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

Expression of purinergic receptors and modulation of P2X7 function by the inflammatory cytokine IFNgamma in human epithelial cells

The cervical epithelial cell line, HeLa, is one of the oldest and most commonly used cell lines in cell biology laboratories. Although a truncated P2X(7) receptor has recently been identified in HeLa cells, the expression of other purinergic receptors or the function of the P2X(7) protein has not been characterized. We here show that HeLa cells express transcripts for most P2X and P2Y purinergic receptors. Treatment of cells with ATP or other P2X(7) agonists does not stimulate cell death, but can induce atypical calcium fluxes and ion currents. Cervical epithelial cells represent an important target for sexually-transmitted pathogens and are commonly exposed to pro-inflammatory cytokines such as IFNgamma. Stimulation of HeLa cells with IFNgamma upregulates expression of P2X(7) mRNA and full-length protein, modifies ATP-dependent calcium fluxes, and renders the cells sensitive to ATP-induced apoptosis, which can be blocked by a P2X(7) antagonist. IFNgamma treatment also increased dramatically the sensitivity of the intestinal epithelial cell line, HCT8, to ATP-induced apoptosis. Significantly, IFNgamma also stimulated P2X(7) expression on human intestinal tissues. Responses to other purinergic receptor ligands suggest that HeLa cells may also express functional P2Y(1), P2Y(2) and P2Y(6) receptors, which could be relevant for modulating ion homeostasis in the cells.

ATP-induced P2X7-associated uptake of large molecules involves distinct mechanisms for cations and anions in macrophages

Macrophages express the P2X(7) receptor and other nucleotide (P2) receptors, and display the phenomenon of extracellular ATP (ATP(e))-induced P2X(7)-dependent membrane permeabilization, which occurs through a poorly understood mechanism. We used patch-clamp recordings, cytoplasmic Ca(2+) measurements and fluorescent dye uptake assays to compare P2X(7)-associated transport phenomena of macrophages and HEK-293 cells transfected with P2X(7) receptors (HEK-P2X(7) cells). Both cell types showed inward currents, increase of free cytoplasmic Ca(2+) concentration and the uptake of cationic dyes upon exposure to ATP(e), as previously described. However, in contrast to the macrophages, HEK-P2X(7) cells did not take up anionic dyes and did not display the 440 pS channels (Z pores) under cell-attached patch-clamping conditions. In addition, the transport mechanism of anionic dyes displayed by macrophages was also able to support dye efflux and, once activated at 37 degrees C, it remained active at 4 degrees C, whereas uptake of cationic dyes was temperature-dependent and unidirectional. Our results indicate that the mechanism of ATP(e)-induced dye uptake, usually called a ;permeabilization phenomenon' and associated with a ;permeabilization pore' can be ascribed to at least two distinct mechanisms in macrophages: a diffusional pathway, possibly associated with the 440 pS Z pores, and a cation uptake mechanism that is not diffusional and should be ascribed to an, as yet, unidentified transport mechanism.

P2Z purinoceptors

In response to tetra-anionic ATP4-, P2Z receptors signal opening of a non-selective plasma membrane pore which permits passage across cell membranes of ions, nucleotides and other small molecules that are usually membrane impermeant. P2Z receptor-induced pores on murine macrophages, macrophage-like cell lines and human culture-matured macrophages are permeable to molecules of up to 831 Da. The function of P2Z receptors is unknown. Also unknown is whether the binding site for ATP4- and the transmembrane pore, the properties that characterize P2Z receptors, reside on a single protein or reflect the activities of two or more proteins. That ATP(4-) -unresponsive cell lines do not express connexin 43 has led Beyer and Steinberg to suggest that opening or surface expression of this gap junction protein is induced by P2Z receptors. Xenopus oocytes, injected with cRNA transcribed from a pool of 100 cDNA clones isolated from a murine macrophage-derived cDNA library, and treated with ATP4-, express a non-selective membrane conductance characteristic of P2Z receptors. The conductance induced with cRNA is smaller than that induced by mRNA from macrophages, suggesting the presence of a dominant subunit of a multicomponent receptor in this pool of 100 cDNA clones.

