Gap junction-mediated intercellular communication in the immune system

Immune cells are usually considered non-attached blood cells, which would exclude the formation of gap junctions. This is a misconception since many immune cells express connexin 43 (Cx43) and other connexins and are often residing in tissue. The role of gap junctions is largely ignored by immunologists as is the immune system in the field of gap junction research. Here, the current knowledge of the distribution of connexins and the function of gap junctions in the immune system is discussed. Gap junctions appear to play many roles in antibody productions and specific immune responses and may be important in sensing danger in tissue by the immune system. Gap junctions not only transfer electrical and metabolical but also immunological information in the form of peptides for a process called cross-presentation. This is essential for proper immune responses to viruses and possibly tumours. Until now only 40 research papers on gap junctions in the immune system appeared and this will almost certainly expand with the increased mutual interest between the fields of immunology and gap junction research.     

Selective permeability of gap junction channels

Gap junctions mediate the transfer of small cytoplasmic molecules between adjacent cells. A family of gap junction proteins exist that form channels with unique properties, and differ in their ability to mediate the transfer of specific molecules. Mutations in a number of individual gap junction proteins, called connexins, cause specific human diseases. Therefore, it is important to understand how gap junctions selectively move molecules between cells. Rules that dictate the ability of a molecule to travel through gap junction channels are complex. In addition to molecular weight and size, the ability of a solute to transverse these channels depends on its net charge, shape, and interactions with specific connexins that constitute gap junctions in particular cells. This review presents some data and interpretations pertaining to mechanisms that govern the differential transfer of signals through gap junction channels.     

Intercellular communication and tissue growth: IX. Junctional membrane structure of hybrids between communication-competent and communication-incompetent cells

Summary The structure of the membrane junctions of the hybrid cell system, examined in the companion paper in respect to competence for communication through cell-to-cell membrane channels, is here examined by freeze-fracture electron microscopy. The junctions of the channel-competent parent cell and of the channel-competent hybrid cells present aggregates of intramembranous particles typical of gap junction; those of the channel-incompetent parent cell and channel-incompetent segregant hybrid cells do not. Competence for junctional communication and for gap junction formation are genetically related. The junctions of the intermediate hybrid cells with incomplete channel-competence (characterized by cell-to-cell transfer of small inorganic ions but not of fluorescein), present special intramembranous fibrillar structures instead of discrete gap-junctional particles. The possibility that these structures may constitute coupling elements with subnormal permeability is discussed in terms of incomplete dominance of the genetic determinants of gap junction.     

Gap junction formation between normal and reaggregated endoderm cells ofXenopus laevis neurulae

Summary Using freeze-fracture electron microscopy and fluorescent dye injection we have analysed the contacts between cells of the deeper endoderm taken from neurulae ofXenopus laevis. Endodermal cells in situ have large 1.5 m diameter gap junctions composed of 8 nm P-face particles and corresponding E-face pits. Beside gap junctions, particle aggregates typical of desmosomal plaques are present but there are no tight junctions. The dissociation of endoderm into single cells involves profound structural alterations in the surface membrane including the complete disappearance of junctional structures among them gap junctions. The reaggregation of endoderm cells leads to the restoration of the surface membrane IMP (Intra Membrane Particle) pattern and, after ca. 30 min, to the establishment of functional pathways allowing for the intercellular transfer of fluorescent dye. Concomitantly gap junctions reappear. The observation that the dissociation and reaggregation of endodermal cells involves IMP alterations which go beyond the cell junctions themselves is discussed as an adaptation of the plasma membrane to changing environmental conditions.     

