Dye transfer between cells of the embryonic chick lens becomes less sensitive to CO2 treatment with development

A Steric “Ball-and-Chain” Mechanism for pH-Mediated Regulation of Gap Junction Channels

Gap junction channels (GJCs) mediate intercellular communication and are gated by numerous conditions such as pH. The electron cryomicroscopy (cryo-EM) structure of Cx26 GJC at physiological pH recapitulates previous GJC structures in lipid bilayers. At pH 6.4, we identify two conformational states, one resembling the open physiological-pH structure and a closed conformation that displays six threads of density, that join to form a pore-occluding density. Crosslinking and hydrogen-deuterium exchange mass spectrometry reveal closer association between the N-terminal (NT) domains and the cytoplasmic loops (CL) at acidic pH. Previous electrophysiologic studies suggest an association between NT residue N14 and H100 near M2, which may trigger the observed movement of M2 toward M1 in our cryo-EM maps, thereby accounting for additional NT-CL crosslinks at acidic pH. We propose that these pH-induced interactions and conformational changes result in extension, ordering, and association of the acetylated NT domains to form a hexameric "ball-and-chain" gating particle.

Calmodulin-Mediated Regulation of Gap Junction Channels

Evidence that neighboring cells uncouple from each other as one dies surfaced in the late 19th century, but it took almost a century for scientists to start understanding the uncoupling mechanism (chemical gating). The role of cytosolic free calcium (Ca2+i) in cell-cell channel gating was first reported in the mid-sixties. In these studies, only micromolar [Ca2+]i were believed to affect gating-concentrations reachable only in cell death, which would discard Ca2+i as a fine modulator of cell coupling. More recently, however, numerous researchers, including us, have reported the effectiveness of nanomolar [Ca2+]i. Since connexins do not have high-affinity calcium sites, the effectiveness of nanomolar [Ca2+]i suggests the role of Ca-modulated proteins, with calmodulin (CaM) being most obvious. Indeed, in 1981 we first reported that a CaM-inhibitor prevents chemical gating. Since then, the CaM role in gating has been confirmed by studies that tested it with a variety of approaches such as treatments with CaM-inhibitors, inhibition of CaM expression, expression of CaM mutants, immunofluorescent co-localization of CaM and gap junctions, and binding of CaM to peptides mimicking connexin domains identified as CaM targets. Our gating model envisions Ca2+-CaM to directly gate the channels by acting as a plug ("Cork" gating model), and probably also by affecting connexin conformation.

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Lens gap junctions in growth, differentiation, and homeostasis

The cells of most mammalian organs are connected by groups of cell-to-cell channels called gap junctions. Gap junction channels are made from the connexin (Cx) family of proteins. There are at least 20 isoforms of connexins, and most tissues express more than 1 isoform. The lens is no exception, as it expresses three isoforms: Cx43, Cx46, and Cx50. A common role for all gap junctions, regardless of their Cx composition, is to provide a conduit for ion flow between cells, thus creating a syncytial tissue with regard to intracellular voltage and ion concentrations. Given this rather simple role of gap junctions, a persistent question has been: Why are there so many Cx isoforms and why do tissues express more than one isoform? Recent studies of lens Cx knockout (KO) and knock in (KI) lenses have begun to answer these questions. To understand these roles, one must first understand the physiological requirements of the lens. We therefore first review the development and structure of the lens, its numerous transport systems, how these systems are integrated to generate the lens circulation, the roles of the circulation in lens homeostasis, and finally the roles of lens connexins in growth, development, and the lens circulation.

Gap junction turnover is achieved by the internalization of small endocytic double-membrane vesicles

Double-membrane-spanning gap junction (GJ) channels cluster into two-dimensional arrays, termed plaques, to provide direct cell-to-cell communication. GJ plaques often contain circular, channel-free domains ( approximately 0.05-0.5 mum in diameter) identified >30 y ago and termed nonjunctional membrane (NM) domains. We show, by expressing the GJ protein connexin43 (Cx43) tagged with green fluorescent protein, or the novel photoconvertible fluorescent protein Dendra2, that NM domains appear to be remnants generated by the internalization of small GJ channel clusters that bud over time from central plaque areas. Channel clusters internalized within seconds forming endocytic double-membrane GJ vesicles ( approximately 0.18-0.27 mum in diameter) that were degraded by lysosomal pathways. Surprisingly, NM domains were not repopulated by surrounding channels and instead remained mobile, fused with each other, and were expelled at plaque edges. Quantification of internalized, photoconverted Cx43-Dendra2 vesicles indicated a GJ half-life of 2.6 h that falls within the estimated half-life of 1-5 h reported for GJs. Together with previous publications that revealed continuous accrual of newly synthesized channels along plaque edges and simultaneous removal of channels from plaque centers, our data suggest how the known dynamic channel replenishment of functional GJ plaques can be achieved. Our observations may have implications for the process of endocytic vesicle budding in general.

Developmental Regulation of the Direct Interaction between the Intracellular Loop of Connexin 45.6 and the C Terminus of Major Intrinsic Protein (Aquaporin-0)

The eye lens is dependent upon a network of gap junction-mediated intercellular communication to facilitate its homeostasis and development. Three gap junction-forming proteins are expressed in the lens of which two are in lens fibers, namely connexin (Cx) 45.6 and 56. Major intrinsic protein (MIP), also known as aquaporin-0 (AQP0), is the most abundant membrane protein in lens fibers. However, its role in the lens is not clear. Our previous studies show that MIP(AQP0) associates with gap junction plaques formed by Cx45.6 and Cx56 during the early stages of embryonic chick lens development but not in late embryonic and adult lenses. We report here that MIP(AQP0) directly interacts with Cx45.6 but not with Cx56. We further identified the intracellular loop of Cx45.6 as the interacting domain for the MIP(AQP0) C terminus. Surface plasmon resonance experiments indicated that the C-terminal domain of MIP(AQP0) interacts with two binding sites within the intracellular loop region of Cx45.6 with a K(D(app)) of 7.5 and 10.3 microm, respectively. The K(D(app)) for the full-length loop region is 7.7 microm. The cleavage at the intracellular loop of Cx45.6 was observed during lens development, and the C terminus of MIP(AQP0) did not interact with the loop-cleaved form of Cx45.6. Thus, the dissociation between these two proteins that occurs in the mature fibers of late lens development is likely caused by this cleavage. Finally this interaction had no impact on Cx45.6-mediated intercellular communication, suggesting that the Cx45.6-MIP(AQP0) interaction plays a novel unidentified role in lens fibers.

