Dynamics of gap junctions observed in living cells with connexin43-GFP chimeric protein

cAMP controls a trafficking mechanism that maintains the neuron specificity and subcellular placement of electrical synapses

Electrical synapses are established between specific neurons and within distinct subcellular compartments, but the mechanisms that direct gap junction assembly in the nervous system are largely unknown. Here, we show that a developmental program tunes cAMP signaling to direct the neuron-specific assembly and placement of electrical synapses in the C. elegans motor circuit. We use live-cell imaging to visualize electrical synapses in vivo and an optogenetic assay to confirm that they are functional. In ventral A class (VA) motor neurons, the UNC-4 transcription factor blocks expression of cAMP antagonists that promote gap junction miswiring. In unc-4 mutants, VA electrical synapses are established with an alternative synaptic partner and are repositioned from the VA axon to soma. cAMP counters these effects by driving gap junction trafficking into the VA axon for electrical synapse assembly. Thus, our experiments establish that cAMP regulates gap junction trafficking for the biogenesis of functional electrical synapses.

Alcohol consumption combined with dietary low-carbohydrate/high-protein intake increased the left ventricular systolic dysfunction risk and lethal ventricular arrhythmia susceptibility in apolipoprotein E/low-density lipoprotein receptor double-knockout mice

Alcohol abuse is positively associated with cardiovascular disease. Dietary low-carbohydrate/high-protein (LCHP) intake confers a greater mortality risk. Here, the impact of ethanol consumption in combination with dietary LCHP intake on left ventricular (LV) systolic function and lethal ventricular arrhythmia susceptibility were investigated in apolipoprotein E/low-density lipoprotein receptor double-knockout (AL) mice. The underlying mechanisms, cardiac sympathovagal balance, beta-adrenergic receptor (ADRB) levels, and gap junction channel protein connexin 43 (Cx43) expression, were examined. Male AL mice fed an LCHP diet with or without ethanol were bred for 16 weeks. Age-matched male AL and wild-type mice received standard chow diet and served as controls. The following were used to assess LV systolic function, lethal ventricular arrhythmia susceptibility, cardiac sympathovagal balance, Cx43 expression, and ADRB levels: The results demonstrated that ethanol consumption in combination with dietary LCHP intake worsened LCHP-induced LV systolic dysfunction in AL mice and enhanced their susceptibility in the ventricular arrhythmia-evoked test. There were concomitant increases in LV weight, LF/HF ratio shown by HRV, TH, ADRB1, ADRB2, and Cx43 expressions by LV fluorescence immunohistochemistry, and LV Cx43 messenger ribonucleic acid expression by PCR. In AL mice, alcohol consumption combined with dietary LCHP intake may thus promote a shift in cardiac sympathovagal balance toward sympathetic predominance, the increases in beta-adrenergic receptors (ADRB1 and ADRB2), and then affect the gap junction channel protein Cx43, which in turn could contribute to increased risks of LV systolic dysfunction and susceptibility to lethal ventricular arrhythmia.

Connexin Type and Fluorescent Protein Fusion Tag Determine Structural Stability of Gap Junction Plaques

Gap junctions (GJs) are made up of plaques of laterally clustered intercellular channels and the membranes in which the channels are embedded. Arrangement of channels within a plaque determines subcellular distribution of connexin binding partners and sites of intercellular signaling. Here, we report the discovery that some connexin types form plaque structures with strikingly different degrees of fluidity in the arrangement of the GJ channel subcomponents of the GJ plaque. We uncovered this property of GJs by applying fluorescence recovery after photobleaching to GJs formed from connexins fused with fluorescent protein tags. We found that connexin 26 (Cx26) and Cx30 GJs readily diffuse within the plaque structures, whereas Cx43 GJs remain persistently immobile for more than 2 min after bleaching. The cytoplasmic C terminus of Cx43 was required for stability of Cx43 plaque arrangement. We provide evidence that these qualitative differences in GJ arrangement stability reflect endogenous characteristics, with the caveat that the sizes of the GJs examined were necessarily large for these measurements. We also uncovered an unrecognized effect of non-monomerized fluorescent protein on the dynamically arranged GJs and the organization of plaques composed of multiple connexin types. Together, these findings redefine our understanding of the GJ plaque structure and should be considered in future studies using fluorescent protein tags to probe dynamics of highly ordered protein complexes.

The carboxyl tail of connexin32 regulates gap junction assembly in human prostate and pancreatic cancer cells

Connexins, the constituent proteins of gap junctions, are transmembrane proteins. A connexin (Cx) traverses the membrane four times and has one intracellular and two extracellular loops with the amino and carboxyl termini facing the cytoplasm. The transmembrane and the extracellular loop domains are highly conserved among different Cxs, whereas the carboxyl termini, often called the cytoplasmic tails, are highly divergent. We have explored the role of the cytoplasmic tail of Cx32, a Cx expressed in polarized and differentiated cells, in regulating gap junction assembly. Our results demonstrate that compared with the full-length Cx32, the cytoplasmic tail-deleted Cx32 is assembled into small gap junctions in human pancreatic and prostatic cancer cells. Our results further document that the expression of the full-length Cx32 in cells, which express the tail-deleted Cx32, increases the size of gap junctions, whereas the expression of the tail-deleted Cx32 in cells, which express the full-length Cx32, has the opposite effect. Moreover, we show that the tail is required for the clustering of cell-cell channels and that in cells expressing the tail-deleted Cx32, the expression of cell surface-targeted cytoplasmic tail alone is sufficient to enhance the size of gap junctions. Our live-cell imaging data further demonstrate that gap junctions formed of the tail-deleted Cx32 are highly mobile compared with those formed of full-length Cx32. Our results suggest that the cytoplasmic tail of Cx32 is not required to initiate the assembly of gap junctions but for their subsequent growth and stability. Our findings suggest that the cytoplasmic tail of Cx32 may be involved in regulating the permeability of gap junctions by regulating their size.

