Single channel behavior of recombinant beta 2 gap junction connexons reconstituted into planar lipid bilayers

Cryo-EM structure of an open conformation of a gap junction hemichannel in lipid bilayer nanodiscs

To mediate cell-to-cell communication via gap junction channels (GJCs), connexins (Cx) traffic as hexameric hemichannels to the plasma membrane, which dock end-to-end between adjacent cell membranes, thereby forming a dodecameric intercellular conduit. Hemichannels also function independently to mediate the passage of contents between the cytoplasm and extracellular space. To generate hemichannels, the mutation N176Y was introduced into the second extracellular loop of Cx26. The electron cryomicroscopy structure of the hexameric hemichannel in lipid bilayer nanodiscs displays an open pore and a 4-helix bundle transmembrane design that is nearly identical to dodecameric GJCs. In contrast to the high resolution of the transmembrane α-helices, the extracellular loops are less well resolved. The conformational flexibility of the extracellular loops may be essential to facilitate surveillance of hemichannels in apposed cells to identify compatible Cx isoforms that enable intercellular docking. Our results also provide a structural foundation for previous electrophysiologic and permeation studies of Cx hemichannels.

Electrical coupling and its channels

Harris explores the development of our current understanding of electrical coupling between cells and the channels that mediate it, highlighting the contributions of the Journal of General Physiology.

As the physiology of synapses began to be explored in the 1950s, it became clear that electrical communication between neurons could not always be explained by chemical transmission. Instead, careful studies pointed to a direct intercellular pathway of current flow and to the anatomical structure that was (eventually) called the gap junction. The mechanism of intercellular current flow was simple compared with chemical transmission, but the consequences of electrical signaling in excitable tissues were not. With the recognition that channels were a means of passive ion movement across membranes, the character and behavior of gap junction channels came under scrutiny. It became evident that these gated channels mediated intercellular transfer of small molecules as well as atomic ions, thereby mediating chemical, as well as electrical, signaling. Members of the responsible protein family in vertebrates—connexins—were cloned and their channels studied by many of the increasingly biophysical techniques that were being applied to other channels. As described here, much of the evolution of the field, from electrical coupling to channel structure–function, has appeared in the pages of the Journal of General Physiology.

Structural determinants and proliferative consequences of connexin 37 hemichannel function in insulinoma cells

Connexin (Cx) 37 suppresses vascular and cancer cell proliferation. The C terminus and a channel able to function are necessary, and neither by itself is sufficient, for Cx37 to mediate growth suppression. Cx37 supports transmembrane and intercellular signaling by forming functional hemichannels (HCs) and gap junction channels (GJCs), respectively. Here we determined whether Cx37 with HC, but not GJC, functionality would suppress proliferation of rat insulinoma (Rin) cells comparably to wild-type Cx37 (Cx37-WT). We mutated extracellular loop residues hypothesized to compromise HC docking but not HC function (six cysteines mutated to alanine, C54A,C61A,C65A, C187A,C192A,C198A (designated as C6A); N55I; and Q58L). All three mutants trafficked to the plasma membrane and formed protein plaques comparably to Cx37-WT. None of the mutants formed functional GJCs, and Cx37-C6A did not form functional HCs. Cx37-N55I and -Q58L formed HCs with behavior and permeation properties similar to Cx37-WT (especially Q58L), but none of the mutants suppressed Rin cell proliferation. The data indicate that determinants of Cx37 HC function differ from other Cxs and that HC functions with associated HC-supported protein-protein interactions are not sufficient for Cx37 to suppress Rin cell proliferation. Together with previously published data, these results suggest that Cx37 suppresses Rin cell proliferation only when in a specific conformation achieved by interaction of the C terminus with a Cx37 pore-forming domain able to open as a GJC.