Chemical gating of gap junction channels; roles of calcium, pH and calmodulin

Both Ca(2+) and H(+) play a role in chemical gating of gap junction channels, but, with the possible exception of Cx46 hemichannels, neither of them is likely to induce gating by a direct interaction with connexins. Some evidence suggests that low pH(i) affects gating via an increase in [Ca(2+)](i); in turn, Ca(2+) is likely to induce gating by activation of CaM, which may act directly as a gating particle. The effective concentrations of both Ca(2+) and H(+) vary depending on cell type, type of connexin expressed and procedure employed to increase their cytosolic concentrations; however, pH(i) as high as 7.2 and [Ca(2+)](i) as low as 150 nM or lower have been reported to be effective in some cells. Some data suggest that Ca(2+) and H(+) affect gating by acting synergistically, but other data do not support synergism. Chemical gating follows the activation of a slow gate distinct from the fast V(j)-sensitive gate, and there is evidence that the chemical/slow gate is V(j)-sensitive. At the single channel level, the chemical/slow gate closes the channels slowly and completely, whereas the fast V(j) gate closes the channels rapidly and incompletely. At least three molecular models of channel gating have been proposed, but all of them are mostly based on circumstantial evidence.

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.

Connexins in the lens: are they to blame in diabetic cataractogenesis?

The pathohistology of the diabetic lens is an enigma. Under normal conditions the lens behaves as a functional syncitium, whereas the diabetic lens exhibits a localized zone of fibre cell swelling and rupture that is confined to the lens outer cortex. Because the lens fibre cells are extensively coupled by gap junction channels, it is believed that the abnormal closure of these channels is responsible for this phenomenon. New evidence concerning regional differences in gap junction gating supports this contention, and it is used to propose a new hypothesis that may explain the cellular changes observed in the diabetic lens.

Regulation of connexin channels by pH. Direct action of the protonated form of taurine and other aminosulfonates

Protonated aminosulfonate compounds directly inhibit connexin channel activity. This was demonstrated by pH-dependent connexin channel activity in Good's pH buffers (MES (4-morpholineethanesulfonic acid)), HEPES, and TAPS (3-({[2-hydroxy-1, 1-bis(hydroxymethyl)ethyl]amino]-1-propanesulfonic acid)) that have an aminosulfonate moiety in common and by the absence of pH-dependent channel activity in pH buffers without an aminosulfonate moiety (maleate, Tris, and bicarbonate). The pH-activity relation was shifted according to the pKa of each aminosulfonate pH buffer. At constant pH, increased aminosulfonate concentration inhibited channel activity. Taurine, a ubiquitous cytoplasmic aminosulfonic acid, had the same effect at physiological concentrations. These data raise the possibility that effects on connexin channel activity previously attributed to protonation of connexin may be mediated instead by protonation of cytoplasmic regulators, such as taurine. Modulation by aminosulfonates is specific for heteromeric connexin channels containing connexin-26; it does not occur significantly for homomeric connexin-32 channels. The identification of taurine as a cytoplasmic compound that directly interacts with and modulates connexin channel activity is likely to facilitate understanding of cellular modulation of connexin channels and lead to the development of reagents for use in structure-function studies of connexin protein.

The differential effects of 12-O-tetradecanoylphorbol-13-acetate on the gap junctions and connexins of the developing mammalian lens

Epithelial cells in primary ovine lens cultures express the gap junction proteins connexin43 (Cx43) and connexin49 (Cx49; a.k.a. MP70), a homologue of mouse connexin50. In contrast, lens cultures of differentiated, fiber-like cells (termed lentoid cells) express Cx49 and connexin46 (Cx46), but not Cx43. To investigate the regulation of lens cell gap junctions by protein kinase C (PKC), differentiating lens cultures were treated with the PKC activator 12-O-tetradecanoylphorbol-13-acetate (beta-TPA). Within 10 min, beta-TPA significantly inhibited the transfer of Lucifer Yellow dye between epithelial, but not lentoid, cells. This inhibition was correlated with the phosphorylation of Cx43 and was followed by the gradual disappearance of Cx43 from cell interfaces. The protein kinase inhibitor staurosporine prevented Cx43 phosphorylation and the loss of Cx43 from intercellular junctions. Following treatment of cultures with beta-TPA for 2-6 hr, Cx49 disappeared from epithelial cell interfaces, and by 24 hr of beta-TPA treatment, levels of Cx49 detected on immunoblots of purified epithelial membrane fractions had also diminished significantly. The beta-TPA-induced loss of Cx49 both from regions of epithelial cell contact and from isolated membranes was correlated with the disappearance of Cx49 mRNA. In contrast to the epithelial connexins, the lentoid connexins Cx49 and Cx46 were unaffected by even extended beta-TPA treatment. In spite of lentoid dye transfer being refractory to beta-TPA, significant levels of PKC-alpha (a beta-TPA-sensitive isoform) were detected in the lentoid cell. The response of lens gap junctions to beta-TPA depends upon the stage of differentiation and the complement of connexins expressed. The contrasting effects of beta-TPA on Cx43 and Cx49 in lens epithelial cells indicate a fundamental difference in the regulation of these connexin proteins in the developing mammalian lens.

Cell-cell coupling between CO2-excited neurons in the dorsal medulla oblongata

Anatomically coupled neurons (17 of 137) and non-coupled neurons (120 of 137), in and near the nucleus tractus solitarius and dorsal motor nucleus (i.e. solitary complex), were studied by rapid perforated patch recording in slices (rat, 150-350 microm thick, postnatal day 0-21) before, during and after exposure to hypercapnic acidosis. Anatomical coupling refers to the intercellular transfer of Lucifer Yellow and Biocytin into adjoining neurons, presumably via gap junctions [see Dean et al. (1997) Neuroscience 80, 21-40]. Eighty-six per cent of the anatomically coupled neurons (12 of 14) were depolarized by hypercapnic acidosis, a response referred to as CO2 excitation or CO2 chemosensitivity. In all, 28% (12 of 43) of the CO2-excited neurons were anatomically coupled to at least one other neuron. None (0 of 39) of the CO2-inhibited neurons were anatomically coupled, and only 4% (two of 46) of the CO2-insensitive neurons were anatomically coupled. Increasing the fractional concentration of CO2 from five to 10 and 15% in constant bicarbonate (26 mM) decreased intracellular pH (control 7.3 7.4, 22-25 degrees C) by approximately 1.0 and 1.5 pH units, respectively, as measured using the pH-sensitive fluorescent dye, 2',7'-bis (2-carboxyethyl)-5,6-carboxyfluorescein. Nine of the anatomically coupled neurons (six CO2-excited, one CO2-insensitive and two unidentified) exhibited spontaneous electrotonic postsynaptic potential-like activity, suggesting that they were also electrotonically coupled. During hypercapnic acidosis, the amplitudes of electrotonic postsynaptic potentials were unchanged, concomitant with small changes in input resistance. The frequency of electrotonic postsynaptic potentials increased during hypercapnic acidosis in many anatomically coupled neurons (eight of nine), indicating that both neurons of the coupled pair were stimulated. Cell-cell coupling occurred preferentially in and between CO2-excited neurons of the solitary complex. Further, CO2-excited neurons were not electrotonically uncoupled during intracellular acidosis, in contrast to the effect that decreased intracellular pH has on many other types of coupled cells. It was not determined whether anatomical coupling was affected by hypercapnic acidosis since dye mixture was always administered under normocapnic conditions. The high correlation between anatomical coupling, electrotonic coupling activity and CO2-induced depolarization suggests that cell-cell coupling is an important electroanatomical feature in CO2-excited neurons of the solitary complex. CO2-excited neurons have been hypothesized to function in central chemoreception for the cardiorespiratory control systems, suggesting that cell cell coupling may contribute in part to central chemoreception of CO2 and H+.

