Chick connexin-56, a novel lens gap junction protein. Molecular cloning and functional expression

Ca2+-Dependent and -Independent Calmodulin Binding to the Cytoplasmic Loop of Gap Junction Connexins

Ca²⁺/calmodulin (Ca²⁺/CaM) interaction with connexins (Cx) is well-established; however, the mechanistic basis of regulation of gap junction function by Ca²⁺/CaM is not fully understood. Ca²⁺/CaM is predicted to bind to a domain in the C-terminal portion of the intracellular loop (CL2) in the vast majority of Cx isoforms and for a number of Cx-s this prediction has proved correct. In this study, we investigate and characterise both Ca²⁺/CaM and apo-CaM binding to selected representatives of each of the α, β and γ connexin family to develop a better mechanistic understanding of CaM effects on gap junction function. The affinity and kinetics Ca²⁺/CaM and apo-CaM interactions of CL2 peptides of β-Cx32, γ-Cx35, α-Cx43, α-Cx45 and α-Cx57 were investigated. All five Cx CL2 peptides were found to have high affinity for Ca²⁺/CaM with dissociation constants (K_d(+Ca)) from 20 to 150 nM. The limiting rate of binding and the rates of dissociation covered a broad range. In addition, we obtained evidence for high affinity Ca²⁺-independent interaction of all five peptides with CaM, consistent with CaM remaining anchored to gap junctions in resting cells. However, for the α-Cx45 and α-Cx57 CL2 peptides, Ca²⁺-dependent association at resting [Ca²⁺] of 50-100 nM is indicated in these complexes as one of the CaM Ca²⁺ binding sites displays high affinity with K_d of 70 and 30 nM for Ca²⁺, respectively. Furthermore, complex conformational changes were observed in peptide-apo-CaM complexes with the structure of CaM compacted or stretched by the peptide in a concentration dependent manner suggesting that the CL2 domain may undergo helix-to-coil transition and/or forms bundles, which may be relevant in the hexameric gap junction. We demonstrate inhibition of gap junction permeability by Ca²⁺/CaM in a dose dependent manner, further cementing Ca²⁺/CaM as a regulator of gap junction function. The motion of a stretched CaM-CL2 complex compacting upon Ca²⁺ binding may bring about the Ca²⁺/CaM block of the gap junction pore by a push and pull action on the CL2 C-terminal hydrophobic residues of transmembrane domain 3 (TM3) in and out of the membrane.

Zebrafish connexin 79.8 (Gja8a): A lens connexin used as an electrical synapse in some neurons

In the mammalian central nervous system, a remarkably small number of connexins is used in electrical synapses, with the majority formed from Cx36. A larger number has been detected in teleosts, with some seeming to serve restricted roles. Here, we report the discovery of a new connexin expressed in the zebrafish lens and a limited set of neurons. Zebrafish cx79.8 (gja8a), previously annotated incorrectly as cx50.5 based on a partial cDNA sequence, is a homologue of mammalian Cx50 (Gja8). We examined its expression through transgenic promoter-reporter constructs, in situ hybridization, and immunolabeling, and examined regulation of coupling in transfected HeLa cells. cx79.8 was expressed most strongly in the lens, but expression was also found in several groups of neurons in the cerebellum and related areas at the midbrain-hindbrain boundary, in cone photoreceptors, and in neurons in the retinal inner nuclear and ganglion cell layers. Labeling in the retina with antibodies against two C-terminal epitopes revealed numerous small punctate spots in the inner plexiform layer and along the somata of cones. Abundant gap junctions were labeled in the outer 1/3 of the lens, but were absent from the center, suggesting that the epitopes or the entire protein was absent from the center. Cx79.8 tracer coupling was strongly regulated by phosphorylation, and was extremely low in control conditions in HeLa cells due to protein phosphatase 2A activity. These properties allow coupling to be strongly restricted in situ, a frequently observed property for electrical synapses. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 548-561, 2017.

Chicken GRIFIN: A homodimeric member of the galectin network with canonical properties and a unique expression profile

Occurrence of the adhesion/growth-regulatory galectins as family sets the challenge to achieve a complete network analysis. Along this route taken for a well-suited model organism (chicken), we fill the remaining gap to characterize its seventh member known from rat as galectin-related inter-fiber protein (GRIFIN) in the lens. Its single-copy gene is common to vertebrates, with one or more deviations from the so-called signature sequence for ligand (lactose) contact. The chicken protein is a homodimeric agglutinin with capacity to bind β-galactosides, especially the histo-blood group B tetrasaccharide, shown by solid-phase/cell assays and a glycan microarray. Mass spectrometric identification of two lactose-binding peptides after tryptic on-bead fragmentation suggests an interaction at the canonical region despite a sequence change from Arg to Val at the site, which impairs reactivity of human galectin-1. RT-PCR and Western blot analyses of specimen from adult chicken organs reveal restriction of expression to the lens, here immunohistochemically throughout its main body. This report sets the stage for detailed structure-activity studies to define factors relevant for affinity beyond the signature sequence and to perform the first complete network analysis of the galectin family in developing and adult organs of a vertebrate.

Regulation of connexin36 gap junction channels by n-alkanols and arachidonic acid

We examined junctional conductance (gj) and its dependence on transjunctional voltage in gap junction (GJ) channels formed of wild-type connexin36 (Cx36) or its fusion form with green fluorescent protein (Cx36-EGFP) transfected in HeLa cells or endogenously expressed in primary culture of pancreatic β-cells. Only a very small fraction (∼0.8%) of Cx36-EGFP channels assembled into junctional plaques of GJs were open under control conditions. We found that short carbon chain n-alkanols (SCCAs) increased gj, while long carbon chain n-alkanols resulted in full uncoupling; cutoff is between heptanol and octanol. The fraction of functional channels and gj increased several fold under an exposure to SCCAs, or during reduction of endogenous levels of arachidonic acid (AA) by exposure to fatty acid-free BSA or cytosolic phospholipase A2 inhibitors. Moreover, uncoupling caused by exogenously applied AA can be rescued by BSA, which binds AA and other polyunsaturated fatty acids (PUFAs), but not by BSA modified with 1,2-cyclohexanedione, which does not bind AA and other PUFAs. We propose that under control conditions, Cx36 GJ channels in HeLa transfectants and β-cells are inhibited by endogenous AA, which stabilizes a closed conformational state of the channel that leads to extremely low fraction of functional channels. In addition, SCCAs increase gj by interfering with endogenous AA-dependent inhibition, increasing open probability and the fraction of functional channels.

Primary cultures of embryonic chick lens cells as a model system to study lens gap junctions and fiber cell differentiation

A major limitation in lens gap junction research has been the lack of experimentally tractable ex vivo systems to study the formation and regulation of fiber-type gap junctions. Although immortalized lens-derived cell lines are amenable to both gene transfection and siRNA-mediated knockdown, to our knowledge none are capable of undergoing appreciable epithelial-to-fiber differentiation. Lens central epithelial explants have the converse limitation. A key advance in the field was the development of a primary embryonic chick lens cell culture system by Drs. Sue Menko and Ross Johnson. Unlike central epithelial explants, these cultures also include cells from the peripheral (preequatorial and equatorial) epithelium, which is the most physiologically relevant population for the study of fiber-type gap junction formation. We have modified the Menko/Johnson system and refer to our cultures as dissociated cell-derived monolayer cultures (DCDMLs). We culture DCDMLs without serum to mimic the avascular lens environment and on laminin, the major matrix component of the lens capsule. Here, I review the features of the DCDML system and how we have used it to study lens gap junctions and fiber cell differentiation. Our results demonstrate the power of DCDMLs to generate new findings germane to the mammalian lens and how these cultures can be exploited to conduct experiments that would be impossible, prohibitively expensive and/or difficult to interpret using transgenic animals in vivo.

