Vasoactive intestinal peptide increases cystic fibrosis transmembrane conductance regulator levels in the apical membrane of Calu-3 cells through a protein kinase C-dependent mechanism

Multiple Regulatory Signals and Components in the Modulation of Bicarbonate Transporters

Bicarbonate transporters are responsible for the appropriate flux of bicarbonate across the plasma membrane to perform various fundamental cellular functions. The functions of bicarbonate transporters, including pH regulation, cell migration, and inflammation, are highlighted in various cellular systems, encompassing their participation in both physiological and pathological processes. In this review, we focused on recently identified modulatory signaling components that regulate the expression and activity of bicarbonate transporters. Moreover, we addressed recent advances in our understanding of cooperative systems of bicarbonate transporters and channelopathies. This current review aims to provide a new, in-depth understanding of numerous human diseases associated with the dysfunction of bicarbonate transporters.

VIP reduction in the pancreas of F508del homozygous CF mice and early signs of Cystic Fibrosis Related Diabetes (CFRD)

Vasoactive intestinal peptide (VIP), a 28-amino acid neuropeptide with potent anti-inflammatory, bronchodilatory and immunomodulatory functions, is secreted by intrinsic neurons innervating all exocrine glands, including the pancreas, in which it exerts a regulatory function in the secretion of insulin and glucagon. Cystic fibrosis-related diabetes (CFRD) is the most common co-morbidity associated with cystic fibrosis (CF), impacting approximately 50% of adult patients. We recently demonstrated a 50% reduction of VIP abundance in the lungs, duodenum and sweat glands of C57Bl/6 CF mice homozygous for the F508del-CFTR disease-causing mutation. VIP deficiency resulted from a reduction in VIPergic and cholinergic innervation, starting before signs of CF disease were observed. As VIP functions as a neuromodulator with insulinotropic effect on pancreatic beta cells, we sought to study changes in VIP in the pancreas of CF mice. Our goal was to examine VIP content and VIPergic innervation in the pancreas of 8- and 17-week-old F508del-CFTR homozygous mice and to determine whether changes in VIP levels would contribute to CFRD development. Our data showed that a decreased amount of VIP and reduced innervation are found in CF mice pancreas, and that these mice also exhibited reduced insulin secretion, up-regulation of glucagon production and high random blood glucose levels compared to same-age wild-type mice. We propose that low level of VIP, due to reduced innervation of the CF pancreas and starting at an early disease stage, contributes to changes in insulin and glucagon secretion that can lead to CFRD development.

Disrupted local innervation results in less VIP expression in CF mice tissues

Vasoactive Intestinal Peptide (VIP) is the major physiological agonist of the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) chloride channel activity. VIP functions as a neuromodulator and neurotransmitter secreted by neurons innervating all exocrine glands. VIP is also a potent vasodilator and bronchodilator that regulates exocrine gland secretions, contributing to local innate defense by stimulating the movement of water and chloride transport across intestinal and tracheobronchial epithelia. Previous human studies have shown that the rich intrinsic neuronal networks for VIP secretion around exocrine glands could be lost in tissues from patients with cystic fibrosis. Our research has since confirmed, in vitro and in vivo, the need for chronic VIP exposure to maintain functional CFTR chloride channels at the cell surface of airways and intestinal epithelium, as well as normal exocrine tissues morphology [1]. The goal of the present study was to examine changes in VIP in the lung, duodenum and sweat glands of 8- and 17-weeks old F508del/F508del mice and to investigate VIPergic innervation in the small intestine of CF mice, before important signs of the disease development. Our data show that a low amount of VIP is found in CF tissues prior to tissue damage. Moreover, we found a specific reduction in VIPergic and cholinergic innervation of the small intestine. The general innervation of the primary and secondary myenteric plexus was lost in CF tissues, with the presence of enlarged ganglionic cells in the tertiary layer. We propose that low amount of VIP in CF tissues is due to a reduction in VIPergic and cholinergic innervation and represents an early defect that constitutes an aggravating factor for CF disease progression.