Distribution and role of gap junctions in normal myocardium and human ischaemic heart disease

In the heart, individual cardiac muscle cells are linked by gap junctions. These junctions form low resistance pathways along which the electrical impulse flows rapidly and repeatedly between all the cells of the myocardium, ensuring their synchronous contraction. To obtain probes for mapping the distribution of gap junctions in cardiac tissue, polyclonal antisera were raised to three synthetic peptides, each matching different cytoplasmically exposed portions of the sequence of connexin43, the major gap-junctional protein reported in the heart. The specificity of each antiserum for the peptide to which it was raised was established by dot blotting. New methods were developed for isolating enriched fractions of gap junctions from whole heart and from dissociated adult myocytes, in which detergent-treatment and raising the temperature (potentially damaging steps in previously described techniques) are avoided. Analysis of these fractions by SDS-polyacrylamide gel electrophoresis revealed major bands at 43 kDa (matching the molecular mass of connexin43) and at 70 kDa. Western blot experiments using our antisera indicated that both the 43-kDa and the 70-kDa bands represent cardiac gap-junctional proteins. Pre-embedding immunogold labelling of isolated gap junctions and post-embedding immunogold labelling of Lowicryl-embedded whole tissue demonstrated the specific binding of the antibodies to ultrastructurally defined gap junctions. One antiserum (raised to residues 131–142) was found to be particularly effective for cytochemical labelling. Using this antiserum for immunofluorescence labelling in combination with confocal scanning laser microscopy enabled highly sensitive detection and three-dimensional mapping of gap junctions through thick slices of cardiac tissue. By means of the serial optical sectioning ability of the confocal microscope, images of the entire gap junction population of complete en face-viewed disks were reconstructed. These reconstructions reveal the presence of large junctions arranged as a peripheral ring around the disk, with smaller junctions in an interior zone: an arrangement that may facilitate efficient intercellular transfer of current. By applying our immunolabelling techniques to tissue from hearts removed from transplant patients with advanced ischaemic heart disease, we have demonstrated that gap junction distribution between myocytes at the border zone of healed infarcts is markedly disordered. This abnormality may contribute to the genesis of reentrant arrhythmias in ischaemic heart disease.     

Intercellular communication between follicular angiotensin receptors and Xenopus laevis oocytes: medication by an inositol 1,4,5- trisphosphate-dependent mechanism

In Xenopus laevis oocytes, activation of angiotensin II (AII) receptors on the surrounding follicular cells sends a signal through gap junctions to elevate cytoplasmic calcium concentration ([Ca2+]i) within the oocyte. The two major candidates for signal transfer through gap junctions into the oocyte during AII receptor stimulation are Ins(1,4,5)P3 and Ca2+. In [3H]inositol-injected follicular oocytes, AII stimulated two- to fourfold increases in phosphoinositide hydrolysis and production of inositol phosphates. Injection of the glycosaminoglycan, heparin, which selectively blocks Ins(1,4,5)P3 receptors, prevented both AII-stimulated and Ins(1,4,5)P3-induced Ca2+ mobilization in Xenopus follicular oocytes but did not affect mobilization of Ca2+ by ionomycin or GTP. These results indicate that the AII-regulated process of gap junction communication between follicular cells and the oocyte operates through an Ins(1,4,5)P3-dependent mechanism rather than through transfer of Ca2+ into the ooplasm and subsequent Ca(2+)-induced Ca2+ release.     

The gap junction proteins: vive la différence!

The intercellular junctions connecting the cytoplasms of fibre cells in the mammalian lens have until recently been regarded as a class of junction which is fundamentally different from that of the gap junctions in other organs. Recent observations, however, suggest that the lens junctions fit protein topology predictions common for all gap junctions. While the homologous peptide portions are predicted to form the channels, the divergent peptide portions of the gap junction polypeptides may adapt channel activity to the special tissue requirements.     

Dye transfer between cells of the embryonic chick lens becomes less sensitive to CO2 treatment with development

During the 3-h developmental stage 14 in the chick, intercellular transfer of iontophoresed fluorescent dyes becomes less sensitive to the lowering of intracellular pH by either CO2 or acetate ions. Up to developmental state 14, dye transfer between lens cells is reversibly blocked by exposure to 50% CO2. Beyond stage 14, dye transfer between these cells is no longer reversibly blocked by elevated pCO2. Electronic coupling is present throughout lens development and is not reversibly blocked by high pCO2 at any stage. The gap junctions joining the lens cells show morphological changes at developmental stage 14. Up to stage 14, all gap junctions observed between chick lens cells have connexon assemblies that appear condensed or crystalline following routine freeze-fracture microscopy. Beyond stage 14, chick lens cells express gap junctions with both the condensed assemblies and the dispersed assemblies characteristic of adult lens gap-junction structure.     

