Luminal purinergic regulatory mechanisms of tracheal ciliary beat frequency

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.

Epithelial cell culture from human adenoids: a functional study model for ciliated and secretory cells

Background. Mucociliary transport (MCT) is a defense mechanism of the airway. To study the underlying mechanisms of MCT, we have both developed an experimental model of cultures, from human adenoid tissue of ciliated and secretory cells, and characterized the response to local chemical signals that control ciliary activity and the secretion of respiratory mucins in vitro. Materials and Methods. In ciliated cell cultures, ciliary beat frequency (CBF) and intracellular Ca²⁺ levels were measured in response to ATP, UTP, and adenosine. In secretory cultures, mucin synthesis and secretion were identified by using immunodetection. Mucin content was taken from conditioned medium and analyzed in the presence or absence of UTP. Results. Enriched ciliated cell monolayers and secretory cells were obtained. Ciliated cells showed a basal CBF of 10.7 Hz that increased significantly after exposure to ATP, UTP, or adenosine. Mature secretory cells showed active secretion of granules containing different glycoproteins, including MUC5AC. Conclusion. Culture of ciliated and secretory cells grown from adenoid epithelium is a reproducible and feasible experimental model, in which it is possible to observe ciliary and secretory activities, with a potential use as a model to understand mucociliary transport control mechanisms.

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.

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.

Computational modelling elucidates the mechanism of ciliary regulation in health and disease

Background

Ciliary dysfunction leads to a number of human pathologies, including primary ciliary dyskinesia, nephronophthisis, situs inversus pathology or infertility. The mechanism of cilia beating regulation is complex and despite extensive experimental characterization remains poorly understood. We develop a detailed systems model for calcium, membrane potential and cyclic nucleotide-dependent ciliary motility regulation.

Results

The model describes the intimate relationship between calcium and potassium ionic concentrations inside and outside of cilia with membrane voltage and, for the first time, describes a novel type of ciliary excitability which plays the major role in ciliary movement regulation. Our model describes a mechanism that allows ciliary excitation to be robust over a wide physiological range of extracellular ionic concentrations. The model predicts the existence of several dynamic modes of ciliary regulation, such as the generation of intraciliary Ca²⁺ spike with amplitude proportional to the degree of membrane depolarization, the ability to maintain stable oscillations, monostable multivibrator regimes, all of which are initiated by variability in ionic concentrations that translate into altered membrane voltage.

Conclusions

Computational investigation of the model offers several new insights into the underlying molecular mechanisms of ciliary pathologies. According to our analysis, the reported dynamic regulatory modes can be a physiological reaction to alterations in the extracellular environment. However, modification of the dynamic modes, as a result of genetic mutations or environmental conditions, can cause a life threatening pathology.

Adenosine activation of A(2B) receptor(s) is essential for stimulated epithelial ciliary motility and clearance

Mucociliary clearance, vital to lung clearance, is dependent on cilia beat frequency (CBF), coordination of cilia, and the maintenance of periciliary fluid. Adenosine, the metabolic breakdown product of ATP, is an important modulator of ciliary motility. However, the contributions of specific adenosine receptors to key airway ciliary motility processes are unclear. We hypothesized that adenosine modulates ciliary motility via activation of its cell surface receptors (A(1), A(2A), A(2B), or A(3)). To test this hypothesis, mouse tracheal rings (MTRs) excised from wild-type and adenosine receptor knockout mice (A(1), A(2A), A(2B), or A(3), respectively), and bovine ciliated bronchial epithelial cells (BBECs) were stimulated with known cilia activators, isoproterenol (ISO; 10 μM) and/or procaterol (10 μM), in the presence or absence of 5'-(N-ethylcarboxamido) adenosine (NECA), a nonselective adenosine receptor agonist [100 nM (A(1), A(2A), A(3)); 10 μM (A(2B))], and CBF was measured. Cells and MTRs were also stimulated with NECA (100 nM or 10 μM) in the presence and absence of adenosine deaminase inhibitor, erythro-9- (2-hydroxy-3-nonyl) adenine hydrochloride (10 μM). Both ISO and procaterol stimulated CBF in untreated cells and/or MTRs from both wild-type and adenosine knockout mice by ~3 Hz. Likewise, CBF significantly increased ~2-3 Hz in BBECs and wild-type MTRs stimulated with NECA. MTRs from A(1), A(2A), and A(3) knockout mice stimulated with NECA also demonstrated an increase in CBF. However, NECA failed to stimulate CBF in MTRs from A(2B) knockout mice. To confirm the mechanism by which adenosine modulates CBF, protein kinase activity assays were conducted. The data revealed that NECA-stimulated CBF is mediated by the activation of cAMP-dependent PKA. Collectively, these data indicate that purinergic stimulation of CBF requires A(2B) adenosine receptor activation, likely via a PKA-dependent pathway.

