Membrane-restricted regulation of Ca2+ release and influx in polarized epithelia

Extracellular Nucleotides and Histamine Suppress TLR3- and RIG-I-Mediated Release of Antiviral IFNs from Human Airway Epithelial Cells

Respiratory viruses stimulate the release of antiviral IFNs from the airway epithelium. Previous studies have shown that asthmatic patients show diminished release of type I and type III IFNs from bronchial epithelia. However, the mechanism of this suppression is not understood. In this study, we report that extracellular nucleotides and histamine, which are elevated in asthmatic airways, strongly inhibit release of type I and type III IFNs from human bronchial airway epithelial cells (AECs). Specifically, ATP, UTP, and histamine all inhibited the release of type I and type III IFNs from AECs induced by activation of TLR3, retinoic acid-inducible gene I (RIG-I), or cyclic GMP-AMP synthase-STING. This inhibition was at least partly mediated by Gq signaling through purinergic P2Y2 and H1 receptors, but it did not involve store-operated calcium entry. Pharmacological blockade of protein kinase C partially reversed inhibition of IFN production. Conversely, direct activation of protein kinase C with phorbol esters strongly inhibited TLR3- and RIG-I-mediated IFN production. Inhibition of type I and type III IFNs by ATP, UTP, histamine, and the proteinase-activated receptor 2 (PAR2) receptor agonist SLIGKV also occurred in differentiated AECs grown at an air-liquid interface, indicating that the suppression is conserved following mucociliary differentiation. Importantly, histamine and, more strikingly, ATP inhibited type I IFN release from human airway cells infected with live influenza A virus or rhinovirus 1B. These results reveal an important role for extracellular nucleotides and histamine in attenuating the induction of type I and III IFNs from AECs and help explain the molecular basis of the suppression of IFN responses in asthmatic patients.

Purinergic receptors in airway hydration

Airway epithelial purinergic receptors control key components of the mucociliary clearance (MCC), the dominant component of pulmonary host defense. In healthy airways, the periciliary liquid (PCL) is optimally hydrated, thus acting as an efficient lubricant layer over which the mucus layer moves by ciliary force. When the hydration of the airway surface decreases, the mucus becomes hyperconcentrated, the PCL collapses, and the "thickened" mucus layer adheres to cell surfaces, causing plaque/plug formation. Mucus accumulation is a major contributing factor to the progression of chronic obstructive lung diseases such as cystic fibrosis (CF) and chronic bronchitis (CB). Mucus hydration is regulated by finely tuned mechanisms of luminal Cl^- secretion and Na⁺ absorption with concomitant osmotically driven water flow. These activities are regulated by airway surface liquid (ASL) concentrations of adenosine and ATP, acting on airway epithelial A_2B and P2Y₂ receptors, respectively. The goal of this article is to provide an overview of our understanding of the role of purinergic receptors in the regulation of airway epithelial ion/fluid transport and the mechanisms of nucleotide release and metabolic activities that contribute to airway surface hydration in healthy and chronically obstructed airways.

Early Effect of Exogenous Na Hyaluronate on Mucociliary Clearance

Background

An animal study is performed to determine the early effect of 1% Na hyaluronate on mucociliary clearance function.

Methods

One percent of Na hyaluronate was introduced into the maxillary sinuses of rabbits by anterior antrostomy. A physiological solution of 1% NaCl was introduced into the maxillary sinuses of a control group to equalize the influence of Na for both groups. The treatment material was sprayed with an atomizer for coating the maxillary sinus lining. Technetium-99m diethylenetriamine pentaacetate dynamic scintigraphic imaging was performed to evaluate mucociliary clearance function on all rabbits 72 hours after the surgical procedure.

Results

Although the mean rate of mucociliary clearance of the Na hyaluronate group was slightly worse than the control group; there was no statistically significant difference between them.

Conclusion

There is no early effect of exogenous 1% Na hyaluronate on mucociliary clearance function.

Computational model for the regulation of extracellular ATP and adenosine in airway epithelia

Extracellular nucleotides are key components of the signaling network regulating airway clearance. They are released by the epithelium into the airway surface liquid (ASL) to stimulate cilia beating activity, mucus secretion and airway hydration. Understanding the factors affecting their availability for purinoceptor activation is an important step toward the development of new therapies for obstructive lung diseases. This chapter presents a mathematical model developed to gain predictive insights into the regulation of ASL nucleotide concentrations on human airway epithelia. The parameters were estimated from experimental data collected on polarized primary cultures of human nasal and bronchial epithelial cells. This model reproduces major experimental observations: (1) the independence of steady-state nucleotide concentrations on ASL height, (2) the impact of selective ectonucleotidase inhibitors on their steady-state ASL concentrations, (3) the changes in ASL composition caused by mechanical stress mimicking normal breathing, (4) and the differences in steady-state concentrations existing between nasal and bronchial epithelia. In addition, this model launched the study of nucleotide release into uncharted territories, which led to the discovery that airway epithelia release, not only ATP, but also ADP and AMP. This study shows that computational modeling, coupled to experimental validation, provides a powerful approach for the identification of key therapeutic targets for the improvement of airway clearance in obstructive respiratory diseases.

17β-Estradiol inhibits phosphorylation of stromal interaction molecule 1 (STIM1) protein: implication for store-operated calcium entry and chronic lung diseases

Sex plays a significant role in the development of lung diseases including asthma, cancer, chronic bronchitis, and cystic fibrosis. In cystic fibrosis, 17β-estradiol (E2) may inhibit store-operated Ca(2+) entry (SOCE) to impinge upon airway secretions, leaving females at greater risk of contracting lung infections. Stromal interaction molecule 1 (STIM1)-mediated SOCE is essential for cell homeostasis and regulates numerous processes including cell proliferation, smooth muscle contraction, and secretion. E2 can signal nongenomically to modulate Ca(2+) signaling, but little is known of the underlying mechanisms. We found that E2 exposure inhibited STIM1 translocation in airway epithelia, preventing SOCE. This correlated with a decrease in STIM1-STIM1 FRET and STIM1 mobility in E2-exposed HEK293T cells co-expressing estrogen receptor α. We also examined the role of STIM1 phosphorylation in E2-mediated inhibition of STIM1 mobility. STIM1 is basally phosphorylated at serine 575, which is required for SOCE. Exposure to E2 significantly decreased STIM1 serine phosphorylation. Mutating serine 575 to an alanine blocked STIM1 phosphorylation, reduced basal STIM1 mobility, and rendered STIM1 insensitive to E2. These data indicate that E2 can signal nongenomically by inhibiting basal phosphorylation of STIM1, leading to a reduction in SOCE.

Molecular modulation of airway epithelial ciliary response to sneezing

Our purpose was to evaluate the effect of the mechanical force of a sneeze on sinonasal cilia function and determine the molecular mechanism responsible for eliciting the ciliary response to a sneeze. A novel model was developed to deliver a stimulation simulating a sneeze (55 mmHg for 50 ms) at 26°C to the apical surface of mouse and human nasal epithelial cells. Ciliary beating was visualized, and changes in ciliary beat frequency (CBF) were determined. To interrogate the molecular cascades driving sneeze-induced changes of CBF, pharmacologic manipulation of intra- and extracellular calcium, purinergic, PKA, and nitric oxide (NO) signaling were performed. CBF rapidly increases by ≥150% in response to a sneeze, which is dependent on the release of adenosine triphosphate (ATP), calcium influx, and PKA activation. Furthermore, apical release of ATP is independent of calcium influx, but calcium influx and subsequent increase in CBF are dependent on the ATP release. Lastly, we observed a blunted ciliary response in surgical specimens derived from patients with chronic rhinosinusitis compared to control patients. Apical ATP release with subsequent calcium mobilization and PKA activation are involved in sinonasal ciliary response to sneezing, which is blunted in patients with upper-airway disease.

Measurements of intracellular calcium signals in polarized primary cultures of normal and cystic fibrosis human airway epithelia

The airways are continuously challenged by a variety of stimuli including bacteria, viruses, allergens, and inflammatory factors that act as agonists for G protein-coupled receptors (GPCR). Intracellular calcium (Ca(2+) (i)) mobilization in airway epithelia in response to extracellular stimuli regulates key airway innate defense functions, e.g., Ca(2+)-activated Cl(-) secretion, ciliary beating, mucin secretion, and inflammatory responses. Because Ca(2+) (i) mobilization in response to luminal stimuli is larger in CF vs. normal human airway epithelia, alterations in Ca(2+) (i) signals have been associated with the pathogenesis of CF airway disease. Hence, assessment of Ca(2+) (i) signaling has become an important area of CF research. This chapter will focus on measurements of cytoplasmic and mitochondrial Ca(2+) signals resulting from GPCR activation in polarized primary cultures of normal and CF human bronchial epithelia (HBE).

