Evidence that the gap junction protein connexin-43 is the ATP-induced pore of mouse macrophages

Extracellular adenosine 5' triphosphate involvement in the death of LAK-engaged human tumor cells via P2X-receptor activation

This study reports that extracellular ATP is a critical factor involved in LAK cell-mediated cytotoxicity. Human colon carcinoma LoVo cells were resistant to LAK cells as well as to ATP, while their multidrug resistant (MDR-1+) derivative, LoVo-Dx cells, were sensitive to both LAK and ATP. LoVo-Dx cells, became resistant to LAK cells and ATP after 48 h pretreatment with Phorbol 12-Myristate-13-Acetate (PMA), while 48 h pretreatment with verapamil in parallel sensitized LoVo cells to LAK cells and to ATP as well. The sensitivity to ATP and LAK cells was not related to the expression of extracellular ecto-ATPase activity on cell targets membranes. Conversely, apyrase, an enzyme with powerful ecto-ATPase activity, abolished the LAK- and ATP-mediated cytotoxicity. Furthermore, ADP-beta-S, an antagonist of ATP, abolished both LAK and ATP-mediated cell killing. Purine binding sites have been detected by radioreceptor assays with ADP-beta[35S] on the cell surface of ATP and LAK-sensitive LoVo-Dx cells. By contrast, no nucleotide receptor was found on the ATP and LAK-resistant cells. Such a putative cytotoxic purinoreceptor has been categorized as P2x purinergic receptor by a panel of synthetic nucleotides. These results demonstrate that extracellular ATP is needed for an efficient LAK cell-mediated killing of tumor cells. We propose that ATP acts as a natural amplifier of physical, or immune cytotoxic damages since it may be released in large amounts from target cells injured by several cytotoxic mediators secreted by LAK effectors.

Phase I trial of extracellular adenosine 5'-triphosphate in patients with advanced cancer

Adenosine 5'-triphosphate (ATP) has antineoplastic activity in vitro and in murine tumor systems, but there are no data in humans defining its potential use as an antineoplastic agent. We conducted a phase I study to determine the spectrum of toxicity, maximum safely tolerated dose (MTD), and pharmacokinetics of intravenous ATP. Fourteen men with advanced cancer received 96-hour infusions of ATP once monthly in doses ranging from 50 to 100 micrograms/kg/minute. Toxicity was assessed by standard National Cancer Institute (NCI) criteria, cardiac function was monitored serially by two-dimensional echocardiography, and whole blood ATP was measured serially in a subset of patients. ATP was generally well tolerated and no significant hematologic toxicity was noted. The dose-limiting toxicity was a cardiopulmonary reaction characterized by chest tightness and dyspnea that resolved within seconds of discontinuing ATP. Dose-limiting cardiopulmonary toxicity occurred in 3 of 3 patients at 100 micrograms/kg/minute, in 3 of 6 patients at 75 micrograms/kg/minute, and 4 of 11 patients at 50 micrograms/kg/minute. Whole blood ATP levels significantly increased with treatment, reaching a steady state by 24 hours and returning to or near baseline by 1 week after treatment. Plateau levels were 63%, 67%, and 116% above base-line at 50, 75, and 100 micrograms/kg/min, respectively. We conclude that prolonged infusions of ATP are feasible with acceptable toxicity and that 50 micrograms/kg/minute is both the MTD and the most appropriate dose rate for subsequent Phase II testing of 96-hour infusions of ATP in patients with advanced cancer.

Porcine aortic endothelial gap junctions: identification and permeation by caged InsP3

Gap junction channels permit the direct intercellular transfer of ions and small molecules and allow electrotonic coupling within tissues. Porcine aortic endothelial cells were extensively coupled, as assessed by gap junctional transfer of Lucifer yellow and the fluorescent calcium indicators fluo-3 and furaptra, but were not permeable to rhodamine B isothiocyanate-dextran 10S. The subunit composition of gap junction channels of porcine aortic endothelial cells was characterised using both northern blot analysis and RT-PCR techniques. Messenger RNA encoding connexins 37 and 43, but not 26, 32 or 40, were found in freshly isolated and cultured porcine aortic endothelial cells. Western blots using antipeptide antibodies raised to unique sequences of connexins 37, 40 and 43 showed the presence of connexins 37 and 43, but no connexin 40 was detected. Immunostaining with anticonnexin 43 antibodies showed extensive punctate fluorescent decoration of contacting membranes, whilst antibodies to connexin 37 showed predominantly intracellular staining. Caged InsP3 was found to readily permeate endothelial gap junctions. These results show that primary cultures of porcine aortic endothelial cells express connexin 37 and 43, and provide strong evidence that the second messenger molecule InsP3 can permeate porcine endothelial gap junctions.

