2a-Cx43.GFP) maintained the expression of Cx43 and Cx43.GFP. Expression of both constructs resulted in functional coupling, as evidenced by electrophysiological and dye-injection analysis. Suppression of cell growth correlated with expression of both Cx43 or Cx43.GFP and retinoic acid treatment. Based on morphology and immunocytochemistry for neurofilament, no difference was observed in the differentiation of N2a cells compared with cells expressing Cx43 constructs. In conclusion, constitutive expression of Cx43 in N2a cells does not alter retinoic acid-induced neuronal differentiation but does enhance growth inhibition.

Quiescent mammary epithelial cells have reduced connexin43 but maintain a high level of gap junction intercellular communication

Gap junctions have been implicated in growth control, but it remains unclear whether cells that enter a quiescent state continue to express connexins and maintain a high level of gap junction intercellular communication (GJIC). To this end, MAC-T cells, a bovine mammary epithelial cell line, were serum starved for 48 h to induce a quiescent (G0) state. In quiescent cells, [3H]thymidine incorporation was reduced by 97.3% from serum-fed controls. Western blotting in conjunction with Phosphorlmager analysis revealed up to a 20-fold decrease in the expression of the gap junction protein connexin43 (Cx43 or alpha 1) and a shift toward the unphosphorylated form in quiescent cells. However, cell-to-cell transfer of the gap junction-permeable fluorescent tracer, Lucifer yellow, was only moderately reduced in quiescent cells. In control cells, Cx43 was predominantly perinuclear, although it was also present at sites of cell-cell apposition. In quiescent cells, intracellular labeling for Cx43 decreased without a corresponding reduction at areas of cell-cell contact. Recovery from serum deprivation resulted in increased thymidine incorporation that corresponded with an elevation in Cx43 protein expression and phosphorylation. In parallel studies, MAC-T cells were also induced to enter a quiescent state through contact inhibition. Despite a 20-fold reduction in 5-bromo-2'-deoxyuridine and a substantial reduction in intracellular Cx43, contact inhibited MAC-T cells also maintained gap junctions and GJIC. These experiments demonstrate that the maintenance of dye coupling in quiescent mammary cells is correlated with a redistribution of intracellular stores of Cx43.

ATP-mediated glia signaling

Glia calcium signaling has recently been identified as a potent modulator of synaptic transmission. We show here that the spatial expansion of calcium waves is mediated by ATP and subsequent activation of purinergic receptors. Ectopic expression of gap junction proteins, connexins (Cxs), leads to an increase in both ATP release and the radius of calcium wave propagation. Cx expression was also associated with a phenotypic transformation, and cortical neurons extended longer neurites when co-cultured with Cx-expressing than with Cx-deficient cells. Purinergic receptor activation mediated both these effects, because treatment with receptor antagonists restored the glia phenotype and slowed neurite outgrowth. These results identify a key role of ATP in both short-term calcium signaling events and in long-term differentiation regulated by glia.

Dynamics of astrocyte adhesion as analyzed by a combination of atomic force microscopy and immuno-cytochemistry: the involvement of actin filaments and connexin 43 in the early stage of adhesion

We observed the time-dependent morphological alteration of astrocytes during their adhesion by atomic force microscopy (AFM) and investigated the relationships between this morphological alteration and the localization of actin filaments and connexin 43 by immunocytochemistry. The fine processes observed as fine ridge-like structures by AFM were closely concerned with actin filaments by immunocytochemistry. During the adhesion of astrocytes, actin filaments appeared to be aligned regularly beyond the borders among different cells. Detectable connexin immunoreactivity was changed in the following regions: 1) the tips of fine cell processes and the cell margin when astrocytes started to adhere; 2) the border of cells when astrocytes tightly adhered; and 3) non-specific sites when astrocytes became a cluster. In the former two cases, the immunopositive spots for connexin were observed to colocalize with the tips of cell processes with actin filaments. These results strongly suggest that connexin associated with actin filaments at the tip of cell processes plays an important role in the early stage of the adhesion of astrocytes. These observations afford valuable clues for understanding the glial communication.

