Gap-junction disassembly and connexin 43 dephosphorylation induced by 18 beta-glycyrrhetinic acid

Regulation of gap junctions by phosphorylation of connexins

Gap junctions are a unique type of intercellular junction found in most animal cell types. Gap junctions permit the intercellular passage of small molecules and have been implicated in diverse biological processes, such as development, cellular metabolism, and cellular growth control. In vertebrates, gap junctions are composed of proteins from the "connexin" gene family. The majority of connexins are modified posttranslationally by phosphorylation, primarily on serine amino acids; however, phosphotyrosine has also been detected in connexin from cells coexpressing nonreceptor tyrosine protein kinases. Connexins are targeted by numerous protein kinases, of which some have been identified: protein kinase C, mitogen-activated protein kinase, and the v-Src tyrosine protein kinase. Phosphorylation has been implicated in the regulation of a broad variety of connexin processes, such as the trafficking, assembly/disassembly, degradation, as well as the gating of gap junction channels. This review examines the consequences of connexin phosphorylation for the regulation of gap junctional communication.

Dephosphorylation and intracellular redistribution of ventricular connexin43 during electrical uncoupling induced by ischemia

Electrical uncoupling at gap junctions during acute myocardial ischemia contributes to conduction abnormalities and reentrant arrhythmias. Increased levels of intracellular Ca(2+) and H(+) and accumulation of amphipathic lipid metabolites during ischemia promote uncoupling, but other mechanisms may play a role. We tested the hypothesis that uncoupling induced by acute ischemia is associated with changes in phosphorylation of the major cardiac gap junction protein, connexin43 (Cx43). Adult rat hearts perfused on a Langendorff apparatus were subjected to ischemia or ischemia/reperfusion. Changes in coupling were monitored by measuring whole-tissue resistance. Changes in the amount and distribution of phosphorylated and nonphosphorylated isoforms of Cx43 were measured by immunoblotting and confocal immunofluorescence microscopy using isoform-specific antibodies. In control hearts, virtually all Cx43 identified immunohistochemically at apparent intercellular junctions was phosphorylated. During ischemia, however, Cx43 underwent progressive dephosphorylation with a time course similar to that of electrical uncoupling. The total amount of Cx43 did not change, but progressive reduction in total Cx43 immunofluorescent signal and concomitant accumulation of nonphosphorylated Cx43 signal occurred at sites of intercellular junctions. Functional recovery during reperfusion was associated with increased levels of phosphorylated Cx43. These observations suggest that uncoupling induced by ischemia is associated with dephosphorylation of Cx43, accumulation of nonphosphorylated Cx43 within gap junctions, and translocation of Cx43 from gap junctions into intracellular pools.

Connexin mimetic peptides reversibly inhibit Ca(2+) signaling through gap junctions in airway cells

The effect of peptides with sequences derived from connexins, the constituent proteins of gap junctions, on mechanically stimulated intercellular Ca(2+) signaling in tracheal airway epithelial cells was studied. Three peptides with sequences corresponding to connexin extracellular loop regions reversibly restricted propagation of Ca(2+) waves to neighboring cells. Recovery of communication began within 10 min of removal of the peptides, with inhibition totally reversed by 20-40 min. The peptides were shown to be more effective in inhibiting Ca(2+) waves than glycyrrhetinic acid or oleamide. Inhibition of intercellular Ca(2+) waves by connexin mimetic peptides did not affect the Ca(2+) response to extracellular ATP. Although the intracellular Ca(2+) response of tracheal epithelial cells to ATP was greatly reduced by either pretreatment with high doses of ATP or application of apyrase, mechanically stimulated intercellular Ca(2+) signaling was not affected by these agents. We conclude that connexin mimetic peptides are effective and reversible inhibitors of gap junctional communication of physiologically significant molecules that underlie Ca(2+) wave propagation in tracheal epithelial cells and propose a potential mechanism for the mode of action of mimetic peptides.

