Regulation of gap junctions by phosphorylation of connexins

Amino terminal glutamate residues confer spermine sensitivity and affect voltage gating and channel conductance of rat connexin40 gap junctions

Connexin40 (Cx40) contains a specific binding site for spermine (affinity approximately 100 microm) whereas connexin43 (Cx43) is unaffected by identical concentrations of intracellular spermine. Replacement of two unique glutamate residues, E9 and E13, from the cytoplasmic amino terminal domain of Cx40 with the corresponding lysine residues from Cx43 eliminated the block by 2 mm spermine, reduced the transjunctional voltage (V(j)) gating sensitivity, and reduced the unitary conductance of this Cx40E9,13K gap junction channel protein. The single point mutations, Cx40E9K and Cx40E13K, predominantly affected the residual conductance state (G(min)) and V(j) gating properties, respectively. Heterotypic pairing of Cx40E9,13K with wild-type Cx40 in murine neuro2A (N2A) cells produced a strongly rectifying gap junction reminiscent of the inward rectification properties of the Kir (e.g. Kir2.x) family of potassium channels. The reciprocal Cx43K9,13E mutant protein exhibited reduced V(j) sensitivity, but displayed much less rectification in heterotypic pairings with wtCx43, negligible changes in the unitary channel conductance, and remained insensitive to spermine block. These data indicate that the connexin40 amino terminus may form a critical cytoplasmic pore-forming domain that serves as the receptor for V(j)-dependent closure and block by intracellular polyamines. Functional reciprocity between Cx40 and Cx43 gap junctions involves other amino acid residues in addition to the E or K 9 and 13 loci located on the amino terminal domain of these two connexins.

Electrotonic coupling between stratum oriens interneurones in the intact in vitro mouse juvenile hippocampus

Using the isolated juvenile (7-14 days) mouse whole hippocampus preparation, which contains intact complex local circuitry, 145 dual whole cell recordings were made from stratum oriens (s.o.) interneurones under infrared microscopy. In 11.7% of paired recordings, evidence for direct electrotonic coupling between the s.o. interneurones was obtained from the response of one interneurone to a long (400-600 ms) constant current pulse passed into the coupled interneurone. When specifically orienting the dual recordings in the transectional plane of the hippocampus, 18.5% of paired recordings showed electrotonic coupling. The coupling coefficient, estimated from averaged data, was 6.9 +/- 4.7%, ranging from 1.3 to 17.6%. The time constant of the electrotonically transmitted hyperpolarization was inversely related to the coupling coefficient between the two neurones. The electrotonic responses of one neurone to constant current pulses injected into the other coupled neurone were intermittent. Spikes in one of the coupled neurones were associated with small electrotonic EPSPs (spikelets) in the other coupled neurone, in those neuronal pairs with coupling coefficients greater than 10%. Failure of spikelet production following a spike in the coupled cell occurred 5-10% of the time. Electrotonic coupling and spikelets persisted in the presence of chemical synaptic transmission blockade by CNQX, APV and bicuculline, or in zero Ca(2+) perfusate, but were abolished by carbenoxolone (100 microm), a gap junctional blocker. These data confirm the existence of electrotonic coupling between s.o. interneurones, presumably via gap junctions located in dendrites.

7-Ketocholesterol modulates intercellular communication through gap-junction in bovine lens epithelial cells

Background

Connexin43 (Cx43) is an integral membrane protein that forms intercellular channels called gap junctions. Intercellular communication in the eye lens relies on an extensive network of gap junctions essential for the maintenance of lens transparency. The association of Cx43 with cholesterol enriched lipid raft domains was recently demonstrated. The objective of this study is to assess if products of cholesterol oxidation (oxysterols) affect gap junction intercellular communication (GJIC).

Results

Primary cultures of lens epithelial cells (LEC) were incubated with 7-ketocholesterol (7-Keto), 25-hydroxycholesterol (25-OH) or cholesterol and the subcellular distribution of Cx43 was evaluated by immunofluorescence confocal microscopy. The levels of Cx43 present in gap junction plaques were assessed by its insolubility in Triton X-100 and quantified by western blotting. The stability of Cx43 at the plasma membrane following incubation with oxysterols was evaluated by biotinylation of cell surface proteins. Gap junction intercellular communication was evaluated by transfer of the dye Lucifer yellow. The results obtained showed that 7-keto induces an accumulation of Cx43 at the plasma membrane and an increase in intercellular communication through gap junction. However, incubation with cholesterol or 25-OH did not lead to significant alterations on subcellular distribution of Cx43 nor in intercellular communication. Data further suggests that increased intercellular communication results from increased stability of Cx43 at the plasma membrane, presumably forming functional gap-junctions, as suggested by decreased solubility of Cx43 in 1% Triton X-100. The increased stability of Cx43 at the plasma membrane seems to be specific and not related to disruption of endocytic pathway, as demonstrated by dextran uptake.

Conclusions

Results demonstrate, for the first time, that 7-keto induces an increase in gap junction intercellular communication, that is most likely due to an increased stability of protein at the plasma membrane and to increased abundance of Cx43 assembled in gap junction plaques.

p38/SAPK2 controls gap junction closure in astrocytes

Astrocyte gap junction communication (GJC) is thought to contribute to death signal propagation following central nervous system injury, noteworthy in some ischemia/anoxia models. The inhibition of p38/stress-activated protein kinase 2 (p38/SAPK2) by a pyrimidyl imidazole derivative has been reported to reduce the extent of the lesion area after cerebral ischemia. Therefore, interleukin-1beta (IL-1beta), which contributes to stroke-induced brain injury and activates p38/SAPK2, and hyperosmolarity induced by sorbitol, a potent stimulus of p38/SAPK2 in non-neuronal cells, were used to investigate a possible involvement of p38/SAPK2 in GJC modulation in mouse cultured astrocytes. Both stimuli inhibited dye coupling within minutes. The IL-1beta effect was transient, while that of sorbitol lasted up to 90 min. Both stimuli induced a rapid p38/SAPK2 activation, the kinetic of which matched that of induction of dye coupling inhibition. Immunocytochemical studies showed that IL-1beta and sorbitol induced a p38/SAPK2 translocation from the nucleus to the cytoplasm. The pharmacological agent SB203580 specifically blocked p38/SAPK2 activation, cytoplasmic translocation and reversed the IL-1beta and sorbitol-induced inhibition of GJC. Further characterization of the p38/SAPK2 mode of action on GJC, performed with sorbitol, revealed an increased phosphorylation of protein kinase C (PKC) substrates abolished by both PKC inhibitors and SB203580. Expression and serine phosphorylation of connexin 43, the main component of astrocyte gap junctions, were unchanged, suggesting the existence of additional intracellular signaling mechanisms modulating the channel gating. Altogether, these results demonstrate that p38/SAPK2 is a central mediator of IL-1beta and sorbitol inhibitory actions on GJC and establish PKC among the distal effectors of p38/SAPK2.

Gap junction communication between cells aggregated on a cellulose-coated polystyrene: influence of connexin 43 phosphorylation

The appropriate functioning of tissues and organ systems depends on intercellular communication such as gap junctions formed by connexin (Cx) protein channels between adjacent cells. We have previously shown that Swiss 3T3 cells aggregated on hydrophilic cellulose substratum Cuprophan (CU) establish short linear gap junctions composed of Cx 43 in cell surface plaques. This phenomenon seems to depend on the high intracellular cyclic AMP (cAMP) concentration triggered by attachment of the cells to CU. We have now used a cellulose-coated polystyrene inducing the same cell behaviour to analyse the gap junction communication between aggregated cells. The transfer of the dye Lucifer Yellow (LY) between cells showed that cells aggregated on cellulose substratum rapidly (within 90 min) establish functional gap junctions. Inhibitors of cAMP protein kinase (PKI) or protein kinase C (GF109203X) both inhibited the diffusion of LY between neighbouring cells. Western blot analysis showed that this change in permeability was correlated with a decrease in Cx 43 phosphorylation. Thus, cellulose substrata seem to induce cell-cell communication through Cx 43 phosphorylation modulated by PKA and PKC. To understand the mechanisms by which a substratum regulates gap junctional communication is critically important for the emerging fields of tissue engineering and biohybrid devices.

