Voltage-dependent gap junction channels are formed by connexin32, the major gap junction protein of rat liver

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

The diverse functional roles and regulation of neuronal gap junctions in the retina

Electrical synaptic transmission through gap junctions underlies direct and rapid neuronal communication in the CNS. The diversity of functional roles that electrical synapses have is perhaps best exemplified in the vertebrate retina, in which gap junctions are formed by each of the five major neuron types. These junctions are dynamically regulated by ambient illumination and by circadian rhythms acting through light-activated neuromodulators such as dopamine and nitric oxide, which in turn activate intracellular signalling pathways in the retina.The networks formed by electrically coupled neurons are plastic and reconfigurable, and those in the retina are positioned to play key and diverse parts in the transmission and processing of visual information at every retinal level.

Transfection of mammalian cells with connexins and measurement of voltage sensitivity of their gap junctions

Vertebrate gap junction channels are formed by a family of more than 20 connexin proteins. These gap junction proteins are expressed with overlapping cellular and tissue specificity, and coding region mutations can cause human hereditary diseases. Here we present a summary of what has been learned from voltage clamp studies performed on cell pairs either endogenously expressing gap junctions or in which connexins are exogenously expressed. General protocols presented here are currently used to transfect mammalian cells with connexins and to study the biophysical properties of the heterologously expressed connexin channels. Transient transfection is accomplished overnight with maximal expression occurring at about 36 h; stable transfectants normally can be generated within three or four weeks through colony selection. Electrophysiological protocols are presented for analysis of voltage dependence and single-channel conductance of gap junction channels as well as for studies of chemical gating of these channels.

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.

Cell-specific expression of connexins and evidence of restricted gap junctional coupling between glial cells and between neurons

The transmembrane connexin proteins of gap junctions link extracellularly to form channels for cell-to-cell exchange of ions and small molecules. Two primary hypotheses of gap junction coupling in the CNS are the following: (1) generalized coupling occurs between neurons and glia, with some connexins expressed in both neurons and glia, and (2) intercellular junctional coupling is restricted to specific coupling partners, with different connexins expressed in each cell type. There is consensus that gap junctions link neurons to neurons and astrocytes to oligodendrocytes, ependymocytes, and other astrocytes. However, unresolved are the existence and degree to which gap junctions occur between oligodendrocytes, between oligodendrocytes and neurons, and between astrocytes and neurons. Using light microscopic immunocytochemistry and freeze-fracture replica immunogold labeling of adult rat CNS, we investigated whether four of the best-characterized CNS connexins are each present in one or more cell types, whether oligodendrocytes also share gap junctions with other oligodendrocytes or with neurons, and whether astrocytes share gap junctions with neurons. Connexin32 (Cx32) was found only in gap junctions of oligodendrocyte plasma membranes, Cx30 and Cx43 were found only in astrocyte membranes, and Cx36 was only in neurons. Oligodendrocytes shared intercellular gap junctions only with astrocytes, with each oligodendrocyte isolated from other oligodendrocytes except via astrocyte intermediaries. Finally, neurons shared gap junctions only with other neurons and not with glial cells. Thus, the different cell types of the CNS express different connexins, which define separate pathways for neuronal versus glial gap junctional communication.

Cell-specific expression of connexins and evidence of restricted gap junctional coupling between glial cells and between neurons

The transmembrane connexin proteins of gap junctions link extracellularly to form channels for cell-to-cell exchange of ions and small molecules. Two primary hypotheses of gap junction coupling in the CNS are the following: (1) generalized coupling occurs between neurons and glia, with some connexins expressed in both neurons and glia, and (2) intercellular junctional coupling is restricted to specific coupling partners, with different connexins expressed in each cell type. There is consensus that gap junctions link neurons to neurons and astrocytes to oligodendrocytes, ependymocytes, and other astrocytes. However, unresolved are the existence and degree to which gap junctions occur between oligodendrocytes, between oligodendrocytes and neurons, and between astrocytes and neurons. Using light microscopic immunocytochemistry and freeze-fracture replica immunogold labeling of adult rat CNS, we investigated whether four of the best-characterized CNS connexins are each present in one or more cell types, whether oligodendrocytes also share gap junctions with other oligodendrocytes or with neurons, and whether astrocytes share gap junctions with neurons. Connexin32 (Cx32) was found only in gap junctions of oligodendrocyte plasma membranes, Cx30 and Cx43 were found only in astrocyte membranes, and Cx36 was only in neurons. Oligodendrocytes shared intercellular gap junctions only with astrocytes, with each oligodendrocyte isolated from other oligodendrocytes except via astrocyte intermediaries. Finally, neurons shared gap junctions only with other neurons and not with glial cells. Thus, the different cell types of the CNS express different connexins, which define separate pathways for neuronal versus glial gap junctional communication.