Role of gap junctions in acetylcholine-induced vasodilation of proximal and distal arteries of the rat mesentery

We have previously shown that myoendothelial gap junctions are more prevalent in distal than in proximal arteries of the rat mesentery. In the present study we have investigated the role of gap junctions in the mechanism of action of endothelium-derived hyperpolarizing factor (EDHF) in these same vessels following relaxation with acetylcholine. Arteries were pre-constricted with phenylephrine and concentration response curves to acetylcholine were constructed in the presence of N(G)-nitro-L-arginine methyl ester (L-NAME; 10(-5) M) and indomethacin (10(-5) M) to prevent effects due to the release of nitric oxide and prostacyclins. Nitric oxide was found to have only a small role in the relaxation of the proximal vessels and was not involved in the relaxations of the distal vessels. 18 alpha-Glycyrrhetinic acid (10(-5) M), a putative gap junction uncoupler, significantly reduced acetylcholine-induced relaxations by 50% in both proximal and distal vessels. Potassium channel antagonists, tetraethylammonium chloride (TEA; 10(-3) M) and barium chloride (10(-4) M), together abolished the dilatory response in the proximal mesenteric arteries, but did not completely block responses in the distal arteries. The data suggest that gap junctions contribute significantly to the acetylcholine-induced relaxation in both proximal and distal arteries of the rat mesentery. We hypothesize that the absence of a correlation between the role of gap junctions and the incidence of myoendothelial gap junctions in these same vessels is due to significant effects of the inhibitors on gap junctions located in the smooth muscle layers of the larger vessels.

Biochemical and functional characterization of intercellular adhesion and gap junctions in fibroblasts

Despite their significance in wound healing, little is known about the molecular determinants of cell-to-cell adhesion and gap junctional communication in fibroblasts. We characterized intercellular adherens junctions and gap junctions in human gingival fibroblasts (HGFs) using a novel model. Calcein-labeled donor cells in suspension were added onto an established, Texas red dextran (10 kDa)-labeled acceptor cell monolayer. Cell-to-cell adhesion required Ca(2+) and was >30-fold stronger than cell-to-fibronectin adhesion at 15 min. Electron micrographs showed rapid formation of adherens junction-like structures at approximately 15 min that matured by approximately 2-3 h; distinct gap junctional complexes were evident by approximately 3 h. Immunoblotting showed that HGF expressed beta-catenin and that cadherins and connexin43 were recruited to the Triton-insoluble cytoskeletal fraction in confluent cultures. Confocal microscopy localized the same molecules to intercellular contacts of acceptor and donor cells. There was extensive calcein dye transfer in a cohort of Texas red dextran-labeled cells, but this was almost completely abolished by the gap junction inhibitor beta-glycyrrhetinic acid and the connexin43 mimetic peptide GAP 27. This donor-acceptor cell model allows large numbers (>10(5)) of cells to form synchronous cell-to-cell contacts, thereby enabling the simultaneous functional and molecular studies of adherens junctions and gap junctions.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

The high production volume chemical challenge program: the relevance of the in vivo micronucleus assay

The in vivo rodent bone marrow micronucleus assay (Mnt) has assumed a pivotal role in screening strategies for the identification of substances potentially carcinogenic to humans. The analysis of the results of the current international 5-year effort to provide toxicological data for high production volume chemicals will play a crucial role in developing future strategies for identifying health hazards. As part of that program, consideration is being given to accepting either in vitro genotoxicity data or results of the Mnt. The present analyses indicate that for hazard identification purposes that, in fact, in vitro genotoxicity test results, such as those derived from the Salmonella mutagenicity assay, may be an acceptable alternative.

Physiological role of gap-junctional hemichannels. Extracellular calcium-dependent isosmotic volume regulation

Hemichannels in the overlapping regions of apposing cells plasma membranes join to form gap junctions and provide an intercellular communication pathway. Hemichannels are also present in the nonjunctional regions of individual cells and their activity is gated by several agents, including calcium. However, their physiological roles are unknown. Using techniques of atomic force microscopy (AFM), fluorescent dye uptake assay, and laser confocal immunofluorescence imaging, we have examined the extracellular calcium-dependent modulation of cell volume. In response to a change in the extracellular physiological calcium concentration (1.8 to Collapse

Key Words

• cell–cell communication
• atomic force microscopy
• scanning probe microscopy
• connexin43
• connexons

