Evidence for heteromeric gap junction channels formed from rat connexin43 and human connexin37

Human connexin26 and connexin30 form functional heteromeric and heterotypic channels

Mutations in GJB2 and GJB6, the genes that encode the human gap junction proteins connexin26 (Cx26) and connexin30 (Cx30), respectively, cause hearing loss. Cx26 and Cx30 are both expressed in the cochlea, leading to the potential formation of heteromeric hemichannels and heterotypic gap junction channels. To investigate their interactions, we expressed human Cx26 and Cx30 individually or together in HeLa cells. When they were expressed together, Cx26 and Cx30 appeared to interact directly (by their colocalization in gap junction plaques, by coimmunoprecipitation, and by fluorescence resonance energy transfer). Scrape-loading cells that express either Cx26 or Cx30 demonstrated that Cx26 homotypic channels robustly transferred both cationic and anionic tracers, whereas Cx30 homotypic channels transferred cationic but not anionic tracers. Cells expressing both Cx26 and Cx30 also transferred both cationic and anionic tracers by scrape loading, and the rate of calcein (an anionic tracer) transfer was intermediate between their homotypic counterparts by fluorescence recovery after photobleaching. Fluorescence recovery after photobleaching also showed that Cx26 and Cx30 form functional heterotypic channels, allowing the transfer of calcein, which did not pass the homotypic Cx30 channels. Electrophysiological recordings of cell pairs expressing different combinations of Cx26 and/or Cx30 demonstrated unique gating properties of cell pairs expressing both Cx26 and Cx30. These results indicate that Cx26 and Cx30 form functional heteromeric and heterotypic channels, whose biophysical properties and permeabilities are different from their homotypic counterparts.

Endothelial connexin 37, connexin 40, and connexin 43 respond uniquely to substrate and shear stress

Endothelial connexins have been linked to atherosclerosis and hypertension; however, little is know about their sensitivity to stimuli and individual functions. This study investigates the responses of endothelial connexin 37, connexin 40, and connexin 43 (Cx37, Cx40, and Cx43) to shear stress and substrate. Human endothelial cells were seeded on adsorbed collagen or a collagen gel containing smooth muscle cells and exposed to static or laminar shear stress. Connexin mRNA, protein, and gap junction communication were examined. Endothelial monolayers were treated with connexin-specific short interfering RNA (siRNA) and evaluated for communication, proliferation, and morphology under static and shear stress. Results show differential responses of Cx37, Cx40, and Cx43 to substrate and shear stress with reduced communication after shear exposure. RNA interference of individual connexins resulted in expression change of nontarget connexins, which suggests linked expression. Gap junction communication under static conditions is reduced following Cx43 siRNA treatment. Endothelial cells are more elongated with RNA interference (RNAi) targeting Cx40. In conclusion, endothelial connexins demonstrated novel sensitivity to mechanical environment and substrate. Individual isotypes show differential responses and RNAi knockdown provides new insight into connexin function and potential roles in the vasculature.

Identification of mutations in members of the connexin gene family as a cause of nonsyndromic deafness in Taiwan

Connexins (Cx), a large family of membrane proteins, are key components of gap junction channels. These channels are critical intercellular pathways through which ions or small molecules are passed, regulating a variety of physiological and developmental processes. One of these processes is hearing. In the current study, a genetic survey was made on 380 Taiwanese individuals, 260 with nonsyndromic deafness and 120 with normal hearing. All the 380 Taiwanese were screened for the presence of mutations in 8 genes of the Cx gene family. These genes included Cx26 (GJB2), Cx29 (GJE1), Cx30 (GJB6), Cx30.3 (GJB4), Cx31 (GJB3), Cx32 (GJB1), Cx43 (GJA1) and pseudogene [rho] of Cx43 (rho GJA1). Mutations were identified in 7 out of the 8 screened genes of the Cx family from 62 of the 260 deaf subjects (23.85%). Of the 17 mutations observed in the Cx gene family, 11 were novel mutations. Fourteen polymorphisms that were not associated with hearing loss were identified in the Cx gene family. The first 2 most frequently occurring mutations were found in the Cx26 (28/62; 45.16%) and the rho Cx43 (17/62; 27.42%), respectively. Nine cases of mutations were found in the Cx30.3 (9/62; 14.52%). In the Cx30, 1 novel mutation was identified in 1 case (1/62; 1.61%). Two patients with mutations of each of Cx29 and Cx43 were found (2/62; 3.23%). One novel mutation of Cx31 was identified in 3 patients with nonsyndromic deafness (3/62; 4.84%). The Cx32 was the only gene without detecting any mutation or polymorphism.Our study provides information for understanding the importance of genetic factors in nonsyndromic deafness of the Taiwanese and may be of use in the improvement of genetic diagnosis of hearing loss in Taiwan.

Sphingosine 1-phosphate induces myoblast differentiation through Cx43 protein expression: a role for a gap junction-dependent and -independent function

Although sphingosine 1-phosphate (S1P) has been considered a potent regulator of skeletal muscle biology, acting as a physiological anti-mitogenic and prodifferentiating agent, its downstream effectors are poorly known. In the present study, we provide experimental evidence for a novel mechanism by which S1P regulates skeletal muscle differentiation through the regulation of gap junctional protein connexin (Cx) 43. Indeed, the treatment with S1P greatly enhanced Cx43 expression and gap junctional intercellular communication during the early phases of myoblast differentiation, whereas the down-regulation of Cx43 by transfection with short interfering RNA blocked myogenesis elicited by S1P. Moreover, calcium and p38 MAPK-dependent pathways were required for S1P-induced increase in Cx43 expression. Interestingly, enforced expression of mutated Cx43(Delta130-136) reduced gap junction communication and totally inhibited S1P-induced expression of the myogenic markers, myogenin, myosin heavy chain, caveolin-3, and myotube formation. Notably, in S1P-stimulated myoblasts, endogenous or wild-type Cx43 protein, but not the mutated form, coimmunoprecipitated and colocalized with F-actin and cortactin in a p38 MAPK-dependent manner. These data, together with the known role of actin remodeling in cell differentiation, strongly support the important contribution of gap junctional communication, Cx43 expression and Cx43/cytoskeleton interaction in skeletal myogenesis elicited by S1P.

N-terminal residues in Cx43 and Cx40 determine physiological properties of gap junction channels, but do not influence heteromeric assembly with each other or with Cx26

The cytoplasmic N-terminal domain in the connexins (Cx) has been implicated in determining several properties including connexin hetero-oligomerization, channel gating and regulation by polyamines. To elucidate the roles of potentially crucial amino acids, we produced site-directed mutants of connexins Cx40 and Cx43 (Cx40E12S,E13G and Cx43D12S,K13G) in which the charged amino acids at positions 12 and 13 were replaced with serine and glycine as found in Cx32. HeLa, N2a and HEK293 cells were transfected and studied by immunochemistry and double whole-cell patch clamping. Immunoblotting confirmed production of the mutant proteins, and immuno-fluorescence localized them to punctuate distributions along appositional membranes. Cx40E12S,E13G and Cx43D12S,K13G formed homotypic gap junction channels that allowed intercellular passage of Lucifer Yellow and electrical current, but these channels exhibited negligible voltage-dependent gating properties. Unlike wild-type Cx40, Cx40E12S,E13G channels were insensitive to block by 2 mM spermine. Affinity purification of material solubilized by Triton X-100 from cells co-expressing mutant Cx43 or mutant Cx40 with wild-type Cx40, Cx43 or Cx26 showed that introducing the mutations did not affect the compatibility or incompatibility of these proteins for heteromeric mixing. Co-expression of Cx40E12S,E13G with wild-type Cx40 or Cx43 dramatically reduced voltage-dependent gating. Thus, whereas the charged amino acids at positions 12 and 13 of Cx40 or Cx43 are not required for gap junction assembly or the compatibility of oligomerization with each other or with Cx26, they strongly influence several physiological properties including those of heteromeric channels.