Biphasic increase of gap junction coupling induced by dipyridamole in the rat aortic A-10 vascular smooth muscle cell line

The rat aortic smooth muscle cell line A-10 was used to investigate the effect of dipyridamole on the gap junction coupling of smooth muscle cells. The scrape loading/dye transfer (SL/DT) technique revealed that dipyridamole concentrations between 5 μM and 100 μM significantly increased gap junction coupling. The adenosine receptor antagonist MRS 1754, as well as the PKA inhibitors Rp-cAMPS and H-89 were able to inhibit the dipyridamole-related increase in coupling, while forskolin and Br-cAMP also induced an enhancement of the gap junction coupling. Regarding the time-dependent behaviour of dipyridamole, a short-term effect characterised by an oscillatory reaction was observed for application times of less than 5 h, while applications times of at least 6 h resulted in a long-term effect, characterised by a constant increase of gap junction coupling to its maximum levels. This increase was not altered by prolonged presence of dipyridamole. In parallel, a short application of dipyridamole for at least 15 min was found to be sufficient to evoke the long-term effect measured 6 h after drug washout. We propose that in both the short-term and long-term effect, cAMP-related pathways are activated. The short-term phase could be related to an oscillatory cAMP effect, which might directly affect connexin trafficking, assembly and/or gap junction gating. The long-term effect is most likely related to the new expression and synthesis of connexins. With previous data from a bovine aortic endothelial cell line, the present results show that gap junction coupling of vascular cells is a target for dipyridamole.

Assembly of connexin43 into gap junctions is regulated differentially by E-cadherin and N-cadherin in rat liver epithelial cells

Cadherins have been thought to facilitate the assembly of connexins (Cxs) into gap junctions (GJs) by enhancing cell-cell contact, however the molecular mechanisms involved in this process have remained unexplored. We examined the assembly of GJs composed of Cx43 in isogenic clones derived from immortalized and nontransformed rat liver epithelial cells that expressed either epithelial cadherin (E-Cad), which curbs the malignant behavior of tumor cells, or neuronal cadherin (N-Cad), which augments the invasive and motile behavior of tumor cells. We found that N-cad expression attenuated the assembly of Cx43 into GJs, whereas E-Cad expression facilitated the assembly. The expression of N-Cad inhibited GJ assembly by causing endocytosis of Cx43 via a nonclathrin-dependent pathway. Knock down of N-Cad by ShRNA restored GJ assembly. When both cadherins were simultaneously expressed in the same cell type, GJ assembly and disassembly occurred concurrently. Our findings demonstrate that E-Cad and N-Cad have opposite effects on the assembly of Cx43 into GJs in rat liver epithelial cells. These findings imply that GJ assembly and disassembly are the down-stream targets of the signaling initiated by E-Cad and N-Cad, respectively, and may provide one possible explanation for the disparate role played by these cadherins in regulating cell motility and invasion during tumor progression and invasion.

E-cadherin differentially regulates the assembly of Connexin43 and Connexin32 into gap junctions in human squamous carcinoma cells

It is as yet unknown how the assembly of connexins (Cx) into gap junctions (GJ) is initiated upon cell-cell contact. We investigated whether the trafficking and assembly of Cx43 and Cx32 into GJs were contingent upon cell-cell adhesion mediated by E-cadherin. We also examined the role of the carboxyl termini of these Cxs in initiating the formation of GJs. Using cadherin and Cx-null cells, and by introducing Cx43 and Cx32, either alone or in combination with E-cadherin, our studies demonstrated that E-cadherin-mediated cell-cell adhesion was neither essential nor sufficient to initiate GJ assembly de novo in A431D human squamous carcinoma cells. However, E-cadherin facilitated the growth and assembly of preformed GJs composed of Cx43, although the growth of cells on Transwell filters was required to initiate the assembly of Cx32. Our results also documented that the carboxyl termini of both Cxs were required in this cell type to initiate the formation of GJs de novo. Our findings also showed that GJ puncta composed of Cx43 co-localized extensively with ZO-1 and actin fibers at cell peripheries and that ZO-1 knockdown attenuated Cx43 assembly. These findings suggest that the assembly of Cx43 and Cx32 into GJs is differentially modulated by E-cadherin-mediated cell-cell adhesion and that direct or indirect cross-talk between carboxyl tails of Cxs and actin cytoskeleton via ZO-1 may regulate GJ assembly and growth.

Mechanisms of gap junction traffic in health and disease

Gap junctions (GJs) allow direct communication between cells. In the heart, GJs mediate the electrical coupling of cardiomyocytes and as such dictate the speed and direction of cardiac conduction. A prominent feature of acquired structural heart disease is remodeling of GJ protein expression and localization concomitant with increased susceptibility to lethal arrhythmias, leading many to hypothesize that the two are causally linked. Detailed understanding of the cellular mechanisms that regulate GJ localization and function within cardiomyocytes may therefore uncover potential therapeutic strategies for a significant clinical problem. This review will outline our current understanding of GJ cell biology with the intent of highlighting cellular mechanisms responsible for GJ remodeling associated with cardiac disease.