Combined effects of ATP and its analogs on the membrane permeability in transformed mouse fibroblasts

Extracellular ATP (0.6 mM) induces a marked decrease in the membrane potential, followed by an increase in cell membrane permeability in transformed mouse fibroblasts. The effects of the ATP analogs, p[CH2]ppA and p[NH]ppA (0.6 mM), on the membrane potential and permeability are much less pronounced. ATP at 0.05 mM has no effect by itself, but markedly increases the analog-induced membrane potential dissipation and permeability. The data suggest that ATP-induced membrane permeation is composed of two processes: One is common to ATP and its analogs and appears to be a receptor-mediated process. The second is unique for ATP, effective even at low concentration (0.05 mM), and might be mediated by cell surface enzymes, for which ATP, but not its analogs, serves as a substrate.

Adenosine 5'-triphosphate (ATP) receptors induce intracellular calcium changes in mouse leydig cells

Cytoplasmic calcium ([Ca(2+)](i)) changes evoked by adenosine 5(1)-triphosphate (ATP) were recorded in cultured individual Leydig cells within 10-18 h after cell dispersion. [Ca(2+)](i) was monitored using Fura-2AM loaded cells with a digital ratio imaging system. Five micromolars ATP induced biphasic [Ca(2+)](i) responses in most cells (94%,n=100), characterized by a fast increase from a basal level (126±5 nMSE,n=60 cells) to a peak (5-7 times above basal levels) within seconds, followed by a slow decrease toward a plateau level (2-3 times above basal) within 5 min. The peak phase of the [Ca(2+)](i) response increased with ATP concentrations (1-100 μM ATP) in a dose-dependent manner with an IC(50) of 5.9±1.2 μM, and it desensitized in a reversible manner with repeated application of 5 μM ATP at <5-min intervals. The [Ca(2+)](i) peak response was dependent on Ca(2+) release from an intracellular pool, whereas the plateau phase was dependent on extracellular [Ca(2+)]. ATP did not appear to induce formation of nonspecific membrane pores, since stimulation for 10 min with ATP (10-100 μM) in the presence of extracellular Lucifer yellow (LY) (5 mg/mL) did not result in dye loading of the cells. [Ca(2+)](i) transients were elicited by other adenosine nucleotides with an order of potencies (ATP>Adenosine diphosphate [ADP]>Adenosine> Adenosine monophosphate [AMP]) that was compatible with the expression of P(2) receptors. [Ca(2+)](i) responses were suppressed by the purinergic P(2) receptor antagonist, suramin. These results provide functional evidence for the expression of purinergic P(2) receptors in Leydig cells.

Connections with connexins: the molecular basis of direct intercellular signaling

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

A cation non-selective channel induced by extracellular ATP in macrophages and phagocytic cells of the thymic reticulum

Extracellular ATP4- can bind to P2Z purinergic receptors including depolarization and cytoplasmic membrane permeabilization to small molecular weight solutes in macrophages, thymocytes, mast cells, phagocytic cells of the thymic reticulum and other cell types. An ATP(4-)-induced cation current has been described in whole-cell records of some of these cells but it is currently not clear whether these currents and the phenomenon of membrane permeabilization are a consequence of only one type of P2Z-associated channel/pore or two different phenomena triggered by one or more receptors. Here we use the outside-out patch-clamp technique to describe a single channel associated with this cation current in two murine phagocytic cells: intraperitoneal macrophages and phagocytic cells of the thymic reticulum. Multi channel currents could be readily observed in 77% of the outside-out patches of macrophages. Single channels of 7.8 pS could usually be resolved only in tail currents. Reversal potential measurements and ion replacement experiments indicated a lack of cation selectivity, similarly to what has already been described for the ATP(4-)-induced whole-cell inward current. No large-conductance channels that could explain the permeabilization to small molecular weight studies solutes was observed under our experimental conditions. A single channel of approx. 5 pS was also observed in phagocytic cells of the thymic reticulum under similar conditions. We conclude that the channel here described is the main carrier of cation current usually associated with the binding of ATP4- to P2Z receptors in whole-cell and outside-out patch-clamp experiments.