Direct high affinity modulation of connexin channel activity by cyclic nucleotides

Connexin channels mediate molecular communication between cells. However, positive identification of biological ligands that directly and noncovalently modulate their activity has been elusive. This study demonstrates a high affinity inhibition of connexin channels by the purine cyclic monophosphates cAMP and cGMP. Purified homomeric connexin-32 and heteromeric connexin-32/connexin-26 channels were inhibited by exposure to nanomolar levels of the nucleotides prior to incorporation into membranes. Access to the site of action, or affinity for the nucleotides, was greatly reduced following incorporation of the connexin channels into membranes, where inhibition required millimolar concentrations of the nucleotides. The high affinity inhibition did not occur with similar concentrations of AMP, ADP, ATP, cTMP, or cCMP. This is the first report of a direct ligand effect on connexin channel function. The high affinity and specificity of the inhibition suggest a biological role in control of connexin channels and also may lead to the application of affinity reagents to study of connexin channel structure-function.

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.

Properties of gap junction channels formed of connexin 45 endogenously expressed in human hepatoma (SKHep1) cells

We have evaluated the voltage dependence and unitary conductance of gap junctional channels that were recorded in a clone isolated from the hepatoma cell line SKHep1. In this clonal population (designated SKHep1A), Northern blots, immunoprecipitation, and immunohistochemical staining demonstrated the expression of connexin (Cx) 45; no other gap junction protein was identified by these techniques, although weak hybridization with Cx40 was detected. Macroscopic junctional conductance (gj) in these cells was low, averaging 1.3 nS, and was steeply voltage dependent. Parameters of voltage sensitivity were as follows: voltage at which voltage-sensitive conductance is reduced by 50%, 13.4 mV; steepness of relation, 0.115 (corresponding to 2.7 gating charges), and voltage-insensitive fraction of residual to total conductance approximately 0.06. Unitary conductance (gamma j) of these junctional channels averaged 32 +/- 8 pS; although gamma j was independent of transjunctional voltage (Vj), at high Vj values (> 50 mV), smaller conductance values were also detected. Open probabilities of the 30-pS channels at various Vj values closely matched the predicted voltage-dependent component of macroscopic gj, the residual conductance at high Vj might be attributable to the smaller conductance events. The voltage dependence of human Cx45 gap junction channels is as steep as that seen for channels formed by Xenopus Cx38 and is much steeper than that previously reported for channels formed of the highly homologous chick Cx45 and for other mammalian connexins expressed either endogenously or exogenously.

Voltage-clamp analysis of gap junctions between embryonic muscles in Drosophila

1. Intercellular communication between embryonic muscle fibres was examined in Drosophila melanogaster. 2. Injection of fluorescent dye revealed extensive coupling between muscle fibres which form a uniform communicating arrangement of cells without restriction at the segmental borders. 3. Dye transfer was blocked by octanol and membrane depolarization suggesting that it is mediated by gap junctions. 4. Double voltage-clamp experiments from cell pairs in situ showed that the ionic coupling is sensitive to the voltage difference between the cytoplasm and the extracellular space (transmembrane voltage, Vi-o) as well as between the cells (transjunctional voltage, Vj). 5. In steady-state conditions, the gap conductance (gj) was maximal for hyperpolarized Vi-o and decreased progressively to near zero as Vi-o became more positive than -50 mV. 6. Gap conductance decreased from a maximal value as Vj increased either in the positive or negative direction (by depolarizing or hyperpolarizing, respectively, one of the cells from a holding potential of -60 mV). In both cases, gj asymptotically approached a non-zero residual value which was different for negative and positive Vj (about 20% of the maximal conductance for negative transmembrane potentials and 10% for positive values). 7. Application of octanol (1 mM) resulted in an almost complete and reversible block of gj.