Establishment of human corneal epithelial cells stably expressing human connexin43

Corneal epithelial cells communicate with each other through gap junctions. Whereas this property is retained in corneal epithelial cells in primary culture, it is often lost in immortalized epithelial cells. However, the life span of primary cultured corneal epithelial cells is short and the availability of human tissue for their preparation is limited. To examine the role of the gap-junction protein connexin43 (Cx43) in human corneal epithelial cells, we set out to establish an immortal human corneal epithelial cell line that stably expresses this protein. An expression vector encoding human Cx43 fused to enhanced green fluorescent protein (EGFP) was constructed and introduced by transfection into SV40-immortalized human corneal epithelial (HCE) cells. Stable transfectants were isolated by selection with the antibiotic G418. The expression and localization of the Cx43-EGFP fusion protein were examined by immunoblot analysis and fluorescence microscopy, respectively, and gap-junctional intercellular communication was monitored on the basis of dye coupling. HCE cells stably expressing Cx43-EGFP manifested intercellular dye transfer, whereas those stably expressing EGFP alone did not. Cx43-EGFP localized to the interfaces of neighboring cells. Stable expression of Cx43-EGFP in HCE cells did not affect the expression of keratins 3 and 12, which is a characteristic of corneal epithelial cells, but it did inhibit cell proliferation. We have established an HCE cell line that stably expresses human Cx43 and forms functional gap junctions. These cells may prove useful for studies of the role of gap junctions in the human corneal epithelium.

The connexin turnover, an important modulating factor of the level of cell-to-cell junctional communication: comparison with other integral membrane proteins

The constituent proteins of gap junctions, called "connexins" (Cxs) in chordates, are generally renewed several times a day, in approximately the same rate range as many other integral plasma membrane proteins and the proteins of other channels, other intercellular junctions or different membrane receptors. This permanent renewal turns on a fine-tuned balance among various processes, such as gene transcription, mRNA stability and processing, protein synthesis and oligomerization, posttranslational modifications, transport to the plasma membrane, anchoring to the cytoskeleton, connexon aggregation and docking, regulation of endocytosis and controlled degradations of the proteins. Subtle changes at one or some of these steps would represent an exquisite level of regulation that extends beyond the rapid channel opening and closure events associated with channel gating; membrane channels and receptors are constantly able to answer to physiological requirements to either up- or downregulate their activity. The Cx turnover rate thereby appears to be a key component in the regulation of any protein, particularly of gap junctional proteins. However, the physiological stimuli that control the assembly of Cxs into gap junctions and their degradation remain poorly understood.

Mechanisms of Cx43 and Cx26 transport to the plasma membrane and gap junction regeneration

Previous reports have suggested that Cx26 exhibits unique intracellular transport pathways en route to the cell surface compared with other members of the connexin family. To directly examine and compare nascent and steady-state delivery of Cx43 and Cx26 to the plasma membrane and gap junction biogenesis we expressed fluorescent-protein-tagged Cx43 and Cx26 in BICR-M1Rk and NRK cells. Static and time-lapse imaging revealed that both connexins were routed through the Golgi apparatus prior to being transported to the cell surface, a process inhibited in the presence of brefeldin A (BFA) or the expression of a dominant-negative form of Sar1 GTPase. During recovery from BFA, time-lapse imaging of nascent connexin Golgi-to-plasma membrane delivery revealed many dynamic post-Golgi carriers (PGCs) originating from the distal side of the Golgi apparatus consisting of heterogeneous vesicles and long, tubular-like extensions. Vesicles and tubular extensions were also observed in HBL-100 cells expressing a human, disease-linked, Golgi-localized Cx26 mutant, D66H-GFP. A diffuse cell surface rim of fluorescent-protein-tagged wild-type connexins was observed prior to the appearance of punctate gap junctions, which suggests that random fusion of PGCs occurred with the plasma membrane followed by lateral diffusion of connexins into clusters. Fluorescence recovery after photobleaching studies revealed that Cx26-YFP was more mobile within gap junction plaques compared with Cx43-GFP. Intriguingly, Cx43-GFP delivery and gap junction regeneration was inhibited by BFA and nocodazole, whereas Cx26-GFP delivery was prevented by BFA but not nocodazole. Collectively, these studies suggest that during gap junction biogenesis two phylogenetically distinct members of the connexin family, Cx43 and Cx26, share common secretory pathways, types of transport intermediates and turnover dynamics but differ in their microtubule-dependence and mobility within the plasma membrane, which might reflect differences in binding to protein scaffolds.

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.

The effects of connexin phosphorylation on gap junctional communication

Gap junctions are specialized membrane domains composed of collections of channels that directly connect neighboring cells providing for the cell-to-cell diffusion of small molecules, including ions, amino acids, nucleotides, and second messengers. Vertebrate gap junctions are composed of proteins encoded by the "connexin" gene family. In most cases examined, connexins are modified post-translationally by phosphorylation. Phosphorylation has been implicated in the regulation of gap junctional communication at several stages of the connexin "lifecycle", such as the trafficking, assembly/disassembly, degradation, as well as, the gating of gap junction channels. Since connexin43 (Cx43) is widely expressed in tissues and cell lines, we understand the most about how it is regulated, and thus, connexin43 phosphorylation is a major focus of this review. Recent reports utilizing new methodologies combined with the latest genome information have shown that activation of several kinases including protein kinase A, protein kinase C, p34(cdc2)/cyclin B kinase, casein kinase 1, mitogen-activated protein (MAP) kinase and pp60(src) kinase can lead to phosphorylation at 12 of the 21 serine and two of the six tyrosine residues in the C-terminal region of connexin43. In several cases, use of site-directed mutants of these sites have shown that these specific phosphorylation events can be linked to changes in gap junctional communication.