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MESH Headings

• Animals
• Calcium/metabolism
• Calcium/pharmacology
• Cattle
• Cell Size/drug effects
• Connexin 43/biosynthesis
• Connexin 43/genetics
• Connexins/biosynthesis
• Cytochalasin D/pharmacology
• Dose-Response Relationship, Drug
• Elasticity
• Endothelium/cytology
• Endothelium/drug effects
• Endothelium/metabolism
• Epithelial Cells/cytology
• Epithelial Cells/drug effects
• Epithelial Cells/metabolism
• Extracellular Space/metabolism
• Fibroblasts/cytology
• Fibroblasts/drug effects
• Fibroblasts/metabolism
• Gap Junctions/drug effects
• Gap Junctions/metabolism
• Humans
• Hypnotics and Sedatives/pharmacology
• Ion Channels/drug effects
• Ion Channels/metabolism
• Mice
• Nucleic Acid Synthesis Inhibitors/pharmacology
• Oleic Acids/pharmacology
• Osmolar Concentration
• Rats
• Water/metabolism

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Grants

• R01 GM056290 NIGMS NIH HHS
• GM056290 NIGMS NIH HHS

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Affiliation(s)

Arjan Pieter Quist Neuroscience Research Institute, University of California, Santa Barbara, California 93106
Seung Keun Rhee Neuroscience Research Institute, University of California, Santa Barbara, California 93106 Department of Biochemistry, Yeungnam University, Kyongsan, 712-749, Korea
Hai Lin Neuroscience Research Institute, University of California, Santa Barbara, California 93106
Ratneshwar Lal Neuroscience Research Institute, University of California, Santa Barbara, California 93106

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Effect of 18beta-glycyrrhetinic acid on electromechanical coupling in the guinea-pig renal pelvis and ureter

We have tested the effect of the gap junction inhibitor, 18beta-glycyrrhetinic acid (18betaGA) on electromechanical coupling in the guinea-pig renal pelvis and ureter by the sucrose gap technique. In the ureter 18betaGA (3 - 30 microM) produced a concentration-dependent inhibition of the spike component of the action potential (AP) and reduced contraction evoked by electrical stimulation. Neurokinin A (NKA) produced a slow depolarization with superimposed APs and phasic contractions of the ureter. 18betaGA (30 microM) markedly inhibited the depolarization and APs evoked by NKA. However the contractile response was more sustained in the presence than in the absence of 18betaGA. At 100 microM, 18betaGA inhibited the mechanical responses to NKA. KCl (80 mM) produced APs and phasic contractions followed by sustained depolarization and tonic contraction. At 30 microM 18betaGA markedly inhibited the KCl-evoked APs and phasic contractions without affecting the sustained responses. At 100 microM 18betaGA inhibited the tonic contraction to KCl. In the renal pelvis 18betaGA (30 microM) inhibited the amplitude of pacemaker potentials and accompanying contractions and induced the appearance of low-amplitude APs not associated with contraction. We conclude that, up to 30 microM, the action of 18betaGA is consistent with an inhibition of cell-to-cell electrical coupling via gap junctions. The single-unit character of smooth muscles in the guinea-pig upper urinary tract is partly converted to a multi-unit pattern. At high concentrations 18betaGA possesses non specific effects which limit its usefulness as a tool for studying the role of gap junctions in smooth muscles. British Journal of Pharmacology (2000) 129, 163 - 169

Levcromakalim causes indirect endothelial hyperpolarization via a myo-endothelial pathway

1. Effects of K+ channel opener, levcromakalim, on vascular endothelial cells were examined. Under voltage- and current-clamp conditions, application of acetylcholine to dispersed endothelial cells isolated from rabbit superior mesenteric artery (dispersed RMAECs) produced hyperpolarization and outward currents. On the other hand, dispersed RMAECs did not respond to levcromakalim. 2. When membrane potential was recorded from endothelium in a mesenteric arterial segment, exposure to levcromakalim in a concentration range of 0.1 to 3 microM caused concentration-dependent hyperpolarization. The hyperpolarization was observed in the absence of external Ca2+ and was inhibited by 10 microM glibenclamide. 3. The presence of 1 mM heptanol did not affect the levcromakalin-induced hyperpolarization, whereas treatment of the mesenteric arterial segment with 20 microM 18 beta-glycyrrhetinic acid significantly reduced the hyperpolarization. The response to acetylcholine of RMAECs in an arterial segment with 18 beta-glycyrrhetinic acid was, however, similar to that without 18 beta-glycyrrhetinic acid. 4. These suggest that although RMAECs themselves are functionally insensitive to levcromakalim, those in an arterial segment are hyperpolarized by levcromakalim via myo-endothelial electrical communication.