Agonists that stimulate secretion promote the recruitment of CFTR into membrane lipid microdomains

The cystic fibrosis transmembrane conductance regulator (CFTR) is a tightly regulated anion channel that mediates secretion by epithelia and is mutated in the disease cystic fibrosis. CFTR forms macromolecular complexes with many proteins; however, little is known regarding its associations with membrane lipids or the regulation of its distribution and mobility at the cell surface. We report here that secretagogues (agonists that stimulate secretion) such as the peptide hormone vasoactive intestinal peptide (VIP) and muscarinic agonist carbachol increase CFTR aggregation into cholesterol-dependent clusters, reduce CFTR lateral mobility within and between membrane microdomains, and trigger the fusion of clusters into large (3.0 µm2) ceramide-rich platforms. CFTR clusters are closely associated with motile cilia and with the enzyme acid sphingomyelinase (ASMase) that is constitutively bound on the cell surface. Platform induction is prevented by pretreating cells with cholesterol oxidase to disrupt lipid rafts or by exposure to the ASMase functional inhibitor amitriptyline or the membrane-impermeant reducing agent 2-mercaptoethanesulfonate. Platforms are reversible, and their induction does not lead to an increase in apoptosis; however, blocking platform formation does prevent the increase in CFTR surface expression that normally occurs during VIP stimulation. These results demonstrate that CFTR is colocalized with motile cilia and reveal surprisingly robust regulation of CFTR distribution and lateral mobility, most likely through autocrine redox activation of extracellular ASMase. Formation of ceramide-rich platforms containing CFTR enhances transepithelial secretion and likely has other functions related to inflammation and mucosal immunity.

CFTR gets together

JGP study shows that pro-secretory agonists prompt CFTR to assemble into large lipid platforms.

JGP study shows that pro-secretory agonists prompt CFTR to assemble into large lipid platforms.

Role of vasoactive intestinal peptide in Aspergillus fumigatus-infected cornea

AIM

To investigate the anti-inflammatory role of vasoactive intestinal peptide (VIP) in Aspergillus fumigatus (A. fumigatus) ketatitis.

METHODS

Expression of VIP was tested by polymerase chain reaction (PCR) in C57BL/6 and BALB/c normal and A. fumigatus infected corneas. C57BL/6 mice were pretreated with recombinant (r) VIP, while BALB/c mice were pretreated with VIP antagonist, and then infected with A. fumigatus. Clinical score was recorded. Expression of pro- and anti-inflammatory cytokines, toll-like receptor 4 (TLR4), lectin-like oxidized low-density lipoprotein receptor 1 (LOX-1), and neutrophil infiltration were tested by PCR, enzyme-linked immunosorbent assay (ELISA), and myeloperoxidase (MPO) assay.

RESULTS

VIP mRNA expression in BALB/c cornea was higher than C57BL/6 cornea at 1 and 3d post infection (p.i.). rVIP treatment of C57BL/6 mice showed alleviated disease and down-regulated expression of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), while IL-10 expression was up-regulated. Neutrophil infiltration and TLR4, IL-17 expression were decreased after rVIP treatment, while LOX-1 expression was up-regulated in C57BL/6. VIP antagonist pretreatment showed increased disease and higher IL-1β, TNF-α, TLR4, IL-17 and MPO levels, while IL-10 and LOX-1 levels were down-regulated in BALB/c mice.

CONCLUSION

rVIP alleviate disease response of C57BL/6 mice. VIP antagonist resulted in worsened disease of BALB/c mice. VIP proposed anti-inflammatory role in A. fumigatus keratitis.

Parasympathetic innervation regulates tubulogenesis in the developing salivary gland

A fundamental question in development is how cells assemble to form a tubular network during organ formation. In glandular organs, tubulogenesis is a multistep process requiring coordinated proliferation, polarization and reorganization of epithelial cells to form a lumen, and lumen expansion. Although it is clear that epithelial cells possess an intrinsic ability to organize into polarized structures, the mechanisms coordinating morphogenetic processes during tubulogenesis are poorly understood. Here, we demonstrate that parasympathetic nerves regulate tubulogenesis in the developing salivary gland. We show that vasoactive intestinal peptide (VIP) secreted by the innervating ganglia promotes ductal growth, leads to the formation of a contiguous lumen, and facilitates lumen expansion through a cyclic AMP/protein kinase A (cAMP/PKA)-dependent pathway. Furthermore, we provide evidence that lumen expansion is independent of apoptosis and involves the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-regulated Cl(-) channel. Thus, parasympathetic innervation coordinates multiple steps in tubulogenesis during organogenesis.

