Calcium mobilization and response recovery following P2-purinoceptor stimulation of rat isolated alveolar type II cells

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.

Purinergic signaling and kinase activation for survival in pulmonary oxidative stress and disease

Stimulus-induced release of endogenous ATP into the extracellular milieu has been shown to occur in a variety of cells, tissues, and organs. Extracellular ATP can propagate signals via P2 receptors that are essential for growth and survival of cells. Abundance of P2 receptors, their multiple isoforms, and their ubiquitous distribution indicate that they transmit vital signals. Pulmonary epithelium and endothelium are rich in both P2X and P2Y receptors. ATP release from lung tissue and cells occurs upon stimulation both in vivo and in vitro. Extracellular ATP can activate signaling cascades composed of protein kinases including extracellular signal-regulated kinase (ERK) and phosphatidylinositol-3-kinase (PI3K). Here we summarize progress related to release of endogenous ATP and nucleotide signaling in pulmonary tissues upon exposure to oxidant stress. Hypoxic, hyperoxic, and ozone exposures cause a rapid increase of extracellular ATP in primary pulmonary endothelial and epithelial cells. Extracellular ATP is critical for survival of these cells in high oxygen and ozone concentrations. The released ATP, upon binding to its specific receptors, triggers ERK and PI3K signaling and renders cells resistant to these stresses. Impairment of ATP release and transmission of such signals could limit cellular survival under oxidative stress. This may further contribute to disease pathogenesis or exacerbation.

A fluorescent microplate assay for exocytosis in alveolar type II cells

The authors describe a simple, reliable, and quantitative assay to monitor exocytotic fusion of lamellar bodies (LBs) in adherent rat alveolar type II (AT II) cells. The assay is based on fluorescence measurements of LB-plasma membrane (PM) fusions modified for the use in multiwell culture plates to obtain a high-sample throughput. In particular, it is based on the presence of a highly light-absorbing dye in the cell supernatants to increase the specificity of fluorescence signals and to yield pseudo-confocal information from the cells. When the assay was tested with agonist-(ATP) and phorbolester-induced stimulation of LB-PM fusions, the authors found a good correlation with direct microscopic investigations based on single cell recordings. To further validate the assay, they used Curosurf at 10 mg/ml. However, it influenced neither the basal nor the ATP-stimulated rate of LB-PM fusions. This was corroborated by the fact that Curosurf had no effect on resting Ca (2+) levels nor the ATP induced Ca (2+) signals. The results cast new light on previous findings that surfactant phospholipids decrease the rate of secretion in AT II cells in a dose-dependent way. The authors conclude that the inhibitory effect exerted by phospholipids might be due to action on a later step in exocytosis, probably associated with exocytotic fusion pore expansion and content release out of fused vesicles.

Exocytosis of lung surfactant: from the secretory vesicle to the air-liquid interface

Exocytosis is fundamental in biology and requires an orchestra of proteins and other constituents to fuse a vesicle with the plasma membrane. Although the molecular fusion machinery appears to be well conserved in evolution, the process itself varies considerably with regard to the diversity of physico-chemical and structural factors that govern the delay between stimulus and fusion, the expansion of the fusion pore, the release of vesicle content, and, finally, its extracellular dispersion. Exocytosis of surfactant is unique in many of these aspects. This review deals with the secretory pathway of pulmonary surfactant from the type II cell to the air-liquid interface, with focus on the distinct mechanisms and regulation of lamellar body (LB) fusion and release. We also discuss the fate of secreted material until it is rearranged into units that finally function to reduce the surface tension in the lung.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.

Neopterin inhibits ATP-induced calcium release in alveolar epithelial cells in vitro

BACKGROUND

Serum neopterin concentrations rise during activation of the cellular immune system. It is suggested that neopterin interacts with cellular redox mechanisms. This induces oxidative stress, which inhibits intracellular Ca2+ transients in various cell types. In type II alveolar epithelial cells, Ca2+ increase is considered involved in the exocytosis of surfactants. This exocytosis is disturbed during inflammation.

AIMS

To clarify whether neopterin affects adenosine triphosphate (ATP)-induced Ca2+ transients in an alveolar epithelial cell line (L2).

METHODS

Ca2+ transients were detected as fura-2 fluorescence by an image analysis system.