The P2Z purinoceptor: an intriguing role in immunity, inflammation and cell death

Many immune and inflammatory cells express a plasma membrane receptor for extracellular ATP, termed the P2Z purinoceptor, which appears to be coupled to a plasma membrane pore. The physiological role of such a molecule is generally unknown, except for the striking susceptibility to ATP-mediated cytotoxicity that it confers. The receptor is upregulated in human monocytes by interferon gamma and is also expressed during macrophage differentiation. Here, Francesco Di Virgilio discusses recent information on this receptor, and suggests a possible role for it in the immune and inflammatory response.

Upregulation of connexin43 gap junctions during early stages of human coronary atherosclerosis

Interactions between cells form the framework for understanding the pathogenesis of atherosclerosis, but little information is available on the role of direct intercellular communication via gap junctions in this process. To investigate gap junction expression in the pathogenesis of human atherosclerosis, lesions representing different stages of the disease were obtained from coronary arteries of hearts removed from patients undergoing cardiac transplantation. Twelve hearts, each providing 1 to 3 segments of artery, were used in the study. Sections were examined by confocal laser scanning microscopy after immunofluorescent labeling with a specific antibody against connexin43, the major gap-junctional protein of smooth muscle cells, to permit high-definition visualization of immunolabeled gap junctions through the depth of the specimen. Double labeling using anti-connexin43 and cell type-specific antibodies demonstrated colocalization of gap junctions with smooth muscle cells but not with macrophages, a relationship confirmed by electron microscopy. Regions of intimal thickening and early atheromatous lesions showed markedly increased expression of connexin43 gap junctions between intimal smooth muscle cells compared with the undiseased vessels. This increase in gap junctions was most marked in regions of intimal thickening, semiquantitative analysis of the confocal digital images revealing a > 10-fold increase compared with the undiseased vessel. The quantity of labeled gap junctions in early atheromatous lesions, although higher than that of the undiseased vessel, was lower than that of intimal thickenings, and this trend toward reduced levels of gap junction immunolabeling with lesion progression continued, the value observed in the most advanced atheromatous lesions being lower than that of the undiseased vessel. As the quantity of gap junctions declined, their distribution became more patchy and the sizes of individual junctions larger. The results suggest that enhanced expression of gap junctions between smooth muscle cells may play a role in maintaining the synthetic phenotype during early growth of the atherosclerotic plaque.

The molecular basis of anisotropy: role of gap junctions

Electrical activation of the heart requires transfer of current from one discrete cardiac myocyte to another, a process that occurs at gap junctions. Recent advances in knowledge have established that, like most differentiated cells, individual cardiac myocytes express multiple gap junction channel proteins that are members of a multigene family of channel proteins called connexins. These proteins form channels with unique biophysical properties. Furthermore, functionally distinct cardiac tissues such as the nodes and bundles of the conduction system and atrial and ventricular muscle express different combinations of connexins. Myocytes in these tissues are interconnected by gap junctions that differ in tissue-specific manner in terms of their number, size, and three-dimensional distribution. These observations suggest that both molecular and structural aspects of gap junctions are critical determinants of the anisotropic conduction properties of different cardiac tissues. Expression of multiple connexins also creates the possibility that "hybrid" channels composed of more than one connexin protein type can form, thus greatly increasing the potential for fine control of intercellular ion flow and communication within the heart.

Repression of the macrophage scavenger receptor in macrophage-smooth muscle cell heterokaryons

Macrophage scavenger receptors mediate the uptake of chemically modified LDL in an unregulated manner, leading to massive intracellular accumulation of lipid and thus a foamy cellular morphology. In atherosclerotic lesions, foam cells originate not only from macrophages but also from smooth muscle cells, yet smooth muscle cells do not normally express scavenger receptors, and when exposed to chemically modified LDL in vitro, lipid accumulation does not occur. The mechanism of conversion of smooth muscle cells into foam cells in the arterial wall is thus still under discussion. To investigate whether direct interaction between macrophages and smooth muscle cells may be involved and to explore the effects of components of the two cell types on the expression of scavenger receptors, we report here experiments using somatic cell hybrids formed by fusion of the two cell types. Immunofluorescent labeling and confocal microscopic techniques were applied to investigate and measure (1) lipid accumulation (using Nile Red staining), (2) the binding and uptake of acetylated LDL (using 1,1'-dioctadecyl-1-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate-labeled acetylated LDL), and (3) receptor expression (assessed using a specific anti-receptor antibody) in smooth muscle cell-macrophage heterokaryons, macrophage-macrophage homokaryons, smooth muscle cell-smooth muscle cell homokaryons, and unfused macrophages and smooth muscle cells. The results demonstrate that scavenger receptor expression becomes repressed in macrophage-smooth muscle cell heterokaryons but not in macrophage-macrophage homokaryons. One possible explanation for the observed repression would be the existence of a negative regulatory cytoplasmic factor produced by smooth muscle cells.