Functional analysis and regulation of purified connexin hemichannels

Gap-junction channels (GJCs) are aqueous channels that communicate adjacent cells. They are formed by head-to-head association of two hemichannels (HCs), one from each of the adjacent cells. Functional HCs are connexin hexamers composed of one or more connexin isoforms. Deafness is the most frequent sensineural disorder, and mutations of Cx26 are the most common cause of genetic deafness. Cx43 is the most ubiquitous connexin, expressed in many organs, tissues, and cell types, including heart, brain, and kidney. Alterations in its expression and function play important roles in the pathophysiology of very frequent medical problems such as those related to cardiac and brain ischemia. There is extensive information on the relationship between phosphorylation and Cx43 targeting, location, and function from experiments in cells and organs in normal and pathological conditions. However, the molecular mechanisms of Cx43 regulation by phosphorylation are hard to tackle in complex systems. Here, we present the use of purified HCs as a model for functional and structural studies. Cx26 and Cx43 are the only isoforms that have been purified, reconstituted, and subjected to functional and structural analysis. Purified Cx26 and Cx43 HCs have properties compatible with those demonstrated in cells, and present methodologies for the functional analysis of purified HCs reconstituted in liposomes. We show that phosphorylation of serine 368 by PKC produces a partial closure of the Cx43 HCs, changing solute selectivity. We also present evidence that the effect of phosphorylation is highly cooperative, requiring modification of several connexin subunits, and that phosphorylation of serine 368 elicits conformational changes in the purified HCs. The use of purified HCs is starting to provide critical data to understand the regulation of HCs at the molecular level.

Functional expression of Ca²⁺ dependent mammalian transmembrane gap junction protein Cx43 in slime mold Dictyostelium discoideum

In this paper, we expressed murine gap junction protein Cx43 in Dictyostelium discoideum by introducing the specific vector pDXA. In the first step, the successful expression of Cx43 and Cx43-eGFP was verified by (a) Western blot (anti-Cx43, anti-GFP), (b) fluorescence microscopy (eGFP-Cx43 co-expression, Cx43 immunostaining), and (c) flow cytometry analysis (eGFP-Cx43 co-expression). Although the fluorescence signals from cells expressing Cx43-eGFP detected by fluorescence microscopy seem relatively low, analysis by flow cytometry demonstrated that more than 60% of cells expressed Cx43-eGFP. In order to evaluate the function of expressed Cx43 in D. discoideum, we examined the hemi-channel function of Cx43. In this series of experiments, the passive uptake of carboxyfluorescein was monitored using flow cytometric analysis. A significant number of the transfected cells showed a prominent dye uptake in the absence of Ca(2+). The dye uptake by transfected cells in the presence of Ca(2+) was even lower than the non-specific dye uptake by non-transformed Ax3 orf+ cells, confirming that Cx43 expressed in D. discoideum retains its Ca(2+)-dependent, specific gating function. The expression of gap junction proteins expressed in slime molds opens a possibility to the biological significance of intercellular communications in development and maintenance of multicellular organisms.

The M34A mutant of Connexin26 reveals active conductance states in pore-suspending membranes

Connexin26 (Cx26) is a member of the connexin family, the building blocks for gap junction intercellular channels. These dodecameric assemblies are involved in gap junction-mediated cell-cell communication allowing the passage of ions and small molecules between two neighboring cells. Mutations in Cx26 lead to the disruption of gap junction-mediated intercellular communication with consequences such as hearing loss and skin disorders. We show here that a mutant of Cx26, M34A, forms an active hemichannel in lipid bilayer experiments. A comparison with the Cx26 wild-type is presented. Two different techniques using micro/nano-structured substrates for the formation of pore-suspending lipid membranes are used. We reconstituted the Cx26 wild-type and Cx26M34A into artificial lipid bilayers and observed single channel activity for each technique, with conductance levels of around 35, 70 and 165 pS for the wild-type. The conductance levels of Cx26M34A were found at around 45 and 70 pS.

Projection structure of a N-terminal deletion mutant of connexin 26 channel with decreased central pore density

Gated gap junction channels are important cellular conduits for establishing and maintaining intercellular communication. The three-dimensional structure of a mutant human connexin 26 (Cx26M34A) by electron cryocrystallography revealed a plug-like density in the channel pore suggesting that physical blockage of the pore may be one mechanism of closure (Oshima et al. 2007, Proc Natl Acad Sci USA 104: 10034-10039). However, it remains to be determined what part of the sequence contributes to the plug. Here, we present the projection structure of an N-terminus deletion of Cx26M34A missing amino acids 2 to 7 (Cx26M34Adel2-7) crystallized in the same two-dimensional crystal form. A 10 A resolution projection map of Cx26M34Adel2-7 revealed that the plug density was dramatically reduced in comparison with that found in full-length Cx26 channel. The difference map between the deletion and full-length Cx26M34A channels strongly suggests that the N-terminus of connexin contributes to the plug for the physical closure of gap junction channels.