Role of gap junctions in the responses to EDHF in rat and guinea-pig small arteries

1. In guinea-pig internal carotid arteries with an intact endothelium, acetylcholine (10 microM) and levcromakalim (10 microM) each hyperpolarized the smooth muscle whereas a 5 mM elevation of extracellular K(+) was without effect. 2. Incubation of the carotid artery with the gap junction inhibitors carbenoxolone (100 microM) or gap 27 (500 microM) essentially abolished the hyperpolarization to acetylcholine but it was without effect on that to levcromakalim. Carbenoxolone had no effect on the acetylcholine-induced endothelial cell hyperpolarization but inhibited the smooth muscle hyperpolarization induced by the endothelial cell K(+) channel opener, 1-ethyl-2-benzimidazolinone (600 microM). 3. In rat hepatic and mesenteric arteries with endothelium, carbenoxolone (100 or 500 microM) depolarized the smooth muscle but did not modify hyperpolarizations induced by KCl or levcromakalim. In the mesenteric (but not the hepatic) artery, the acetylcholine-induced hyperpolarization was inhibited by carbenoxolone. 4. Phenylephrine (1 microM) depolarized the smooth muscle cells of intact hepatic and mesenteric arteries, an effect enhanced by carbenoxolone. Gap 27 did not have a depolarizing action. In the presence of phenylephrine, acetylcholine-induced hyperpolarization of both hepatic and mesenteric artery myocytes was partially inhibited by each of the gap junction inhibitors. 5. Collectively, the data suggest that gap junctions play some role in the EDHF (endothelium-derived hyperpolarizing factor) response in rat hepatic and mesenteric arteries. However, in the guinea-pig internal carotid artery, electrotonic propagation of endothelial cell hyperpolarizations via gap junctions may be the sole mechanism underlying the response previously attributed to EDHF.

Structural determinants of developmental toxicity in hamsters

A CASE/MULTICASE structure activity relationship (SAR) model of developmental toxicity of chemicals in hamsters (HaDT) was developed. The model exhibited a predictive performance of 74%. The model's overall predictivity and informational content were similar to those of an SAR model of mutagenicity in Salmonella. However, unlike the Salmonella mutagenicity model, the HaDT model did not identify overtly chemically reactive moieties as associated with activity. Moreover, examination of the number and nature of significant structural determinants suggested that developmental toxicity in hamsters was not the result of a unique mechanism or attack on a specific molecular target. The analysis also indicated that the availability of experimental data on additional chemicals would improve the performance of the SAR model.

Inhibition of gap junction communication in alveolar epithelial cells by 18alpha-glycyrrhetinic acid

Cultured alveolar epithelial cells exhibit gap junction intercellular communication (GJIC) and express regulated levels of connexin (Cx) 43 mRNA and protein. Newly synthesized radiolabeled Cx43 protein equilibrates with phosphorylated Cx43 isoforms; these species assemble to form both connexons and functional gap junction plaques. The saponin 18alpha-glycyrrhetinic acid (GA) rapidly and reversibly blocks GJIC at low concentrations (5 microM). Extended exposure to 18alpha-GA at higher concentrations causes inhibition of GJIC and time- and dose-dependent reductions in both Cx43 protein and mRNA expression. The latter toxic effects are paralleled by disassembly of gap junction plaques and are reversed less readily than acute effects on GJIC. These observations demonstrate 18alpha-GA-sensitive regulation of intercellular communication in epithelial cells from the mammalian lung and suggest a role for Cx43 expression and phosphorylation in acute and chronic regulation of GJIC between alveolar epithelial cells.