Nucleotides as extracellular signalling molecules

There is now wide acceptance that ATP and other nucleotides are ubiquitous extracellular chemical messengers. ATP and diadenosine polyphosphates can be released from synaptosomes. They act on a large and diverse family of P2 purinoceptors, four of which have been cloned. This receptor family can be divided into two distinct classes: ligand-gated ion channels for P2X receptors and G protein-coupled receptors for P2Y, P2U, P2T and P2D receptors. The P2Y, P2U and P2D receptors have a fairly wide tissue distribution, while the P2X receptor is mainly found in neurons and muscles and the P2T and P2Z receptors confined to platelets and immune cells, respectively. Inositol phosphate and calcium signalling appear to be the predominant mechanisms for transducing the G-protein linked P2 receptor signals. Multiple P2 receptors are expressed by neurons and glia in the CNS and also in neuroendocrine cells. ATP and other nucleotides may therefore have important roles not only as a neurotransmitter but also as a neuroendocrine regulatory messenger.

Adenosine 5'-triphosphate promotes mineralization in differentiating chick limb-bud mesenchymal cell cultures

When chick limb-bud mesenchymal cells are plated in micromass culture, they differentiate to form a mineralizable cartilage matrix. Previous studies have demonstrated that, when the total inorganic phosphate concentration of the medium is adjusted to 3-4 mM by adding inorganic phosphate to the basal medium, the mineralized matrix formed resembles that of chick calcified cartilage in ovo. When the high-energy phosphates adenosine 5'-triphosphate (ATP) or creatine phosphate are used as supplements in place of inorganic phosphate, the mineralized matrix as analyzed by electron microscopy and Fourier transform infrared microscopy is also similar to that in ovo. This is in marked contrast to the mineralized matrix formed in the presence of 2.5-5 mM beta-glycerophosphate, where mineral deposition is random and mineral crystal sizes in general are larger. This is also in contrast to the known ability of ATP to inhibit mineral deposition in solution in the absence of cells. In the differentiating mesenchymal cell culture system, ATP does not alter the rate of cell proliferation (DNA content), the rate of matrix synthesis (3H-leucine uptake), the mean crystallite length, or the rate of mineral deposition (45Ca uptake) when contrasted with cultures supplemented with inorganic phosphate. However, ATP does increase the mineral to matrix ratio, especially around the edge of the culture, where a type I collagen matrix is presented. It is suggested that ATP promotes mineral deposition by providing a high-energy phosphate source, which may be used to phosphorylate extracellular matrix proteins and to regulate calcium flux through cell membranes.

Permeabilization of cells of hemopoietic origin by extracellular ATP4-: elimination of osteoclasts, macrophages, and their precursors from isolated bone cell populations and fetal bone rudiments

Skeletal tissues contain, apart from cells of the osteogenic and chondrogenic lineage, cells of hemopoietic origin, e.g., macrophages, osteoclasts, and their precursors. In the present study we examined the sensitivity for extracellular ATP4- of the above-mentioned cell types in freshly isolated, bone-derived cell populations and in explanted fetal metatarsal bones. Cells of hemopoietic origin reacted to the presence of ATP4- with an increased permeability for impermeant cytotoxic molecules, e.g., ethidium bromide (EB), thiocyanate (KSCN), and an increased non-ion selective membrane conductance. As a consequence, these cells could be killed by a short treatment with adenosine-5' triphosphate (ATP)+KSCN. On the other hand, cells of nonhemopoietic origin (e.g., osteoblasts, chondrocytes) were found to be insensitive to ATP4- in this respect. These cells survived the treatment without apparent damage to their alkaline phosphatase activities, osteogenic potentials, and osteoclast induction capacities. The elimination of the endogenous cells of hemopoietic origin from bone tissue or cell populations derived therefrom offers the possibility to study the properties and functions of osteogenic or chondrogenic cells without interference by the presence of cells of hemopoietic origin. It also allows the study of interactions between osteogenic cells and selected cell populations of hemopoietic origin in coculture experiments.

