Aberrant hemichannel properties of Cx26 mutations causing skin disease and deafness

Mutations in the human GJB2 gene, which encodes connexin26 (Cx26), underlie various forms of hereditary deafness and skin disease. While it has proven difficult to discern the exact pathological mechanisms that cause these disorders, studies have shown that the loss or abnormal function of Cx26 protein has a profound effect on tissue homeostasis. Here, we used the Xenopus oocyte expression system to examine the functional characteristics of a Cx26 mutation (G45E) that results in keratitis-ichthyosis-deafness syndrome (KIDS) with a fatal outcome. Our data showed that oocytes were able to express both wild-type Cx26 and its G45E variant, each of which formed hemichannels and gap junction channels. However, Cx26-G45E hemichannels displayed significantly greater whole cell currents than wild-type Cx26, leading to cell lysis and death. This severe phenotype could be rescued in the presence of elevated Ca²⁺ levels in the extracellular milieu. Cx26-G45E could also form intercellular channels with a similar efficiency as wild-type Cx26, however, with increased voltage sensitive gating. We also compared Cx26-G45E with a previously described Cx26 mutant, A40V, which has an overlapping human phenotype. We found that both dominant Cx26 mutants elicited similar functional consequences and that cells coexpressing mutant and wild-type connexins predominantly displayed mutant-like behavior. These data suggest that mutant hemichannels may act on cellular homeostasis in a manner that can be detrimental to the tissues in which they are expressed. connexin     

Loss-of-function and residual channel activity of connexin26 mutations associated with non-syndromic deafness

Connexins are the protein subunits of gap junction channels that allow a direct signaling pathway between networks of cells. The specific role of connexin channels in the homeostasis of different organs has been validated by the association of mutations in several human connexins with a variety of genetic diseases. Several connexins are present in the mammalian cochlea and at least four of them have been proposed as genes causing sensorineural hearing loss. We have started our functional analysis by selecting nine mutations in Cx26 that are associated with non-syndromic recessive deafness (DFNB1). We have observed that both human Cx26 wild-type (HCx26wt) and the F83L polymorphism, found in unaffected controls, generated electrical conductance between paired Xenopus oocytes, which was several orders of magnitude greater than that measured in water-injected controls. In contrast, most recessive Cx26 mutations (identified in DFNB1 patients) resulted in a simple loss of channel activity. In addition, the V37I mutation, originally identified as a polymorphism in heterozygous unaffected individuals, was devoid of function and thus may be pathologically significant. Unexpectedly, we have found that the recessive mutation V84L retained functional activity in both paired Xenopus oocytes and transfected HeLa cells. Furthermore, both the magnitude of macroscopic junctional conductance and its voltage-gating properties were indistinguishable from those of HCx26wt. The identification of functional differences of disease causing mutations may lead to define which permeation or gating properties of Cx26 are necessary for normal auditory function in humans and will be instrumental in identifying the molecular steps leading to DFNB1.     

Human connexin26 (GJB2) deafness mutations affect the function of gap junction channels at different levels of protein expression.

Mutations in the connexin26 (GJB2) gene account for about half of inherited non-syndromic deafness cases in Western countries. The connexin26 protein is a subunit of gap junctions that form a network of intercellular communication among supporting cells and fibrocytes in the mammalian inner ear. Here we describe functional implications of mutations in the coding region of connexin26 genes (M1V, M34T, L90P, R127H, F161S, P173R, and R184P), identified in patients and stably transfected in human HeLa cells. While all mutated connexin26 cDNAs were transcribed, only M34T, L90P, R127H, F161S, and R184P were translated in HeLa cells. Analysis of indirect immunofluorescence showed membranous localization, strong for M34T, L90P, R127H, and very weak for F161S, but no signal corresponding to M1V, P173R and R184P. Tracer coupling experiments revealed diffusion of microinjected neurobiotin into neighbouring cells in the case of M34T and R127H, whereas M1V, L90P, F161S, P173R and R184P mutants did not show intercellular coupling. The results of oligomerisation studies suggested a partly disturbed assembly of hemichannels in M34T and L90P mutants but complete absence of hemichannel formation in the R184P mutant. The R127H mutation did not affect channel formation and is likely to represent a polymorphism. Our results show that mutations in the connexin26 gene can affect gap junctional intercellular communication at the level of protein translation, trafficking or assembly of hemichannels.     