A connexin-32 mutation associated with Charcot-Marie-Tooth disease does not affect channel formation in oocytes

Members of the connexin family differ most in their carboxy-termini, both with respect to sequence and length. In order to assess the contribution of this region to channel function, a series of carboxy-terminal deletion mutants were tested in the paired-oocyte expression system. Connexin-32 can be truncated by 64 amino acids without detectable loss of its known channel properties. Removal of additional amino acids results in a progressive loss of function over a stretch of 4 amino acids. In addition to this effect of length the charge of the carboxy-terminus appears to be another determinant of channel function. One of the fully functional deletion mutants, carrying a stop codon after amino acid-219, had been reported to be associated with Charcot-Marie-Tooth disease. The implications of this finding are discussed.

Gap junction channels: distinct voltage-sensitive and -insensitive conductance states

All mammalian gap junction channels are sensitive to the voltage difference imposed across the junctional membrane, and parameters of voltage sensitivity have been shown to vary according to the gap junction protein that is expressed. For connexin43, the major gap junction protein in the cardiovascular system, in the uterus, and between glial cells in brain, voltage clamp studies have shown that transjunctional voltages (Vj) exceeding +/- 50 mV reduce junctional conductance (gj). However, substantial gj remains at even very large Vj values; this residual voltage-insensitive conductance has been termed gmin. We have explored the mechanism underlying gmin using several cell types in which connexin43 is endogenously expressed as well as in communication-deficient hepatoma cells transfected with cDNA encoding human connexin43. For pairs of transfectants exhibiting series resistance-corrected maximal gj (gmax) values ranging from < 2 to > 90 nS, the ratio gmin/gmax was found to be relatively constant (about 0.4-0.5), indicating that the channels responsible for the voltage-sensitive and -insensitive components of gj are not independent. Single channel studies further revealed that different channel sizes comprise the voltage-sensitive and -insensitive components, and that the open times of the larger, more voltage-sensitive conductance events declined to values near zero at large voltages, despite the high gmin. We conclude that the voltage-insensitive component of gj is ascribable to a voltage-insensitive substate of connexin43 channels rather than to the presence of multiple types of channels in the junctional membrane. These studies thus demonstrate that for certain gap junction channels, closure in response to specific stimuli may be graded, rather than all-or-none.

Human connexin43 gap junction channels. Regulation of unitary conductances by phosphorylation

Connexin43 is the major gap protein in the heart and cardiovascular system. Single channel recordings of human connexin43 gap junction channels exogenously expressed in transfected SKHep1 cells demonstrate two discrete classes of channel events, with unitary conductances of predominantly 60 to 70 and 90 to 100 pS when recorded with an internal solution containing CsCl as the major current-carrying ionic species and at moderate transjunctional voltages (< 60 mV). Human connexin43 expressed in SKHep1 cells displays multiple electrophoretic mobilities (apparent M(r), approximately 41 to 45 kD) when resolved in Western blots. Treatment of connexin43 from these cells with alkaline phosphatase collapses the bands into a single 41-kD species; application of alkaline phosphatase to the cell interior through patch pipettes yields channels that are predominantly of the larger unitary conductance. The smaller 60- to 70-pS unitary conductance values correspond to the most common channel size seen in cultured rat cardiac myocytes; these channels were more frequently observed after treatment with the phosphatase inhibitor okadaic acid, which was shown to increase phosphorylation of human connexin43 in these cells under similar conditions. Exposure to the protein kinase inhibitor staurosporine shifted the proportion of events toward the largest unitary conductance and resulted in decreased phosphorylation of human connexin43 in seryl residues in these cells. Thus, the unitary conductance of human connexin43 gap junction channels covaries with the phosphorylation state of the protein. This change in unitary conductance appears to be a unique effect of phosphorylation on gap junction channels, since it has not been observed for other ion channels that have thus far been evaluated.