Introduction to section I: overview of approaches to study cystic fibrosis pathophysiology

Mutation of the CFTR chloride channel was identified as the genetic basis of cystic fibrosis over 20 years ago; however, correlation of the pathophysiological changes occurring in CF lung disease with the mutation of a chloride channel is ongoing. The failure of innate lung defense in CF, and the subsequent cyclical microbial colonization of airways, explains the gross anatomical changes that occur in CF pathophysiology. However, ongoing research is focused on how the lack of the CFTR channel explains the failure of innate lung defense. Hydration status of the mucus blanket is key to understanding this link, and this series of chapters details the recent progress that has been made in understanding the interplay between ion transport activity and innate lung defense, and the initiation of CF lung pathophysiology.

Mechanisms and function of DUOX in epithelia of the lung

The human lung produces considerable amounts of H(2)O(2). In the normal uninflamed epithelium of both the airways and the alveoli, mucosal release of H(2)O(2) is readily detected both in cell cultures in vitro and in the exhaled breath of humans. The dual oxidases DUOX1 and DUOX2 are the H(2)O(2)-producing isoforms of the NADPH oxidase family found in epithelial cells. The DUOXs are prominently expressed at the apical cell pole of ciliated cells in the airways and in type II cells of the alveoli. Recent studies focused on the functional consequences of H(2)O(2) release by DUOX into the lung lining fluid. In the airways, a major function of DUOX is to support lactoperoxidase (LPO) to generate bactericidal OSCN(-), and there are indications that the DUOX/LPO defense system is critically dependent on the function of the CFTR Cl(-) channel, which provides both SCN(-) (for LPO function) and HCO(3)(-) (for pH adjustment) to the airway surface liquid. Although DUOX is also functional in the alveolar epithelium, no comparable heme peroxidase is present in the alveolus, and thus DUOX-mediated H(2)O(2) release by alveolar cells may have other functions, such as cellular signaling.

Thrombin promotes release of ATP from lung epithelial cells through coordinated activation of rho- and Ca2+-dependent signaling pathways

Extracellular ATP controls key aspects of lung function via activation of epithelial cell purinergic receptors, but how ATP is released from cells remains poorly understood. To identify mechanistic components upstream of ATP release, we examined the effect of selected G protein coupled-receptor activation on ATP release from lung epithelial cells. The protease-activated receptor (PAR) agonist thrombin elicited a rapid Ca(2+)-dependent release of ATP from A549 cells. In contrast, the P2Y(2) receptor agonist UTP caused negligible ATP release, despite promoting a robust Ca(2+) response. Agonist-elicited ATP release was associated with Rho activation and was reduced in cells transfected with dominant negative mutants of p115-Rho GEF or RhoA, and by inhibitors of Rho kinase (ROCK). However, RhoA activation alone did not promote ATP release if temporally separated from Ca(2+) mobilization. PAR3 was the only PAR subtype detected in A549 cells by reverse transcription-PCR. Transfection of cells with human PAR3 cDNA increased thrombin-promoted ATP release, inositol phosphate formation, and RhoA activation. Conversely, small interference RNA against PAR3 diminished thrombin-evoked responses. Thrombin-elicited ATP release was accompanied by an enhanced cellular uptake of propidium iodide in a Ca(2+)- and ROCK-dependent manner and was inhibited by connexin/pannexin hemichannel blockers. Our data suggest that thrombin promotes ATP release from A549 cells via Rho- and Ca(2+)-dependent activation of connexin/pannexin hemichannels. The relevance of these findings is highlighted by the observation that exposure of primary cultures of well differentiated human bronchial epithelial cells to thrombin resulted in robust ATP release, which was inhibited by ROCK inhibitors and by connexin/pannexin hemichannel blockers.

P2Y receptors mediate Ca2+ signaling in duodenocytes and contribute to duodenal mucosal bicarbonate secretion

Since little is known about the role of P2Y receptors (purinoceptors) in duodenal mucosal bicarbonate secretion (DMBS), we sought to investigate the expression and function of these receptors in duodenal epithelium. Expression of P2Y(2) receptors was detected by RT-PCR in mouse duodenal epithelium and SCBN cells, a duodenal epithelial cell line. UTP, a P2Y(2)-receptor agonist, but not ADP (10 microM), significantly induced murine duodenal short-circuit current and DMBS in vitro; these responses were abolished by suramin (300 microM), a P2Y-receptor antagonist, or 2-aminoethoxydiphenyl borate (2-APB; 100 microM), a store-operated channel blocker. Mucosal or serosal addition of UTP induced a comparable DMBS in wild-type mice, but markedly impaired response occurred in P2Y(2) knockout mice. Acid-stimulated DMBS in vivo was significantly inhibited by suramin (1 mM) or PPADS (30 microM). Both ATP and UTP, but not ADP (1 microM), raised cytoplasmic-free Ca(2+) concentrations ([Ca(2+)](cyt)) with similar potencies in SCBN cells. ATP-induced [Ca(2+)](cyt) was attenuated by U-73122 (10 microM), La(3+) (30 microM), or 2-APB (10 microM), but was not significantly affected by nifedipine (10 microM). UTP (1 microM) induced a [Ca(2+)](cyt) transient in Ca(2+)-free solutions, and restoration of external Ca(2+) (2 mM) raised [Ca(2+)](cyt) due to capacitative Ca(2+) entry. La(3+) (30 microM), SK&F96365 (30 microM), and 2-APB (10 microM) inhibited UTP-induced Ca(2+) entry by 92, 87, and 94%, respectively. Taken together, our results imply that activation of P2Y(2) receptors enhances DMBS via elevation of [Ca(2+)](cyt) that likely results from an initial increase in intracellular Ca(2+) release followed by extracellular Ca(2+) entry via store-operated channel.

Apical adenosine regulates basolateral Ca2+-activated potassium channels in human airway Calu-3 epithelial cells

In airway epithelial cells, apical adenosine regulates transepithelial anion secretion by activation of apical cystic fibrosis transmembrane conductance regulator (CFTR) via adenosine receptors and cAMP/PKA signaling. However, the potent stimulation of anion secretion by adenosine is not correlated with its modest intracellular cAMP elevation, and these uncorrelated efficacies have led to the speculation that additional signaling pathways may be involved. Here, we showed that mucosal adenosine-induced anion secretion, measured by short-circuit current (Isc), was inhibited by the PLC-specific inhibitor U-73122 in the human airway submucosal cell line Calu-3. In addition, the Isc was suppressed by BAPTA-AM (a Ca2+ chelator) and 2-aminoethoxydiphenyl borate (2-APB; an inositol 1,4,5-trisphosphate receptor blocker), but not by PKC inhibitors, suggesting the involvement of PKC-independent PLC/Ca2+ signaling. Ussing chamber and patch-clamp studies indicated that the adenosine-induced PLC/Ca2+ signaling stimulated basolateral Ca2+-activated potassium (KCa) channels predominantly via A2B adenosine receptors and contributed substantially to the anion secretion. Thus, our data suggest that apical adenosine activates contralateral K+ channels via PLC/Ca2+ and thereby increases the driving force for transepithelial anion secretion, synergizing with its modulation of ipsilateral CFTR via cAMP/PKA. Furthermore, the dual activation of CFTR and KCa channels by apical adenosine resulted in a mixed secretion of chloride and bicarbonate, which may alter the anion composition in the secretion induced by secretagogues that elicit extracellular ATP/adenosine release. Our findings provide novel mechanistic insights into the regulation of anion section by adenosine, a key player in the airway surface liquid homeostasis and mucociliary clearance.

Localized cytosolic alkalization and its functional impact in ciliary cells

Using confocal microscopy we demonstrate that ciliary cells from airway epithelium maintain two qualitatively distinct cytosolic regions in terms of pH regulation. While the bulk of the cytosol is stringently buffered and is virtually insensitive to changes in extracellular pH (pHo), the values of cytosolic pH in the vicinity of the ciliary membrane is largely determined by pHo. Variation of pHo from 6.2 up to 8.5 failed to affect ciliary beat frequency (CBF). Application of NH(4)Cl induced profound localized alkalization near cilia, which did not depress ciliary activity, but resulted in strong and prolonged enhancement of CBF. Calmodulin and protein kinase A (PKA) functionality was essential for the alkalization-induced CBF enhancement. We suggest that the ability of airway epithelium to sustain unusually strong but localized cytosolic alkalization near cilia facilitates CBF enhancement through altering the binding constants of Ca2+ to calmodulin and promotion of Ca2+-calmodulin complex formation. The NH4Cl-induced elevations in cytosolic pH and Ca2+ concentration act synergistically to activate calmodulin-dependent processes, cAMP pathway, and, thereby, stimulate CBF.