Mechanism of regulation of the gap junction protein connexin 43 by protein kinase C-mediated phosphorylation

Phosphorylation of the gap junction protein connexin 43 (Cx43) by protein kinase C (PKC) decreases dye coupling in many cell types. We report an investigation of the regulation by PKC of Cx43 gap junctional hemichannels (GJH) expressed in Xenopus laevis oocytes. The activity of GJH was assessed from the uptake of hydrophilic fluorescent probes. PKC inhibitors increased probe uptake in isolated oocytes expressing recombinant Cx43, indicating that the regulatory effect occurs at the hemichannel level. We identified by mutational analysis the carboxy-terminal (CT) domain sequences involved in this response. We found that 1) Ser368 is responsible for the regulation of Cx43 GJH solute permeability by PKC-mediated phosphorylation, 2) CT domain residues 253-270 and 288-359 are not necessary for the effect of PKC, and 3) the prolinerich CT region is not involved in the effect of phosphorylation by PKC. Our results demonstrate that Ser368 (but not Ser372) is involved in the regulation of Cx43 solute permeability by PKC-mediated phosphorylation, and we conclude that different molecular mechanisms underlie the regulation of Cx43 by intracellular pH and PKC-mediated phosphorylation.

Functional Expression in Xenopus Oocytes of Gap-junctional Hemichannels Formed by a Cysteine-less Connexin 43

Gap-junctional channels are formed by two connexons or gap-junctional hemichannels in series, with each connexon conformed by six connexin molecules. As with other membrane proteins, structural information on connexons can potentially be obtained with techniques that take advantage of the highly specific thiol chemistry by positioning Cys residues at locations of interest, ideally in an otherwise Cys-less protein. It has been shown that conserved Cys residues located in the extracellular loops of connexins are essential for the docking of connexons from adjacent cells, preventing the formation of functional gap-junctional channels. Here we engineered a Cys-less version of connexin 43 (Cx43) and assessed its function using a Xenopus oocyte expression system. The Cys-less protein was expressed at the plasma membrane and did not form gap-junctional channels but formed hemichannels that behave similarly to those formed by Cx43 in terms of permeation to carboxyfluorescein. The carboxyfluorescein permeability of Cys-less hemichannels was increased by protein kinase C inhibition, like the wild-type Cx43 hemichannels. We generated a protein with a single Cys in a position (residue 34) thought to face the channel pore and show that thiol modification of the Cys abolishes the carboxyfluorescein permeability. We conclude that Cysless Cx43 forms regulated functional hemichannels, and therefore Cys-less Cx43 is a useful tool for future structural studies.

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.

Epidermal growth factor regulates ubiquitination, internalization and proteasome-dependent degradation of connexin43

Connexins are membrane-spanning proteins that form gap junction channels between adjacent cells. Connexin43 (Cx43), the most widely expressed member of the connexin family in tissues and cell lines, has a rapid turnover rate and its degradation involves both the lysosomal and ubiquitin-proteasome pathway. It was previously shown that the proteasome is involved in regulating the number of functional gap junctions at the plasma membrane. However, little is known about how proteasome-dependent turnover of Cx43 is controlled. Epidermal growth factor (EGF) induces hyperphosphorylation of Cx43 and a rapid, transient decrease in gap junctional intercellular communication. In this study, we show that, along with inhibition of gap junctional intercellular communication, EGF induces disorganization, internalization and degradation of Cx43 gap junction plaques in IAR20 rat liver epithelial cells. These EGF-induced modifications of Cx43 were counteracted by the MEK1 inhibitor PD98059, indicating that the effects were mediated by the mitogen-activated protein kinase pathway. The EGF-induced destruction of Cx43 was proteasome-dependent, because the loss of Cx43 protein was counteracted by the proteasome inhibitor MG132 but not the lysosomal inhibitor leupeptin. Furthermore, EGF induced ubiquitination of Cx43, which was associated with the Cx43 hyperphosphorylation. The EGF-induced Cx43 ubiquitination was counteracted by PD98059. The EGF-induced internalization of Cx43 was blocked by hypertonic sucrose treatment, indicating that EGF mediates internalization of Cx43 via a clathrin-dependent mechanism. Our results indicate that ubiquitination of Cx43 occurs at the plasma membrane before Cx43 internalization. Taken together, these data provide the first evidence that EGF-induced phosphorylation of Cx43 induces binding of ubiquitin and targets Cx43 for internalization and degradation in a proteasome-dependent manner.

Regulation of purified and reconstituted connexin 43 hemichannels by protein kinase C-mediated phosphorylation of Serine 368

Indirect evidence suggests that the permeability of connexin 43 (Cx43) gap-junctional channels (connexons) to small organic molecules (M(r) < 1,000) is decreased by protein kinase C (PKC)-mediated phosphorylation of Ser-368. However, it is currently unknown whether this effect is produced directly by phosphorylation of this residue or whether cytoplasmic regulatory factors are required for the decrease in Cx43 gap-junctional channel permeability. Here we studied the effects of PKC-mediated phosphorylation on purified recombinant wild-type Cx43 and a PKC-unresponsive mutant (S368A). Our studies show that (a) PKC phosphorylates Ser-368, (b) the phosphorylation by PKC of purified and reconstituted connexons abolishes sucrose and Lucifer Yellow permeability, (c) the regulation of Cx43 by PKC is the direct result of phosphorylation of Ser-368 and does not involve intermediary regulatory factors, and (d) phosphorylation of Ser-368 produces a conformational change in purified Cx43 as demonstrated by changes in intrinsic Trp fluorescence and proteolytic digestion pattern. We conclude that phosphorylation of Ser-368 by PKC induces a conformational change of Cx43 that results in a decrease in connexon permeability.

Electron microprobe analysis of ouabain-exposed ciliary epithelium: PE-NPE cell couplets form the functional units

Aqueous humor is secreted by the bilayered ciliary epithelium. Solutes and water enter the pigmented ciliary epithelial (PE) cell layer, cross gap junctions into the nonpigmented ciliary epithelial (NPE) cell layer, and are released into the aqueous humor. Electrical measurements suggest that heptanol reduces transepithelial ion movement by interrupting PE-NPE communication and that gap junctions may be a regulatory site of aqueous humor formation. Several lines of evidence also suggest that net ciliary epithelial transport is strongly region dependent. Divided rabbit iris-ciliary bodies were incubated in chambers under control and experimental conditions, quick-frozen, cryosectioned, and freeze-dried. Elemental intracellular contents of NPE and PE cells were determined by electron probe X-ray microanalysis. With or without heptanol, ouabain produced concentration- and time-dependent changes more markedly in anterior than in posterior epithelium. Without heptanol, there were considerable cell-to-cell variations in Na gain and K loss. However, contiguous NPE and PE cells displayed similar changes, even when nearby cell pairs were little changed by ouabain in aqueous, stromal, or both reservoirs. In contrast, with heptanol present, ouabain added to aqueous or both reservoirs produced much larger changes in NPE than in PE cells. The results indicate that 1) heptanol indeed interrupts PE-NPE junctions, providing an opportunity for electron microprobe analysis of the sidedness of modification of ciliary epithelial secretion; 2) Na and K undergo faster turnover in anterior than in posterior epithelium; and 3) PE-NPE gap junctions differ from PE-PE and NPE-NPE junctions in permitting ionic equilibration between adjoining ouabain-stressed cells.