Connexin43 with a cytoplasmic loop deletion inhibits the function of several connexins

Connexins (Cx) form gap junction channels mediating direct intercellular communication. To study the role of amino acids within the cytoplasmic loop, we produced a recombinant adenovirus containing Cx43 with a deletion of amino acids 130-136 (Cx43del(130-136)). Cx43del(130-136) expressed alone in HeLa cells localized within the cytoplasm and did not allow transfer of ions, neurobiotin or Lucifer yellow. When co-expressed with wild type Cx43, Cx43del(130-136) blocked electrical coupling and transfer of neurobiotin or Lucifer yellow. Cx43del(130-136) and Cx43 co-localized by immunofluorescence and were co-purified from Triton X-100-solubilized cell extracts. Intercellular transfer mediated by Cx37 and Cx45 (but not Cx26 or Cx40) was inhibited when co-expressed with Cx43del(130-136). Cx43del(130-136) co-localized with Cx37, Cx40, or Cx45, but not Cx26. These data suggest that Cx43del(130-136) produces connexin-specific inhibition of intercellular communication through formation of heteromeric connexons that are non-functional and/or retained in the cytoplasm.

Connexin-specific cell-to-cell transfer of short interfering RNA by gap junctions

The purpose of this study was to determine whether oligonucleotides the size of siRNA are permeable to gap junctions and whether a specific siRNA for DNA polymerase beta (pol beta) can move from one cell to another via gap junctions, thus allowing one cell to inhibit gene expression in another cell directly. To test this hypothesis, fluorescently labelled oligonucleotides (morpholinos) 12, 16 and 24 nucleotides in length were synthesized and introduced into one cell of a pair using a patch pipette. These probes moved from cell to cell through gap junctions composed of connexin 43 (Cx43). Moreover, the rate of transfer declined with increasing length of the oligonucleotide. To test whether siRNA for pol beta was permeable to gap junctions we used three cell lines: (1) NRK cells that endogenously express Cx43; (2) Mbeta16tsA cells, which express Cx32 and Cx26 but not Cx43; and (3) connexin-deficient N2A cells. NRK and Mbeta16tsA cells were each divided into two groups, one of which was stably transfected to express a small hairpin RNA (shRNA), which gives rise to siRNA that targets pol beta. These two pol beta knockdown cell lines (NRK-kcdc and Mbeta16tsA-kcdc) were co-cultured with labelled wild type, NRK-wt or Mbeta16tsA-wt cells or N2A cells. The levels of pol beta mRNA and protein were determined by semiquantitative RT-PCR and immunoblotting. Co-culture of Mbeta16tsA-kcdc cells with Mbeta16tsA-wt, N2A or NRK-wt cells had no effect on pol beta levels in these cells. Similarly, co-culture of NRK-kcdc with N2A cells had no effect on pol beta levels in the N2A cells. In contrast, co-culture of NRK-kcdc with NRK-wt cells resulted in a significant reduction in pol beta in the wt cells. The inability of Mbeta16tsA-kcdc cells to transfer siRNA is consistent with the fact that oligonucleotides of the 12 nucleotide length were not permeable to Cx32/Cx26 channels. This suggested that Cx43 but not Cx32/Cx26 channels allowed the cell-to-cell movement of the siRNA. These results support the novel hypothesis that non-hybridized and possible hybridized forms of siRNA can move between mammalian cells through connexin-specific gap junctions.

Functional consequences of heterogeneous gap junction channel formation and its influence in health and disease

The capacity of multiple connexins to hetero-oligomerize into functional heterogeneous gap junction channels has been demonstrated in vivo, in vitro, and in nonmammalian expression systems. These heterogeneous channels display gating activity, channel conductances, selectivity and regulatory behaviors that are sometimes not predicted by the behaviors of the corresponding homogeneous channels. Such observations suggest that heteromerization of gap junction proteins offers an efficient cellular strategy for finely regulating cell-to-cell communication. The available evidence strongly indicates that heterogeneous gap junction assembly is important to normal growth and differentiation, and may influence the appearance of several disease states. Definitive evidence that heterogeneous gap junction channels differentially regulate electrical conduction in excitable cells is absent. This review examines the prevalence, regulation, and implications of gap junction channel hetero-oligomerization.

Connexin phosphorylation as a regulatory event linked to gap junction internalization and degradation

Gap junction proteins, connexins, are dynamic polytopic membrane proteins that exhibit unprecedented short half-lives of only a few hours. Consequently, it is well accepted that in addition to channel gating, gap junctional intercellular communication is regulated by connexin biosynthesis, transport and assembly as well as the formation and removal of gap junctions from the cell surface. At least nine members of the 20-member connexin family are known to be phosphorylated en route or during their assembly into gap junctions. For some connexins, notably Cx43, evidence exists that phosphorylation may trigger its internalization and degradation. In recent years it has become apparent that the mechanisms underlying the regulation of connexin turnover are quite complex with the identification of many connexin binding molecules, a multiplicity of protein kinases that phosphorylate connexins and the involvement of both lysosomal and proteasomal pathways in degrading connexins. This paper will review the evidence that connexin phosphorylation regulates, stimulates or triggers gap junction disassembly, internalization and degradation.

Structural organization of gap junction channels

Gap junctions were initially described morphologically, and identified as semi-crystalline arrays of channels linking two cells. This suggested that they may represent an amenable target for electron and X-ray crystallographic studies in much the same way that bacteriorhodopsin has. Over 30 years later, however, an atomic resolution structural solution of these unique intercellular pores is still lacking due to many challenges faced in obtaining high expression levels and purification of these structures. A variety of microscopic techniques, as well as NMR structure determination of fragments of the protein, have now provided clearer and correlated views of how these structures are assembled and function as intercellular conduits. As a complement to these structural approaches, a variety of mutagenic studies linking structure and function have now allowed molecular details to be superimposed on these lower resolution structures, so that a clearer image of pore architecture and its modes of regulation are beginning to emerge.

The permeability of gap junction channels to probes of different size is dependent on connexin composition and permeant-pore affinities

Gap junctions have traditionally been characterized as nonspecific pores between cells passing molecules up to 1 kDa in molecular mass. Nonetheless, it has become increasingly evident that different members of the connexin (Cx) family mediate quite distinct physiological processes and are often not interchangeable. Consistent with this observation, differences in permeability to natural metabolites have been reported for different connexins, although the physical basis for selectivity has not been established. Comparative studies of different members of the connexin family have provided evidence for ionic charge selectivity, but surprisingly little is known about how connexin composition affects the size of the pore. We have employed a series of Alexa dyes, which share similar structural characteristics but range in size from molecular weight 350 to 760, to probe the permeabilities and size limits of different connexin channels expressed in Xenopus oocytes. Correlated dye transfer and electrical measurements on each cell pair, in conjunction with a three-dimensional mathematical model of dye diffusion in the oocyte system, allowed us to obtain single channel permeabilities for all three dyes in six homotypic and four heterotypic channels. Cx43 and Cx32 channels passed all three dyes with similar efficiency, whereas Cx26, Cx40, and Cx45 channels showed a significant drop-off in permeability with the largest dye. Cx37 channels only showed significant permeability for the smaller two dyes, but at two- to sixfold lower levels than other connexins tested. In the heterotypic cases studied (Cx26/Cx32 and Cx43/Cx37), permeability characteristics were found to resemble the more restrictive parental homotypic channel. The most surprising finding of the study was that the absolute permeabilities calculated for all gap junctional channels in this study are, with one exception, at least 2 orders of magnitude greater than predicted purely on the basis of hindered pore diffusion. Consequently, affinity between the probes and the pore creating an energetically favorable in-pore environment, which would elevate permeant concentration within the pore and hence the flux, is strongly implicated.