Selective interactions among the multiple connexin proteins expressed in the vertebrate lens: the second extracellular domain is a determinant of compatibility between connexins

Gap junctions are collections of intercellular channels composed of structural proteins called connexins (Cx). We have examined the functional interactions of the three rodent connexins present in the lens, Cx43, Cx46, and Cx50, by expressing them in paired Xenopus oocytes. Homotypic channels containing Cx43, Cx46, or Cx50 all developed high conductance. heterotypic channels composed of Cx46 paired with either Cx43 or Cx50 were also well coupled, whereas Cx50 did not form functional channels with Cx43. We also examined the functional response of homotypic and heterotypic channels to transjunctional voltage and cytoplasmic acidification. We show that all lens connexins exhibited sensitivity to cytoplasmic acidification as well as to voltage, and that voltage-dependent closure of heterotypic channels for a given connexin was dramatically influenced by its partner connexins in the adjacent cell. Based on the observation that Cx43 can discriminate between Cx46 and Cx50, we investigated the molecular determinants that specify compatibility by constructing chimeric connexins from portions of Cx46 and Cx50 and testing them for their ability to form channels with Cx43. When the second extracellular (E2) domain in Cx46 was replaced with the E2 of Cx50, the resulting chimera could no longer form heterotypic channels with Cx43. A reciprocal chimera, where the E2 of Cx46 was inserted into Cx50, acquired the ability to functionally interact with Cx43. Together, these results demonstrate that formation of intercellular channels is a selective process dependent on the identity of the connexins expressed in adjacent cells, and that the second extracellular domain is a determinant of heterotypic compatibility between connexins.

Molecular cloning and functional characterization of chick lens fiber connexin 45.6

The avian lens is an ideal system to study gap junctional intercellular communication in development and homeostasis. The lens is experimentally more accessible in the developing chick embryo than in other organisms, and chick lens cells differentiate well in primary cultures. However, only two members of the connexin gene family have been identified in the avian lens, whereas three are known in the mammalian system. We report here the molecular cloning and characterization of the third lens connexin, chick connexin45.6 (ChCx45.6), a protein with a predicted molecular mass of 45.6 kDa. ChCx45.6 was encoded by a single copy gene and was expressed specifically in the lens. There were two mRNA species of 6.4 kilobase (kb) and 9.4 kb in length. ChCx45.6 was a functional connexin protein, because expression in Xenopus oocyte pairs resulted in the development of high levels of conductance with a characteristic voltage sensitivity. Antisera were raised against ChCx45.6 and chick connexin56 (ChCx56), another avian lens-specific connexin, permitting the examination of the distribution of both proteins. Immunofluorescence localization showed that both ChCx45.6 and ChCx56 were abundant in lens fibers. Treatment of lens membranes with alkaline phosphatase resulted in electrophoretic mobility shifts, demonstrating that both ChCx45.6 and ChCx56 were phosphoproteins in vivo.

Ion concentrations, fluxes and electrical properties of the embryonic chicken lens

The membrane properties of embryonic chicken lenses were characterized using isotopic and electrical techniques. The lenses had a relatively high water content (80%) and large extracellular space (12.5%). Isotopic uptake measurements indicated that the lens cytoplasm contained 118 mM K+ and 26 mM Cl-. A value for intracellular Na+ of 14 mM was obtained using Na(+)-sensitive microelectrodes. A double-exponential model was used to fit the efflux of 86Rb+, 22Na+, 36Cl- and [3H]mannitol (an extracellular space marker) from the lens. When perfused with artificial aqueous humor (AAH) solution, embryonic lenses exhibited membrane potentials of between -20 and -40 mV. The more negative values were generally observed in lenses from older embryos. A ouabain-sensitive component, contributing -7 mV to the membrane potential, was also identified. The relatively depolarized membrane potentials suggested that the lens membranes were only weakly selective for K+ over Na+. To test this further, lenses were perfused with AAH containing varying concentrations of K+. The resulting changes in potential were interpreted in terms of the Goldman model. The best fit of the Goldman potential equation indicated that, in the presence of ouabain, the chicken lens membranes had a relative permeability to K+, Na+ and Cl- of 1.0, 0.36, 0.51 respectively. Replacing most or all of the Na+ in the AAH caused only a small change in the membrane potential rather than the large hyperpolarization towards the K+ equilibrium potential predicted by the Goldman model. Including the K+ ionophore valinomycin in the low Na(+)-AAH solutions caused a large increase in 86Rb+ efflux but did not result in additional hyperpolarization. This suggested that the insensitivity of the membrane potential to reduced extracellular Na+ was not due to voltage or pH inactivation of lens K+ channels.

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.

Single-membrane and cell-to-cell permeability properties of dissociated embryonic chick lens cells

Ion channels are believed to play an important role in the maintenance of lens transparency. In order to ascribe junctional and nonjunctional permeability properties to specific lens cell types, embryonic chick lenses were enzymatically dissociated into cell clusters, cell pairs and single cells, and both cell-to-cell and single-membrane permeability properties were characterized with the patch-clamp technique. Double patch-clamp experiments and single patch-clamp experiments with Lucifer yellow in the pipette demonstrated that the cells in the dissociated preparation were well coupled, the average conductance between pairs being 42 +/- 27 nS. Double patch-clamp experiments also revealed single cell-to-cell channel events with a predominant unitary conductance of 286 +/- 38 pS. Whole-cell measurements of surface membrane conductance indicate heterogeneity within the population of dissociated embryonic chick lens cells: 63% of the cells have a voltage-independent leak current, 14% of the cells have a potassium-selective inward-rectifier current, and 23% of the cells have a current which turns off with positive voltage on a time scale on the order of seconds. The time constant for this turnoff is voltage dependent.

The crystalline lens. A system networked by gap junctional intercellular communication

The vertebrate eye lens is a solid cyst of cells which grows throughout life by addition of new cells at the surface. The older cells, buried by the newer generations, differentiate into long, prismatic fibers, losing their cellular organelles and filling their cytoplasms with high concentrations of soluble proteins, the crystallins. The long-lived lens fibers are interconnected by gap junctions, both with themselves and with an anterior layer of simple cuboidal epithelial cells at the lens surface. This network of gap junctions joins the lens cells into a syncytium with respect to small molecules, permitting metabolic co-operation: intercellular diffusion of ions, metabolites, and water. In contact with nutrients at the lens surface, the epithelial cells retain their cellular organelles, and are able to provide the metabolic energy to maintain correct ion and metabolite concentrations within the lens fiber cytoplasms, such that the crystallins remain in solution and do not aggregate (cataract). Gap junctions are formed by a family of integral membrane channel-forming proteins called connexins. Gap junctions between lens epithelial cells are composed of a connexin which is common between many different cell types, notably myocardial cells and connective tissue fibroblasts. The gap junctions between epithelial cells and lens fibers have not yet been biochemically characterized. The gap junctions formed between lens fibers are composed of at least two different connexins, one of which has not been detected between other cell types. The unusual physiology and longevity of the lens fibers may require the special set of connexins which are found joining these cells.