Histamine inhibits conducted vasodilation through endothelium‐derived NO production in arterioles of mouse skeletal muscle

Conducted vasodilation along arterioles manifests the spread of hyperpolarization through gap junction channels along endothelium. Whereas histamine increases the permeability of capillary and venular endothelium, its effect on the integrity of arteriolar endothelium is unknown. We tested whether histamine could inhibit conducted vasodilation. In second-order arterioles (2A) supplying the cremaster muscle of C57BL6, PECAM-1-/-, and eNOS-/- mice (8-12 wk), neither resting (16+/-2 microm) nor maximal (38+/-2 microm) diameters were different. Acetylcholine (ACh) microiontophoresis (1 microA, 500 ms pulse) triggered vasodilation that was conducted >1400 microm along arterioles. Neither local (14+/-2 microm) nor conducted vasodilation (10+/-2 microm) was different among mice. Histamine (5 microM) had no effect on resting diameter or local vasodilation to ACh yet inhibited conduction by >50% in C57BL6 and PECAM-1-/- mice (P<0.05); this effect was abolished after blockade of NO synthase or of soluble guanylate cyclase. Washout of histamine restored conduction, though recovery was longer (P<0.05) in PECAM-1-/- mice vs. C57BL6 mice. Remarkably, conducted vasodilation in eNOS-/- mice was insensitive to histamine. These findings indicate that histamine inhibits cell-to-cell coupling through an NO-dependent mechanism and suggests a dynamic interaction among intercellular adhesion molecules and gap junction channels along arteriolar endothelium.

Junctional adhesion molecules and interendothelial junctions

Similar to epithelia, endothelial cells are linked to each other via intercellular junctional complexes including gap junctions, adherens junctions and tight junctions. While polarized epithelial cells show a high degree of spatial sorting of junctional complexes, endothelia organize their junctions randomly. For this reason the nature of endothelial contacts may be highly adaptable to the need of permeability and leukocyte transmigration. For instance, high endothelial venules (HEVs) in lymphoid organs, where lymphocytes continuously exit the bloodstream, generally show more leaky contacts than brain with its impermeable blood-brain barrier. We recently identified an Ig superfamily molecule named JAM-2 which is specifically expressed in junctions of lymphatic endothelial cells and HEVs. We showed that JAM-2 belongs to the novel CTX molecular family and we now cloned the human equivalent of JAM-2. The presence of JAM-2 at sites of constitutive lymphocyte circulation argues for a role of this molecule in facilitating transmigration. This is supported by the increased transmigration in vitro across endothelial cells overexpressing JAM-2 at intercellular contacts.

Phosphorylation of serine 262 in the gap junction protein connexin-43 regulates DNA synthesis in cell-cell contact forming cardiomyocytes

Mitogenic stimulation of cardiomyocytes is associated with decreased gap junction coupling and protein kinase C (PKC)-mediated phosphorylation of the gap junction protein connexin43 (Cx43). Identification of and interference with the amino acid(s) that becomes phosphorylated in response to stimulation are important steps towards defining the relationship between Cx43 phosphorylation and cell cycle. Using immunoblotting and phosphospecific antibodies we were able to show that serine-262 (S262) on Cx43 becomes phosphorylated in response to growth factor or PKC stimulation of cardiomyocytes. To examine the effect of Cx43, S262 phosphorylation and cell-cell contact (and/or coupling) on DNA synthesis, we overexpressed wild-type (wt) or mutant Cx43, carrying a S262-to-alanine (S262A, simulating the unphosphorylated state) or a S262-to-aspartate (S262D, simulating constitutive phosphorylation) substitutions in cultures of cell-cell contact forming or isolated cardiomyocytes. Overexpression of wt-Cx43 caused a significant decrease in DNA synthesis irrespective of the presence of cell-cell contact. In cell-cell contact forming cultures, the S262D mutation reversed while the S262A mutation increased the inhibitory effect of Cx43. In the absence of cell-cell contact, the S262-Cx43 mutations had no significant effect on Cx43 inhibition of DNA synthesis. Dye-coupling, evaluated by scrape-loading, indicated increased gap junction permeability in S262A (compared to wt or S262D) overexpressing myocytes. We conclude that Cx43 inhibits cardiomyocyte DNA synthesis irrespectively of cell-cell contact or coupling. Cell-cell contact, and possibly gap junction-mediated communication is required, however, in order to reverse Cx43 inhibition of DNA synthesis by S262 phosphorylation.

Ouabain exerts biphasic effects on connexin functionality and expression in vascular smooth muscle cells

1. We have compared the effects of ouabain on the maintenance of gap junctional communication in rat aortic A7r5 smooth muscle cells, monkey COS-1 fibroblasts and human HeLa epithelial cells. 2. Ouabain (1 mM) interrupted dye coupling between confluent A7r5 cells within approximately 1 h, and high concentrations of ouabain were similarly required to reduce coupling between COS-1 cells selected to express the rat alpha1 Na+/K+-ATPase subunit, which is ouabain resistant. By contrast, low concentrations of ouabain (1-10 microM) attenuated dye transfer in wild-type COS-1 and HeLa cells, whose endogenous alpha1 subunits possess relatively high affinity for the glycoside (Ki approximately 0.3 vs approximately 100 microM) Ouabain-induced reductions in dye transfer therefore correlated with the ability of the glycoside to bind to the Na+/K+-ATPase isoenzymes expressed in these different cell lines. 3. No consistent relationship between inhibition of intercellular dye transfer and secondary changes in [Ca2+]i or pHi could be identified following incubation with ouabain. 4. In separate experiments, the effects of ouabain on real-time trafficking of connexin (Cx) protein were monitored by time-lapse microscopy of A7r5 cells transfected to express a fluorescent Cx43-green fluorescent protein (GFP) and the ability of the glycoside to modulate endogenous expression of Cx40 and Cx43 evaluated in A7r5 cells by immunochemical and Western blot analysis. 5. Ouabain (1 mM) depressed vesicular trafficking of Cx43-GFP after approximately 1 h, and caused a time-dependent loss of endogenous Cx40 and Cx43 protein that was first evident at 2 h and almost complete after 4 h. These effects of ouabain on Cx expression were reversed 90 min following washout of the glycoside. 6. We conclude that ouabain exerts biphasic effects on intercellular communication that involve an initial decrease in gap junctional permeability followed by a global reduction in the expression of Cx protein. Further studies are necessary to establish to what extent these actions of ouabain reflect inversion of the normal [Na+]i/[K+]i ratio and/or conversion of the Na+/K+-ATPase into a general signal transducer that regulates downstream protein synthesis.

Novel mechanism of tumorigenesis: Increased transforming growth factor-?1 suppresses the expression of connexin 43 in BALB/cJ mice after implantation of poly-L-lactic acid

Poly-L-lactic acid (PLLA) is a widely used promising material for surgical implants such as tissue-engineered scaffolds. In this study, we aimed to determine the in vivo effect of PLLA plates on the cellular function of subcutaneous tissue in the two mouse strains, BALB/cJ and SJL/J, higher and lower tumorigenic strains, respectively. Gap-junctional intercellular communication (GJIC) and the expression of connexin 43 (Cx43) protein were significantly suppressed, whereas the secretion of transforming growth factor-beta 1 (TGF-beta 1) level was significantly increased in PLLA-implanted BALB/cJ mice compared with BALB/cJ controls. However, no significant difference in TGF-beta 1 secretion was observed between the SJL/J-implanted and SJL/J control mice. We found for the first time that a significant difference was observed between the two strains; thus, the PLLA increased the secretion of TGF-beta 1 and suppressed the mRNA expression of Cx43 at the earlier stage after implantation into the higher-tumorigenic strain, BALB/cJ mice. This novel mechanism might have a vital role in the inhibition of GJIC and promote the tumorigenesis in BALB/cJ mice.