Gap junction protein connexin40 is preferentially expressed in vascular endothelium and conductive bundles of rat myocardium and is increased under hypertensive conditions

Gap junction channels consisting of connexin protein mediate electrical coupling between cardiac cells. Expression of two connexins, connexin40 (Cx40) and connexin43 (Cx43), has been studied in ventricular myocytes from normal and hypertensive rats. Polyclonal affinity-purified rabbit antibodies to Cx43 and Cx40 have been used for immunohistochemical analysis on frozen sections from rat heart. These studies revealed coexpression of Cx43 and Cx40 in ventricular myocytes. In addition, Cx40 is preferentially expressed in three distinct regions: first, in the endothelial layer of the heart blood vessels but not in the smooth muscle layer of the arteries; second, in the ventricular conductive myocardium, particularly in the atrioventricular bundle and bundle branches, where Cx43 is not observed; and third, in the myocyte layers close to the ventricular cavities. These results suggest that Cx40 is preferentially expressed in the fast conducting areas of myocardial tissue. Expression of both Cx40 and Cx43 was also found in immunoblots from normal and hypertensive rat myocardiocytes. Under hypertensive conditions (ie, in spontaneous hypertensive rats and in transgenic rats that exhibit hypertension due to expression of an exogenous renin gene), we found a 3.1-fold increase in Cx40 expression, compared with normal myocardium. Furthermore, we detected a 3.3-fold decrease in Cx43 protein level in transgenic hypertensive rats. The coexpression of Cx40 and Cx43 proteins in rat myocytes, their spatial distribution, and the increased amount of Cx40 protein during cardiac hypertrophy suggest that Cx40 may be involved in mediating fast conduction under normal and pathological conditions. The increased expression of Cx40 in hypertrophic heart may be a compensatory mechanism to increase conduction velocity.

Voltage dependence of liver gap-junction channels reconstituted into liposomes and incorporated into planar bilayers

The voltage dependence of rat liver gap junctions was investigated using non-denaturing solubilization and reconstitution of gap-junction protein into proteoliposomes in controlled conditions of connexon aggregation. The presence of liver connexin 32 in reconstituted proteoliposomes was checked with specific antibodies. The proteoliposomes were inserted into planar lipid bilayers by fusion. The single-channel conductance was voltage independent, and its magnitude was 700-1900 pS in 1 M NaCl, as expected from other reports, assuming that conductance is linear with ion activity. The channels were open at zero voltage and completely closed above 40 mV in either direction. This steep voltage dependence corresponded to an open/closed-state voltage difference of 19 mV and to 3.5 gating charges moving through the field. When several channels were inserted into the bilayer, a large fraction of the membrane conductance became voltage insensitive. These results show that the isolated channel units are highly voltage dependent and are consistent with the assumption that aggregated connexons interact through links which prevent voltage-sensitive conformational changes.

Ion channels in single bilayers induced by rat connexin32

The gap junction channel mediates an important form of intercellular communication, but its detailed study is hindered by inaccessibility in situ. We show here that connexin32, the major protein composing junctional channels in rat liver, forms ion channels in single bilayer membranes. The properties of these reconstituted connexin32 channels are characterized and compared with those of gap junction channels. The demonstration that connexin32 forms channels in single membranes has implications for assembly and regulation of junctional channels, and permits detailed study of the gating, permeability and modulation of this channel-forming protein.

Reconstitution of channels from preparations enriched in lens gap junction protein MP70

Detergent-solubilized ovine lens membrane proteins, enriched in the 70-kDa gap junction component (MP70), were reconstituted into planar lipid bilayers and analyzed for channel activities. Three distinct activities were found. Those showing conductance steps of 290 pS (symmetrical 150-mM KCl solutions) had properties similar to those reported earlier for MIP26 (Ehring, G.R., Zampighi, G., Horwitz, J., Bok, D., Hall, J.E. 1990. J. Gen. Physiol. 96:631-664.) of which minor amounts were normally present in the detergent-solubilized preparations. Two novel channel activities had unitary conductances of 90 and 45 pS, were halothane sensitive and did not discriminate between sodium and potassium ions. The 90-pS channel was asymmetrically voltage dependent, and its properties would be consistent with the expected properties of junctional hemichannels.