Internal acidification modulates membrane and junctional resistance in the isolated lens of the frog Rana pipiens

The normal internal pH (pHi) of the amphibian lens, measured using ion-sensitive microelectrodes, is 7.1 (pHo = 7.4) and the membranes appear to be relatively impermeable to hydrogen ions. Perifusing the lens with 100% CO2 appeared to be the most efficient way of decreasing pHi, which fell to 6.3 after an exposure lasting 30 min. Accompanying this acidification, there was a rapid depolarization of membrane potential (Em), a decrease in membrane resistance (Rm) and increase in internal or bulk resistance (Ri). These changes did not occur if the external pH alone was decreased. All changes were reversible, although the time course of Ri recovery was faster than the others. The decrease in membrane resistance could be prevented if the chloride concentration in the external solution was reduced, suggesting that internal acidification opens chloride channels in the amphibian lens. Since chloride ions are normally close to equilibrium across amphibian lens membranes, it is suggested that the pH-induced depolarization is due to a decrease in potassium conductance. The increase in internal resistance on perifusing with CO2 is most likely due to a closing of gap junctions between the fibre cells. The relationship between internal conductance and pHi was very similar to that obtained in other tissues and could be fitted by the Hill equation with n = 6 and pK = 6.9. Fibre junctional conductance seems sensitive to small changes in hydrogen ion concentration around the resting pH. Two agents, aspirin and cyanate, that are believed to influence cataract development, slowed the recovery of Em, Rm and Ri during recovery from an acid load.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical analysis of connexin43 intracellular transport, phosphorylation, and assembly into gap junctional plaques

We previously demonstrated that the gap junction protein connexin43 is translated as a 42-kD protein (connexin43-NP) that is efficiently phosphorylated to a 46,000-Mr species (connexin43-P2) in gap junctional communication-competent, but not in communication-deficient, cells. In this study, we used a combination of metabolic radiolabeling and immunoprecipitation to investigate the assembly of connexin43 into gap junctions and the relationship of this event to phosphorylation of connexin43. Examination of the detergent solubility of connexin43 in communication-competent NRK cells revealed that processing of connexin43 to the P2 form was accompanied by acquisition of resistance to solubilization in 1% Triton X-100. Immunohistochemical localization of connexin43 in Triton-extracted NRK cells demonstrated that connexin43-P2 (Triton-insoluble) was concentrated in gap junctional plaques, whereas connexin43-NP (Triton-soluble) was predominantly intracellular. Using either a 20 degrees C intracellular transport block or cell-surface protein biotinylation, we determined that connexin43 was transported to the plasma membrane in the Triton-soluble connexin43-NP form. Cell-surface biotinylated connexin43-NP was processed to Triton-insoluble connexin43-P2 at 37 degrees C. Connexin43-NP was also transported to the plasma membrane in communication defective, gap junction-deficient S180 and L929 cells but was not processed to Triton-insoluble connexin43-P2. Taken together, these results demonstrate that gap junction assembly is regulated after arrival of connexin43 at the plasma membrane and is temporally associated with acquisition of insolubility in Triton X-100 and phosphorylation to the connexin43-P2 form.

Connexin46, a novel lens gap junction protein, induces voltage-gated currents in nonjunctional plasma membrane of Xenopus oocytes

Gap junctions are composed of a family of structural proteins called connexins, which oligomerize into intercellular channels and function to exchange low molecular weight metabolites and ions between adjacent cells. We have cloned a new member of the connexin family from lens cDNA, with a predicted molecular mass of 46 kD, called rat connexin46 (Cx46). Since a full-length cDNA corresponding to the 2.8-kb mRNA was not obtained, the stop codon and surrounding sequences were confirmed from rat genomic DNA. The RNA coding for this protein is abundant in lens fibers and detectable in both myocardium and kidney. Western analysis of both rat and bovine lens membrane proteins, using the anti-MP70 monoclonal antibody 6-4-B2-C6 and three anti-peptide antibodies against Cx46 demonstrates that Cx46 and MP70 are different proteins. Immunocytochemistry demonstrates that both proteins are localized in the same lens fiber junctional maculae. Synthesis of Cx46 in either reticulocyte lysate or Xenopus oocytes yields a 46-kD polypeptide; all anti-Cx46 antisera recognize a protein in rat lens membranes 5-10 kD larger, suggesting substantive lenticular posttranslational processing of the native translation product. Oocytes that have synthesized Cx46 depolarize and lyse within 24 h, a phenomenon never observed after expression of rat connexins 32 or 43 (Cx32 and Cx43). Lysis is prevented by osmotically buffering the oocytes with 5% Ficoll. Ficoll-buffered oocytes expressing Cx46 are permeable to Lucifer Yellow but not FITC-labeled BSA, indicating the presence of selective membrane permeabilities. Cx43-expressing oocytes are impermeable to Lucifer Yellow. Voltage-gated whole cell currents are measured in oocytes injected with dilute concentrations of Cx46 but not Cx43 mRNA. These currents are activated at potentials positive to -10 mV. Unlike other connexins expressed in Xenopus oocytes, these results suggest that unprocessed Cx46 induces nonselective channels in the oolemma that are voltage dependent and opened by large depolarizations.

MP26, a protein of intercellular junctions in the bovine lens: electrophoretic and chromatographic characterization

We have characterized the membrane protein of apparent molecular weight 26 kD from bovine lenses (MP26 or MIP) with respect to six different electrophoretic and chromatographic procedures. These include one- and two-dimensional gel electrophoretic procedures, as well as SDS-hydroxylapatite chromatography. The two-dimensional gels include isoelectric focusing with both conventional ampholytes and buffer focusing methods. With buffer focusing, the membranes are solubilized without the use of SDS and the isoelectric focusing is performed in the absence of SDS. As specific probes for MP26, a monoclonal antibody and an anti-MP26 rabbit serum were used, the latter prepared against electrophoretically purified MP26. These separation techniques were adapted to MP26 in order to permit a more detailed characterization of this protein and to search for any heterogeneity in this size range, specifically other junctional proteins or protein fragments. We have found evidence for charge heterogeneity in MP26, but no evidence for multiple membrane proteins of Mr 26,000 in urea-treated membranes. The charge heterogeneity appears to be related to a phosphorylation of MP26. The results reported here aid the interpretation of a variety of data, especially findings on the reconstitution of MP26 in artificial membranes and results from work with polyclonal MP26 antibodies. These investigations are all designed to evaluate the proposed role of MP26 as a protein of cell-to-cell channels in the lens fiber cell.