Activation of ERK1/2 by extracellular nucleotides in macrophages is mediated by multiple P2 receptors independently of P2X7-associated pore or channel formation

Macrophages express several P2X and P2Y nucleotide receptors and display the phenomenon of ATP-induced P2X7-dependent membrane permeabilization, which occurs through a poorly understood mechanism. Several P2 receptors are known to be coupled to the activation of mitogen-activated protein kinases (MAPKs) and Ca2+ signaling. Here, we use macrophages to investigate the phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) by nucleotides and the involvement of MAPKs and intracellular Ca2+ concentration in ATP-induced membrane permeabilization. Short-term (5 min) pre-exposure to oxidized ATP (oATP), a P2X7 antagonist that does not inhibit P2X7-associated inward currents or membrane permeabilization, inhibits the activation of ERK1/2 by ATP, ADP, the P2X7 agonist 2'-3'-O-(4-benzoylbenzoyl)-ATP (BzATP), but not by UTP and UDP. We conclude that macrophages display several P2Y receptors coupled to the ERK1/2 pathway and that oATP antagonizes the action of purine nucleotides, possibly binding to P2X7 and/or other purine-binding P2Y receptors. We also show that BzATP and ATP activate ERK1/2 by two different pathways since ERK1/2 activation by BzATP, but not by ATP, is blocked by the tryrosine kinase inhibitor, genistein, and the Src protein kinase inhibitor, tyrphostin. However, the activation of ERK1/2 by ATP is blocked by the protein kinase C (PKC) inhibitor, chelerythrine chloride. Under the same conditions, membrane permeabilization is not blocked by genistein, tyrphostin, or chelerythrine chloride, indicating that tyrosine kinase, Src protein kinase, and PKC are not required for pore opening. Membrane permeabilization is independent of ERK1/2 activation since chelerythrine, or short-term exposure to oATP or PD98059, efficiently block ERK1/2 activation without inhibiting membrane permeabilization. In addition, membrane permeabilization is not inhibited by SB203580 and SB202190, two inhibitors of p38 MAPK, nor by intracellular BAPTA, which blocks ATP-induced Ca2+ signals. These results suggest that multiple P2 receptors lead to ERK1/2 activation, that ligation of the same receptors by agonists with different affinities can lead to differential stimulation of separate pathways, and that MAPKs and intracellular Ca2+ fluxes are independent of P2X7-associated pore formation.

Multiple P2X and P2Y receptor subtypes in mouse J774, spleen and peritoneal macrophages

We investigated P2 receptor expression and function in macrophages from mouse, and in the J774 cell line, and revealed a larger spectrum of P2 receptor subtypes than previously recognised. The nucleotides adenosine triphosphate (ATP), adenosine diphosphate, uridine triphosphate and uridine diphosphate evoked an increase in intracellular calcium and the activation of a potassium current. The sensitivity of these responses to the antagonists suramin, PPADS, MRS 2179 and Cibacron blue suggest the presence of at least three functional P2Y receptor subtypes, most probably P2Y(2), P2Y(4) and P2Y(6). ATP also activated P2X receptors, giving rise to a rapidly activating cation conductance. This response was insensitive to the antagonists suramin and Cibacron blue, was potentiated by Zn(2+) and inhibited by acidification suggesting involvement of P2X(4) receptors. In low divalent cation solution, responses to ATP became larger, and dibenzoyl-ATP became more potent than ATP, indicating the presence of P2X(7) receptors. Immunofluorescence, flow cytometry, Western blots and RT-PCR show that P2X(4) and P2X(7) receptors are the most prominent in both macrophage types, while the expression of the other P2X subunits is variable and sometimes weak or undetectable. These techniques also demonstrated the presence of mRNA for P2Y(1), P2Y(2), P2Y(4) and P2Y(6) receptors along with protein expression for the three subtypes we investigated, namely, P2Y(1), P2Y(2) and P2Y(4).

Are second messengers crucial for opening the pore associated with P2X7 receptor?

Stimulation of the P2X7 receptor by ATP induces cell membrane depolarization, increase in intracellular Ca2+ concentration, and, in most cases, permeabilization of the cell membrane to molecules up to 900 Da. After the activation of P2X7, at least two phenomena occur: the opening of low-conductance (8 pS) cationic channels and pore formation. At least two conflicting hypotheses have been postulated to reconcile these findings: 1) the P2X7 pore is formed as a result of gradual permeability increase (dilation) of cationic channels, and 2) the P2X7 pore represents a distinct channel, possibly activated by a second messenger and not directly by extracellular nucleotides. In this study, we investigated whether second messengers are necessary to open the pore associated with the P2X7 receptor in cells that expressed the pore activity by using the patch-clamp technique in whole cell and cell-attached configurations in conjunction with fluorescent imaging. In peritoneal macrophages and 2BH4 cells, we detected permeabilization and single-channel currents in the cell-attached configuration when ATP was applied outside the membrane patch in a condition in which oxidized ATP and Lucifer yellow were maintained within the pipette. Our data support Ca2+ as a second messenger associated with pore formation because the permeabilization depended on the presence of intracellular Ca2+ and was blocked by BAPTA-AM. In addition, MAPK inhibitors (SB-203580 and PD-98059) blocked the permeabilization and single-channel currents in these cells. Together our data indicate that the P2X7 pore depends on second messengers such as Ca2+ and MAP kinases.