Connexin phosphorylation as a regulatory event linked to gap junction channel assembly

Gap junctions, composed of proteins from the connexin family, allow for intercellular communication between cells and are important in development and maintenance of cell homeostasis. Phosphorylation has been implicated in the regulation of gap junctional communication at several stages of the cell cycle and the connexin "lifecycle", such as trafficking, assembly/disassembly, degradation, as well as in the gating of "hemi" channels or intact gap junction channels. This review focuses on how phosphorylation can regulate the early stages of the connexin life cycle through assembly of functional gap junctional channels. The availability of sequences from the human genome databases has indicated that the number of connexins in the gene family is approximately 20, but we know mostly about how connexin43 (Cx43) is regulated. Recent technologies and investigations of interacting proteins have shown that activation of several kinases including protein kinase A, protein kinase C (PKC), p34(cdc2)/cyclin B kinase, casein kinase 1 (CK1), mitogen-activated protein kinase (MAPK) and pp60(src) kinase can lead to phosphorylation of the majority of the 21 serine and two of the tyrosine residues in the C-terminal region of Cx43. While many studies have correlated changes in kinase activity with changes in gap junctional communication, further research is needed to directly link specific phosphorylation events with changes in connexin oligomerization and gap junction assembly.

Disrupted traffic of connexin 43 in human testicular seminoma cells: overexpression of Cx43 induces membrane location and cell proliferation decrease

Connexins, the constitutive proteins of gap junctions, are considered to be tumour suppressive agents and are often impaired in the tumourigenic processes. In the present study, the expression of connexin 43 (Cx43), which is involved in the control of spermatogenesis through Sertoli/germ cell coupling, has been investigated in human testicular seminoma cells (tumours and the JKT-1 cell line). Cx43 was immunolocalized in the Golgi apparatus without membrane expression and was detected by immunoblotting in JKT-1 as exclusive 70 kD bands. No mutation could be found by sequencing the transcript obtained by RT-PCR. Transfection with a Cx43-V5 vector reproduced the same gel shift, identifying these 70 kD bands as Cx43. The Cx43-70 kD bands were also expressed in normal testicular tissue, associated with the classical 43 kD isoforms. Stable transfection of JKT-1 with a Cx43-GFP vector allowed restoration of Cx43 membrane expression, functional cell coupling, and inhibition of the cell proliferation rate. Storage of Cx43 in the Golgi apparatus may correspond during spermatogenesis to an intermittent physiological process that becomes permanent in malignant seminoma cells as a result of the tumourigenic process. By preventing Cx43 membrane expression, this disrupted traffic may itself participate in tumour promotion.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Forskolin-induced cyclic AMP signaling in single adherent bovine oviductal cells: effect of dichlorodiphenyltrichloroethane (DDT) and tris(4-chlorophenyl)methanol (TCPM)

The influence of tris(4-chlorophenyl)methanol (TCPM) and dichlorodiphenyltrichloroethane (o,p'DDT) on forskolin induced cAMP signalling in single adherent bovine oviductal cells was investigated. An increase in the intracellular cAMP levels was measured indirectly by an increase in the 520/580 nm fluorescence emission ratio of the protein kinase A fluorosensor (FICRhR). FICRhR was microinjected into single cells, and the 520/580 nm fluorescence emission ratio was monitored by image cytometry with an image analysis system as a measure of intracellular cAMP concentration ([cAMP](i)). Applications of dibutyryl cAMP and forskolin caused time- and dose-dependent effects on [cAMP](i) in single oviductal cells. The addition of 16 or 32 microM TCPM or DDT for 1 h to the culture medium decreased the intracellular cAMP concentration significantly, whereas 8 microM was not able to influence the [cAMP](i). In the presence of both pesticides at 16 microM the forskolin (30 microM)-induced [cAMP](i) was significantly reduced after 1 h of incubation. It is suggested that TCPM can have the same influence compared with DDT on cells responsible for reproduction.

Irsogladine malate up-regulates gap junctional intercellular communication between pancreatic cancer cells via PKA pathway

INTRODUCTION

Gap junctions (GJs) are intercellular channels that aid communication between coupling cells and may play a critical role in cell differentiation and growth. Connexins (Cxs) are structural proteins of GJs. Though several reports have demonstrated that Cx expression decreases in various malignant tumors, a pancreatic cancer cell line, PANC-1, was reported to express Cx43 mRNA. It is known that irsogladine malate (IM) can up-regulate gap junctional intercellular communication (GJIC). We examined the effects of IM on GJ between pancreatic cancer cells (PC cells) and the mechanism of GJ up-regulation.

METHODOLOGY

GJIC between PC cells (PANC-1) was evaluated by dye transfer methods. The expression of Cx43 was estimated by Western blot analysis with immunoprecipitation sample and immunohistochemical analysis. Intracellular cAMP level was estimated by enzyme-linked immunoassay.

RESULTS

IM increased cell coupling in a dose-dependent manner (0M-10 ). Western blot analysis of Cx43 revealed that PANC-1 cells expressed Cx43 protein. Treatment with IM was found to move localization of Cx43 immunoreactive spots from the cytoplasm to boundary lesions with neighboring cells, but no major change was seen in the phosphorylation state of Cx43. Intracellular cAMP level was increased by IM. The PKA inhibitor H-89 and adenylyl cyclase inhibitor SQ22536 inhibited the effects of IM.