Isoelectric points and post-translational modifications of connexin26 and connexin32

The isoelectric points of the gap junction proteins connexin26 (Cx26) and connexin32 (Cx32) were determined by isoelectric focusing in free fluids. The isoelectric points were significantly more acidic than predicted from amino acid sequences and different from each other, allowing homomeric channels to be resolved separately. The isoelectric points of the homomeric channels bracketed the isoelectric points of heteromeric Cx26/Cx32 channels. For heteromeric channels, Cx26 and Cx32 were found in overlapping, pH-focused fractions, indicating quaternary structure was retained. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used to identify post-translational modifications of Cx26 and Cx32 cytoplasmic domains, including the first reported post-translational modifications of Cx26. Suspected modifications were hydroxylation and/or phosphorylation near the amino terminus of both connexins, gamma-carboxyglutamate residues in the cytoplasmic loop of both connexins, phosphorylation in the carboxyl-terminal domain of Cx32, and palmitoylation at the carboxyl-terminus of Cx32. These modifications contribute to the measured acidic isoelectric points of Cx26 and Cx32, whereas their low molecular masses would not appreciably change connexin SDS-PAGE mobility. Most of these modifications have not previously been identified for connexins and may be instrumental in guiding and understanding novel aspects of channel trafficking and molecular mechanisms of channel regulation.

Gap junction channels reconstituted in two closely apposed lipid bilayers

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

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

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

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.

Roles of Met-34, Cys-64, and Arg-75 in the assembly of human connexin 26. Implication for key amino acid residues for channel formation and function

Connexins form a family of membrane proteins that assemble into communication channels and directly connect the cytoplasms of adjoining cells. Malfunctioning of connexin channels often cause disease, such as the mutations M34T and R75W in human connexin 26, which are associated with hereditary deafness. Another residue known to be essential for normal channel activity in the connexin is Cys-64. To obtain structural and functional insights of connexin 26, we studied the roles of these three residues by expressing mutant connexins in insect Sf9 and HeLa cells. The M34T and M34A mutants both formed gap junction plaques, but dye transfer assays showed that the M34A mutant had a significantly reduced permeability, suggesting that for proper channel function a side chain of adequate size is required at this position. We propose that Met-34 is located in the innermost helix of the channel, where it ensures a fully open channel structure via interactions with other transmembrane helices. Gap junction channels formed by the R75W and R75D mutants dissociated upon solubilization in dodecyl maltoside, whereas the R75A mutant remained hexameric. All gap junctions formed by Arg-75 mutants also showed only negligible activity in dye transfer experiments. These results suggest that residue Arg-75 plays a role in subunit interactions needed to retain a functional and stable connexin hexamer. The C64S mutant was suggested to be defective in oligomerization and/or protein folding even in the presence of wild-type connexin.

Dual mechanism of intercellular communication in HOBIT osteoblastic cells: a role for gap-junctional hemichannels

Intercellular communication allows tissue coordination of cell metabolism and sensitivity to extracellular stimuli. Paracrine stimulation and cell-to-cell coupling through gap junctions induce the formation of complex cellular networks, which favors the intercellular exchange of nutrients and second messengers. Intercellular Ca2+ signaling was investigated in human osteoblast-like initial transfectant (HOBIT) cells, a human osteoblastic cell line in which cells retain most of the osteoblastic differentiation markers. HOBIT cells express connexin43 (Cx43) clustered at the cell-to-cell boundary and display functional intercellular coupling as assessed by the intercellular transfer of Lucifer yellow. Mechanical stimulation of a single cell induced a wave of increased Ca2+ that was radially propagated to surrounding cells. Treatment of cells with thapsigargin blocked mechanically induced signal propagation. Intercellular Ca2+ spreading and dye transfer were inhibited by 18alpha-glycyrrhetinic acid (18-GA), showing the involvement of gap junctions in signal propagation. Pretreatment of cells with suramin or with apyrase decreased the extent of wave propagation, suggesting that ATP-mediated paracrine stimulation contribute to cell-to-cell signaling. The functional expression of gap-junctional hemichannels was evidenced in experiments of Mn2+ quenching, extracellular dye uptake, and intracellular Ca2+ release, activated by uptake of inositol 1,4,5-trisphosphate (InsP3) from the external medium. Gap-junctional hemichannels were activated by low extracellular Ca2+ concentrations and inhibited by 18-GA. A role for Cx hemichannels in adenosine triphosphate (ATP) release and paracrine stimulation is suggested.