Coupling of airway ciliary activity and mucin secretion to mechanical stresses by purinergic signaling

The mucociliary clearance system is comprised of three components, ion transport activities controlling the height of airway surface liquid (ASL), mucin secretion, and ciliary activity. These activities in humans are controlled principally by local agonists, extracellular nucleotides and nucleosides released from the epithelium. Importantly, mechanical stresses stimulate goblet cell mucin secretion, ciliary beating, and Cl- and fluid secretion through mechanically induced nucleotide release. Emerging evidence also implicates co-secretion of nucleotides and mucin from goblet cells as a source of extracellular agonist. At rest, ATP is released onto airway surfaces at approximately 370fmol/mincm2, but only approximately 3% of released ATP is recovered in ASL. Secreted UTP meets with a similar fate. A wide variety of hydrolytic and transphosphorylating ecto-enzymes convert the triphosphate nucleotides into ADP, AMP, and adenosine, UDP, UMP, and uridine. Of these, ATP, adenosine, UTP, and UDP act as agonists at apical P2Y2 (ATP, UTP), P2Y6 (UDP), and A2B (adenosine) receptors on ciliated and/or goblet cells to regulate mucociliary clearance.

Efficient mucociliary transport relies on efficient regulation of ciliary beating

The respiratory mucociliary epithelium is a synchronized and highly effective waste-disposal system. It uses mucus as a vehicle, driven by beating cilia, to transport unwanted particles, trapped in the mucus, away from the respiratory system. The ciliary machinery can function in at least two different modes: a low rate of beating that requires only ATP, and a high rate of beating regulated by second messengers. The mucus propelling velocity is linearly dependent on ciliary beat frequency (CBF). The linear dependence implies that a substantial increase in transport efficiency requires an equally substantial rise in CBF. The ability to enhance beating in response to various physiological cues is a hallmark of mucociliary cells. An intricate signaling network controls ciliary activity, which relies on interplay between calcium and cyclic nucleotide pathways.

Calcium-mediated, purinergic stimulation and polarized localization of calcium-sensitive adenylyl cyclase isoforms in human airway epithelia

Purinergic stimulation of human airway epithelia results in a prolonged increase in ciliary beat frequency that depends on calcium-mediated cAMP production [Lieb, T., Wijkstrom Frei, C., Frohock, J.I., Bookman, R.J. and Salathe, M. (2002) Prolonged increase in ciliary beat frequency after short-term purinergic stimulation in human airway epithelial cells. J. Physiol. (Lond.) 538, 633-646]. Here, fully differentiated human airway epithelial cells in culture are shown to express calcium-stimulated transmembrane adenylyl cyclase (tmAC) isoforms (types 1, 3, and 8) by reverse transcription polymerase chain reaction. Immunohistochemistry of tracheal sections and fully differentiated airway epithelial cell cultures revealed polarized expression of these tmACs, with types 1 and 8 localized to the apical membrane and thus at the position required for ciliary regulation. Real-time, ciliated-cell specific cAMP production by tmACs upon apical, purinergic stimulation with UTP was confirmed using fluorescent energy resonance transfer between fluorescently tagged PKA subunits.

The lactoperoxidase system links anion transport to host defense in cystic fibrosis

Chronic respiratory infections in cystic fibrosis result from CFTR channel mutations but how these impair antibacterial defense is less clear. Airway host defense depends on lactoperoxidase (LPO) that requires thiocyanate (SCN-) to function and epithelia use CFTR to concentrate SCN- at the apical surface. To test whether CFTR mutations result in impaired LPO-mediated host defense, CF epithelial SCN- transport was measured. CF epithelia had significantly lower transport rates and did not accumulate SCN- in the apical compartment. The lower CF [SCN-] did not support LPO antibacterial activity. Modeling of airway LPO activity suggested that reduced transport impairs LPO-mediated defense and cannot be compensated by LPO or H2O2 upregulation.