Nuclear ion channels in cardiac myocytes

The paradigm that nucleocytoplasmic transport of ions occurs without a diffusional barrier has been challenged by the recent demonstration with patch-clamp techniques of the existence of ion channels in the nuclear envelope of murine zygotes and hepatocytes. This report demonstrates the existence of nuclear ion channels (NIC) in murine ventricular cardiac myocytes. NIC conductance (gamma), calculated from current histogram peaks, was 106-532 pS at 22-36 degrees C. In nucleus-attached patches, replacement of cytoplasmic K+ with Na+ reduced NIC activity within 30 s, suggesting that intranuclear-delimited mechanisms mediate this phenomenon. In excised, inside-out patches K+ was as permeable as Na+ through NIC. NIC activity was observed in 0-4 mM Mg2+ and/or ATP2-, with or without 0-1 mM Ca2+, indicating a minor direct role of these ions. However, in non-responsive excised inside-out patches, NIC activity appeared when the catalytic subunit of the cAMP-dependent protein kinase was applied to the nucleoplasmic side of the patch, in the presence of Mg2+ and ATP2-, indicating an important role for phosphorylation-dependent process(es) in NIC function--an observation supported by the depressing effects of protein kinase inhibitor on responsive NIC. The concept that nucleopore complexes are solely responsible for nucleocytoplamic transport leads to the speculation that these structures are the physical substrate for NIC.

Molecular cloning and functional expression of mouse connexin40, a second gap junction gene preferentially expressed in lung

From a mouse genomic library, a clone has been isolated that codes for a connexin-homologous sequence of 358 amino acids. Because of its theoretical molecular mass of 40.418 kD it is named connexin40 (Cx40). Based on both protein and nucleotide sequence, mouse Cx40 is more closely related to mouse Cx43 (alpha subgroup of connexins) than to mouse Cx32 (beta subgroup). The highest overall homology detected, however, was to chick Cx42 (67% amino acid and 86% nucleotide identity), raising the possibility that Cx40 may be the mouse analogue. The coding region of Cx40 is uninterrupted by introns and is detected as a single copy gene in the mouse genome. High stringency hybridization of Northern blots with the coding sequence of Cx40 identified a single transcript of 3.5 kb that is at least 16-fold more abundant in lung-similar to mouse Cx37-than in other adult tissues (kidney, heart, and skin). In embryonic kidney, skin, and liver the level of the Cx40 transcript is two- to fourfold higher than in the corresponding adult tissues. Microinjection of Cx40 cRNA into Xenopus oocytes induced functional cell-to-cell channels between pairs. These channels show a symmetrical and markedly cooperative closure in response to transjunctional voltage (Boltzmann parameters of Vo = +/- 35 mV; A = 0.32) which is also fast relative to other connexin channels recorded similarly (tau = 580 ms at Vj of +/- 50 mV). Although Cx40-expressing oocytes did not couple efficiently with oocytes expressing endogenous connexins, they did couple well to Cx37-expressing oocytes. The heterotypic channels which formed had voltage-gating properties modified from those of the original homotypic forms. Transfection of mouse Cx40 DNA, under control of the SV-40 early promoter, into coupling-deficient human HeLa or SK-Hep-1 cells resulted in expression of the expected transcript and restoration of fluorescent dye transfer in transfected clones.

Multiple-channel conductance states and voltage regulation of embryonic chick cardiac gap junctions

We used the double whole-cell voltage-clamp technique on ventricle cell pairs isolated from 7-day chick heart to measure the conductance of their gap junctions (Gj) and junctional channels (gamma j) with a steady-state voltage difference (Vj) applied across the junction. Currents were recorded from single gap junction channels (ij) as symmetrical rectangular signals of equal size and opposite sign in the two cells, and gamma j was measured from ij/Vj. We observed channel openings at six reproducible conductance levels with means of 42.6, 80.7, 119.6, 157.7, 200.4 and 240.3 pS. More than half of all openings were to the 80- and 160-pS conductance levels. The probability that a high conductance event (e.g., 160 or 240 pS) results from the random simultaneous opening of several 40-pS channels is small, based on their frequency of occurrence and on the prevalence of shifts between small and large conductance states with no intervening 40-pS steps. Our results are consistent with three models of embryonic cardiac gap junction channel configuration: a homogeneous population of 40-pS channels that can open cooperatively in groups of up to six; a single population of large channels with a maximal conductance near 240 pS and five smaller substates; or several different channel types, each with its own conductance. Gj was determined from the junctional current (Ij) elicited by rectangular pulses of applied transjunctional voltage as Ij/Vj. It was highest near 0 Vj and was progressively reduced by application of Vj between 20 and 80 mV or -20 and -80 mV. In response to increases in Vj, Gj decayed in a voltage- and time-dependent fashion. After a 6-sec holding period at 0 Vj, the initial conductance (G(init) measured immediately after the onset of an 80-mV step in Vj was nearly the same as that measured by a 10-mV prepulse. However, during 6-sec pulses of Vj greater than +/- 20 mV, Gj declined over several seconds from G(init)to a steady-state value (Gss). At potentials greater than +/- 20 mV the current decay could be fit with biexponential curves with the slow decay time constant (tau 2) 5-20 times longer than tau 1. For the response to a step to 80 mV Vj, for example, tau 1 = 127 msec and tau 2 = 2.6 sec. The rate of current decay in response to smaller positive or negative steps in Vj was slower, the magnitude of the decline was smaller, and the ratio tau 2/tau 1 decreased.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical coupling between cells of the insect Aedes albopictus

