Regulation by P2 agonists of the intracellular calcium concentration in epithelial cells freshly isolated from rat trachea

CALHM1-Mediated ATP Release and Ciliary Beat Frequency Modulation in Nasal Epithelial Cells

Mechanical stimulation of airway epithelial cells causes apical release of ATP, which increases ciliary beat frequency (CBF) and speeds up mucociliary clearance. The mechanisms responsible for this ATP release are poorly understood. CALHM1, a transmembrane protein with shared structural features to connexins and pannexins, has been implicated in ATP release from taste buds, but it has not been evaluated for a functional role in the airway. In the present study, Calhm1 knockout, Panx1 knockout, and wild-type mouse nasal septal epithelial cells were grown at an air-liquid interface (ALI) and subjected to light mechanical stimulation from an air puff. Apical ATP release was attenuated in Calhm1 knockout cultures following mechanical stimulation at a pressure of 55 mmHg for 50 milliseconds (p < 0.05). Addition of carbenoxolone, a PANX1 channel blocker, completely abolished ATP release in Calhm1 knockout cultures but not in wild type or Panx1 knockout cultures. An increase in CBF was observed in wild-type ALIs following mechanical stimulation, and this increase was significantly lower (p < 0.01) in Calhm1 knockout cultures. These results demonstrate that CALHM1 plays a newly defined role, complementary to PANX1, in ATP release and downstream CBF modulation following a mechanical stimulus in airway epithelial cells.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Rhinovirus-induced IL-1β release from bronchial epithelial cells is independent of functional P2X7

Airway epithelial cell defenses to viral infections are often compromised in disease or injury. Danger molecules, including ATP, are released during infection and contribute to nucleotide receptor-dependent inflammatory responses, largely through P2X(7). Although respiratory epithelium has been shown to express a variety of nucleotide receptors, the functional contribution of P2X(7) to the epithelial cell inflammatory response is unclear. We used human donor bronchial epithelial cells (BECs) and primary brushed epithelium to explore responses upon nucleotide and Toll-like receptor stimulation. P2X(7) messenger RNA and protein were observed in unprimed BECs, whereas inflammatory cytokine stimulation increased both messenger RNA and protein. Functional pore activity characteristic of P2X(7) was observed in BECs, and IL-1β was rapidly released by BECs after Toll-like receptor 3 agonist, polyinosine-polycytidylic acid, priming followed by ATP administration, although no change was observed in IL-18 release. BECs produced more IL-1β after stimulation with polyinosine-polycytidylic acid than LPS, showing a different preferential response than monocytes. In addition, blockade of nucleotide receptors with oxidized ATP significantly increased human rhinovirus (HRV) recovered 24 hours after infection in BECs, whereas 2'-3'-O-(4-benzoylbenzoyl) ATP treatment of brushed epithelial cells and respiratory cell lines nonsignificantly decreased HRV recovery. IL-1β release was detected after HRV infection in both BECs and brushed cells, but BzATP did not significantly increase IL-1β release further. BEC processing of pro-IL-1β to the mature, cleaved, 17-kD form was confirmed by Western blotting. These results support the expression of functional P2X(7) in human lung epithelium, although its role in epithelial pathogen defense is likely independent of IL-1 family cytokine processing.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

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.

Attenuated P2X7 pore function as a risk factor for virus-induced loss of asthma control

RATIONALE

Upper respiratory tract infection is a guideline accepted risk domain for the loss of asthma control. The ionotrophic nucleotide receptor P2X(7) regulates compartmentalized acute inflammation and the immune response to airway pathogens.

OBJECTIVES

We hypothesized that variability in P2X(7) function contributes to neutrophilic airway inflammation during a cold and thereby is linked to acute asthma.

METHODS

Research volunteers with asthma were enrolled at the onset of a naturally occurring cold and monitored through convalescence, assessing symptoms, lung function, and airway inflammation. P2X(7) pore activity in whole blood samples was measured using a genomically validated flow cytometric assay.