Mechanisms and function of intercellular calcium signaling

Intercellular Ca2+ waves initiated by mechanical or chemical stimuli propagate between cells via gap junctions. The ability of a wide diversity of cells to display intercellular Ca2+ waves suggests that these Ca2+ waves may represent a general mechanism by which cells communicate. Although Ca2+ may permeate gap junctions, the intercellular movement of Ca2+ is not essential for the propagation of Ca2+ waves. The messenger that moves from one cell to the next through gap junctions appears to be IP3 and a regenerative mechanism for IP3 may be required to effect multicellular communication. Extracellularly mediated Ca2+ signaling also exists and this could be employed to supplement or replace gap junctional communication. The function of intercellular Ca2+ waves may be the coordination of cooperative cellular responses to local stimuli.

ATP- and UTP-induced currents in macrophages and macrophage polykaryons

We have investigated the currents induced by extracellular ATP (ATPo), extra-cellular UTP, and other related compounds in macrophages. At potentials of -20 to -60 mV, a typical response to ATPo puffs consists of a fast-activating inward current followed by a transient outward current. The phenomenon lasts 5-20 s, but for sustained exposure to ATP the inward current persists for up to 10 min (our longest recording time). Both currents are inhibited by Mg2+, suggesting that the phenomenon is mediated by ATP4-. The outward current can be ascribed to a Ca(2+)-dependent K+ conductance, and release of Ca2+ from intracellular stores is at least in part responsible for this current. The inward current has a reversal potential of approximately 0 mV, and it is nonspecific for monovalent cations. UTP, a nucleotide that induces an increase in the cytoplasmic concentration of free Ca2+ but does not permeabilize macrophages, and ATP-gamma-S can also induce inward and outward current similar to those described for ATP, but higher doses are required. Adenosine and AMP produce no detectable effect, whereas ADP induces a small outward current. The implications of these results to the phenomenon of ATPo-induced permeabilization of macrophage membranes to large molecules are discussed.

Intercellular calcium signaling induced by extracellular adenosine 5′-triphosphate and mechanical stimulation in airway epithelial cells

Airway epithelial cells in culture respond to extracellular adenosine 5′-triphosphate (ATP) by increasing their intracellular Ca2+ concentration ([Ca2+]i). The effective concentration of ATP that elicited a Ca2+ response equal to 50% of the maximal response (EC50) was 0.5 microM. Release of ATP from a pipette to form a local gradient of ATP increased [Ca2+]i of individual cells in a sequential manner. Cells closest to the pipette showed an immediate increase in [Ca2+]i while more distal cells displayed a delayed increase in [Ca2+]i. This response to the local release of ATP appeared as a wave of increasing [Ca2+]i that spread to several cells and, in this respect, was similar to the intercellularly communicated Ca2+ waves initiated by mechanical stimulation in airway epithelial cells (Sanderson et al., Cell Regul. 1, 585–596, 1990). In the presence of a unidirectional fluid flow, the Ca2+ response to a local release of ATP was biased such that virtually all the cells responding with an increase in [Ca2+]i were downstream of the release site. By contrast, an identical fluid flow did not bias the radial propagation of intercellular Ca2+ waves induced by mechanical stimulation. Suramin, a P2-purinergic receptor antagonist, did attenuate the Ca2+ response induced by ATP but did not block the propagation of mechanically induced Ca2+ waves. Cells from young cultures (3-5 days) or those at the leading edge of an outgrowth elevated their [Ca2+]i in response to ATP. However, these cells do not respond to mechanical stimulation by the propagation of a Ca2+ wave. From these results we conclude that the intercellular Ca2+ waves elicited by mechanical stimulation are not the result of ATP or another compound released from the stimulated cell, diffusing through the extracellular fluid. This conclusion is consistent with previous experimental evidence suggesting that intercellular Ca2+ signaling in epithelial cells is mediated by the movement of inositol trisphosphate through gap junctions.

Structure, sequence and expression of the mouse Cx43 gene encoding connexin 43

Gap junctions, membrane channels that mediate the diffusion of ions and small molecules between cells, are hypothesized to play a role in development and growth regulation. The Cx43 gene (encoding connexin 43) is one member of the gap junction gene family whose transcripts are expressed in a highly regionalized manner during mouse development. We cloned and sequenced Cx43 cDNAs from a 7.5-day mouse embryo cDNA library. These cDNA clones encode the authentic 43-kDa connexin. Analysis of RNA isolated from different regions of the 7.5-day mouse embryo revealed that Cx43 transcripts are differentially expressed, with expression detected in the embryo proper, but not in the extraembryonic region containing the ectoplacental cone. Using one of the newly isolated mouse Cx43 cDNA probes, we screened a mouse genomic DNA library and cloned the Cx43 gene. Restriction mapping and sequencing of the cloned genomic inserts revealed that Cx43 contains two exons and a 10.5-kb intron located in the 5' untranslated region (5'-UTR). We mapped the Cx43 transcription start point (tsp) by RNase protection and primer extension analyses and showed that transcripts expressed in the 7.5-day mouse embryo and in adult tissues are initiated from the same tsp. The DNA sequence immediately upstream from the tsp contains a putative AP1-binding site and a degenerate TATA consensus sequence. A comparison of mouse, rat, human and bovine Cx43s showed that the 3'-UTR has an unexpectedly high degree of sequence homology. This includes conservation of four AUUUA motifs, a sequence associated with transcript instability in immediate early genes.(ABSTRACT TRUNCATED AT 250 WORDS)