1. Cell pairs of an insect cell line (Aedes albopictus, clone C6/36) were used to study the electrical properties of intercellular junctions. A double voltage-clamp approach was adopted to control the voltage gradient between the cells and measure the intracellular current flow. 2. Determinations of junctional conductance (gj) revealed two types of intercellular contacts, gap junctions and cytoplasmic bridges. Identification occurred by means of functional criteria, i.e. the dependency of gj on (i) junctional membrane potential, (ii) non-junctional membrane potential, and (iii) heptanol. 3. In cell pairs with putative gap junctions, gj was dependent on the junctional membrane potential (Vj). When determined at the beginning of voltage pulses, gj was insensitive to Vj; when determined at the end of 15 s pulses, it depended on Vj in a bell-shaped manner (70% decrease for a change in Vj of +/- 75 mV). 4. These cell pairs also showed a dependency of gj on the non-junctional membrane potential (Vm). When determined immediately after changing the non-junctional membrane potential in both cells, gj was not affected by Vm; when determined 30 s later, gj was modified by Vm in a S-shaped fashion (100% decrease when Vm was depolarized to +50 mV). 5. Exposure to 3 mM-heptanol gave rise to complete and reversible block of gj in cell pairs with putative gap junctions. 6. Cell pairs susceptible to uncoupling by heptanol revealed junctional currents indicative of the operation of gap junction channels. The single-channel conductance, determined at a Vm of -50 to -70 mV, was 133 pS. 7. In the case of putative cytoplasmic bridges, gj was insensitive to the junctional and non-junctional membrane potential. In addition, it was not affected by 3 mM-heptanol. 8. While most cell pairs showed functional properties characteristic of gap junctions or cytoplasmic bridges, few cell pairs exhibited junctional currents compatible with the co-existence of both junctional structures.

Biophysics of gap junctions

Gap junction channels, now known to be formed of connexins, connect the interiors of apposed cells. These channels can be opened and closed by various physiological stimuli and experimental treatments. They are permeable to ions and neutral molecules up to a size of about 1 kDa or 1.5 nm diameter, including second messengers and metabolites. The processes of gating and of permeation are the subject of this review. Voltage is a readily applied stimulus, and transjunctional voltages, or those between cytoplasm and exterior, affect most junctions. Single channel transitions between open and closed states are rapid and presumably involve a charge movement as occurs with channels of electrically excitable channels of nerve and muscle. Identification of gating domains and charges by domain replacement and site-directed mutagenesis is being pursued. Raising cytoplasmic H+ or Ca2+ concentrations rapidly reduces junctional conductance, and this action is generally reversible, at least in part. A number of lipophilic alcohols, fatty acids and volatile anesthetics have similar actions. Phosphorylation also modulates junctional conductance, and in several cases, sites of phosphorylation are known. These gating processes appear similar to those induced by voltage. Permeability measurement indicates that the channel is aqueous and that permeation is by diffusion with only minor interactions with the channel wall. Differences among junctions are known, but further characterization of connexin and cell specificity is required.

