Distribution and dynamics of gap junction channels revealed in living cells

The dynamic Nexus: gap junctions control protein localization and mobility in distinct and surprising ways

Gap junction (GJ) channels permit molecules, such as ions, metabolites and second messengers, to transfer between cells. Their function is critical for numerous cellular interactions, providing exchange of metabolites, signaling molecules, and ionic currents. GJ channels are composed of Connexin (Cx) hexamers paired across extracellular space and typically form large rafts of clustered channels, called plaques, at cell appositions. Cxs together with molecules that interact with GJ channels make up a supramolecular structure known as the GJ Nexus. While the stability of connexin localization in GJ plaques has been studied, mobility of other Nexus components has yet to be addressed. Colocalization analysis of several nexus components and other membrane proteins reveal that certain molecules are excluded from the GJ plaque (Aquaporin 4, EAAT2b), while others are quite penetrant (lipophilic molecules, Cx30, ZO-1, Occludin). Fluorescence recovery after photobleaching of tagged Nexus-associated proteins showed that mobility in plaque domains is affected by mobility of the Cx proteins. These novel findings indicate that the GJ Nexus is a dynamic membrane organelle, with cytoplasmic and membrane-embedded proteins binding and diffusing according to distinct parameters.

Lens ion homeostasis relies on the assembly and/or stability of large connexin 46 gap junction plaques on the broad sides of differentiating fiber cells

The eye lens consists of layers of tightly packed fiber cells, forming a transparent and avascular organ that is important for focusing light onto the retina. A microcirculation system, facilitated by a network of gap junction channels composed of connexins 46 and 50 (Cx46 and Cx50), is hypothesized to maintain and nourish lens fiber cells. We measured lens impedance in mice lacking tropomodulin 1 (Tmod1, an actin pointed-end capping protein), CP49 (a lens-specific intermediate filament protein), or both Tmod1 and CP49. We were surprised to find that simultaneous loss of Tmod1 and CP49, which disrupts cytoskeletal networks in lens fiber cells, results in increased gap junction coupling resistance, hydrostatic pressure, and sodium concentration. Protein levels of Cx46 and Cx50 in Tmod1(-/-);CP49(-/-) double-knockout (DKO) lenses were unchanged, and electron microscopy revealed normal gap junctions. However, immunostaining and quantitative analysis of three-dimensional confocal images showed that Cx46 gap junction plaques are smaller and more dispersed in DKO differentiating fiber cells. The localization and sizes of Cx50 gap junction plaques in DKO fibers were unaffected, suggesting that Cx46 and Cx50 form homomeric channels. We also demonstrate that gap junction plaques rest in lacunae of the membrane-associated actin-spectrin network, suggesting that disruption of the actin-spectrin network in DKO fibers may interfere with gap junction plaque accretion into micrometer-sized domains or alter the stability of large plaques. This is the first work to reveal that normal gap junction plaque localization and size are associated with normal lens coupling conductance.

Blockade of gap junction hemichannel protects secondary spinal cord injury from activated microglia-mediated glutamate exitoneurotoxicity

We previously demonstrated that activated microglia release excessive glutamate through gap junction hemichannels and identified a novel gap junction hemichannel blocker, INI-0602, that was proven to penetrate the blood-brain barrier and be an effective treatment in mouse models of amyotrophic lateral sclerosis and Alzheimer disease. Spinal cord injury causes tissue damage in two successive waves. The initial injury is mechanical and directly causes primary tissue damage, which induces subsequent ischemia, inflammation, and neurotoxic factor release resulting in the secondary tissue damage. These lead to activation of glial cells. Activated glial cells such as microglia and astrocytes are common pathological observations in the damaged lesion. Activated microglia release glutamate, the major neurotoxic factor released into the extracellular space after neural injury, which causes neuronal death at high concentration. In the present study, we demonstrate that reduction of glutamate-mediated exitotoxicity via intraperitoneal administration of INI-0602 in the microenvironment of the injured spinal cord elicited neurobehavioral recovery and extensive suppression of glial scar formation by reducing secondary tissue damage. Further, this intervention stimulated anti-inflammatory cytokines, and subsequently elevated brain-derived neurotrophic factor. Thus, preventing microglial activation by a gap junction hemichannel blocker, INI-0602, may be a promising therapeutic strategy in spinal cord injury.