Connexin43 in rat pituitary: localization at pituicyte and stellate cell gap junctions and within gonadotrophs

Immunohistochemical methods were employed to investigate the cellular and ultrastructural localization of the gap junction protein connexin43 (Cx43) in rat pituitary. Western blots of pituitary homogenates probed with anti-Cx43 antibodies showed the presence of Cx43 in both anterior and posterior pituitary lobes. By light microscopy (LM), Cx43-immunoreactive (Cx43-IR) puncta were found in all areas of the posterior lobe, but at greater concentrations in peripheral regions of this structure. By electron microscopy (EM), immunogold labelling for Cx43 was seen at gap junctions between thin cytoplasmic processes of pituicytes. No immunoreactivity was detected in the intermediate lobe. The anterior lobe contained puncta similar to but more sparsely scattered than those in the posterior lobe, and by EM analysis these were demonstrated to correspond to labelled gap junctions between stellate cells. In addition, anti-Cx43 antibodies produced intracellular labelling in a small percentage of endocrine cells, which were distributed throughout the anterior lobe and determined by double immunostaining methods to be cells containing luteinizing hormone. By EM, labelling within these cells was associated with predominantly large secretory granules and other loosely organized organelles. The results indicate that gap junctions in the pituitary are composed of Cx43 and that this or a related protein may have a novel intracellular function within gonadotrophs.

Cytolytic effects and biochemical changes induced by extracellular ATP to isolated hepatocytes

Cell death, as estimated by the release of lactate dehydrogenase (LDH), was induced by incubating isolated hepatocytes for 60 min in the presence of extracellular ATP (ecATP), while AMP, adenosine, GTP and UTP were without any significant effects, even when tested at 3 mM (final concentration). At such a concentration, the release of LDH induced by ecATP, but also by ecADP, reached almost 50% and 30%, respectively. Since UTP and GTP (which have no lytic effects) were able to activate phosphorylase a at the same rate as ATP, we excluded the possibility that an increase of free cytosolic Ca2+ triggers the onset of a process leading to cell lysis. Moreover, such a lytic ability of ecATP (1.7 mM) can not be the result of a previous complexation of ionic iron (making it catalytically available for a Fenton reaction), because Desferal, a strong iron chelator, did not modify the cytolytic effect of the ecATP observed after 60 min of incubation. A major cellular function such as protein synthesis was impaired in a dose-dependent way by incubating hepatocytes during 60 min in the presence of ecATP. The inhibition was already observed at 0.1 mM ecATP, a dose without any effect on cell viability. The biological relevance of such metabolic impairment, however, remains to be elucidated.

Epidermal Langerhans cells are resistant to the permeabilizing effects of extracellular ATP: in vitro evidence supporting a protective role of membrane ATPase

Extracellular adenosine 5'-triphosphate (ATPo) can induce pore formation in cell membranes, leading to cell permeabilization and eventual cell death. In this study, we examined the sensitivity of human epidermal Langerhans cells to ATP-induced permeabilization and tested the possibility that the Mg(++)- or Ca(++)-dependent plasma membrane ectonucleotidase (mATPase) on Langerhans cells provides protection against the cytotoxic effects of ATPo. Membrane permeability was assessed by using the fluorescent tracer propidium iodide, which confers red nuclear fluorescence to permeabilized cells. Langerhans cells were identified within human epidermal cell suspensions with fluorescein isothiocyanate-conjugated MoAb against CD1a or human leukocyte antigen-DR (HLA-DR) antigens. Cultured human keratinocytes and J774 macrophages were both highly sensitive to permeabilization induced by incubation with ATP (0.5 to 20 mM at 37 degrees C), whereas Langerhans cells were relatively resistant. The non-hydrolyzable ATP analog, adenosine 5'-(beta,gamma-imido) triphosphate, but not other nucleotides such as ADP, AMP, GTP, or UTP, was also able to induce permeabilization comparable to that of ATP, thereby suggesting that ATP hydrolysis is not required for this effect. ATP4- is the moiety most likely responsible for permeabilization, because propidium iodide uptake occurred only when the pH of the medium was > or = 7.4. Permeabilization induced by ATP was augmented by chelation of divalent cations with ethylene-diamine-tetraacetic acid and by the addition of lanthanum or cerium (0.01 to 1 mM). Finally, incubation with the adenosine analog, 5'-p-fluorosulfonylbenzoyl-adenosine (1 mM), inhibited mATPase staining of Langerhans cells in human epidermal sheets, but markedly augmented ATP-induced permeabilization of Langerhans cells. The results indicate that epidermal LC are resistant to the lytic effects of ATPo and that mATPase is involved in such resistance.