A carboxyl terminal domain of connexin43 is critical for gap junction plaque formation but not for homo- or hetero-oligomerization

We have initiated a series of experiments to analyze the biosynthesis and oligomerization of Cx43 in cells containing other connexins through the expression of site-directed mutants and chimeric connexin polypeptides. Here we report studies concerning a mutant of Cx43 (Cx43tr) that has been truncated after amino acid 251 to remove most of the Cx43 carboxy-terminal region. In stably transfected HeLa cells, full length Cx43 localized primarily to appositional membranes while much more Cx43tr was observed in the cytoplasm. Both Cx43 and Cx43tr showed similar oligomerization profiles based on centrifugation through sucrose gradients. HeLaCx43tr cells showed limited transfer of microinjected Lucifer Yellow but did show electrical coupling. Co-expression of Cx43tr with Cx43 or Cx45 led to Cx43tr localization at appositional membranes and co-localization with the other connexins. Moreover, cells co-expressing Cx43tr with Cx43 or Cx45 showed extensive intercellular dye coupling. Thus, Cx43tr was able to oligomerize and form functional channels when expressed alone or with a compatible connexin, but it only formed plaques when co-expressed. These results suggest that the carboxyl tail of Cx43 is not important for oligomerization, but they implicate critical residues in the formation of gap junction plaques.

Shear stress-induced upregulation of connexin 43 expression in endothelial cells on upstream surfaces of rat cardiac valves

Endothelial expression of the gap junction proteins, connexin (Cx) 37, Cx40, and Cx43, varies within the vascular network. While previous studies suggest that shear stress may upregulate Cx43, it is not well understood if shear stress affects the expression of all endothelial connexins and to what extent. Endothelial cells on the upstream and downstream surfaces of cardiac valves are subjected to considerably different intensities of shear stress. We therefore reasoned that we could determine the extent hemodynamic forces affect the expression of Cx37, Cx40, and Cx43 by comparing their immunohistochemical distribution on the upstream and downstream surfaces of rat cardiac valves. We found 70- to 200-fold greater expression of Cx43 in the endothelial cells on the upstream than on the downstream surfaces. However, Cx37 was expressed almost equally in the endothelial cells on upstream and downstream surfaces, and Cx40, a major connexin in most vascular endothelial cells, was not detected on either surface. In addition to the heterogeneity in Cx43 expression, endothelial cells on the upstream surface were 35% to 65% smaller than those on the corresponding downstream surface. These results suggest that shear stress may affect endothelial cell size and Cx43 expression but not Cx37 expression.

The conduction of dilation along an arteriole is diminished in the cremaster muscle of hypertensive hamsters

Arteriolar vasomotor responses can include a component that conducts along the vessel through gap junction channels. This study examined conducted vasomotor responses in arterioles of the hypertensive hamster. The cremaster muscle of normotensive (CHF-148) and spontaneously hypertensive (CHF-H4) hamsters was exteriorized. Micropipettes containing phenylephrine (0.1 M) or acetylcholine (ACh; 1.0 M) were positioned along second-order arterioles and diameter responses were recorded locally for every 0.4 mm upstream to 1.6 mm. Substantative local constrictions to phenylephrine(PE) were poorly conducted to the 0.4-mm site in normotensive and hypertensive hamsters. Local dilation to ACh decayed by 3 +/- 1 microm/mm as it conducted along arterioles of the normotensive hamster. In contrast, conducted dilation decayed by 7 +/- 1 microm/mm (p < 0.05) in the hypertensive hamster. This hypertension-induced increase in decay was reversed by alpha-adrenergic receptor blockade (phentolamine: 1 microM). However, arteriolar constriction to global alpha(1)- (PE) and alpha(2)- (clonidine) adrenergic agonists was unaffected by hypertension. Rather, sympathetic nervous activity was elevated in the hypertensive hamster as indicated by a greater reduction in arterial pressure upon sympathetic ablation (hexamethonium infusion: 30 mg/kg). This study provides the first evidence that vascular cell-cell communication is altered by the elevated sympathetic nervous activity observed in the hypertensive hamster.

Adenoviral delivery of human connexin37 induces endothelial cell death through apoptosis

Gap junction channels formed of connexins directly link the cytoplasm of adjacent cells and have been implicated in intercellular signaling that may regulate the functions of vascular cells. To facilitate connexin manipulation and analysis of their roles in adult endothelial cells, we developed adenoviruses containing the vascular connexins (Cx37, Cx40, and Cx43). We infected cultured human umbilical vein endothelial cells with control or connexin adenoviruses. Connexin expression was verified by immunoblotting and immunofluorescence. Infection with the Cx37 adenovirus (but not control or other connexin adenoviruses) led to a dose-dependent death of the endothelial cells that was partially antagonized by the gap junction blocker alpha-glycyrrhetinic acid and altered the intercellular transfer of Lucifer yellow and neurobiotin. Cell morphology, Annexin V and TUNEL staining, and caspase 3 assays all implicated apoptosis in the cell death. These data suggest that connexin-specific alterations of intercellular communication may modulate endothelial cell growth and death.

Connexin43 and connexin26 form gap junctions, but not heteromeric channels in co-expressing cells

Many cells contain two (or more) gap junction proteins that are able to oligomerize with each other to form heteromeric gap junction channels and influence the properties of intercellular communication. Cx26 and Cx43 are found together in a number of cell types, but previous data have suggested that they might not form heteromeric connexons. We studied the possible interactions of these connexins by co-expression in three different cell lines. Analysis of N2aCx26/Cx43 cell pairs by double whole-cell patch-clamp methods showed that these cells were coupled, but contained only a small number of sizes of single channels consistent with those formed by homomeric Cx26 or Cx43 channels. Immunofluorescence studies showed that both connexins localized to appositional membranes, but in largely distinct domains. Analysis of Triton X-100-solubilized connexons from co-expressing cells by centrifugation through sucrose gradients or by affinity purification using a Ni-NTA column showed no evidence of mixing of Cx26 and Cx43. These results contrast with our observations in cells co-expressing other connexins with Cx43 and suggest that Cx26 and Cx43 do not form heteromeric hemichannels. Moreover, the incorporation of Cx26 and Cx43 into oligomers and into the membrane were similarly affected by treatment of co-expressing cells with brefeldin A or nocodazole, suggesting that the lack of mixing is due to incompatibility of these connexins, not to differences in biosynthetic trafficking.

Selective permeability of gap junction channels

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

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.