Modulation of intercellular communication in macrophages: possible interactions between GAP junctions and P2 receptors

Gap junctions are connexin-formed channels that play an important role in intercellular communication in most cell types. In the immune system, specifically in macrophages, the expression of connexins and the establishment of functional gap junctions are still controversial issues. Macrophages express P2X(7) receptors that, once activated by the binding of extracellular ATP, lead to the opening of transmembrane pores permeable to molecules of up to 900 Da. There is evidence suggesting an interplay between gap junctions and P2 receptors in different cell systems. Thus, we used ATP-sensitive and -insensitive J774.G8 macrophage cell lines to investigate this interplay. To study junctional communication in J774-macrophage-like cells, we assessed cell-to-cell communication by microinjecting Lucifer Yellow. Confluent cultures of ATP-sensitive J774 cells (ATP-s cells) are coupled, whereas ATP-insensitive J774 cells (ATP-i cells), derived by overexposing J774 cells to extracellular ATP until they do not display the phenomenon of ATP-induced permeabilization, are essentially uncoupled. Western-blot and reverse-transcription polymerase chain reaction assays revealed that ATP-s and ATP-i cells express connexin43 (Cx43), whereas only ATP-s cells express the P2X(7) receptor. Accordingly, ATP-i cells did not display any detectable ATP-induced current under whole-cell patch-clamp recordings. Using immunofluorescence microscopy, Cx43 reactivity was found at the cell surface and in regions of cell-cell contact of ATP-s cells, whereas, in ATP-i cells, Cx43 immunoreactivity was only present in cytosolic compartments. Using confocal microscopy, it is shown here that, in ATP-s cells as well as in peritoneal macrophages, Cx43 and P2X(7) receptors are co-localized to the membrane of ATP-s cells and peritoneal macrophages.

Extracellular ATP induces oscillations of intracellular Ca2+ and membrane potential and promotes transcription of IL-6 in macrophages

The effects of low concentrations of extracellular ATP on cytosolic Ca(2+), membrane potential, and transcription of IL-6 were studied in monocyte-derived human macrophages. During inflammation or infection many cells secrete ATP. We show here that application of 10 microM ATP or 10 microM UTP induces oscillations in cytosolic Ca(2+) with a frequency of approximately 12 min(-1) and oscillations in membrane potential. RT-PCR analysis showed expression of P2Y(1), P2Y(2), P2Y(11), P2X(1), P2X(4), and P2X(7) receptors, large-conductance (KCNMA1 and KCNMB1-4), and intermediate-conductance (KCNN4) Ca(2+)-activated K(+) channels. The Ca(2+)oscillations were unchanged after removal of extracellular Ca(2+), indicating that they were mainly due to movements of Ca(2+) between intracellular compartments. Comparison of the effects of different nucleotides suggests that the Ca(2+) oscillations were elicited by activation of P2Y(2) receptors coupled to phospholipase C. Patch-clamp experiments showed that ATP induced a transient depolarization, probably mediated by activation of P2X(4) receptors, followed by membrane potential oscillations due to opening of Ca(2+)-activated K(+) channels. We also found that 10 microM ATP gamma S increased transcription of IL-6 approximately 40-fold within 2 h. This effect was abolished by blockade of P2Y receptors with 100 microM suramin. Our results suggest that ATP released from inflamed, damaged, or metabolically impaired cells represents a "danger signal" that plays a major role in activating the innate immune system.