RESULTS

Cells were exposed for 100 sec to ATP (1 microM, repeated four times). The first application of ATP induced an increase of the fluorescence ratio by approximately 100%, while the following stimulations resulted in smaller transients. In a second set of experiments, L2 cells were exposed to ATP or ATP + neopterin (100 nM), alternately. Simultaneous application of neopterin inhibited Ca2+ transients almost completely.

CONCLUSIONS

Inhibition of Ca2+ transients by neopterin may lead to suppressed exocytosis of surfactant proteins in alveolar epithelial cells. This might contribute to the deterioration of pulmonary functions in the course of inflammatory processes.

Modulation of pulmonary alveolar type II cell phenotype and communication by extracellular matrix and KGF

The alveolar epithelium consists of two cell types, alveolar type I (AT1) and alveolar type II (AT2) cells. We have recently shown that 7-day-old cultures of AT2 cells grown on a type I collagen/fibronectin matrix develop phenotypic characteristics of AT1 cells, display a distinct connexin profile, and coordinate mechanically induced intercellular Ca(2+) changes via gap junctions (25). In this study, we cultured AT2 cells for 7 days on matrix supplemented with laminin-5 and/or in the presence of keratinocyte growth factor. Under these conditions, cultured AT2 cells display AT2 type morphology, express the AT2-specific marker surfactant protein C, and do not express AT1-specific cell marker aquaporin 5, all consistent with maintenance of AT2 phenotype. These AT2-like cells also coordinate mechanically induced intercellular Ca(2+) signaling, but, unlike AT1-like cells, do so by using extracellular nucleotide triphosphate release. Additionally, cultured cells that retain AT2 cell-specific markers express connexin profiles different from cultured cells with AT1 characteristics. The parallel changes in intercellular Ca(2+) signaling with cell differentiation suggest that cell signaling mechanisms are an intrinsic component of lung alveolar cell phenotype. Because lung epithelial injury is accompanied by extracellular matrix and growth factor changes, followed by extensive cell division, differentiation, and migration of AT2 progenitor cells, we suggest that similar changes may be vital to the lung recovery and repair process in vivo.

Secretion in alveolar type II cells at the interface of constitutive and regulated exocytosis

Long-term, simultaneous, measurements of cytoplasmic free Ca(2+) concentrations and single exocytotic fusion events in surfactant-secreting type II cells were performed. All fusion (constitutive, phorbol ester-induced, and agonist-induced) was Ca(2+)-dependent. Kinetic analysis revealed that agonist (adenosine triphosphate [ATP])-induced fusion exhibited a kinetic pattern that correlated well with the Ca(2+) signal. The effects of Ca(2+) release from intracellular stores (early) and Ca(2+) entry (late) could be demonstrated for the first time by dissecting the slow (10-to-15-min) fusion response to ATP into these two components. Bath Ba(2+) or Sr(2+) could replace Ca(2+) to elicit a fusion response in thapsigargin-pretreated cells lacking ATP-induced Ca(2+) release from stores. Although the late response was partially inhibited by interrupting the phospholipase D-protein kinase C axis, a high Ca(2+) dependence of the entire secretory course was demonstrated by a significant correlation between the integrated Ca(2+) signal and the fusion response. There was also a highly significant correlation between constitutive and ATP-stimulated fusion activity in individual cells. We propose a common mechanistic model for all types of fusion in this slow secretory cell, in which constitutive and regulated forms of exocytosis are subject to the same principles of regulation.

Inhibition of ATP-induced surfactant exocytosis by dihydropyridine (DHP) derivatives: a non-stereospecific, photoactivated effect and independent of L-type Ca2+ channels

Purinergic stimulation of surfactant secretion via exocytosis of lamellar bodies is mediated by an elevation of the intracellular Ca2+ concentration ([Ca2+](i)). We tested the dihydropyridine (DHP) analogues isradipine (+/-enantiomers), nifedipine and Bay K 8644 (racemic forms) on ATP-induced surfactant secretion and [Ca2+](i) in single type II cells, using FM1-43 and fura-2 fluorescence. None of the DHPs (2 microM) had an effect on ATP-induced surfactant secretion in the dark. They did, however, inhibit secretion in a concentration-dependent manner during illumination, particularly with UV light. This effect was not stereospecific, because it was mimicked by (-)-isradipine. In addition, (+)- or (-)-isradipine, but not nifedipine or Bay K 8644, elicited a slow increase of [Ca2+](i) during illumination with UV light, which was reversible by exposure to dark. None of the DHPs inhibited the ATP-induced Ca2+ signal. In perforated patch clamp experiments, depolarizing voltage steps did not induce L-type Ca2+ (Sr(2+)) currents, even in the presence of the agonist Bay K 8644 (1 microM). We conclude that impairment of ATP-induced surfactant secretion by all tested DHPs and alterations of Ca2+ homeostasis by isradipine are photoactivated effects, independent of L-type Ca2+ channels.