Mechanosensitivity of mouse tracheal ciliary beat frequency: roles for Ca2+, purinergic signaling, tonicity, and viscosity

Mechanosensitivity is hypothesized to participate in the regulation of ciliary beat frequency (CBF) in airway epithelia. To investigate this hypothesis, CBF in excised mouse trachea was monitored (microscopy image analysis) while varying mucosal shear (perfusate velocity and/or viscosity; planar flow). CBF increased within minutes of step increase to steady shear stress as small as 10(-3) Pa and decreased within minutes of shear reduction (<or=10(-4) Pa). CBF response was directional, being less with cephalad vs. caudal flow, and was reduced in trachea from mutant mice lacking P2Y2 receptors, as well as by administration of the Ca2+ chelator EGTA, the Ca2+ channel inhibitor La3+, the nucleotide phosphohydrolase apyrase, the metabolically stabilized adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine, the osmotic agent mannitol, and the viscosity modifier dextran. Brief exposure to exogenous ATP, a candidate mediator, augmented CBF response, although augmentation declined with higher ATP concentration (5.0 vs. 0.1 mM) or longer ATP exposure before shear (55 vs. 20 min). Prolonged extended exposure (45 min) to the metabolically stabilized ATP analog ATPgammaS [adenosine 5'-(3-thiotriphosphate), 0.1 mM] inhibited CBF response to shear. Furthermore, neither ATP nor ATPgammaS substantially increased CBF in the relative absence of shear. With viscosity increase or shear withdrawal apyrase evoked CBF stimulation, inhibitable by the adenosine receptor antagonist 8-(p-sulfophenyl)theophylline. Thus CBF response to shear is finely tuned, directional, La3+ sensitive, likely dependent on extracellular Ca2+ and ATP, involving P2Y2 and adenosine receptor activations, influenced by shear history, tonicity, viscosity, and metabolism/exposure of ATP, and thus reflective of a complex interplay of physical and biochemical actions.

Relationships between cystic fibrosis transmembrane conductance regulator, extracellular nucleotides and cystic fibrosis

Cystic fibrosis (CF) is one of the most common lethal autosomal recessive genetic diseases in the Caucasian population, with a frequency of about 1 in 3000 livebirths. CF is due to a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene encoding the CFTR protein, a cyclic adenosine 5'-monophosphate (cAMP)-regulated chloride channel localized in the apical membrane of epithelial cells. CFTR is a multifunctional protein which, in addition to be a Cl-channel, is also a regulator of multiple ion channels and other proteins. In particular CFTR has been reported to play a role in the outflow of adenosine 5'-triphosphate (ATP) from cells, but this remains controversial. Extracellular nucleotides are signaling molecules that regulate ion transport and mucociliary clearance by acting on P2 nucleotide receptors, in particular the P2Y(2) receptor. Nucleotides activating the P2Y(2) receptor represent thus one pharmacotherapeutic strategy to treat CF disease, via improvement of mucus hydration and mucociliary clearance in airways. Phase II clinical trials have recently shown that aerosolized denufosol (INS37217, Inspire(R)) improves pulmonary function in CF patients: denufosol was granted orphan drug status and phase III trials are planned. Here, we review what is known about the relationship between extracellular nucleotides and CFTR, the role of extracellular nucleotides in epithelial pathophysiology and their putative role as therapeutic agents.

E-NTPDases in human airways: Regulation and relevance for chronic lung diseases

Chronic obstructive lung diseases are characterized by the inability to prevent bacterial infection and a gradual loss of lung function caused by recurrent inflammatory responses. In the past decade, numerous studies have demonstrated the importance of nucleotide-mediated bacterial clearance. Their interaction with P2 receptors on airway epithelia provides a rapid ‘on-and-off’ signal stimulating mucus secretion, cilia beating activity and surface hydration. On the other hand, abnormally high ATP levels resulting from damaged epithelia and bacterial lysis may cause lung edema and exacerbate inflammatory responses. Airway ATP concentrations are regulated by ecto nucleoside triphosphate diphosphohydrolases (E-NTPDases) which are expressed on the mucosal surface and catalyze the sequential dephosphorylation of nucleoside triphosphates to nucleoside monophosphates (ATP → ADP → AMP). The common bacterial product, Pseudomonas aeruginosa lipopolysaccharide (LPS), induces an acute reduction in azide-sensitive E-NTPDase activities, followed by a sustained increase in activity as well as NTPDase 1 and NTPDase 3 expression. Accordingly, chronic lung diseases, including cystic fibrosis (CF) and primary ciliary dyskinesia, are characterized by higher rates of nucleotide elimination, azide-sensitive E-NTPDase activities and expression. This review integrates the biphasic regulation of airway E-NTPDases with the function of purine signaling in lung diseases. During acute insults, a transient reduction in E-NTPDase activities may be beneficial to stimulate ATP-mediated bacterial clearance. In chronic lung diseases, elevating E-NTPDase activities may represent an attempt to prevent P2 receptor desensitization and nucleotide-mediated lung damage.