Connexins/connexons. Cell-free expression

With a few exceptions, all secretory and plasma membrane proteins studied to date are synthesized in the endoplasmic reticulum (ER) membrane. Then, they are transported by successive vesicle budding and fusion from the ER through the Golgi stacks to the plasma membrane following the general intracellular transport route referred to as secretory pathway (originally reviewed in 1). Gap junction connexins have been shown to follow this pathway.

Biosynthesis and structural composition of gap junction intercellular membrane channels

Gap junction channels assemble as dodecameric complexes, in which a hexameric connexon (hemichannel) in one plasma membrane docks end-to-end with a connexon in the membrane of a closely apposed cell to provide direct cell-to-cell communication. Synthesis, assembly, and trafficking of the gap junction channel subunit proteins referred to as connexins, largely appear to follow the general secretory pathway for membrane proteins. The connexin subunits can assemble into homo-, as well as distinct hetero-oligomeric connexons. Assembly appears to be based on specific signals located within the connexin polypeptides. Plaque formation by the clustering of gap junction channels in the plane of the membrane, as well as channel degradation are poorly understood processes that are topics of current research. Recently, we tagged connexins with the autofluorescent reporter green fluorescent protein (GFP), and its cyan (CFP), and yellow (YFP) color variants and combined this reporter technology with single, and dual-color, high resolution deconvolution microscopy, computational volume rendering, and time-lapse microscopy to examine the detailed organization, structural composition, and dynamics of gap junctions in live cells. This technology provided for the first time a realistic, three-dimensional impression of gap junctions as they appear in the plasma membranes of adjoining cells, and revealed an excitingly detailed structural organization of gap junctions never seen before in live cells. Here, I summarize recent progress in areas encompassing the synthesis, assembly and structural composition of gap junctions with a special emphasis on the recent results we obtained using cell-free translation/ membrane-protein translocation, and autofluorescent reporters in combination with live-cell deconvolution microscopy.

Developmental expression and assembly of connexins into homomeric and heteromeric gap junction hemichannels in the mouse mammary gland

During the development of the mammary gland, duct-lining epithelial cells progress through a program of expansive proliferation, followed by a terminal differentiation that allows for the biosynthesis and secretion of milk during lactation. The role of gap junction proteins, connexins, in the development and function of this secretory epithelium was investigated. Connexins, Cx26 and Cx32, were differentially expressed throughout pregnancy and lactation in alveolar cells. Cx26 poly-(A)(+) RNA and protein levels increased from early pregnancy, whereas Cx32 was detectable only during lactation. At this time, immunolocalization of connexins by confocal microscopy and immunogold labeling of high-pressure frozen freeze-substituted tissue showed that both connexins colocalized to the same junctional plaque. Analysis of gap junction hemichannels (connexons) isolated from lactating mammary gland plasma membranes by a rate-density centrifugation procedure, followed by immunoprecipitation and by size-exclusion chromatography, showed that Cx26 and Cx32 were organized as homomeric and heteromeric connexons. Structural diversity in the assembly of gap junction hemichannels demonstrated between pregnant and lactating mammary gland may account for differences in ionic and molecular signaling that may physiologically influence the onset and/or maintenance of the secretory phenotype of alveolar epithelial cells.