Intercellular Ca2+ signaling in alveolar epithelial cells through gap junctions and by extracellular ATP

Inter- and extracellular-mediated changes in intracellular Ca2+ concentration ([Ca2+]i) can ensure coordinated tissue function in the lung. Cultured rat alveolar epithelial cells (AECs) have been shown to respond to secretagogues with increases in [Ca2+]i and have been shown to be gap junctionally coupled. However, communication of [Ca2+]i changes in AECs is not well defined. Monolayers of AECs were mechanically perturbed and monitored for [Ca2+]i changes. Perturbation of AECs was administered by a glass probe to either mechanically stimulate or mechanically wound individual cells. Both approaches induced a change in [Ca2+]i in the stimulated cell that was propagated to neighboring cells (Ca2+ waves). A connexin mimetic peptide shown to uncouple gap junctions eliminated Ca2+ waves in mechanically stimulated cells but had no effect on mechanically wounded cells. In contrast, apyrase, an enzyme that effectively removes ATP from the extracellular milieu, had no effect on mechanically stimulated cells but severely restricted mechanically wounded Ca2+ wave propagation. We conclude that AECs have the ability to communicate coordinated Ca2+ changes using both gap junctions and extracellular ATP.

Cellular activation by Ca2+ release from stores in the endoplasmic reticulum but not by increased free Ca2+ in the cytosol

Ca(2+) release from intracellular stores and/or transmembrane influx can increase the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). Such changes in [Ca(2+)](i) might transduce signals regulating transcription, motility, secretion, and so on. Surfactant secretagogues such as ATP and ionomycin stimulate the release and transmembrane influx of Ca(2+), both of which increase [Ca(2+)](i). The addition of surfactant protein A (SP-A) or depleting cellular Ca(2+) inhibited both surfactant secretion and Ca(2+) transients. Current results suggest that Ca(2+) signalling stimulates surfactant secretion by type II pneumocytes, but not via increased [Ca(2+)](i). Treatment of cells with a Ca(2+) chelator, bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA-AM), stimulated secretion but decreased [Ca(2+)](i). Adding SP-A or depleting Ca(2+) inhibited BAPTA-AM-induced secretion. When studied directly, Ca(2+) in the endoplasmic reticulum store ([Ca(2+)](l)) decreased in response to BAPTA, ionomycin and thapsigargin, and increased in response to SP-A. Phorbol ester (PMA) induced surfactant secretion without altering [Ca(2+)](i) or [Ca(2+)](l) and was unaffected by Ca(2+) depletion. The addition of PMA to Ca(2+)-releasing secretagogues increased secretion, but combining two Ca(2+)-releasing secretagogues did not. These results suggest that (1) Ca(2+) signalling of type II cell surfactant secretion reflects changes in [Ca(2+)](l), not [Ca(2+)](i), (2) PMA elicits secretion differently from Ca(2+)-releasing secretagogues, and (3) SP-A inhibits secretion by enhancing Ca(2+) sequestration within endoplasmic reticulum stores. Whether other cell types signal via changes in [Ca(2+)](l) is unknown.