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.

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.

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.

4alpha-Phorbol negates the inhibitory effects of phorbol-12-myristate-13-acetate on human cilia and alters the phosphorylation of PKC

In medium 199, ciliary beat frequency (CBF) in human nasal epithelium declines to 60% of baseline by 2 h and 1 nM phorbol-12-myristate-13-acetate (PMA) doubles the rate of decline by activating protein kinase C (PKC). We find that a reported negative control for PMA, 4alpha-phorbol (1 pM-1 nM)+/-1 nM PMA, not only maintains CBF at baseline, but arrests a pre-existing PMA-induced decline in CBF and alters the profile of multiple phosphorylated PKC species. Thus, 4alpha-phorbol not only potently prevents PMA from inhibiting CBF but also has potent effects on the phosphorylation of PKC.

Regulator of G-protein signaling protein 2 modulates purinergic calcium and ciliary beat frequency responses in airway epithelia

In ciliated airway epithelial cells, purinergic stimulation increases both intracellular calcium ([Ca(2+)](i)) and ciliary beat frequency (CBF). Because regulator of G-protein signaling protein 2 (RGS2) terminates Galphaq-mediated phospholipase C activation, we examined its role in regulating purinergic signaling in human and ovine airway epithelial cells. RT-PCR of both human and ovine epithelial cell RNA yielded fragments of expected size ( approximately 491 bp) and sequence, confirming RGS2 message. Immunofluorescence demonstrated RGS2 protein expression in cultured airway epithelial cells of both species. Overexpression of an EGFP-RGS2 fusion protein (increasing RGS2 protein levels 1.8 times control, n = 28 cells) resulted in a reduced [Ca(2+)](i) and CBF response to 10 micro M ATP (human: 58 +/- 9% and 49 +/- 8% lower, respectively; n = 8 measurements, 4 cells; ovine: 56 +/- 12% and 53 +/- 16% lower, respectively; n = 5 measurements, 4 cells). Reducing RGS2 protein levels using antisense oligonucleotides increased the response of both [Ca(2+)](i) and CBF to ATP in human cells by 57 +/- 10% and 47 +/- 11%, respectively (n = 10 measurements, 6 cells), and in ovine cells by 88 +/- 13% and 48 +/- 9%, respectively (n = 10 measurements, 5 cells). These data provide functional evidence that RGS2 modulates purinergic signaling in human and ovine ciliated airway epithelial cells.

The mechanism of ciliary stimulation by acetylcholine: roles of calcium, PKA, and PKG

Stimulation of ciliary cells through muscarinic receptors leads to a strong biphasic enhancement of ciliary beat frequency (CBF). The main goal of this work is to delineate the chain of molecular events that lead to the enhancement of CBF induced by acetylcholine (ACh). Here we show that the Ca(2+), cGMP, and cAMP signaling pathways are intimately interconnected in the process of cholinergic ciliary stimulation. ACh induces profound time-dependent increase in cGMP and cAMP concentrations mediated by the calcium-calmodulin complex. The initial strong CBF enhancement in response to ACh is mainly governed by PKG and elevated calcium. The second phase of CBF enhancement induced by ACh, a stable moderately elevated CBF, is mainly regulated by PKA in a Ca(2+)-independent manner. Inhibition of either guanylate cyclase or of PKG partially attenuates the response to ACh of [Ca(2+)](i), but completely abolishes the response of CBF. Inhibition of PKA moderately attenuates and significantly shortens the responses to ACh of both [Ca(2+)](i) and CBF. In addition, PKA facilitates the elevation in [Ca(2+)](i) and cGMP levels induced by ACh, whereas an unimpeded PKG activity is essential for CBF enhancement mediated by either Ca(2+) or PKA.

Prolonged increase in ciliary beat frequency after short-term purinergic stimulation in human airway epithelial cells