CONCLUSION

These results suggest that IM up-regulates GJIC between PC cells via regulation of the PKA pathway. It also suggests a useful adjuvant of IM to pancreatic cancer therapy.

Lighting up gap junction channels in a flash

Gap junction intercellular communication channels permit the exchange of small regulatory molecules and ions between neighbouring cells and coordinate cellular activity in diverse tissue and organ systems. These channels have short half-lives and complex assembly and degradation pathways. Much of the recent work elucidating gap junction biogenesis has featured the use of connexins (Cx), the constituent proteins of gap junctions, tagged with reporter proteins such as Green Fluorescent Protein (GFP) and has illuminated the dynamics of channel assembly in live cells by high-resolution time-lapse microscopy. With some studies, however, there are potential short-comings associated with the GFP chimeric protein technologies. A recent report by Gaietta et al., has highlighted the use of recombinant proteins with tetracysteine tags attached to the carboxyl terminus of Cx43, which differentially labels 'old' and 'new' connexins thus opening up new avenues for studying temporal and spatial localisation of proteins and in situ trafficking events.

Evidence for a role of the N-terminal domain in subcellular localization of the neuronal connexin36 (Cx36)

The expression and functional properties of connexin36 (Cx36) have been investigated in two neuroblastoma cell lines (Neuro2A, RT4-AC) and primary hippocampal neurons transfected with a Cx36-enhanced green fluorescent protein (EGFP) expression vector. Transfected cells express Cx36-EGFP mRNA, and Cx36-EGFP protein is localized in the perinuclear area and cell membrane. Upon differentiation of cell lines, Cx36-EGFP protein was detectable in processes with both axonal and dendritic characteristics. Small gap junction plaques were found between adjacent cells, and electrophysiological recordings demonstrated that the electrical properties of these gap junctions were virtually indistinguishable from those reported for native Cx36. Mutagenesis of Cx36 led to the identification of a structural element that interferes with normal protein localization. In contrast, site directed mutagenesis of putative protein phosphorylation motifs did not alter subcellular localization. This excludes phosphorylation/dephosphorylation as a major regulatory step in Cx36 protein transport.

Desmosomes: interconnected calcium-dependent structures of remarkable stability with significant integral membrane protein turnover

Desmosomes are prominent cell adhesion structures that are major stabilizing elements, together with the attached cytoskeletal intermediate filament network, of the cytokeratin type in epithelial tissues. To examine desmosome dynamics in tightly coupled cells and in situations of decreased adhesion, fluorescent desmosomal cadherin desmocollin 2a (Dsc2a) chimeras were stably expressed in human hepatocellular carcinoma-derived PLC cells (clone PDc-13) and in Madin-Darby canine kidney cells (clone MDc-2) for the continuous monitoring of desmosomes in living cells. The hybrid polypeptides integrated specifically and without disturbance into normal-appearing desmosomes that occurred in association with typical cytokeratin filament bundles. Tracking of labeled adhesion sites throughout the cell cycle by time-lapse fluorescence microscopy revealed that they were immobile and that they maintained their structural integrity for long periods of time. Time-space diagrams further showed that desmosomal positioning was tightly controlled, even during pronounced cell shape changes, although the desmosomal arrays extended and contracted, suggesting that they were interconnected by a flexible system with intrinsic elasticity. Double-fluorescence microscopy detecting Dsc2a chimeras together with fluorescent cytokeratin 18 chimeras revealed the association and synchronous movement of labeled desmosomes and fluorescent cytokeratin filaments. Only a minor destabilization of desmosomes was observed during mitosis, demonstrated by increased diffuse plasma membrane fluorescence and the fusion of desmosomes into larger structures. Desmosomes did not disappear completely at any time in any cell, and residual cytokeratin filaments remained in association with adhesion sites throughout cell division. On the other hand, a rapid loss of desmosomes was observed upon calcium depletion, with irreversible uptake of some desmosomal particles. Simultaneously, diffusely distributed desmosomal cadherins were detected in the plasma membrane that retained the competence to nucleate the reformation of desmosomes after the cells were returned to a standard calcium-containing medium. To examine the molecular stability of desmosomes, exchange rates of fluorescent chimeras were determined by fluorescence recovery after photobleaching, thereby identifying considerable Dsc2a turnover with different rates of fluorescence recovery for PDc-13 cells (36±17% recovery after 30 minutes) and MDc-2 cells (60±20% recovery after 30 minutes). Taken together, our observations suggest that desmosomes are pliable structures capable of fine adjustment to functional demands despite their overall structural stability and relative immobility.

Differential effects of TPA and retinoic acid on cell-cell communication in human bronchial epithelial cells

Understanding how normal and immortalized bronchial epithelial cells respond to modulators of gap junctional communication will increase our understanding of the process of tumor promotion. In the present study we compared to effects of retinoic acid (RA) and 12-O-tetradecanoylphorbol-13-acetate (TPA) on the rate of fluorescent dye transfer via gap junctions in primary human tracheo-bronchial epithelial cells (TBE) and SV40 large T-antigen immortalized, non-tumorigenic bronchial epithelial cells (BEAS-2B). RA in the physiological range (0.001-1 microM) inhibited cell proliferation (DNA synthesis, mitotic index) more in primary TBE cells than BEAS-2B cells. Also in RA-treated cells, decreased cell proliferation was coupled to decreased gap junctional communication (GJC) in TBE but not in BEAS-2B cells. TPA strongly suppressed GJC and proliferation in primary TBE cells, whereas BEAS-2B exhibited increased GJC and retained a significant fraction of cells undergoing DNA synthesis. Our studies show that an uncoupling of GJC and cell proliferation is associated with a differential response to the growth inhibitory effects of RA and phorbol esters in immortalized compared to primary human bronchial epithelial cells.