Inhibition of chlamydial infectious activity due to P2X7R-dependent phospholipase D activation

Chlamydia trachomatis survives within host cells by inhibiting fusion between Chlamydia vacuoles and lysosomes. We show here that treatment of infected macrophages with ATP leads to killing of chlamydiae through ligation of the purinergic receptor, P2X(7)R. Chlamydial killing required phospholipase D (PLD) activation, as PLD inhibition led to rescue of chlamydiae in ATP-treated macrophages. However, there was no PLD activation nor chlamydial killing in ATP-treated P2X(7)R-deficient macrophages. P2X(7)R ligation exerts its effects by promoting fusion between Chlamydia vacuoles and lysosomes. P2X(7)R stimulation also resulted in macrophage death, but fusion with lysosomes preceded macrophage death and PLD inhibition did not prevent macrophage death. These results suggest that P2X(7)R ligation leads to PLD activation, which is directly responsible for inhibition of infection.

Procedures to characterize and study P2Z/P2X7 purinoceptor: flow cytometry as a promising practical, reliable tool

The expression of P2Z/P2X7 purinoceptor in different cell types is well established. This receptor is a member of the ionotropic P2X receptor family, which is composed by seven cloned receptor subtypes (P2X1 - P2X7). Interestingly, the P2Z/P2X7 has a unique feature of being linked to a non-selective pore which allows the passage of molecules up to 900 Da depending on the cell type. Early studies of P2Z/P2X7 purinoceptor were exclusively based on classical pharmacological studies but the recent tools of molecular biology have enriched the analysis of the receptor expression. The majority of assays and techniques chosen so far to study the expression of P2Z/P2X7 receptor explore directly or indirectly the effects of the opening of P2Z/P2X7 linked pore. In this review we describe the main techniques used to study the expression and functionality of P2Z/P2X7 receptor. Additionally, the increasing need and importance of a multifunctional analysis of P2Z/P2X7 expression based on flow cytometry technology is discussed, as well as the adoption of a more complete analysis of P2Z/P2X7 expression involving different techniques.

Gap junctions in the cardiovascular and immune systems

Gap junctions are clusters of intercellular channels directly connecting the cytoplasm of adjacent cells. These channels are formed by proteins named connexins and are present in all metazoan organisms where they serve diverse functions ranging from control of cell growth and differentiation to electric conduction in excitable tissues. In this overview we describe the presence of connexins in the cardiovascular and lympho-hematopoietic systems giving the reader a summary of the topics to be covered throughout this edition and a historical perspective of the discovery of gap junctions in the immune system.

Extracellular ATP in the lymphohematopoietic system: P2Z purinoceptors off membrane permeabilization

The effects of extracellular nucleosides and nucleotides on many organs and systems have been recognized for almost 50 years. The effects of extracellular ATP (ATPo), UTPo, ADPo, and other agonists are mediated by P2 purinoceptors. One of the most dramatic effects of ATPo is the permeabilization of plasma membranes to low molecular mass solutes of up to 900 Da. This effect is evident in several cells of the lymphohematopoietic system and is supposed to be mediated by P2Z, an ATP(4-)-activated purinoceptor. Here, we review some basic information concerning P2 purinoceptors and focus our attention on P2Z-associated phenomena displayed by macrophages. Using fluorescent dye uptake, measurement of free intracellular Ca2+ concentration and electrophysiological recordings, we elucidate some of the events that follow the application of ATP to the extracellular surface of macrophages. We propose a regulatory mechanism for the P2Z-associated permeabilization pore. The presence of P2 purinoceptors in cells of the lymphohematopoietic system makes them potential candidates to mediate immunoregulatory events.

P2Z purinoceptor-associated pores induced by extracellular ATP in macrophages and J774 cells

Millimolar concentrations of extracellular ATP (ATPo) can induce the permeabilization of plasma membranes of macrophages and other bone marrow-derived cells to low-molecular-weight solutes, a phenomenon that is the hallmark of P2Z purinoceptors. However, patch-clamp and whole cell electrophysiological experiments have so far failed to demonstrate the existence of any ATPo-induced P2Z-associated pores underlying this permeabilization phenomenon. Here, we describe ATPo-induced pores of 409 +/- 33 pS recorded using cell-attached patch-clamp experiments performed in macrophages and J774 cells. These pores are voltage dependent and display several properties of the P2Z-associated permeabilization phenomenon: they are permeable to both large cations and anions, such as tris(hydroxymethyl)aminomethane, N-methyl-D-glucamine, and glutamate; their opening is favored at temperatures higher than 30 degrees C; they are blocked by oxidized ATP and Mg2+; and they can be triggered by 3'-O-(4-benzoylbenzoyl)-ATP but not by UTP or ADP. We conclude that the pores described in this report are associated with the P2Z permeabilization phenomenon.