Functional Domain Mapping and Selective Trans-dominant Effects Exhibited by Cx26 Disease-causing Mutations

Mutations in Cx26 are a major cause of autosomal dominant and recessive forms of sensorineural deafness. Some mutations in Cx26 are associated not only with deafness but also with skin disease. We examined the subcellular localization and function of two green fluorescent protein (GFP)-tagged Cx26 point mutants that exhibit both phenotypes, G59A-GFP and D66H-GFP. D66H-GFP was retained within the brefeldin A-insensitive trans-Golgi network, whereas a population of G59A-GFP was transported to the cell surface. Neither G59A nor D66H formed gap junctions that were permeable to small fluorescent dyes, suggesting they are loss-of-function mutations. When co-expressed with wild-type Cx26, both G59A and D66H exerted dominant-negative effects on Cx26 function. G59A also exerted a trans-dominant negative effect on co-expressed wild type Cx32 and Cx43, whereas D66H exerted a trans-dominant negative effect on Cx43 but not Cx32. We propose that the severity of the skin disease is dependent on the specific nature of the Cx26 mutation and the trans-dominant selectivity of the Cx26 mutants on co-expressed connexins. Additional systematic mutations at residue D66, in which the overall charge of this motif was altered, suggested that the first extracellular loop is critical for Cx26 transport to the cell surface as well as function of the resulting gap junction channels.

Cardiac connexins Cx43 and Cx45: formation of diverse gap junction channels with diverse electrical properties

HeLa cells expressing rat connexin43 (Cx43) and/or mouse Cx45 were studied with the dual voltage-clamp technique. Different types of cell pairs were established and their gap junction properties determined, i.e. the dependence of the instantaneous and steady-state conductances (gj,inst, gj,ss) on the transjunctional voltage (Vj) and the kinetics of inactivation of the gap junction current (Ij). Pairs of singly transfected cells showed homogeneous behaviour at both Vj polarities. Homotypic Cx43-Cx43 and Cx45-Cx45 cell pairs yielded distinct symmetrical functions gj,inst=f(Vj) and gj,ss=f(Vj). Heterotypic Cx43-Cx45 preparations exhibited asymmetric functions gj,inst=f(Vj) and gj,ss=f(Vj) suggesting that connexons Cx43 and Cx45 gate with positive and negative Vj, respectively. Preparations containing a singly (Cx43 or Cx45) or doubly (Cx43/45) transfected cell showed quasi-homogeneous behaviour at one Vj polarity and heterogeneous behaviour at the other polarity. The former yielded Boltzmann parameters intermediate between those of Cx43-Cx43, Cx45-Cx45 and Cx43-Cx45 preparations; the latter could not be explained by homotypic and heterotypic combinations of homomeric connexons. Each pair of doubly transfected cells (Cx43/Cx45) yielded unique functions gj,inst=f(Vj) and gj,ss=f(Vj). This can not be explained by combinations of homomeric connexons. We conclude that Cx43 and Cx45 form homomeric-homotypic, homomeric-heterotypic channels as well as heteromeric-homotypic and heteromeric-heterotypic channels. This has implications for the impulse propagation in specific areas of the heart.

Human mesenchymal stem cells make cardiac connexins and form functional gap junctions

Human mesenchymal stem cells (hMSCs) are a multipotent cell population with the potential to be a cellular repair or delivery system provided that they communicate with target cells such as cardiac myocytes via gap junctions. Immunostaining revealed typical punctate staining for Cx43 and Cx40 along regions of intimate cell-to-cell contact between hMSCs. The staining patterns for Cx45 rather were typified by granular cytoplasmic staining. hMSCs exhibited cell-to-cell coupling to each other, to HeLa cells transfected with Cx40, Cx43 and Cx45 and to acutely isolated canine ventricular myocytes. The junctional currents (I(j)) recorded between hMSC pairs exhibited quasi-symmetrical and asymmetrical voltage (V(j)) dependence. I(j) records from hMSC-HeLaCx43 and hMSC-HeLaCx40 cell pairs also showed symmetrical and asymmetrical V(j) dependence, while hMSC-HeLaCx45 pairs always produced asymmetrical I(j) with pronounced V(j) gating when the Cx45 side was negative. Symmetrical I(j) suggests that the dominant functional channel is homotypic, while the asymmetrical I(j) suggests the activity of another channel type (heterotypic, heteromeric or both). The hMSCs exhibited a spectrum of single channels with transition conductances (gamma(j)) of 30-80 pS. The macroscopic I(j) obtained from hMSC-cardiac myocyte cell pairs exhibited asymmetrical V(j) dependence, while single channel events revealed gamma(j) of the size range 40-100 pS. hMSC coupling via gap junctions to other cell types provides the basis for considering them as a therapeutic repair or cellular delivery system to syncytia such as the myocardium.

Coupling of astrocyte connexins Cx26, Cx30, Cx43 to oligodendrocyte Cx29, Cx32, Cx47: Implications from normal and connexin32 knockout mice

Oligodendrocytes in vivo form heterologous gap junctions with astrocytes. These oligodendrocyte/astrocyte (A/O) gap junctions contain multiple connexins (Cx), including Cx26, Cx30, and Cx43 on the astrocyte side, and Cx32, Cx29, and Cx47 on the oligodendrocyte side. We investigated connexin associations at A/O gap junctions on oligodendrocytes in normal and Cx32 knockout (KO) mice. Immunoblotting and immunolabeling by several different antibodies indicated the presence of Cx32 in liver and brain of normal mice, but the absence of Cx32 in liver and brain of Cx32 KO mice, confirming the specificity and efficacy of the antibodies, as well as allowing the demonstration of Cx32 expression by oligodendrocytes. Oligodendrocytes throughout brain were decorated with numerous Cx30-positive puncta, which also were immunolabeled for both Cx32 and Cx43. In Cx32 KO mice, astrocytic Cx30 association with oligodendrocyte somata was nearly absent, Cx26 was partially reduced, and Cx43 was present in abundance. In normal and Cx32 KO mice, oligodendrocyte Cx29 was sparsely distributed, whereas Cx47-positive puncta were densely localized on oligodendrocyte somata. These results demonstrate that astrocyte Cx30 and oligodendrocyte Cx47 are widely present at A/O gap junctions. Immunolabeling patterns for these six connexins in Cx32 KO brain have implications for deciphering the organization of heterotypic connexin coupling partners at A/O junctions. The persistence and abundance of Cx43 and Cx47 at these junctions after Cx32 deletion, together with the paucity of Cx29 normally present at these junctions, suggests Cx43/Cx47 coupling at A/O junctions. Reductions in Cx30 and Cx26 after Cx32 deletion suggest that these astrocytic connexins likely form junctions with Cx32 and that their incorporation into A/O gap junctions is dependent on the presence of oligodendrocytic Cx32.

Specific amino-acid residues in the N-terminus and TM3 implicated in channel function and oligomerization compatibility of connexin43

To identify signals that convey connexin oligomerization compatibility, we have aligned amino-acid sequences of alpha and beta group connexins (Cx) and compared the physico-chemical properties of each homologous amino-acid residue. Four positions were identified that consistently differed between alpha and beta-type connexins; two are located in the N-terminal domain (P1 and P2, corresponding to residues 12 and 13 of the Cx43 sequence), and two in the third trans-membrane-spanning domain TM3 (P3 and P4, corresponding to residues 152 and 153 of the Cx43 sequence). Replacement of each of these residues in Cx43 (an alpha-type connexin) with the corresponding residues of Cx32 (a beta-type connexin) resulted in the assembly of all variants into gap junctions; however, only the P4 variant was functional, as indicated by lucifer yellow dye transfer assays. The other three variants exerted a moderate to severe dose-dependent, dominant-negative effect on co-expressed wild-type (wt) Cx43 channel activity. Moreover, a significant dose-dependent, trans-dominant inhibition of channel activity was observed when either one of the N-terminal variants was co-expressed with wt Cx32. Assembly analyses indicated that dominant and trans-dominant inhibitory effects appeared to be based on the oligomerization of wt and variant connexins into mixed connexons. Interestingly, the identified N-terminal amino acids coincide with the position of naturally occurring, disease-causing missense mutations of several beta-connexin genes (Cx26, Cx30, Cx31, Cx32). Our results demonstrate that three of the identified discriminative amino-acid residues (positions 12, 13 and 152) are crucial for Cx43 channel function and suggest that the N-terminal amino-acid residues at position 12/13 are involved in the oligomerization compatibility of alpha and beta connexins.