Cyclic AMP regulates bicarbonate secretion in cholangiocytes through release of ATP into bile

BACKGROUND & AIMS

Bicarbonate secretion is a primary function of cholangiocytes. Either adenosine 3',5'-cyclic monophosphate (cAMP) or cytosolic Ca(2+) can mediate bicarbonate secretion, but these are thought to act through separate pathways. We examined the role of the inositol 1,4,5-trisphosphate receptor (InsP3R) in mediating bicarbonate secretion because this is the only intracellular Ca(2+) release channel in cholangiocytes.

METHODS

Intrahepatic bile duct units (IBDUs) were microdissected from rat liver then luminal pH was examined by confocal microscopy during IBDU microperfusion. Cyclic AMP was increased using forskolin or secretin, and Ca(2+) was increased using acetylcholine (ACh) or adenosine triphosphate (ATP). Apyrase was used to hydrolyze extracellular ATP, and suramin was used to block apical P2Y ATP receptors. In selected experiments, IBDUs were pretreated with short interfering RNA (siRNA) to silence expression of specific InsP3R isoforms.

RESULTS

Both cAMP and Ca(2+) agonists increased luminal pH. The effect of ACh on luminal pH was reduced by siRNA for basolateral (types I and II) but not apical (type III) InsP3R isoforms. The effect of forskolin on luminal pH was reduced by a cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor and by siRNA for the type III InsP3R. Luminal apyrase or suramin blocked the effects of forskolin but not ACh on luminal pH.

CONCLUSIONS

Cyclic AMP-induced ductular bicarbonate secretion depends on an autocrine signaling pathway that involves CFTR, apical release of ATP, stimulation of apical nucleotide receptors, and then activation of apical, type III InsP3Rs. The primary role of CFTR in bile duct secretion may be to regulate secretion of ATP rather than to secrete chloride and/or bicarbonate.

ATP release and its prejunctional modulation

We studied some properties of the release of noradrenaline and ATP in isolated sympathetically innervated tissues. Release was elicited by electric stimulation and assessed as overflow of tritiated compounds (after labelling with [3H]noradrenaline) and enzymically measured ATP, respectively. Evans blue, which inhibits ectonucleotidases, greatly increased the evoked overflow of ATP, indicating that a major part of the ATP was metabolized after release. Much of the ATP was postjunctional in origin. The neural fraction was isolated when postjunctional release was suppressed by prazosin (alpha 1-adrenoceptor antagonist) and suramin (P2 purinoceptor antagonist). Comparison of neural ATP and [3H]-noradrenaline release showed that prostaglandin E2 reduced the release of both co-transmitters to a similar extent. Activation of prejunctional alpha 2-adrenoceptors, however, preferentially reduced the release of [3H]noradrenaline, and activation of prejunctional A1 purinoceptors reduced preferentially the release of ATP. Nucleotides such as ATP depressed the release of [3H]noradrenaline through two receptors: the well-known prejunctional A1 receptors and a separate group of prejunctional P2 purinoceptors. P2 antagonists increased the release of [3H]-noradrenaline. Overall, the results indicate differential storage, release and modulation of release of the two sympathetic co-transmitters. They also indicate that postganglionic sympathetic axons possess receptors for both co-transmitters: alpha 2 and P2 autoreceptors.

Airway surface dehydration in cystic fibrosis: pathogenesis and therapy

Cystic fibrosis (CF) lung disease reflects the failure of airways defense against chronic bacterial infection. Studies of CF cultures, transgenic mice, and CF patients suggest that the initiating event in CF airways disease pathogenesis is reduced airway surface liquid (ASL) volume, i.e., dehydration. CF ASL volume regulation depends on a single extracellular signaling system, ATP, which renders CF airways more vulnerable to disease-causing insults (e.g., viruses) than are normal airways, which regulate ASL volume by dual ATP and adenosine signaling pathways. Clinical studies have explored the hypothesis that treating the dehydration of CF airways will be therapeutically beneficial. Inhaled hypertonic saline osmotically draws water onto airway surfaces, improves mucus clearance and pulmonary function, and reduces acute exacerbations in CF patients. Thus, rehydration therapies may slow the progression of CF lung disease in patients with established bacterial infection and may prevent the onset of CF lung disease if initiated early in life.

Endogenous ATP release inhibits electrogenic Na⁺ absorption and stimulates Cl⁻ secretion in MDCK cells

Our previous studies with a line of Madin-Darby canine kidney (MDCK) cells (FL-MDCK) transfected with FLAG-labeled alpha, beta, and gamma subunits of epithelial Na(+) channel (ENaC) showed that, although most of the short-circuit current (I (sc)) was amiloride sensitive (AS-I (sc)), there was also an amiloride-insensitive component (NS-I (sc)) due to Cl(-) secretion (Morris and Schafer, J Gen Physiol 120:71-85, 2002). In the present studies, we observed a progressive increase in NS-I (sc) and a corresponding decrease in AS-I (sc) during experiments. There was a significant negative correlation between AS-I (sc) and NS-I (sc) both in the presence and absence of treatment with cyclic adenosine monophosphate (cAMP). NS-I (sc) could be attributed to both cystic fibrosis transmembrane conductance regulator (CFTR) and a 4, 4'-diisothiocyano-2, 2'-disulfonic acid stilbene (DIDS)-sensitive Ca(2+)-activated Cl(-) channel (CaCC). Continuous perfusion of both sides of the Ussing chamber with fresh rather than recirculated bathing solutions, or addition of hexokinase (6 U/ml), prevented the time-dependent changes and increased AS-I (sc) by 40-60%, with a proportional decrease in NS-I (sc). Addition of 100 muM adenosine triphosphate (ATP) in the presence of luminal amiloride produced a transient four-fold increase in NS-I (sc) that was followed by a sustained increase of 50-60% above the basal level. ATP release from the monolayers, measured by bioluminescence, was found to occur across the apical but not the basolateral membrane, and the apical release was tripled by cAMP treatment. These data show that constitutive apical ATP release, which occurs under both basal and cAMP-stimulated conditions, underlies the time-dependent rise in Cl(-) secretion and the proportional fall in ENaC-mediated Na(+) absorption in FL-MDCK cells. Thus, endogenous ATP release can introduce a significant confounding variable in experiments with this and similar epithelial cells, and it may underlie at least some of the observed interaction between Cl(-) secretion and Na(+) absorption.

Forces applied by cilia measured on explants from mucociliary tissue

Forces applied by intact mucus-propelling cilia were measured for the first time that we know of using a combined atomic force microscopy (AFM) and electrooptic system. The AFM probe was dipped into a field of beating cilia and its time-dependent deflection was recorded as it was struck by the cilia while the electrooptic system simultaneously and colocally measured the frequency to ensure that no perturbation was induced by the AFM probe. Using cilia from frog esophagus, we measured forces of approximately 0.21 nN per cilium during the effective stroke. This value, together with the known internal structure of these cilia, leads to the conclusion that most dynein arms along the length of the axoneme contribute to the effective stroke of these cilia.

Calcium signaling in human airway goblet cells following purinergic activation

Despite the general importance of Ca(2+) signaling in signal transduction, and of goblet cell mucin hypersecretion in inflammatory pulmonary diseases, measurement of airway goblet cell intracellular Ca(2+) (Ca(i)(2+)) has not been reported. In this article, we describe the results of experiments measuring Ca(i)(2+) in primary cultures of human bronchial goblet cells after stimulation with the purinergic agonist adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) and phorbol 12-myristate 13-acetate (PMA). Ca(2+) signaling in human goblet cells after purinergic stimulation follows the classic paradigm of a Ca(i)(2+) transient from a basal activity of 110 nM to a peak response of 260.1 +/- 41.2 nM within 2 min, followed by a long superbasal plateau (155.3 +/- 0.2 nM) between 10 and 15 min. The rise in Ca(i)(2+) appears to result from a mobilization of intracellular stores, because the transient was nearly abolished by inhibition of PLC with the phosphatidylinositol-specific PLC inhibitor U-73122, and it was not affected significantly by removal of extracellular Ca(2+). Loading goblet cells with BAPTA inhibited the ATPgammaS-induced Ca(2+) transient by 86.0 +/- 13.1%, relative to control. Finally, in contrast to the massive effects of high doses of PMA (300 nM) on mucin secretion from goblet cells, phorbol ester stimulated a small (27.1 +/- 7% of the ATPgammaS control peak), brief rise in Ca(i)(2+). This diminutive signal likely denotes a local Ca(2+) gradient, which may be associated with the mucin granule exocytotic process.