A catalytic antibody produces fluorescent tracers of gap junction communication in living cells

The antibody 38C2 efficiently catalyzed a retro-Michael reaction to convert a novel, cell-permeable fluorogenic substrate into fluorescein within living cells. In vitro, the antibody converted the substrate to fluorescein with a k(cat) of 1.7 x 10(-5) s(-1) and a catalytic proficiency (k(cat)/k(uncat)K(m)) of 1.4 x 10(10) m(-1) (K(m) = 7 microm). For hybridoma cells expressing antibody or Chinese Hamster Ovarian (CHO) cells injected with antibody, incubation of the substrate in the extracellular medium resulted in bright intracellular fluorescence distinguishable from autofluorescence or noncatalyzed conversion of substrate. CHO cells loaded with antibody were 12 times brighter than control cells, and more than 85% of injected cells became fluorescent. The fluorescein produced by the antibody traveled into neighboring cells through gap junctions, as demonstrated by blocking dye transfer using the gap junction inhibitor oleamide. The presence of functional gap junctions in CHO cells was confirmed through oleamide inhibition of lucifer yellow transfer. These studies demonstrate the utility of the intracellular antibody reaction, which could generate tracer dyes in specific cells within complex multicellular environments simply by bathing the system in substrate.     

2-Methoxyethanol inhibits gap junctional communication in rat myometrial myocytes

The glycol ethers 2-methoxyethanol (2-ME) and 2-ethoxyethanol (2-EE) prolong gestation in rodents. Because gap junctions in the myometrium likely facilitate parturition, the present study examined inhibition of gap junctional communication by 2-ME and 2-EE in myometrial smooth-muscle cell cultures. To measure gap junctional communication, the fluorescent dye Lucifer yellow was injected into cultured cells and the transfer of the dye to adjacent cells was scored with epifluorescence microscopy. The data are presented as the percentage of cells adjacent to the microinjected cell that exhibited dye following microinjection. A 30 min treatment with 32 or 63 mmol/L 2-ME decreased dye transfer to 71% and 63%, respectively (p0.05; control 90%). Similarly, 2-EE inhibited dye transfer, although myometrial cells were less sensitive to 2-EE compared to 2-ME. Dye transfer returned to control levels after 2 h in the continued presence of 2-ME. The primary metabolite of 2-ME, methoxyacetic acid (MAA), had no effect on dye transfer at concentrations equimolar to 2-ME. Because 2-ME and 2-EE inhibited gap junctional communication only at high concentrations and because the inhibition reversed in the continued presence of the compounds, it is suggested that glycol ethers delay parturition by a mechanism independent of a direct action on myometrial gap junctions.     

LAMP, a new imaging assay of gap junctional communication unveils that Ca2+ influx inhibits cell coupling

Using a new class of photo-activatible fluorophores, we have developed a new imaging technique for measuring molecular transfer rates across gap junction connexin channels in intact living cells. This technique, named LAMP, involves local activation of a molecular fluorescent probe, NPE-HCCC2/AM, to optically label a cell. Subsequent dye transfer through gap junctions from labeled to unlabeled cells was quantified by fluorescence microscopy. Additional uncagings after prior dye transfers reached equilibrium enabled multiple measurements of dye transfer rates in the same coupled cell pair. Measurements in the same cell pair minimized variation due to differences in cell volume and number of gap junctions, allowing us to track acute changes in gap junction permeability. We applied the technique to study the regulation of gap junction coupling by intracellular Ca(2+) ([Ca(2+)](i)). Although agonist or ionomycin exposure can raise bulk [Ca(2+)](i) to levels higher than those caused by capacitative Ca(2+) influx, the LAMP assay revealed that only Ca(2+) influx through the plasma membrane store-operated Ca(2+) channels strongly reduced gap junction coupling. The noninvasive and quantitative nature of this imaging technique should facilitate future investigations of the dynamic regulation of gap junction communication.     