Regulation of lens gap junctions by Transforming Growth Factor beta

Using cultured lens epithelial cells, we discovered a new type of cross-talk between the FGF and TGF-β pathways, as well as a novel role for TGF-β and p38 kinase in the regulation of gap junctional intercellular communication. Our findings provide an explanation for how pathologically increased TGF-β signaling could contribute to cataract formation.

Gap junction–mediated intercellular communication (GJIC) is essential for the proper function of many organs, including the lens. GJIC in lens epithelial cells is increased by FGF in a concentration-dependent process that has been linked to the intralenticular gradient of GJIC required for lens transparency. Unlike FGF, elevated levels of TGF-β are associated with lens dysfunction. We show that TGF–β1 or -2 up-regulates dye coupling in serum-free primary cultures of chick lens epithelial cells (dissociated cell-derived monolayer cultures [DCDMLs]) via a mechanism distinct from that utilized by other growth factors. Remarkably, the ability of TGF-β and of FGF to up-regulate GJIC is abolished if DCDMLs are simultaneously exposed to both factors despite undiminished cell–cell contact. This reduction in dye coupling is attributable to an inhibition of gap junction assembly. Connexin 45.6, 43, and 56–containing gap junctions are restored, and intercellular dye coupling is increased, if the activity of p38 kinase is blocked. Our data reveal a new type of cross-talk between the FGF and TGF-β pathways, as well as a novel role for TGF-β and p38 kinase in the regulation of GJIC. They also provide an explanation for how pathologically increased TGF-β signaling could contribute to cataract formation.

Upregulation and maintenance of gap junctional communication in lens cells

The cells of the lens are joined by an extensive network of gap junction intercellular channels consisting of connexins 43, 46, and 50. We have proposed, and experimentally supported, the hypothesis that fibroblast growth factor (FGF) signaling is required for upregulation of gap junction-mediated intercellular coupling (GJIC) at the lens equator. The ability of FGF to increase GJIC in cultured lens cells requires sustained activation of extracellular signal-regulated kinase (ERK). In other cell types, activation of ERK has been shown to block GJIC mediated by connexin43 (Cx43). Why ERK signaling does not block lens cell coupling is not known. Another unresolved issue in lens gap junction regulation is how connexins, synthesized before the loss of biosynthetic organelles in mature lens fiber cells, avoid degradation during formation of the organelle-free zone. We have addressed these questions using serum-free cultures (termed DCDMLs) of primary embryonic chick lens epithelial cells. We show that FGF stimulates ERK in DCDMLs via the canonical Ras/Raf1 pathway, and that the reason that neither basal nor growth factor-stimulated GJIC is blocked by activation of ERK is because it is not mediated by Cx43. In fibroblastic cells, the normally rapid rate of degradation of Cx43 after its transport to the plasma membrane is reduced by treatments that either directly (ALLN; epoxomicin) or indirectly (generation of oxidatively un/mis-folded proteins by arsenic compounds) prevent the ubiquitin/proteasome system (UPS) from acting on its normal substrates. We show here that Cx45.6 and Cx56, the chick orthologs of mammalian Cx50 and Cx46, behave similarly in DCDMLs. When organelles lyse during the maturation of fiber cells, they release into the cytosol a large amount of new proteins that have the potential to saturate the capacity, and/or compromise the function, of the UPS. This would serve to spare gap junctions from degradation during formation of the organelle-free zone, thereby preserving GJIC between mature fiber cells despite the lack of de novo connexin synthesis.

The cytoplasmic accumulations of the cataract-associated mutant, Connexin50P88S, are long-lived and form in the endoplasmic reticulum

Mutant connexins have been linked to hereditary congenital cataracts. One such mutant causes a proline-to-serine substitution at position 88 in human connexin 50 (CX50P88S). In transfected cells, CX50P88S does not form gap junctions, but localizes in cytoplasmic multilamellar structures. We studied the dynamics of formation and the stability of these structures in HeLa cells stably transfected with CX50P88S containing a tetracysteine motif appended to its C-terminus (HeLa-CX50P88S(Cys)(4) cells). The tetracysteine motif binds the membrane-permeable biarsenical compounds, FlAsH and ReAsH, which become fluorescent upon binding allowing detection of CX50P88S(Cys)(4) by fluorescence microscopy or by transmission electron microscopy after the ReAsH-driven fluorescent photoconversion of diaminobenzidine. CX50P88S structures were long-lived. Pulse labeling of HeLa-CX50P88S(Cys)(4) cells with FlAsH followed by a chase and ReAsH labeling showed a differential distribution of the labels, with older CX50P88S surrounded by newly synthesized protein. Formation of CX50P88S accumulations was not affected by treatments that block ER-to-Golgi transport. Transmission electron microscopy and tomographic reconstruction revealed that CX50P88S accumulations corresponded to closely apposed circular or semicircular membrane stacks that were sometimes continuous with the rough endoplasmic reticulum. These results suggest that CX50P88S accumulations originate from the rough endoplasmic reticulum and that mutant protein is sequentially added resulting in long-lived cytoplasmic particles. The persistence of these particles in the lens may cause light scattering and the pulverulent cataracts observed in affected individuals.

Lens fiber connexin turnover and caspase-3-mediated cleavage are regulated alternately by phosphorylation

Lens connexins are phosphorylated in vivo; however, the function and regulation of the phosphorylation remain largely unknown. We have previously identified an in vivo phosphorylation site, Ser(364), at the COOH terminus of lens connexin (Cx) Cx45.6 and phosphorylation appears to regulate connexin protein turnover. To assess the specific mechanism of Ser(364) phosphorylation in Cx45.6, exogenous wild type and Ser(364) mutant Cx45.6 were expressed in primary lens cultures through retroviral infection. Cx45.6 turnover was attenuated primarily by proteasomal inhibitors and to a lesser extent by lysosomal inhibitors. Furthermore, the level of Cx45.6 protein in ubiquitin co-expressed cells was significantly reduced as compared to the cells expressing Cx45.6 alone. Moreover, overexpression of ubiquitin led to a more significant decrease in wild type Cx45.6 than Cx45.6(S364A), a mutant deficient of phosphorylation site at Ser(364), although we did not detect any difference in the levels of ubiquitination between wild type and mutant Cx45.6. Interestingly, the mutant mimicking constitutive phosphorylation, Cx45.6(S364D), partially prevented the cleavage of Cx45.6 by caspase-3. Together, our data suggest that phosphorylation of Cx45.6 at Ser(364) appears to stimulate Cx45.6 turnover primarily through proteasome pathway and this phosphorylation inhibits the cleavage of Cx45.6 by caspase-3. These findings provide further insights into regulatory mechanism of the specific phosphorylation of connexins in the lens.

