Mode of Ca2+ action on ciliary beat frequency in single ovine airway epithelial cells

Inhibition of ezrin phosphorylation by NSC305787 attenuates procaterol-stimulated ciliary beating in airway cilia

Ciliary beating in the airway epithelium plays an important role in preventing infection by eliminating small particles and pathogens. Stimulation of β2 adrenergic receptor (β2AR) increases [cAMP]i levels and strongly activates this ciliary beating. β2AR is localized to the apical membrane of the airways by indirectly binding to ezrin, an actin-binding protein. Ezrin takes active phosphorylated and inactive dephosphorylated states at Thr-567. Previously we showed that procaterol-stimulated ciliary beating was impaired in the ezrin-knockdown mice. In this study, we examined the roles of ezrin and its phosphorylation in regulating ciliary beating by using NSC305787, an ezrin inhibitor, in normal human airway epithelial cells (NHBE). We found that NSC305787 inhibits the phosphorylation of ezrin with an IC50 of 50 μM in NHBE. Treatment with NSC305787 for 4 h or more decreased the expression of β2AR in the cell membrane and induced vesicle- or dot-like expression of ezrin and β2AR inside the cell. As a result, inhibition of ezrin phosphorylation by NSC305787 attenuated the effect of procaterol-induced activation of ciliary beating in both frequency and distance indices.

A fixed 20:1 combination of cafedrine/theodrenaline increases cytosolic Ca2+ concentration in human tracheal epithelial cells via ryanodine receptor-mediated Ca2+ release

Mucociliary clearance is a pivotal physiological mechanism that protects the lung by cleaning the airways from pollution and colonization, thereby preventing infection. Ciliary function is influenced by various signal transduction cascades, and Ca2+ represents a key second messenger. A fixed 20:1 combination of cafedrine and theodrenaline has been widely used to treat perioperative hypotension and emergency hypotensive states since the 1960s; however, its effect on the intracellular Ca2+ concentration ([Ca2+]i) of respiratory epithelium remains unknown. Therefore, human tracheal epithelial cells were exposed to the clinically applied 20:1 mixture of cafedrine/theodrenaline and the individual substances separately. [Ca2+]i was assessed by FURA-2 340/380 fluorescence ratio. Pharmacological inhibitors were applied to elucidate relevant signal transduction cascades, and reverse transcription polymerase chain reaction (RT-PCR) was performed on murine tracheal epithelium to analyze ryanodine receptor (RyR) subtype expression. All three pharmacological preparations instantaneously induced a steep increase in [Ca2+]i that quickly returned to its baseline value despite the persistence of each substance. Peak [Ca2+]i following the administration of 20:1 cafedrine/theodrenaline, cafedrine alone, and theodrenaline alone increased in a dose-dependent manner, with median effective concentrations of 0.35 mM (7.32 mM cafedrine and 0.35 mM theodrenaline), 3.14 mM, and 3.45 mM, respectively. When extracellular Ca2+ influx was inhibited using a Ca2+-free buffer solution, the peak [Ca2+]i following the administration of cafedrine alone and theodrenaline alone were reduced but not abolished. No alteration in [Ca2+]i compared with baseline [Ca2+]i was observed during β-adrenergic receptor inhibition. Depletion of caffeine-sensitive stores and inhibition of RyR, but not IP3 receptors, completely abolished any increase in [Ca2+]i. However, [Ca2+]i still increased following the depletion of mitochondrial Ca2+ stores using 2,4-dinitrophenol. RT-PCR revealed RyR-2 and RyR-3 expression on murine tracheal epithelium. Although our experiments showed that cafedrine/theodrenaline, cafedrine alone, or theodrenaline alone release Ca2+ from intracellular stores through mechanisms that are exclusively triggered by β-adrenergic receptor stimulation, which most probably lead to RyR activation, clinical plasma concentrations are considerably lower than those used in our experiments to elicit an increase in [Ca2+]i; therefore, further studies are needed to evaluate the ability of cafedrine/theodrenaline to alter mucociliary clearance in clinical practice.

E-cigarette aerosols of propylene glycol impair BK channel activity and parameters of mucociliary function

Propylene glycol (PG) is a common delivery vehicle for nicotine and flavorings in e-cigarette (e-cig) liquids and is largely considered safe for ingestion. However, little is known about its effects as an e-cig aerosol on the airway. Here, we investigated whether pure PG e-cig aerosols in realistic daily amounts impact parameters of mucociliary function and airway inflammation in a large animal model (sheep) in vivo and primary human bronchial epithelial cells (HBECs) in vitro. Five-day exposure of sheep to e-cig aerosols of 100% PG increased mucus concentrations (% mucus solids) of tracheal secretions. PG e-cig aerosols further increased the activity of matrix metalloproteinase-9 (MMP-9) in tracheal secretions. In vitro exposure of HBECs to e-cig aerosols of 100% PG decreased ciliary beating and increased mucus concentrations. PG e-cig aerosols further reduced the activity of large conductance, Ca2+-activated, and voltage-dependent K+ (BK) channels. We show here for the first time that PG can be metabolized to methylglyoxal (MGO) in airway epithelia. PG e-cig aerosols increased levels of MGO and MGO alone reduced BK activity. Patch-clamp experiments suggest that MGO can disrupt the interaction between the major pore-forming BK subunit human Slo1 (hSlo1) and the gamma regulatory subunit LRRC26. PG exposures also caused a significant increase in mRNA expression levels of MMP9 and interleukin 1 beta (IL1B). Taken together, these data show that PG e-cig aerosols cause mucus hyperconcentration in sheep in vivo and HBECs in vitro, likely by disrupting the function of BK channels important for airway hydration.

Mucociliary Wnt signaling promotes cilia biogenesis and beating

It is widely thought that Wnt/Lrp6 signaling proceeds through the cytoplasm and that motile cilia are signaling-inert nanomotors. Contrasting both views, we here show in the mucociliary epidermis of X. tropicalis embryos that motile cilia transduce a ciliary Wnt signal that is distinct from canonical β-catenin signaling. Instead, it engages a Wnt-Gsk3-Ppp1r11-Pp1 signaling axis. Mucociliary Wnt signaling is essential for ciliogenesis and it engages Lrp6 co-receptors that localize to cilia via a VxP ciliary targeting sequence. Live-cell imaging using a ciliary Gsk3 biosensor reveals an immediate response of motile cilia to Wnt ligand. Wnt treatment stimulates ciliary beating in X. tropicalis embryos and primary human airway mucociliary epithelia. Moreover, Wnt treatment improves ciliary function in X. tropicalis ciliopathy models of male infertility and primary ciliary dyskinesia (ccdc108, gas2l2). We conclude that X. tropicalis motile cilia are Wnt signaling organelles that transduce a distinct Wnt-Pp1 response.

Vegetable glycerin e-cigarette aerosols cause airway inflammation and ion channel dysfunction

Vegetable glycerin (VG) and propylene glycol (PG) serve as delivery vehicles for nicotine and flavorings in most e-cigarette (e-cig) liquids. Here, we investigated whether VG e-cig aerosols, in the absence of nicotine and flavors, impact parameters of mucociliary function in human volunteers, a large animal model (sheep), and air-liquid interface (ALI) cultures of primary human bronchial epithelial cells (HBECs). We found that VG-containing (VG or PG/VG), but not sole PG-containing, e-cig aerosols reduced the activity of nasal cystic fibrosis transmembrane conductance regulator (CFTR) in human volunteers who vaped for seven days. Markers of inflammation, including interleukin-6 (IL6), interleukin-8 (IL8) and matrix metalloproteinase-9 (MMP9) mRNAs, as well as MMP-9 activity and mucin 5AC (MUC5AC) expression levels, were also elevated in nasal samples from volunteers who vaped VG-containing e-liquids. In sheep, exposures to VG e-cig aerosols for five days increased mucus concentrations and MMP-9 activity in tracheal secretions and plasma levels of transforming growth factor-beta 1 (TGF-β1). In vitro exposure of HBECs to VG e-cig aerosols for five days decreased ciliary beating and increased mucus concentrations. VG e-cig aerosols also reduced CFTR function in HBECs, mechanistically by reducing membrane fluidity. Although VG e-cig aerosols did not increase MMP9 mRNA expression, expression levels of IL6, IL8, TGFB1, and MUC5AC mRNAs were significantly increased in HBECs after seven days of exposure. Thus, VG e-cig aerosols can potentially cause harm in the airway by inducing inflammation and ion channel dysfunction with consequent mucus hyperconcentration.