Electrical synapses in the mammalian brain

Many neurons in the mammalian central nervous system communicate through electrical synapses, defined here as gap junction-mediated connections. Electrical synapses are reciprocal pathways for ionic current and small organic molecules. They are often strong enough to mediate close synchronization of subthreshold and spiking activity among clusters of neurons. The most thoroughly studied electrical synapses occur between excitatory projection neurons of the inferior olivary nucleus and between inhibitory interneurons of the neocortex, hippocampus, and thalamus. All these synapses require the gap junction protein connexin36 (Cx36) for robust electrical coupling. Cx36 appears to interconnect neurons exclusively, and it is expressed widely along the mammalian neuraxis, implying that there are undiscovered electrical synapses throughout the central nervous system. Some central neurons may be electrically coupled by other connexin types or by pannexins, a newly described family of gap junction proteins. Electrical synapses are a ubiquitous yet underappreciated feature of neural circuits in the mammalian brain.

Protective role of gap junctions in preconditioning against myocardial infarction

The aim of the present study was to examine the hypothesis that acceleration of gap junction (GJ) closure during ischemia contributes to anti-infarct tolerance afforded by preconditioning (PC). First, the effects of PC on GJ communication during ischemia were assessed. Isolated buffer-perfused rabbit hearts were subjected to 5-min global ischemia with or without PC with two cycles of 5-min ischemia/5-min reperfusion or a GJ blocker (2 mM heptanol), and then the tissue excised from the ischemic region was incubated in anoxic buffer containing lucifer yellow (LY; 2.5 mg/ml), a tracer of GJ permeability, for 20 min at 37°C. PC and heptanol significantly reduced the area to which LY was transported in the ischemic myocardium by 39% and by 54%, respectively. In the second series of experiments, three GJ blockers (heptanol, 18β-glycyrrhetinic acid, and 2,3-butanedione monoxime) infused after the onset of ischemia reduced infarct size after 30-min ischemia/2-h reperfusion to an extent equivalent to that in the case of PC. In the third series of experiments, Western blotting for connexin43 (Cx43) showed that PC shortened the time to the onset of ischemia-induced Cx43 dephosphorylation but reduced the extent of Cx43 dephosphorylation during a 30-min period of ischemia. Calphostin C, a protein kinase C (PKC) inhibitor, abolished preservation of phosphorylated Cx43 but not the early onset of Cx43 dephosphorylation after ischemia in the preconditioned myocardium. These results suggest that PC-induced reduction of GJ permeability during ischemia, presumably by PKC-mediated Cx43 phosphorylation, contributes to infarct size limitation.

Ouabain exerts biphasic effects on connexin functionality and expression in vascular smooth muscle cells

1. We have compared the effects of ouabain on the maintenance of gap junctional communication in rat aortic A7r5 smooth muscle cells, monkey COS-1 fibroblasts and human HeLa epithelial cells. 2. Ouabain (1 mM) interrupted dye coupling between confluent A7r5 cells within approximately 1 h, and high concentrations of ouabain were similarly required to reduce coupling between COS-1 cells selected to express the rat alpha1 Na+/K+-ATPase subunit, which is ouabain resistant. By contrast, low concentrations of ouabain (1-10 microM) attenuated dye transfer in wild-type COS-1 and HeLa cells, whose endogenous alpha1 subunits possess relatively high affinity for the glycoside (Ki approximately 0.3 vs approximately 100 microM) Ouabain-induced reductions in dye transfer therefore correlated with the ability of the glycoside to bind to the Na+/K+-ATPase isoenzymes expressed in these different cell lines. 3. No consistent relationship between inhibition of intercellular dye transfer and secondary changes in [Ca2+]i or pHi could be identified following incubation with ouabain. 4. In separate experiments, the effects of ouabain on real-time trafficking of connexin protein were monitored by time-lapse microscopy of A7r5 cells transfected to express a fluorescent Cx43-green fluorescent protein (GFP) and the ability of the glycoside to modulate endogenous expression of connexins (Cx) 40 and 43 evaluated in A7r5 cells by immunochemical and Western blot analysis. 5. Ouabain (1 mM) depressed vesicular trafficking of Cx43-GFP after approximately 1 h, and caused a time-dependent loss of endogenous Cx40 and Cx43 protein that was first evident at 2 h and almost complete after 4 h. These effects of ouabain on Cx expression were reversed approximately 90 min following washout of the glycoside. 6. We conclude that ouabain exerts biphasic effects on the intercellular communication that involve an initial decrease in gap junctional permeability followed by a global reduction in the expression of Cx protein. Further studies are necessary to establish to what extent these actions of ouabain reflect inversion of the normal [Na+]i/[K+]i ratio and/or conversion of the Na+/K+-ATPase into a general signal transducer that regulates downstream protein synthesis.

Calmodulin and protein kinase C regulate gap junctional coupling in lens epithelial cells

The mechanisms regulating the permeability of lens epithelial cell gap junctions in response to calcium ionophore or ATP agonist-mediated increases in cytosolic Ca2+ (Cai2+) have been investigated using inhibitors of calmodulin (CaM) and PKC. Cell-to-cell transfer of the fluorescent dye AlexaFluor594 decreased after the rapid and sustained increase in Cai2+ (to micromolar concentrations) observed after the addition of ionophore plus Ca2+ but was prevented by pretreatment with inhibitors of CaM but not PKC. In contrast, the delayed, transient decrease in cell-to-cell coupling observed after the addition of ATP that we have reported previously (Churchill G, Lurtz MM, and Louis CF. Am J Physiol Cell Physiol 281: C972-C981, 2001) could be prevented by either the direct or indirect inhibition of PKC but not by inhibition of CaM. Surprisingly, there was no change in the relative proportion of the different phosphorylated forms of lens connexin43 after this ATP-dependent transient decrease in cell-to-cell coupling. Although BAPTA-loaded cells did not display the ATP-dependent transient increase in Cai2+, the delayed, transient decrease in cell-to-cell dye transfer was still observed, indicating it was Cai2+ independent. Thus CaM-mediated inhibition of lens gap junctions is associated with sustained, micromolar Cai2+ concentrations, whereas PKC-mediated inhibition of lens gap junctions is associated with agonist activation of second messenger pathways that are independent of changes in Cai2+.

Oxidative metabolism, gap junctions and the ionizing radiation-induced bystander effect

Evidence accumulated over the past two decades has indicated that exposure of cell populations to ionizing radiation results in significant biological effects occurring in both the irradiated and nonirradiated cells in the population. This phenomenon, termed the 'bystander response', has been shown to occur both in vitro and in vivo and has been postulated to impact both the estimation of risks of exposure to low doses/low fluences of ionizing radiation and radiotherapy. Several mechanisms involving secreted soluble factors, oxidative metabolism and gap-junction intercellular communication have been proposed to regulate the radiation-induced bystander effect. Our current knowledge of the biochemical and molecular events involved in the latter two processes is reviewed in this article.

Sequestration of connexin43 in the early endosomes: An early event of Leydig cell tumor progression

Connexins form gap junction channels that allow intercellular communication between neighboring cells. Compelling evidence has revealed that Cx are tumor-suppressor genes and reduced Cx expression has been related with uncontrolled cell growth in tumors and transformed cells. In the present study, we addressed Cx transcriptional and posttranscriptional regulations during the earlier stage of testicular tumors confined to Leydig cells in a transgenic mice model. In situ hybridization indicated that connexin43 (Cx43) mRNA was highly expressed either at early tumorogenesis (3 m) characterized by intense proliferation of Leydig cells, or at advanced tumorogenesis (6-7 m) when tumor cells completely invaded the testis. In contrast, Cx43 protein analyzed by Western blotting or classic immunohistochemical analyses was present at the beginning of tumor progression, but was dramatically reduced as tumor advanced. Application of high-resolution deconvolution microscopy to testis sections demonstrates that cells that proliferate exhibited an aberrant cytoplasmic Cx43 localization, in contrast to the expected plasma membrane Cx43 localization in normal Leydig cells. Dual immunofluorescence labeling with specific markers of cellular compartments shows that cytoplasmic Cx43 signal was mainly sequestered within early endosomes. Altogether, this study provides the first evidence that impaired Cx43 trafficking in endosomes is an early event associated with uncontrolled cell proliferation that could serve as a neoplastic marker.