Enhancement of tumor ablation by a selected HSV-1 thymidine kinase mutant

With the advent of gene therapy, herpes simplex virus type I (HSV-1) thymidine kinase (TK) has garnered much interest as a suicide gene for cancer ablation. As a means to improve the overall efficacy of the prodrug-gene activation approach, as well as to reduce ganciclovir-mediated toxicity, a large library of mutant thymidine kinases was generated and screened for the ability to enhance in vitro cell sensitivity to the prodrugs, ganciclovir (GCV) and acyclovir (ACV). Enzyme kinetics of one thymidine kinase mutant from this library that contains six amino acid substitutions at or near the active site reveals a distinct mechanism for providing enhanced prodrug-mediated killing in mammalian cells. In in vitro rat C6 cell prodrug sensitivity assays the TK mutant (mutant 30) achieves nanomolar IC50 values with GCV and ACV, in contrast to IC50values of 30 microM and >100 microM, respectively, for wild-type TK. In a mouse xenograft tumor model, growth of mutant 30 expressing tumors is restricted by ganciclovir at a dose at least 10- fold lower than one that impedes growth of wild-type TK-expressing tumors. Furthermore, in the presence of GCV a substantial bystander effect is observable when only 20% of the tumor cells express mutant 30 whereas no restriction in tumor growth is seen in tumors bearing the wild-type TK under the same conditions. The enhanced sensitization to prodrugs conferred by mutant 30 is apparently due to a 35-fold increase in thymidine Km which results in reduced competition between prodrug and thymidine at the active site. This provides mutant 30 a substantial kinetic advantage despite very high Kms for both ganciclovir and acyclovir. Molecular modeling of the mutations within the active site suggests that a tyrosine substitution at alanine 168 (A168) alters thymidine and prodrug interactions by causing catalytically important residues to move. The use of mutant 30 in place of the wild-type TK should provide a more effective gene therapy of cancer.

Tentative novel mechanism of the bystander effect in glioma gene therapy with HSV-TK/GCV system

Although many works support gap junctional intercellular communication (GJIC) having a close relation to bystander cell killing in herpes simplex virus thymidine kinase (HSV-TK) gene and ganciclovir (GCV) treatment, our previous work suggested that other factors involved in bystander effect besides GJIC exist. To confirm our primary work, we evaluated the mode of the bystander cell (C6) co-cultured with TK-positive cells (TF10.2) in our designed "insert plates" in which two cell lines could be separated but share the same medium. Another method that we used was adding the supernatant from the medium of GCV-treated TF10.2 cells to the wild type C6. Growth inhibition of the bystander cells was observed despite the absence of GJIC. In addition, apoptotic cell death of TK+ cells and bystander cells was obvious. These studies suggested that other pathways besides cell-cell contacts may play a role in bystander cell killing; the factors released from TK-positive cells could induce apoptosis of bystander cells.

Cytoskeletal assembly and ATP release regulate astrocytic calcium signaling

We have studied the role of actin fiber assembly on calcium signaling in astrocytes. We found that (1) after astrocytes have been placed in culture, it takes several hours for organization of the definitive actin cytoskeleton. Actin organization and the number of cells engaged in calcium signaling increased in parallel. (2) Disruption of the actin cytoskeleton attenuated the calcium wave propagation; cytochalasin D treatment reduced the number of astrocytes engaged in calcium signaling. (3) Propagation of calcium waves depends on cytoskeletal function; inhibition of myosin light chain kinase suppressed wave activity. (4) Astrocytic calcium signaling is mediated by release of ATP and purinergic receptor stimulation, because agents that interfere with this cascade attenuated or reduced calcium signaling. Because purinergic receptors are fully functional shortly after plating and not affected by cytochalasin D, these observations indicate that cytoskeleton organization is a prerequisite for interastrocytic calcium signaling mediated by release of ATP.