Dominant-negative connexin43-EGFP inhibits calcium-transient synchronization of primary neonatal rat cardiomyocytes

Recent studies using mice with genetically engineered gap junction protein connexin (Cx) genes have provided evidence that reduced gap-junctional coupling in ventricular cardiomyocytes predisposes to ventricular arrhythmia. However, the pathological processes of arrhythmogenesis due to abnormalities in gap junctions are poorly understood. We have postulated a hypothesis that dysfunction of gap junctions at the single-cell level may affect synchronization of calcium transients among cardiomyocytes. To examine this hypothesis, we developed a novel system in which gap-junctional intercellular communication in primary neonatal rat cardiomyocytes was inhibited by a mutated (Delta130-137) Cx43 fused with enhanced green fluorescent protein (Cx43-EGFP), and calcium transients were imaged in real time while the mutated Cx43-EGFP-expressing cardiomyocytes were identified. The mutated Cx43-EGFP inhibited dye coupling not only in the liver epithelial cell line IAR 20 but also in primary neonatal rat cardiomyocytes in a dominant-negative manner, whereas wild-type Cx43-EGFP made functional gap junctions in otherwise communication-deficient HeLa cells. The mutated Cx43-EGFP induced desynchronization of calcium transients among cardiomyocytes with significantly higher frequency than wild-type Cx43-EGFP. These results suggest that dysfunction of gap-junctional intercellular communication at the single-cell level could hamper synchronous beating among cardiomyocytes as a result of desynchronization of calcium transients.

Organization of cyclic AMP-dependent connexin 43 in Swiss 3T3 cells attached to a cellulose substratum

We have previously shown that the adenylyl cyclase, which produces cyclic AMP (cAMP) in Swiss 3T3 cells, is activated by their attachment to a cellulose substratum (Cuprophan, CU). This substratum adsorbs vitronectin poorly, prevents cell spreading and causes them to aggregate. By contrast, cells spread out on polystyrene and contain low concentrations of cAMP. We have found that Connexin 43 (Cx 43) gap junction plaques are involved in this cell aggregation. MDL 12330 A, a specific inhibitor of adenylyl cyclase, prevented cell aggregation on CU and abolished Cx 43 channel clustering. But forskolin, a direct activator of adenylyl cyclase, and SBr cAMP, a cell-permeable analogue of cAMP, caused Cx 43 channel clustering in cells attached to polystyrene. Hence, Cx 43 channel clustering is regulated by cAMP in Swiss 3T3 cells. In addition, neither brefeldin A nor monensin (inhibitors of transit through the endoplasmic reticulum and Golgi apparatus), abolished Cx 43 channel clustering in cells aggregated on CU. Thus, the Cx 43 that form clusters in cells attached to CU are not dependent upon the trafficking of Cx 43 from intracellular storage sites, but are probably reorganised from the plasma membrane.

Ser364 of connexin43 and the upregulation of gap junction assembly by cAMP

The assembly of gap junctions (GJs) is a process coordinated by growth factors, kinases, and other signaling molecules. GJ assembly can be enhanced via the elevation of cAMP and subsequent stimulation of connexon trafficking to the plasma membrane. To study the positive regulation of GJ assembly, fibroblasts derived from connexin (Cx)43 knockout (KO) and wild-type (WT) mice were transfected with WT Cx43 (WTCx43) or mutant Cx43. GJ assembly between untransfected WT fibroblasts or stably transfected WTCx43/KO fibroblasts was increased two- to fivefold by 8Br-cAMP, and this increase could be blocked by inhibition of cAMP-dependent protein kinase (PKA) or truncation of the Cx43 COOH terminus (CT). Although serine 364 (S364) of the Cx43 CT was determined to be a major site of phosphorylation, the molar ratio of Cx43 phosphorylation was not increased by 8Br-cAMP. Importantly, GJ assembly between either S364ECx43/KO or S364ECx43/WT fibroblasts was stimulated by 8Br-cAMP, but that between S364ACx43/KO or S364PCx43/KO fibroblasts was not stimulated, indicating that phosphorylation or a negative charge at S364 is required for enhancement of GJ assembly by cAMP. Furthermore, GJ assembly between S364ACx43/WT fibroblasts could be stimulated by 8Br-cAMP, but could not be between S364PCx43/WT fibroblasts. Thus, S364PCx43 interferes with enhanced GJ assembly when coexpressed with WTCx43.

Accumulation of cyclic AMP in Swiss 3T3 cells adhering to a cellulose biomaterial substratum through interaction with adenylyl cyclase

Controlling cell shape induced by cell-substrata interaction appears of prime importance to influence subsequent biological processes such as cell migration, proliferation, differentiation or apoptosis. Studies on Swiss 3T3 fibroblasts have recently provided evidence that cell spreading is mediated by integrins and RhoA. Our previous studies showed that on Cuprophan, a cellulose membrane (CU) to which vitronectin adhesive protein is poorly adsorbed, Swiss 3T3 cells are rounded and undergo cAMP-dependent aggregation. In contrast, on a polyacrylonitrile membrane (AN69) that favours the adsorption of vitronectin and fibronectin, cells spread out and contain low concentrations of cAM P. We have now examined the parts played by the three components in the cAMP pathway (receptors, G-proteins and adenylyl cyclase itself) in cAM P-dependent cell aggregation on CU. Experiments with intact cells showed no interaction between the CU and receptors, or between the CU and G-proteins. Assays on membrane preparations plus the Mn-ATP substrate, which uncouples G-proteins and adenylyl cyclase, demonstrated that activation of the cAMP pathway by CU depends primarily on the catalytic activity of the adenylyl cyclase. These investigations provide essential data for the development of biomaterials that favour cell functionality.

