Properties and regulation of gap junctional hemichannels in the plasma membranes of cultured cells

Isolated epithelial cells from amphibian urinary bladder express functional gap junctional hemichannels

Exposure of the urinary bladder epithelium of Necturus maculosus (NUB) to protease and collagenase yields approximately 50% isolated polarized cells. These cells express a membrane current slowly activated by depolarization or by removal of external divalent cations. The biophysical and pharmacological properties of the current are largely consistent with those of gap junctional hemichannels. After removal of divalent cations, the cells can also be loaded with 5(6)-carboxyfluorescein, a hydrophilic fluorescent anionic dye, and exposure to dye reduces the current in a manner dependent on membrane voltage and side of application. In contrast, Necturus gallbladder (NGB) cells exhibit no membrane conductance attributable to gap junctional hemichannels, although previous studies reveal the persistence of gap junction plaques on the plasma membrane. We conclude that functional gap junctional hemichannels can be expressed on the surface of certain isolated epithelial cells and that this is not a necessary consequence of the isolation procedure. These structures may contribute to cell damage under pathological conditions involving cell detachment.

Connexins regulate calcium signaling by controlling ATP release

Forced expression of gap junction proteins, connexins, enables gap junction-deficient cell lines to propagate intercellular calcium waves. Here, we show that ATP secretion from the poorly coupled cell lines, C6 glioma, HeLa, and U373 glioblastoma, is potentiated 5- to 15-fold by connexin expression. ATP release required purinergic receptor-activated intracellular Ca2+ mobilization and was inhibited by Cl- channel blockers. Calcium wave propagation also was reduced by purinergic receptor antagonists and by Cl- channel blockers but insensitive to gap junction inhibitors. These observations suggest that cell-to-cell signaling associated with connexin expression results from enhanced ATP release and not, as previously believed, from an increase in intercellular coupling.

Connexin-aequorin chimerae report cytoplasmic calcium environments along trafficking pathways leading to gap junction biogenesis in living COS-7 cells

The cytoplasmic calcium environments along membrane trafficking pathways leading to gap junction intercellular communication channels at the plasma membrane were studied. Connexins, the constitutive proteins of gap junctions, were fused at their carboxyl terminus to the calcium-sensitive photoprotein aequorin. The cellular location of the chimeric proteins was determined by immunolocalization and subcellular fractionation. The generation of functional gap junctions by the connexin chimerae was monitored by the ability of the cells to exchange small dyes. Although aequorin fused to connexin-26 was nonfunctional, its ability to report Ca2+ and to form functional gap junctions was rescued by replacement of its cytoplasmic carboxyl tail with that of connexin-43. In COS-7 cells expressing these connexin-aequorin chimerae, calcium levels below the plasma membrane were higher (approximately 5 microM) than those in the cytoplasm (approximately 100 nM); gap junctions were able to transfer dyes under these conditions. Cytoplasmic levels of free calcium surrounding the ERGIC/Golgi reported by connexin-43 chimera (approximately 420 nM) were twice those measured by connexin-32 chimera (approximately 200 nM); both chimerae measured calcium levels substantially higher than those reported by a connexin-26 chimera (approximately 130 nM). Dispersion of the ERGIC and Golgi complex by brefeldin A led to a marked reduction in calcium levels. The results show that the various connexin chimerae were located in spatially different subcellular stores and that the ERGIC/Golgi regions of the cell maintain heterogeneous cytoplasmic domains of calcium. The implications of the subplasma-membrane Ca2+ levels on the gating of gap junctions are discussed.

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.

Visualization and functional blocking of gap junction hemichannels (connexons) with antibodies against external loop domains in astrocytes

Astrocytes constitute a metabolically and electrically coupled syncytium which is essential for the regulation of ionic homeostasis of the interstitial brain fluid and for coordinating responses to neuronal activities. The structural avenues that couple individual astrocytes are provided by gap junctions which consist of transmembraneous channels that bridge the extracellular space. Each gap junction channel is composed of two hemichannels (connexons) that dock to each other via their extracellular loops. Here we report on the characterization of external loop antibodies that visualize hemichannels in specific plasma membranes of vital astrocytes. Hemichannels were found to be concentrated on cytoplasmic processes and filopodia of subconfluent cells. Specificity of hemichannel-binding was evaluated by blockage of Lucifer Yellow (LY) dye-transfer and Ca2+ transmission, as well as LY uptake under calcium-deficient conditions. Our data indicate that hemichannels are accessible from the extracytoplasmic side either for direct visualization or functional manipulation. The availability of such a probe will allow in vivo experiments which require selective and/or temporal blocking of gap junctions in animal models.