Autocellular coupling by gap junctions in cultured astrocytes: a new view on cellular autoregulation during process formation

Neocortical astrocytes make two types of gap junctions, intercellular ones create a functional syncytium, while reflexive gap junctions mediate autocellular coupling and serve unknown functions (Rohlmann and Wolff, 1996). Here, the question is addressed whether solitary astrocytes in vitro express connexin43 (Cx43) and establish gap junctions in the absence of intercellular contacts. In all media conditions tested, immunocytochemistry visualized Cx43-expression and gap junctions irrespective of the presence or absence of intercellular contacts. Reflexive gap junctions were associated with mechanical junctions (adherent spots and fascia adherens) connecting surface membranes and cytoskelal components, respectively. Both were characteristically located along incompletely separated borders between developing processes and/or branches. In addition, Cx43-immunoreactivity was found on some non-junctional membranes: i) intracellular vesicle clusters sited to forming processes and at the basis of filopodia; ii) the surface membrane of filopodial subpopulations usually appearing in bunches. Results suggest changes in the resumptive role of Cx43 in cultivated astrocytes: 1) Cx43 is not confined to intercellular gap junctions, it may even selectively compose reflexive ones; 2) from intracellular stores (vesicle aggregates), Cx43 may be incorporated into the surface membrane of filopodia; 3) by contacting other parts of the same cell surface (or neighboring cells), filopodia and membrane patches carrying Cx43-half channels may be essential in initial steps of gap junction formation; 4) the distribution of reflexive gap junctions is compatible with the hypothesis that autocellular coupling serves reorganization of cytoskeleton during the formation of cell processes and branches; 5) in general, gap junctions may be important for coordinating the cytoskeleton across intercellular contacts and within cells with complex shape.

Relationship of cytoskeletal filaments to annular gap junction expression in human adrenal cortical tumor cells in culture

In addition to the well-characterized surface gap junctions expressed at contact sites between cells, annular gap junction profiles have been localized within the cytoplasm of some cell populations. To study and characterize these annular profiles, gap junction protein type was demonstrated with Western blot and immunocytochemistry. The distribution of annular gap junctions and the relationships to cytoskeletal elements were demonstrated with immunocytochemical, transmission electron microscopic, or image analysis with confocal microscopy techniques. SW-13 adrenal cortical tumor cells expressed alpha1 gap junctions at areas of cell to cell contact. In addition, alpha1 gap junction annular profiles were seen within the cytoplasm. Actin and myosin II were found closely associated with these annular gap junctions, while no physical association between tubulin- or vimentin-containing fibers and gap junction protein could be established. Disruption of microfilaments with cytochalasin B treatment (10 microg/ml, 1 h) resulted in a decrease in the average number and an increase in the average size of annular gap junctions compared to control populations. The results are consistent with a role for cytoskeletal elements containing actin and myosin II in annular gap junction turnover.

Gap junction turnover, intracellular trafficking, and phosphorylation of connexin43 in brefeldin A-treated rat mammary tumor cells

Intercellular gap junction channels are thought to form when oligomers of connexins from one cell (connexons) register and pair with connexons from a neighboring cell en route to forming tightly packed arrays (plaques). In the current study we used the rat mammary BICR-M1Rk tumor cell line to examine the trafficking, maturation, and kinetics of connexin43 (Cx43). Cx43 was conclusively shown to reside in the Golgi apparatus in addition to sites of cell-cell apposition in these cells and in normal rat kidney cells. Brefeldin A (BFA) blocked Cx43 trafficking to the surface of the mammary cells and also prevented phosphorylation of the 42-kD form of Cx43 to 44- and 46-kD species. However, phosphorylation of Cx43 occurred in the presence of BFA while it was still a resident of the ER or Golgi apparatus yielding a 43-kD form of Cx43. Moreover, the 42- and 43-kD forms of Cx43 trapped in the ER/Golgi compartment were available for gap junction assembly upon the removal of BFA. Mammary cells treated with BFA for 6 h lost preexisting gap junction "plaques," as well as the 44- and 46-kD forms of Cx43 and functional coupling. These events were reversible 1 h after the removal of BFA and not dependent on protein synthesis. In summary, we provide strong evidence that in BICR-M1Rk tumor cells: (a) Cx43 is transiently phosphorylated in the ER/Golgi apparatus, (b) Cx43 trapped in the ER/Golgi compartment is not subject to rapid degradation and is available for the assembly of new gap junction channels upon the removal of BFA, (c) the rapid turnover of gap junction plaques is correlated with the loss of the 44- and 46-kD forms of Cx43.