Dominant cataracts result from incongruous mixing of wild-type lens connexins

Gap junctions are composed of proteins called connexins (Cx) and facilitate both ionic and biochemical modes of intercellular communication. In the lens, Cx46 and Cx50 provide the gap junctional coupling needed for homeostasis and growth. In mice, deletion of Cx46 produced severe cataracts, whereas knockout of Cx50 resulted in significantly reduced lens growth and milder cataracts. Genetic replacement of Cx50 with Cx46 by knockin rescued clarity but not growth. By mating knockin and knockout mice, we show that heterozygous replacement of Cx50 with Cx46 rescued growth but produced dominant cataracts that resulted from disruption of lens fiber morphology and crystallin precipitation. Impedance measurements revealed normal levels of ionic gap junctional coupling, whereas the passage of fluorescent dyes that mimic biochemical coupling was altered in heterozygous knockin lenses. In addition, double heterozygous knockout lenses retained normal growth and clarity, whereas knockover lenses, where native Cx46 was deleted and homozygously knocked into the Cx50 locus, displayed significantly deficient growth but maintained clarity. Together, these findings suggest that unique biochemical modes of gap junctional communication influence lens clarity and lens growth, and this biochemical coupling is modulated by the connexin composition of the gap junction channels.

Decreased intercellular dye-transfer and downregulation of non-ablated connexins in aortic endothelium deficient in connexin37 or connexin40

Vascular endothelial cells are coupled by gap junctions that permit cell-to-cell transfer of small molecules, including signals that may be important for vasomotor responses. Connexin37 (Cx37) and connexin40 (Cx40) are the predominant gap-junction proteins present in mouse endothelium. We examined the effect of eliminating Cx37, Cx40, or both, on interendothelial communication in mouse aorta. Intercellular transfer of biocytin and [2-(4-nitro-2,1,3-benzoxadiazol-7-yl)aminoethyl]trimethylammonium (NBD-TMA) was used to assess gap-junction-mediated coupling. Ablation of Cx40 generally had a greater effect on dye-transfer than ablation of Cx37. The effect of Cx40 ablation on dye-transfer was age dependent. There was a 27-fold reduction in biocytin transfer in embryonic Cx40-/- aortic endothelium, a much larger change than in aortas of 6-7-week-old Cx40-/- animals, which showed a 3.5-fold reduction. By contrast, there was no reduction in biocytin transfer in embryonic Cx37-/- endothelium. Embryonic aortas lacking both Cx37 and Cx40 showed a complete loss of endothelial dye-transfer. Surprisingly, elimination of Cx40 resulted in up to a 17-fold drop in endothelial Cx37 on western blots, whereas deletion of Cx37 reduced endothelial Cx40 up to 4.2-fold. By contrast, in the medial layer, both Cx37 and Cx43 increased approximately fourfold in Cx40-/- aortas. Declines in non-ablated endothelial connexins were not mediated by changes in connexin mRNA levels, suggesting a post-transcriptional effect. Our results indicate that Cx37 and Cx40 are the only functional connexins expressed in mouse aortic endothelium and are collectively crucial for endothelial communication. Furthermore, Cx37 and Cx40 are codependent on each other for optimal expression in vascular endothelium.

Distribution and dynamics of gap junction channels revealed in living cells

To study the structural composition and dynamics of gap junctions in living cells, we tagged their subunit proteins, termed connexins, with the autofluorescent tracer green fluorescent protein (GFP) and its cyan (CFP) and yellow (YFP) color variants. Tagged connexins assembled normally and channels were functional. High-resolution fluorescence images of gap junction plaques assembled from CFP and YFP tagged connexins revealed that the mode of channel distribution is strictly dependent on the connexin isoforms. Co-distribution as well as segregation into well-separated domains was observed. Based on accompanying studies we propose that channel distribution is regulated by intrinsic, connexin isoform specific signals. High-resolution time-lapse images revealed that gap junctions, contrary to previous expectations, are dynamic assemblies of channels. Channels within clusters and clusters themselves are mobile and constantly undergo structural rearrangements. Movements are complex and allow channels to move, comparable to other plasma membrane proteins not anchored to cytoskeletal elements. Comprehensive analysis, however, demonstrated that gap junction channel movements are not driven by diffusion described to propel plasma membrane protein movement. Instead, recent studies suggest that movements of gap junction channels are indirect and predominantly propelled by plasma membrane lipid flow that results from metabolic endo- and exocytosis.

Functional analysis of a dominant mutation of human connexin26 associated with nonsyndromic deafness

Cx26 has been implicated in dominant (DFNA3) and recessive (DFNB1) forms of nonsyndromic sensorineural deafness. While most homozygous DFNB1 Cx26 mutations result in a simple loss of channel activity, it is less clear how heterozygous mutations in Cx26 linked to DFNA3 cause hearing loss. We have tested the ability of one dominant mutation (W44C) to interfere with wild-type human Cx26 (HCx26wt). HCx26wt induced robust electrical conductance between paired oocytes, and facilitated dye transfer between transfected HeLa cells. In contrast, oocyte pairs injected with only W44C were not electrically coupled above background levels, and W44C failed to dye couple transfected HeLa cells. Moreover, W44C dramatically inhibited intercellular conductance of HCx26wt when co-expressed in an equal ratio, and the low levels of residual conductance displayed altered gating properties. A nonfunctional recessive mutation (W77R) did not inhibit the ability of HCx26wt to form functional channels when co-injected in the same oocyte pairs, nor did it alter HCx26wt gating. These results provide evidence for a functional dominant negative effect of the W44C mutant on HCx26wt and explain how heterozygous Cx26 mutations could contribute to autosomal dominant deafness, by resulting in a net loss, and/or alteration, of Cx26 function.

Heteromeric mixing of connexins: compatibility of partners and functional consequences

Cx43 is widely expressed in many different cell types, and many of these cells also express other connexins. If these connexins are capable of mixing, the functional properties of channels containing heteromeric connexons may substantially influence intercellular communication between such cells. We used biochemical strategies (sedimentation through sucrose gradients, co-immunoprecipitation, or co-purification by Ni-NTA chromatography) to examine heteromeric mixing of Cx43 with other connexins (including Cx26, Cx37, Cx40, Cx45, and Cx56) in transfected cells. These analyses showed that all of the tested connexins except Cx26 formed heteromeric connexons with Cx43. We used the double whole-cell patch-camp technique to analyze the electrophysiological properties of gap junction channels in pairs of co-expressing cells. Cx37 and Cx45 made a large variety of functional heteromeric combinations with Cx43 based on detection of many different single channel conductances. Most of the channel event sizes observed in cells co-expressing Cx40 and Cx43 were similar to those of homomeric Cx43 or Cx40 hemichannels in homo- or hetero-typic configurations. Our data suggest several different possible consequences of connexin co-expression: (1) some combinations of connexins may form heteromeric connexons with novel proeprties; (2) some connexins may form heteromeric channels that do not have unique properties, and (3) some connexins may be incompatible for heteromeric mixing.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.