Stimulation of ovine airway epithelial cells with 10 microM ATP for 1 min at 25 degrees C transiently increased both cytoplasmic calcium (fura-2 epifluorescence microscopy) and ciliary beat frequency (CBF; differential interference contrast microscopy) with a similar time course. Identical purinergic stimulation of human airway epithelial cells at 25 or 35 degrees C, however, lead to an increase in CBF that outlasted the calcium transient at least 20 min. While a nitric oxide synthase inhibitor had no effect, pre-treatment of human cells with inhibitors of cAMP-dependent kinase (PKA), 10 microM myristoylated PKA-inhibitory peptide and 1 microM KT-5720, as well as an inhibitor of adenylyl cyclase, 1 mM SQ22536, blocked the prolonged, but not calcium-coupled CBF increase. Addition of PKA inhibitors after purinergic stimulation only partially reduced CBF from its elevated plateau. Prolonged CBF increases did not depend on adenosine production as 10 microM UTP had an effect similar to ATP and 8-sulphophenyl-theophylline did not block them. After increasing human CBF in a PKA-dependent manner to a stable plateau with forskolin (10 microM), ATP caused only a transient, calcium-coupled CBF increase. Calcium transients were necessary for both short-term and prolonged CBF changes as ATP failed to produce CBF increases after emptying calcium stores with 1 microM thapsigargin. These data suggest that in human, but not ovine airway epithelial cells, ATP-induced calcium transients activate a signalling cascade including adenylyl cyclase and PKA. The resulting prolonged CBF stimulation does not rely only on PKA activity, suggesting that the decay of CBF is influenced by ciliary phosphatase activity.

Differential effects of UTP, ATP, and adenosine on ciliary activity of human nasal epithelial cells

The purinergic regulation of ciliary activity was studied using small, continuously superfused explants of human nasal epithelium. The P2Y(2) purinoceptor (P2Y(2)-R) was identified as the major purinoceptor regulating ciliary beat frequency (CBF); UTP (EC(50) = 4.7 microM), ATP, and adenosine-5'-O-(3-thiotriphosphate) elicited similar maximal responses, approximately twofold over baseline. ATP, however, elicited a post-peak sustained plateau in CBF (1.83 +/- 0.1-fold), whereas the post-peak CBF response to UTP declined over 15 min to a low-level plateau (1.36 +/- 0.16-fold). UDP also stimulated ciliary beating, probably via P2Y(6)-R, with a maximal effect approximately one-half that elicited by P2Y(2)-R stimulation. Not indicated were P2Y(1)-R-, P2Y(4)-R-, or P2Y(11)-R-mediated effects. A(2B)-receptor agonists elicited sustained responses in CBF approximately equal to those from UTP/ATP [5'-(N-ethylcarboxamido)adenosine, EC(50) = 0.09 microM; adenosine, EC(50) = 0.7 microM]. Surprisingly, ADP elicited a sustained stimulation in CBF. The ADP effect and the post-peak sustained portion of the ATP response in CBF were inhibited by the A(2)-R antagonist 8-(p-sulfophenyl)theophylline. Hence, ATP affects ciliary activity through P2Y(2)-R and, after an apparent ectohydrolysis to adenosine, through A(2B)AR.

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.

ATP release mechanisms, ATP receptors and purinergic signalling along the nephron

1. Autocrine and paracrine signalling along the nephron of the kidney has been a widely held hypothesis for several decades. The lumen of the nephron is an ideal autocrine and paracrine signalling microenvironment. Any agonist, filtered at the glomerulus or released in the proximal tubule or other proximal segments, is subsequently trapped in the lumen of the nephron and present to interact with luminal receptors. Similar signalling in the renal interstitium is also possible and likely. Indeed, receptors for many autocrine and paracrine agonists have been characterized on the luminal membrane and serosal membrane of multiple nephron segments. 2. An important family of autocrine and paracrine agonists in the kidney are the purinergic agonists. Extracellular ATP, as well as its metabolites (ADP, 5'-AMP and adenosine), is released by renal epithelial cells. These compounds are also freely filtered at the glomerulus and are found in the final urine. Receptors for ATP and adenosine are also expressed on the luminal and serosal side of many nephron segments. 3. The present review discusses purinergic signalling by nucleotide agonists in an integrated manner, from ATP release to ATP receptors to extracellular ATP-mediated effects on renal epithelial function. These themes are the focus of our laboratory in normal and polycystic kidneys as well as in normal and diseased epithelial cells from other tissues. The physiological roles of extracellular purinergic signalling in the kidney and other tissues are only beginning to emerge.