Monitoring channel activities of proteoliposomes with SecA and Cx26 gap junction in single oocytes

Establishing recordable channels in membranes of oocytes formed by expressing exogenous complementary DNA (cDNA) or messenger RNA (mRNA) has contributed greatly to understanding the molecular mechanisms of channel functions. Here, we report the extension of this semi-physiological system for monitoring the channel activity of preassembled membrane proteins in single cell oocytes by injecting reconstituted proteoliposomes along with substrates or regulatory molecules. We build on the observation that SecA from various bacteria forms active protein-conducting channels with injection of proteoliposomes, protein precursors, and ATP–Mg²⁺. Such activity was enhanced by reconstituted SecYEG–SecDF•YajC liposome complexes that could be monitored easily and efficiently, providing correlation of in vitro and intact cell functionality. In addition, inserting reconstituted gap junction Cx26 liposomes into the oocytes allowed the demonstration of intracellular/extracellular Ca²⁺-regulated hemi-channel activities. The channel activities can be detected rapidly after injection, can be monitored for various effectors, and are dependent on specific exogenous lipid compositions. This simple and effective functional system with low endogenous channel activity should have broad applications for monitoring the specific channel activities of complex interactions of purified membrane proteins with their effectors and regulatory molecules.     

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.     

Correlative studies of gating in Cx46 and Cx50 hemichannels and gap junction channels

Transjunctional voltage (V(j)) gating of gap junction (GJ) channels formed of connexins has been proposed to occur by gating of the component hemichannels. We took advantage of the ability of Cx46 and Cx50 to function as unapposed hemichannels to identify gating properties intrinsic to hemichannels and how they contribute to gating of GJ channels. We show that Cx46 and Cx50 hemichannels contain two distinct gating mechanisms that generate reductions in conductance for both membrane polarities. At positive voltages, gating is similar in Cx46 and Cx50 hemichannels, primarily showing increased transitioning to long-lived substates. At negative voltages, Cx46 currents deactivate completely and the underlying single hemichannels exhibit transitions to a fully closed state. In contrast, Cx50 currents do not deactivate completely at negative voltages and the underlying single hemichannels predominantly exhibit transitions to various substates. Transitions to a fully closed state occur, but are infrequent. In the respective GJ channels, both forms of gating contribute to the reduction in conductance by V(j). However, examination of gating of mutant hemichannels and GJ channels in which the Asp at position 3 was replaced with Asn (D3N) showed that the positive hemichannel gate predominantly closes Cx50 GJs, whereas the negative hemichannel gate predominantly closes Cx46 GJs in response to V(j). We also report, for the first time, single Cx50 hemichannels in oocytes to be inwardly rectifying, high conductance channels (gamma = 470 pS). The antimalarial drug mefloquine, which selectively blocks Cx50 and not Cx46 GJs, shows the same selectivity in Cx50 and Cx46 hemichannels indicating that the actions of such uncoupling agents, like voltage gating, are intrinsic hemichannel properties.     

Cx40.8, a Cx43-like protein, forms gap junction channels inefficiently and may require Cx43 for its association at the plasma membrane

In addition to having a Cx43 ortholog, the zebrafish genome also contains a Cx43-like gene, Cx40.8. Here, we investigate the expression of cx40.8 in zebrafish fins and the function of Cx40.8 in HeLa cells. We find that cx40.8 is present in the same population of dividing cells as cx43. Unlike Cx43, dye coupling assays suggest that Cx40.8 only inefficiently forms functional gap junction channels. However, co-transfection reveals that Cx40.8 can co-localize with Cx43 in gap junction plaques, and that the resulting plaques contain functional gap junction channels. Together, these data suggest the possibility that Cx40.8 may functionally interact with Cx43 to regulate cell proliferation in vivo.

Structured summary

MINT-7266123: cx40.8 (genbank_protein_gi:68354404) and cx43 (uniprotkb:O57474) colocalize (MI:0403) by fluorescence microscopy (MI:0416)     

Regulation of gap junction coupling through the neuronal connexin Cx35 by nitric oxide and cGMP

Gap-junctional coupling among neurons is subject to regulation by a number of neurotransmitters including nitric oxide. We studied the mechanisms by which NO regulates coupling in cells expressing Cx35, a connexin expressed in neurons throughout the central nervous system. NO donors caused potent uncoupling of HeLa cells stably transfected with Cx35. This effect was mimicked by Bay 21-4272, an activator of guanylyl cyclase. A pharmacological analysis indicated that NO-induced uncoupling involved both PKG-dependent and PKG-independent pathways. PKA was involved in both pathways, suggesting that PKG-dependent uncoupling may be indirect. In vitro, PKG phosphorylated Cx35 at three sites: Ser110, Ser276, and Ser289. A mutational analysis indicated that phosphorylation on Ser110 and Ser276, sites previously shown also to be phosphorylated by PKA, had a significant influence on regulation. Ser289 phosphorylation had very limited effects. We conclude that NO can regulate coupling through Cx35 and that regulation is indirect in HeLa cells.     