Roles of Ca2+and protein kinase C in the excitatory response to serotonin in embryonic molluscan ciliary cells

We examined the roles of Ca2+and protein kinase C (PKC) in the cilio-excitatory response to serotonin in pedal ciliary cells from Helisoma trivolvis embryos. Serotonin (5-hydroxytryptamine; 5-HT; 100 µmol/L) induced an increase in ciliary beat frequency (CBF) was abolished by microinjected BAPTA (50 mmol/L), but was only partially inhibited by the phospholipase C inhibitor U-73122 (10 µmol/L). The diacylglycerol analogs 1-oleoyl-2-acetyl-sn-glycerol (100 µmol/L) and 1,2-dioctanoyl-sn-glycerol (100 µmol/L) caused increases in [Ca2+]ithat were smaller than those induced by serotonin. In the absence of extracellular Ca2+, 1,2-dioctanoyl-sn-glycerol (100 µmol/L) failed to elicit an increase in both CBF and [Ca2+]i. In contrast, the serotonin-induced increase in CBF persisted in the absence of extracellular Ca2+, although the increase in [Ca2+]iwas abolished. PKC inhibitors bisindolylmaleimide (10 and 100 nmol/L) and calphostin C (10 nmol/L) partially inhibited the serotonin-induced increase in CBF, but didn’t affect the serotonin-induced change in [Ca2+]i. These findings suggest that an intracellular store-dependent increase in [Ca2+]imediates the cilio-excitatory response to serotonin. Furthermore, although PKC is able to cause an increase in [Ca2+]ithrough calcium influx, it contributes to the cilio-excitatory response to 5-HT through a different mechanism.

The role of epithelial P2Y2 and P2Y4 receptors in the regulation of intestinal chloride secretion

UTP-induced chloride secretion by the intestinal mucosa mounted in Ussing chambers was assessed by measurement of the short-circuit current (I(sc)) in the presence of phloridzin in the case of jejunum or amiloride in the case of colon to eliminate any contribution of electrogenic Na(+) movement to the net ionic transport. Since we have previously demonstrated the absence of chloride-secretory response to apical UTP in the jejunum from P2Y(4)-null mice, in the present study we studied the response to basolateral UTP in the jejunum and to either apical or basolateral UTP in the colon, in both P2Y(2)- and P2Y(4)-deficient mice. In the jejunum, the chloride-secretory response to basolateral UTP was partially reduced in both P2Y(2)- (40%) and P2Y(4)- (60%) null mice. In the colon, both apical or basolateral UTP increased the I(sc). That response was abolished in a chloride-free medium. The colonic chloride-secretory response to either basolateral or apical UTP was abolished in P2Y(4)-deficient mice, but not significantly affected in P2Y(2)-deficient mice. The chloride-secretory response to forskolin was potentiated by prior basolateral addition of UTP and this potentiation was abolished in P2Y(4)-null mice. The jejunum of mice homozygous for the DeltaF508 mutation of cystic fibrosis transmembrane conductance regulator was responsive to UTP, but the magnitude of that response was smaller than in the wild-type littermates. In conclusion, the P2Y(4) receptor fully mediates the chloride-secretory response to UTP in both small and large intestines, except at the basolateral side of the jejunum, where both P2Y(2) and P2Y(4) receptors are involved.

Membrane capacitance and conductance changes parallel mucin secretion in the human airway epithelium

Measurement of the magnitude and kinetics of exocytosis from intact epithelia has historically been difficult. Using well-differentiated cultures of human bronchial epithelial cells, we describe the use of transepithelial impedance analysis to enable the real-time quantification of mucin secretagogue-induced changes in membrane capacitance (surface area) and conductance. ATPgammaS, UTP, ionomycin, and PMA induced robust increases in total cellular capacitance that were demonstrated to be dominated by a specific increase in apical membrane surface area. The UTP-induced increase in capacitance occurred in parallel with goblet cell emptying and the secretion of mucin and was associated with decreases in apical and basolateral membrane resistances. The magnitude and kinetics of the capacitance increases were dependent on the agonist and the sidedness of the stimulation. The peak increase in capacitance induced by UTP was approximately 30 mucin granule fusions per goblet cell. Secretagogue-induced decreases in apical membrane resistance were independent of exocytosis, although each of the secretagogues induced profound reductions in basolateral membrane resistance. Transepithelial impedance analysis offers the potential to study morphological and conductance changes in cultured human bronchial epithelial cells.

Cystic fibrosis airway epithelial Ca2+ i signaling: the mechanism for the larger agonist-mediated Ca2+ i signals in human cystic fibrosis airway epithelia

In cystic fibrosis (CF) airways, abnormal epithelial ion transport likely initiates mucus stasis, resulting in persistent airway infections and chronic inflammation. Mucus clearance is regulated, in part, by activation of apical membrane receptors coupled to intracellular calcium (Ca(2+)(i)) mobilization. We have shown that Ca(2+)(i) signals resulting from apical purinoceptor (P2Y(2)-R) activation are increased in CF compared with normal human airway epithelia. The present study addressed the mechanism for the larger apical P2Y(2)-R-dependent Ca(2+)(i) signals in CF human airway epithelia. We show that the increased Ca(2+)(i) mobilization in CF was not specific to P2Y(2)-Rs because it was mimicked by apical bradykinin receptor activation, and it did not result from a greater number of P2Y(2)-R or a more efficient coupling between P2Y(2)-Rs and phospholipase C-generated inositol 1,4,5-trisphosphate. Rather, the larger apical P2Y(2)-R activation-promoted Ca(2+)(i) signals in CF epithelia resulted from an increased density and Ca(2+) storage capacity of apically confined endoplasmic reticulum (ER) Ca(2+) stores. To address whether the ER up-regulation resulted from ER retention of misfolded DeltaF508 CFTR or was an acquired response to chronic luminal airway infection/inflammation, three approaches were used. First, ER density was studied in normal and CF sweat duct human epithelia expressing high levels of DeltaF508 CFTR, and it was found to be the same in normal and CF epithelia. Second, apical ER density was morphometrically analyzed in airway epithelia from normal subjects, DeltaF508 homozygous CF patients, and a disease control, primary ciliary dyskinesia; it was found to be greater in both CF and primary ciliary dyskinesia. Third, apical ER density and P2Y(2)-R activation-mobilized Ca(2+)(i), which were investigated in airway epithelia in a long term culture in the absence of luminal infection, were similar in normal and CF epithelia. To directly test whether luminal infection/inflammation triggers an up-regulation of the apically confined ER Ca(2+) stores, normal airway epithelia were chronically exposed to supernatant from mucopurulent material from CF airways. Supernatant treatment expanded the apically confined ER, resulting in larger apical P2Y(2)-R activation-dependent Ca(2+)(i) responses, which reproduced the increased Ca(2+)(i) signals observed in CF epithelia. In conclusion, the mechanism for the larger Ca(2+)(i) signals elicited by apical P2Y(2)-R activation in CF airway epithelia is an expansion of the apical ER Ca(2+) stores triggered by chronic luminal airway infection/inflammation. Greater ER-derived Ca(2+)(i) signals may provide a compensatory mechanism to restore, at least acutely, mucus clearance in CF airways.