Synthesis, assembly and structure of gap junction intercellular channels

Gap junction membrane channels assemble as dodecameric complexes, in which a hexameric hemichannel (connexon) in one plasma membrane docks end to end with a connexon in the membrane of a closely apposed cell. Steps in the synthesis, assembly and turnover of gap junction channels appear to follow the general secretory pathway for membrane proteins. In addition to homo-oligomeric connexons, different connexin polypeptide subunits can also assemble as hetero-oligomers. The ability to form homotypic and heterotypic channels that consist of two identical or two different connexons, respectively, adds even greater versatility to the functional modulation of gap junction channels. Electron cryocrystallography of recombinant gap junction channels has recently provided direct evidence for alpha-helical folding of at least two of the transmembrane domains within each connexin subunit. The potential to correlate the structure and biochemistry of gap junction channels with recently identified human diseases involving connexin mutations makes this a particularly exciting area of research.

The ion selectivity of a membrane conductance inactivated by extracellular calcium in Xenopus oocytes

1. The ion selectivity of a membrane ion conductance that is inactivated by extracellular calcium (Ca2+o) in Xenopus oocytes has been studied using the voltage-clamp technique. 2. The reversal potential of the Ca2+o-sensitive current (Ic) was measured using voltage ramps (-80 to +40 mV) as a function of the external concentration (12-240 mM) of NaCl or KCl. The direction and amplitude of the shifts in reversal potentials are consistent with permeability ratios of 1:0.99:0.24 for K+:Na+:Cl-. 3. Current-voltage (I-V ) relations of Ic, determined during either voltage ramps of 0.5 s duration or at steady state, displayed pronounced rectification at both hyperpolarized and depolarized potentials. However, instantaneous I-V relations showed less rectification and could be fitted by the constant field equation assuming the above K+:Na+:Cl- permeability ratios. 4. Ion substitution experiments indicated that relatively large organic monovalent cations and anions are permeant through Ic channels with the permeability ratios K+:NMDG+:TEA+:TPA+:TBA+:Gluc- = 1:0.45:0. 35:0.2:0.2:0.2. 5. External amiloride (200 microM), gentamicin (220 microM), flufenamic acid (40 microM), niflumic acid (100 microM), Gd3+ (0.3 microM) or Ca2+ (200 microM) caused reversible block of Ic without changing its reversal potential. 6. Preinjection of oocytes with antisense oligonucleotide against connexin 38, the Xenopus hemi-gap-junctional protein, inhibited Ic by 80 % without affecting its ion selectivity, thus confirming and extending the recent suggestion of Ebihara that Ic represents current carried through hemi-gap-junctional channels. 7. In vitro and in vivo maturation of oocytes resulted in a significant decrease in Ic conductance to 7 % and 2 % of control values, respectively. This developmental downregulation of Ic minimizes any toxic effect Ic activation would have when the mature egg is released into Ca2+o-free pond water. 8. The results of this study are discussed in relation to other Ca2+o-inactivated conductances seen in a wide variety of cell types and which have previously been interpreted as arising either from Ca2+o-masked channels or from changes in the ion selectivity of voltage-gated Ca2+ or K+ channels.

Differential ganciclovir-mediated cytotoxicity and bystander killing in human colon carcinoma cell lines expressing herpes simplex virus thymidine kinase

The two human colon carcinoma cell lines HT-29 and SW620, which stably express herpes simplex virus thymidine kinase (HSV-TK), are sensitized to the cytotoxic effects of the antiviral drug ganciclovir (GCV). Compared with HT-29 cells, SW620 cells were more sensitive to lower GCV concentrations (<1 microM), accumulated GCV triphosphate more rapidly, and incorporated higher levels of GCV into DNA. Following a 24-hr exposure to 10 microM GCV, bystander killing was as much as sixfold greater in SW620 cells than HT-29 cells. This bystander effect was dependent on the level of HSV-TK expression, the number of cells expressing HSV-TK, and the overall confluency of the cells. However, bystander killing did not correlate with gap junctional intercellular communication as determined by microinjection of Lucifer Yellow fluorescent dye. SW620 cells were coupled to <3% adjacent cells (compared with >50% for HT-29 cells), but were still able to transfer phosphorylated GCV to bystander cells as soon as 4 hr after drug was added. These results emphasize the importance of cell-specific metabolism in HSV-TK/GCV-mediated cytotoxicity and may suggest a novel mechanism for bystander killing.