Phosphorylation in the C-terminus of the rat connexin46 (rCx46) and regulation of the conducting activity of the formed connexons

To analyse the role of PKC-dependent phosphorylation in the C-terminus of rCx46 in regulation of rCx46 connexons, truncated mutants rCx46(45.3) and rCx46(44.2) which end before and after PKC-dependent phosphorylation sites respectively were generated. Both rCx46(45.3) and rCx46(44.2) formed connexons in Xenopus oocytes similar to Cx46(wt)-connexons. They were activated by depolarisation above -40 mV and at voltages above 50 mV, inactivation was spontaneously observed or induced by PKC activator TPA, suggesting that inactivation does not require PKC-dependent phosphorylation in the C-terminus. Three casein-kinase-II-(CKII)-dependent phosphorylation sites were also identified. rCx46(37.7) and rCx46(28.2) respectively without two or all of these sites were generated. rCx46(37.7)-connexons were similar to rCx46(wt)-connexons. rCx46(28.2)-connexons comparable to rCx46(wt)-connexons were observed after injection of 50 times more rCx46(28.2)-mRNA (25 ng per oocyte). CKII-blocker inhibited depolarisation-evoked currents in oocytes injected with 0.5 ng per oocyte rCx46(37.7)-mRNA or rCx46(wt)-mRNA. Injection of 25 ng per oocyte rCx46(37.7)-mRNA or rCx46(wt)-mRNA overcame the effect of CKII-inhibitor. We propose that CKII-dependent phosphorylation in the C-terminus accelerates formation of rCx46-connexons.

Promotion of lens epithelial-fiber differentiation by the C-terminus of connexin 45.6 – a role independent of gap junction communication

We previously reported that, among the three connexins expressed in chick lens, overexpression of connexin (Cx) 45.6, not Cx43 or Cx56, stimulates lens cell differentiation; however, the underlying mechanism responsible for this effect is unclear. Here, we took advantage of naturally occurring loss-of-gap-junction function mutations of Cx50 (ortholog of chick Cx45.6) and generated the corresponding site mutants in Cx45.6: Cx45.6(D47A) and Cx45.6(P88S). In contrast to wild-type Cx45.6, the mutants failed to form functional gap junctions, and Cx45.6(P88S) and, to a lesser degree, Cx45.6(D47A) functioned in a dominant-negative manner. Interestingly, overexpression of both mutants incapable of forming gap junctions significantly increased epithelial-fiber differentiation to a level comparable to that of wild-type Cx45.6. To map the functional domain of Cx45.6, we generated a C-terminus chimera as well as deletion mutants. Overexpression of Cx56*45.6C, the mutant in which the C-terminus of Cx56 was replaced with that of Cx45.6, had a stimulatory effect on lens cell differentiation similar to that of Cx45.6. However, cells overexpressing Cx45.6*56C, the mutant in which C-terminus of Cx45.6 was replaced with that of Cx56, and Cx45.6(–C), in which the C-terminus was deleted, failed to promote differentiation. Taken together, we conclude that the expression of Cx45.6, but not Cx45.6-dependent gap junction channels, is involved in lens epithelial-fiber cell differentiation, and the C-terminal domain of Cx45.6 plays a predominant role in mediating this process.

Molecular basis of voltage dependence of connexin channels: An integrative appraisal

The importance of electrical and molecular signaling through connexin (Cx) channels is now widely recognized. The transfer of ions and other small molecules between adjacent cells is regulated by multiple stimuli, including voltage. Indeed, Cx channels typically exhibit complex voltage sensitivity. Most channels are sensitive to the voltage difference between the cell interiors (or transjunctional voltage, V(j)), while other channels are also sensitive to absolute inside-outside voltage (i.e., the membrane potential, V(m)). The first part of this review is focused on the description of the distinct forms of voltage sensitivity and the gating mechanisms that regulate hemichannel activity, both individually and as components of homotypic and heterotypic gap junctions. We then provide an up to date and precise picture of the molecular and structural aspects of how V(j) and V(m) are sensed, and how they, therefore, control channel opening and closing. Mutagenic strategies coupled with structural, biochemical and electrophysical studies are providing significant insights into how distinct forms of voltage dependence are brought about. The emerging picture indicates that Cx channels can undergo transitions between multiple conductance states driven by distinct voltage-gating mechanisms. Each hemichannel may contain a set of two V(j) gates, one fast and one slow, which mediate the transitions between the main open state to the residual state and to the fully closed state, respectively. Eventually, a V(m) gate regulates channel transitions between the open and closed states. Clusters of charged residues within separate domains of the Cx molecule have been identified as integral parts of the V(j) and V(m) sensors. The charges at the first positions of the amino terminal cytoplasmic domain determine the magnitude and polarity of the sensitivity to fast V(j)-gating, as well as contributing to the V(j)-rectifying properties of ion permeation. Additionally, important advances have been made in identifying the conformational rearrangements responsible for fast V(j)-gating transitions to the residual state in the Cx43 channel. These changes involve an intramolecular particle-receptor interaction between the carboxy terminal domain and the cytoplasmic loop.

Decreased gap junctional communication in neurobiotin microinjected lens epithelial cells after taxol treatment

The aim of the study was to examine gap-junction-mediated intercellular communication after experimentally induced aggregations of microtubules in cultured bovine lens epithelial cells. Intercellular communication between lens cells appears to be crucial for normal lens homeostasis. However, investigations on the maintenance of direct ion and metabolite exchange via gap junctions and its quantified dependency of cytoskeletal microtubules have not been available under conditions leading to bundling of microtubules. Thus, metabolic coupling of neighboring lens epithelial cells was quantified following microinjections of neurobiotin into single cells under various conditions. In controls, intensive gap-junction-mediated intercellular communication could be documented by dye-spreading of microinjected neurobiotin. In contrast, taxol treatment for 1-3 days impaired, but did not completely block gap-junction-mediated intercellular communication. After depletion of taxol, a complete recovery of intercellular communication was achieved. In addition, confocal laser scanning microscopy and rapid-freeze deep-etch electron microscopy revealed a displacement of actin-filaments from the perinuclear cytoplasm, accompanied by an abnormal aggregation of microtubules after taxol treatment, including impeded translocation of connexin 43 from the cytoplasm into the plasma membrane. Incubation of cells with nocodazole destroyed the microtubule network, accompanied by a clear reduction of plasma-membrane-integrated connexin 43 and significant impairment of dye spreading. Thus, in lens epithelial cells intercellular communication at gap junctions made by connexin 43 depends on the integrity of the microtubule network through the translocation of connexins to the plasma membrane.