Inositol trisphosphate analogues selective for types I and II inositol trisphosphate receptors exert differential effects on vasopressin-stimulated Ca2+ inflow and Ca2+ release from intracellular stores in rat hepatocytes

Previous studies have shown that adenophostin A is a potent initiator of the activation of SOCs (store-operated Ca2+ channels) in rat hepatocytes, and have suggested that, of the two subtypes of Ins(1,4,5)P3 receptor predominantly present in rat hepatocytes [Ins(1,4,5)P3R1 (type I receptor) and Ins(1,4,5)P3R2 (type II receptor)], Ins(1,4,5)P3R1s are required for SOC activation. We compared the abilities of Ins(1,4,6)P3 [with higher apparent affinity for Ins(1,4,5)P3R1] and Ins(1,3,6)P3 and Ins(1,2,4,5)P4 [with higher apparent affinities for Ins(1,4,5)P3R2] to activate SOCs. The Ins(1,4,5)P3 analogues were microinjected into single cells together with fura 2, and dose-response curves for the activation of Ca2+ inflow and Ca2+ release from intracellular stores obtained for each analogue. The concentration of Ins(1,4,6)P3 which gave half-maximal stimulation of Ca2+ inflow was substantially lower than that which gave half-maximal stimulation of Ca2+ release. By contrast, for Ins(1,3,6)P3 and Ins(1,2,4,5)P3, the concentration which gave half-maximal stimulation of Ca2+ inflow was substantially higher than that which gave half-maximal stimulation of Ca2+ release. The distribution of Ins(1,4,5)P3R1 and Ins(1,4,5)P3R2 in rat hepatocytes cultured under the same conditions as those employed for the measurement of Ca2+ inflow and release was determined by immunofluorescence. Ins(1,4,5)-P3R1s were found predominantly at the cell periphery, whereas Ins(1,4,5)P3R2s were found at the cell periphery, the cell interior and nucleus. It is concluded that the idea that a small region of the endoplasmic reticulum enriched in Ins(1,4,5)P3R1 is required for the activation of SOCs is consistent with the present results for hepatocytes.

Intracellular Ca2+ regulates the phosphorylation and the dephosphorylation of ciliary proteins via the NO pathway

The phosphorylation profile of ciliary proteins under basal conditions and after stimulation by extracellular ATP was investigated in intact tissue and in isolated cilia from porcine airway epithelium using anti-phosphoserine and anti-phosphothreonine specific antibodies. In intact tissue, several polypeptides were serine phosphorylated in the absence of any treatment (control conditions). After stimulation by extracellular ATP, changes in the phosphorylation pattern were detected on seven ciliary polypeptides. Serine phosphorylation was enhanced for three polypeptides (27, 37, and 44 kD), while serine phosphorylation was reduced for four polypeptides (35, 69, 100, and 130 kD). Raising intracellular Ca²⁺ with ionomycin induced identical changes in the protein phosphorylation profile. Inhibition of the NO pathway by inhibiting either NO syntase (NOS), guanylyl cyclase (GC), or cGMP-dependent protein kinase (PKG) abolished the changes in phosphorylation induced by ATP. The presence of PKG within the axoneme was demonstrated using a specific antibody. In addition, in isolated permeabilized cilia, submicromolar concentrations of cGMP induced protein phosphorylation. Taken together, these results suggest that the axoneme is an integral part of the intracellular NO pathway. The surprising observation that ciliary activation is accompanied by sustained dephosphorylation of ciliary proteins via NO pathway was not detected in isolated cilia, suggesting that the protein phosphatases were either lost or deactivated during the isolation procedure. This work reveals that any pharmacological manipulation that abolished phosphorylation and dephosphorylation also abolished the enhancement of ciliary beating. Thus, part or all of the phosphorylated polypeptides are likely directly involved in axonemal regulation of ciliary beating.

P2Y2-receptor-mediated activation of a contralateral, lanthanide-sensitive calcium entry pathway in the human airway epithelium

1. Receptor-mediated calcium entry (RMCE) was examined in well-differentiated cultures of normal human bronchial epithelial cells (HBECs). Changes in intracellular free Ca(2+) ([Ca(2+)](i)) were quantified using fluorescence ratio imaging of Fura-2-loaded cells during perfusion with Ca(2+) mobilizing agonists. 2. Initial studies revealed an agonist potency of ATP=uridine triphosphate (UTP) >ADP=uridine diphosphate, consistent with purinergic activation of an apical P2Y(2)-receptor mediating the increase in [Ca(2+)](i) in HBECs. 3. Apical UTP (30 microm) induced a sustained period of elevated [Ca(2+)](i) between 300 and 600 s following agonist stimulation that extracellular Ca(2+) free studies indicated was dominated by Ca(2+) influx. 4. RMCE was inhibited by 100 nm La(3+) (83+/-3%) or Gd(3+) (95+/-7%) (P<0.005, n=4-11) and was partially attenuated by Ni(2+) (1 mm) (58.7+/-5.0%, P<0.005, n=9). 5. RMCE was also partially sensitive (< 25% inhibition, P<0.01) to the cation channel blockers SKF96365 (30 microm) and econazole (30 microm), but was insensitive to both verapamil (1 microm) and ruthenium red (10 microm). 6. Using either a sided Ca(2+) readdition protocol or unilateral La(3+), established that the RMCE pathway was located exclusively on the basolateral membrane. 7. The pharmacological sensitivity of the P2Y(2)-receptor activated Ca(2+) entry pathway in the human airway epithelium is inconsistent with the established profile of TRP channel families and is therefore likely to be of an as-yet uncharacterized molecular identity.

The mitochondrial barriers segregate agonist-induced calcium-dependent functions in human airway epithelia

In airway epithelia, purinergic receptor (P2Y₂-R) stimulation of intracellular calcium (Ca²⁺_i)–regulated ion transport is restricted to the membrane domain ipsilateral to receptor activation, implying compartmentalization of Ca²⁺_i signaling. Because mitochondria can spatially restrict cellular Ca²⁺_i signals, immunocytochemical, electron microscopic, and fluorescent studies of mitochondria localization were performed in human airway epithelia. Although concentrated at the apical domain, mitochondria were found distributed at both the apical and the basolateral poles and in close association with the endoplasmic reticulum. The role of mitochondria in locally restricting P2Y₂-R–induced Ca²⁺_i signals was investigated by measuring changes in mitochondrial Ca²⁺ (Ca²⁺_m) in human airway epithelial monolayers. P2Y₂-R activation induced Ca²⁺_m accumulation in mitochondria confined to the domain ipsilateral to P2Y₂-R stimulation, which was blocked by mitochondrial uncoupling with 1 μM CCCP and 2.5 μg/ml oligomycin. The role of mitochondria in restricting the cellular cross-talk between basolateral P2Y₂-R–dependent Ca²⁺_i mobilization and apical membrane Ca²⁺-activated Cl⁻ secretion was investigated in studies simultaneously measuring Ca²⁺_i and Cl⁻ secretion in cystic fibrosis human airway epithelial monolayers. Activation of basolateral P2Y₂-Rs produced similar increases in Ca²⁺_i in monolayers without and with pretreatment with uncouplers, whereas Ca²⁺_i-activated Cl⁻ secretion was only efficiently triggered in mitochondria-uncoupled conditions. We conclude that (a) mitochondria function as a Ca²⁺_i-buffering system in airway epithelia, compartmentalizing Ca²⁺_i-dependent functions to the membrane ipsilateral to receptor stimulation; and (b) the mitochondria provide structural barriers that protect the airway epithelia against nonspecific activation of Ca²⁺_i-modulated functions associated with Ca²⁺_i signals emanating from the apical or the basolateral membrane domains.

Two types of store-operated Ca2+ channels with different activation modes and molecular origin in LNCaP human prostate cancer epithelial cells

The one or more coupling mechanisms of store-operated channels (SOCs) to endoplasmic reticulum (ER) Ca2+ store depletion as well as the molecular identity of SOCs per se still remain a mystery. Here, we demonstrate the co-existence of two populations of molecular distinct endogenous SOCs in LNCaP prostate cancer epithelial cells, which are preferentially activated by either active inositol 1,4,5-trisphosphate (IP3)-mediated or passive thapsigargin-facilitated store depletion and have different ER store content sensitivity. The first population, called SOC(CC) (for "conformational coupling"), is characterized by preferential IP3 receptor-dependent mode of activation, as judged from sensitivity to cytoskeleton modifications, and dominant contribution of transient receptor potential (TRP) TRPC1 within it. The second one, called SOC(CIF) (for "calcium influx factor"), depends on Ca(2+)-independent phospholipase A2 for activation with probable CIF involvement and is mostly represented by TRPC4. The previously identified SOC constituent in LNCaP cells, TRPV6, seems to play equal role in both SOC populations. These results provide new insight into the nature of SOCs and their representation in the single cell type as well as permit reconciliation of current SOC activation hypotheses.