The CFTR Amplifier Nesolicaftor Rescues TGF-β1 Inhibition of Modulator-Corrected F508del CFTR Function

Highly effective cystic fibrosis transmembrane conductance regulator (CFTR) modulators have led to dramatic improvements in lung function in many people with cystic fibrosis (PwCF). However, the efficacy of CFTR modulators may be hindered by persistent airway inflammation. The cytokine transforming growth factor-beta1 (TGF-β1) is associated with worse pulmonary disease in PwCF and can diminish modulator efficacy. Thus, strategies to augment the CFTR response to modulators in an inflammatory environment are needed. Here, we tested whether the CFTR amplifier nesolicaftor (or PTI-428) could rescue the effects of TGF-β1 on CFTR function and ciliary beating in primary human CF bronchial epithelial (CFBE) cells. CFBE cells homozygous for F508del were treated with the combination of elexacaftor/tezacaftor/ivacaftor (ETI) and TGF-β1 in the presence and absence of nesolicaftor. Nesolicaftor augmented the F508del CFTR response to ETI and reversed TGF-β1-induced reductions in CFTR conductance by increasing the expression of CFTR mRNA. Nesolicaftor further rescued the reduced ciliary beating and increased expression of the cytokines IL-6 and IL-8 caused by TGF-β1. Finally, nesolicaftor augmented the F508del CFTR response to ETI in CFBE cells overexpressing miR-145, a negative regulator of CFTR expression. Thus, CFTR amplifiers, but only when used with highly effective modulators, may provide benefit in an inflamed environment.

Losartan reduces cigarette smoke-induced airway inflammation and mucus hypersecretion

The aim was to determine whether losartan reduces cigarette smoke (CS)-induced airway inflammation and mucus hypersecretion in an in vitro model and a small clinical trial. Primary human bronchial epithelial cells (HBECs) were differentiated at the air-liquid interface (ALI) and exposed to CS. Expression of transforming growth factor (TGF)-β1 and the mucin MUC5AC, and expression or activity of matrix metalloproteinase (MMP)-9 were measured after CS exposure. Parameters of mucociliary clearance were evaluated by measuring airway surface liquid volumes, mucus concentrations, and conductance of cystic fibrosis transmembrane conductance regulator (CFTR) and large conductance, Ca²⁺-activated and voltage-dependent potassium (BK) channels. Nasal cells were collected from study participants and expression of MUC5AC, TGF-β1, and MMP-9 mRNAs was measured before and after losartan treatment. In vitro, CS exposure of HBECs caused a significant increase in mRNA expression of MUC5AC and TGF-β1 and MMP-9 activity and decreased CFTR and BK channel activities, thereby reducing airway surface liquid volumes and increasing mucus concentrations. Treatment of HBECs with losartan rescued CS-induced CFTR and BK dysfunction and caused a significant decrease in MUC5AC expression and mucus concentrations, partially by inhibiting TGF-β signalling. In a prospective clinical study, cigarette smokers showed significantly reduced mRNA expression levels of MUC5AC, TGF-β1, and MMP-9 in the upper airways after 2 months of losartan treatment. Our findings suggest that losartan may be an effective therapy to reduce inflammation and mucus hypersecretion in CS-induced chronic airway diseases.

Caspofungin induces the release of Ca2+ ions from internal stores by activating ryanodine receptor-dependent pathways in human tracheal epithelial cells

The antimycotic drug caspofungin is known to alter the cell function of cardiomyocytes and the cilia-bearing cells of the tracheal epithelium. The objective of this study was to investigate the homeostasis of intracellular Ca²⁺ concentration ([Ca²⁺]_i) after exposure to caspofungin in isolated human tracheal epithelial cells. The [Ca²⁺]_i was measured using the ratiometric fluoroprobe FURA-2 AM. We recorded two groups of epithelial cells with distinct responses to caspofungin exposure, which demonstrated either a rapid transient rise in [Ca²⁺]_i or a sustained elevation of [Ca²⁺]_i. Both patterns of Ca²⁺ kinetics were still observed when an influx of transmembraneous Ca²⁺ ions was pharmacologically inhibited. Furthermore, in extracellular buffer solutions without Ca²⁺ ions, caspofungin exposure still evoked this characteristic rise in [Ca²⁺]_i. To shed light on the origin of the Ca²⁺ ions responsible for the elevation in [Ca²⁺]_i we investigated the possible intracellular storage of Ca²⁺ ions. The depletion of mitochondrial Ca²⁺ stores using 25 µM 2,4-dinitrophenol (DNP) did not prevent the caspofungin-induced rise in [Ca²⁺]_i, which was rapid and transient. However, the application of caffeine (30 mM) to discharge Ca²⁺ ions that were presumably stored in the endoplasmic reticulum (ER) prior to caspofungin exposure completely inhibited the caspofungin-induced changes in [Ca²⁺]_i levels. When the ER-bound IP₃ receptors were blocked by 2-APB (40 µM), we observed a delayed transient rise in [Ca²⁺]_i as a response to the caspofungin. Inhibition of the ryanodine receptors (RyR) using 40 µM ryanodine completely prevented the caspofungin-induced elevation of [Ca²⁺]_i. In summary, caspofungin has been shown to trigger an increase in [Ca²⁺]_i independent from extracellular Ca²⁺ ions by liberating the Ca²⁺ ions stored in the ER, mainly via a RyR pathway.

[Ca2+ ]i modulation of cAMP-stimulated ciliary beat frequency via PDE1 in airway ciliary cells of mice

NEW FINDINGS

What is the central question of this study? The ciliary beat frequency (CBF) of the airway is controlled by [Ca²⁺ ]_i . However, the effects of a reduction in [Ca²⁺ ]_i on CBF are still controversial (an increase, a decrease or no change). What is the main finding and its importance? This study demonstrated that [Ca²⁺ ]_i directly regulates CBF (direct action) and also indirectly regulates CBF via cAMP accumulation controlled by Ca²⁺ -dependent PDE1 activity (indirect action). The final CBF is determined by the balance of direct and indirect actions. PDE1 plays crucial roles in the regulation of airway CBF. ABSTRACT: [Ca²⁺ ]_i plays crucial roles in the regulation of ciliary beat frequency (CBF) and ciliary bend angle (CBA) of airway cilia. Moreover, Ca²⁺ -dependent PDE1A existing in the CBF-regulating metabolon of cilia modifies the CBF by regulating the cAMP accumulation. This study demonstrated that the CBF is regulated by a direct and an indirect action of [Ca²⁺ ]_i ; the direct action changes CBF mediated via [Ca²⁺ ]_i , and the indirect action changes CBF mediated via cAMP, the accumulation of which is controlled by PDE1 activity. Upon reducing [Ca²⁺ ]_i to various levels, the direct action decreases CBF and the indirect action increases CBF. The final CBF is determined by the extent of cAMP accumulation, which is determined by the amount of inhibition of PDE1 activity, dependent on a reduction in [Ca²⁺ ]_i ; a slight decrease induced by a nominally Ca²⁺ -free solution (no cAMP accumulation via PDE1) decreases CBF, and an extreme decrease induced by 50 μm BAPTA-AM increases CBF via cAMP accumulation by inhibiting PDE1 in a similar manner to a PDE1 inhibitor (8MmIBMX). The increase in CBA in response to a reduction in [Ca²⁺ ]_i is smaller than the increase in CBF, because no PDE1A exists in the CBA-regulating metabolon. On the contrary, an increase in [Ca²⁺ ]_i induced by ionomycin, which decreases cAMP accumulation by PDE1A activation, caused a slower procaterol-stimulated increase in CBF than that decreased by a Ca²⁺ -free solution. A decrease in [Ca²⁺ ]_i stimulates cAMP accumulation, whereas an increase in [Ca²⁺ ]_i inhibits cAMP accumulation in airway ciliary cells. Thus, changes in [Ca²⁺ ]_i modulate CBF and CBA via cAMP accumulation by controlling the activity of PDE1.