Biochemical and functional characterization of intercellular adhesion and gap junctions in fibroblasts

Despite their significance inwound healing, little is known about the molecular determinants ofcell-to-cell adhesion and gap junctional communication in fibroblasts.We characterized intercellular adherens junctions and gap junctions inhuman gingival fibroblasts (HGFs) using a novel model. Calcein-labeleddonor cells in suspension were added onto an established, Texas red dextran (10 kDa)-labeled acceptor cell monolayer. Cell-to-cell adhesionrequired Ca²⁺ and was >30-fold stronger thancell-to-fibronectin adhesion at 15 min. Electron micrographs showedrapid formation of adherens junction-like structures at ~15 min thatmatured by ~2-3 h; distinct gap junctional complexes wereevident by ~3 h. Immunoblotting showed that HGF expressed -cateninand that cadherins and connexin43 were recruited to theTriton-insoluble cytoskeletal fraction in confluent cultures. Confocalmicroscopy localized the same molecules to intercellular contacts ofacceptor and donor cells. There was extensive calcein dye transfer in acohort of Texas red dextran-labeled cells, but this was almostcompletely abolished by the gap junction inhibitor -glycyrrhetinicacid and the connexin43 mimetic peptide GAP 27. Thisdonor-acceptor cell model allows large numbers (>10⁵) ofcells to form synchronous cell-to-cell contacts, thereby enabling thesimultaneous functional and molecular studies of adherens junctions andgap junctions.

     

Intercellular junctions and transfer of small molecules in primary vascular endothelial cultures

The ultrastructure of gap and tight junctions and the cell-to-cell transfer of small molecules were studied in primary cultures and freshly isolated sheets of endothelial cells from calf aortae and umbilical veins. In thin sections and in freeze-fracture replicas, the gap and tight junctions in the freshly isolated cells from both sources appeared similar to those found in the intimal endothelium. Most of the interfaces in replicas had complex arrays of multiple gap junctions either intercalated within tight junction networks or interconnected by linear particle strands. The particle density in the center of most gap junctions was noticeably reduced. In confluent monolayers, after 3-5 days in culture, gap and tight junctions were present, although reduced in complexity and apparent extent. Despite the relative simplicity of the junctions, the cell-to-cell transfer of potential changes, dye (Lucifer Yellow CH), and nucleotides was readily detectable in cultures of both endothelial cell types. The extent and rapidity of dye transfer in culture was only slightly less than that in sheets of freshly isolated cells, perhaps reflecting a reduced gap junctional area combined with an increase in cell size in vitro.     

Patterns of junctional communication during development of the early amphibian embryo

Cell-cell communication through gap junctions was examined in Xenopus laevis embryos between the 16-cell and early blastula stages using Lucifer Yellow, Fluorescein, lead EDTA and dicyanoargentate as probes of junctional permeability. Injections were made into cells whose position was identified with respect to the primary cleavage axis and the grey crescent. FITC dextrans revealed cytoplasmic bridges between the injected cell and its sister only. In the animal pole at the 16-cell stage at the future dorsal side of the embryo, Lucifer Yellow was frequently and extensively transferred between cells through gap junctions. At the future ventral side gap junctional transfer of Lucifer Yellow was significantly less frequent and less extensive. The asymmetry of transfer between future dorsal and ventral sides of the animal pole was more marked at the 32-cell stage. In the vegetal pole also at the 32-cell stage, a dorsoventral difference in junctional permeability to Lucifer Yellow was observed. At the 64-cell stage the transfer of Lucifer Yellow was relatively frequent between cells lying in the same radial segment in the animal pole; transfer into cells outside each segment was infrequent, except at the grey crescent. At the 128-cell stage, Lucifer transfer between future dorsal or future ventral cells in the equatorial region was infrequent. A high incidence of transfer was restored at the future dorsal side at the 256-cell stage. At the 32-cell stage, fluorescein was infrequently transferred between animal pole cells although lead EDTA moved from cell to cell with high, comparable frequency in future dorsal and ventral regions. Dicyanoargentate always transferred extensively, both at the 32- and 64-cell stages. Treatment of embryos with methylamine raised intracellular pH by 0.15 units, increased the electrical conductance of the gap junction and produced a 10-fold increase in the frequency of Lucifer Yellow transfer through gap junctions in future ventral regions of the animal pole at the 32-cell stage.     