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.

Endothelin-1 decreases gap junctional intercellular communication by inducing phosphorylation of connexin 43 in human ovarian carcinoma cells

Endothelin-1 (ET-1) is overexpressed in ovarian carcinoma and acts as an autocrine factor selectively through the ETA receptor (ETAR) to promote tumor cell proliferation, survival, neovascularization, and invasiveness. Loss of gap junctional intercellular communication (GJIC) is critical for tumor progression by allowing the cells to escape growth control. Exposure of HEY and OVCA 433 ovarian carcinoma cell lines to ET-1 led to a 50-75% inhibition in intercellular communication and to a decrease in the connexin 43 (Cx43)-based gap junction plaques. To investigate the phosphorylation state of Cx43, ovarian carcinoma cell lysates were immunoprecipitated and transient tyrosine phosphorylation of Cx43 was detected in ET-1-treated cells. BQ 123, a selective ETAR antagonist, blocked the ET-1-induced Cx43 phosphorylation and cellular uncoupling. Gap junction closure was prevented by tyrphostin 25 and by the selective c-Src inhibitor, PP2. Furthermore, the increased Cx43 tyrosine phosphorylation was correlated with ET-1-induced increase of c-Src activity, and PP2 suppressed the ET-1-induced Cx43 tyrosine phosphorylation, indicating that inhibition of Cx43-based GJIC is mainly mediated by the Src tyrosine kinase pathway. In vivo, the inhibition of human ovarian tumor growth in nude mice induced by the potent ETAR antagonist, ABT-627, was associated with a reduction of Cx43 phosphorylation. These findings indicate that the signaling mechanisms involved in GJIC disruption on ovarian carcinoma cells depend on ETAR activation, which leads to the Cx43 tyrosine phosphorylation mediated by c-Src, suggesting that ETAR blockade may contribute to the control of ovarian carcinoma growth and progression also by preventing the loss of GJIC.

Epidermal growth factor receptor is a common mediator of quinone-induced signaling leading to phosphorylation of connexin-43: role of glutathione and tyrosine phosphatases

Rat liver epithelial cells were exposed to three quinones with different properties: menadione (2-methyl-1,4-naphthoquinone, vitamin K3), an alkylating as well as redox-cycling quinone, the strongly alkylating p-benzoquinone (BQ), and the non-arylating redox-cycler, 2,3-dimethoxy-1,4-naphthoquinone (DMNQ). All three quinones induced the activation of extracellular signal-regulated kinase (ERK) 1 and ERK 2 via the activation of epidermal growth factor receptor (EGFR) and MAPK/ERK kinases (MEK) 1/2. ERK activation resulted in phosphorylation at Ser-279 and Ser-282 of the gap junctional protein, connexin-43, known to result in the loss of gap junctional intercellular communication. Another EGFR-dependent pathway was stimulated, leading to the activation of the antiapoptotic kinase Akt via phosphoinositide 3-kinase. The activation of EGFR-dependent signaling by these quinones was by different mechanisms: (i) menadione, but not BQ or DMNQ, inhibited a protein-tyrosine phosphatase regulating the EGFR, as concluded from an EGFR dephosphorylation assay; (ii) although menadione-induced activation of ERK was unimpaired by pretreatment of cells with N-acetyl cysteine, activation by BQ and DMNQ was prevented; (iii) cellular glutathione (GSH) levels were strongly depleted by BQ. The mere depletion of GSH by application of diethyl maleate EGFR-dependently activated ERK and Akt, thus mimicking BQ effects. GSH levels were only moderately decreased by menadione and not affected by DMNQ. In summary, EGFR-dependent signaling was mediated by protein-tyrosine phosphatase inactivation (menadione), GSH depletion (BQ), and redox-cycling (DMNQ), funneling into the same signaling pathway.

The metabolic inhibitor antimycin A can disrupt cell-to-cell communication by an ATP- and Ca(2+)-independent mechanism

In cardiac myocytes of new-born rats, the degree of intercellular communication through gap junctional channels closely depends on the metabolic state of the cells. In contrast, in stably transfected HeLa cells expressing rat cardiac connexin43 (Cx43, the main channel-forming protein present in ventricular myocytes), a major part of junctional communication persisted in ATP-depleted conditions, in the presence of a metabolic inhibitor (KCN) or of a broad spectrum inhibitor of protein kinases (H7). However, another metabolic inhibitor, antimycin A, which like cyanide inhibits electron transfer in the respiratory chain, totally interrupted cell-to-cell communication between Cx43-HeLa cells, even in whole-cell conditions, when ATP (5 mM) was present. Antimycin A caused a modest increase in cytosolic calcium concentration; however, junctional uncoupling still occurred when this rise was prevented. Conditions of ischemic insult (e.g. ischemia or chemical hypoxia) frequently cause the activation of protein kinases, particularly of Src and MAP kinases, and such activations are known to markedly disrupt gap junctional communication. Antimycin-induced junctional uncoupling occurred even in the presence of inhibitors of these kinases. Antimycin A appears able to cause junctional uncoupling either through the ATP depletion it induces as a metabolic poison or via a direct action on gap junction constituents.

The relationship between connexins, gap junctions, tissue architecture and tumour invasion, as studied in a novel in vitro model of HPV-16-associated cervical cancer progression

Disruption of gap junctional intercellular communication (GJIC) and/or connexins (gap junction proteins) is frequently reported in malignant cell lines and tumours. Certain human papillomaviruses (HPV) associated with the development of cancers, especially of the cervix, have previously been reported to downregulate GJIC in vitro. There is also evidence for reduced gap junctions in cervical dysplasia. However, many squamous hyperproliferative conditions, including HPV-induced warts, often show extensive upregulation of certain connexins. The association between HPV and GJIC, and the mechanism and consequence of deregulated GJIC in cervical tumour progression, remains unclear. Therefore, using a variety of nonmalignant and malignant cell lines and an organotypic raft-culture system, we investigated the relationship between HPV, gap junctions and tumour progression. Established cervical tumour cell lines carrying HPV were unable to communicate via gap junctions (when assayed by dye-transfer techniques). This correlated with lack of connexin protein expression, while transfection with connexins 26 or 43 led to functional gap junction membrane plaques. On the other hand, immortal but nonmalignant cell lines that contained episomal or integrated HPV-16, but required feeder-layer and growth-factor support, were consistently well coupled, and expressed multiple connexins at membrane junctions. In vitro selection of feeder-layer and growth-factor-independent variants eventually lead to loss of GJIC, which correlated with loss of membrane and increased cytoplasmic connexin 43 localization. However, this was preceded by loss of differentiation and stromal invasion, as assayed on the organotypic raft-culture model. Using this model, a comparison between noncoupled, well-coupled and connexin-transfected cell lines revealed no firm correlation between GJIC and dysplasia, but GJIC appeared to favour increased stratification. These findings demonstrate that loss of GJIC is frequent, but appears to occur more as a consequence of, rather than being the cause of, epithelial dysplasia, and may be influenced by, but is not directly attributable to, HPV.

ECM-induced gap junctional communication enhances mammary epithelial cell differentiation

The relationship between gap junctional intercellular communication (GJIC) and mammary cell (CID-9) differentiation in vitro was explored. CID-9 cells differentiate and express beta-casein in an extracellular matrix (ECM)- and hormone-dependent manner. In response to interaction with the ECM, cells in culture modulated the expression of their gap junction proteins at the transcriptional and post-translational levels. In the presence of EHS-matrix, connexins (Cx)26, 32 and 43 localized predominantly to the plasma membrane, and enhanced GJIC [as measured by Lucifer Yellow (LY) dye transfer assays] was noted. Inhibition of GJIC of cells on EHS-matrix with 18 alpha glycyrrhetinic acid (GA) resulted in reversible downregulation of beta-casein expression. In the presence of cAMP, cells cultured on plastic expressed beta-casein, upregulated Cx43 and Cx26 protein levels and enhanced GJIC. This was reversed in the presence of 18 alpha GA. cAMP-treated cells plated either on a non-adhesive PolyHEMA substratum or on plastic supplemented with function-blocking anti-beta 1 integrin antibodies, maintained beta-casein expression. These studies suggest that cell-ECM interaction alone may induce differentiation through changes in cAMP levels and formation of functional gap junctions. That these events are downstream of ECM signalling was underscored by the fact that enhanced GJIC induced partial differentiation in mammary epithelial cells in the absence of an exogenously provided basement membrane and in a beta 1-integrin- and adhesion-independent manner.