A low [Ca2+]i-induced enhancement of cAMP-activated ciliary beating by PDE1A inhibition in mouse airway cilia

This study demonstrated that PDE1 (phosphodiesterase 1) existing in the ciliary beat frequency (CBF)-regulating metabolon regulates CBF in procaterol-stimulated lung airway ciliary cells of mouse. Procaterol (an β₂-agonist) increased the ciliary bend angle (CBA) and CBF via cAMP accumulation in the ciliary cells of mice: interestingly, the time course of CBF increase was slower than that of CBA increase. However, IBMX (3-isobutyl-1-methylxanthine, an inhibitor of PDE) increased CBA and CBF in an identical time course. Lowering an intracellular Ca²⁺ concentration ([Ca²⁺]_i) caused by switching to an EGTA-containing Ca²⁺-free solution from normal one elevated the procaterol-induced increasing rate of CBF. These observations suggest that Ca²⁺-dependent PDE1 controls cAMP-stimulated CBF increase. Either application of 8MmIBMX (8-methoxymethyl-IBMX, a selective PDE1 inhibitor), BAPTA-AM (an intracellular Ca²⁺ chelator), or calmidazolium (an inhibitior of calmodulin) alone increased CBA and CBF in the lung airway ciliary cells and increased cAMP contents in the isolated lung cells, and like IBMX, each application of the compound made the time courses of CBA and CBF increase stimulated by procaterol identical. The immunoelectron microscopic examinations revealed that PDE1A exists in the space between the nine doublet tubules ring and plasma membrane in the lung airway cilium, where the outer dynein arm (a molecular motor regulating CBF) functions. In conclusion, PDE1A is a key factor slowing the time course of the procaterol-induced increase in CBF via degradation of cAMP in the CBF-regulating metabolon of the mouse lung airway cilia.

Marked increases in mucociliary clearance produced by synergistic secretory agonists or inhibition of the epithelial sodium channel

Mucociliary clearance (MCC) is a critical host innate defense mechanism in airways, and it is impaired in cystic fibrosis (CF) and other obstructive lung diseases. Epithelial fluid secretion and absorption modify MCC velocity (MCCV). We tested the hypotheses that inhibiting fluid absorption accelerates MCCV, whereas inhibiting fluid secretion decelerates it. In airways, ENaC is mainly responsible for fluid absorption, while anion channels, including CFTR and Ca²⁺-activated chloride channels mediate anion/fluid secretion. MCCV was increased by the cAMP-elevating agonists, forskolin or isoproterenol (10 μM) and by the Ca²⁺-elevating agonist, carbachol (0.3 μM). The CFTR-selective inhibitor, CFTR_inh-172, modestly reduced MCCV-increases induced by forskolin or isoproterenol but not increases induced by carbachol. The ENaC inhibitor benzamil increased basal MCCV as well as MCCV increases produced by forskolin or carbachol. MCC velocity was most dramatically accelerated by the synergistic combination of forskolin and carbachol, which produced near-maximal clearance rates regardless of prior treatment with CFTR or ENaC inhibitors. In CF airways, where CFTR-mediated secretion (and possibly synergistic MCC) is lost, ENaC inhibition via exogenous agents may provide therapeutic benefit, as has long been proposed.

Roflumilast partially reverses smoke-induced mucociliary dysfunction

BACKGROUND

Phosphodiesterases (PDEs) break down cAMP, thereby regulating intracellular cAMP concentrations and diffusion. Since PDE4 predominates in airway epithelial cells, PDE4 inhibitors can stimulate Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) by increasing cAMP. Tobacco smoking and COPD are associated with decreased CFTR function and impaired mucociliary clearance (MCC). However, the effects of the PDE4 inhibitor roflumilast on smoke-induced mucociliary dysfunction have not been fully explored.

METHODS

Primary normal human bronchial epithelial cells (NHBE) from non-smokers, cultured at the air-liquid interface (ALI) were used for most experiments. Cultures were exposed to cigarette smoke in a Vitrocell VC-10 smoking robot. To evaluate the effect of roflumilast on intracellular cAMP concentrations, fluorescence resonance energy transfer (FRET) between CFP- and YFP-tagged protein kinase A (PKA) subunits was recorded. Airway surface liquid (ASL) was measured using light refraction scanning and ciliary beat frequency (CBF) employing infrared differential interference contrast microscopy. Chloride conductance was measured in Ussing chambers and CFTR expression was quantified with qPCR.

RESULTS

While treatment with 100 nM roflumilast had little effect alone, it increased intracellular cAMP upon stimulation with forskolin and albuterol in cultures exposed to cigarette smoke and in control conditions. cAMP baselines were lower in smoke-exposed cells. Roflumilast prolonged cAMP increases in smoke-exposed and control cultures. Smoke-induced reduction in functional, albuterol-mediated chloride conductance through CFTR was improved by roflumilast. ASL volumes also increased in smoke-exposed cultures in the presence of roflumilast while it did not in its absence. Cigarette smoke exposure decreased CBF, an effect rescued with roflumilast, particularly when used together with the long-acting ß-mimetic formoterol. Roflumilast also enhanced forskolin-induced CBF stimulation in ASL volume supplemented smoked and control cells, confirming the direct stimulatory effect of rising cAMP on ciliary function. In active smokers, CFTR mRNA expression was increased compared to non-smokers and ex-smokers. Roflumilast also increased CFTR mRNA levels in cigarette-smoke exposed cell cultures.

CONCLUSIONS

Our results show that roflumilast can rescue smoke-induced mucociliary dysfunction by reversing decreased CFTR activity, augmenting ASL volume, and stimulating CBF, the latter particularly in combination with formoterol. As expected, CFTR mRNA expression was not indicative of apical CFTR function.

Airway Surface Dehydration by Transforming Growth Factor β (TGF-β) in Cystic Fibrosis Is Due to Decreased Function of a Voltage-dependent Potassium Channel and Can Be Rescued by the Drug Pirfenidone

Transforming growth factor β1 (TGF-β1) is not only elevated in airways of cystic fibrosis (CF) patients, whose airways are characterized by abnormal ion transport and mucociliary clearance, but TGF-β1 is also associated with worse clinical outcomes. Effective mucociliary clearance depends on adequate airway hydration, governed by ion transport. Apically expressed, large-conductance, Ca(2+)- and voltage-dependent K(+) (BK) channels play an important role in this process. In this study, TGF-β1 decreased airway surface liquid volume, ciliary beat frequency, and BK activity in fully differentiated CF bronchial epithelial cells by reducing mRNA expression of the BK γ subunit leucine-rich repeat-containing protein 26 (LRRC26) and its function. Although LRRC26 knockdown itself reduced BK activity, LRRC26 overexpression partially reversed TGF-β1-induced BK dysfunction. TGF-β1-induced airway surface liquid volume hyper-absorption was reversed by the BK opener mallotoxin and the clinically useful TGF-β signaling inhibitor pirfenidone. The latter increased BK activity via rescue of LRRC26. Therefore, we propose that TGF-β1-induced mucociliary dysfunction in CF airways is associated with BK inactivation related to a LRRC26 decrease and is amenable to treatment with clinically useful TGF-β1 inhibitors.