VIP regulates CFTR membrane expression and function in Calu-3 cells by increasing its interaction with NHERF1 and P-ERM in a VPAC1- and PKCε-dependent manner

Vasoactive intestinal peptide (VIP) is a topical airway gland secretagogue regulating fluid secretions, primarily by stimulating cystic fibrosis transmembrane conductance regulator (CFTR)-dependent chloride secretion that contributes to the airways innate defense mechanism. We previously reported that prolonged VIP stimulation of pituitary adenylate cyclase-activating peptide receptors (VPAC1) in airway cells enhances CFTR function by increasing its membrane stability. In the present study, we identified the key effectors in the VIP signaling cascade in the human bronchial serous cell line Calu-3. Using immunocytochemistry and in situ proximity ligation assays, we found that VIP stimulation increased CFTR membrane localization by promoting its colocalization and interaction with the scaffolding protein Na+/H+ exchange factor 1 (NHERF1), a PDZ protein known as a positive regulator for CFTR membrane localization. VIP stimulation also increased phosphorylation, by protein kinase Cε of the actin-binding protein complex ezrin/radixin/moesin (ERM) and its interaction with NHERF1 and CFTR complex. On the other hand, it reduced intracellular CFTR colocalization and interaction with CFTR associated ligand, another PDZ protein known to compete with NHERF1 for CFTR interaction, inducing cytoplasmic retention and lysosomal degradation. Reducing NHERF1 or ERM expression levels by specific siRNAs prevented the VIP effect on CFTR membrane stability. Furthermore, iodide efflux assays confirmed that NHERF1 and P-ERM are necessary for VIP regulation of the stability and sustained activity of membrane CFTR. This study shows the cellular mechanism by which prolonged VIP stimulation of airway epithelial cells regulates CFTR-dependent secretions.

Cystic fibrosis transmembrane conductance regulator dysfunction in VIP knockout mice

Vasoactive intestinal peptide (VIP), a neuropeptide, controls multiple functions in exocrine tissues, including inflammation, and relaxation of airway and vascular smooth muscles, and regulates CFTR-dependent secretion, which contributes to mucus hydration and local innate defense of the lung. We had previously reported that VIP stimulates the VPAC1 receptor, PKCϵ signaling cascade, and increases CFTR stability and function at the apical membrane of airway epithelial cells by reducing its internalization rate. Moreover, prolonged VIP stimulation corrects the molecular defects associated with F508del, the most common CFTR mutation responsible for the genetic disease cystic fibrosis. In the present study, we have examined the impact of the absence of VIP on CFTR maturation, cellular localization, and function in vivo using VIP knockout mice. We have conducted pathological assessments and detected signs of lung and intestinal disease. Immunodetection methods have shown that the absence of VIP results in CFTR intracellular retention despite normal expression and maturation levels. A subsequent loss of CFTR-dependent chloride current was measured in functional assays with Ussing chamber analysis of the small intestine ex vivo, creating a cystic fibrosis-like condition. Interestingly, intraperitoneal administration of VIP corrected tissue abnormalities, close to the wild-type phenotype, as well as associated defects in the vital CFTR protein. The results show in vivo a primary role for VIP chronic exposure in CFTR membrane stability and function and confirm in vitro data.

Calcium-dependent aggregation and fusion of phosphatidylcholine liposomes induced by complexes of flavonoids with divalent iron

It was found that complexes of the flavonoids quercetin, taxifolin, catechin and morin with divalent iron initiated an increase in light scattering in a suspension of unilamellar 100nm liposomes. The concentration of divalent iron in the suspension was 10μM. Liposomes were prepared from 1-palmitoyl-2-oleoylglycero-3-phoshpatidylcholine. The fluorescent resonance energy transfer (FRET) analysis of liposomes labeled with NBD-PE and lissamine rhodamine B dyes detected a slow lipid exchange in liposomes treated with flavonoid-iron complexes and calcium, while photon correlation spectroscopy and freeze-fracture electron microscopy revealed the aggregation and fusion of liposomes to yield gigantic vesicles. Such processes were not found in liposomes treated with phloretin because this flavonoid is unable to interact with iron. Rutin was also unable to initiate any marked changes because this water-soluble flavonoid cannot interact with the lipid bilayer. The experimental data and computer calculations of lipophilicity (cLogP) as well as the charge distribution on flavonoid-iron complexes indicate that the adhesion of liposomes is provided by an iron link between flavonoid molecules integrated in adjacent bilayers. It is supposed that calcium cations facilitate the aggregation and fusion of liposomes because they interact with the phosphate moieties of lipids.