Gap junctions and hemichannels composed of connexins: potential therapeutic targets for neurodegenerative diseases

Microglia are macrophage-like resident immune cells that contribute to the maintenance of homeostasis in the central nervous system (CNS). Abnormal activation of microglia can cause damage in the CNS, and accumulation of activated microglia is a characteristic pathological observation in neurologic conditions such as trauma, stroke, inflammation, epilepsy, and neurodegenerative diseases. Activated microglia secrete high levels of glutamate, which damages CNS cells and has been implicated as a major cause of neurodegeneration in these conditions. Glutamate-receptor blockers and microglia inhibitors (e.g., minocycline) have been examined as therapeutic candidates for several neurodegenerative diseases; however, these compounds exerted little therapeutic benefit because they either perturbed physiological glutamate signals or suppressed the actions of protective microglia. The ideal therapeutic approach would hamper the deleterious roles of activated microglia without diminishing their protective effects. We recently found that abnormally activated microglia secrete glutamate via gap-junction hemichannels on the cell surface. Moreover, administration of gap-junction inhibitors significantly suppressed excessive microglial glutamate release and improved disease symptoms in animal models of neurologic conditions such as stroke, multiple sclerosis, amyotrophic lateral sclerosis, and Alzheimer's disease. Recent evidence also suggests that neuronal and glial communication via gap junctions amplifies neuroinflammation and neurodegeneration. Elucidation of the precise pathologic roles of gap junctions and hemichannels may lead to a novel therapeutic strategies that can slow and halt the progression of neurodegenerative diseases.

Green-to-red photoconvertible fluorescent proteins: tracking cell and protein dynamics on standard wide-field mercury arc-based microscopes

Background

Green fluorescent protein (GFP) and other FP fusions have been extensively utilized to track protein dynamics in living cells. Recently, development of photoactivatable, photoswitchable and photoconvertible fluorescent proteins (PAFPs) has made it possible to investigate the fate of discrete subpopulations of tagged proteins. Initial limitations to their use (due to their tetrameric nature) were overcome when monomeric variants, such as Dendra, mEos, and mKikGR were cloned/engineered.

Results

Here, we report that by closing the field diaphragm, selective, precise and irreversible green-to-red photoconversion (330-380 nm illumination) of discrete subcellular protein pools was achieved on a wide-field fluorescence microscope equipped with standard DAPI, Fluorescein, and Rhodamine filter sets and mercury arc illumination within 5-10 seconds. Use of a DAPI-filter cube with long-pass emission filter (LP420) allowed the observation and control of the photoconversion process in real time. Following photoconversion, living cells were imaged for up to 5 hours often without detectable phototoxicity or photobleaching.

Conclusions

We demonstrate the practicability of this technique using Dendra2 and mEos2 as monomeric, photoconvertible PAFP representatives fused to proteins with low (histone H2B), medium (gap junction channel protein connexin 43), and high (α-tubulin; clathrin light chain) dynamic cellular mobility as examples. Comparable efficient, irreversible green-to-red photoconversion of selected portions of cell nuclei, gap junctions, microtubules and clathrin-coated vesicles was achieved. Tracking over time allowed elucidation of the dynamic live-cycle of these subcellular structures. The advantage of this technique is that it can be performed on a standard, relatively inexpensive wide-field fluorescence microscope with mercury arc illumination. Together with previously described laser scanning confocal microscope-based photoconversion methods, this technique promises to further increase the general usability of photoconvertible PAFPs to track the dynamic movement of cells and proteins over time.

Migrating cells retain gap junction plaque structure and function

Cell migration is an essential process in organ development, differentiation, and wound healing, and it has been hypothesized that gap junctions play a pivotal role in these cell processes. However, the changes in gap junctions and the capacity for cell communication as cells migrate are unclear. To monitor gap junction plaques during cell migration, adrenocortical cells were transfected with cDNA encoding for the connexin 43-green fluorescent protein. Time-lapse imaging was used to analyze cell movements and concurrent gap junction plaque dynamics. Immunocytochemistry was used to analyze gap junction morphology and distribution. Migration was initiated by wounding the cell monolayer and diffusional coupling was demonstrated by monitoring Lucifer yellow dye transfer and fluorescence recovery after photobleaching (FRAP) in cells at the wound edge and in cells located some distance from the wound edge. Gap junction plaques were retained at sites of contact while cells migrated in a "sheet-like" formation, even when cells dramatically changed their spatial relationship to one another. Consistent with this finding, cells at the leading edge retained their capacity to communicate with contacting cells. When cells detached from one another, gap junction plaques were internalized just prior to cell process detachment. Although gap junction plaque internalization clearly was a method of gap junction removal during cell separation, cells retained gap junction plaques and continued to communicate dye while migrating.