Rhythmicity, recurrence, and recovery of flagellar beating

The eukaryotic flagellum beats with apparently unfailing periodicity, yet responds rapidly to stimuli. Like the human heartbeat, flagellar oscillations are now known to be noisy. Using the alga C. reinhardtii, we explore three aspects of nonuniform flagellar beating. We report the existence of rhythmicity, waveform noise peaking at transitions between power and recovery strokes, and fluctuations of interbeat intervals that are correlated and even recurrent, with memory extending to hundreds of beats. These features are altered qualitatively by physiological perturbations. Further, we quantify the recovery of periodic breaststroke beating from transient hydrodynamic forcing. These results will help constrain microscopic theories on the origins and regulation of flagellar beating.

Soluble adenylyl cyclase in health and disease

The second messenger cAMP is integral for many physiological processes. Soluble adenylyl cyclase (sAC) was recently identified as a widely expressed intracellular source of cAMP in mammalian cells. sAC is evolutionary, structurally, and biochemically distinct from the G-protein-responsive transmembranous adenylyl cyclases (tmAC). The structure of the catalytic unit of sAC is similar to tmAC, but sAC does not contain transmembranous domains, allowing localizations independent of the membranous compartment. sAC activity is stimulated by HCO(3)(-), Ca²⁺ and is sensitive to physiologically relevant ATP fluctuations. sAC functions as a physiological sensor for carbon dioxide and bicarbonate, and therefore indirectly for pH. Here we review the physiological role of sAC in different human tissues with a major focus on the lung. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease, guest edited by J. Buck and L.R. Levin.

Ca²⁺ signaling and fluid secretion by secretory cells of the airway epithelium

Cytoplasmic Ca(2+) is a master regulator of airway physiology; it controls fluid, mucus, and antimicrobial peptide secretion, ciliary beating, and smooth muscle contraction. The focus of this review is on the role of cytoplasmic Ca(2+) in fluid secretion by airway exocrine secretory cells. Airway submucosal gland serous acinar cells are the primary fluid secreting cell type of the cartilaginous conducting airways, and this review summarizes the current state of knowledge of the molecular mechanisms of serous cell ion transport, with an emphasis on their regulation by intracellular Ca(2+). Many neurotransmitters that regulate secretion from serous acinar cells utilize Ca(2+) as a second messenger. Changes in intracellular Ca(2+) concentration regulate the activities of ion transporters and channels involved in transepithelial ion transport and fluid secretion, including Ca(2+)-activated K(+) channels and Cl(-) channels. We also review evidence of interactions of Ca(2+) signaling with other signaling pathways (cAMP, NO) that impinge upon different ion transport pathways, including the cAMP/PKA-activated cystic fibrosis (CF) transmembrane conductance regulator (CFTR) anion channel. A better understanding of Ca(2+) signaling and its targets in airway fluid secretion may identify novel strategies to intervene in airway diseases, for example to enhance fluid secretion in CF airways.

IFN-γ-mediated reduction of large-conductance, Ca2+-activated, voltage-dependent K+ (BK) channel activity in airway epithelial cells leads to mucociliary dysfunction

Effective mucociliary clearance (MCC) depends in part on adequate airway surface liquid (ASL) volume to maintain an appropriate periciliary fluid height that allows normal ciliary activity. Apically expressed large-conductance, Ca(2+)-activated, and voltage-dependent K(+) (BK) channels provide an electrochemical gradient for Cl(-) secretion and thus play an important role for adequate airway hydration. Here we show that IFN-γ decreases ATP-mediated apical BK activation in normal human airway epithelial cells cultured at the air-liquid interface. IFN-γ decreased mRNA levels of KCNMA1 but did not affect total protein levels. Because IFN-γ upregulates dual oxidase (DUOX)2 and therefore H2O2 production, we hypothesized that BK inactivation could be mediated by BK oxidation. However, DUOX2 knockdown did not affect the IFN-γ effect on BK activity. IFN-γ changed mRNA levels of the BK β-modulatory proteins KCNMB2 (increased) and KCNMB4 (decreased) as well as leucine-rich repeat-containing protein (LRRC)26 (decreased). Mallotoxin, a BK opener only in the absence of LRRC26, showed that BK channels lost their association with LRRC26 after IFN-γ treatment. Finally, IFN-γ caused a decrease in ciliary beating frequency that was immediately rescued by apical fluid addition, suggesting that it was due to ASL volume depletion. These data were confirmed with direct ASL measurements using meniscus scanning. Overexpression of KCNMA1, the pore-forming subunit of BK, overcame the reduction of ASL volume induced by IFN-γ. Key experiments were repeated in cystic fibrosis cells and showed the same results. Therefore, IFN-γ induces mucociliary dysfunction through BK inactivation.

Primary ciliary dyskinesia assessment by means of optical flow analysis of phase-contrast microscopy images

Primary ciliary dyskinesia implies cilia with defective or total absence of motility, which may result in sinusitis, chronic bronchitis, bronchiectasis and male infertility. Diagnosis can be difficult and is based on an abnormal ciliary beat frequency (CBF) and beat pattern. In this paper, we present a method to determine CBF of isolated cells through the analysis of phase-contrast microscopy images, estimating cilia motion by means of an optical flow algorithm. After having analyzed 28 image sequences (14 with a normal beat pattern and 14 with a dyskinetic pattern), the normal group presented a CBF of 5.2 ± 1.6 Hz, while the dyskinetic patients presented a 1.9 ± 0.9 Hz CBF. The cutoff value to classify a dyskinetic specimen was set to 3.45 Hz (sensitivity 0.86, specificity 0.93). The presented methodology has provided excellent results to objectively diagnose PCD.

Enhancement of Ca(2+) influx and ciliary beating by membrane hyperpolarization due to ATP-sensitive K(+) channel opening in mouse airway epithelial cells

Among the several types of cells composing the airway epithelium, the ciliary cells are responsible for one of the most important defense mechanisms of the airway epithelium: the transport of inhaled particles back up into the throat by coordinated ciliary movement. Changes in the cytoplasmic Ca(2+) concentration ([Ca(2+)]i) are the main driving force controlling the ciliary activity. In mouse ciliary cells, membrane hyperpolarization from -20 to -60 mV under whole-cell voltage-clamp induced a slow but significant [Ca(2+)]i rise in a reversible manner. This rise was completely inhibited by the removal of Ca(2+) from the extracellular solution. Application of diazoxide, an ATP-dependent K(+) channel opener, dose-dependently induced a membrane hyperpolarization (EC50 = 2.3 μM), which was prevented by the addition of 5 μM glibenclamide. An inwardly rectifying current was elicited by the application of 10 μM diazoxide and suppressed by subsequent addition of 5 μM glibenclamide. Moreover, the application of 10 μM diazoxide induced a significant [Ca(2+)]i rise and facilitated ciliary movement. Multi-cell reverse-transcription polymerase chain reaction analyses and immunocytochemical staining suggested that the subunit combination of Kir6.2/SUR2B and possibly also Kir6.1/SUR2B is expressed in ciliary cells. The confocal Ca(2+) imaging analyses suggested that the [Ca(2+)]i rise induced by diazoxide occurred preferentially in the apical submembrane region. In conclusion, the application of a KATP channel opener to airway ciliary cells induces membrane hyperpolarization and thereby induces a [Ca(2+)]i rise via the facilitation of Ca(2+) influx through the non-voltage-dependent Ca(2+) permeable channels. Therefore, a KATP opener may be beneficial in facilitating ciliary movement.

Methods for analysis of calcium/calmodulin signaling in cilia and flagella

The axonemal microtubules of cilia/flagella act as a scaffold for assembly of the protein complexes that ultimately regulate dynein activity to control the size and shape of ciliary bends. Despite our general understanding of the contribution of microtubule sliding to ciliary and flagellar motility, many questions regarding the regulation of dynein remain unanswered. For example, we know that the second messenger calcium plays an important role in modulating dynein activity in response to extracellular cues, but it remains unclear how calcium-binding proteins anchored to the axoneme contribute to this regulation. Recent work has focused on determining the identity and specific functions of these axonemal calcium-binding proteins. Here, we review our current knowledge of calcium-mediated motility and highlight key experiments that have substantially aided our understanding of calcium signaling within the axoneme.