VIP-dependent increase in F508del-CFTR membrane localization is mediated by PKCε

The most common cystic fibrosis causing mutation F508del induces early degradation and reduced trafficking of cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels to the apical membrane of epithelial cells. In the human nasal epithelial cells JME/CF15, we previously reported that vasoactive intestinal peptide (VIP) exposure corrects trafficking and membrane insertion of functional F508del-CFTR channels at 37°C. Correction of trafficking was PKA dependent, whereas enhanced membrane localization involved PKC. In the present study, we have identified PKCε as the isoform involved in VIP-dependent F508del-CFTR membrane insertion. Iodide effluxes were used to monitor the presence of VIP-rescued functional F508del-CFTR channels at the surface of JME/CF15 cells maintained at 37°C. Iodide efflux peaks measured in response to stimulation with forskolin were insensitive to PKC α, β, γ, δ, ζ inhibitors. In contrast, efflux peaks were completely inhibited by pretreatment with the PKCε inhibitor peptide EAVSLKPT with an IC50 of 4.9 μM or by PKCε small interfering RNA (siRNA). Immunostaining and confocal microscopy confirmed that membrane localization of F508del-CFTR induced by VIP was abolished in the presence of EAVSLKPT but not with other isoform inhibitors. In recombinant baby hamster kidney cells, endogenously expressing PKCε but no VIP receptor, wild-type, and F508del-CFTR sensitivity to cpt-cAMP stimulation was increased by PMA treatment. Biotinylation assays and immunoblots confirmed that PMA (0.5–2 h) induced a greater than threefold increase in membrane CFTR, whereas forskolin had no effect. The PMA effect was abolished by specifically inhibiting PKCε (EAVSLKPT IC50 = 5.7 μM) but not other PKC isoforms. Taken together, these results indicate that stimulating PKCε by VIP or PMA increases membrane insertion and activity of WT- and F508del-CFTR.

Activated PKC{delta} and PKC{epsilon} inhibit epithelial chloride secretion response to cAMP via inducing internalization of the Na+-K+-2Cl- cotransporter NKCC1

The basolateral Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) is a key determinant of transepithelial chloride secretion and dysregulation of chloride secretion is a common feature of many diseases including secretory diarrhea. We have previously shown that activation of protein kinase C (PKC) markedly reduces transepithelial chloride secretion in human colonic T84 cells, which correlates with both functional inhibition and loss of the NKCC1 surface expression. In the present study, we defined the specific roles of PKC isoforms in regulating epithelial NKCC1 and chloride secretion utilizing adenoviral vectors that express shRNAs targeting human PKC isoforms (α, δ, ε) (shPKCs) or LacZ (shLacZ, non-targeting control). After 72 h of adenoviral transduction, protein levels of the PKC isoforms in shPKCs-T84 cells were decreased by ∼90% compared with the shLacZ-control. Activation of PKCs by phorbol 12-myristate 13-acetate (PMA) caused a redistribution of NKCC1 immunostaining from the basolateral membrane to intracellular vesicles in both shLacZ- and shPKCα-T84 cells, whereas the effect of PMA was not observed in shPKCδ- and shPKCε- cells. These results were further confirmed by basolateral surface biotinylation. Furthermore, activation of PKCs by PMA inhibited cAMP-stimulated chloride secretion in the uninfected, shLacZ- and shPKCα-T84 monolayers, but the inhibitory effect was significantly attenuated in shPKCδ- and shPKCε-T84 monolayers. In conclusion, the activated novel isoforms PKCδ or PKCε, but not the conventional isoform PKCα, inhibits transepithelial chloride secretion through inducing internalization of the basolateral surface NKCC1. Our study reveals that the novel PKC isoform-regulated NKCC1 surface expression plays an important role in the regulation of chloride secretion.