Characterization of gap junctions between cultured leptomeningeal cells

Leptomeningeal cells in intact meninges or dissociated and cultured for 2 h to several weeks were dye-coupled (Lucifer yellow), and voltage-clamped pairs of freshly dissociated leptomeningeal cells were well coupled electrically. Unitary conductances of junctional channels were predominantly 40-90 pS. Junctional conductance was reversibly reduced by 2 mM halothane, 1 mM heptanol and 100% CO2 and was increased by 1 mM 8 Br-cAMP. Two gap junction proteins, connexin 26 and connexin 43, were identified between leptomeningeal cells using immunocytochemical methods; Northern blot analyses of RNA isolated from cultured leptomeningeal cells showed specific hybridization to cDNAs encoding connexins 26 and 43, but not to a cDNA encoding connexin 32. These studies demonstrate co-expression of two connexins in a single cell type in the nervous system; biophysical properties do not differ significantly from those of astrocytes and cardiac myocytes, which express only connexin 43.

Connexin32 gap junction channels in stably transfected cells. Equilibrium and kinetic properties

Communication-deficient cells (the SKHep1 cell line) were stably transfected with a plasmid containing cDNA which encodes the major gap junction protein of rat liver, connexin32. Application of the dual whole-cell voltage clamp technique with patch electrodes to pairs of transfected SKHep1 cells revealed strong sensitivity of junctional conductance (gj) to transjunctional voltages (Vjs) of either polarity, with the ratio of minimal to maximal gj (gmin/gmax) being approximately 0.1 at the highest Vjs. Steady-state gj values as a function of voltages of either polarity were well fit by the Boltzmann equation. V0, the voltage at which gj was reduced by 50%, was approximately 25-30 mV; A, the Boltzmann parameter describing voltage dependence, was approximately 0.06 (corresponding to an energy difference between states of approximately 1 kCal/mol and to approximately 2 gating charges moving through the field). The kinetics of the transjunctional voltage dependence were slow (tau greater than 5 s at 20-40 mV, tau = 2 s at and beyond 70 mV). Voltage sensitivity of the opening rate constant (alpha) was approximately 30% lower than that of the closing rate constant (beta) over the Vj range 0-70 mV; at higher voltages, voltage sensitivity of alpha and beta saturated. The kinetic response of gj to a paradigm in which gj was first rendered low by a prepulse of opposite polarity indicated that the voltage sensors are likely to be arranged in series. Transitions between open and closed states in response to transjunctional voltages of either polarity are single order processes; transitions from one closed state to the other involve passage through the open state.

Gap junctions formed by connexins 26 and 32 alone and in combination are differently affected by applied voltage

Gap junctions are formed by a family of homologous proteins termed connexins. Their channels are dodecamers, and homomeric forms differ in their properties with respect to control by voltage and other gating stimuli. We report here the properties of coupling from expression of connexin complementary RNAs (cRNAs; sense to mRNA, antisense to cDNA) in Xenopus oocyte pairs in which endogenous coupling was blocked by injection of DNA oligonucleotides antisense to the mRNA of Cx38, the principal endogenous connexin. We found that a connexin recently sequenced from rat liver, Cx26, formed functional gap junctions whose conductance exhibited voltage dependence with unusual characteristics suggestive of two gating mechanisms. Junctional conductance (gj) was increased to a small degree by depolarization and decreased by hyperpolarization of either cell in a coupled pair, indicating dependence on the potential between the inside and outside of the cells (Vi-o). These changes were fast compared with the resolution of their measurement (ca. 10 ms). On a slower timescale, large transjunctional potentials (Vj) of either sign caused a more substantial decrease in conductance similar to that previously reported for several other gap junctions. Homotypic junctions formed of another connexin, Cx32, exhibited a similar slow dependence on Vj but no dependence on Vi-o. In contrast, heterotypic junctions between an oocyte expressing Cx26 and one expressing Cx32 were electrically asymmetric; they exhibited a greater fast change in gj, which depended, however, on Vj, such that gj increased with relative positivity on the Cx26 side and decreased with relative negativity on the Cx26 side. There was also a large slow decrease in gj in response to Vj for relative positivity on the Cx26 side but not for Vj of the opposite sign. These data indicate that properties of the hemichannels contributed by the two connexins in the heterotypic case were changed from their properties in homotypic junctions. The fast change in gj may involve a mechanism analogous to that at fast rectifying electrical synapses. Experiments in which oocytes expressing Cx32 were paired with oocytes expressing both Cx26 and Cx32 demonstrated that asymmetric junctions would form between oocytes expressing both connexins, thereby confirming their potential relevance in vivo, where the same coupled cells are known to express both proteins.