Methods to measure and analyze ciliary beat activity: Ca2+ influx-mediated cilia mechanosensitivity

Airway ciliary beat activity (CBA) plays a pivotal role in protecting the body by removing mucus and pathogens from the respiratory tract. Since CBA is complicated and cannot be characterized by merely frequency, we recorded CBA using laser confocal line scanning and defined six parameters for describing CBA. The values of these parameters were all above 0 when measured in beating ciliated cells from mouse tracheae. We subsequently used 10 μM adenosine-5'-triphosphate (ATP) to stimulate ciliated cells and simultaneously recorded intracellular Ca(2+) levels and CBA. We found that intracellular Ca(2+) levels first increased, followed by an increase in CBA. Among the six parameters, frequency, amplitude, and integrated area significantly increased, whereas rise time, decay time, and full duration at half maximum markedly decreased. The results suggest that these six parameters are appropriate for assessing CBA and that increased intracellular Ca(2+) levels might enhance CBA. We next used our established methods to observe changes in mechanically stimulated cilia tips. We found that mechanical stimulation-induced changes in both intracellular Ca(2+) levels and CBA were not only similar to those induced by ATP, but were also blocked by treatment with a Ca(2+) chelator, BAPTA-AM, (10 μM) for 10 min. Moreover, while the same blockage was observed under Ca(2+)-free conditions, addition of 2 mM Ca(2+) into the chamber restored increases in both intracellular Ca(2+) levels and CBA. Taken together, we have provided a novel method for real-time measurement and complete analysis of CBA as well as demonstrated that mechanical stimulation of cilia tips resulted in Ca(2+) influx that led to increased intracellular Ca(2+) levels, which in turn triggered CBA enhancement.

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.

Development and validation of a method of cilia motility analysis for the early diagnosis of primary ciliary dyskinesia

BACKGROUND

Primary ciliary dyskinesia (PCD) is a clinically uniform entity, but cilia motility and structure can vary between patients, making the diagnostic difficult. The aim of this study was to evaluate the sensitivity and specificity in diagnosing PCD of a system of high-resolution digital high-speed video analysis with proprietary software that we developed for analysis of ciliary motility (Desinsoft-Bio 200). The secondary aim was to correlate nasal ciliary activity with clinical and structural abnormalities in PCD.

MATERIAL AND METHODS

We analysed nasal mucociliary transport, cilia ultrastructure, nasal ciliary beat frequency and beat pattern studied by high-resolution digital high-speed video in 25 cases of PCD (11 Kartagener syndrome), 27 secondary ciliary dyskinesia and 34 healthy volunteers.

RESULTS

Nasal mucociliary transport was defective in both primary and secondary ciliary dyskinesia. Ciliary immotility was observed only in 6 patients with Kartagener syndrome and correlated with the absence of dynein. We observed a correlation between partial dynein deficiencies and ciliary dyskinesia. Cilia activity and structure was normal in secondary ciliary dyskinesia.

CONCLUSION

Nasal mucociliary transport showed high sensitivity for PCD diagnosis with a low specificity. High-resolution digital high-speed video has a high sensitivity and specificity for diagnosing PCD. This system of video analysis is more useful than ultrastructural study and mucociliary transport for PCD screening. Dynein absence is correlated with cilia immotility and is more common in patients with Kartagener syndrome.

Ciliary beat co-ordination by calcium

Motile cilia in the airway epithelium are the engine for mucociliary clearance, the mechanism responsible for cleaning the airways from inhaled particles. Human airway epithelial cilia appear to have a slow constitutive rate of beating, driven by inherent and spontaneous dynein ATPase activity. Additionally, cilia can increase their beating frequency by activation of several different control mechanisms. One of these controllers is calcium. Its intracellular concentration is regulated by purinergic and acetylcholine receptors. Besides the rate regulatory effect of calcium on ciliary beat, calcium is also involved in synchronizing the beat among cilia of one single cell as well as between cilia on different cells. This article gives an overview of the complex effects of calcium on the beating of motile cilia in the airways.

Functional apical large conductance, Ca2+-activated, and voltage-dependent K+ channels are required for maintenance of airway surface liquid volume

Large conductance, Ca(2+)-activated, and voltage-dependent K(+) (BK) channels control a variety of physiological processes in nervous, muscular, and renal epithelial tissues. In bronchial airway epithelia, extracellular ATP-mediated, apical increases in intracellular Ca(2+) are important signals for ion movement through the apical membrane and regulation of water secretion. Although other, mainly basolaterally expressed K(+) channels are recognized as modulators of ion transport in airway epithelial cells, the role of BK in this process, especially as a regulator of airway surface liquid volume, has not been examined. Using patch clamp and Ussing chamber approaches, this study reveals that BK channels are present and functional at the apical membrane of airway epithelial cells. BK channels open in response to ATP stimulation at the apical membrane and allow K(+) flux to the airway surface liquid, whereas no functional BK channels were found basolaterally. Ion transport modeling supports the notion that apically expressed BK channels are part of an apical loop current, favoring apical Cl(-) efflux. Importantly, apical BK channels were found to be critical for the maintenance of adequate airway surface liquid volume because continuous inhibition of BK channels or knockdown of KCNMA1, the gene coding for the BK α subunit (KCNMA1), lead to airway surface dehydration and thus periciliary fluid height collapse revealed by low ciliary beat frequency that could be fully rescued by addition of apical fluid. Thus, apical BK channels play an important, previously unrecognized role in maintaining adequate airway surface hydration.

Nucleotide-mediated airway clearance

A thin layer of airway surface liquid (ASL) lines the entire surface of the lung and is the first point of contact between the lung and the environment. Surfactants contained within this layer are secreted in the alveolar region and are required to maintain a low surface tension and to prevent alveolar collapse. Mucins are secreted into the ASL throughout the respiratory tract and serve to intercept inhaled pathogens, allergens and toxins. Their removal by mucociliary clearance (MCC) is facilitated by cilia beating and hydration of the ASL by active ion transport. Throughout the lung, secretion, ion transport and cilia beating are under purinergic control. Pulmonary epithelia release ATP into the ASL which acts in an autocrine fashion on P2Y(2) (ATP) receptors. The enzymatic network describes in Chap. 2 then mounts a secondary wave of signaling by surface conversion of ATP into adenosine (ADO), which induces A(2B) (ADO) receptor-mediated responses. This chapter offers a comprehensive description of MCC and the extensive ramifications of the purinergic signaling network on pulmonary surfaces.

Hysteresis modeling for calcium-mediated ciliary beat frequency in airway epithelial cells

A hysteresis model is proposed to describe calcium-mediated Ciliary beat frequency (CBF) in airway epithelial cells. In this dynamic model, the kinetics of coupling between calcium and CBF is posited as a two-step configuration. First, Ca²+ directly binds to or indirectly acts on the axonemal proteins to modulate the activity of axonemal proteins. This step can be modeled by a Hill function in biochemistry. In the second step, the activity of axonemal proteins interacts with the sliding velocity of axonemal microtubules, the equivalent to regulating the CBF. The well-known Bouc-Wen model for hysteresis in mechanical engineering, which can only generate the stable clockwise hysteresis loops, is modified to describe the counter clockwise hysteresis loops commonly observed in the biological experiments. Based upon this new hysteresis model, the dynamic behavior of calcium-regulated CBF in epithelial airway cells is investigated through simulation studies. The numerical results demonstrate that the CBF dynamics in airway epithelial cells predicted by the hysteresis model is more consistent with the experimental observations than that predicted by previous static model in the literature.