Metabolism of P2 receptor agonists in human airways: implications for mucociliary clearance and cystic fibrosis

Extracellular nucleotides are among the most potent mediators of mucociliary clearance (MCC) in human lungs. However, clinical trials revealed that aerosolized nucleotides provide only a transient improvement of MCC to patients diagnosed with cystic fibrosis (CF). In this study, we identified the mechanism that eliminates extracellular nucleotides from human airways. Polarized primary cultures of human bronchial epithelial cells were impermeable to extracellular nucleotides but rapidly dephosphorylated ATP into ADP, AMP, and adenosine. The half-life of a therapeutic ATP concentration (0.1 mm) was approximately 20 s within the periciliary liquid layer. The mucosal epithelial surface eliminated P2 receptor agonists (ATP = UTP > ADP > UDP) at 3-fold higher rates than the serosal surface. We also showed that mucosal (not serosal) ectoATPase activity increases toward areas most susceptible to airway obstruction (nose < bronchi << bronchioles). Bronchial cultures from patients with CF, primary ciliary dyskinesia, or alpha1-antitrypsin deficiency exhibited 3-fold higher mucosal (not serosal) ectoATPase activity than normal cultures. Time course experiments indicated that CF enhances ATP elimination and adenosine accumulation on the mucosal surface. Furthermore, nonspecific alkaline phosphatase was identified as the major regulator of airway nucleotide concentrations in CF, primary ciliary dyskinesia, and alpha1-antitrypsin deficiency. The ectoAT-Pase activity and mRNA expression of mucosally restricted nonspecific alkaline phosphatase were 3-fold higher on bronchial cultures from these patients than from healthy subjects. This study demonstrates that the duration of nucleotide-mediated MCC is limited by epithelial ectonucleotidases throughout human airways, with the efficiency of this mechanism enhanced in chronic inflammatory lung diseases, including CF.

Purinergic stimulation induces Ca2+-dependent activation of Na+-K+-2Cl- cotransporter in human nasal epithelia

Increasing evidence suggests that P2 receptors (P2Rs) in airway epithelial cells perform critical functions in auto- or paracrine regulation of fluid and mucus secretion. In the present study, we characterized the effects of P2R stimulation on Na(+)-K(+)-2Cl(-) cotransporter (NKCC) activity in normal human nasal epithelial (NHNE) cells. [Ca(2+)](i) and pH(i) were measured in primary cultures of NHNE cells using a double perfusion chamber, which enabled us to analyze membrane-specific transporter activities. NKCC activities were estimated by the pH(i) reduction due to Na(+)-dependent and bumetanide-sensitive intracellular uptake of NH(4)(+). NKCC activities were observed in the basolateral membrane, but not in the luminal membrane, of NHNE cells. Interestingly, P2Rs were expressed in both membranes, and the stimulation of either luminal or basolateral P2R increased NKCC activity. Blockades of luminal Cl(-) channels, basolateral K(+) channels, or protein kinase C did not affect the activation of NKCC by basolateral P2R stimulation. The effects of luminal P2R stimulation were partially reduced by Cl(-) channel blockers. However, chelation of intracellular Ca(2+) by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) treatment completely blocked the stimulatory effects of luminal and basolateral P2Rs on NKCC. In addition, increasing [Ca(2+)](i) by treatment with ionomycin-stimulated NKCC activity. These results provide evidence that stimulation of P2Rs directly activates basolateral NKCC by Ca(2+)-dependent pathways in NHNE cells, which is an important aspect of the purinergic regulation of ion and fluid secretions in human airway epithelia under physiologic and pathologic conditions.

Adenosine triphosphate is released during injurious mechanical ventilation and contributes to lung edema

BACKGROUND

Extracellular nucleotides mediate many cellular functions and are released in response to mechanical stress in vitro. It is unknown whether adenosine triphosphate (ATP) is released in vivo during mechanical ventilation (MV). We hypothesized that stress from high-pressure MV would increase airway ATP, contributing to MV-associated lung edema.

METHODS

Rats were randomized to nonventilated control (n = 6) or 30 minutes of MV with low (15 cm H(2)0, n = 7) or high (40 cm H(2)0, n = 6) pressure. Additional groups received intratracheal ATP (n = 7) or saline (n = 7) before low-pressure MV.

RESULTS

Low-pressure MV did not affect lung edema or bronchoalveolar lavage (BAL) ATP levels. In contrast, high-pressure MV significantly increased BAL ATP and produced alveolar edema; lactate dehydrogenase was unchanged. Intratracheal ATP administration significantly increased lung water during low-pressure MV.

CONCLUSION

High-pressure MV increases BAL ATP concentration without altering lactate dehydrogenase, suggesting that release is not from cell lysis. Intratracheal ATP increases lung water, implicating nucleotides in MV-associated lung edema.

Neonatal 6-hydroxydopamine administration to mice is fatal

Depletion of dopamine in adult rats by treatment with the neurotoxin 6-hydroxydopamine (6-OHDA) causes severe deficits in feeding, drinking, and movement that often lead to death. However, when neonatal rats are treated similarly, they survive normally, suggesting that compensatory adaptation to dopamine depletion occurs. In contrast, dopamine-deficient mice that have a selective genetic deficiency in dopamine production die 2-4 weeks after birth. Thus, we tested the hypothesis that killing dopaminergic neurons with 6-OHDA might promote survival of dopamine-deficient mice. Body weights, motor coordination, catecholamine levels, and survival were monitored for several weeks after bilateral administrations of 6-OHDA to 3-day-old mice. Some treated mice were raised in a heated chamber to help them conserve energy. The results demonstrate that regardless of genotype or environmental temperature, bilateral neonatal 6-OHDA lesions are lethal to mice.

Acute mechanism of medium chain fatty acid-induced enhancement of airway epithelial permeability

The localization of viral receptors to the basolateral surface of airway epithelia is an obstacle to the effectiveness of luminal viral-mediated gene transfer to the lung. The tight junction (TJ) serves as a rate-limiting barrier to the penetration of viral vectors. We have previously identified the sodium salt of the medium chain fatty acid (MCFA) capric acid (C10) as an agent that can enhance the ability of adenoviral vectors to transduce well differentiated (WD) primary human airway epithelial (HAE) cells. Previous studies have suggested that intracellular calcium (Ca(i)2+) levels may play a central role in the long-term C10-mediated increases in junctional permeability. In this study, we investigated the effects of C10 and lauric acid (C12) on Ca(i)2+ in WD primary HAE cells and determined whether these effects were necessary for the acute MCFA-induced reduction in transepithelial resistance (R(T)) and increased permeability. In addition, we characterized the effects of C10 and C12 on components localized to the TJ, including ZO-1, junctional adhesion molecule (JAM), and the claudin family of transmembrane proteins. In addition to rapidly decreasing R(T), C10 and C12 increased cellular and paracellular permeability. C10 induced a rapid, sustained increase in Ca(i)2+. However, buffering Ca(i)2+ did not block the effects of C10 on R(T). Both C10 and C12 caused reorganization of claudins-1, -4, JAM, and beta-catenin, but not ZO-1. These data suggest that C10 and C12 exert their acute effects on airway TJs via a Ca(2+)-independent mechanism of action and may alter junctional permeability via direct effects on the claudin family of TJ proteins.

Human airway ecto-adenylate kinase. A mechanism to propagate ATP signaling on airway surfaces

Mechanically induced ATP release from human airway epithelial cells regulates mucociliary clearance through cell surface nucleotide receptors. Ectoenzymes detected on these cells were recently shown to terminate ATP-mediated responses by sequential dephosphorylation of extracellular ATP into ADP, AMP, and adenosine. We now demonstrate that an ecto-adenylate kinase (ecto-AK) contributes to the metabolism of adenine nucleotides on human airway epithelial surfaces by the reversible reaction: ATP + AMP 2ADP. This phosphotransferase exhibited a bilateral distribution on polarized primary cultures of human bronchial epithelial cells with a 4-fold higher activity on the mucosal surface. Ecto-AK presented an absolute requirement for magnesium and adenine-based nucleotides. UMP, GMP, and CMP could not substitute for AMP as gamma-phosphate acceptor, and UDP could not replace ADP. Apparent K(m) and V(max) values were 23 +/- 5 microM and 1.1 +/- 0.1 nmol x min(-1) x cm(-2) for ATP and 43 +/- 6 microM and 0.5 +/- 0.1 nmol x min(-1) x cm(-2) for ADP. Ecto-AK accounted for 20% of [gamma-(32)P]ATP dephosphorylation, and the impermeant AK inhibitor, diadenosine pentaphosphate, reduced ADPase activity by more than 70% on both epithelial surfaces. Time course experiments on ATP metabolism demonstrated that ecto-AK significantly prolongs effective ATP and ADP concentrations on airway epithelial surfaces for P2 receptor signaling and reduces by 6-fold adenosine production. Our data suggest a role for this nucleotide entrapment cycle in the propagation of purine-mediated mucociliary clearance on human airway epithelial surfaces.