Isoform composition of connexin channels determines selectivity among second messengers and uncharged molecules

Intercellular connexin channels (gap junction channels) have long been thought to mediate molecular signaling between cells, but the nature of the signaling has been unclear. This study shows that connexin channels from native tissue have selective permeabilities, partially based on pore diameter, that discriminate among cytoplasmic second messenger molecules. Permeability was assessed by measurement of selective loss/retention of tracers from liposomes containing reconstituted connexin channels. The tracers employed were tritiated cyclic nucleotides and a series of oligomaltosaccharides derivatized with a small uncharged fluorescent moiety. The data define different size cut-off limits for permeability through homomeric connexin-32 channels and through heteromeric connexin-32/connexin-26 channels. Connexin-26 contributes to a narrowed pore. Both cAMP and cGMP were permeable through the homomeric connexin-32 channels. cAMP was permeable through only a fraction of the heteromeric channels. Surprisingly, cGMP was permeable through a substantially greater fraction of the heteromeric channels than was cAMP. The data suggest that isoform stoichiometry and/or arrangement within a connexin channel determines whether cyclic nucleotides can permeate, and which ones. This is the first evidence for connexin-specific selectivity among biological signaling molecules.

Peptides homologous to extracellular loop motifs of connexin 43 reversibly abolish rhythmic contractile activity in rabbit arteries

1. Phenylephrine (10 microM) evoked rises in tension in isolated rings of endothelium-denuded rabbit superior mesenteric artery. These increases consisted of a tonic component with superimposed rhythmic activity, the frequency of which generally remained constant over time but whose amplitude exhibited cycle-to-cycle variability. 2. The amplitude, but not the frequency, of the rhythmic activity was affected by a series of short peptides possessing sequence homology with extracellular loops 1 and 2 of connexin 43 (Cx43). Oscillatory behaviour was abolished at concentrations of 100-300 microM (IC50 of 20-30 microM), without change in average tone. No synergy was evident between peptides corresponding to the extracellular loops, and cytoplasmic loop peptides were biologically inactive. 3. The putative gap junction inhibitor heptanol mimicked the action of the extracellular loop peptides and abolished rhythmic activity at concentrations of 100-300 microM without effects on frequency. However, in marked contrast to the peptides, heptanol completely inhibited the contraction evoked by phenylephrine (IC50, 283 +/- 28 microM). 4. The presence of mRNA encoding Cx32, Cx40 and Cx43 was detected in the rabbit superior mesenteric artery by reverse transcriptase-polymerase chain reaction. Western blot analysis showed that Cx43 was the major connexin in the endothelium-denuded vessel wall. 5. We conclude that intercellular communication between vascular smooth muscle cells via gap junctions is essential for synchronized rhythmic activity in isolated arterial tissue, whereas tonic force development appears to be independent of cell-cell coupling. The molecular specificity of the peptide probes employed in the study suggests that the smooth muscle relaxant effects of heptanol may be non-specific and unrelated to inhibition of gap junctional communication.

Connexin46 is retained as monomers in a trans-Golgi compartment of osteoblastic cells

Connexins are gap junction proteins that form aqueous channels to interconnect adjacent cells. Rat osteoblasts express connexin43 (Cx43), which forms functional gap junctions at the cell surface. We have found that ROS 17/2.8 osteosarcoma cells, UMR 106-01 osteosarcoma cells, and primary rat calvarial osteoblastic cells also express another gap junction protein, Cx46. Cx46 is a major component of plasma membrane gap junctions in lens. In contrast, Cx46 expressed by osteoblastic cells was predominantly localized to an intracellular perinuclear compartment, which appeared to be an aspect of the TGN as determined by immunofluorescence colocalization. Hela cells transfected with rat Cx46 cDNA (Hela/Cx46) assembled Cx46 into functional gap junction channels at the cell surface. Both rat lens and Hela/Cx46 cells expressed 53-kD (nonphosphorylated) and 68-kD (phosphorylated) forms of Cx46; however, only the 53-kD form was produced by osteoblasts. To examine connexin assembly, monomers were resolved from oligomers by sucrose gradient velocity sedimentation analysis of 1% Triton X-100-solubilized extracts. While Cx43 was assembled into multimeric complexes, ROS cells contained only the monomer form of Cx46. In contrast, Cx46 expressed by rat lens and Hela/Cx46 cells was assembled into multimers. These studies suggest that assembly and cell surface expression of two closely related connexins were differentially regulated in the same cell. Furthermore, oligomerization may be required for connexin transport from the TGN to the cell surface.

Identification of a pore lining segment in gap junction hemichannels

The ability of certain connexins to form open hemichannels has been exploited to study the pore structure of gap junction (hemi)channels. Cysteine scanning mutagenesis was applied to cx46 and to a chimeric connexin, cx32E(1)43, which both form patent hemichannels when expressed in Xenopus oocytes. The thiol reagent maleimido-butyryl-biocytin was used to probe 12 cysteine replacement mutants in the first transmembrane segment and two in the amino-terminal segment. Maleimido-butyryl-biocytin was found to inhibit channel activity with cysteines in two equivalent positions in both connexins: I33C and M34C in cx32E(1)43 and I34C and L35C in cx46. These two positions in the first transmembrane segment are thus accessible from the extracellular space and consequently appear to contribute to the pore lining. The data also suggest that the pore structure is complex and may involve more than one transmembrane segment.