Multiple pathways in the trafficking and assembly of connexin 26, 32 and 43 into gap junction intercellular communication channels

The assembly of gap junctions was investigated in mammalian cells expressing connexin (Cx) 26, 32 and 43 fused to green, yellow or cyan fluorescent proteins (GFP, YFP, CFP). Targeting of Cx32-CFP and 43-GFP to gap junctions and gap junctional communication was inhibited in cells treated with Brefeldin A, a drug that disassembles the Golgi. However gap junctions constructed of Cx26-GFP were only minimally affected by Brefeldin A. Nocodazole, a microtubule disruptor, had little effect on the assembly of Cx43-GFP gap junctions, but perturbed assembly of Cx26-GFP gap junctions. Co-expression of Cx26-YFP and Cx32-CFP in cells treated with Brefeldin A resulted in assembly of gap junctions constructed of Cx26-YFP. Two amino acids that distinguish Cx26 from Cx32 in transmembrane domains were mutated in Cx32 to investigate underlying mechanisms determining trafficking routes to gap junctions. One mutation, Cx32I28L, conferred on it partial Cx26-like trafficking properties as well the post-translational membrane insertion characteristics of Cx26, suggesting that a key determinant regulating trafficking was present in the first transmembrane domain. The results provide a protein trafficking basis for specifying and regulating connexin composition of gap junctions and thus selectivity of intercellular signaling, with Cx32 and 43 trafficking through the secretory pathway and Cx26 also following an alternative pathway.

cAMP enhances Cx43 gap junction formation and function and reverses choline deficiency apoptosis

Previously, it had been shown that acute choline deficiency (CD) induced apoptosis in cultured rat liver epithelial cells, whereas cells that are adapted to survive in low-choline-containing medium acquire resistance to CD apoptosis and undergo malignant transformation. Thus, understanding the mechanisms of action of CD could increase our understanding of the role of choline, an essential nutrient, in the process of malignant transformation. The present experiments were designed to test the hypothesis that CD might function as a pro-apoptotic trigger by altering the localization of connexin 43 gap junction protein and gap junctional intercellular communication (GJIC). Established liver epithelial cells (WB cells; Hep3B cells) were maintained in a defined, serum-free medium control (70 microM choline) or choline deficient medium (CD, 5 microM choline) and the localization of connexin 43 protein (Cx43) was studied by immunocytochemistry and Western blotting. In nontumorigenic WB cells, CD apoptosis was associated with retention of Cx43 in the golgi/ER region of the cytoplasm and decreased GJIC as measured using a preloading fluorescent dye transfer method (calcein AM/DiIC(18)). Cells maintained in CD in the presence of 8-bromoadenosine 3':5'-cyclic monophosphate exhibited restoration of Cx43 at the plasma membrane and increased GJIC and inhibition of apoptosis. These studies show that CD apoptosis in nontumorigenic liver epithelial cells is associated with alterations to Cx43 and GJIC and that an uncoupling of Cx43 localization and GJIC is related to resistance to CD apoptosis in transformed liver epithelial cells.

Comparative analysis and application of fluorescent protein-tagged connexins

In order to examine connexin transport, assembly, and turnover in living cells, we tagged green fluorescent protein or its color variants to several members of the connexin family of proteins. When green fluorescent protein was tagged to the carboxyl terminal end of connexin43 (Cx43-GFP), the resulting fusion protein was transported and assembled into functional gap junctions. However, when GFP was tagged to the amino terminal end of Cx43 (GFP-Cx43), this chimera was biosynthesized, transported to the plasma membrane, but failed to form gap junction channels that could transfer Lucifer yellow. Single cells that expressed Cx43-GFP were capable of transporting this fusion protein to the cell surface in the absence of cell-cell contact. Imaging of Cx43-yellow (Y)FP (Cx43-YFP) was quite efficient; however, the low quantum yield Cx43-BFP and the requirement for ultraviolet excitation made this chimera less suitable for time-lapse imaging. Cx43-cyan C(FP) (Cx43-CFP) was more suitable for imaging than Cx43-blue (B)FP and could be effectively separated from Cx43-YFP. The versatility of tagging GFP to the carboxyl terminal end of other members of the connexin family was established when Cx32-GFP and Cx26-YFP were found to assemble into gap junctions capable of transferring Lucifer yellow. Finally, we are examining the effectiveness of using a new red fluorescent protein (DsRed) fused to connexins in combination with Cx-GFP to simultaneously examine the kinetics, transport and turnover of two connexins. Together, our studies suggest that tagging fluorescent proteins to the carboxyl terminal end of connexins is an effective and valuable approach for studying the life cycle and dynamics of connexins in living cells.