Endocytic processing of connexin43 gap junctions: a morphological study

Gap junctions are plasma membrane areas enriched in channels that provide direct intercellular communication. Gap junctions have a high turnover rate; however, the mechanisms by which gap junctions are degraded are incompletely understood. In the present study, we show that in response to phorbol ester treatment, the gap junction channel protein Cx43 (connexin43) is redistributed from the plasma membrane to intracellular vesicles positive for markers for early and late endosomes and for the endolysosomal protease cathepsin D. Immunoelectron microscopy studies indicate that the double membranes of internalized gap junctions undergo separation and cutting, resulting in multivesicular endosomes enriched in Cx43 protein. Using preloading of BSA-gold conjugates to mark lysosomes, we provide evidence suggesting that the degradation process of the double-membrane structure of annular gap junctions occurs prior to transport of Cx43 to the lysosome. The results further suggest that bafilomycin A1, an inhibitor of vacuolar H+-ATPases, causes accumulation of Cx43 in early endosomes. Taken together, these findings indicate that internalized gap junctions undergo a maturation process from tightly sealed double-membrane vacuoles to connexin-enriched multivesicular endosomes with a single limiting membrane. The results further suggest that along with the processing of the double-membrane structure of annular gap junctions, connexins are trafficked via early and late endosomes, finally resulting in their endolysosomal degradation.

Cx31 is assembled and trafficked to cell surface by ER-Golgi pathway and degraded by proteasomal or lysosomal pathways

Gap junctions, consisting of connexins, allow the exchange of small molecules (less than 1 KD) between adjacent cells, thus providing a mechanism for synchronizing the responses of groups of cells to environmental stimuli. Connexin 31 is a member of the connexin family. Mutations on connexin 31 are associated with erythrokeratodermia variabilis, hearing impairment and peripheral neuropathy. However, the pathological mechanism for connexin 31 mutants in these diseases are still unknown. In this study, we analyzed the assembly, trafficking and metabolism of connexin 31 in HeLa cells stably expressing connexin 31. Calcein transfer assay showed that calcein transfer was inhibited when cells were treated with Brefeldin A or cytochalasin D, but not when treated with nocodazole or a-glycyrrhetinic acid, suggesting that Golgi apparatus and actin filaments, but not microtubules, are crucial to the trafficking and assembly of connexin 31, as well as the formation of gap junction intercellular communication by connexin 31. Additionally, a-glycyrrhetinic acid did not effectively inhibit gap junctional intercellular communication formed by connexin 31. Pulse-chase assay revealed that connexin 31 had a half-life of about 6 h. Moreover, Western blotting and fluorescent staining demonstrated that in HeLa cells stably expressing connexin 31, the amount of connexin 31 was significantly increased after these cells were treated with proteasomal or lysosomal inhibitors. These findings indicate that connexin 31 was rapidly renewed, and possibly degraded by both proteasomal and lysosomal pathways.

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.

Sequence and phylogenetic analyses of 4 TMS junctional proteins of animals: connexins, innexins, claudins and occludins

Connexins and probably innexins are the principal constituents of gap junctions, while claudins and occludins are principal tight junctional constituents. All have similar topologies with four alpha-helical transmembrane segments (TMSs), and all exhibit well-conserved extracytoplasmic cysteines that either are known to or potentially can form disulfide bridges. We have conducted sequence, topological and phylogenetic analyses of the proteins that comprise the connexin, innexin, claudin and occludin families. A multiple alignment of the sequences of each family was used to derive average hydropathy and similarity plots as well as phylogenetic trees. Analyses of the data generated led to the following evolutionary and functional suggestions: (1) In all four families, the most conserved regions of the proteins from each family are the four TMSs although the extracytoplasmic loops between TMSs 1 and 2, and TMSs 3 and 4 are usually well conserved. (2) The phylogenetic trees revealed sets of orthologues except for the innexins where phylogeny primarily reflects organismal source, probably due to a lack of relevant organismal sequence data. (3) The two halves of the connexins exhibit similarities suggesting that they were derived from a common origin by an internal gene duplication event. (4) Conserved cysteyl residues in the connexins and innexins may point to a similar extracellular structure involved in the docking of hemichannels to create intercellular communication channels. (5) We suggest a similar role in homomeric interactions for conserved extracellular residues in the claudins and occludins. The lack of sequence or motif similarity between the four different families indicates that, if they did evolve from a common ancestral gene, they have diverged considerably to fulfill separate, novel functions. We suggest that internal duplication was a general evolutionary strategy used to generate new families of channels and junctions with unique functions. These findings and suggestions should serve as guides for future studies concerning the structures, functions and evolutionary origins of junctional proteins.

Genetic tags for labelling live cells: gap junctions and beyond

The availability of green fluorescent protein (GFP) as a tracer for observing proteins in living cells has revolutionized cell biology and spurred an intensive search for GFP variants with novel characteristics, additional autofluorescent proteins and alternative techniques of protein labelling. Two recent studies - one on tagging with tetracysteine motifs and labelling with biarsenic fluorophores of different colours, and the other on GFP tagging and fluorescence recovery after photobleaching (FRAP) - show how membrane channels are added and removed from gap junctions by using different fluorescent tags to distinguish between newly synthesized and older protein populations.