Gap Junction Assembly: Multiple Connexin Fluorophores Identify Complex Trafficking Pathways

The assembly of gap junction channels was studied using mammalian cells expressing connexin (Cx) 26, 32 and 43 in which the carboxyl terminus was fused to green, yellow or cyan fluorescent proteins (GFP, YFP, CFP). Intracellular targeting of Cx32-CFP and 43-GFP to gap junctions was disrupted by brefeldin A treatment and resulted in a severe loss of gap junctional intercellular communication reflected by low intercellular dye transfer. Cells expressing Cx43-GFP exposed to nocodazole showed normal targeting to gap junctions and dye transfer. Cx32 and 43 thus appear to be transported and assembled into gap junctions via the classical secretory pathway. In contrast, we found that assembly of Cx26-GFP into functional gap junctions was relatively unaffected by treatment of cells with brefeldin A, but was extremely sensitive to nocodazole treatment. Coexpression of Cx26-YFP and Cx32-CFP indicated a different intracellular distribution that was accentuated in the presence of brefeldin A, with the gap junctions in these cells constructed predominantly of Cx26-YFP. A site specific mutation in the first transmembrane domain that distinguished Cx32 from Cx26 (Cx32128L) resulted in the adoption of the trafficking properties of Cx26 as well as its unusual post-translational membrane integration characteristics. The results indicate that multiple intracellular connexin trafficking routes exist and provide a further mechanism for regulating the connexin composition of gap junctions and thus specificity in intercellular signalling.     

Gap Junction Assembly: Multiple Connexin Fluorophores Identify Complex Trafficking Pathways

The assembly of gap junction channels was studied using mammalian cells expressing connexin (Cx) 26, 32 and 43 in which the carboxyl terminus was fused to green, yellow or cyan fluorescent proteins (GFP, YFP, CFP). Intracellular targeting of Cx32-CFP and 43-GFP to gap junctions was disrupted by brefeldin A treatment and resulted in a severe loss of gap junctional intercellular communication reflected by low intercellular dye transfer. Cells expressing Cx43-GFP exposed to nocodazole showed normal targeting to gap junctions and dye transfer. Cx32 and 43 thus appear to be transported and assembled into gap junctions via the classical secretory pathway. In contrast, we found that assembly of Cx26-GFP into functional gap junctions was relatively unaffected by treatment of cells with brefeldin A, but was extremely sensitive to nocodazole treatment. Coexpression of Cx26-YFP and Cx32-CFP indicated a different intracellular distribution that was accentuated in the presence of brefeldin A, with the gap junctions in these cells constructed predominantly of Cx26-YFP. A site specific mutation in the first transmembrane domain that distinguished Cx32 from Cx26 (Cx32128L) resulted in the adoption of the trafficking properties of Cx26 as well as its unusual post-translational membrane integration characteristics. The results indicate that multiple intracellular connexin trafficking routes exist and provide a further mechanism for regulating the connexin composition of gap junctions and thus specificity in intercellular signalling.     

Gap junction assembly: multiple connexin fluorophores identify complex trafficking pathways

The assembly of gap junction channels was studied using mammalian cells expressing connexin (Cx) 26, 32 and 43 in which the carboxyl terminus was fused to green, yellow or cyan fluorescent proteins (GFP, YFP, CFP). Intracellular targeting of Cx32-CFP and 43-GFP to gap junctions was disrupted by brefeldin A treatment and resulted in a severe loss of gap junctional intercellular communication reflected by low intercellular dye transfer. Cells expressing Cx43-GFP exposed to nocodazole showed normal targeting to gap junctions and dye transfer. Cx32 and 43 thus appear to be transported and assembled into gap junctions via the classical secretory pathway. In contrast, we found that assembly of Cx26-GFP into functional gap junctions was relatively unaffected by treatment of cells with brefeldin A, but was extremely sensitive to nocodazole treatment. Coexpression of Cx26-YFP and Cx32-CFP indicated a different intracellular distribution that was accentuated in the presence of brefeldin A, with the gap junctions in these cells constructed predominantly of Cx26-YFP. A site specific mutation in the first transmembrane domain that distinguished Cx32 from Cx26 (Cx32128L) resulted in the adoption of the trafficking properties of Cx26 as well as its unusual post-translational membrane integration characteristics. The results indicate that multiple intracellular connexin trafficking routes exist and provide a further mechanism for regulating the connexin composition of gap junctions and thus specificity in intercellular signalling.     