Aerosolization of P2Y(2)-receptor agonists enhances mucociliary clearance in sheep

The purpose of this study was to determine whether aerosolized INS316 (UTP) stimulates lung mucociliary clearance (MCC) in sheep and, if so, to compare its effects with INS365, a novel P2Y(2)-receptor agonist. In the first series of studies, we used a previously described roentgenographic technique to measure tracheal mucus velocity (TMV), an index of MCC, before and for 4 h after aerosolization of INS316 (10(-1) M and 10(-2) M) and INS365 (10(-1) M and 10(-2) M), or normal saline in a randomized crossover fashion (n = 6). In a second series of studies, we compared the ability of these agents to enhance total lung clearance. For these tests, the clearance of inhaled technetium-labeled human serum albumin was measured serially over a 2-h period after aerosolization of 10(-1) M concentration of each agent (n = 7). Aerosolization of both P2Y(2)-receptor agonists induced significant dose-related increases in TMV (P < 0.05) compared with saline. The greatest increase in TMV was observed between 15 and 30 min after drug treatment. The highest dose (10(-1) M) of INS316 produced a greater overall stimulation of TMV than did INS365 (10(-1) M). Both compounds, compared with saline, induced a significant increase in MCC (P < 0.05) within 20 min of treatment. This enhancement in MCC began to plateau at 60 min. Although the response to INS316 started earlier, there was no significant difference between the clearance curves for the two compounds. We conclude that inhaled P2Y(2)-receptor agonists can increase lung MCC in sheep and that for P2Y(2)-receptor stimulation TMV accurately reflects changes in whole lung MCC.

Intracellular calcium oscillations regulate ciliary beat frequency of airway epithelial cells

The effect of ATP-induced Ca2+ oscillations on ciliary activity was examined in airway epithelial cells by simultaneously measuring the ciliary beat frequency (CBF) and the intracellular Ca2+ concentration ([Ca2+]i) near the base of the cilia. Exposure to extracellular ATP (ATPo) induces a rapid and large increase in both [Ca2+]i and CBF, followed by oscillations in [Ca2+]i and a sustained elevation in CBF. After each Ca2+ oscillation, the [Ca2+]i returned to near basal values. By contrast, the CBF remained elevated during these Ca2+ oscillations, although each Ca2+ oscillation induced small variations in CBF. During Ca2+ oscillations, increases in CBF closely followed the rising phase of increases in [Ca2+]i, but declines in CBF lagged behind declines in [Ca2+]i. Higher frequency Ca2+ oscillations reduced variations in CBF, producing a stable and sustained elevation in CBF. The maximal CBF was induced by Ca2+ oscillations and was 15% greater than the CBF induced by the substantially larger initial [Ca2+]i increase. These data demonstrate that the rate of CBF is not directly dependent on the absolute [Ca2+]i, but is dependent on the differential changes in [Ca2+]i and suggest that CBF in airway epithelial cells is regulated by frequency-modulated Ca2+ signaling.

Interplay between the NO pathway and elevated [Ca2+]i enhances ciliary activity in rabbit trachea

1. Average intracellular calcium concentration ([Ca2+]i) and ciliary beat frequency (CBF) were simultaneously measured in rabbit airway ciliated cells in order to elucidate the molecular events that lead to ciliary activation by purinergic stimulation. 2. Extracellular ATP and extracellular UTP caused a rapid increase in both [Ca2+]i and CBF. These effects were practically abolished by a phospholipase C inhibitor (U-73122) or by suramin. 3. The effects of extracellular ATP were not altered: when protein kinase C (PKC) was inhibited by either GF 109203X or chelerythrine chloride, or when protein kinase A (PKA) was inhibited by RP-adenosine 3', 5'-cyclic monophosphothioate triethylamine (Rp-cAMPS). 4. Activation of PKC by phorbol 12-myristate, 13-acetate (TPA) had little effect on CBF or on [Ca2+]i, while activation of PKA by forskolin or by dibutyryl-cAMP led to a small rise in CBF without affecting [Ca2+]i. 5. Direct activation of protein kinase G (PKG) with dibutyryl-cGMP had a negligible effect on CBF when [Ca2+]i was at basal level. However, dibutyryl-cGMP strongly elevated CBF when [Ca2+]i was elevated either by extracellular ATP or by ionomycin. 6. The findings suggest that the initial rise in [Ca2+]i induced by extracellular ATP activates the NO pathway, thus leading to PKG activation. In the continuous presence of elevated [Ca2+]i the stimulated PKG then induces a robust enhancement in CBF. In parallel, activated PKG plays a central role in Ca2+ influx via a still unidentified mechanism, and thus, through positive feedback, maintains CBF close to its maximal level in the continuous presence of ATP.

Airway epithelium of transplanted lungs with and without direct bronchial artery revascularization

OBJECTIVE

Normal systemic blood flow to the airways and lung parenchyma of transplanted lungs can only be re-established by direct bronchial artery revascularization. The purpose of the present study was to investigate whether such direct bronchial artery revascularization would preserve ciliary function, previously shown to be reduced in lungs transplanted without revascularization.