Cilia motility and structure in primary and secondary ciliary dyskinesia

BACKGROUND

Primary ciliary dyskinesia (PCD) is a clinically uniform entity, although cilia motility and structure can vary among patients, making diagnosis difficult. Chronic sinusitis, bronchiectasis, sinus hypoplasia, secretory otitis media, and low fertility are common in PCD patients. The aim of this work was to correlate nasal ciliary activity with clinical and structural abnormalities in PCD. A secondary aim was to evaluate the usefulness of high-resolution digital high-speed video (DHSV) in the diagnosis of PCD.

METHODS

We analyzed nasal mucociliary transport and cilia ultrastructure by electron microscopy and studied nasal ciliary beat frequency (CBF) and beat pattern using high-resolution DHSV imaging in 34 healthy volunteers, 25 individuals with PCD (including 11 with Kartagener's syndrome [KS]with situs inversus), and 27 with secondary ciliary dyskinesia (SCD).

RESULTS

Nasal mucociliary transport was defective in the PCD and SCD patients. Ciliary immotility was observed in only six KS patients and was correlated with the absence of dynein. We observed a correlation between partial dynein deficiency and an uncoordinated, stiffly vibrating ciliary beat. Cilia activity and structure were normal in the SCD patients.

CONCLUSION

Nasal mucociliary transport showed a sensitivity of 100% for the diagnosis of PCD but has low specificity. High-resolution DHSV imaging has high sensitivity and specificity for the diagnosis of PCD. Video analysis is probably more useful than the study of mucociliary transport and cilia ultrastructure in screening for PCD. The absence of dynein was correlated with ciliary immotility and was more common in KS patients.

Induction of ciliated cells from avian embryonic stem cells using three-dimensional matrix

We have devised a simple three-dimensional (3D) tissue-culturing method to induce ciliogenesis from avian embryonic stem (ES) cells by using avian fertilized eggs. Unlike the previous reported techniques, this method does not require trypsinization, which would reduce the viability of the cells; it also does not require an air-liquid interface to induce ciliogenesis and to maintain the growth of the induced ciliated cells. ES cells seeded and attached on this collagen-coated chitosan 3D gel grew spontaneously and robustly. Following 2 weeks in culture with inhibition of embryoid body formation, cells with noticeable and vigorous beating cilia were observed. We measured the ciliary beat frequencies of these ES-differentiated ciliated cells for 40 days. These results were consistent with all reported measurements made for other species of ciliated cells, including human, from our previous study. These data imply that the cilia of these ES-derived ciliated cells, beating at their intrinsic basal autorhythmic rate, preserve the integrity of the regulatory mechanisms of ciliary beat frequency. In conclusion, we have shown that ES cells cultured in a 3D tissue-engineered scaffold is a promising approach for developing an in vitro cell model that closely mimics the in vivo ciliated cell natural milieu. This cell model can potentially be the source of ciliated cells for cell-based high-throughput screening and discovery of pulmonary drugs.

Decreased soluble adenylyl cyclase activity in cystic fibrosis is related to defective apical bicarbonate exchange and affects ciliary beat frequency regulation

Human airway cilia contain soluble adenylyl cyclase (sAC) that produces cAMP upon HCO(3)(-)/CO(2) stimulation to increase ciliary beat frequency (CBF). Because apical HCO(3)(-) exchange depends on cystic fibrosis transmembrane conductance regulator (CFTR), malfunctioning CFTR might impair sAC-mediated CBF regulation in cells from patients with cystic fibrosis (CF). By Western blot, sAC isoforms are equally expressed in normal and CF airway epithelial cells, but CBF decreased more in CF than normal cells upon increased apical HCO(3)(-)/CO(2) exposure in part because of greater intracellular acidification from unbalanced CO(2) influx (estimated by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) fluorescence). Importantly, ciliated cell-specific cAMP production (estimated by FRET fluorescence ratio changes of tagged cAMP-dependent protein kinase (PKA) subunits expressed under a ciliated cell-specific promoter) in response to increased apical HCO(3)(-)/CO(2) perfusion was higher in normal compared with CF cells. Inhibition of bicarbonate influx via CFTR (CFTR(inh)172) and inhibition of sAC (KH7) and PKA activation (H89) led to larger CBF declines in normal cells, now comparable with changes seen in CF cells. These inhibitors also reduced FRET changes in normal cells to the level of CF cells with the expected exception of H89, which does not prevent dissociation of the fluorescently tagged PKA subunits. Basolateral permeabilization and subsequent perfusion with HCO(3)(-)/CO(2) rescued CBF and FRET changes in CF cells to the level of normal cells. These results suggest that CBF regulation by sAC-produced cAMP could be impaired in CF, thereby possibly contributing to mucociliary dysfunction in this disease, at least during disease exacerbations when airway acidification is common.

Adrenomedullin regulates sperm motility and oviductal ciliary beat via cyclic adenosine 5'-monophosphate/protein kinase A and nitric oxide

Cilium and flagellum beating are important in reproduction and defects in their motion are associated with ectopic pregnancy and infertility. Adrenomedullin (ADM) is a polypeptide present in the reproductive system. This report demonstrates a novel action of ADM in enhancing the flagellar/ciliary beating of human spermatozoa and rat oviductal ciliated cells. At the concentration found in the seminal plasma, it increases the progressive motility of spermatozoa. ADM binds to its classical receptor, calcitonin receptor-like receptor/receptor activity-modifying protein complex on spermatozoa. ADM treatment increases the protein kinase A activities, the cyclic adenosine monophosphate, and nitric oxide levels of spermatozoa and oviductal cells. Pharmacological activators and inhibitors confirmed that the ADM-induced flagella/ciliary beating was protein kinase A dependent. Whereas nitric oxide donors had no effect on sperm motility, they potentiated the motility-inducing action of protein kinase A activators, demonstrating for the first time the synergistic action of nitric oxide and protein kinase A signaling in flagellar/ciliary beating. The ADM-induced motility enhancement effect in spermatozoa also depended on the up-regulation of intracellular calcium, a known key regulator of sperm motility and ciliary beating. In conclusion, ADM is a common activator of flagellar/ciliary beating. The study provides a physiological basis on possible use of ADM as a fertility regulation drug.

Effect of cyclic AMP-elevating agents on airway ciliary beat frequency in central and lateral airways in rat precision-cut lung slices

In patients with chronic obstructive pulmonary disease (COPD), mucociliary clearance of the respiratory tract is impaired due to enhanced mucus secretion and deterioration of normal ciliary activity. We investigated the effects of cyclic AMP-elevating agents with a different mode of action on ciliary beat frequency (CBF) in rat large central and small lateral airways by comparing the phosphodiesterase-4 (PDE4) inhibitors rolipram and roflumilast to the beta(2)-adrenoceptor agonist terbutaline and the adenylyl cyclase activator forskolin. Rat precision-cut lung slices were prepared and effects of cyclic AMP-elevating agents on CBF were assessed for up to 4h. In central airways a time- and concentration-dependent increase in CBF was seen for roflumilast (59+/-4%, 1microM, 60min), rolipram (55+/-4%, 1microM, 60min), terbutaline (64+/-8%, 10microM, 60min) and forskolin (55+/-8%, 100microM, 60min). Only roflumilast and rolipram increased CBF in lateral airways, with a similar time course and maximum efficacy (roflumilast 48+/-5%, rolipram 54+/-2%). Incubation of lateral airways with terbutaline (10microM, +11%) or forskolin (100microM, +1%) had negligible effects. As a major novel finding this study reveals that PDE4 inhibitors increased CBF in central as well as in lateral airways, while terbutaline and forskolin affected CBF in proximal airways only.

Zinc Increases Ciliary Beat Frequency in a Calcium-Dependent Manner

Background

Dynamic regulation of respiratory ciliary beat frequency (CBF) is regulated by fluxes in intracellular calcium (Ca2+). P2X receptors (P2XR) are extracellular ATP-gated, Ca2+-permeable, nonselective cation channels. Zinc increases intracellular Ca2+ in a sodium (Na+)-free environment through activation of P2XR channels. We hypothesize that topical zinc increases CBF in a Ca2+-dependent fashion as a result of this mechanism.