Role of cytoskeletal elements in the recruitment of Cx43-GFP and Cx26-YFP into gap junctions

Cytoskeletal elements may be important in connexin transport to the cell surface, cell surface gap junction plaque formation and/or gap junction internalization. In this study, fluorescence recovery after photobleaching was used to examine the role of microfilaments and microtubules in the recruitment and coalescence of green fluorescent protein-tagged Cx43 (Cx43-GFP) or yellow fluorescent tagged-Cx26 (Cx26-YFP) into gap junctions in NRK cells. In untreated cells, both Cx26-YFP and Cx43-GFP were recruited into gap junctions within photobleached areas of cell-cell contact within 2 hrs. However, disruption of microfilaments with cytochalasin B inhibited the recruitment and assembly of both Cx26-YFP and Cx43-GFP into gap junctions within photobleached areas. Surprisingly, disruption of microtubules with nocodazole inhibited the recruitment of Cx43-GFP into gap junctions but had limited effect on the transport and clustering of Cx26-YFP into gap junctions within the photobleached regions of cell-cell contact. These results suggest that the recruitment of Cx43-GFP and Cx26-YFP to the cell surface or their lateral clustering into gap junctions plaques is dependent in part on the presence of intact actin microfilaments while Cx43-GFP was more dependent on intact microtubules than Cx26-YFP.     

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.     

Identification of cells expressing Cx43, Cx30, Cx26, Cx32 and Cx36 in gap junctions of rat brain and spinal cord

We have identified cells expressing Cx26, Cx30, Cx32, Cx36 and Cx43 in gap junctions of rat central nervous system (CNS) using confocal light microscopic immunocytochemistry and freeze-fracture replica immunogold labeling (FRIL). Confocal microscopy was used to assess general distributions of connexins, whereas the 100-fold higher resolution of FRIL allowed co-localization of several different connexins within individual ultrastructurally-defined gap junction plaques in ultrastructurally and immunologically identified cell types. In >4000 labeled gap junctions found in >370 FRIL replicas of gray matter in adult rats, Cx26, Cx30 and Cx43 were found only in astrocyte gap junctions; Cx32 was only in oligodendrocytes, and Cx36 was only in neurons. Moreover, Cx26, Cx30 and Cx43 were co-localized in most astrocyte gap junctions. Oligodendrocytes shared intercellular gap junctions only with astrocytes, and these heterologous junctions had Cx32 on the oligodendrocyte side and Cx26, Cx30 and Cx43 on the astrocyte side. In 4 and 18 day postnatal rat spinal cord, neuronal gap junctions contained Cx36, whereas Cx26 was present in leptomenigeal gap junctions. Thus, in adult rat CNS, neurons and glia express different connexins, with "permissive" connexin pairing combinations apparently defining separate pathways for neuronal vs. glial gap junctional communication.     

Comparative characterization of the 21-kD and 26-kD gap junction proteins in murine liver and cultured hepatocytes