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.

Cx40 and Cx43 expression ratio influences heteromeric/ heterotypic gap junction channel properties

In cells that coexpress connexin (Cx)40 and Cx43, the ratio of expression can vary depending on the cellular environment. We examined the effect of changing Cx40:Cx43 expression ratio on functional gap junction properties. Rin cells transfected with Cx40 or Cx43 (Rin40, Rin43) were cocultured with 6B5n, A7r5, A7r540C1, or A7r540C3 cells for electrophysiological and dye coupling analysis. Cx40:Cx43 expression ratio in 6B5n, A7r5, A7r540C1, and A7r540C3 cells was ~1:1, 3:1, 5:1, and 10:1, respectively. When Rin43 cells were paired with coexpressing cells, there was an increasing asymmetry of voltage-dependent gating and a shift toward smaller conductance events as Cx40:Cx43 ratio increased in the coexpressing cell. These observations could not be predicted by linear combinations of Cx40 and Cx43 properties in proportion to the expressed ratios of the two Cxs. When Rin40 cells were paired with coexpressing cells, the net voltage gating and single-channel conductance behavior were similar to those of Rin40/Rin40 cell pairs. Dye permeability properties of cell monolayers demonstrated that as Cx40:Cx43 expression ratio increased in coexpressing cells the charge and size selectivity of dye transfer reflected that of Rin40 cells, as would be predicted. These data indicate that the electrophysiological properties of heteromeric/heterotypic channels are not directly related to the proportions of Cx constituents expressed in the cell; however, the dye permeability of these same channels can be predicted by the relative Cx contributions.

Connexin43 and connexin45 form heteromeric gap junction channels in which individual components determine permeability and regulation

Two gap junction proteins, connexin43 (Cx43) and connexin45 (Cx45), are coexpressed in many cardiac and other cells. Homomeric channels formed by these proteins differ in unitary conductance, permeability, and regulation. We sought to determine the ability of Cx43 and Cx45 to oligomerize with each other to form heteromeric gap junction channels and to determine the functional and regulatory properties of these heteromeric channels. HeLa cells were transfected with Cx45 or (His)(6)-tagged Cx43 or sequentially transfected with both connexins. Immunoblots verified production of the transfected connexins, and immunofluorescence demonstrated that they were colocalized in the HeLa-Cx43(His)(6)/Cx45 cells. Connexons were solubilized from HeLa-Cx43(His)(6)/Cx45 cells by using Triton X-100 and were applied to a Ni(2+)-NTA column, which binds the His(6) sequence. Cx45 was coeluted from the column with Cx43(His)(6), demonstrating that some hemichannels contain both connexins. Single-channel recordings showed that the HeLa-Cx43(His)(6)/Cx45 cells exhibited single-channel conductances that were not observed in cells expressing either connexin alone. Dye-coupling experiments showed that HeLa-Cx43(His)(6) cells readily passed Lucifer yellow and N-(2-aminoethyl)biotinamide hydrochloride (neurobiotin); in contrast, HeLa-Cx45 and HeLa-Cx43(His)(6)/Cx45 cells showed extensive intercellular passage of neurobiotin but little coupling with Lucifer yellow. Treatment with the protein kinase C activator 12-O-tetradecanoylphorbol 13-acetate reduced junctional conductance in cells expressing Cx43, Cx45, or both connexins, but it reduced the extent of neurobiotin transfer only in HeLa-Cx43(His)(6) and HeLa-Cx43(His)(6)/Cx45 cells but not in the HeLa-Cx45 cells. Thus, biochemical and electrophysiological evidence suggests that Cx43 and Cx45 extensively mix to form heteromeric channels; however, individual connexin components dominate aspects of the physiological behavior of these channels.

Multimeric connexin interactions prior to the trans-Golgi network

Cells that express multiple connexins have the capacity to form heteromeric (mixed) gap junction hemichannels. We used a dominant negative connexin construct, consisting of bacterial β-galactosidase fused to the C terminus of connexin43 (Cx43/β-gal), to examine connexin compatibility in NIH 3T3 cells. Cx43/β-gal is retained in a perinuclear compartment and inhibits Cx43 transport to the cell surface. The intracellular connexin pool induced by Cx43/β-gal colocalized with a medial Golgi apparatus marker and was readily disassembled by treatment with brefeldin A. This was unexpected, since previous studies indicated that Cx43 assembly into hexameric hemichannels occurs in the trans-Golgi network (TGN) and is sensitive to brefeldin A. Further analysis by sucrose gradient fractionation showed that Cx43 and Cx43/β-gal were assembled into a subhexameric complex. Cx43/β-gal also specifically interacted with Cx46, but not Cx32, consistent with the ability of Cx43/β-gal to simultaneously inhibit multiple connexins. We confirmed that interactions between Cx43/β-gal and Cx46 reflect the ability of Cx43 and Cx46 to form heteromeric complexes, using HeLa and alveolar epithelial cells, which express both connexins. In contrast, ROS osteoblastic cells, which differentially sort Cx43 and Cx46, did not form Cx43/Cx46 heteromers. Thus, cells have the capacity to regulate whether or not compatible connexins intermix.

Heterotypic gap junction channel formation between heteromeric and homomeric Cx40 and Cx43 connexons

Recent evidence indicating formation of functional homomeric/heterotypic gap junction channels by connexin40 (Cx40) and connexin43 (Cx43) raises the question of whether data previously interpreted as support for heteromeric channel formation by these connexins might not instead reflect the activity of homomeric/heterotypic channels. To address this question and to further characterize the behavior of these channels, we used dual whole cell voltage-clamp techniques to examine the junctions formed between cells that express only Cx40 (Rin40) or Cx43 (Rin43) and compared the results with those obtained when either of these cell types was paired with cells that naturally express both connexins (A7r5 cells). Rin40/Rin43 cell pairs formed functional gap junctions that displayed a strongly asymmetric voltage-dependent gating response. Single-channel event amplitudes ranged between 34 and 150 pS, with 90- to 130-pS events predominating. A7r5/Rin43 and A7r5/Rin40 cell pairs had voltage-dependent gating responses that varied greatly, with most pairs demonstrating strong asymmetry. These cell pairs exhibited a variety of single-channel events that were not consistent with homomeric/homotypic Cx40 or Cx43 channels or homomeric/heterotypic Cx40/Cx43 channels. These data indicate that Cx40 and Cx43 form homomeric/heterotypic as well as heteromeric/heterotypic channels that display unique gating and conductance properties.

Gap junction channels formed by coexpressed connexin40 and connexin43

Many cardiovascular cells coexpress multiple connexins (Cx), leading to the potential formation of mixed (heteromeric) gap junction hemichannels whose biophysical properties may differ from homomeric channels containing only one connexin type. We examined the potential interaction of connexin Cx43 and Cx40 in HeLa cells sequentially stably transfected with these two connexins. Immunoblots verified the production of comparable amounts of both connexins, cross-linking showed that both connexins formed oligomers, and immunofluorescence showed extensive colocalization. Moreover, Cx40 copurified with (His)(6)-tagged Cx43 by affinity chromatography of detergent-solubilized connexons, demonstrating the presence of both connexins in some hemichannels. The dual whole cell patch-clamp method was used to compare the gating properties of gap junctions in HeLa Cx43/Cx40 cells with homotypic (Cx40-Cx40 and Cx43-Cx43) and heterotypic (Cx40-Cx43) gap junctions. Many of the observed single channel conductances resembled those of homotypic or heterotypic channels. The steady-state junctional conductance (g(j,ss)) in coexpressing cell pairs showed a reduced sensitivity to the voltage between cells (V(j)) compared with homotypic gap junctions and/or an asymmetrical V(j) dependence reminiscent of heterotypic gap junctions. These gating properties could be fit using a combination of homotypic and heterotypic channel properties. Thus, whereas our biochemical evidence suggests that Cx40 and Cx43 form heteromeric connexons, we conclude that they are functionally insignificant with regard to voltage-dependent gating.