Neurotransmitter coupling through gap junctions in the retina

Although all bipolar cells in the retina probably use the excitatory transmitter glutamate, approximately half of the cone bipolar cells also contain elevated levels of the inhibitory transmitter glycine. Some types of cone bipolar cells make heterologous gap junctions with rod amacrine cells, which contain elevated levels of glycine, leading to the hypothesis that the bipolar cells obtain their glycine from amacrine cells. Experimental support for this hypothesis is now provided by three independent lines of evidence. First, the glycine transporter GLYT1 is expressed by the glycine-containing amacrine cells but not by the glycine-containing bipolar cells, suggesting that only the amacrine cells are functionally glycinergic. Second, the gap-junction blocker carbenoxolone greatly reduces exogenous 3H-glycine accumulation into the bipolar cells but not the amacrine cells. Moreover, when the endogenous glycine stores in both cell classes are depleted by incubating the retina with a glycine-uptake inhibitor, carbenoxolone blocks the subsequent glycine replenishment of the bipolar cells but not the amacrine cells. Third, intracellular injection of rod amacrine cells with the gap-junction permeant tracer Neurobiotin secondarily labels a heterogenous population of cone bipolar cells, all of which show glycine immunoreactivity. Taken together, these findings indicate that the elevated glycine in cone bipolar cells is not derived by high-affinity uptake or de novo synthesis but is obtained by neurotransmitter coupling through gap junctions with glycinergic amacrine cells. Thus transmitter content may be an unreliable indicator of transmitter function for neurons that make heterologous gap junctions.

Endothelium-dependent hyperpolarization and intercellular electrical coupling in guinea-pig mesenteric arterioles

1. Using the conventional whole-cell clamp method, the electrical responses of individual smooth muscle and endothelial cells to acetylcholine (ACh) were observed in multicellular preparations where the two types of cells remained in close apposition. 2. In both types of cells, ACh induced similar hyperpolarizing responses which, when recorded in current clamp mode, had two phases (an initial fast and a second slower phase). 3. After blocking gap junctions, including myoendothelial junctions, with 18beta-glycyrrhetinic acid, ACh induced an outward current with two phases in voltage-clamped endothelial cells. The outward current appeared around -90 mV and increased linearly with the membrane depolarization. 4. In smooth muscle cells, ACh failed to induce a membrane current after gap junctions had been blocked with 18beta-glycyrrhetinic acid. The inhibition of ACh-induced response by 18beta-glycyrrhetinic acid was observed using either sharp or patch electrodes. 5. Nominally Ca2+-free solution reduced the initial phase and abolished the second phase of ACh-induced responses of endothelial cells. Both phases were also reduced by charybdotoxin (CTX). 6. Our results indicate that in guinea-pig mesenteric arterioles, ACh hyperpolarizes endothelial cells by activating Ca2+-activated K+ channels which are sensitive to CTX. On the other hand, hyperpolarizing responses detected in smooth muscle cells seem to originate in endothelial cells and conduct to the muscle layer via myoendothelial gap junctions.

Heterogeneous loss of connexin43 protein in ischemic dog hearts

INTRODUCTION

Ischemia causes cell decoupling in the myocardium. Prolonged ischemia activates proteases and causes degradation of structural proteins as well as gap junctions. There is little information about the degradation of gap junction protein during the early time period after acute ischemia. The purpose of the present study was to investigate connexin43 (Cx43) protein degradation and distribution patterns in the canine left ventricular wall during 1 to 6 hours of ischemia.

METHODS AND RESULTS

Ischemia of canine left ventricular myocardium was induced by ligation of the left anterior descending coronary artery. Following a period of in situ ischemia of up to 6 hours, samples were harvested, and standard paraffin slides were prepared for Cx43 and wheat germ agglutinin double labeling. Cx43 distribution was visualized by confocal microscopy. In controls, homogeneous distribution of Cx43 staining was determined. Ischemia caused a loss of Cx43 with a heterogeneous pattern by mixing foci of infarcted cells among normal cardiac myocytes. To determine if the changes were induced by heterogeneous reduction in the blood supply, an in vitro ischemic model was studied to induce more homogeneous ischemia. Western blot analysis of these in vitro ischemic tissue samples revealed a reduction of Cx43 protein concentration with a 50% decay time of 4.8 hours. Cx43 dephosphorylation was detected after 1 hour of in vitro ischemia. Heterogeneous loss of Cx43 was found in the in vitro ischemic tissue. There were no significant changes in Cx43 staining density during the first hour of ischemia at a time when dephosphorylation of the protein was observed. After 1 hour of ischemia, Cx43 was reduced at intercalated disk areas, and, after 6 hours, most Cx43 disappeared at intercalated disk areas, while small amounts of Cx43 remained at side-to-side junctions.