Ultrastructural observations of typical gap junctions in human foetal lens nucleus

The ultrastructure of gap junctions throughout the human foetal lens was observed. By freeze-fracture analysis, we observed numerous gap junctions in both lens cortex and lens nucleus. Comparison between lens cortex and lens nucleus showed that the gap junctions of lens nucleus are characterized by extreme mosaics of closely apposed P- and E-faces in junctional areas, though no significant difference in the area of gap junctions was observed between lens cortex and lens nucleus. In addition, some morphological variations, such as the smooth domains without particles or pits in junctional areas and the reticulated figures of gap junctions, were observed only in the lens nucleus. We also observed by thin-section electron microscopy that cell membranes of human foetal lens nucleus, as observed in the lens cortex, are mainly composed of continuous lipid bilayer and junctional structures. We concluded that characteristic morphology of lens gap junctions, as observed in the cortex of human foetal lens, is mostly preserved in the human foetal lens nucleus, although some depth-dependent alterations were also observed.

Differential phosphorylation of the gap junction protein connexin43 in junctional communication-competent and -deficient cell lines

Connexin43 is a member of the highly homologous connexin family of gap junction proteins. We have studied how connexin monomers are assembled into functional gap junction plaques by examining the biosynthesis of connexin43 in cell types that differ greatly in their ability to form functional gap junctions. Using a combination of metabolic radiolabeling and immunoprecipitation, we have shown that connexin43 is synthesized in gap junctional communication-competent cells as a 42-kD protein that is efficiently converted to a approximately 46-kD species (connexin43-P2) by the posttranslational addition of phosphate. Surprisingly, certain cell lines severely deficient in gap junctional communication and known cell-cell adhesion molecules (S180 and L929 cells) also expressed 42-kD connexin43. Connexin43 in these communication-deficient cell lines was not, however, phosphorylated to the P2 form. Conversion of S180 cells to a communication-competent phenotype by transfection with a cDNA encoding the cell-cell adhesion molecule L-CAM induced phosphorylation of connexin43 to the P2 form; conversely, blocking junctional communication in ordinarily communication-competent cells inhibited connexin43-P2 formation. Immunohistochemical localization studies indicated that only communication-competent cells accumulated connexin43 in visible gap junction plaques. Together, these results establish a strong correlation between the ability of cells to process connexin43 to the P2 form and to produce functional gap junctions. Connexin43 phosphorylation may therefore play a functional role in gap junction assembly and/or activity.

Properties of channels reconstituted from the major intrinsic protein of lens fiber membranes

Detergent-solubilized plasma membrane protein of either adult bovine or calf lens and high-performance liquid chromatography-purified major intrinsic protein (MIP) of the lens were reconstituted into unilamellar vesicles and planar lipid bilayers. Freeze-fracture studies showed that the density of intramembrane particles in the vesicles was proportional to the protein/lipid ratio. At high ratios, these particles crystallized into tetragonal arrays as does MIP in lens fibers. Channels induced by either purified MIP or detergent-solubilized protein had essentially identical properties. The conductance of multichannel membranes was maximal near 0 mV and decreased to 0.49 +/- 0.08 of the maximum value at voltages greater than 80 mV. The dependence of the conductance on voltage was well fit by a two-state Boltzmann distribution. Voltage steps greater than 30 mV elicited an ohmic current step followed by a slow (seconds) biexponential decrease. The amplitudes and time constants depended on the magnitude but not the sign of the voltage. Steps from 100 mV to voltages less than 30 mV caused the channels to open exponentially with a millisecond time constant. Analysis of latency to first closure after a voltage step gave nearly the same time constants as multichannel kinetics. Single-channel conductance is proportional to salt concentration from 0.1 to 1.0 M in KCl. In 0.1M KCl, the channel had two preferred conductance states with amplitudes of 380 and 160 pS, as well as three additional substates. Multi- and single-channel data suggest that the channel has two kinetically important open states. The channel is slightly anion selective. The properties of the channel do not vary appreciably from pH 7.4 to 5.8 or from pCa 7 to 2. We propose that a channel with these properties could contribute to maintenance of lens transparency and fluid balance.

Expression of the gap junction protein connexin43 in embryonic chick lens: molecular cloning, ultrastructural localization, and post-translational phosphorylation

Lens epithelial cells are physiologically coupled to each other and to the lens fibers by an extensive network of intercellular gap junctions. In the rat, the epithelial-epithelial junctions appear to contain connexin43, a member of the connexin family of gap junction proteins. Limitations on the use of rodent lenses for the study of gap junction formation and regulation led us to examine the expression of connexin43 in embryonic chick lenses. We report here that chick connexin43 is remarkably similar to its rat counterpart in primary amino acid sequence and in several key structural features as deduced by molecular cDNA cloning. The cross-reactivity of an anti-rat connexin43 serum with chick connexin43 permitted definitive immunocytochemical localization of chick connexin43 to lens epithelial gap junctional plaques and examination of the biosynthesis of connexin43 by metabolic radiolabeling and immunoprecipitation. We show that chick lens cells synthesize connexin43 as a single, 42-kD species that is efficiently posttranslationally converted to a 45-kD form. Metabolic labeling of connexin43 with 32P-orthophosphate combined with dephosphorylation experiments reveals that this shift in apparent molecular weight is due solely to phosphorylation. These results indicate that embryonic chick lens is an appropriate system for the study of connexin43 biosynthesis and demonstrate for the first time that connexin43 is a phosphoprotein.

Membrane and junctional properties of dissociated frog lens epithelial cells

Individual cells and cell pairs were isolated from frog lens epithelium. Individual cells were whole cell voltage clamped and the current-voltage relationship was determined. The cells had a mean resting voltage of -54.3 mV and a mean input resistance of 1.4 G omega. The current-voltage relationship was linear near the cell resting voltage, but showed decreased resistance with large depolarization or hyperpolarization. Junctional currents between pairs of cells were recorded using the dual whole cell voltage-clamp technique. The corrected junctional resistance was 15.5 M omega (64.5 nS). The junctional current-voltage relationship was linear. A combination of ATP and cAMP, in the electrodes, stabilized junctional resistance. Currents recorded when uncoupling was nearly complete, showed evidence of single connexion gating events. A single-channel conductance of about 100 pS was prominent. Dye spread between isolated cell pairs was demonstrated using Lucifer Yellow CH in a whole cell configuration. Photodamage to the cells due to the dye was apparent. Dye loaded cells, in the presence of exciting light, showed decreased resting voltages, decreased input resistances and morphological changes. Glutathione (20 mM) delayed this damage.