METHODS

Twenty-five single lung transplanted patients were included in this study. Complete direct bronchial artery revascularization was achieved in eight patients. In 16 patients the procedure had either failed (n = 10) or was not attempted (n = 6). In one patient the result of the revascularization was unknown. Airway epithelium samples were obtained from the native and the transplanted lungs during bronchoscopic examinations. Airway erythema and excessive secretion were registered. The epithelium samples underwent histological examination and ciliary beat frequency was measured in vitro by video recording. Transbronchial biopsies from the transplanted lungs were examined for signs of rejection and bronchitis.

RESULTS

No differences in ciliary beat frequency nor in the distribution of ciliated/de-ciliated columnar epithelium cells between native lungs and transplanted lungs with or without successful direct bronchial artery revascularization could be demonstrated. In 38% of the transplanted lungs without successful revascularization metaplastic or squamous epithelium was present, while lungs with successful revascularization had only normal columnar epithelium. Ongoing rejection or airway erythema did not influence ciliary beat frequency. Excessive secretion in the airways was the only finding associated with significantly increased ciliary beat frequency.

CONCLUSIONS

Ciliary beat frequency of epithelium cells of transplanted lungs did not differ from that of native lungs and consequently direct bronchial artery revascularization did not have any demonstrable important influence. Excessive secretion in the airways was associated with increased ciliary beat frequency. The histological findings also showed that the abundance of ciliated cells was preserved in transplanted bronchi irrespective of bronchial artery revascularization. However, epithelium metaplasia was only seen in transplanted bronchi without revascularization.

Extracellular ATP directly gates a cation-selective channel in rabbit airway ciliated epithelial cells

1. A membrane conductance activated by extracellular ATP was identified and characterized in freshly dissociated rabbit airway ciliated cells using the whole-cell and outside-out patch configurations of the patch-clamp technique. 2. In solutions designed to maximize currents through voltage-gated calcium channels, there were no indications of voltage-gated Ba2+ currents. 3. Extracellular ATP (but not UTP or ADP) activated a membrane conductance which remained activated for several minutes in the presence of ATP. The conductance was permeable to monovalent and divalent cations with approximate relative permeabilities (P) for PBa : PCs : PTEA of 4 : 1 : 0.1. Permeability to Cl- was negligible. 4. Including GDP-beta-S in the intracellular solution did not inhibit the effects of ATP, nor did GTP-gamma-S irreversibly activate the conductance. 5. In outside-out membrane patches, with GDP-beta-S in the pipette solution, ATP activated ion channels which had a chord conductance of approximately 6 pS in symmetrical 150 mM CsCl solutions at -120 mV. 6. Suramin (100 microM) inhibited the whole-cell currents activated by ATP (200 microM) by 93 +/- 3 %. Similar effects of suramin were observed on ATP-activated channels in outside-out membrane patches. 7. Extracellular ATP had a priming action on the response to subsequent exposure to ATP. At -40 mV, the time to half-maximal current activation (t1/2) was 46 +/- 9 s during the first exposure to 200 microM ATP and decreased to 5 +/- 3 s during a second exposure to the same concentration of ATP. The priming action of ATP was not inhibited by including GDP-beta-S in the intracellular solution. 8. The initial rate of activation increased with the concentration of ATP, and was voltage sensitive. During the first exposure to 200 microM ATP, t1/2 at +40 mV was 4-fold longer than t1/2 at -40 mV. 9. Half-maximal activation of the conductance shifted from 210 +/- 30 to 14 +/- 4 microM added ATP when CaCl2 in the extracellular solution was reduced from 1.58 to 0. 01 mM. The Hill coefficient for ATP was 1.2 in both solutions.10. These observations suggest that a form of ATP uncomplexed with divalent cations directly gates an ion channel (P2X receptor) in rabbit airway ciliated cells, which serves as a pathway for Ca2+ influx. This purinoceptor may contribute to sustained ciliary activation during prolonged exposures to ATP.