Distinct axonemal processes underlie spontaneous and stimulated airway ciliary activity

Cilia are small organelles protruding from the cell surface that beat synchronously, producing biological transport. Despite intense research for over a century, the mechanisms underlying ciliary beating are still not well understood. Even the nature of the cytosolic molecules required for spontaneous and stimulated beating is debatable. In an effort to resolve fundamental questions related to cilia beating, we developed a method that integrates the whole-cell mode of the patch-clamp technique with ciliary beat frequency measurements on a single cell. This method enables to control the composition of the intracellular solution while the cilia remain intact, thus providing a unique tool to simultaneously investigate the biochemical and physiological mechanism of ciliary beating. Thus far, we investigated whether the spontaneous and stimulated states of cilia beating are controlled by the same intracellular molecular mechanisms. It was found that: (a) MgATP was sufficient to support spontaneous beating. (b) Ca(2+) alone or Ca(2+)-calmodulin at concentrations as high as 1 microM could not alter ciliary beating. (c) In the absence of Ca(2+), cyclic nucleotides produced a moderate rise in ciliary beating while in the presence of Ca(2+) robust enhancement was observed. These results suggest that the axonemal machinery can function in at least two different modes.

The cellular and molecular basis of store-operated calcium entry

The impact of calcium signalling on so many areas of cell biology reflects the crucial role of calcium signals in the control of diverse cellular functions. Despite the precision with which spatial and temporal details of calcium signals have been resolved, a fundamental aspect of the generation of calcium signals -- the activation of 'store-operated channels' (SOCs) -- remains a molecular and mechanistic mystery. Here we review new insights into the exchange of signals between the endoplasmic reticulum (ER) and plasma membrane that result in activation of calcium entry channels mediating crucial long-term calcium signals.

Modulation of cytosolic Ca(2+) concentration in airway epithelial cells by Pseudomonas aeruginosa

Modulation of cytosolic (intracellular) Ca(2+) concentration (Ca(i)) may be an important host response when airway epithelial cells are exposed to Pseudomonas aeruginosa. We measured Ca(i) in Calu-3 cells exposed from the apical or basolateral surface to cytotoxic and noncytotoxic strains of P. aeruginosa. Apical addition of either noncytotoxic strains or cytotoxic strains failed to affect Ca(i) over a 3-h time period, nor were changes observed after basolateral addition of noncytotoxic strains. In contrast, basolateral addition of cytotoxic strains caused a slow increase in Ca(i) from 100 nM to 200 to 400 nM. This increase began after 20 to 50 min and persisted for an additional 30 to 75 min, at which time the cells became nonviable. P. aeruginosa-induced increases in Ca(i) were blocked by the addition of the Ca channel blocker La(3+) to the basolateral but not to the apical chamber. Likewise, replacing the basolateral but not the apical medium with Ca-free solution prevented P. aeruginosa-mediated changes in Ca(i). With isogenic mutants of PA103, we demonstrated that the type III secretion apparatus, the type III-secreted effector ExoU, and type IV pili were necessary for increased Ca(i). We propose that translocation of ExoU through the basolateral surface of polarized airway epithelial cells via the type III secretion apparatus leads to release of Ca stored in the endoplasmic reticulum and activation of Ca channels in the basolateral membranes of epithelial cells.

Cellular mechanisms for apical ATP effects on intracellular pH in human bronchial epithelium

The effect of external ATP on intracellular pH (pH(i)) was investigated using a pH imaging system in a human bronchial epithelial cell line (16HBE14o-) loaded with BCECF-AM. The steady-state pH(i) of 16HBE14o- epithelial monolayers was 7.137 +/- 0.027 (n = 46). Apical addition of ATP (10(-4) M) to epithelial monolayers induced a rapid and sustained pH(i) decrease of 0.164 +/- 0.024 pH units (n = 17; P < 0.001). The intracellular acidification was rapidly reversed upon removal of external ATP. In contrast, the non-hydrolysable ATP analogue AMP-PNP did not produce any significant change in pH(i). Inhibition of purinoreceptors by suramin did not affect the acidification induced by apical ATP. Inhibition of Na+-H+ exchange by apical Na+ removal or addition of amiloride (0.5 mM) reduced the apical ATP-induced pH(i) decrease, suggesting the involvement of a Na+-H+ exchanger or surface pH effects on the ATP-induced pH(i) response. Inhibitors of proton channels such as ZnCl2 (10(-4) M) also partially inhibited the ATP response. The pH(i) response to ATP was dependent on the external pH (pH(o)), with increasing acidification produced at lower pH(o) values. Neither the basal pH(i) nor the ATP-induced intracellular acidification was affected by thapsigargin (a Ca2+-ATPase inhibitor), chelerythrine chloride (a protein kinase C (PKC) inhibitor), RpcAMP (a protein kinase A (PKA) inhibitor) or PMA (a PKC activator). Therefore, the intracellular acidification of human bronchial epithelial cells induced by apical ATP does not involve signalling via Ca2+, PKC or PKA nor binding to a purinoreceptor. We interpret the effect of ATP to produce an intracellular acidification as a three step process: activation of H+ channels, inhibition of Na+-H+ exchange and influx of protonated ATP.

Stimulation of Cl- secretion via membrane-restricted Ca2+ signaling mediated by P2Y receptors in polarized epithelia

Extracellular nucleotides such as ATP have been shown to regulate ion transport processes in a variety of epithelia. This effect is mediated by the activation of plasma membrane P2Y receptors, which leads to Ca(2+) signaling cascade. Ion transport processes (e.g. activation of apical calcium-dependent Cl(-) channels) are then stimulated via an increase in [Ca(2+)](i). Many polarized epithelia express apical and/or basolateral P2Y receptors. To test whether apical and basolateral stimulation of P2Y receptors elicit polarized Ca(2+) signaling and anion secretion, we simultaneously measured the two parameters in polarized epithelia. Although activation of P2Y receptors located at both apical and basolateral membranes evoked an increase in [Ca(2+)](i), only apical P2Y receptors-coupled Ca(2+) release stimulated an increase in anion secretion. Moreover, the calcium influx evoked by apical and basolateral P2Y receptor stimulation is predominately via the basolateral membrane domain. It appears that the apical P2Y receptor-regulated Ca(2+) release and activation of apical Cl(-) channels is compartmentalized in polarized epithelia with basolateral P2Y-stimulated Ca(2+) release failing to activate anion secretion. These data suggest that there may be two distinct ATP-releasable Ca(2+) pools, each coupled to apical and basolateral membrane receptor but linked to the same calcium influx pathway located at the basolateral membrane.

Intracellular stores maintain stable cytosolic Ca(2+) gradients in epithelial cells by active Ca(2+) redistribution

A stable localized region of high calcium concentration near the plasma membrane has been postulated to exist as an outcome of prolonged calcium influx and to play a crucial role in regulation of cellular life. However, the mechanism supporting this phenomenon is a perplexing problem. We show here that a sustained localized region of high cytosolic Ca(2+) concentration is formed near the plasma membrane. Calcium influx, calcium uptake by intracellular stores and calcium release from the stores are essential for this phenomenon. Our results strongly suggest that the mechanism of formation of stable calcium gradient near the plasma membrane involves a process of active redistribution-uptake of entering calcium into intracellular stores and its release from the stores toward the plasma membrane.

Polarized expression and function of P2Y ATP receptors in rat bile duct epithelia

Extracellular nucleotides may be important regulators of bile ductular secretion, because cholangiocytes express P2Y ATP receptors and nucleotides are found in bile. However, the expression, distribution, and function of specific P2Y receptor subtypes in cholangiocytes are unknown. Thus our aim was to determine the subtypes, distribution, and role in secretion of P2Y receptors expressed by cholangiocytes. The molecular subtypes of P2Y receptors were determined by RT-PCR. Functional studies measuring cytosolic Ca2+ (Ca) signals and bile ductular pH were performed in isolated, microperfused intrahepatic bile duct units (IBDUs). PCR products corresponding to P2Y1, P2Y2, P2Y4, P2Y6, and P2X4 receptor subtypes were identified. Luminal perfusion of ATP into IBDUs induced increases in Ca that were inhibited by apyrase and suramin. Luminal ATP, ADP, 2-methylthioadenosine 5'-triphosphate, UTP, and UDP each increased Ca. Basolateral addition of adenosine 5'-O-(3-thiotriphosphate) (ATP-gamma-S), but not ATP, to the perifusing bath increased Ca. IBDU perfusion with ATP-gamma-S induced net bile ductular alkalization. Cholangiocytes express multiple P2Y receptor subtypes that are expressed at the apical plasma membrane domain. P2Y receptors are also expressed on the basolateral domain, but their activation is attenuated by nucleotide hydrolysis. Activation of ductular P2Y receptors induces net ductular alkalization, suggesting that nucleotide signaling may be an important regulator of bile secretion by the liver.