Outward potassium current oscillations in macrophage polykaryons: extracellular calcium entry and calcium-induced calcium release

Outward current oscillations associated with transient membrane hyperpolarizations were induced in murine macrophage polykaryons by membrane depolarization in the absence of external Na+. Oscillations corresponded to a cyclic activation of Ca(2+)-dependent K+ currents (IKCa) probably correlated with variations in intracellular Ca2+ concentration. Addition of external Na+ (8 mM) immediately abolished the outward current oscillations, suggesting that the absence of the cation is necessary not only for their induction but also for their maintenance. Oscillations were completely blocked by nisoldipine. Ruthenium red and ryanodine reduced the number of outward current cycles in each episode, whereas quercetin prolonged the hyperpolarization 2- to 15-fold. Neither low molecular weight heparin nor the absence of a Na+ gradient across the membrane had any influence on oscillations. The evidence suggests that Ca2+ entry through a pathway sensitive to Ca2+ channel blockers is elicited by membrane depolarization in Na(+)-free medium and is essential to initiate oscillations, which are also dependent on the cyclic release of Ca2+ from intracellular Ca(2+)-sensitive stores; Ca2+ ATPase acts by reducing intracellular Ca2+, thus allowing slow deactivation of IKCa. Evidence is presented that neither a Na+/Ca2+ antiporter nor Ca2+ release from IP3-sensitive Ca2+ stores participate directly in the mechanism of oscillation.

Role of cyclic ADP-ribose in ATP-activated potassium currents in alveolar macrophages

There is growing evidence that extracellular ATP causes a dramatic change in the membrane conductance of a variety of inflammatory cells. In the present study, using the nystatin perforated patch recording configuration, we found that ATP (0.3-30 microM) induced a transient outward current in a concentration-dependent manner and that the reversal potential of the ATP-induced outward current was close to the K+ equilibrium potential, indicating that the membrane behaves like a K+ electrode in the presence of ATP. The first application of ATP to alveolar macrophages perfused with Ca2+-free external solution could induce the outward current, but the response to ATP was diminished with successive applications. Intracellular perfusion with a Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid, also diminished the response. When cyclic ADP-ribose (cADPR) was applied to the macrophage cytoplasm, a transient outward current was elicited. Thereafter, the successive outward current was inhibited, suggesting the involvement of cADPR in the response. Intracellular perfusion with inositol 1,4, 5-trisphosphate also induced a transient outward current, but the successive current was not inhibited. The ATP-induced outward current was abolished when 8-amino-cADPR (as a blocker of cADPR, 10(-6)-10(-5) M) was introduced into the cytoplasm. Homogenates of alveolar macrophages showed both ADP-ribosyl cyclase and cADPR hydrolase activities, and CD38 (ADP-ribosyl cyclase/cADPR hydrolase) expression was confirmed by reverse transcriptase-polymerase chain reaction and Western blot analyses. These results indicate that ATP activates K+ currents by releasing Ca2+ from cADPR-sensitive internal Ca2+ stores.

Characterization of P2Z purinergic receptors on phagocytic cells of the thymic reticulum in culture

The thymic microenvironment is under intrinsic and extrinsic control circuits by several elements including hormones, neuropeptides, lymphokines, innervation and cell contact. P2 purinergic receptors have been described in a number of cells including macrophages, thymocytes, and other cells of the immune-inflammatory system. Here, we use the whole-cell patch-clamp technique and dye permeabilization assays to investigate the presence of ionic channels and purinergic receptors in one microenvironmental thymic component, namely the phagocytic cell of the thymic reticulum. At holding potentials ranging from -30 to -60 mV, applications of extracellular ATP in the vicinity of the cell membrane induce a transient and fast-activating inward current followed in most cells by an outward current. The whole event lasts 5-20 s. The inward current has a reversal potential close to 0 mV and the outward current can be ascribed to a Ca2+ -dependent K+ conductance. Both currents are inhibited by Mg2+, suggesting that the phenomenon is mediated by ATP4-. ATP-gamma-S can also induce both inward and outward currents. Exposure of phagocytic cells of the thymic reticulum to 5 mM ATP for 10 min induced permeabilization to lucifer yellow but not to the larger dyes trypan blue and rhodamine-dextran, suggesting a molecular weight cut-off smaller than 900. These observations lead us to conclude that phagocytic cells of the thymic reticulum express P2Z purinergic receptors that can mobilize Ca2+, induce the opening of ionic channels and permeabilize the cell membrane.

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.