Specific labeling of connexin43 in NRK cells using tyramide-based signal amplification and fluorescence photooxidation

Imaging of gap junction proteins, the connexins, has been performed in tissue culture cells both by labeling of connexins with immunocytochemical tags and by cloning and expressing chimeras of connexins and fluorescent proteins such as Green Fluorescent Protein. These two approaches have been used to gain information about protein localization or trafficking at light microscopic resolution. Electron microscopy provides higher resolution; however, analysis of electron micrographs of unlabeled connexins has been generally limited to recognition of gap junction structures. Immunolabeling of gap junction proteins in whole cells at the electron microscopic level has been difficult to achieve because of the fixation sensitivity of most gap junction antibodies. To obtain reasonable sensitivity, immunoperoxidase procedures are typically employed, and these suffer from relatively poor resolution. Here we describe the combination of tyramide signal amplification techniques and fluorescence photooxidation for higher resolution immunolocalization studies for correlative light and electron microscopic imaging. By using correlative microscopy, we can not only localize connexin pools or structures, but also discover what other cellular substructures interact with gap junction proteins. The use of tyramide signal amplification techniques is necessary to increase fluorescence levels that have decreased due to increased specimen fixation required to maintain cell ultrastructure. The fluorescence photooxidation technique provides a high-resolution method for staining of proteins in cells. Unlike colloidal gold-based methods, fluorescence photooxidation allows for three-dimensional localization using high-voltage electron microscopy.

High resolution, fluorescence deconvolution microscopy and tagging with the autofluorescent tracers CFP, GFP, and YFP to study the structural composition of gap junctions in living cells

High-resolution, fluorescence deconvolution (DV) microscopy was implemented to obtain a detailed view of the organization and structural composition of gap junctions assembled from one or two different connexin isotypes in live and fixed cells. To visualize gap junctions, the structural protein components of gap junction channels, the connexin polypeptides alpha1(Cx43), beta1(Cx32), and beta2(Cx26), were tagged on their C-termini with the autofluorescent tracers green fluorescent protein (GFP), and its cyan (CFP), and yellow (YFP) color variants. Tagged connexins were expressed in transiently transfected HeLa cells. Comprehensive analysis including dye-transfer analysis demonstrated that the tagged connexins trafficked, assembled, and packed normally into functional gap junction channel plaques. Such gap junction plaques were examined by single, dual, and triple-color DV microscopy. High-resolution images and three-dimensional volume reconstructions of gap junction plaques were obtained by this technique, which revealed several new aspects of gap junction structure. Specifically, the studies demonstrated that the mode of channel distribution strictly depends on the connexin isotypes. Here we present such images, and volume reconstructions in context with images obtained by other light, and electron microscopic techniques, such as laser scanning confocal, conventional wide-field fluorescence, thin section, and freeze-fracture electron microscopy. In addition, we give a simple description of the principal mechanisms of DV microscopy, name advantages and disadvantages, and discuss issues such as dual-color imaging using CFP and YFP, spatial resolution, colocalization, and avoiding imaging artifacts.

Looking at Receptors: What Have Fluorescent Receptors and Channels Told Us?

The receptors and channels that reside on the surface of a neuron enable it to respond to and integrate a wide variety of signals. Electrophysiology has made it possible to study the behavior of these channels in remarkable detail. For instance, patch-clamp recording has made it possible in many instances to actually resolve the opening and closing of individual channels. Similarly, immuncytochemistry has provided us with static images of where these proteins are in a neuron. Nevertheless, we know remarkably little about how these proteins are actually used by living cells. Fundamental questions concerning how long they are at the surface, how localized they are, how quickly they are internalized in response to activation, or how free they are to move about on the surface remain to be addressed. One way to answer such questions is to fluorescently label these proteins and image them in living cells. The discovery of the jellyfish green fluorescent protein has recently made this feasible, and the new views it is providing are the topic of this review.

Cyclic AMP and LDL trigger a rapid enhancement in gap junction assembly through a stimulation of connexin trafficking

Given the rapid turnover of connexin proteins, gap junction (GJ) assembly represents an important means of regulating the extent of GJ communication between cells. This report describes an increase in the level of GJ assembly within one hour following treatment with cAMP-elevating reagents or low density lipoprotein (LDL). Dye transfer methods and freeze-fracture with electron microscopy were used to assay junctional permeability and structure, respectively, subsequent to the dissociation, recovery and reaggregation of Novikoff hepatoma cells. Reaggregating cells in the presence of agents that increase cAMP levels (8-Br-cAMP, forskolin and IBMX) enhanced both dye transfer rates between cells and the extent of GJ formation 2- to 3-fold. These data and studies with the protein kinase A inhibitor, H-89, indicate that cAMP signaling plays a key role in enhanced assembly. The response to LDL parallels that to cAMP and relies on the activity of both adenylyl cyclase and protein kinase A. Immunoblot analysis revealed no change in the level of connexin43 (Cx43) or its phosphorylation states over a period of 2.5 hours. However, three agents (brefeldin A, monensin and nocodazole), that inhibit intracellular membrane trafficking by different mechanisms, all blocked the enhanced assembly of GJs when triggered by either elevated cAMP or exposure to LDL. Related studies, which employed trafficking inhibitors at different stages in GJ assembly, suggested that Cx43 trafficking during enhanced assembly is regulated, in part, by cell contact. Intracellular sources of Cx43 were characterized by colabeling for several markers of cytoplasmic membrane systems. We conclude that an increase in GJ assembly: (i) occurs rapidly in the presence of elevated cAMP or LDL, (ii) does not require an increase in Cx43 levels or major changes in Cx43 phosphorylation and (iii) is dependent upon the trafficking of Cx43 from intracellular storage sites.

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.