Physiological role of gap-junctional hemichannels. Extracellular calcium-dependent isosmotic volume regulation

Hemichannels in the overlapping regions of apposing cells plasma membranes join to form gap junctions and provide an intercellular communication pathway. Hemichannels are also present in the nonjunctional regions of individual cells and their activity is gated by several agents, including calcium. However, their physiological roles are unknown. Using techniques of atomic force microscopy (AFM), fluorescent dye uptake assay, and laser confocal immunofluorescence imaging, we have examined the extracellular calcium-dependent modulation of cell volume. In response to a change in the extracellular physiological calcium concentration (1.8 to Collapse

Key Words

• cell–cell communication
• atomic force microscopy
• scanning probe microscopy
• connexin43
• connexons

Collapse

MESH Headings

• Animals
• Calcium/metabolism
• Calcium/pharmacology
• Cattle
• Cell Size/drug effects
• Connexin 43/biosynthesis
• Connexin 43/genetics
• Connexins/biosynthesis
• Cytochalasin D/pharmacology
• Dose-Response Relationship, Drug
• Elasticity
• Endothelium/cytology
• Endothelium/drug effects
• Endothelium/metabolism
• Epithelial Cells/cytology
• Epithelial Cells/drug effects
• Epithelial Cells/metabolism
• Extracellular Space/metabolism
• Fibroblasts/cytology
• Fibroblasts/drug effects
• Fibroblasts/metabolism
• Gap Junctions/drug effects
• Gap Junctions/metabolism
• Humans
• Hypnotics and Sedatives/pharmacology
• Ion Channels/drug effects
• Ion Channels/metabolism
• Mice
• Nucleic Acid Synthesis Inhibitors/pharmacology
• Oleic Acids/pharmacology
• Osmolar Concentration
• Rats
• Water/metabolism

Collapse

Grants

• R01 GM056290 NIGMS NIH HHS
• GM056290 NIGMS NIH HHS

Collapse

Affiliation(s)

Arjan Pieter Quist Neuroscience Research Institute, University of California, Santa Barbara, California 93106
Seung Keun Rhee Neuroscience Research Institute, University of California, Santa Barbara, California 93106 Department of Biochemistry, Yeungnam University, Kyongsan, 712-749, Korea
Hai Lin Neuroscience Research Institute, University of California, Santa Barbara, California 93106
Ratneshwar Lal Neuroscience Research Institute, University of California, Santa Barbara, California 93106

Collapse

Cell-free synthesis for analyzing the membrane integration, oligomerization, and assembly characteristics of gap junction connexins

For gap junction channels to function, their subunit proteins, referred to as connexins, have to be synthesized and inserted into the cell membrane in their native configuration. Like other transmembrane proteins, connexins are synthesized and inserted cotranslationally into the endoplasmic reticulum membrane. Membrane insertion is followed by their assembly and transport to the plasma membrane. Finally, the end-to-end pairing of two half-channels, referred to as connexons, each provided by one of two neighboring cells, and clustering of the channels into larger plaques complete the gap junction channel formation. Gap junction channel formation is further complicated by the potential assembly of homo- as well as heterooligomeric connexons, and the pairing of identical or different connexons into homo- and heterotypic gap junction channels. In this article, I describe the cell-free synthesis approach that we have used to study the biosynthesis of connexins and gap junction channels. Special emphasis is placed on the synthesis of full-length, membrane-integrated connexins, assembly into gap junction connexons, homo- as well as heterooligomerization, and characterization of connexin-specific assembly signals.

TPA induced expression and function of human connexin 26 by post-translational mechanisms in stably transfected neuroblastoma cells

Connexin 26 (Cx26) has been proposed to be a tumor suppressor gene and its expression may modulate development, cell growth and differentiation in various tissues, including the brain. 12-O-tetradecanoylphorbol-13-acetate (TPA) may serve as either tumor promoter (in mammary gland amd skin) or as a differentiating agent (in neuroblastoma and leukemic cells) and may also modulate expression, function and phosphorylation of gap junctions. In this study, to determine the effects of TPA on Cx26 expression and its function in neuroblastoma, we transfected N2A mouse neuroblastoma cells (which are gap junction deficient) with the coding region of human Cx26 gene (which lacks TPA response elements) and examined the changes of expression and function of Cx26 following 10 nM TPA treatment. Individual clones of transfectants stably expressed distinct levels of exogenous Cx26 as judged by Northern and Western blots, immunocytochemistry and electrophysiological recordings. Cx26 channels displayed unitary conductances of about 140-155 pS. Increase of Cx26 expression following TPA treatment was markedly observed using immunocytochemistry and Western blots of membrane fractions although it was not detected in Northern or Western blots of whole cells. This increase in Cx26 expression in the plasma membrane was accompanied by an increase of function as evidenced in measurements of junctional conductance. These results suggest that induction of exogenous Cx26 in neuroblastoma cells by TPA treatment is controlled by post-translational mechanisms.