Voltage-dependent properties of electrical synapses formed between identified leech neurones in vitro

1. The voltage-dependent properties of rectifying and non-rectifying electrical synapses formed between identified leech neurones were quantified during their regeneration in vitro. 2. Junctional conductance increased with time in culture. This was evaluated by making comparisons between cell pairs maintained in vitro for differing amounts of time, as well as by taking repeated measurements from a single cell pair at different time intervals. 3. Non-rectifying electrical synapses were formed between certain identified neurones of the same type. Thus, Leydig cells cultured with Leydig cells established non-rectifying electrical connections, as did Retzius cells, longitudinal motoneurones (L cells) and anterior pagoda (AP) cells, each paired with its own cell type. 4. Rectifying synapses developed when sensory neurones (P cells or N cells) were paired with the other neurones mentioned above that form non-rectifying connections between themselves. The cell combinations examined were L cell-P cell. Leydig cell-N cell, and AP cell-P cell. The direction of current flow across these rectifying synapses was consistently from the sensory neurone to the other cell in the pair. 5. Non-rectifying connections early in the process of synapse regeneration (1-3 days) showed non-linearities greater than those observed in established non-rectifying synapses. There was a subtle, but clear, voltage dependence even at the later stages of synapse formation (4-18 days). 6. In contrast to non-rectifying connections, rectifying synapses formed between cells at early times in culture showed less voltage dependence than those observed at later times. 7. The marked non-linearities of the non-rectifying connections at early stages in synapse formation along with the reduced voltage dependence of the rectifying connections within the same time period revealed unexpected similarities between the two. At the early stages of synapse formation, the two types of electrical synapse were essentially indistinguishable for one direction of junctional current.

NCAM in the differentiation of embryonic lens tissue

The role of the neural cell adhesion molecule (NCAM)2 in ocular lens differentiation was investigated in chicken embryos. Changes in expression of NCAM were documented by immunohistology of frozen sections. This analysis revealed that NCAM diminished during lens fiber differentiation, in contrast to the gap junction-associated protein MP26 which became more abundant. The form of NCAM expressed was determined by Western blot analysis of proteins extracted from the different regions of the Embryonic Day 6 lenses. All regions expressed NCAM with an apparent molecular weight of 140 kDa and relatively low levels of polysialylation. The function of NCAM in lens differentiation was investigated using antibodies that inhibit NCAM-mediated adhesion. Two parameters that change during maturation of the lens epithelial cells were monitored: the thickness of the tissue, indicating the length of lens cells, and the particle arrangement of gap junctions, reflecting the state of junctional differentiation. When epithelial cell explants of Embryonic Day 6 lenses were cultured for 5 days, the cells elongated and displayed an increase in the loose, random intramembranous particle arrangements characteristic of maturing lens fiber gap junctions. When the explants were cultured in the presence of anti-NCAM Fabs, the epithelia were thinner than in matched controls and had particle arrangements characteristic of a less mature state. The expression of NCAM during lens differentiation and the effects of attenuating NCAM function suggest that adhesion mediated by NCAM is an essential event in lens cell differentiation.

The structural organization and protein composition of lens fiber junctions

The structural organization and protein composition of lens fiber junctions isolated from adult bovine and calf lenses were studied using combined electron microscopy, immunolocalization with monoclonal and polyclonal anti-MIP and anti-MP70 (two putative gap junction-forming proteins), and freeze-fracture and label-fracture methods. The major intrinsic protein of lens plasma membranes (MIP) was localized in single membranes and in an extensive network of junctions having flat and undulating surface topologies. In wavy junctions, polyclonal and monoclonal anti-MIPs labeled only the cytoplasmic surface of the convex membrane of the junction. Label-fracture experiments demonstrated that the convex membrane contained MIP arranged in tetragonal arrays 6-7 nm in unit cell dimension. The apposing concave membrane of the junction displayed fracture faces without intramembrane particles or pits. Therefore, wavy junctions are asymmetric structures composed of MIP crystals abutted against particle-free membranes. In thin junctions, anti-MIP labeled the cytoplasmic surfaces of both apposing membranes with varying degrees of asymmetry. In thin junctions, MIP was found organized in both small clusters and single membranes. These small clusters also abut against particle-free apposing membranes, probably in a staggered or checkerboard pattern. Thus, the structure of thin and wavy junctions differed only in the extent of crystallization of MIP, a property that can explain why this protein can produce two different antibody-labeling patterns. A conclusion of this study is that wavy and thin junctions do not contain coaxially aligned channels, and, in these junctions, MIP is unlikely to form gap junction-like channels. We suggest MIP may behave as an intercellular adhesion protein which can also act as a volume-regulating channel to collapse the lens extracellular space. Junctions constructed of MP70 have a wider overall thickness (18-20 nm) and are abundant in the cortical regions of the lens. A monoclonal antibody raised against this protein labeled these thicker junctions on the cytoplasmic surfaces of both apposing membranes. Thick junctions also contained isolated clusters of MIP inside the plaques of MP70. The role of thick junctions in lens physiology remains to be determined.

Inhibition of chicken embryo lens differentiation and lens junction formation in culture by pp60v-src

A culture system was developed which permitted the differentiation of chicken lens epithelial cells to lentoid bodies which contained several cell layers, accumulated high levels of delta-crystallin, and produced extensive gap junctions. This differentiation process was prevented when the cells were infected with a temperature-sensitive src mutant of Rous sarcoma virus and maintained at the permissive temperature. These transformed cells continued to proliferate and also synthesized the major lens gap junction protein, MP28, at near-normal rates. However, this MP28 was not assembled to produce gap junctions. Cultures shifted to the nonpermissive temperature formed lentoid bodies similar to those in uninfected lens cultures, including the establishment of gap junctions containing MP28.

Membrane and junctional properties of the isolated frog lens epithelium

The isolated frog lens epithelium can be maintained intact in both appearance and electrical properties for more than 24 hours. The mean resting membrane potential was -80 mV and the cells were depolarized by both high potassium and low calcium Ringer's solution in a manner very similar to that of the whole lens. The epithelial cells were found to be well coupled using both electrical and dye-injection techniques. Electrical coupling was measured using separate current-injection and voltage-measuring electrodes and the relationship between the induced voltage and distance from the current-passing electrode could be well fitted by a Bessel Function solution to the cable equation. The values obtained from the fit for the membrane and internal resistances were 1.95 omega m2 and 25 omega m, respectively. Exposure to octanol (500 microM) or low external Ca2+ (less than 1 microM) failed to disrupt significantly the intercellular flow of current. There was evidence to suggest that raised intracellular calcium does, however, uncouple the cells. Dye coupling was investigated by microinjecting Lucifer Yellow CH into single epithelial cells. Diffusion into surrounding cells was rapid and, in control medium, occurred in a radially symmetrical manner. In contrast to the electrical coupling data, dye transfer appeared to be blocked by exposure to 500 microM octanol and was severely restricted on perfusing with low external calcium. Differences between the electrical and dye-coupling experiments indicate either that there are two types of junction within the cell and only the larger type, permeable to Lucifer Yellow, is capable of being uncoupled or that there is only one large type of junction which can be partially closed by uncoupling agents.