Cardiac malformation in neonatal mice lacking connexin43

Gap junctions are made up of connexin proteins, which comprise a multigene family in mammals. Targeted mutagenesis of connexin43 (Cx43), one of the most prevalent connexin proteins, showed that its absence was compatible with survival of mouse embryos to term, even though mutant cell lines showed reduced dye coupling in vitro. However, mutant embryos died at birth, as a result of a failure in pulmonary gas exchange caused by a swelling and blockage of the right ventricular outflow tract from the heart. This finding suggests that Cx43 plays an essential role in heart development but that there is functional compensation among connexins in other parts of the developing fetus.

Mechanisms and function of intercellular calcium signaling

Intercellular Ca2+ waves initiated by mechanical or chemical stimuli propagate between cells via gap junctions. The ability of a wide diversity of cells to display intercellular Ca2+ waves suggests that these Ca2+ waves may represent a general mechanism by which cells communicate. Although Ca2+ may permeate gap junctions, the intercellular movement of Ca2+ is not essential for the propagation of Ca2+ waves. The messenger that moves from one cell to the next through gap junctions appears to be IP3 and a regenerative mechanism for IP3 may be required to effect multicellular communication. Extracellularly mediated Ca2+ signaling also exists and this could be employed to supplement or replace gap junctional communication. The function of intercellular Ca2+ waves may be the coordination of cooperative cellular responses to local stimuli.

Dynamics of connexin43 phosphorylation in pp60v-src-transformed cells

Connexin43 phosphorylation was analysed in non-transformed and pp60v-src-transformed Rat-1 fibroblasts. Connexin43 appeared to be the primary connexin expressed in these cells. Although gap-junctional communication was disrupted in pp60v-src-transformed cells, they contained more connexin43 protein and RNA than their non-transformed counterpart. Connexin43 was phosphorylated within minutes of its synthesis in both cell types and appeared to be degraded while in the phosphorylated state. Phosphopeptide and phosphoamino acid analyses suggested that connexin43 in both cell types contained at least five fragments with serine phosphorylation. The major difference in connexin43 phosphorylation between the pp60v-src-transformed and non-transformed cells was that, whereas approx. 70% of the phosphorylated connexin43 in the former contained phosphotyrosine, this phosphoamino acid was not detected in connexin43 isolated from the latter cells. These data support the hypothesis that phosphorylation of connexin43 on tyrosine is critical for the blockade of gap-junctional communication which occurs concomitantly with transformation by the pp60v-src oncogene.

Coexpression of gap junction proteins in the cumulus-oocyte complex

The connexins constitute a family of proteins that make up the intercellular membrane channels of gap junctions. We had previously reported the presence of two members of this protein family, connexins 32 and 43, in mouse one-cell zygotes (Barron et al., Dev Genet 10:318-323, 1989; Valdimarsson et al., Mol Reprod Dev 30:18-26, 1991), implying that both must be present in the mature oocyte and could be involved in mediating the intercellular coupling that occurs between the oocyte and cumulus granulosa during oogenesis. In the present report we provide evidence for this, based on an analysis of the cumulus-oocyte complex (COC) using reverse transcription-polymerase chain reaction (RT-PCR) and immunocytochemistry with a confocal microscope. Transcripts of both connexin32 (Cx32) and connexin43 (Cx43) were detected by RT-PCR in both components of the COC. Cx32 mRNA in the oocyte declined precipitously following human chorionic gonadotropin (hCG) stimulation of pregnant mare serum gonadotropin (PMSG)-primed ovaries, whereas there was no obvious change in Cx43 mRNA. Peptide-specific antibodies against both connexins provided diffuse cytoplasmic staining of oocytes as well as some punctate staining near the oocyte surface, which could not be unequivocally resolved as cumulus-oocyte gap junctions. However, the two antibodies did provide clear evidence of Cx32 and Cx43 in gap junction-like structures between cumulus cells. We could find no evidence of the incorporation of the oocyte's store of Cx32 into gap junctions during postfertilization development.(ABSTRACT TRUNCATED AT 250 WORDS)

Gap junctions in the brain: where, what type, how many and why?