Intercellular communication in a positional field. Ultrastructural correlates and tracer analysis of communication between insect epidermal cells.

The junctional membrane in the epidermal cells of the larval beetle (Tenebrio molitor L.) is comprised of macular gap junctions embedded in septate junctions. Ultrastructural and morphometric analysis of the distribution of gap junctions within the segmental epidermis suggests that this junction alone could account for the high electrotonic coupling recorded for the epidermal sheet. Analysis of the lanthanum-impregnated septate junction makes it doubtful that this junction serves as a communicating channel between beetle cells. A new model for the septate junction is presented in which pleated septa, less than 30 A thick, connect adjacent plasma membranes; the septa themselves are interconnected by two interseptal platforms that are coplanar with the plasma membranes. Iontophoretic injection of organic tracers into single epidermal cells suggests that only molecules of less than MW 1000 can transfer between cells through low-resistance junctions.     

A Functional Assay for Gap Junctional Examination; Electroporation of Adherent Cells on Indium-Tin Oxide

In this technique, cells are cultured on a glass slide that is partly coated with indium-tin oxide (ITO), a transparent, electrically conductive material. A variety of molecules, such as peptides or oligonucleotides can be introduced into essentially 100% of the cells in a non-traumatic manner.  Here, we describe how it can be used to study intercellular, gap junctional communication. Lucifer yellow penetrates into the cells when an electric pulse, applied to the conductive surface on which they are growing, causes pores to form through the cell membrane. This is electroporation. Cells growing on the nonconductive glass surface immediately adjacent to the electroporated region do not take up Lucifer yellow by electroporation but do acquire the fluorescent dye as it is passed to them via gap junctions that link them to the electroporated cells. The results of the transfer of dye from cell to cell can be observed microscopically under fluorescence illumination. This technique allows for precise quantitation of gap junctional communication. In addition, it can be used for the introduction of peptides or other non-permeant molecules, and the transfer of small electroporated peptides via gap junctions to inhibit the signal in the adjacent, non-electroporated cells is a powerful demonstration of signal inhibition.     

Reduced gap junctional communication is associated with the lethal condition characteristic of DDK mouse eggs fertilized by foreign sperm

Communication through gap junctions was examined in 8-cell zygotes generated by fertilization of eggs of the DDK inbred strain of mice with spermatozoa of the C3H strain. These zygotes spontaneously begin to extrude cells at the late 16-cell stage and 95% die by the blastocyst stage. The transfer of Lucifer Yellow between cells of DDK/C3H zygotes that had not yet begun to express the defect was significantly slower than in DDK/DDK controls or in controls from other strains. Treatment with the weak base methylamine, to raise intracellular pH, speeded the transfer of Lucifer in all strains; transfer between cells of DDK/C3H zygotes became as fast as that between cells of control zygotes. DDK/C3H zygotes cultured in methylamine either from the 4- to 8-cell stage to the early 16-cell stage (19h) or from the early to the late 16-cell stage (6 h) showed significant rescue to the blastocyst stage. Once spontaneous decompaction of cells from DDK/C3H zygotes had begun (the late 16-cell stage onwards) methylamine treatment was no longer able to bring about rescue. We conclude that zygotes developed from eggs of the DDK strain fertilized by foreign spermatozoa are characterized physiologically by defective gap junctional communication. Improving gap junctional communication is sufficient to allow many zygotes to maintain the compacted state, suggesting a link between compaction and communication through gap junctions.