Exocrine specific expression of Connexin32 is dependent on the basic helix-loop-helix transcription factor Mist1

Gap junctions are intercellular channels that provide direct passage of small molecules between adjacent cells. In pancreatic acini, the connexin26 (Cx26) and connexin32 (Cx32) proteins form functional channels that coordinate the secretion of digestive enzymes. Although the function of Cx26/Cx32 gap junctions are well characterized, the regulatory circuits that control the spatial and temporal expression patterns of these connexin genes are not known. In an effort to identify the molecular pathways that regulate connexin gene expression, we examined Cx26 and Cx32 gene activities in mice lacking the basic helix-loop-helix transcription factor Mist1 (Mist1KO). Mist1, Cx26 and Cx32 are co-expressed in most exocrine cell types, and acinar cells from Mist1KO mice exhibit a highly disorganized cellular architecture and an altered pattern of expression for several genes involved in regulated exocytosis. Analysis of Mist1KO mice revealed a dramatic decrease in both connexin proteins, albeit through different molecular mechanisms. Cx32 gene transcription was greatly reduced in all Mist1KO exocrine cells, while Cx26 gene expression remained unaffected. However, in the absence of Cx32 protein, Cx26 did not participate in gap junction formation, leading to a complete lack of intercellular communication among Mist1KO acinar cells. Additional studies testing Mist1 gene constructs in pancreatic exocrine cells confirmed that Mist1 transcriptionally regulates expression of the Cx32 gene. We conclude that Mist1 functions as a positive regulator of Cx32 gene expression and, in its absence, acinar cell gap junctions and intercellular communication pathways become disrupted.

Lysosomal and proteasomal degradation play distinct roles in the life cycle of Cx43 in gap junctional intercellular communication-deficient and -competent breast tumor cells

The present study was designed to determine the specific roles played by lysosomes and proteasomes in the degradation of Cx43 in both gap junctional intercellular communication-deficient MDA-MB-231 and -competent BICR-M1Rk cells. In MDA-MB-231 cells, immunolocalization and brefeldin A protein transport blocking studies revealed that there was a propensity for newly synthesized Cx43 to be transported to lysosomes. On the other hand, light and electron microscopic analysis of BICR-M1Rk cells showed that Cx43 gap junctions were prevalent with a subpopulation of intracellular Cx43 localized to lysosomes. In both cell types, Western blots revealed a notable increase in total cellular Cx43 in response to lysosome inhibitors. Interestingly, lactacystin inhibition of proteosomal degradation in MDA-MB-231 cells resulted in a marked increase in phosphorylated Cx43 at the expense of non-phosphorylated Cx43, and this change corresponded with an increase in "oversized" gap junction plaques. In BICR-M1Rk cells, lactacystin treatment partially prevented the BFA-induced loss of gap junctions. Together, our data suggests that lysosomes play a key role in not only degrading internalized gap junction in BICR-M1Rk cells but also in degrading Cx43 delivered from early secretory compartments to lysosomes in MDA-MB-231 cells. Overall proteasomal degradation regulates the stability of phosphorylated Cx43 and appears to promote the internalization of Cx43 from the cell surface.

Aberrant Connexin 43 endocytosis by the carcinogen lindane involves activation of the ERK/mitogen-activated protein kinase pathway

Although worldwide concerns have emerged about environmental factors that display carcinogenic and reprotoxic effects, little is known about the mechanism(s) by which these chemicals alter testicular function. Using the 42GPA9 Sertoli cell line, we recently reported that one widely used lipid-soluble pesticide, Lindane impairs gap junctional intercellular communication by promoting the intracellular localization of Connexin 43 (Cx43), a tumor suppressor. We showed here that this chemical triggered the accumulation of Cx43 within Rab5 positive endosomes. Interestingly, evidence is provided that Lindane-induced Cx43 endocytosis did not stem on alteration of Cx43 partition in lipid rafts. Lindane induced concomitantly Cx43 phosphorylation and activation of extracellular signal-regulated kinases (ERK) but not of JNK and p38 mitogen- activated protein kinases. Inhibition of ERK pathway by PD98059, a MEK1-specific inhibitor, prevented Lindane-induced Cx43 phosphorylation, restored Cx43 membranous localization and gap junction coupling. Altogether, these findings provide the first evidence that Lindane-altered Cx43 endocytosis requires ERK activation. Such inappropriate activation of the mitogenic MAPK pathway and inactivation of the tumor suppressor Cx43 by Lindane may participate in the promotion of neoplastic cell growth.

Identification of functional regulatory regions of the connexin32 gene promoter

Connexin32 (Cx32) is the predominant gap junction protein expressed in adult rat hepatocytes. This study investigated transcriptional regulation of the rat Cx32 gene in MH(1)C(1) rat hepatoma cells using transient expression assays in conjunction with promoter mutagenesis and 5' nested deletion analysis. Site-directed mutagenesis of the -736 and -187 hepatocyte nuclear factor-1 (HNF-1) sites, the -196 and -116 Sp1 sites, and the -729 and -329 Yin Yang 1 (YY1) sites all significantly reduced promoter activity. We have defined the contribution of each individual site to promoter activity in the intact cell. A novel upstream region of the Cx32 promoter (-1042 to -758) was cloned and shown to contain negative regulatory elements. The transcription factors HNF-1 and Sp1 have important functional roles in the transcriptional regulation of basal and cell-specific Cx32 expression. The multifunctional transcription factor YY1 is also implicated.

Ischemic preconditioning preserves connexin 43 phosphorylation during sustained ischemia in pig hearts in vivo

During myocardial ischemia, connexin 43 (Cx43) is dephosphorylated in vitro, and the subsequent opening of gap junctions formed by two opposing Cx43 hexamers was suggested to propagate ischemia/reperfusion injury. Reduction of infarct size (IS) by ischemic preconditioning (IP) involves activation of protein kinase C (PKC) and p38 mitogen activated protein kinase (MAPK), both of which can phosphorylate Cx43. We now studied in anesthetized pigs whether IP impacts on Cx43 phosphorylation by measuring the density of non-phosphorylated and total Cx43 (confocal laser) during normoperfusion and 90-min ischemia in non-preconditioned and preconditioned hearts. Co-localization of PKCalpha, p38MAPKalpha, and p38MAPKbeta with Cx43 and the activity of p38MAPK were assessed. IP by 10 min ischemia and 15 min reperfusion reduced IS. Non-phosphorylated Cx43 remained unchanged during ischemia in preconditioned hearts, while it increased from 35+/-3 to 75+/-8 AU (P<0.05) in non-preconditioned hearts. Co-localization of PKCalpha, p38MAPKalpha, and p38MAPKbeta with Cx43 during ischemia increased only in preconditioned hearts. While the ischemia-induced increase in p38MAPKalpha activity was comparable in preconditioned and non-preconditioned hearts, p38MAPKbeta activity was increased only in preconditioned hearts. Blockade of p38MAPK by SB203580 attenuated the IS-reduction and the increased p38MAPK-Cx43 co-localization by IP. We conclude that IP increases co-localization of protein kinases with Cx43 and preserves phosphorylation of Cx43 during ischemia.