Affinity-purified antibodies to mouse liver 26- and 21-kD gap junction proteins have been used to characterize gap junctions in liver and cultured hepatocytes. Both proteins are colocalized in the same gap junction plaques as shown by double immunofluorescence and immunoelectron microscopy. In the lobules of rat liver, the 21-kD immunoreactivity is detected as a gradient of fluorescent spots on apposing plasma membranes, the maximum being in the periportal zone and a faint reaction in the perivenous zone. In contrast, the 26-kD immunoreactivity is evenly distributed in fluorescent spots on apposing plasma membranes throughout the rat liver lobule. Immunoreactive sites with anti-21 kD shown by immunofluorescence are also present in exocrine pancreas, proximal tubules of the kidney, and the epithelium of small intestine. The 21-kD immunoreactivity was not found in thin sections of myocardium and adult brain cortex. Subsequent to partial rat hepatectomy, both the 26- and 21-kD proteins first decrease and after approximately 2 d increase again. By comparison of the 26- and 21-kD immunoreactivity in cultured embryonic mouse hepatocytes, we found (a) the same pattern of immunoreactivity on apposing plasma membranes and colocalization within the same plaque, (b) a similar decrease after 1 d and subsequent increase after 3 d of both proteins, (c) cAMP-dependent in vitro phosphorylation of the 26-kD but not of the 21-kD protein, and (d) complete inhibition of intercellular transfer of Lucifer Yellow in all hepatocytes microinjected with anti-26 kD and, in most cases, partial inhibition of dye transfer after injection of anti-21 kD. Our results indicate that both the 26-kD and the 21-kD proteins are functional gap junction proteins.     

Permeability of homotypic and heterotypic gap junction channels formed of cardiac connexins mCx30.2, Cx40, Cx43, and Cx45

We examined the permeabilities of homotypic and heterotypic gap junction (GJ) channels formed of rodent connexins (Cx) 30.2, 40, 43, and 45, which are expressed in the heart and other tissues, using fluorescent dyes differing in net charge and molecular mass. Combining fluorescent imaging and electrophysiological recordings in the same cell pairs, we evaluated the single-channel permeability (P(gamma)). All homotypic channels were permeable to the anionic monovalent dye Alexa Fluor-350 (AF(350)), but mCx30.2 channels exhibited a significantly lower P(gamma) than the others. The anionic divalent dye Lucifer yellow (LY) remained permeant in Cx40, Cx43, and Cx45 channels, but transfer through mCx30.2 channels was not detected. Heterotypic channels generally exhibited P(gamma) values that were intermediate to the corresponding homotypic channels. P(gamma) values of mCx30.2/Cx40, mCx30.2/Cx43, or mCx30.2/Cx45 heterotypic channels for AF(350) were similar and approximately twofold higher than P(gamma) values of mCx30.2 homotypic channels. Permeabilities for cationic dyes were assessed only qualitatively because of their binding to nucleic acids. All homotypic and heterotypic channel configurations were permeable to ethidium bromide and 4,6-diamidino-2-phenylindole. Permeability for propidium iodide was limited only for GJ channels that contain at least one mCx30.2 hemichannel. In summary, we have demonstrated that Cx40, Cx43, and Cx45 are permeant to all examined cationic and anionic dyes, whereas mCx30.2 demonstrates permeation restrictions for molecules with molecular mass over approximately 400 Da. The ratio of single-channel conductance to permeability for AF(350) was approximately 40- to 170-fold higher for mCx30.2 than for Cx40, Cx43, and Cx45, suggesting that mCx30.2 GJs are notably more adapted to perform electrical rather than metabolic cell-cell communication.     

Segment-specific expression of the gap junction gene connexin31 during hindbrain development

 During segmentation of the mouse hindbrain (d8.0–8.5 pc), expression of the gap junction gene connexin31 (cx31) is precisely restricted to rhombomeres (r) 3 and 5. Shortly afterwards, during the turning process, cx31 expression in rhombomere 3 decreases and is no longer detectable at d9.5 pc, whereas expression in rhombomere 5 is maintained until about d10.0 pc. So far, cx31 is the first gap junction gene found to be expressed in rhombomeres. Its precise segmental and temporal expression pattern may reflect a critical requirement of cx31 channels for these odd numbered rhombomeres to acquire distinct cell identities. Received: 9 June 1997 / Accepted: 30 July 1997     

Mouse Cx50, a functional member of the connexin family of gap junction proteins, is the lens fiber protein MP70.

The crystalline lens is an attractive system to study the biology of intercellular communication; however, the identity of the structural components of gap junctions in the lens has been controversial. We have cloned a novel member of the connexin family of gap junction proteins, Cx50, and have shown that it is likely to correspond to the previously described lens fiber protein MP70. The N-terminal amino acid sequence of MP70 closely matches the sequence predicted by the clone. Cx50 mRNA is detected only in the lens, among the 12 organs tested, and this distribution is indistinguishable from that of MP70 protein. A monoclonal antibody directed against MP70 and an anti-Cx50 antibody produced against a synthetic peptide identify the same proteins on western blots and produce identical patterns of immunofluorescence on frozen sections of rodent lens. We also show that expression of Cx50 in paired Xenopus oocytes induces high levels of voltage-dependent conductance. This indicates that Cx50 is a functional member of the connexin family with unique physiological properties. With the cloning of Cx50, all known participants in gap junction formation between various cell types in the lens are available for study and reconstitution in experimental systems.     