Expression of connexin48.5, connexin44.1, and connexin43 during zebrafish (Danio rerio) lens development

Connexins (Cx), the protein units of gap junctions, play important roles in lens development and homeostasis. Here, we report the mRNA expression patterns of zebrafish Cx48.5, Cx44.1, Cx43 during lens development. The expression of all three connexins in the adult lens was first confirmed by reverse transcriptase-polymerase chain reaction. By whole-mount in situ hybridization, we detected Cx48.5 expression throughout the lens, except the lateral lens epithelium, at 36 hours postfertilization (hpf). The pattern remained the same at 2 days postfertilization (dpf). By 3 and 4 dpf, Cx48.5 expression was restricted to the differentiating lens fibers in the equatorial and medial regions. Cx44.1 was expressed in a similar manner as Cx48.5 from 36 hpf to 4 dpf. However, Cx44.1 expression was also detected in the lens at 24 hpf. Cx43 expression was detected throughout the lens at 24 and 36 hpf but became restricted to the lateral epithelium at later stages.

Interaction of major intrinsic protein (aquaporin-0) with fiber connexins in lens development

We observed that chick lens-fiber gap-junction-forming proteins, connexin (Cx) 45.6 and Cx56, were associated with an unknown protein, which was then identified as major intrinsic protein (MIP), also known as aquaporin-0 (AQP0), the most abundant membrane protein in lens fibers. A 1063 bp cDNA of chick MIP(AQP0) was identified that encodes a 262 amino acid protein with a predicted molecular weight of 28.1 kDa. Dual immunofluorescence and confocal microscopy of sagittal and coronal sections of the lens tissues showed that MIP(AQP0) consistently localized with gap junction plaques formed by Cx45.6 and Cx56 during the early stages of embryonic chick lens development. Immunoprecipitation combined with immunoblotting analyses revealed that MIP(AQP0) was associated with Cx45.6 and Cx56 at these developmental stages. The specificity of this interaction was further confirmed with the silver staining of the protein components of immunoprecipitates. The pull-down analysis of lens lysates revealed that C-terminus of MIP(AQP0) probably interacted with these two fiber connexins. In late embryonic and adult lenses, however, uniform co-distribution of MIP(AQP0) and fiber connexins was largely disrupted, except for the area surrounding the actively differentiating bow regions, as was revealed by immunofluorescence and immunoprecipitation experiments. The interaction of MIP(AQP0) with lens fiber connexins in differentiating lens cells but not in mature lens fibers suggests a potential role for MIP(AQP0) in the facilitation of fiber connexins for the formation of gap junctions during lens development.

Plasma membrane channels formed by connexins: their regulation and functions

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

Loss of function and impaired degradation of a cataract-associated mutant connexin50

A mutant human connexin50 (hCx50), hCx50P88S, has been linked to cataracts inherited as an autosomal dominant trait. The functional, biochemical and cellular behavior of wild-type and mutant hCx50 were examined in transfected cells. hCx50P88S was unable to induce gap junctional currents by itself, and it abolished gap junctional currents when co-expressed with wild-type (wt) hCx50. Cells transfected with hCx50P88S showed cytoplasmic accumulations of Cx50 immunoreactivity in addition to staining at appositional membranes; these accumulations did not significantly co-localize with markers for the endoplasmic reticulum, Golgi apparatus, lysosomes, endosomes or vimentin filaments. Immunoelectron microscopy studies localized hCx50P88S to cytoplasmic membrane stacks in close vicinity to the endoplasmic reticulum. In contrast, aggresome-like accumulations were induced by treatment of wt hCx50-transfected cells with proteasomal inhibitors. The formation of hCx50P88S accumulations in transiently transfected cells was not blocked by treatment with Brefeldin A suggesting that they form before Cx50 transits through the Golgi apparatus to the plasma membrane. Treatment of HeLa-hCx50P88S cells with cycloheximide demonstrated the presence of a very stable pool of hCx50P88S. Taken together, these results suggest that the P to S mutation at amino acid residue 88 causes a defect that leads to decreased degradation and subsequent accumulation of hCx50P88S in a cellular structure different from aggresomes.

A novel role for FGF and extracellular signal-regulated kinase in gap junction-mediated intercellular communication in the lens

Gap junction-mediated intercellular coupling is higher in the equatorial region of the lens than at either pole, a property believed to be essential for lens transparency. We show that fibroblast growth factor (FGF) upregulates gap junctional intercellular dye transfer in primary cultures of embryonic chick lens cells without detectably increasing either gap junction protein (connexin) synthesis or assembly. Insulin and insulin-like growth factor 1, as potent as FGF in inducing lens cell differentiation, had no effect on gap junctions. FGF induced sustained activation of extracellular signal-regulated kinase (ERK) in lens cells, an event necessary and sufficient to increase gap junctional coupling. We also identify vitreous humor as an in vivo source of an FGF-like intercellular communication-promoting activity and show that FGF-induced ERK activation in the intact lens is higher in the equatorial region than in polar and core fibers. These findings support a model in which regional differences in FGF signaling through the ERK pathway lead to the asymmetry in gap junctional coupling required for proper lens function. Our results also identify upregulation of intercellular communication as a new function for sustained ERK activation and change the current paradigm that ERKs only negatively regulate gap junction channel activity.

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.

Molecular cloning, expression analysis, and functional characterization of connexin44.1: A zebrafish lens gap junction protein

The connexin family of genes codes for proteins that oligomerize into a connexon of six subunits to form one half of the gap junction channel. Gap junctions are plasma membrane structures that mediate intercellular communication by joining the cytoplasm of two cells, allowing the passage of small molecules and metabolites, and contributing significantly to the maintenance of tissue homeostasis. The signaling mediated by these junctions appears to be necessary for the correct timing of key developmental events. This communication is especially important in the avascular lens where the intercellular passage of metabolites, second messengers, and ions is necessary to maintain the correct ionic balance in the lens fibre cells, and prevent cataract formation. To characterize the role that the connexin genes play in development, a novel connexin was cloned from zebrafish. A genomic clone was isolated that contained a 1,173 base open reading frame. The nucleotide sequence in this open reading frame shows extensive sequence similarity to mouse connexin50 (Cx50), chicken Cx45.6, sheep Cx49, and human Cx50. The protein encoded by this open reading frame contains 391 amino acids, with a predicted molecular weight of 44.1 kDa and a typical connexin transmembrane topology. By using the LN54 radiation hybrid panel, the Cx44.1 gene was mapped to linkage group 1. Whole-mount in situ hybridization and Northern blot analyses were performed on zebrafish embryos at various developmental stages to characterize the developmental expression of the Cx44.1 message. The ocular lens was the only tissue in which Cx44.1 transcripts were detected. The transcripts were first detected in the lens around 24 hr post fertilization and remained detectable until 120 hr post fertilization. Electrophysiological analysis of Cx44.1 channels revealed gating properties that were virtually identical to the mouse and chicken orthologues of Cx44.1.