Protein kinase C induced calcium influx and sustained enhancement of ciliary beating by extracellular ATP

The major purpose of this work was to determine protein kinase C (PKC) influence on ciliary beat frequency (CBF) and to assess participation of PKC in purinergic ciliary stimulation. The experiments were performed by simultaneous measurement of [Ca2+]i and CBF on tissue culture of frog esophagus epithelium. The PKC activators TPA and DiC8 produced significant elevation of [Ca2+]i and strong frequency enhancement. The calcium elevation was inhibited by lowering the extracellular calcium level, or by La3+, but was unaffected by verapamil and the phospholipase C inhibitor U-73122, suggesting that Ca2+ influx was via non-voltage-operated calcium channels. The inhibition of [Ca2+]i elevation resulted in corresponding inhibition of CBF enhancement. The effect of TPA was blocked by the selective PKC inhibitors chelerythrine, calphostin C, and GF109203X, and by the enzyme downregulation. The downregulation of PKC, or the enzyme inhibitors did not affect the immediate response to extracellular ATP but caused rapid decay of initially stimulated [Ca2+]i and CBF to the basal level. These results suggest that PKC produces CBF enhancement via activation of calcium influx through non-voltage-operated calcium channels. This calcium influx seems to be responsible for the duration of ciliary stimulation produced by the extracellular ATP.

Purinergically induced membrane fluidization in ciliary cells: characterization and control by calcium and membrane potential

To examine the role of membrane dynamics in transmembrane signal transduction, we studied changes in membrane fluidity in mucociliary tissues from frog palate and esophagus epithelia stimulated by extracellular ATP. Micromolar concentrations of ATP induced strong changes in fluorescence polarization, possibly indicating membrane fluidization. This effect was dosage dependent, reaching a maximum at 10-microM ATP. It was dependent on the presence of extracellular Ca2+ (or Mg2+), though it was insensitive to inhibitors of voltage-gated calcium channels. It was inhibited by thapsigargin and by ionomycin (at low extracellular Ca2+ concentration), both of which deplete Ca2+ stores. It was inhibited by the calcium-activated potassium channel inhibitors quinidine, charybdotoxin, and apamine and was reduced considerably by replacement of extracellular Na+ with K+. Hyperpolarization, or depolarization, of the mucociliary membrane induced membrane fluidization. The degree of membrane fluidization depended on the degree of hyperpolarization or depolarization of the ciliary membrane potential and was considerably lower than the effect induced by extracellular ATP. These results indicate that appreciable membrane fluidization induced by extracellular ATP depends both on an increase in intracellular Ca2+, mainly from its internal stores, and on hyperpolarization of the membrane. Calcium-dependent potassium channels couple the two effects. In light of recent results on the enhancement of ciliary beat frequency, it would appear that extracellular ATP-induced changes both in ciliary beat frequency and in membrane fluidity are triggered by similar signal transduction pathways.

Biochemical and pharmacological characterization of cyclic nucleotide phosphodiesterase in airway epithelium

According to their respective elution order, specificity for cAMP and cGMP, their sensitivity to calmodulin, and their modulation by cGMP and rolipram, four cyclic nucleotide phosphodiesterases (PDE) were separated from the cytosol: PDE I (calmodulin-sensitive), PDE II (stimulated by cGMP, PDE IV (cGMP specific-PDE and inhibited by rolipram) and PDE V (cGMP specific). PDE IV (Km = 1.4 microM) was competitively inhibited by rolipram (Ki = 1.2 microM) whereas PDE V (Km = 0.83 microM) was competitively inhibited by zaprinast in the mumolar range (Ki = 0.12 microM). Moreover the microsomal fraction contained three PDE isoforms: PDE II, PDE III (inhibited by cGMP or indolidan) and PDE IV. These results show that cAMP degradation in cytosolic and membrane fractions is modulated by cGMP and selectively inhibited by rolipram and, in addition, by indolidan in membrane fractions.

Simultaneous measurement of ciliary beating and intracellular calcium

A novel system for measuring, simultaneously, ciliary beating and intracellular free calcium is presented. The advantages and dynamic nature of the system are demonstrated by measuring the effects of the calcium ionophore lonomycin and of extracellular ATP on ciliated rabbit trachea. The results are discussed with regard to the ciliary and calcium stimulation.

The role of airway mucus in pulmonary toxicology

Airway mucus is a complex airway secretion whose primary function as part of the mucociliary transport mechanism is to to serve as renewable and transportable barrier against inhaled particulates and toxic agents. The rheologic properties necessary for this function are imparted by glycoproteins, or mucins. Some respiratory disease states, e.g., asthma, cystic fibrosis, and bronchitis, are characterized by quantitative and qualitative changes in mucus biosynthesis that contribute to pulmonary pathology. Similar alterations in various aspects of mucin biochemistry and biophysics, leading to mucus hypersecretion and altered mucus rheology, result from inhalation of certain air pollutants, such as ozone, sulfur dioxide, nitrogen dioxide, and cigarette smoke. The consequences of these pollutant-induced alterations in mucus biology are discussed in the context of pulmonary pathophysiology and toxicology.

Mechanisms and function of intercellular calcium signaling

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

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

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