MEASUREMENTS AND MAIN RESULTS

Thirty-five participants with mild to moderate allergic asthma were enrolled and 31 completed all visits. P2X(7) pore function correlated with the change in nasal lavage neutrophil counts during the cold (R(s) = 0.514, P = 0.004) and was inversely related to the change in asthma symptoms (R(s) = -0.486, P = 0.009). The change in peak expiratory flow recordings, precold use of inhaled corticosteroids, and P2X(7) pore function were multivariate predictors of asthma symptoms (P = 0.001, < 0.001 and = 0.003 respectively). Attenuated P2X(7) activity was associated with the risk of losing asthma control (crude odds ratio, 11.0; 95% confidence interval, 1.1-106.4) even after adjustment for inhaled corticosteroids and rhinovirus (odds ratio, 15.0).

CONCLUSIONS

A whole blood P2X(7) pore assay robustly identifies participants with loss-of-function genotypes. Using this assay as an epidemiologic tool, attenuated P2X(7) pore activity may be a novel biomarker of virus-induced loss of asthma control.

P2X7 and phospholipid signalling: The search of the “missing link” in epithelial cells

The purinergic receptor P2X(7) is widely expressed in epithelial cells. This receptor shares in common with the other P2X receptors the ability to form a non-selective cation channel. On the other hand, the COOH terminus of P2X(7) seems to allow this receptor to couple to a spectrum of downstream effectors responsible for the regulation of cell death and pore formation among other functions. However, the coupling of P2X(7) to these downstream effectors, as well as the identity of possible adapters directly interacting with the receptor, remains poorly understood. Here we review the ability of P2X(7) to activate phospholipid signalling pathways in epithelial cells and propose this step as a possible link between the receptor and other downstream effectors. The P2X(7) ability to control the cellular levels of several lipid messengers (PA, AA, DAG, ceramide, etc.) through the modulation of phospholipases (C, A(2), D) and neutral sphingomyelinase is described. These pathways are sometimes regulated independently of the channel function of the receptor. Recent data concerning P2X(7) localization in lipid rafts is also discussed in relation to the coupling to these pathways and dissociation from channel function.

Pore properties and pharmacological features of the P2X receptor channel in airway ciliated cells

Airway ciliated cells express an ATP-gated P2X receptor channel of unknown subunit composition (P2X(cilia)) which is modulated by Na+ and by long exposures to ATP. P2X(cilia) was investigated by recording currents from freshly dissociated rabbit airway ciliated cells with the patch-clamp technique in the whole-cell configuration. During the initial continuous exposure to extracellular ATP, P2X(cilia) currents gradually increase in magnitude (priming), yet the permeability to N-methyl-D-glucamine (NMDG) does not change, indicating that priming does not arise from a progressive change in pore diameter. Na+, which readily permeates P2X(cilia) receptor channels, was found to inhibit the channel extracellular to the electric field. The rank order of permeability to various monovalent cations is: Li+, Na+, K+, Rb+, Cs+, NMDG+ and TEA+, with a relative permeability of 1.35, 1.0, 0.99, 0.91, 0.79, 0.19 and 0.10, respectively. The rank order for the alkali cations follows an Eisenman series XI for a high-strength field site. Ca2+ has been estimated to be 7-fold more permeant than Na+. The rise in [Ca2+]i in ciliated cells, induced by the activation of P2X(cilia), is largely inhibited by either Brilliant Blue G or KN-62, indicating that P2X7 may be a part of P2X(cilia). P2X(cilia) is augmented by Zn2+ and by ivermectin, and P2X4 receptor protein is detected by immunolabelling at the basal half of the cilia, strongly suggesting that P2X4 is a component of P2X(cilia) receptor channels. Taken together, these results suggest that P2X(cilia) is either assembled from P2X4 and P2X7 subunits, or formed from modified P2X4 subunits.