Functional properties, developmental regulation, and chromosomal localization of murine connexin36, a gap-junctional protein expressed preferentially in retina and brain

Retinal neurons of virtually every type are coupled by gap-junctional channels whose pharmacological and gating properties have been studied extensively. We have begun to identify the molecular composition and functional properties of the connexins that form these 'electrical synapses,' and have cloned several that constitute a new subclass (gamma) of the connexin family expressed predominantly in retina and brain. In this paper, we present a series of experiments characterizing connexin36 (Cx36), a member of the gamma subclass that was cloned from a mouse retinal cDNA library. Cx36 has been localized to mouse chromosome 2, in a region syntenic to human chromosome 5, and immunocytochemistry showed strong labeling in the ganglion cell and inner nuclear layers of the mouse retina. Comparison of the developmental time course of Cx36 expression in mouse retina with the genesis of the various classes of retinal cells suggests that the expression of Cx36 occurs primarily after cellular differentiation is complete. Because photic stimulation can affect the gap-junctional coupling between retinal neurons, we determined whether lighting conditions might influence the steady state levels of Cx36 transcript in the mouse retina. Steady-state levels of Cx36 transcript were significantly higher in animals reared under typical cyclic-light conditions; exposure either to constant darkness or to continuous illumination reduced the steady-state level of mRNA approximately 40%. Injection of Cx36 cRNA into pairs of Xenopus oocytes induced intercellular conductances that were relatively insensitive to transjunctional voltage, a property shared with other members of the gamma subclass of connexins. Like skate Cx35, mouse Cx36 was unable to form heterotypic gap-junctional channels when paired with two other rodent connexins. In addition, mouse Cx36 failed to form voltage-activated hemichannels, whereas both skate and perch Cx35 displayed quinine-sensitive hemichannel activity. The conservation of intercellular channel gating contrasts with the failure of Cx36 to make hemichannels, suggesting that the voltage-gating mechanisms of hemichannels may be distinct from those of intact intercellular channels.

Casein kinase II phosphorylates lens connexin 45.6 and is involved in its degradation

Connexin (Cx) 45.6, an avian counterpart of rodent Cx50, is phosphorylated in vivo, but the sites and function of the phosphorylation have not been elucidated. Our peptide mapping experiments showed that the Ser(363) site in the carboxyl (COOH) terminus of Cx45.6 was phosphorylated and that this site is within casein kinase (CK) II consensus sequence, although showing some similarity to CKI sequence. The peptide containing Ser(363) could be phosphorylated in vitro by CKII, but not by CKI. Furthermore, CKII phosphorylated Cx45.6 in embryonic lens membrane and the fusion protein containing the COOH terminus of Cx45.6. Two-dimensional peptide mapping experiments showed that one of the Cx45.6 peptides phosphorylated in vivo migrated to the same spot as one of those phosphorylated by CKII in vitro. Furthermore, CKII activity could be detected in lens lysates. To assess the function of this phosphorylation event, exogenous wild type and mutant Cx45.6 (Ser(363) --> Ala) were expressed in lens primary cultures by retroviral infection. The mutant Cx45.6 was shown to be more stable having a longer half-life compared with wild type Cx45.6. Together, the evidence suggests that CKII is likely a kinase responsible for the Ser(363) phosphorylation, leading to the destablization and degradation of Cx45.6. The connexin degradation induced by phosphorylation has a broad functional significance in the regulation of gap junctions in vivo.

Gap junctions containing alpha8-connexin (MP70) in the adult mammalian lens epithelium suggests a re-evaluation of its role in the lens

A missense mutation in one of the three lens connexins, alpha8-connexin, has been recently shown to be the genetic basis of the zonular pulverant lens cataract. This connexin had been considered to be expressed only in lens fibre cells. The present studies show that alpha8-connexin is also expressed in the lens epithelial cell layer. For this study, the distribution of gap junctions in the adult bovine lens has been investigated by confocal immunofluorescence microscopy using antibodies against alpha8-connexin (MP70) and alpha1-connexin (Cx43). In addition to the anticipated localisation of alpha8-connexin to the broad faces of lens fibre cells as reported in other species, alpha8-connexin was also found colocalized with alpha1-connexin at plaques in the lateral epithelial-epithelial plasma membranes of the bovine lens. These data suggest that mixed alpha8-connexin/alpha1-connexin plaques are between epithelial cells at their apico-lateral plasma membranes, rather than between epithelial and fibre cells. Indeed, freeze fracture analyses of the epithelial-fibre cell interface failed to reveal gap junctions connecting the epithelium and the underlying fibre cells. Importantly, microdissection and subsequent immunoblotting of lens epithelium samples confirmed the immunolocalisation results. The data suggest mature mammalian lens epithelial cells could form either heteromeric, heterotypic and/or mixed homomeric-homotypic gap junctional complexes with unique physiological properties, an important point when considering the role of epithelial cell connexins in cataractogenesis.

Trafficking, assembly, and function of a connexin43-green fluorescent protein chimera in live mammalian cells

To examine the trafficking, assembly, and turnover of connexin43 (Cx43) in living cells, we used an enhanced red-shifted mutant of green fluorescent protein (GFP) to construct a Cx43-GFP chimera. When cDNA encoding Cx43-GFP was transfected into communication-competent normal rat kidney cells, Cx43-negative Madin-Darby canine kidney (MDCK) cells, or communication-deficient Neuro2A or HeLa cells, the fusion protein of predicted length was expressed, transported, and assembled into gap junctions that exhibited the classical pentalaminar profile. Dye transfer studies showed that Cx43-GFP formed functional gap junction channels when transfected into otherwise communication-deficient HeLa or Neuro2A cells. Live imaging of Cx43-GFP in MDCK cells revealed that many gap junction plaques remained relatively immobile, whereas others coalesced laterally within the plasma membrane. Time-lapse imaging of live MDCK cells also revealed that Cx43-GFP was transported via highly mobile transport intermediates that could be divided into two size classes of <0.5 microm and 0.5-1.5 microm. In some cases, the larger intracellular Cx43-GFP transport intermediates were observed to form from the internalization of gap junctions, whereas the smaller transport intermediates may represent other routes of trafficking to or from the plasma membrane. The localization of Cx43-GFP in two transport compartments suggests that the dynamic formation and turnover of connexins may involve at least two distinct pathways.

Heptanol exerts epileptiform effects in identified neurons of the buccal ganglia of Helix pomatia

1-Heptanol (0.2-5.0 mM) known to block electrical contacts was tested under epileptic and non-epileptic conditions in the buccal ganglia of Helix pomatia. Synchronicity of epileptiform activity was not affected. In concentrations below 1 mM, heptanol accelerated epileptiform activity induced by pentylenetetrazol. In concentrations above 1 mM, it evoked epileptiform activity without admixture of an epileptogenic drug. Coupling coefficient was increased and decreased in low and high concentration ranges of heptanol, respectively. The measured decrease of coupling is interpreted as a result of the activation of 'epileptiform' membrane conductances accompanied by decreased length constants of neuronal fibers.

Co-expression of lens fiber connexins modifies hemi-gap-junctional channel behavior

Lens fiber cells contain two gap junction proteins (Cx56 and Cx45.6 in the chicken). Biochemical studies have suggested that these two proteins can form heteromeric connexons. To investigate the biophysical properties of heteromeric lens connexons, Cx56 was co-expressed with Cx45.6 (or its mouse counterpart, Cx50) in Xenopus oocytes. Whole-cell and single-channel currents were measured in single oocytes by conventional two-microelectrode voltage-clamp and patch clamp techniques, respectively. Injection of Cx56 cRNA induced a slowly activating, nonselective cation current that activated on depolarization to potentials higher than -10 mV. In contrast, little or no hemichannel current was induced by injection of Cx50 or Cx45.6 cRNA. Co-expression of Cx56 with Cx45.6 or Cx50 led to a shift in the threshold for activation to -40 or -70 mV, respectively. It also slowed the rate of deactivation of the hemichannel currents. Moreover, an increase in the unitary conductance, steady state probability of hemichannel opening and mean open times at negative potentials, was observed in (Cx56 + Cx45.6) cRNA-injected oocytes compared with Cx56 cRNA-injected oocytes. These results indicate that co-expression of lens fiber connexins gives rise to novel channels that may be explained by the formation of heteromeric hemichannels that contain both connexins.