Membrane ecto-ATPase on epidermal Langerhans cells

Epidermal Langerhans cells (LC) possess a plasma membrane Mg(++)- or Ca(++)-dependent ecto-ATPase (mATPase), the function of which is not known. On the other hand, extracellular ATP (ATPo) has been shown to cause a rapid increase in cell-membrane permeability for selected cells, thus inducing the entry of exogenous substances, leading eventually to cell death. ATPo has also been shown to be a mediator of T cell-mediated cytotoxicity. We have observed human LC to be resistant to the permeabilizing effects of ATPo when compared to cultured human keratinocytes and J774 macrophages. In addition, our preliminary evidence in vitro suggests that mATPase on LC may provide protection against the lytic effects of ATPo. Finally, we propose that this latter mechanism may also operate in vivo.

P2Y purinoceptors in normal NIH 3T3 and in NIH 3T3 overexpressing c-ras

The ability of purinergic agonists to induce Ca2+ responses has been tested in two lines of murine fibroblasts: normal NIH 3T3 fibroblasts and NIH 115.14, a clone expressing high levels [1] of the c-ras protooncogene. Both kinds of cells are responsive to ATP in the range 1 microM-1 mM; ADP and ATP gamma S are almost as potent as ATP, while AMP is unable to elicit a response. Ca2+ measurements performed in single cells by image analysis show great variability among cells but in each individual responding cell the Ca2+ rise occurs in an all-or-none fashion. The transient Ca2+ response does not depend on influx from the extracellular medium. Electrophysiological experiments reveal the activation of an outward current (at -50 mV) by ATP, probably due to Ca(2+)-activated K+ channels, confirming the absence of a substantial Ca2+ influx. Finally, stimulation by ATP produces a small but significant increase in the production of inositol phosphates. These results indicate that these cell lines possess purinergic receptors which are not integral membrane channels and which are coupled to InsP3 formation and may be therefore classified as P2Y.

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.

Characterization of ATP receptor which mediates norepinephrine release in PC12 cells

PC12 cells, a rat pheochromocytoma cell line, has been reported to release norepinephrine in response to extracellular ATP in the presence of extracellular Ca2+. The potency order of ATP analogues was adenosine 5'-O-(3-thiotriphosphate) greater than ATP greater than adenosine 5'-O-(1-thiotriphosphate) = 2-methylthioadenosine 5'-triphosphate (MeSATP) greater than 2'- and 3'-O-(4-benzoyl-benzoyl)ATP (BzATP) greater than ADP greater than 5-adenylylimidodiphosphate. Adenosine 5'-O-(2-thiodiphosphate), beta, gamma-methyleneadenosine 5'-triphosphate, AMP and adenosine were inactive. The ATP action in the absence of extracellular Ca2+, suggests a small but appreciable contribution of intracellular Ca2+ mobilization, for norepinephrine release. However, for some ATP derivatives, like BzATP, almost no contribution of the phospholipase C-Ca2+ pathway is suggested, based on their low activity in inositol phosphates production. To identify the ATP-receptor protein, PC12 cell membranes were photoaffinity-labeled with [32P]BzATP. SDS-PAGE analysis showed that a 53-kDa protein labeling was inhibited by ATP and its derivatives, as well as by P2-antagonists, suramin and reactive blue 2, which inhibit the nucleotide-induced norepinephrine release. The inhibitory activity of the nucleotides was, in parallel with their potency, to induce norepinephrine release. Despite their inability to release norepinephrine, GTP and GTP gamma S inhibited the BzATP labeling, suggesting the participation of a putative G protein in the ATP-receptor-mediated actions. We suggest that the 53-kDa protein on the PC12 cell surface is an ATP receptor, which mediates the norepinephrine release, depending, mainly, on extracellular Ca2+ gating.