Connexin 36 in bovine retina: lack of phosphorylation but evidence for association with phosphorylated proteins

In vertebrate retina interneuronal communication through gap junctions is involved in light adaptation and in the transfer of visual information from the rod pathway to the cone pathway. Reports over the last two decades have indicated that these gap junctions are regulated by cyclic nucleotide-dependent protein kinases suggesting that the gap junction proteins, connexins, are phosphorylated. Though all the connexins involved in light adaptation and information transfer from rod to cone pathway are not yet known, connexin 36 has been shown to be definitively involved in the latter process. We have therefore attempted to investigate the cyclic nucleotide-dependent phosphorylation of this connexin in bovine retina. We found several soluble and membrane proteins in bovine retina whose phosphorylation was regulated by cyclic nucleotides. However, no protein of about 36 kDa with cyclic nucleotide-regulated phosphorylation was found in gap junction-enriched membrane preparations. A 36-kDa phosphorylated protein was found in gap junction-enriched membranes phosphorylated in the presence of calcium. However, this protein was not immunoprecipitated by anti-connexin 36 antibodies indicating that it was not connexin 36 in spite of its similarity in molecular weight. Immunoprecipitation did reveal phosphorylated proteins coimmunoprecipitated with connexin 36. Two of these proteins were identified as beta and alpha tubulin subunits. Though cyclic GMP and calcium did not greatly influence the association of these proteins with connexin 36, the results suggest the possibility of connexin 36 associating with other proteins. Together, these observations indicate that interneuronal communication at gap junctions made by connexin 36 may not be regulated by direct phosphorylation of connexin 36, but possibly by phosphorylation of associated proteins.

Opposing effects of nitric oxide on different connexins expressed in the vascular system

Gap junctions--clusters of intercellular channels built by connexins (Cx)--are thought to be important for vascular cell functions such as differentiation, control of tone, or growth. In the vascular system, gap junctions can be formed by four different connexins (Cx37, Cx40, Cx43 and Cx45). The permeability of these connexin-formed gap junctions determines the amount of intercellular coupling and can be modulated by several vasoactive substances such as prostacyclin or nitric oxide (NO). We demonstrate here that NO has specific effects on certain connexins. Using two different techniques--injection of a fluorescent dye in single cells as well as detection of the de novo formation of gap junctions by a flow cytometry based technique--we found that NO decreases the functional coupling in Cx37 containing gap junctions whereas it increases the de novo formation of gap junctions containing Cx40. We conclude that NO, in addition to its known vasomotor effects, has a novel role in controlling intercellular coupling resulting in opposing effects depending on the specific connexin expressed in the cells.

Oscillating fluid flow regulates gap junction communication in osteocytic MLO-Y4 cells by an ERK1/2 MAP kinase-dependent mechanism

The present work was designed to investigate the effects of oscillating fluid flow on gap junctional intercellular communication (GJIC) and the gap junction protein connexin (Cx) 43 in osteocyte-like MLOY-4 cells. Cells were exposed for 1 h to oscillating fluid flow at a shear stress of +/-10 dyn/cm(2) and a frequency of 1 Hz in a parallel plate flow chamber. Control cells were incubated in the chamber but were not exposed to oscillating fluid flow. Functional analysis of GJIC indicated that MLOY-4 cells exposed to oscillating fluid flow established more gap junctions with an independent population of dye-labeled cells than did control cells. Phosphorylation of Cx43 was quantified by immunoprecipitation with an anti-Cx43 antibody followed by immunoblot analysis using an anti-phosphoserine antibody. Phosphoserine was normalized to Cx43 in each sample. Compared to control cells, phosphoserine content of Cx43 increased approximately twofold in cells exposed to oscillating fluid flow. The possible role of the extracellular signal regulated kinase (ERK1/2) in the flow-induced upregulation of GJIC was also investigated. The ERK1/2 inhibitor PD-98059 significantly attenuated the effects of oscillating fluid flow on MLOY-4 cells GJIC. These results indicate that oscillating fluid flow regulates GJIC in MLOY-4 cells via the ERK1/2 MAP kinase. In addition, increased serine phosphorylation of Cx43 correlates with the flow-induced increase in GJIC.

Evidence for long-lasting cholinergic control of gap junctional communication between adrenal chromaffin cells

We investigated long-lasting interactions that may occur between two forms of intercellular signaling: cholinergic synaptic transmission and gap junction-mediated coupling in the rat adrenal medulla. The junctional coupling between chromaffin cells was studied during reduced or blocked synaptic transmission in adrenal slices. First, cholinergic synaptic activity was reduced by pharmacological treatment. Bath-application of the nicotinic receptor antagonists hexamethonium, the oxystilbene derivative F3, or alpha-bungarotoxin, acting at distinct neuronal-like postsynaptic nicotinic acetylcholine receptors (nAChRs), significantly increased the incidence of Lucifer yellow passage (dye coupling) between chromaffin cells (p > 0.7 in treated slices vs p = 0.4 in controls). Dye coupling was associated with an elevated macroscopic conductance of the junctional current measured by dual patch-clamp. Pharmacological inhibition of protein trafficking from the trans-Golgi network to the plasma membrane by either brefeldin A or nocodazole pretreatment prevented the effects of nAChR antagonists on dye coupling. Interestingly, this upregulation of gap junction-mediated coupling in response to reduced synaptic activity is of physiological relevance, because it is found in the newborn rat, in which cholinergic synaptic transmission has not yet matured. This mechanism may also be of importance in pathological conditions, because chronic blockade of synaptic transmission after surgical denervation of the adrenal gland also resulted in increased dye coupling between chromaffin cells. In conclusion, our pharmacological, physiological, and pathological data concur to demonstrate that gap junction-mediated intercellular communication between chromaffin cells undergoes persistent adaptation in response to impairment of synaptic activity. These results strongly suggest that gap junctional communication between chromaffin cells is under tonic inhibitory control exerted by cholinergic synaptic inputs.

Connexin43 phosphorylation at S368 is acute during S and G2/M and in response to protein kinase C activation

Phorbol esters such as 12-O-tetradeconylphorbol-13-acetate (TPA) activate protein kinase C, increase Connexin43 (Cx43) phosphorylation, and decrease cell-cell communication via gap junctions in many cell types. Previous work has implicated protein kinase C (PKC) in the direct phosphorylation of Cx43 at S368, which results in a change in single channel behavior that contributes to a decrease in intercellular communication. We have examined Cx43 phosphorylation in several cell lines with an antibody specific for phosphorylated S368. We show that this antibody detects Cx43 only when it is phosphorylated at S368 and, consistent with previous results, TPA treatment causes a dramatic increase in phosphorylation at S368. However, in some cell types, the increased phosphorylation at S368 did not cause a detectable shift in migration as compared with the nonphosphorylated Cx43. Immunofluorescence showed increased S368 immunolabeling in cytoplasmic and plasma membrane structures in response to TPA. Immunoblot analysis of synchronized cells showed increased phosphorylation at S368 during S and G2/M phases of the cell cycle. S-phase cells contained more total Cx43 but assembled fewer functional gap junctional channels than G0-phase cells. Since M-phase cells also communicate poorly and contain few assembled gap junctions, phosphorylation at S368 appears to be negatively correlated with gap junction assembly. Thus, both gap junctional communication and S368 phosphorylation change during S phase and G2/M, implying that phosphorylation at S368 might play a role in key cell-cycle events.