Intercellular communication in cultured human vascular smooth muscle cells

Intercellular communication through gap junction channels plays a fundamental role in regulating vascular myocyte tone. We investigated gap junction channel expression and activity in myocytes from the physiologically distinct vasculature of the human internal mammary artery (IMA, conduit vessel) and saphenous vein (SV, capacitance vessel). Northern and Western blots documented the presence of connexin43 (Cx43) in frozen tissues and cultured cells from both vessels. Northern blots also confirmed the presence of Cx40 mRNA in cultured IMA and SV myocytes. Dual whole cell patch-clamp experiments revealed that macroscopic junctional conductance was voltage dependent and characteristic of that observed for Cx43. In the majority of records, in both vessels, single-channel activity was dominated by a main-state conductance of 120 pS, with subconducting events comprising less than 10% of the amplitude histograms. However, some records showed "atypical" unitary events that had a conductance similar to Cx40 (approximately 140-160 pS), but gating behavior like that of Cx43. As such, it is conceivable that the presence and coexpression of Cx40 and Cx43 in IMA and SV myocytes may result in heteromeric channel formation. Nonetheless, in terms of gating, Cx43-like behavior clearly dominates.

Heterocellular gap junctional communication between alveolar epithelial cells

We analyzed the pattern of gap junction protein (connexin) expression in vivo by indirect immunofluorescence. In normal rat lung sections, connexin (Cx)32 was expressed by type II cells, whereas Cx43 was more ubiquitously expressed and Cx46 was expressed by occasional alveolar epithelial cells. In response to bleomycin-induced lung injury, Cx46 was upregulated by alveolar epithelial cells, whereas Cx32 and Cx43 expression were largely unchanged. Given that Cx46 may form gap junction channels with either Cx43 or Cx32, we examined the ability of primary alveolar epithelial cells cultured for 6 days, which express Cx43 and Cx46, to form heterocellular gap junctions with cells expressing other connexins. Day 6 alveolar epithelial cells formed functional gap junctions with other day 6 cells or with HeLa cells transfected with Cx43 (HeLa/Cx43), but they did not communicate with HeLa/Cx32 cells. Furthermore, day 6 alveolar epithelial cells formed functional gap junction channels with freshly isolated type II cells. Taken together, these data are consistent with the notion that type I and type II alveolar epithelial cells communicate through gap junctions compatible with Cx43.

Gap junctions in the rabbit sinoatrial node

In comparison to the cellular basis of pacemaking, the electrical interactions mediating synchronization and conduction in the sinoatrial node are poorly understood. Therefore, we have taken a combined immunohistochemical and electrophysiological approach to characterize gap junctions in the nodal area. We report that the pacemaker myocytes in the center of the rabbit sinoatrial node express the gap junction proteins connexin (Cx)40 and Cx46. In the periphery of the node, strands of pacemaker myocytes expressing Cx43 intermingle with strands expressing Cx40 and Cx46. Biophysical properties of gap junctions in isolated pairs of pacemaker myocytes were recorded under dual voltage clamp with the use of the perforated-patch method. Macroscopic junctional conductance ranged between 0.6 and 25 nS with a mean value of 7.5 nS. The junctional conductance did not show a pronounced sensitivity to the transjunctional potential difference. Single-channel recordings from pairs of pacemaker myocytes revealed populations of single-channel conductances at 133, 202, and 241 pS. With these single-channel conductances, the observed average macroscopic junctional conductance, 7.5 nS, would require only 30-60 open gap junction channels.

The carboxyl terminal domain regulates the unitary conductance and voltage dependence of connexin40 gap junction channels

Chemical regulation of connexin (Cx) 40 and Cx43 follows a ball-and-chain model, in which the carboxyl terminal (CT) domain acts as a gating particle that binds to a receptor affiliated with the pore. Moreover, Cx40 channels can be closed by a heterodomain interaction with the CT domain of Cx43 and vice versa. Here, we report similar interactions in the establishment of the unitary conductance and voltage-dependent profile of Cx40 in N2A cells. Two mean unitary conductance values ("lower conductance" and "main") were detected in wild-type Cx40. Truncation of the CT domain at amino acid 248 (Cx40tr248) caused the disappearance of the lower-conductance state. Coexpression of Cx40tr248 with the CT fragment of either Cx40 (homodomain interactions) or Cx43 (heterodomain interactions) rescued the unitary conductance profile of Cx40. In the N2A cells, the time course of macroscopic junctional current relaxation was best described by a biexponential function in the wild-type Cx40 channels, but it was reduced to a single-exponential function after truncation. However, macroscopic junctional currents recorded in the oocyte expression system were not significantly different between the wild-type and mutant channels. Concatenation of the CT domain of Cx43 to amino acids 1 to 248 of Cx40 yielded a chimeric channel with unitary conductance and voltage-gating profile indistinguishable from that of wild-type Cx40. We conclude that residence of Cx40 channels in the lower-conductance state involves a ball-and-chain type of interaction between the CT domain and the pore-forming region. This interaction can be either homologous (Cx40 truncation with Cx40CT) or heterologous (with the Cx43CT).

Alteration of Cx43:Cx40 expression ratio in A7r5 cells

Connexins (Cx) 40 and 43 are coexpressed by several cell types at ratios that vary as a function of development, aging, and disease. Because these connexins form heteromeric channels, changes in expression ratio might be expected to significantly alter the connexin composition of the gap junction channel population and, therefore, gap junction function. To examine this possibility, we stably transfected A7r5 cells, which naturally coexpress Cx43 and Cx40, with a vector encoding antisense Cx43. Cx43 mRNA continued to be expressed in the antisense transfected clones, although levels were inversely related to the number of copies of antisense DNA incorporated into the genome. Protein levels, quantified in the clones with the highest and lowest Cx43:Cx40 mRNA ratios, were not well predicted by the mRNA levels, although the trends predicted by the Cx43:Cx40 mRNA ratio were preserved. Electrical coupling did not differ significantly between clones, but the clone with elevated Cx43:Cx40 protein expression ratio and unchanged Cx43 banding pattern was significantly better dye coupled than the parental A7r5 cells. These results suggest that as the Cx43:Cx40 ratio increases, provided alterations of Cx43 banding pattern (phosphorylation) have not occurred, permeability to large molecules increases even though electrical coupling remains nearly constant.