Voltage-dependent gating and single-channel conductance of adult mammalian atrial gap junctions

In the heart, the rapid propagation and synchronization of action potentials necessary for a normal heart rhythm and an effective cardiac output are mediated by specialized ionic channels that link adjacent cells and are known collectively as gap junctions. Cardiac gap junctions are gated by various physiological and pharmacological agents, but the role of voltage in their gating is unclear. Whereas embryonic or neonatal ventricular cells have voltage-gated gap junctions, adult cells are reported to have only voltage-independent gap junctions. We studied the voltage dependence of adult rat atrial gap junctions by individually voltage clamping each cell of a connected cell pair and controlling the transjunctional voltage (Vj), measuring transjunctional current (Ij), and calculating junctional conductance (gj). Two distinct populations of cell pairs were observed: highly coupled pairs with the peak gjs ranging from 3.4 to 40 nS and weakly coupled pairs with the peak gjs ranging from 0.3 to 2.0 nS. gj was dependent on Vj, and Ij decayed exponentially, with the time constants being voltage dependent. Voltage dependence was most apparent when cells were poorly coupled. The gj did not decrease to zero. The normalized conductance--Vj plot was fit with a two-state Boltzmann model as a first approximation, resulting in a half-inactivation potential and gating charge of 42.5 mV and 1.14 eV, respectively, for the weakly coupled cell pairs. For highly coupled cell pairs, the half-inactivation potential shifted to 53.3 mV. Single gap junctional channels had a gj of 36.2 +/- 7.6 pS (range, 27-49 pS), which was Vj independent.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution pattern of connexin 43, a gap junctional protein, during the differentiation of mouse heart myocytes

In the cardiac muscle, the electrical coupling of myocytes by means of gap (or communicating) junctions, allows the action potentials to be propagated. Connexin 43 (CX 43) is the major constitutive protein of the gap junctions in the mammalian myocardium. In this organ, the abundance of CX 43 and of its messenger, as well as the spatial expression of this protein, are developmentally regulated. These findings are complemented by the results presented in this article, which deals with the distribution of CX 43 in the ventricular myocytes of mouse heart during differentiation, between the 11 days post coitum embryo stage and adulthood. By immunoelectron microscopy experiments on ultrathin sections of cardiac ventricular tissue of one-week-old mouse, we have provided confirmation that the anti-CX 43 antibodies used here specifically recognized the gap junctions. Double labeling immunofluorescence experiments have been undertaken to localize, within the same cells, either CX 43 and desmin, or CX 43 and Con A or WGA receptor sites. From the earliest stage investigated (11 days post coitum) onwards, expression of CX 43 is always associated with desmin-positive cells, that is, with the myocytes. Up to birth, there is in the ventricular wall a gradient of expression of CX 43 which is superimposable on a gradient of expression of desmin. Immunoreactivity to anti-CX 43 and anti-desmin antibodies is high in the sub-endocardial trabeculae and low (or even undetectable for CX 43, in the early stages) in the sub-epicardial cell layers. In the embryonic stages, the expression sites of CX 43 are visible in the form of small dots, whose abundance increases as development proceeds. During these stages, the immunoreactive sites are distributed in a relatively homogeneous pattern throughout the membrane of the myocytes. One week after birth, the CX 43 expression is restricted to the two ends of the myocytes (where the intercalated discs develop), and the adjacent lateral regions. This polarization of CX 43 is more pronounced at the two and three weeks post natal stages and in the fully differentiated ventricular myocytes (adult stage) CX 43 is only present in the intercalated discs.

Aspects of gap junction structure and assembly

The development of ideas about gap junction structure is summarized, including some recent results obtained by use of atomic force microscopy. Particular attention is paid to novel aspects of the biosynthesis and assembly of connexons and to the formation of new junctions.

Cardiac myocytes express multiple gap junction proteins

Electrical propagation in the normal heart occurs via intercellular transfer of current at gap junctions. Alterations in intercellular coupling in the diseased heart are critical in the pathogenesis of reentrant ventricular arrhythmias. Until recently only a single gap junction protein was known to couple cardiac myocytes. We have now identified and sequenced two additional distinct gap junction proteins (connexins) expressed in the mammalian heart. The sequences differ in their predicted cytoplasmic regulatory domains. Expression of all three connexins by canine ventricular myocytes has been confirmed by Northern blotting and by immunohistochemistry with connexin-specific antisera. Immunoelectron microscopy confirmed that all three connexins are localized to myocyte gap junctions. The presence of multiple connexins in myocyte gap junctions suggests novel mechanisms for regulating cardiac electrical coupling.