Stimulation of ATP secretion in the liver by therapeutic bile acids

ATP receptors are ubiquitously expressed and are potential targets for the therapy of a number of disorders. However, delivery of ATP or other nucleotides to specific tissues is problematic, and no pharmacological means to stimulate the release of endogenous ATP has been described. We examined the effects of the bile acid ursodeoxycholic acid (UDCA) on ATP release into bile, since this bile acid is the only agent known to be of therapeutic benefit in secretory disorders of the liver, and since its mechanism of action is not established. Both UDCA and its taurine conjugate stimulated secretion of ATP by isolated rat hepatocytes, and produced measurable increases in ATP in bile of isolated rat liver. Perfusion of ATP into microdissected bile-duct segments induced Ca(2+) signalling in bile-duct epithelia, while perfusion of bile acid did not. Thus UDCA may promote bile flow by inducing hepatocytes to release ATP into bile, which then stimulates fluid and electrolyte secretion by bile-duct epithelia downstream via changes in cytosolic Ca(2+). Moreover, these findings demonstrate the feasibility of using pharmacological means to induce secretion of endogenous ATP. Since the liver and other epithelial organs express luminal ATP receptors, these findings more generally suggest that a mechanism exists for pharmacological activation of this paracrine signalling pathway. This strategy may be useful for treatment of cystic fibrosis and other secretory disorders of the liver and other epithelial tissues.

Extracellular nucleotide signaling along the renal epithelium

During the past two decades, several cell membrane receptors, which preferentially bind extracellular nucleotides, and their analogs have been identified. These receptors, collectively known as nucleotide receptors or "purinergic" receptors, have been characterized and classified on the basis of their biological actions, their pharmacology, their molecular biology, and their tissue and cell distribution. For these receptors to have biological and physiological relevance, nucleotides must be released from cells. The field of extracellular ATP release and signaling is exploding, as assays to detect this biological process increase in number and ingenuity. Studies of ATP release have revealed a myriad of roles in local regulatory (autocrine or paracrine) processes in almost every tissue in the body. The regulatory mechanisms that these receptors control or modulate have physiological and pathophysiological roles and potential therapeutic applications. Only recently, however, have ATP release and nucleotide receptors been identified along the renal epithelium of the nephron. This work has set the stage for the study of their physiological and pathophysiological roles in the kidney. This review provides a comprehensive presentation of these issues, with a focus on the renal epithelium.

Protease-activated receptor-2-mediated inhibition of ion transport in human bronchial epithelial cells

A cytoprotective role for protease-activated receptor-2 (PAR2) has been suggested in a number of systems including the airway, and to this end, we have studied the role that PARs play in the regulation of airway ion transport, using cultures of normal human bronchial epithelial cells. PAR2 activators, added to the basolateral membrane, caused a transient, Ca2+-dependent increase in short-circuit current ( I sc), followed by a sustained inhibition of amiloride-sensitive I sc. These phases corresponded with a transient increase in intracellular Ca2+ concentration and then a transient increase, followed by decrease, in basolateral K+ permeability. After PAR2 activation and the addition of amiloride, the forskolin-stimulated increase in I sc was also attenuated. By contrast, PAR2 activators added to the apical surface of the epithelia or PAR1 activators added to both the apical and basolateral surfaces were without effect. PAR2 may, therefore, play a role in the airway, regulating Na+ absorption and anion secretion, processes that are central to the control of airway surface liquid volume and composition.

Polarized signaling via purinoceptors in normal and cystic fibrosis airway epithelia

Airway epithelia are confronted with distinct signals emanating from the luminal and/or serosal environments. This study tested whether airway epithelia exhibit polarized intracellular free calcium (Ca(2+)(i)) and anion secretory responses to 5' triphosphate nucleotides (ATP/UTP), which may be released across both barriers of these epithelia. In both normal and cystic fibrosis (CF) airway epithelia, mucosal exposure to ATP/UTP increased Ca(2+)(i) and anion secretion, but both responses were greater in magnitude for CF epithelia. In CF epithelia, the mucosal nucleotide-induced response was mediated exclusively via Ca(2+)(i) interacting with a Ca(2+)-activated Cl(-) channel (CaCC). In normal airway epithelia (but not CF), nucleotides stimulated a component of anion secretion via a chelerythrine-sensitive, Ca(2+)-independent PKC activation of cystic fibrosis transmembrane conductance regulator. In normal and CF airway epithelia, serosally applied ATP or UTP were equally effective in mobilizing Ca(2+)(i). However, serosally applied nucleotides failed to induce anion transport in CF epithelia, whereas a PKC-regulated anion secretory response was detected in normal airway epithelia. We conclude that (1) in normal nasal epithelium, apical/basolateral purinergic receptor activation by ATP/UTP regulates separate Ca(2+)-sensitive and Ca(2+)-insensitive (PKC-mediated) anion conductances; (2) in CF airway epithelia, the mucosal ATP/UTP-dependent anion secretory response is mediated exclusively via Ca(2+)(i); and (3) Ca(2+)(i) regulation of the Ca(2+)-sensitive anion conductance (via CaCC) is compartmentalized in both CF and normal airway epithelia, with basolaterally released Ca(2+)(i) failing to activate CaCC in both epithelia.

Role of calcium and calmodulin in ciliary stimulation induced by acetylcholine

The goal of this work was to elucidate the molecular events underlying stimulation of ciliary beat frequency (CBF) induced by acetylcholine (ACh) in frog esophagus epithelium. ACh induces a profound increase in CBF and in intracellular Ca(2+) concentration ([Ca(2+)](i)) through M(1) and M(3) muscarinic receptors. The [Ca(2+)](i) slowly decays to the basal level, while CBF stabilizes at an elevated level. These results suggest that ACh triggers Ca(2+)-correlated and -uncorrelated modes of ciliary stimulation. ACh response is abolished by the phospholipase C (PLC) inhibitor U-73122 and by depletion of intracellular Ca(2+) stores but is unaffected by reduction of extracellular Ca(2+) concentration and by blockers of Ca(2+) influx. Therefore, ACh activates PLC and mobilizes Ca(2+) solely from intracellular stores. The calmodulin inhibitors W-7 and calmidazolium attenuate the ACh-induced increase in [Ca(2+)](i) but completely abolish the elevation in CBF. Therefore, elevation of [Ca(2+)](i) is necessary for CBF enhancement but does not lead directly to it. The combined effect of Ca(2+) elevation and of additional factors, presumably mobilized by Ca(2+)-calmodulin, results in a robust CBF enhancement.

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

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

A primitive ATP receptor from the little skate Raja erinacea

P2Y ATP receptors are widely expressed in mammalian tissues and regulate a broad range of activities. Multiple subtypes of P2Y receptors have been identified and are distinguished both on a molecular basis and by pharmacologic substrate preference. Functional evidence suggests that hepatocytes from the little skate Raja erinacea express a primitive P2Y ATP receptor lacking pharmacologic selectivity, so we cloned and characterized this receptor. Skate hepatocyte cDNA was amplified with degenerate oligonucleotide probes designed to identify known P2Y subtypes. A single polymerase chain reaction product was found and used to screen a skate liver cDNA library. A 2314-base pair cDNA clone was generated that contained a 1074-base pair open reading frame encoding a 357-amino acid gene product with 61-64% similarity to P2Y(1) receptors and 21-37% similarity to other P2Y receptor subtypes. Pharmacology of the putative P2Y receptor was examined using the Xenopus oocyte expression system and revealed activation by a range of nucleotides. The receptor was expressed widely in skate tissue and was expressed to a similar extent in other primitive organisms. Phylogenetic analysis suggested that this receptor is closely related to a common ancestor of the P2Y subtypes found in mammals, avians, and amphibians. Thus, the skate liver P2Y receptor functions as a primitive P2Y ATP receptor with broad pharmacologic selectivity and is related to the evolutionary forerunner of P2Y(1) receptors of higher organisms. This novel receptor should provide an effective comparative model for P2Y receptor pharmacology and may improve our understanding of nucleotide specificity among the family of P2Y ATP receptors.