ATP regulation of ciliary beat frequency in rat tracheal and distal airway epithelium

Ciliary beat frequency (CBF) was measured by video-optical microscopy in rat tracheal and distal airway ciliary cells using a slice preparation. In tracheal ciliary cells (tracheal slice), ATP or 2-methylthio ATP (MeSATP) increased CBF, which was inhibited by suramin (100 microm, an inhibitor of purinergic receptor). Ionomycin (5 microm) or thapsigargin (2 microm) increased CBF similarly. Ca2+-free solution or addition of Ni2+ (1 mm) decreased CBF gradually by approximately 25% and subsequent stimulation with ATP (10 microm) increased CBF transiently. The purinergic agonist experiments demonstrated that ATP increases CBF in tracheal ciliary cells via both P2X and P2Y receptors. ATP increased the intracellular calcium concentration ([Ca2+]i) in tracheal ciliary cells. However, in distal airway ciliary cells (lung slice), ATP did not increase CBF and [Ca2+]i, although a Ca2+-free solution decreased CBF, and ionomycin (5 microm) or thapsigargin (2 microm) increased it. Moreover, acetylcholine (100 microm) did not increase CBF in distal airway ciliary cells, although it increased CBF in tracheal ciliary cells. Terbutaline (10 microm), a selective beta2-adrenergic agonist, increased CBF in both tracheal and distal airway ciliary cells. These observations suggest that the Ca2+-mobilization mechanisms via purinergic or muscarinic receptors of the distal airway ciliary cell may be different from those of the tracheal ciliary cell. In conclusion, the CBF increase is differently regulated in the tracheal and distal airway epithelia of the rat.

Hypo-osmotic potentiation of acetylcholine-stimulated ciliary beat frequency through ATP release in rat tracheal ciliary cells

The ciliary beat frequency (CBF) of rat tracheal ciliary cells in a slice preparation was measured using video-enhanced contrast (VEC) microscopy. Acetylcholine (ACh) increased CBF mediated via intracellular Ca2+ concentration ([Ca2+]i) in a dose-dependent manner. An adequate hypo-osmotic stress (-40 mosM) potentiated ACh-stimulated CBF increase in tracheal ciliary cells and shifted the ACh dose-response curve to the left (lower concentration side). This potentiation was independent of hypo-osmotic stresses applied ranging from -20 mosM to -90 mosM. A hypo-osmotic stress induces ATP release in many cell types. The present study demonstrated that suramin (an inhibitor of purinergic receptors) and apyrase (an ATPase/ADPase) eliminate the hypo-osmotic potentiation of ACh-stimulated CBF increase and that ATP increased [Ca2+]i and CBF, as well as potentiating ACh-stimulated rises in [Ca2+]i and CBF increase. Moreover, the apical surface of tracheal ciliary cells were stained immunopositive for the P2X4 purinergic receptor. A hypo-osmotic stress (-40 mosM) transiently increased [Ca2+]i and potentiated the ACh-stimulated [Ca2+]i increase. The hypo-osmotic potentiation of ACh-stimulated CBF increase was not detected under Ca2+-free conditions. These observations suggest that a hypo-osmotic stress stimulates ATP release from the trachea. The released ATP may induce further increases in [Ca2+]i and CBF in ACh-stimulated tracheal ciliary cells, which may be mediated by purinergic receptors, such as P2X4.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

Multiple effects of trichloroethanol on calcium handling in rat submandibular acinar cells

The effect of trichloroethanol (TCEt), the active metabolite of chloral hydrate, on the intracellular concentration of calcium ([Ca(2+)](i)) was investigated in rat submandibular glands (RSMG) acini loaded with fura-2. TCEt (1 - 10 mM) increased the [Ca(2+)](i) independently of the presence of calcium in the extracellular medium. Dichloroethanol (DCEt) and monochloroethanol (MCEt) reproduced the stimulatory effect of TCEt but at much higher concentrations (about 6 fold higher for DCEt and 20 fold higher for MCEt). TCEt mobilized an intracellular pool of calcium, which was depleted by a pretreatment with thapsigargin, an inhibitor of the sarcoplasmic and endoplasmic reticulum calcium-dependent ATPases, but not with FCCP, an uncoupler of mitochondria. TCEt 10 mM inhibited by 50% the thapsigargin-sensitive microsomal Ca(2+)-ATPase. DCEt 10 mM and MCEt 10 mM inhibited the ATPase by 20 and 10%, respectively. TCEt inhibited the increase of the [Ca(2+)](i) and the production of inositol phosphates in response to carbachol, epinephrine and substance P. TCEt inhibited the uptake of calcium mediated by the store-operated calcium channel (SOCC). ATP and Bz-ATP increased the [Ca(2+)](i) in RSMG acini and this effect was blocked by extracellular magnesium, by Coomassie blue and by oxydized ATP (oATP). TCEt potentiated the increase of the [Ca(2+)](i) and of the uptake of extracellular calcium in response to ATP and Bz-ATP. TCEt had no effect on the uptake of barium and of ethidium bromide in response to purinergic agonists. These results suggest that TCEt, at sedative concentrations, exerts various effects on the calcium regulation: (1) it mobilizes a thapsigargin-sensitive intracellular pool of calcium in RSMG acini; (2) it inhibits the uptake of calcium via the SOCC; (3) it inhibits the activation by G protein-coupled receptors of a polyphosphoinositide-specific phospholipase C. It does not interfere with the activation of the ionotropic P2X receptors. The use of chloral hydrate should be avoided in studies exploring the in vivo responses to sialagogues.