Methods

The apical surface of mouse sinonasal air–liquid interface cultures were bathed in zinc in a Na+-free solution with or without Ca2+. High-speed digital video imaging captured and analyzed CBF at a sampling rate of 100 frames/s.

Results

CBF significantly increased fourfold over baseline from 5.99 ± 3.16 Hz to 22.4 ± 4.33 Hz in the presence of zinc chloride (50 micromoles) and calcium chloride (3 mM). This effect is abolished in the presence of extracellular Na+ and was pH dependent.

Conclusions

Zinc stimulates CBF in the presence of Ca2+ likely through activation of P2X receptors. Thus, zinc represents a promising agent for stimulation of mucociliary clearance.

Calcium regulation of ciliary motility analysis of axonemal calcium-binding proteins

Substantial data have contributed to a model in which the axonemal microtubules act as a scaffold for the assembly of molecules that form a signal transduction pathway that ultimately regulates dynein. We have also known for some time that for virtually all motile cilia and flagella, the second messenger, calcium, impacts upon these signaling pathways to modulate beating in response to extracellular cues. Yet we are only beginning to identify the axonemal proteins that bind this second messenger and determine their role in regulating dynein-driven microtubule sliding to alter the size and shape of ciliary bends. Here, we review our current understanding of calcium regulation of motility, emphasizing recent advances in the detection and characterization of calcium-binding proteins anchored to the axoneme.

Ciliogenesis in cryopreserved mammalian tracheal epithelial cells cultured at the air-liquid interface

To determine air-liquid interface (ALI) culture derived from cryopreserved mammalian tracheal ciliated cells is a viable ciliated cell model for the investigations of regulatory mechanisms of ciliary beat frequency (CBF), two studies were performed using ovine and porcine tracheae obtained from local slaughterhouses. The protease-digested tracheal ciliated cells were harvested and cultured at the ALI using collagen-coated, porous membrane inserts. In study 1, the ALI culturing protocols were established using non-cryopreserved ovine tracheal ciliated cells. Ciliogenesis was documented with immuno-histology and electron micrographs. Vigorous beating cilia were video-recorded. CBF was measured by laser light scattering. The functional integrity of the autonomic receptors of the ciliated cells was confirmed with the stimulatory responses of CBF using luminal methacholine and basolateral terbutaline. In study 2, porcine tracheal ciliated cells stored in liquid nitrogen for a minimum of 4 weeks were used. The cryopreserved cells were thawed and cultured using the ALI protocol established in study 1. After two months, cilia outgrowths were confirmed using video microscopy and scanning electron micrograph (SEM). The trans-epithelial resistances were 28.5 kOmega (n=4). Luminal applications of 1 microM and 10 microM methacholine stimulated CBF from a baseline of 7.4+/-0.2 Hz to 8.4+/-0.8 Hz and 7.7+/-0.4 Hz, respectively (n=5). Basolateral applications of 1 microM and 10 microM terbutaline stimulated CBF from a baseline of 7.5+/-0.3 Hz to 8.2+/-0.4 Hz and 8.0+/-0.4 Hz, respectively (n=5). These data demonstrated that a ciliated cell bank can be established using cryopreserved ciliated cells for pulmonary drug discovery and toxicological screening.

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.

In vitro culturing of porcine tracheal mucosa as an ideal model for investigating the influence of drugs on human respiratory mucosa

It has been previously shown that fresh mucosa from different mammals could serve as raw material for in vitro culturing with the differentiation of cilia, which are the most important morphological structures for the function of the mucociliary system. Increasing legal restrictions on the removal of human tissue and changing surgical techniques have led to a lack of fresh human mucosa for culturing. Most of the animals that have been used as donors up to now are genetically not very close to human beings and must all be sacrificed for such studies. We, therefore, established a modified system of culturing mucosa cells from the trachea of pigs, which is available as a regular by-product after slaughtering. With respect to the possibility of developing “beating” cilia, it could be shown that the speed of cell proliferation until adhesion to the coated culture dishes, the formation of conjunctions of cell clusters and the proliferation of cilia were comparable for porcine and human mucosa. Moreover, it could be demonstrated that the porcine cilia beat frequency of 7.57 ± 1.39 Hz was comparable to the human mucosa cells beat frequency of 7.3 ± 1.4 Hz and that this beat frequency was absolutely constant over the investigation time of 360 min. In order to prove whether the reaction to different drugs is comparable between the porcine and human cilia, we initially tested benzalkonium chloride, which is known to be toxic for human cells, followed by naphazoline, which we found in previous studies on human mucosa to be non-toxic. The results clearly showed that the functional and morphological reactions of the porcine ciliated cells to these substances were similar to the reaction we found in the in vitro cultured human mucosa.

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.

Soluble adenylyl cyclase is localized to cilia and contributes to ciliary beat frequency regulation via production of cAMP

Ciliated airway epithelial cells are subject to sustained changes in intracellular CO(2)/HCO(3)(-) during exacerbations of airway diseases, but the role of CO(2)/HCO(3)(-)-sensitive soluble adenylyl cyclase (sAC) in ciliary beat regulation is unknown. We now show not only sAC expression in human airway epithelia (by RT-PCR, Western blotting, and immunofluorescence) but also its specific localization to the axoneme (Western blotting and immunofluorescence). Real time estimations of [cAMP] changes in ciliated cells, using FRET between fluorescently tagged PKA subunits (expressed under the foxj1 promoter solely in ciliated cells), revealed CO(2)/HCO(3)(-)-mediated cAMP production. This cAMP production was specifically blocked by sAC inhibitors but not by transmembrane adenylyl cyclase (tmAC) inhibitors. In addition, this cAMP production stimulated ciliary beat frequency (CBF) independently of intracellular pH because PKA and sAC inhibitors were uniquely able to block CO(2)/HCO(3)(-)-mediated changes in CBF (while tmAC inhibitors had no effect). Thus, sAC is localized to motile airway cilia and it contributes to the regulation of human airway CBF. In addition, CO(2)/HCO(3)(-) increases indeed reversibly stimulate intracellular cAMP production by sAC in intact cells.

CCSP regulates cross talk between secretory cells and both ciliated cells and macrophages of the conducting airway

Pulmonary host defense employs a combination of biochemical and biophysical activities to recognize, inactivate, and mediate clearance of environmental agents as well as modulate the overall response to such challenge. Dysregulation of the inflammatory arm of this response is associated with chronic lung diseases (CLD) including cystic fibrosis and chronic obstructive lung disease. Although mechanisms mediating immunoregulation are incompletely characterized, decrements in levels of the nonciliated secretory cell product Clara cell secretory protein (CCSP) in numerous CLD and identification of proinflammatory state in mice homozygous for a null allele of the CCSP gene (CCSP−/−) suggest a central role for the nonciliated secretory cell in this process. In an effort to determine the molecular basis for immunoregulatory defects associated with CCSP deficiency, we utilized difference gel electrophoresis in combination with matrix-assisted laser desorption ionization time-of-flight to compare the proteomes of wild-type and CCSP−/− mice. We demonstrate a shift in the isoelectric point of the immunomodulatory protein annexin A1 (ANXA1) to more acidic isoforms in CCSP−/− mice. Similar ANXA1 mRNA and protein abundance in wild-type and CCSP−/− tissue and identical localization of ANXA1 protein to alveolar macrophages and the ciliary bed of ciliated cells demonstrated that CCSP deficiency was associated exclusively with altered posttranslational modification of ANXA1. These results suggest that both long- and short-range paracrine signaling between nonciliated secretory cells and cells of the immune system and epithelium impact modification of cell type-specific proteins and implicate nonciliated secretory cells in a regulatory axis that might integrate critical aspects of host defense.