CONCLUSION

Cx43 undergoes both distribution and concentration changes following acute cardiac ischemia. The loss of Cx43 protein is heterogeneous. Cx43 dephosphorylation occurred within 1 hour following ischemia.

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

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

Growth control of 3T3 fibroblast cell lines established from connexin 43-deficient mice

Connexins are considered to be involved in cell growth control, on the basis of studies mainly with tumorigenic cells. To study the role of connexin genes in normal cell growth control, we established fibroblast cell lines from connexin 43 (Cx43)-deficient mice and characterized their growth. Embryonic fibroblasts from wild-type mice (Cx43+/+) and those with heterozygous (Cx43+/-) and homozygous (Cx43+/-) deficiencies of the Cx43 gene were cultured and passaged by a 3T3 protocol (every 3 d, 3 x 10(5) cells/60-mm dish). All cell lines showed a growth crisis during passages 6-15 and then started to grow well. All cell lines grew at similar rates under the 3T3 protocol, but Cx43-deficient (Cx43-/-) cell lines tended to grow faster when they were plated at 10(5) cells per dish. Cx43-/- cells did not express Cx43 and showed little gap-junctional intercellular communication (GJIC), confirming that Cx43 is the major connexin responsible for GJIC of these fibroblasts. While all Cx43+/+ and Cx43+/- cell lines expressed Cx43 protein, some of them showed very little GJIC. Those cell lines with high GJIC showed higher levels of the P2 form of Cx43 protein, and more Cx43 was localized in the plasma membrane than in cell lines with lower GJIC levels. We investigated effects of serum concentration on cell growth in these cell lines. Although different cell lines responded differentially to these agents, there was no clear relationship between Cx43 expression and cell growth stimulation by them. This suggests that Cx43 expression alone is not a strong regulator of mouse fibroblast growth.

Blockade by 18beta-glycyrrhetinic acid of intercellular electrical coupling in guinea-pig arterioles

1. Intercellular electrical communication between smooth muscle and endothelial cells was examined in guinea-pig mesenteric arterioles using the whole-cell patch-clamp method. The time course of the current required to impose a 10 mV voltage clamp step was used to determine the extent of electrical coupling between them. Currents recorded from both smooth muscle and endothelial cells relaxed in a multi-exponential manner, indicating the existence of electrical coupling between cells. 2. 18beta-Glycyrrhetinic acid, a gap junction blocker, quickly blocked electrical communication at 40 microM, while neither heptanol nor octanol did so at concentrations of up to 1 mM. 3. In the current clamp mode, repetitive spikes, induced by 10 mM Ba2+ solutions, could be recorded from both kinds of cells. After blocking gap junctions, spikes could only be recorded from the smooth muscle cell layer, indicating that they had been conducted through myoendothelial junctions. 4. In endothelial cells, acetylcholine (ACh, 3 microM) induced hyperpolarizing responses, which had two phases (an initial fast and a second slower phase) in the current clamp condition. This ACh response persisted in the presence of 18beta-glycyrrhetinic acid, although this compound seemed to make the membrane slightly leaky. 5. After blocking gap junctions, the membrane potential of a single cell in a multicellular preparation could be well clamped. Thus, 18beta-glycyrrhetinic acid may be useful in studying the function of both arteriolar smooth muscle and endothelial cells while they remain located within a multicellular preparation.

Inhibition of the gap junctional component of endothelium-dependent relaxations in rabbit iliac artery by 18-alpha glycyrrhetinic acid

The gap junction inhibitor 18-alpha-glycyrrhetinic acid (alpha-GA, 100 microM) attenuated endothelium-dependent relaxations to acetylcholine and cyclopiazonic acid by approximately 20% in rings of pre-constricted rabbit iliac artery. The nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 300 microM) inhibited relaxations to both agents by approximately 65% and these were further attenuated by alpha-GA to < 10% of control. In endothelium-denuded preparations, relaxations to sodium nitroprusside were not affected by alpha-GA. Heterocellular gap junctional communication may therefore account for nitric oxide-independent relaxations evoked both by receptor-dependent and -independent mechanisms in rabbit iliac artery.