Isolated epithelial cells from amphibian urinary bladder express functional gap junctional hemichannels

Exposure of the urinary bladder epithelium of Necturus maculosus (NUB) to protease and collagenase yields approximately 50% isolated polarized cells. These cells express a membrane current slowly activated by depolarization or by removal of external divalent cations. The biophysical and pharmacological properties of the current are largely consistent with those of gap junctional hemichannels. After removal of divalent cations, the cells can also be loaded with 5(6)-carboxyfluorescein, a hydrophilic fluorescent anionic dye, and exposure to dye reduces the current in a manner dependent on membrane voltage and side of application. In contrast, Necturus gallbladder (NGB) cells exhibit no membrane conductance attributable to gap junctional hemichannels, although previous studies reveal the persistence of gap junction plaques on the plasma membrane. We conclude that functional gap junctional hemichannels can be expressed on the surface of certain isolated epithelial cells and that this is not a necessary consequence of the isolation procedure. These structures may contribute to cell damage under pathological conditions involving cell detachment.

Normal differentiation of cultured lens cells after inhibition of gap junction-mediated intercellular communication

The cells of the vertebrate lens are linked to each other by gap junctions, clusters of intercellular channels that mediate the direct transfer of low-molecular-weight substances between the cytosols of adjoining cells. Although gap junctions are detectable in the unspecialized epithelial cells that comprise the anterior face of the organ, both their number and size are greatly increased in the secondary fiber cells that differentiate from them at the lens equator. In other organs, gap junctions have been shown to play an important role in tissue development and differentiation. It has been proposed, although not experimentally tested, that this may be true in the lens as well. To investigate the function of gap junctions in the development of the lens, we have examined the effect of the gap junction blocker 18beta-glycyrrhetinic acid (betaGA) on the differentiation of primary cultures (both dissociated cell-derived monolayers and central epithelium explants) of embryonic chick lens epithelial cells. We found that betaGA greatly reduced gap junction-mediated intercellular transfer of Lucifer yellow and biocytin throughout the 8-day culture period. betaGA did not, however, affect the differentiation of these cells into MP28-expressing secondary fibers. Furthermore, inhibition of gap junctions had no apparent effect on either of the two other types of intercellular (adherens and tight) junctions present in the lens. We conclude that the high level of gap junctional intercellular communication characteristic of the lens equator in vivo is not required for secondary fiber formation as assayed in culture. Up-regulation of gap junctions is therefore likely to be a consequence rather than a cause of lens fiber differentiation and may primarily play a role in lens physiology.

Monovalent ion selectivity sequences of the rat connexin43 gap junction channel

The relative permeability sequences of the rat connexin 43 (rCx43) gap junction channel to seven cations and chloride were examined by double whole cell patch clamp recording of single gap junction channel currents in rCx43 transfected neuroblastoma 2A (N2A) cell pairs. The measured maximal single channel slope conductances (gammaj, in pS) of the junctional current-voltage relationships in 115 mM XCI were RbC1 (103) > or = CsC1 (102) > KC1 (97) > NaC1 (79) > or = LiC1 (78) > TMAC1 (65) > TEAC1 (53) and for 115 mM KY were KBr (105) > KC1 (97) > Kacetate (77) > Kglutamate (61). The single channel conductance- aqueous mobility relationships for the test cations and anions were linear. However, the predicted minimum anionic and cationic conductances of these plots did not accurately predict the rCx43 channel conductance in 115 mM KC1. Instead, the conductance of the rCx43 channel in 115 mM KC1 was accurately predicted from cationic and anionic conductance-mobility plots by applying a mobility scaling factor Dx/Do, which depends upon the relative radii of the permeant ions to an estimated pore radius. Relative permeabilities were determined for all of the monovalent catious and anions tested from asymmetric salt reversal potential measurements and the Goldman-Hodgkin-Katz voltage equation. These experiments estimate the relative chloride to potassium permeability to be 0.13. The relationship between the relative cation permeability and hydrated radius was modeled using the hydrodynamic equation assuming a pore radius of 6.3 +/- 0.4 A. Our data quantitatively demonstrate that the rCx43 gap junction channel is permeable to monovalent atomic and organic cations and anions and the relative permeability sequences are consistent with an Eisenman sequence II or I, respectively. These predictions about the rCx43 channel pore provide a useful basis for future investigations into the structural determinants of the conductance and permeability properties of the connexin channel pore.

The gap-junction protein connexin 56 is phosphorylated in the intracellular loop and the carboxy-terminal region

The lens gap-junction protein, connexin 56, is modified by phosphorylation. Two-dimensional mapping of tryptic phosphopeptides of 32P-labeled connexin 56 from primary chicken-lens cultures showed that treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) induced an increase in phosphorylation of connexin 56 at specific constitutively phosphorylated sites. Treatment with 8-Br-cAMP or forskolin did not induce substantial changes in connexin 56 phosphorylation. Two phosphorylation sites within connexin 56, S493 and S118, were identified after HPLC purification and peptide sequencing of tryptic phosphopeptides from bacterially expressed connexin 56 fusion proteins phosphorylated by protein kinase C or protein kinase A in vitro. Comparisons of the two-dimensional maps of tryptic phosphopeptides from in vitro phosphorylated connexin 56 fusion proteins and in vivo phosphorylated connexin 56 showed that S493 and S118 were constitutively phosphorylated in lentoid-containing cultures, and that treatment with TPA induced an increase in phosphorylation of the peptides containing S118. It is suggested that phosphorylation of connexin 56 at S118 is involved in the TPA-induced decrease in intercellular communication and acceleration of connexin 56 degradation.

Gap junction channel gating at bass retinal electrical synapses

To further characterize the properties of retinal horizontal cell electrical synapses, we have studied the gating characteristics of gap junctions between cone-driven horizontal cells from the hybrid striped bass retina using double whole-cell voltage-clamp techniques. In a total of 105 cell pairs, the macroscopic conductance ranged from 0.4-100 nS with most cell pairs exhibiting junctional conductances between 10 and 30 nS. The junctional current-voltage relationship showed that peak or instantaneous currents (Iinst) were linear within the Vj range of +/- 100 mV and that steady-state junctional currents (Iss) exhibited rectification with increasing voltage beginning around +/- 30-40 mV Vj. The normalized junctional current-voltage relationship was well fit by a two-state Boltzmann distribution, with an effective gating charge of 1.9 charges/channel, a half-maximal voltage of approximately +/- 55 mV, and a normalized residual conductance of 0.28. The decay of junctional current followed a single exponential time course with the time constant decreasing with increasing Vj. Recovery of junctional current from voltage-dependent inactivation takes about 1 s following a pulse of 80 mV, and is about five times slower than the inactivation time course at the same Vj. Single-channel analysis showed that the unitary conductance of junctional channels is 50-70 pS. The overall open probability decreased in a voltage-dependent manner. Both the mean channel open time and the frequency of channel opening decreased, while the channel closure time increased. The ratio of closed time/total recording time significantly increased as Vj increased. Increased Vj reduced the number of events at all levels and shifted the unitary conductance to a lower level. Kinetic analysis of channel open duration showed that the distribution of channel open times was best fit by two exponentials and increased Vj significantly reduced the slower time constant. These results indicate that bass retina horizontal cells exhibit voltage-dependent inactivation of macroscopic junctional current. The inactivation occurs at the single-channel level mainly by increasing the rate of closure of voltage-sensitive channels.