Innexin genes and gap junction proteins in the locust frontal ganglion

Gap junctions (GJs) belong to one of the most conserved cellular structures in multicellular organisms. They probably serve similar functions in all Metazoa, providing one of the most common forms of intercellular communication. GJs are widely distributed in embryonic cells and tissues and have been attributed an important role in development, modulating cell growth and differentiation. These channels have been also implicated in mediating electrical synaptic signaling; Coupling through GJs is now accepted as a major pathway that supports network behavior and contributes to physiological rhythms. Here we focus on the physiology and molecular biology of GJs in a recently established model for the study of rhythm-generating networks and their role in behavior: the frontal ganglion (FG) of the desert locust, Schistocerca gregaria. Four novel genes of the invertebrate GJs (innexin) gene family were found to be expressed in the FG: Sg-inx1, Sg-inx2, Sg-inx3 and Sg-inx4. Immunohistochemistry revealed that some of the neurons in the FG express at least one innexin protein, INX1. We also established the presence of functional gap junction proteins in the FG and demonstrated functional electrical coupling between the neurons in the FG. This study forms the basis for further investigation of the role of GJs in network development and behavior.     

Preconditioning in the absence or presence of sustained ischemia modulates myocardial Cx43 protein levels and gap junction distribution

In the heart, brief repeated episodes of ischemia prior to a sustained occlusion (ischemic preconditioning; PC) significantly delay the onset of necrosis and arrhythmogenesis. Ischemia has been reported to influence gap junction organization and connexin43 (Cx43) content, but whether PC affects these structures is not known. We investigated the effect of PC (2 cycles of 5-min ischemia plus 10-min reperfusion) followed by prolonged reperfusion without concomitant regional coronary occlusion on the myocardial Cx43 content and its spatial distribution in rabbit hearts. We also compared the effect of sustained ischemia with or without PC on Cx43 spatial distribution. In experiments with PC only, there was an initial decrease in Cx43 levels within the ischemic zone followed by a progressive increase after 48 h reperfusion. End-to-end immunolabeling of Cx43 was augmented in the ischemic region between 24 and 48 h reperfusion; labeling was not uniquely confined to myocyte abutments, but was also dispersed along the sarcolemma. Cx43 immunolabelling was more intense and diffuse in hearts subjected to PC before sustained coronary occlusion (compared to non-PC). These data indicate that gap junctions are significantly altered during brief episodes of ischemia. Reorganization of the gap junction complex could contribute to PC-mediated reductions in cardiac arrhythmias.     

Immunocytochemical localization of the gap junction 26 K protein in mouse liver plasma membranes

Specific binding sites for anti-26 K antibodies directed against the liver gap junction protein (26 K) were localized by immunoelectron microscopy in gap junction plaques purified from hepatic plasma membranes. Using immunofluorescence microscopy we found discrete fluorescent spots on plasma membranes in cross sections of liver tissues after incubation with anti-26 K antibodies. This is consistent with the notion of specific binding to gap junction plaques. Quantitative binding of anti-26 K antibodies was indirectly measured by the protein A-gold technique. We found that urea/detergent-treated, purified gap junction plaques bind 30-fold more anti-26 K antibodies than preimmune serum. Anti-26 K antibodies also bind specifically to native gap junction plaques within hepatic plasma membranes although only about one fifth as efficiently as to purified plaques. Possibly the anti-26 K antibodies raised after injection of SDS-denatured 26 K protein into rabbits recognize the cytoplasmic face of urea/detergent-treated plaques better than that of native plaques. Some, if not most, of the vesicular structures in preparations of purified plaques appear to be derived from split gap junction plaques and are probably sheets of gap junction hemichannels. In some vesicles the former cytoplasmic face of the hemichannels is turned outside, other vesicles have the former cell surface turned outside. The anti-26 K antibodies do not recognize any 26 K protein on the sheets of partially split gap junction plaques, on the heterogeneous vesicular structures, or on non-junctional areas of hepatic plasma membranes. These results suggest that the conformation of the 26 K protein in plaques must be different from that of the 26 K protein in earlier biosynthetic steps of plaque assembly.