Junctional communication between isolated pairs of canine atrial cells is mediated by homogeneous and heterogeneous gap junction channels

INTRODUCTION

The expression of multiple connexins (Cxs) in the canine right atria raises the possibility that heterogeneous gap junction channels might be formed.

METHODS AND RESULTS

We compared the unitary conductance (gamma(j)) of gap junction channels between isolated canine atrial cell pairs with those of homogeneous cardiac gap junction channels expressed in other systems. After partial uncoupling with halothane (2 mmol/L), the (gamma)j calculations for atrial isolated cardiocytes ranged from 30 to 220 pS and their distribution in event histograms was spread over the entire range, with a small peak at approximately 100 pS. This distribution deviates from the discrete peaks calculated from (gamma)j of homogeneous channels. All-points histograms of junctional current traces revealed distinct open-state levels. Some of these are related to the main open state of connexin43 (Cx43) (approximately 100 pS), observed between canine ventricular cells, or connexin40 (Cx40) (approximately 215 pS) observed between transfected N2A cells under similar recording conditions. Intermediate values for (gamma)j were not observed in recordings from ventricular cells, which express mostly Cx43, nor in those from N2A cells expressing Cx40, but were observed consistently between atrial cells. Because they were measured as first openings from the nonconductance state, these intermediate values most likely represent main conductance states of heterogeneous channels rather than subconductance states of homogeneous channels.

CONCLUSION

This suggests that regulation of cell-to-cell coupling in the heart depends not only on posttranslational modulation of preexisting Cxs, but also on the intracellular assembly mechanisms, and the way individual Cxs interact with others within a connexon and/or with other connexons from adjacent cells.

Gating connexin 43 channels reconstituted in lipid vesicles by mitogen-activated protein kinase phosphorylation

The regulation of gap junctional permeability by phosphorylation was examined in a model system in which connexin 43 (Cx43) gap junction hemichannels were reconstituted in lipid vesicles. Cx43 was immunoaffinity-purified from rat brain, and Cx43 channels were reconstituted into unilamellar phospholipid liposomes. The activities of the reconstituted channels were measured by monitoring liposome permeability. Liposomes containing the Cx43 protein were fractionated on the basis of permeability to sucrose using sedimentation in an iso-osmolar density gradient. The gradient allowed separation of the sucrose-permeable and -impermeable liposomes. Liposomes that were permeable to sucrose were also permeable to the communicating dye molecule lucifer yellow. Permeability, and therefore activity of the reconstituted Cx43 channels, were directly dependent on the state of Cx43 phosphorylation. The permeability of liposomes containing Cx43 channels was increased by treatment of liposomes with calf intestinal phosphatase. Moreover, liposomes formed with Cx43 that had been dephosphorylated by calf intestinal phosphatase treatment showed increased permeability to sucrose. The role of phosphorylation in the gating mechanism of Cx43 channels was supported further by the observation that phosphorylation of Cx43 by mitogen-activated protein kinase reversibly reduced the permeability of liposomes containing dephosphorylated Cx43. Our results show a direct correlation between gap junctional permeability and the phosphorylation state of Cx43.