Mucus secretion from individual submucosal glands of the ferret trachea

Mucus secretion from individual tracheal glands in adult ferrets was studied with time-lapse optical imaging of mucus droplets under an oil layer. Density of functional glands (determined by responses to 1 muM carbachol) was 1.5 +/- 0.3 per mm(2) (n = 6). Secretion rates (in pl.min(-1).gland(-1)) were as follows: 4.1 +/- 0.7 basal (unstimulated; n = 27, 669 glands), 338 +/- 70 to 10 microM forskolin (n = 8, 90 glands), 234 +/- 13 to 1 microM VIP (n = 6, 57 glands), 183 +/- 92 to 10 microM isoproterenol (n = 3, 33 glands), 978 +/- 145 to 1 microM carbachol (n = 11, 131 glands), and 1,348 +/- 325 to 10 muM phenylephrine (n = 7, 74 glands). The potency (EC(50), in microM) and efficacy (V(max), in pl x min(-1) x gland(-1)) were 7.6 (EC(50)) and 338 +/- 16 (V(max)) to forskolin, 1.0 (EC(50)) and 479 +/- 19 (V(max)) to VIP, 0.6 (EC(50)) and 1,817 +/- 268 (V(max)) to carbachol, and 3.7 (EC(50)) and 1,801 +/- 95 (V(max)) to phenylephrine. Although carbachol and phenylephrine were equally effective secretagogues, only carbachol caused contractions of the trachealis muscle. Synergy was demonstrated between 300 nM isoproterenol and 100 nM carbachol, which, when combined, produced a secretion rate almost fourfold greater than predicted from their additive effect. The dependence of fluid secretion on Cl(-) and HCO(3)(-) varied depending on the mode of stimulation. Secretion stimulated by VIP or forskolin was reduced by approximately 60% by blocking either anion, while carbachol-stimulated secretion was blocked 68% by bumetanide and only 32% by HEPES replacement of HCO(3)(-). These results provide parametric data for comparison with fluid secretion from glands in ferrets lacking CFTR.

Rescue of functional F508del cystic fibrosis transmembrane conductance regulator by vasoactive intestinal peptide in the human nasal epithelial cell line JME/CF15

F508del is the most common cystic fibrosis-causing mutation that induces early degradation and poor trafficking of cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels to the apical membrane of epithelial cells. Our previous work in bronchial serous cells showed that vasoactive intestinal peptide (VIP) stimulation of the VPAC(1) receptor enhances CFTR-dependent chloride secretion by increasing its membrane insertion by a protein kinase C (PKC)-dependent pathway. In the present study, we investigated the effect of VIP on F508del-CFTR activity and membrane insertion in the human nasal epithelial cell line JME/CF15, which also expresses the VPAC(1) receptor. At reduced temperature (27 degrees C), which rescues F508del-CFTR trafficking, acute stimulation by VIP of rescued F508del-CFTR channels was protein kinase A (PKA)- and PKC-dependent. One hour of treatment with VIP strongly increased F508del-CFTR activity, with iodide efflux peaks three times higher than with untreated cells. At 37 degrees C, VIP-treated cells, but not untreated controls, showed significant iodide efflux peaks that were sensitive to the CFTR inhibitor 3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl)methylene]-2-thioxo-4-thiazolidinone (CFTR(inh)-172). Immunostaining, biotinylation assays, and Western blots confirmed a VIP-induced maturation and membrane insertion of F508del-CFTR at 37 degrees C. The corrector effect of VIP was abolished by the PKA inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamidedihydrochloride (H89), whereas Galpha(s) stimulation by cholera toxin significantly increased F508del-CFTR trafficking. On the other hand, membrane localization, but not maturation, of F508del-CFTR was significantly reduced by the PKC inhibitor bisindolylmaleimide X and the G(i/o) protein inhibitor pertussis toxin. VIP treatment had no effect on intracellular calcium or proteasome activity. These results indicate that, in human nasal cells, VIP rescues trafficking and membrane insertion of functional F508del-CFTR channels at physiological temperature by stimulating both PKA- and PKC-dependent pathways.