The effect of endogenous nitric oxide on mechanical ciliostimulation of human nasal mucosa

BACKGROUND

Endogenous nitric oxide (NO) production by the inducible NO-synthase is enhanced in the nasal respiratory epithelium of patients with allergic rhinitis. Recent experimental data suggest endogenous NO to be strongly involved in the regulation of ciliary activity, the driving force of the mucociliary transport system.

OBJECTIVE

In this study, we investigated the effect of endogenous NO on mechanical stimulation of ciliary activity in a nasal mucosa explant model.

METHODS

Cultures of nasal mucosa explants were incubated with TNF-alpha and bacterial lipopolysaccharides (LPS) to enhance endogenous NO production. Direct in vitro NO imaging was performed by the fluorescent NO-indicator DAF-2 DA and laser scanning confocal microscopy. Ciliary beat frequency (CBF) was determined using a photoelectric technique. Mechanical stimulation was performed by two consecutive flow increments in a closed perfusion chamber. Endogenous NO-synthesis was blocked by l-NAME before the second flow stimulation.

RESULTS

Under control conditions the mean rise of CBF relative to baseline was 30.2% during the first flow increment and 30.7% during the second flow increment. Blocking of the endogenous NO synthesis in TNF-alpha/LPS-stimulated cultures reduced baseline CBF by 10.6+/-2.1% (P<0.05) but the effect of mechanical ciliostimulation on CBF remained unchanged (36.0% vs. 38.2%).

CONCLUSION

In conclusion, endogenous NO- and Ca(2+)-dependent mechanical stimulation of ciliary activity probably use independent intracellular signalling pathways. The combination of both effects on ciliary activity is likely to improve the local defence against inhaled allergens in patients with nasal allergies.

Regulation of mammalian ciliary beating

Recent advances in our understanding of the structure-function relationship of motile cilia with the 9 + 2 microtubular arrangement have helped explain some of the mechanisms of ciliary beat regulation by intracellular second messengers. These second messengers include cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) as well as calcium and pH. cAMP activates protein kinase A (PKA), which is localized to the axoneme. The cAMP-dependent phosphorylation of PKA's main target, originally described as p29 in Paramecium, seems to increase ciliary beat frequency (CBF) directly. The mechanism by which cGMP increases CBF is less well defined but involves protein kinase G and possibly PKA. Protein kinase C inhibits ciliary beating. The regulation mechanisms of CBF by calcium remain somewhat controversial, favoring an immediate, direct action of calcium on ciliary beating and a second cyclic nucleotide-dependent phase. Finally, intracellular pH likely affects CBF through direct influences on dynein arms.

Apical oxidative hyaluronan degradation stimulates airway ciliary beating via RHAMM and RON

Hyaluronan (HA) is synthesized in high-molecular-weight form at the apical pole of airway epithelial cells, covering the luminal surface. When human airway epithelial cells grown and redifferentiated at the air-liquid interface (ALI) were exposed to xanthine/xanthine oxidase (X/XO), ciliary beat frequency (CBF) increased. This effect was blocked by superoxide dismutase (SOD) and catalase. Inhibition of hyaluronan synthesis inhibited the CBF response to X/XO, while addition of exogenous HA amplified it. A functionally blocking antibody to the receptor for hyaluronic acid-mediated motility (RHAMM) reduced the CBF response to X/XO. Since RHAMM has no transmembrane domain and thus cannot signal on its own, the association of RHAMM with recepteur d'origine nantais (RON), a member of the hepatocyte growth factor receptor family, was explored. Immunohistochemistry of human airway epithelium showed co-localization of RHAMM and RON at the apex of ciliated cells. Physical association of RHAMM and RON was confirmed with co-immunoprecipitations. Macrophage-stimulating protein (MSP), an agonist of RON, stimulated CBF. Genistein, a nonspecific tyrosine kinase inhibitor, and MSP beta chain (beta-MSP), a specific RON inhibitor, blocked the X/XO-induced CBF increase. HA present in the apical secretions of human airway epithelial cells was shown to degrade upon exposure to X/XO, a process inhibited by SOD. Low-molecular-weight HA fragments stimulated CBF, an effect blocked by anti-RHAMM antibody and genistein. These data suggest that high molecular form HA is broken down by reactive oxygen species to form low-molecular-weight fragments that signal via RHAMM and RON to stimulate CBF.

Use of confocal linescan to document ciliary beat frequency

We present a method to document ciliary beat frequency with the linescan function of a scanning confocal microscope, using ciliated tracheal cells and free-swimming rotifers as examples. Depending on the clarity of the original data, the ciliary beat frequency can be determined from the confocal linescan directly or from an intensity linescan analysis of the original data. Fast Fourier transform treatment of the data can be used to verify the derived ciliary beat frequency. The linescan approach allows analysis of simple ciliary movements displayed by the ciliated tracheal cells, as well as complex movements performed by free-swimming rotifers while feeding.

Real-time analysis of cAMP-mediated regulation of ciliary motility in single primary human airway epithelial cells

Airway ciliary beat frequency regulation is complex but in part influenced by cyclic adenosine monophosphate (cAMP)-mediated changes in cAMP-dependent kinase activity, yet the cAMP concentration required for increases in ciliary beat frequency and the temporal relationship between ciliary beat frequency and cAMP changes are unknown. A lentiviral gene transfer system was developed to express a fluorescence resonance energy transfer (FRET)-based cAMP sensor in ciliated cells. Expression of fluorescently tagged cAMP-dependent kinase subunits from the ciliated-cell-specific foxj1 promoter enhanced expression in fully differentiated ciliated human airway epithelial cells, and permitted simultaneous measurements of ciliary beat frequency and cAMP (represented by the FRET ratio). Apical application of forskolin (1 microM, 10 microM, 20 microM) and, in permeabilized cells, basolateral cAMP (20 microM, 50 microM, 100 microM) caused dose-dependent, albeit similar and simultaneous-increases in cAMP and ciliary beat frequency. However, decreases in cAMP preceded decreases in ciliary beat frequency, suggesting that either cellular cAMP decreases before ciliary cAMP or the dephosphorylation of target proteins by phosphatases occur at a rate slower than the rate of cAMP hydrolysis.

Nervous control of ciliary beating by Cl-, Ca2+ and calmodulin inTritonia diomedea

In vertebrates, motile cilia line airways, oviducts and ventricles. Invertebrate cilia often control feeding, swimming and crawling, or gliding. Yet control and coordination of ciliary beating remains poorly understood. Evidence from the nudibranch mollusc, Tritonia diomedea, suggests that locomotory ciliated epithelial cells may be under direct electrical control. Here we report that depolarization of ciliated pedal epithelial (CPE)cells increases ciliary beating frequency (CBF), and elicits CBF increases similar to those caused by dopamine and the neuropeptide, TPep-NLS. Further,four CBF stimulants (zero external Cl-, depolarization, dopamine and TPep-NLS) depend on a common mode of action, viz. Ca2+influx, possibly through voltage-gated Ca2+ channels, and can be blocked by nifedipine. Ca2+ influx alone, however, does not provide all the internal Ca2+ necessary for CBF change. Ryanodine receptor(RyR) channel-gated internal stores are also necessary for CBF excitation. Caffeine can stimulate CBF and is sensitive to the presence of the RyR blocker dantrolene. Dantrolene also reduces CBF excitation induced by dopamine and TPep-NLS. Finally, W-7 and calmidazolium both block CBF excitation by caffeine and dopamine, and W-7 is effective at blocking TPep-NLS excitation. The effects of calmidazolium and W-7 suggest a role for Ca2+-calmodulin in regulating CBF, either directly or via Ca2+-calmodulin dependent kinases or phosphodiesterases. From these results we hypothesize dopamine and TPep-NLS induce depolarization-driven Ca2+ influx and Ca2+ release from internal stores that activates Ca2+-calmodulin, thereby increasing CBF.