Immunocytochemical analysis of connexin expression in the healthy and diseased cardiovascular system

Gap junctions play essential roles in the normal function of the heart and arteries, mediating the spread of the electrical impulse that stimulates synchronized contraction of the cardiac chambers, and contributing to co-ordination of activities between cells of the arterial wall. In common with other multicellular systems, cardiovascular tissues express multiple connexin isotypes that confer distinctive channel properties. This review highlights how state-of-the-art immunocytochemical and cellular imaging techniques, as part of a multidisciplinary approach in gap junction research, have advanced our understanding of connexin diversity in cardiovascular cell function in health and disease. In the heart, spatially defined patterns of expression of three connexin isotypes-connexin43, connexin40, and connexin45-underlie the precisely orchestrated patterns of current flow governing the normal cardiac rhythm. Derangement of gap junction organization and/or reduced expression of connexin43 are associated with arrhythmic tendency in the diseased human ventricle, and high levels of connexin40 in the atrium are associated with increased risk of developing atrial fibrillation after coronary by-pass surgery. In the major arteries, endothelial gap junctions may simultaneously express three connexin isotypes, connexin40, connexin37, and connexin43; underlying medial smooth muscle, by contrast, predominantly expresses connexin43, with connexin45 additionally expressed at restricted sites. In normal arterial smooth muscle, the abundance of connexin43 gap junctions varies according to vascular site, and shows an inverse relationship with desmin expression and positive correlation with the quantity of extracellular matrix. Increased connexin43 expression between smooth muscle cells is closely linked to phenotypic transformation in early human coronary atherosclerosis and in the response of the arterial wall to injury. Current evidence thus suggests that gap junctions in both their guises, as pathways for cell-to-cell signaling in the vessel wall and as pathways for impulse conduction in the heart, contribute to the initial pathogenesis and eventual clinical manifestation of human cardiovascular disease.

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.

Site-directed mutations in the transmembrane domain M3 of human connexin37 alter channel conductance and gating

Connexin37 (Cx37) is expressed principally in endothelial cells. We have introduced individual point mutations (Cx37-V156D or Cx37-K162E) in the putative pore lining segment M3 of a polymorphic human Cx37 (Cx37-S319) and expressed them in N2A and RIN cells. RT-PCR and immunofluorescence microscopy were used to confirm the expression of the proteins. Stably transfected cells were subjected to electrophysiological studies. Experiments were performed on cell pairs using the dual whole cell patch-clamp method. Single channel records showed that both mutants display a variety of conductive states (Cx37-V156D, 47-250 pS; Cx37-K162E, 58-342 pS) in contrast to the typical high conductance of 340-375 pS and subconductive state of 60-80 pS reported for Cx37-S319. Analysis of the macroscopic data for Cx37-K162E revealed a broadened Vo indicating the influence of the mutation on voltage gating. Our data indicate that substitution of a conserved residue with a charged residue could cause changes in the main state and/or in the size of the pore. It is possible that these particular residues in the M3 domain interact electrostatistically with several of the other domains in the Cx37 protein.

Heteromeric connexons formed by the lens connexins, connexin43 and connexin56

In the eye lens, three connexins have been detected in epithelial cells and bow region/differentiating fiber cells, suggesting the possible formation of heteromeric gap junction channels. To study possible interactions between Cx56 and Cx43, we stably transfected a normal rat kidney cell line (NRK) that expresses Cx43 with Cx56 (NRK-Cx56). Similar to the lens, several bands of Cx56 corresponding to phosphorylated forms were detected by immunoblotting in NRK-Cx56 cells. Immunofluorescence studies showed co-localization of Cx56 with Cx43 in the perinuclear region and at appositional membranes. Connexin hexamers in NRK-Cx56 cells contained both Cx43 and Cx56 as demonstrated by sedimentation through sucrose gradients. Immunoprecipitation of Cx56 from sucrose gradient fractions resulted in co-precipitation of Cx43 from NRK-Cx56 cells suggesting the presence of relatively stable interactions between the two connexins. Double whole-cell patch-clamp experiments showed that the voltage-dependence of Gmin in NRK-Cx56 cells differed from that in NRK cells. Moreover, stable interactions between Cx43 and Cx56 were also demonstrated in the embryonic chicken lens by co-precipitation of Cx43 in Cx56 immunoprecipitates. These data suggest that Cx43 and Cx56 form heteromeric connexons in NRK-Cx56 cells as well as in the lens in vivo leading to differences in channel properties which might contribute to the variations in gap junctional intercellular communication observed in different regions of the lens.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Directional rectification of gap junctional voltage gating between dieters cells in the inner ear of guinea pig

Deiters cells (DCs) are the cochlear supporting cells in inner ear and contain multiple gap junction connexin genes, which when mutated can induce hearing loss. In the present study, the gap junctions between DCs were investigated by a double voltage clamp technique. Besides asymmetric responses to the polarities of transjunctional voltage (V(j)) and transmembrane potential (V(m)), the channels were also sensitive to which cell side was stimulated in a cell pair, i.e. voltage gating had directional dependence. The direction-dependent voltage gating could result in asymmetric current flow between the cells and influenced K(+) passage. Multiple connexins may constitute non-homotypic channels with directional dependence of voltage gating to mediate functional gap junction pathways in the cochlea. This may explain how a single connexin mutation can produce hearing loss.

Connexin-specific distribution within gap junctions revealed in living cells

To study the organization of gap junctions in living cells, the connexin isotypes alpha(1)(Cx43), beta(1)(Cx32) and beta(2)(Cx26) were tagged with the autofluorescent tracer green fluorescent protein (GFP) and its cyan (CFP) and yellow (YFP) color variants. The cellular fate of the tagged connexins was followed by high-resolution fluorescence deconvolution microscopy and time-lapse imaging. Comprehensive analyses demonstrated that the tagged channels were functional as monitored by dye transfer, even under conditions where the channels were assembled solely from tagged connexins. High-resolution images revealed a detailed structural organization, and volume reconstructions provided a three-dimensional view of entire gap junction plaques. Specifically, deconvolved dual-color images of gap junction plaques assembled from CFP- and YFP-tagged connexins revealed that different connexin isotypes gathered within the same plaques. Connexins either codistributed homogeneously throughout the plaque, or each connexin isotype segregated into well-separated domains. The studies demonstrate that the mode of channel distribution strictly depends on the connexin isotypes. Based on previous studies on the synthesis and assembly of connexins I suggest that channel distribution is regulated by intrinsic connexin isotype specific signals.

Properties of gap junction channels formed by Cx46 alone and in combination with Cx50

Gap junctions formed of connexin46 (Cx46) and connexin50 (Cx50) in lens fiber cells are crucial for maintaining lens transparency. We determined the functional properties of homotypic Cx46, heterotypic Cx46/Cx50, and heteromeric Cx46/Cx50 channels in a communication-deficient neuroblastoma (N2A) cell line, using dual whole-cell recordings. N2A cultures were stably and/or transiently transfected with Cx46, Cx50, and green fluorescent protein (EGFP). The macroscopic voltage sensitivity of homotypic Cx46 conformed to the two-state model (Boltzmann parameters: G(min) = 0.11, V(0) = +/- 48.1 mV, gating charge = 2). Cx46 single channels showed a main-state conductance of 140 +/- 8 pS and multiple subconductance states ranging from < or =10 pS to 60 pS. Conservation of homotypic properties in heterotypic Cx46/Cx50 cell pairs allowed the determination of a positive relative gating polarity for the dominant gating mechanisms in Cx46 and Cx50. Observed gating properties were consistent with a second gating mechanism in Cx46 connexons. Moreover, rectification was observed in heterotypic cell pairs. Some cell pairs in cultures simultaneously transfected with Cx46 and Cx50 exhibited junctional properties not observed in other preparations, suggesting the formation of heteromeric channels. We conclude that different combinations of Cx46 and Cx50 within gap junction channels lead to unique biophysical properties.

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.