The influence of pH on membrane conductance and intercellular resistance in the rat lens

1. The conductance of the rat lens was measured using a two-internal-microelectrode technique. The voltage response to a step of current consisted of two components arising from bulk and membrane resistance respectively. 2. The potassium permeability was calculated by applying Goldman theory to 86Rb+ efflux data. 3. The internal pH (pHi) and internal free calcium (pCai) were measured directly using single- and double-barrelled ion-sensitive microelectrodes. 4. Lens pHi was 6.9 in control solution (external pH, pHo = 7.3) and was reduced on lowering pHo. The presence of propionate or 100% CO2 in the external solution accentuated this effect. 5. Internal acidification was accompanied by a depolarization of membrane potential, an increase in membrane and cell-to-cell resistance and a decrease in potassium permeability. The acidification had no effect on pCai. 6. The intracellular pH was increased by perifusing with trimethylamine or NH4Cl. Both treatments induced a membrane depolarization with little change in potassium permeability. Subsequent removal of NH4Cl led to a sustained decrease in pHi. 7. In every case where pHi decreased, the changes in membrane potential and conductance could be explained largely on the basis of a decrease in potassium permeability. The concomitant increase in cell-to-cell resistance was less pronounced and probably insufficient to uncouple the lens system.

Inhibition of chicken embryo lens differentiation and lens junction formation in culture by pp60v-src

A culture system was developed which permitted the differentiation of chicken lens epithelial cells to lentoid bodies which contained several cell layers, accumulated high levels of delta-crystallin, and produced extensive gap junctions. This differentiation process was prevented when the cells were infected with a temperature-sensitive src mutant of Rous sarcoma virus and maintained at the permissive temperature. These transformed cells continued to proliferate and also synthesized the major lens gap junction protein, MP28, at near-normal rates. However, this MP28 was not assembled to produce gap junctions. Cultures shifted to the nonpermissive temperature formed lentoid bodies similar to those in uninfected lens cultures, including the establishment of gap junctions containing MP28.

Expression of the gene for main intrinsic polypeptide (MIP): separate spatial distributions of MIP and beta-crystallin gene transcripts in rat lens development

The main intrinsic polypeptide (MIP) is the major protein present in the lens fiber cell membrane and is the product of a gene which, as far as is known, is expressed only in the lens. We have used in situ hybridization and immunofluorescence microscopy to characterize the expression of this gene during the course of development in the rat. At progressive stages of lens morphogenesis, we find that synthesis of the protein is closely tied to the accumulation of MIP mRNA in cells that are committed to terminal differentiation, first in the elongating presumptive primary lens fibers and later in the secondary fibers as they differentiate from the anterior epithelial cells. The transcripts accumulate in the basal cytoplasm of the primary fibers and in the cytoplasm which surrounds the cell nucleus in the secondary fibers. We have compared this pattern of expression with that of a gene for a cytoplasmic protein, beta-crystallin beta-A1/A3. In sharp contrast to the localized concentrations seen for the MIP mRNA, beta-A1/A3 transcripts are relatively uniformly distributed throughout the cytoplasm. Neither MIP nor crystallin gene appears to be transcriptionally active in the undifferentiated epithelial cell, but transcripts from the beta-A1/A3 gene appear earlier in fiber cell differentiation than do those from the gene for MIP.

Junctions between lens cells in differentiating cultures: structure, formation, intercellular permeability, and junctional protein expression

We previously described cultures of chick embryo lens cells which displayed a marked degree of differentiation. In this report, the junctions found between the lens fiber-like cells in the differentiated "lentoids" are characterized in several ways. Thin-section methods with electron microscopy first demonstrated that numerous, large junctions between lentoid cells accompanied the other differentiated features of these cells. Freeze-fracture techniques, including quantitative analysis, then revealed that (a) junctional particles were loosely arranged as is typical of fiber cells, (b) the population of individual junctional areas in culture was indistinguishable from that found in 10- to 12-day chick embryo lenses, and (c) apparent junction formation occurred during the development of the lens cells, with lacy arrays of particles being associated with fiber-like junctions. In addition, gap junctions with hexagonally packed particles, typical of lens epithelial cells, largely disappeared during the course of differentiation. Injection of tracer dyes into lentoid cells resulted in rapid intercellular movement of dye, consistent with functional cell-to-cell channels connecting lentoid cells. During the development of the lens cells in culture, as junction formation occurred, an increase of approximately eight-fold in MP28 protein was observed within the cells. These combined results indicate that (a) extensive lens fiber junctions and functional cell-to-cell channels are found between differentiated lentoid lentoid cells in vitro, (b) lens fiber junctions appear to form during the course of lens cell differentiation in culture, (c) a significant increase occurs in the putative junctional protein before the cultures are highly developed, (d) the increased levels of MP28 and junction formation may be required for the full expression of the differentiated state in the lens fiber cell, and (e) this culture system should prove to be valuable for additional experiments on lens junctions and for other studies requiring the development of lens fiber cells in vitro.

Electrophysiological properties of gap junctions between dissociated pairs of rat hepatocytes

Physiological properties of isolated pairs of rat hepatocytes were examined within 5 h after dissociation. These cells become round when separated, but cell pairs still display membrane specializations. Most notably, canaliculi are often present at appositional membranes which are flanked by abundant gap and tight junctions. These cell pairs are strongly dye-coupled; Lucifer Yellow CH injected into one cell rapidly diffuses to the other. Pairs of hepatocytes are closely coupled electrically. Conductance of the junctional membrane is not voltage sensitive: voltage clamp studies demonstrate that gj is constant in response to long (5 s) transjunctional voltage steps of either polarity (to greater than +/- 40 mV from rest). Junctional conductance (gj) between hepatocyte pairs is reduced by exposure to octanol (0.1 mM) and by intracellular acidification. Normal intracellular pH (pHi), measured with a liquid ion exchange microelectrode, was generally 7.1-7.4, and superfusion with saline equilibrated with 100% CO2 reduced pHi to 6.0-6.5. In the pHi range 7.5-6.6, gj was constant. Below pH 6.6, gj steeply decreased and at 6.1 coupling was undetectable. pHi recovered when cells were rinsed with normal saline; in most cases gj recovered in parallel so that gj values were similar for pHs obtained during acidification or recovery. The low apparent pK and very steep pHi-gj relation of the liver gap junction contrast with higher pKs and more gradually rising curves in other tissues. If H+ ions act directly on the junctional molecules, the channels that are presumably homologous in different tissues must differ with respect to reactive sites or their environment.

Sensitivity of gap junctional conductance to H ions in amphibian embryonic cells is independent of voltage sensitivity

In vertebrate embryos gap junctional conductance (gj) is reduced by transjunctional voltage (Vj) and by cytoplasmic acidification; in each case sensitivity is comparable to those of other channels gated by voltage and ligand-receptor binding. We show here that the mechanisms by which Vj and intracellular pH (pHi) gate gj are apparently independent. Partial reduction of gj by lowering pHi neither attenuates nor enhances further reduction by Vj. Certain drugs irreversibly (glutaraldehyde, 1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline) or reversibly (retinoic acid) abolish dependence of gj on pHi without appreciably affecting kinetic properties of voltage dependence or the shape of the steady-state Vj-gj relation. These findings suggest that the mechanisms by which pHi and Vj act on the gap junction are at least partially distinct and presumably involve separate regions of the junctional macromolecules.