Connections with connexins: the molecular basis of direct intercellular signaling

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

Longitudinal body wall muscles are electrically coupled across the segmental boundary in the third instar larva of Drosophila melanogaster

Longitudinal body wall muscles in the third instar larva of the fruitfly, Drosophila melanogaster, were systematically examined for electrical and dye coupling. These muscle cells were found to be electrically coupled but rarely dye-coupled across the segmental boundary. The inter-segmental coupling coefficients between muscle #6s and muscle #7s across the segmental boundary were 0.33 +/- 0.09 (mean +/- S.D., n = 12) and 0.43 +/- 0.09 (n = 5), respectively, which are much larger than values previously reported in Drosophila but similar to those reported in the blowfly and hawkmoth. By contrast, the intra-segmental coupling coefficient between muscles #6 and #7 was smaller, 0.16 +/- 0.08 (n = 28). Other muscle cells which had apparent physical contacts with these longitudinal muscles were examined but were not electrically coupled to them. Nerve-evoked as well as miniature excitatory junctional potentials were found also electrotonically spread across the segmental boundary. The inter-segmental coupling between muscle #6s was not blocked by the gap junction inhibitors halothane or 1-octanol. Functional significance of this electrical coupling is apparently in coordination of larval body movements.

Molecular cloning of sheep connexin49 and its identity with MP70

The nucleotide sequence of the sheep homologue of the lens-specific mouse connexin50, chicken connexin45.6, and human connexin50 has been obtained following screening of a sheep genomic library. This connexin comprises 1323 nucleotides, coding for a protein of 440 amino acid residues and a predicted molecular weight of 49,160 daltons, so by convention is termed sheep connexin49. A connexin49 cDNA probe detected a single major band with a mobility of 6.8 kb in sheep lens RNA, but not in RNA isolated from five other sheep organs. The N-terminal amino acid sequence of sheep connexin49 is identical to that of mouse connexin50 and closely matches that of MP70, indicating the identity of sheep connexin49 with MP70. The nucleotide and translated amino acid sequences of connexin49 have 69-87% and 76%-87% identity respectively with chicken connexin45.6, human connexin50 and mouse connexin50. Like other members of this lens connexin family, sheep connexin49 coding region is completely contained within one exon, and the sequence of the N-terminal region, the four transmembrane domains and the two extracellular loops are highly conserved.

Chondrocytes isolated from mature articular cartilage retain the capacity to form functional gap junctions

The distribution, expression, and functionality of gap junctions was examined in bovine chondrocytes (BCs) isolated from mature articular cartilage. BC cells displayed immunoreactivity for connexin 43 (Cx43), a specific gap junction protein. Cx43 protein expression was confirmed by Western blot analysis, and Cx43 mRNA was detected by nuclease protection assay. Additionally, BCs were shown to be functionally coupled, as revealed by dye transfer studies, and octanol, a gap junction uncoupler, greatly attenuated coupling. Furthermore, confocal microscopy of fluo-3 loaded BC cells revealed that deformation-induced cytosolic Ca2+ ion (Ca2+) signals propagated from cell-to-cell via gap junctions. To our knowledge, this is the first evidence suggesting that chondrocytes isolated from adult articular cartilage express functional gap junctions.

Purification of bovine lens cell-to-cell channels composed of connexin44 and connexin50

Cell-to-cell channels composed of connexin44 and connexin50 were purified from plasma membranes of calf and fetal bovine lenses. The channels were treated with the nonionic detergents octyl-beta-D-glucopyranoside and decyl-beta-D-maltopyranoside, and the channel/detergent complexes purified by ion and gel filtration column chromatography. In negative staining, the channels appeared as annuli 11 +/- 0.6 nm (s.d., n = 105) in diameter and as 16 +/- 0.8 nm (s.d., n = 96) long particles which corresponded to top and side views of ‘complete’ cell-to-cell channels. The purified cell-to-cell channels were composed principally of a protein, called MP70, that appeared as a diffuse 55–75 kDa band in SDS-PAGE. Dephosphorylation with alkaline phosphatase transformed the diffuse 55–75 kDa band into two distinct bands of almost equal intensity. Immunoblotting showed the bands to be connexin44 and connexin50, respectively. The antibodies also recognized weaker bands composed of the unphosphorylated form of both connexins. The connexins appear to be processed independently ‘in vivo’. The unphosphorylated form of connexin50 was present in channels and membranes from fetal, calf and adult bovine lenses, while unphosphorylated connexin44 only in channels purified from fetal lenses. Therefore, lens cell-to-cell channels are composed principally of equal amounts of phosphorylated connexins 44 and 50 that appear to be assembled in the same channel (‘hybrid’).

Molecular cloning, tissue distribution, and hormonal control in the ovary of Cx41 mRNA, a novel Xenopus connexin gene transcript

Gap junctions (GJ) are aggregates of intercellular channels, composed of connexin (Cx) protein, between adjacent cells. The vertebrate ovarian follicle contains homocellular (granulosa cell-granulosa cell) and heterocellular (granulosa cell-oocyte) GJ. However, the function of GJ during final oocyte differentiation (maturation) is controversial. The objectives of this study are to reexamine the number and identity of Cx genes that are expressed in the Xenopus ovary, and to examine the potential role of GJ in oocyte maturation by determining the temporal association between changes in ovarian Cx mRNA content and the process of maturation. We used reverse transcriptase-polymerase chain reaction to amplify ovarian cDNA fragments using degenerate Cx primers. We amplified three Cx-like fragments: one was novel and two corresponded to known Cx of Xenopus ovaries (Cx38 and 43). The novel fragment was used to screen an ovarian cDNA library. One positive clone was identified and its nucleotide sequence determined. Its deduced amino acid sequence showed that it corresponded to a novel Cx, Cx41, belonging to the Group II class of Cx. Xenopus Cx41 showed the highest homology to rat Cx37 (65% identity, 80% similarity). Also, the last 10 C-terminal amino acids of Cx41 were identical to those of rat, mouse, and human Cx37. Cx41 transcripts were detected by riboprobe mapping in ovarian somatic cells, heart, leg muscle, liver and eye, but not in brain or in oocytes of any developmental stage. Full-grown follicles incubated in vitro with human chorionic gonadotropin became committed to mature within 1-4 hr, and physical signs of maturation (germinal vesicle breakdown) were seen at 4-5 hr. Significant reductions in the levels of Cx41 and 43, but not 38 transcripts were seen at 4 hr, after oocytes had committed to mature. Thus, if availability of Cx mRNA determines availability of Cx protein and GJ, our results would suggest that irreversible commitment to maturation occurred prior to major declines in follicular GJ during the periovulatory period. The present study is the first to report the presence of at least two hormone-responsive Cx gene transcripts (Cx41 and 43 in Xenopus) in ovaries of a single animal species.