Immunorecognition, ultrastructure and phosphorylation status of astrocytic gap junctions and connexin43 in rat brain after cerebral focal ischaemia

Gap junctions between astrocytes support a functional syncytium that is thought to play an important role in neural homeostasis. In order to investigate regulation of this syncytium and of connexin43 (Cx43), a principal astrocytic gap junction protein, we determined the sequelae of gap junction and Cx43 disposition in a rat cerebral focal ischaemia model with various ischaemia/reperfusion times using sequence-specific anti-Cx43 antibodies (designated 13-8300, 18A, 16A and 71-0700) that exhibit differential recognition of Cx43, perhaps reflecting functional aspects of gap junctions. Antibody 13-8300 specifically detects only an unphosphorylated form of Cx43 in both Western blots and tissue sections. In hypothalamus after brief (15 min) ischaemic injury, Cx43 at intact gap junctions undergoes dephosphorylation, accompanied by reduced epitope recognition by antibodies 16A and 71-0700. Tissue examined 24 h after reperfusion showed that these effects were reversible. Astrocytic gap junction internalization occurring 1 h after ischaemia was accompanied by decreased immunodetection with 13-8300. At this time, gap junctions were absent in the ischaemic core, coinciding with a loss of Cx43 recognition with 18A and 13-8300, but elevated labelling of internalized Cx43 with 16A and 71-0700. Unphosphorylated Cx43 persisted at intact gap junctions confined to a thin corridor at the ischaemic penumbra which contained presumptive apoptotic cell profiles. Similar results were obtained in ischaemic striatum and cerebral cortex, though with a delayed time course that depended on the severity of the ischaemic insult. These results demonstrate that astrocytic Cx43 epitope masking, dephosphorylation and cellular redistribution occur after ischaemic brain injury, proceed as a temporally and spatially ordered sequence of events and culminate in differential patterns of Cx43 modification and sequestration at the lesion centre and periphery. These observations suggest an attempt by astrocytes in the vicinity of injury to remodel the junctional syncytium according to altered tissue homeostatic requirements.

Rapid determination of gap junction formation using HeLa cells microinjected with cDNAs encoding wild-type and chimeric connexins

A procedure for rapidly determining the functionality of gap junctions constructed of recombinant connexins in communication-deficient HeLa cells is described. Nuclear microinjection of cDNA encoding wild-type connexins (Cx) 26, 32, 43, and a range of connexin-aequorin (Cx-Aeq) chimerase resulted in generation of gap junction intercellular communication channels. Expression of recombinant protein was detected in > 95% of cells 18-72 h following nuclear microinjection, and the functionality of the channels generated was determined according to their ability to transfer the fluorescent dye tracers Lucifer yellow and propidium iodide. The dye transfer results obtained correlated closely with other published studies using stably transfected cells and yet are obtained as rapidly as 18 h following microinjection of cDNA. Expression of a truncated form of Cx43 (Cx43 delta 244) by this new method indicated diminished intercellular transfer of both dyes and supports a channel-gating mechanism that postulates interaction between the carboxyl tail and the intracellular loop.

The sleep-inducing lipid oleamide deconvolutes gap junction communication and calcium wave transmission in glial cells

Oleamide is a sleep-inducing lipid originally isolated from the cerebrospinal fluid of sleep-deprived cats. Oleamide was found to potently and selectively inactivate gap junction-mediated communication between rat glial cells. In contrast, oleamide had no effect on mechanically stimulated calcium wave transmission in this same cell type. Other chemical compounds traditionally used as inhibitors of gap junctional communication, like heptanol and 18beta-glycyrrhetinic acid, blocked not only gap junctional communication but also intercellular calcium signaling. Given the central role for intercellular small molecule and electrical signaling in central nervous system function, oleamide- induced inactivation of glial cell gap junction channels may serve to regulate communication between brain cells, and in doing so, may influence higher order neuronal events like sleep induction.

Intercellular communication, tumor promotion and non-genotoxic carcinogenesis: relationships based upon structural considerations

An SAR model for inhibition of metabolic cooperation (iMC) was developed. The structural and physicochemical features associated with the ability to cause iMC are primarily lipophilic moieties consistent with the possibility that they represent receptor-binding ligands. There are also significant parallels between the structural descriptors associated with iMC and those associated with tumor promotion and with carcinogenesis in rodents. Overall, the present study provides structural evidence that iMC is a feature associated with the carcinogenic process.