Gap junctions represent well-documented means of intercellular communication in various tissues, including the brain, where they function as portals allowing the exchange of electrolytes, second messengers and metabolites between cells. In view of the enormous recent surge of information dealing with the cellular and molecular biology of gap junctions in non-nervous tissue, as well as current interest in the cell biology of glia, this review is intended to provide an overview of the molecular and functional implications of gap-junction-mediated intercellular communication in the nervous system.

Connexin trafficking and the control of gap junction assembly in mouse preimplantation embryos

Gap junction assembly in the preimplantation mouse embryo is a temporally regulated event, beginning a few hours after the third cleavage during the morphogenetic event known as compaction. Recently, we demonstrated that both mRNA and protein corresponding to connexin43, a gap junction protein, accumulate through preimplantation development beginning at least as early as the 4-cell stage. Using an antibody raised against a synthetic C-terminal peptide of connexin43, this protein was shown to assemble into gap junction-like plaques beginning at compaction (G. Valdimarsson, P. A. De Sousa, E. C. Beyer, D. L. Paul and G. M. Kidder (1991). Molec. Reprod. Dev. 30, 18–26). The purpose of the present study was to follow the fate of nascent connexin43 during preimplantation development, from synthesis to plaque insertion, and to learn more about the control of gap junction assembly during compaction. Cell fractionation and reverse transcription-polymerase chain reaction were employed to show that connexin43 mRNA is in polyribosomes at the 4-cell stage, suggesting that synthesis of connexin43 begins at least one cell cycle in advance of when gap junctions first form. The fate of nascent connexin43 was then followed throughout preimplantation development by means of laser confocal microscopy, using two other peptide (C-terminal)-specific antibodies. As was reported previously, connexin43 could first be detected in gap junction-like plaques beginning in the 8-cell stage, at which time considerable intracellular immunoreactivity could be seen as well. Later, connexin43 becomes differentially distributed in the apposed plasma membranes of morulae and blastocysts: a zonular distribution predominates between outside blastomeres and trophectoderm cells whereas plaque-like localizations predominate between inside blastomeres and cells of the inner cell mass. The cytoplasmic immunoreactivity in morulae was deemed to be nascent connexin en route to the plasma membrane since it could be abolished by treatment with cycloheximide, and redistributed by treatment with monensin or brefeldin-A, known inhibitors of protein trafficking. Treatment of uncompacted 8-cell embryos with either monensin or brefeldin-A inhibited the appearance of gap junction-like structures and the onset of gap junctional coupling in a reversible manner. These data demonstrate that the regulated step in the onset of gap junction assembly during compaction is downstream of transcription and translation and involves mobilization of connexin43 through trafficking organelles to plasma membranes.

Growth retardation in glioma cells cocultured with cells overexpressing a gap junction protein

To examine the role of gap-junctional intercellular communication in controlling cell proliferation, we have transfected C6 glioma cells with connexin 43 cDNA. The growth of transfected clones was dramatically reduced compared with nontransfected glioma cells. To further characterize the role of gap junctions in controlling proliferation, we have examined the growth of C6 cells cocultured with transfected cells overexpressing connexin 43. Although C6 cells grew at their normal rate when cocultured with nontransfected C6 cells, when cocultured with connexin 43-overexpressing cells they displayed a dramatic reduction in growth rate. Furthermore, a significant, dose-dependent reduction in cell proliferation was noted when C6 cells were cultured in medium conditioned by transfected cells. This effect correlated with the level of connexin 43 expression. These results suggest that the decreased cell proliferation rate of transfected cells and C6 cells cultured with them is due to the secretion of a growth inhibitory factor(s) and that the secretion of this factor may be linked to the level of gap junctional intercellular communication.