Ocular lens gap junctions: protein expression, assembly, and structure-function analysis

Recent advances in understanding lens fiber gap junction formation are reviewed. These include studies of junctional protein expression in the embryonic lens, and of age related changes affecting gap junction structure and composition in the adult lens. An in vitro assembly system based on detergent solubilized pore complexes and endogenous lipids has been developed to provide information on the molecular interactions involved in gap junction formation and to provide material for structure analysis. Important information on the electrical properties of lens gap junction channels is obtained using electrophysiological techniques including planar lipid bilayer analysis and patch clamping.

Cell-to-cell communication in osteoblastic networks: cell line-dependent hormonal regulation of gap junction function

We have characterized the distribution, expression, and hormonal regulation of gap junctions in primary cultures of rat osteoblast-like cells (ROBs), and three osteosarcoma cell lines, ROS 17/2.8, UMR-106, and SAOS-2, and a continuous osteoblastic cell line, MC3T3-E1. All cell lines we examined were functionally coupled. ROS 17/2.8 were the more strongly coupled, while ROB and MC3T3-E1 were moderately coupled and UMR-106 and SAOS-2 were weakly coupled. Exposure to parathyroid hormone (PTH) for 1 h increased functional coupling in ROB cells in a concentration-dependent manner. Furthermore, PTH(3-34), an analog of PTH with binds to the PTH receptor and thus attenuates PTH-stimulated cAMP accumulation, also attenuated PTH-stimulated functional coupling in ROB. This suggests that PTH increases functional coupling partly through a cAMP-dependent mechanism. A 1 h exposure to PTH did not affect coupling in ROS 17/2.8, UMR-106, MC3T3-E1, or SAOS-2. To examine whether connexin43 (Cx43), a specific gap junction protein, is present in functionally coupled osteoblastic cells, we characterized Cx43 distribution and expression. Indirect immunofluorescence with antibodies to Cx43 revealed that ROS 17/2.8, ROB, and to a lesser extent MC3T3-E1 and UMR-106, expressed Cx43 immunoreactivity. SAOS-2 showed little if any Cx43 immunoreactivity. Cx43 mRNA and Cx43 protein were detected by Northern blot analysis and immunoblot analysis, respectively, in all cell lines examined, including SAOS-2. Our findings suggest that acute exposure to PTH regulates gap junction coupling, in a cell-line dependent manner, in osteoblastic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental expression of connexins in the chick embryo myocardium and other tissues

BACKGROUND

Connexins are cell surface proteins that form specialized regions of cell-cell communication called gap junctions. These allow impulse conduction in in voluntary muscle tissue such as the heart, but also allow the formation of communities of like cells during development of organs.

METHODS

We used an antipeptide antibody to connexin 43 in immunolocalization studies and an anti-peptide antibody to an external loop domain common to most connexins in Western blotting of total heart protein to measure the accumulation of connexins in the heart as it develops from 33 hours to 21 days (hatching), and in the adult.

RESULTS

Immunolocalization revealed that connexin 43 is widely distributed in the earliest organ rudiments. It is especially prominent in the neural tube and its derivatives, in the lens and nasal placodes, in the foregut and its derivatives, in the somites, in the mesonephric tubules, and in the heart and major arteries. Heart tissue staining grew more intense with development through day 8. However, at day 11 and day 15, and in the adult, heart staining diminished. Endocardium and valve tissue did not stain. Western blotting of heart homogenates with the antibody directed against the external loop domain peptide showed 26, 32, 43, 45, and 56 kilodalton connexins, which changed in relative abundance, displaying unique patterns during development.

CONCLUSIONS

Our results show patterns of connexin immunolocalization in early germ layers and organ rudiments that are similar to those known in the mouse, but with certain differences. Our results show a distinctive pattern of multiple connexin gene expression in the developing heart from days 2-21.

Modulation of connexin43 expression: effects on cellular coupling

INTRODUCTION

Gap junctions connect cardiac myocytes allowing propagation of action potentials. They contain intercellular channels formed by multiple different connexin proteins. The arrangement and type of gap junctions and the types, function, and interaction of connexin proteins determine intercellular resistance and can thereby influence conduction velocity and the potential for reentrant arrhythmias. Our goal was to develop genetically manipulable models to test the effects of altering expression of a major cardiac connexin (connexin43) on intercellular coupling and expression of other connexin proteins.

METHODS AND RESULTS

BHK cells that are poorly coupled and BWEM cells that are well coupled were stably transfected with plasmids containing connexin43 cDNA in antisense and sense orientations. RNA blots confirmed expression of the transfected transcripts. Immunoblots showed that connexin43 protein was reduced in the BHK antisense transfectants and increased in the BHK sense transfectants compared to the parental cells. It was not detectably changed in the BWEM antisense transfectant line compared to the BWEM parental cells. Transfection of connexin43 cDNA did not affect production of connexin45 mRNA and protein nor did transfection induce expression of other previously unexpressed connexin mRNAs. Cell coupling was assessed by intercellular diffusion of microinjected Lucifer yellow in confluent cell populations. Lucifer yellow passed to a mean of 3 +/- 3 neighboring parental BHK cells, to 8 +/- 8 neighbors in the sense connexin43 transfected BHK cells, and to only 2 +/- 2 neighbors in the antisense connexin43 transfected BHK cells (P < 0.05). In contrast, dye transfer did not differ significantly between the parental BWEM cells (mean transfer = 19 +/- 14 cells) and the BWEM connexin43 antisense transfectants (mean transfer = 15 +/- 12 cells) (P = 0.20).

CONCLUSIONS

These data demonstrate that stable transfection with connexin43 cDNA constructs can result in detectable changes in connexin43 expression and cellular coupling without inducing compensatory changes in the cell's connexin phenotype and, therefore, may provide a basis for future attempts at specifically modulating connexin expression and intercellular resistance in cardiac tissues.

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.

Selective dye and ionic permeability of gap junction channels formed by connexin45

Gap junctions are thought to mediate the direct intercellular coupling of adjacent cells by the gating of an aqueous pore permeable to ions and molecules of up to 1 kD or 8 to 14 A in diameter. We performed ion-substitution and dye-transfer experiments to determine the relative Cl-/K+ conductance and dye permeability of anionic fluorescein derivatives in chick connexin45 (Cx45) channels. We demonstrate that Cx45 forms a 26 +/- 6-picosiemen (pS) channel with a maximum detectable Cl- permeability of 0.2 relative to K+ or Cs+. Although homogeneous channel conductances were observed in multichannel recordings, the open probability estimates were indicative of nonhomogeneous gating behavior and occasional cooperativity. A second conductance state of 19 +/- 4 pS begins to predominate at higher voltages. Cx45 gap junctions are permeable to 2',7'-dichlorofluorescein but are not permeable to the more polar 6-carboxyfluorescein dye. These observations suggest that the Cx45 pore diameter is approximately 10 A and is associated with a fixed negative charge within the junctional channel.