Breaking human cytomegalovirus major immediate-early gene silence by vasoactive intestinal peptide stimulation of the protein kinase A-CREB-TORC2 signaling cascade in human pluripotent embryonal NTera2 cells

The triggering mechanisms underlying reactivation of human cytomegalovirus (HCMV) in latently infected persons are unclear. During latency, HCMV major immediate-early (MIE) gene expression breaks silence to initiate viral reactivation. Using quiescently HCMV-infected human pluripotent embryonal NTera2 cells (NT2) to model HCMV reactivation, we show that vasoactive intestinal peptide (VIP), an immunomodulatory neuropeptide, immediately and dose-dependently (1 to 500 nM) activates HCMV MIE gene expression. This response requires the MIE enhancer cyclic AMP response elements (CRE). VIP quickly elevates CREB Ser133 and ATF-1 Ser63 phosphorylation levels, although the CREB Ser133 phosphorylation level is substantial at baseline. VIP does not change the level of HCMV genomes in nuclei, Oct4 (pluripotent cell marker), or hDaxx (cellular repressor of HCMV gene expression). VIP-activated MIE gene expression is mediated by cellular protein kinase A (PKA), CREB, and TORC2. VIP induces PKA-dependent TORC2 Ser171 dephosphorylation and nuclear entry, which likely enables MIE gene activation, as TORC2 S171A (devoid of Ser171 phosphorylation) exhibits enhanced nuclear entry and desilences the MIE genes in the absence of VIP stimulation. In conclusion, VIP stimulation of the PKA-CREB-TORC2 signaling cascade activates HCMV CRE-dependent MIE gene expression in quiescently infected NT2 cells. We speculate that neurohormonal stimulation via this signaling cascade is a possible means for reversing HCMV silence in vivo.

Trafficking of immature DeltaF508-CFTR to the plasma membrane and its detection by biotinylation

Recent studies suggest that immature, core-glycosylated DeltaF508-CFTR [the predominant mutant form of the CFTR (cystic fibrosis transmembrane conductance regulator)] can reach the plasma membrane under some conditions. In the present study we investigated this possibility since it has implications for understanding how therapeutics rescue the trafficking of mutant CFTR and perhaps other misfolded proteins. Core-glycosylated CFTR was labelled and pulled down on streptavidin beads after exposure to sulfo-NHS-SS-biotin [biotin attached to a reactive NHS (N-hydroxysuccinimide) ester with a disulfide spacer; molecular mass=606.7 Da]; however, intracellular proteins were also detected in the precipitates. When the R domain of CFTR was expressed in the cytosol of BHK (baby-hamster kidney) cells as a soluble polypeptide it was also labelled after surface biotinylation and pulled down on streptavidin beads. Intracellular biotinylation was reduced when cells were treated with sulfo-NHS-LC-biotin (biotin attached to a reactive NHS ester with an aminocaproic acid spacer) or sulfo-NHS-PEO(12)-biotin [biotin attached to a reactive NHS ester with a poly(ethylene glycol) spacer], but the reduction could be explained by the lower reactivity of these reagents. The R domain was detected on Western blots after loading <0.25% of the pulldown sample ( approximately 0.01% of total lysate protein), a fraction that could be ascribed to cells that were permeable to ethidium homodimer-1 (molecular mass=856.8 Da) and propidium iodide (molecular mass=668.6 Da). When BHK cells were incubated at 29 degrees C to rescue DeltaF508-CFTR trafficking, and then biotinylated and sorted to remove permeable cells, labelling of core-glycosylated DeltaF508-CFTR was no longer detected although a weak signal was still observed using CFBE (cystic fibrosis bronchial epithelial) cells. These results suggest that there is weak surface expression of immature DeltaF508-CFTR on airway epithelial cells and demonstrate the need to remove permeable cells when studying CFTR glycoforms by surface biotinylation.

Purinergic receptors in airway epithelia

Nucleotides and nucleosides within the airway surface liquid regulate mucociliary clearance (MCC) activities, the primary innate defense mechanism that removes foreign particles and pathogens from airway surfaces. Nucleotide and nucleoside actions in the airways are mediated mainly by two purinergic receptor subtypes, the Gq-coupled ATP/UTP-sensing P2Y2 receptor and the Gs-coupled A2b adenosine receptor. Activation of the A2b receptor results in cyclic AMP-dependent activation of the cystic fibrosis transmembrane regulator (CFTR) Cl- channel and stimulation of ciliary beat frequency. Agonist occupation of the P2Y2 receptor promotes inhibition of Na+ absorption as well as CFTR-dependent and CFTR-independent Cl- secretion, ciliary beating, and mucin secretion.