Modulation by propranolol of the uptake of ethidium bromide by rat submandibular acinar cells exposed to a P2X(7) agonist or to maitotoxin

We have compared the formation of pores in rat submandibular acinar cells in response to 2',3'-O-(4-benzoylbenzoyl) adenosine 5'-triphosphate (Bz-ATP) and maitotoxin. Bz-ATP (100 microM) permeabilized the cells to ethidium bromide. The uptake of ethidium increased to 29+/-1% of maximal uptake in 10 min. DL-Propranolol (300 microM) inhibited the Bz-ATP-induced uptake of ethidium bromide by 40% without affecting the P2X(7)-gated cation channel. The inhibitory effect of DL-propranolol on the formation of pores by Bz-ATP was reproduced by D-propranolol, an optical isomer with very poor beta-blocking activity. Tenidap, an antiinflammatory drug, enhanced the permeabilization in response to Bz-ATP. Propanolol inhibited the response to tenidap plus Bz-ATP. The effect of propranolol was reproduced by labetolol, a beta-adrenergic antagonist with membrane-stabilizing properties, but not by atenolol, which blocks beta-adrenergic receptors but has no effect on the stability of the membrane. In the presence of extracellular calcium, maitotoxin also increased the uptake of ethidium bromide. Tenidap had no effect on this response, which was delayed by propranolol. In conclusion, we have shown that propranolol, in a range of 10-300 microM, inhibits the pore-forming activity of the P2X(7) receptor without affecting the opening of the cation channel coupled to this receptor. This inhibition is not related to its beta-adrenergic blocking activity but rather to its membrane-stabilizing properties. Propranolol also delays the uptake of ethidium bromide in response to maitotoxin. This is in agreement with the current view that P2X(7) agonists and maitotoxin share a common pore.

Bromoenol lactone enhances the permeabilization of rat submandibular acinar cells by P2X7 agonists

The permeabilizing effect of P2X(7) agonists was tested in rat submandibular acinar cells using the uptake of ethidium bromide as an index. The uptake of ethidium bromide by acini incubated at 37 degrees C in the presence of 1 mM ATP increased with time and reached after 5 min about 10% of maximal uptake measured in the presence of digitonin. The response to ATP was dose-dependent (half-maximal concentration around 40 microM) and it was decreased when the temperature was lowered to 25 degrees C. Benzoyl-ATP reproduced the response to ATP (half-maximal concentration around 10 microM). UTP or 2-methylthioATP had no effect. The permeabilization in response to ATP was blocked by oxidized ATP and by magnesium and inhibited by Coomassie blue. ATP increased the activity of a calcium-insensitive phospholipase A(2) (iPLA(2)). Bromoenol lactone (BEL) inhibited the iPLA(2) stimulated by ATP but potentiated the uptake of ethidium bromide in response to the purinergic agonist. From these results it is concluded that the activation of P2X(7) receptors permeabilizes rat submandibular acinar cells. The pore-forming activity of the receptor might be negatively regulated by the concomitant activation of the iPLA(2) by the receptor.