A lens intercellular junction protein, MP26, is a phosphoprotein

The major protein present in the plasma membrane of the bovine lens fiber cell (MP26), thought to be a component of intercellular junctions, was phosphorylated in an in vivo labeling procedure. After fragments of decapsulated fetal bovine lenses were incubated with [32P]orthophosphate, membranes were isolated and analyzed by SDS PAGE and autoradiography. A number of lens membrane proteins were routinely phosphorylated under these conditions. These proteins included species at Mr 17,000 and 26,000 as well as a series at both 34,000 and 55,000. The label at Mr 26,000 appeared to be associated with MP26, since (a) boiling the membrane sample in SDS led to both an aggregation of MP26 and a loss of label at Mr 26,000, (b) the label at 26,000 was resistant to both urea and nonionic detergents, and (c) two-dimensional gels showed that a phosphorylated Mr 24,000 fragment was derived from MP26 with V8 protease. Studies with proteases also provided for a localization of most label within approximately 20 to 40 residues from the COOH-terminus of MP26. Published work indicates that the phosphorylated portion of MP26 resides on the cytoplasmic side of the membrane, and that this region of MP26 contains a number of serine residues. The same region of MP26 was labeled when isolated lens membranes were reacted with a cAMP-dependent protein kinase prepared from the bovine lens. After the in vivo labeling of lens fragments, phosphoamino acid analysis of MP26 demonstrated primarily labeled serines, with 5-10% threonines and no tyrosines. Treatments that lowered the intracellular calcium levels in the in vivo system led to a selective reduction of MP26 phosphorylation. In addition, forskolin and cAMP stimulated the phosphorylation of MP26 and other proteins in concentrated lens homogenates. These findings are of interest because MP26 appears to serve as a protein of cell-to-cell channels in the lens, perhaps as a lens gap junction protein.

Evidence for two physiologically distinct gap junctions expressed by the chick lens epithelial cell

Lens epithelial cells communicate with two different cell types. They communicate with other epithelial cells via gap junctions on their lateral membranes, and with fiber cells via junctions on their apices. We tested independently these two routes of cell-cell communication to determine if treatment with a 90% CO2-equilibrated medium caused a decrease in junctional permeability; the transfer of fluorescent dye was used as the assay. We found that the high-CO2 treatment blocked intraepithelial dye transfer but not fiber-to-epithelium dye transfer. The lens epithelial cell thus forms at least two physiologically distinct classes of gap junctions.

The distribution of the main intrinsic membrane polypeptide in ocular lens

The Main Intrinsic Polypeptide (MIP) of the ocular lens fiber cell plasma membrane was immunocytochemically localized at the ultrastructural level on ultrathin frozen sections of rat lens, and on extracted, gradient-purified bovine lens membranes. The results indicate that both the junctional and non-junctional membrane domains of the cortical lens fiber cell are MIP immunoreactive. Frozen thin section immunocytochemistry of the lens epithelium and hepatocytes, also using anti-MIP antibodies, revealed that these cells, and their intercellular junctions, are not MIP-immunoreactive. From these findings we conclude that 1) MIP, a putative fiber cell junctional protein, is present throughout the plasma membrane of the lens fiber cell, and is not confined to the fiber cell junctional domain, 2) MIP is not a detectable component of the lens epithelial cell membrane, or its intercellular junctions, 3) MIP is not detectable in gap junctions of hepatocytes.

Gap junction structures after experimental alteration of junctional channel conductance

Gap junctions are known to present a variety of different morphologies in electron micrographs and x-ray diffraction patterns. This variation in structure is not only seen between gap junctions in different tissues and organisms, but also within a given tissue. In an attempt to understand the physiological meaning of some aspects of this variability, gap junction structure was studied following experimental manipulation of junctional channel conductance. Both physiological and morphological experiments were performed on gap junctions joining stage 20-23 chick embryo lens epithelial cells. Channel conductance was experimentally altered by using five different experimental manipulations, and assayed for conductance changes by observing the intercellular diffusion of Lucifer Yellow CH. All structural measurements were made on electron micrographs of freeze-fracture replicas after quick-freezing of specimens from the living state; for comparison, aldehyde-fixed specimens were measured as well. Analysis of the data generated as a result of this study revealed no common statistically significant changes in the intrajunctional packing of connexons in the membrane plane as a result of experimental alteration of junctional channel conductance, although some of the experimental manipulations used to alter junctional conductance did produce significant structural changes. Aldehyde fixation caused a dramatic condensation of connexon packing, a result not observed with any of the five experimental uncoupling conditions over the 40-min time course of the experiments.

Identification of the calmodulin-binding components in bovine lens plasma membranes

Lens membranes, purified from calf lenses, have been labeled by covalent cross-linking to membrane-bound 125I-calmodulin with dithiobis(succinimidyl propionate). Electrophoretic analysis in sodium dodecyl sulfate demonstrated two major 125I-containing products of Mr = 49 000 and 36 000. That the formation of these two components was specifically inhibited by unlabeled calmodulin, or calmodulin antagonists, would indicate that the formation of these components was calmodulin-specific. The size of these two 125I-labeled components was unchanged over a range of 125I-calmodulin or dithiobis(succinimidyl propionate) concentrations indicating that they represent 1:1 complexes between 125I-calmodulin (Mr = 17 000) and Mr-32 000 and Mr-19 000 lens membrane components respectively. Although formation of both cross-linked components exhibited an absolute dependence on Mg2+, the autoradiographic intensity of these components was enhanced when Ca2+ was included with Mg2+ during the cross-linking reaction. Labeling was maximal in 10 mM MgCl2 and approximately 1 microM Ca2+. Treatment of lens membranes with chymotrypsin resulted in the cleavage of MP26 (the major lens membrane protein), with the appearance of a major proteolytic fragment of Mr = 22 000. This proteolysis was not associated with any significant change in either the size or amount of the 125I-calmodulin-labeled membrane components. These results suggest that calmodulin interacts with two membrane proteins, but not significantly with MP26, in the intact lens cell membrane. Our results indicate the need to maintain caution in interpreting direct calcium plus calmodulin effects on MP26 and lens cell junctions.

The effects of cytoplasmic acidification upon electrical coupling in the organ of Corti

The supporting cells of the organ of Corti are joined to one another by gap junctions, and electrical coupling among them is known to be good. It is demonstrated here, using an in vitro preparation, that electrical communication between Hensen's cells can be modified by treatments which are known to cause cytoplasmic acidification. Treatment of the preparation with 100% CO2-saturated medium causes a drop in membrane potential, increase in input resistance, and decrease in steady-state coupling ratio. These measures return to pretreatment levels upon washout of the CO2 medium. Also, direct injection of H+ into a Hensen's cell uncouples that cell from the supporting cell network. An increase in coupling ratio is sometimes observed immediately before and after uncoupling due to CO2 treatment. In fact, in some cases it is possible to solely increase coupling ratios with limited CO2 treatments, although prolonged treatment with CO2 invariably produces uncoupling. This phenomenon may be due to an increase in cell resistance without a change in junctional conductance. A few possible roles for gap junctions in the inner ear are suggested, and the significance of the present results discussed.