Gap junctions in normal and neoplastic mammary gland

Progress in the characterization of gap junctions and their constituent connexin sub-units is leading to a greater understanding of the structure, function, and regulation of this cell-cell communication channel. Although much of the experimental evidence generated to date is correlative, recent work utilizing reverse genetic approaches to manipulate connexin gene function has provided direct evidence that intercellular communication via gap junctions plays key roles in development, cellular differentiation, and organogenesis. Pathogenic mutations in human connexin genes have now been identified. Furthermore, a considerable body of experimental evidence correlates a loss of junctional communication with progression to a malignant phenotype. Although the cell biology of the mammary gland has been extensively studied, the role(s) of gap junctions in the development, differentiation, and maintenance of this tissue are unknown. Gap junctions were first reported in the mammary gland following freeze-fracture and electron microscopic analyses. The development of anti-connexin antibodies and the cloning of individual connexin isoforms have enabled this work to be extended, but there are contradictory reports in the temporal expression patterns of these proteins within mammary epithelium. In addition, a recent report in this Journal has implied by immunocytochemistry that there is up-regulation of connexin protein in some human breast tumours, a novel observation which may be inconsistent with the proposed tumour suppressor role for gap junctions.

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.

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.

Connexin expression systems: to what extent do they reflect the situation in the animal?

Intercellular communication is mediated by specialized cell-cell contact areas known as gap junctions. Connexins are the constitutive proteins of gap junction intercellular channels. Various cell expression systems are used to express connexins and, in turn, these expression systems can then be tested for their ability to form functional cell-cell channels. In this review, expression of murine endogenous connexins in primary cells and established cell lines is compared with results obtained by expression of exogenous connexins in Xenopus oocytes and cultured mammalian cells. In addition, first reports on characterization of connexin-deficient mice are discussed.

Multiple connexin proteins in single intercellular channels: connexin compatibility and functional consequences

In vertebrates, the protein subunits of intercellular channels found in gap junctions are encoded by a family of genes called connexins. These channels span two plasma membranes and result from the association of two half channels, or connexons, which are hexameric assemblies of connexins. Physiological analysis of channel formation and gating has revealed unique patterns of connexin-connexin interaction, and uncovered novel functional characteristics of channels containing more than one type of connexin protein. Structure-function studies have further demonstrated that unique domains within connexins participate in the regulation of different functional properties of intercellular channels. Thus, gap junctional channels can contain more than one connexin, and this structural heterogeneity has functional consequences in vitro. Moreover, emerging evidence for the existence of intercellular channels containing multiple connexins in native tissues suggests that the functional diversity generated by connexin-connexin interaction could contribute to complex communication patterns that have been observed in vivo.

Channel-forming activity of immunoaffinity-purified connexin32 in single phospholipid membranes

Connexin32, a member of the family of proteins that forms gap junction channels between cells, was immunoaffinity-purified from rat liver using a monoclonal antibody, under nondenaturing conditions and reconstituted into unilamellar phospholipid liposomes and bilayers. Gel-filtration studies indicate that the connexin32 is purified predominantly in structures of a size consistent with that of single hemichannels and too small to be junctional channels (dimers of hemichannels). Purified connexin formed channels permeable to sucrose and to Lucifer Yellow. The permeability was reversibly reduced by acidic pH and unaffected by several agents that modulate coupling between cells. Modeling of the distribution of the permeability in the liposomes indicates that it is mediated by connexin structures that distribute among the liposomes as single hemichannels. Bilayer recordings of the purified connexin show high conductance channels with asymmetric voltage sensitivity. The results show that immunopurified connexin32 can form channels, in single phospholipid membranes, that have permeability similar to that of gap junction channels and thus can be utilized in studies of permeability and its regulation to investigate its role in normal physiological function, development, and disease.

Connections with connexins: the molecular basis of direct intercellular signaling

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

Heteromeric connexons in lens gap junction channels

Gap junction channels are formed by paired oligomeric membrane hemichannels called connexons, which are composed of proteins of the connexin family. Experiments with transfected cell lines and paired Xenopus oocytes have demonstrated that heterotypic intercellular channels which are formed by two connexons, each composed of a different connexin, can selectively occur. Studies by Stauffer [Stauffer, K. A. (1995) J. Biol. Chem. 270, 6768-6772] have shown that recombinant Cx26 and Cx32 coinfected into insect cells may form heteromeric connexons. By solubilizing and subfractionating individual connexons from ovine lenses, we show by immunoprecipitation that connexons can contain two different connexins forming heteromeric assemblies in vivo.