Regulation of epidermal growth factor-induced connexin 43 gap junction communication by big mitogen-activated protein kinase1/ERK5 but not ERK1/2 kinase activation

The gap junction protein, Cx43, plays a pivotal role in coupling cells electrically and metabolically, and the putative phosphorylation sites that modulate its function are reflected as changes in gap junction communication. Growth factor stimulation has been correlated with a decrease in gap junction communication and a parallel activation of ERK1/2; the inhibition of epidermal growth factor (EGF)-induced Cx43 gap junction uncoupling was observed by using the MEK1/2 inhibitor, PD98059. Because 1) BMK1/ERK5, another MAPK family member also activated by growth factors, possesses a phosphorylation motif similar to ERK1/2, and 2) it has been reported that PD98059 can inhibit not only MEK1/2-ERK1/2 but also MEK5-BMK1 activation, we investigated whether BMK1 can regulate EGF-induced Cx43 gap junction uncoupling and phosphorylation, comparing this to the role of ERK1/2 on Cx43 function and phosphorylation induced by EGF. Selective activation or inactivation of ERK1/2 by using a constitutively active form or a dominant negative form of MEK1 did not regulate Cx43 gap junction coupling. In contrast, we found that BMK1, selectively activated by constitutively active MEK5alpha, induced gap junction uncoupling, and the inhibition of BMK1 activation by transfection of dominant negative BMK1 prevented EGF-induced gap junction uncoupling. Activated BMK1 selectively phosphorylates Cx43 on Ser-255 in vitro and in vivo, but not on S279/S282, which are reported as the consensus phosphorylation sites for MAPK. Furthermore, by co-immunoprecipitation, we found that BMK1 directly associates with Cx43 in vivo. These data indicate that BMK1 is more important than ERK1/2 in EGF-mediated Cx43 gap junction uncoupling by association and Cx43 Ser- 255 phosphorylation.

Besides affecting intracellular calcium signaling, 2-APB reversibly blocks gap junctional coupling in confluent monolayers, thereby allowing measurement of single-cell membrane currents in undissociated cells

2-aminoethoxydiphenyl borate (2-APB) has been widely used as a blocker of the IP3 receptor and TRP channels, including store-operated calcium channels. We now show in monolayers of normal rat kidney cells (NRK/49F) that 2-APB completely and reversibly blocks gap junctional intercellular communication at concentrations similar to that required for inhibition of PGF2alpha-induced increases in intracellular calcium. Gap junctional conductances between NRK cells were estimated with single-electrode patch-clamp measurements and were fully blocked by 2-APB (50 microM), when applied extracellularly but not via the patch pipette. Half maximal inhibition (IC50) of electrical coupling in NRK cells was achieved at 5.7 microM. Similar results were obtained for human embryonic kidney epithelial cells (HEK293/tsA201) with an IC50 of 10.3 microM. Using 2-APB as an electrical uncoupler of monolayer cells, we could thus measure inward rectifier potassium, L-type calcium, and calcium-dependent chloride membrane currents in confluent NRK monolayers, with properties similar to those in dissociated NRK cells in the absence of 2-APB. The electrical uncoupling action described here is a new 2-APB property that promises to provide a powerful pharmacological tool to study single-cell properties in cultured confluent monolayers and intact tissues by electrical and chemical uncoupling of the cells without the need of prior dissociation.

Modulation of perch connexin35 hemi-channels by cyclic AMP requires a protein kinase A phosphorylation site

Retinal neurons are coupled via gap junctions, which function as electrical synapses that are gated by ambient light conditions. Gap junctions connecting either horizontal cells or AII amacrine cells are inhibited by the neurotransmitter dopamine, via the activation of the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling pathway. Fish connexin35 (Cx35) and its mouse ortholog, Cx36, are good candidates to undergo dopaminergic modulation, because they have been detected in the inner plexiform layer of the retina, where Type II amacrine cells establish synaptic contacts. We have taken advantage of the ability of certain connexins to form functional connexons (hemi-channels), when expressed in Xenopus oocytes, to investigate whether pharmacological elevation of cAMP modulates voltage-activated hemi-channel currents in single oocytes. Injection of perch Cx35 RNA into Xenopus oocytes induced outward voltage-dependent currents that were recorded at positive membrane potentials. Incubation of oocytes with 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP), a membrane permeable cAMP analog, resulted in a dose-dependent and reversible inhibition of hemi-channel currents at the more positive voltage steps. In contrast, treatment with 8-Br-cAMP did not have any effect on hemi-channel currents induced by skate Cx35. Amino acid sequence comparison of the two fish connexins revealed, in the middle cytoplasmic loop of perch Cx35, the presence of a PKA consensus sequence that was absent in the skate connexin. The results obtained with two constructs in which the putative PKA phosphorylation site was either suppressed (perch Cx35R108Q) or introduced (skate Cx35Q108R) indicate that it is responsible for the inhibition of hemi-channel currents. These studies demonstrate that perch Cx35 is a target of the cAMP/PKA signaling pathway and identify a consensus PKA phosphorylation site that is required for channel gating.

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.

Protein kinase C-epsilon mediates phorbol ester-induced phosphorylation of connexin-43

We have used adenoviral vectors to express dominant negative variants of protein kinase C epsilon (PKCepsilon) or mitogen kinase kinase 1 (MKK1) to investigate their involvement in phorbol ester-induced connexin-43 (Cx43) phosphorylation in cardiomyocytes. Stimulation of cardiomyocytes with phorbol 12-myristate 13-acetate (PMA) increased the fraction of the slower migrating (> or = 45 kDa) and more extensively phosphorylated Cx43 species. Expression of dominant negative MKK1 did not prevent the effect of PMA on Cx43 phosphorylation. Selective inhibition of PKCE significantly decreased baseline levels of Cx43 phosphorylation and the PMA-induced accumulation of > or = 45 kDa Cx43. Thus, production of the more extensively phosphorylated species of Cx43 in cardiomyocytes by PMA requires activation of PKCepsilon.

Cloning and functional expression of a novel human connexin-25 gene

Gap junctions are intercellular, water-filled channels composed of transmembrane proteins called connexins, six of which are arranged radially and dock with six homologous proteins in an adjacent cell to form an approximate 16 A pore. Through this pore cell-to-cell transfer of small water-soluble molecules up to about 1000 daltons occurs along concentration gradients. Connexins comprise a multigene family that share consensus sequences in the trans-membrane domains and the first and second extracellular loops. Comparison of the protein sequences of known human connexins with the draft nucleotide sequence of the human genome revealed two clones from chromosome 6 which showed strong similarity to highly conserved connexin sequences. Detailed analysis revealed the presence of a 672 nt open reading frame in these clones, encoding a 223 amino acid polypeptide with a predicted molecular weight of about 25 kD. This is smaller than other known human connexins. The ORF of the potential connexin25 was amplified by semi-nested PCR using human genomic DNA as a template. To confirm that this new gene encodes a connexin, Cx25 was transfected into a gap junction deficient subclone of the human HeLa cell line. After selection of transformants, cells were microinjected with the fluorescent dye Lucifer yellow. Transfectants but not controls successfully transferred dye, demonstrating that this new gene encodes a functional connexin.

Defective activation of c-Src in cystic fibrosis airway epithelial cells results in loss of tumor necrosis factor-alpha-induced gap junction regulation

Tumor necrosis factor-alpha (TNF-alpha) signaling is central to the transmission of the innate immune response and subsequent activation of the adaptive immune system. The functioning of both systems is required for optimal clearance of pathogens from the airways. In cystic fibrosis (CF), dysfunction of the CF transmembrane conductance regulator (CFTR) is associated with recurrent pulmonary infections despite an intense inflammatory and immune response. We reported recently that TNF-alpha decreased gap junction connectivity in non-CF airway cells, a mechanism that was absent in CF cells expressing the DeltaPhe-508 mutant of CFTR. We have now identified the tyrosine kinase c-Src as a possible pathway between the mediators of inflammation and the gap junction protein connexin43 (Cx43). Indeed, TNF-alpha increased the proportion of activated c-Src in non-CF airway cells. Moreover, pharmacological antagonists and expression in non-CF cells of a dominant negative construct of c-Src prevented Cx43 channel closure by TNF-alpha. Finally, gap junction channel closure was prevented by expression of a Cx43 mutant lacking tyrosine phosphorylation sites for c-Src. Additional experiments showed that activation of c-Src was defective in CF airway cells but rescued in CFTR-corrected CF cells. These data suggest that CFTR dysfunction is associated with altered TNF-alpha signaling, resulting in the persistence of gap junction connectivity in CF airway cells. We propose that altered regulation of c-Src may contribute to the dysregulated inflammatory response that is characteristic of the CF phenotype.