Cyclosporin A protects lung function from hyperoxic damage

Cyclosporin A (CsA), an inhibitor of protein phosphatase 2B (calcineurin), has been shown to play a role in exocytosis and neutrophil mobility. Hyperoxia (>95% oxygen for 72 h) causes lung injury and reduces lung compliance. This model is indicative of deficiencies in surfactant and elicits a vigorous immune response leading to further damage. We examined the effects of CsA on surfactant-secreting lung alveolar type II cells. CsA enhances ATP-stimulated increases in whole cell capacitance in the presence of 2 mM extracellular Ca2+. This measurement corresponds with increases in exocytosis. Because of its effect on the immune system and exocytosis from type II cells, CsA was examined for its protective effects against hyperoxia-induced lung damage in mice. We found that CsA (50 mg. kg-1. day-1) attenuated hyperoxia-induced reductions in lung compliance when administered before or during 72 h of >95% oxygen (P < 0.05). CsA (10 mg. kg-1. day-1) also had a protective effect against hyperoxia-induced changes in neutrophil infiltration, capillary congestion, edema, and hyaline membrane formation. Wet lung weight-to-dry lung weight ratios did not show any significant changes after hyperoxia or hyperoxia plus CsA (P < 0. 05). CsA may be useful to treat patients undergoing prolonged high-oxygen therapy and possibly other lung injuries.

Dynamics of surfactant release in alveolar type II cells

Pulmonary surfactant, secreted via exocytosis of lamellar bodies (LB) by alveolar type II (AT II) cells, maintains low alveolar surface tension and is therefore essential for normal lung function. Here we describe real-time monitoring of exocytotic activity in these cells by visualizing and quantifying LB fusion with the plasma membrane (PM). Two approaches were used. First, fluorescence of LysoTracker Green DND-26 (LTG) in LB disappeared when the dye was released after exocytosis. Second, phospholipid staining by FM 1-43 resulted in bright fluorescence when this dye entered the LB through the fusion pore. Both processes were restricted to and colocalized with LB and occurred simultaneously. In AT II cells, FM 1-43 offered the unique advantage to independently define the moment and cellular location of single exocytotic events as well as the amount of material released, and to monitor its extracellular fate. Furthermore, both dyes could be used in combination with fura-2. The results indicate considerable diversity in the dynamics of LB exocytosis. In the majority of cells stimulated with ATP and isoproterenol, the first fusion of LB coincided with the rise of [Ca2+]i, but subsequent response of other LB in the same cell considerably outlasted this signal. In other cells, however, the onset of exocytosis was delayed by several minutes. After LB fusion, release of surfactant from LB into an aqueous solution was slow. In summary, stimulated exocytosis in AT II cells occurs at a much slower rate than in most other secretory cells but is still a more dynamic process than predicted from conventional measurements of surfactant released into cell supernatants.

Adenosine 5'-triphosphate (ATP) receptors induce intracellular calcium changes in mouse leydig cells

Cytoplasmic calcium ([Ca(2+)](i)) changes evoked by adenosine 5(1)-triphosphate (ATP) were recorded in cultured individual Leydig cells within 10-18 h after cell dispersion. [Ca(2+)](i) was monitored using Fura-2AM loaded cells with a digital ratio imaging system. Five micromolars ATP induced biphasic [Ca(2+)](i) responses in most cells (94%,n=100), characterized by a fast increase from a basal level (126±5 nMSE,n=60 cells) to a peak (5-7 times above basal levels) within seconds, followed by a slow decrease toward a plateau level (2-3 times above basal) within 5 min. The peak phase of the [Ca(2+)](i) response increased with ATP concentrations (1-100 μM ATP) in a dose-dependent manner with an IC(50) of 5.9±1.2 μM, and it desensitized in a reversible manner with repeated application of 5 μM ATP at <5-min intervals. The [Ca(2+)](i) peak response was dependent on Ca(2+) release from an intracellular pool, whereas the plateau phase was dependent on extracellular [Ca(2+)]. ATP did not appear to induce formation of nonspecific membrane pores, since stimulation for 10 min with ATP (10-100 μM) in the presence of extracellular Lucifer yellow (LY) (5 mg/mL) did not result in dye loading of the cells. [Ca(2+)](i) transients were elicited by other adenosine nucleotides with an order of potencies (ATP>Adenosine diphosphate [ADP]>Adenosine> Adenosine monophosphate [AMP]) that was compatible with the expression of P(2) receptors. [Ca(2+)](i) responses were suppressed by the purinergic P(2) receptor antagonist, suramin. These results provide functional evidence for the expression of purinergic P(2) receptors in Leydig cells.

Arachidonic acid and eicosapentaenoic acid stimulate type II pneumocyte surfactant secretion

Arachidonic acid and its leukotriene metabolites have been shown to stimulate surfactant secretion by alveolar type II cells. The present study was undertaken to determine the effects of various unsaturated fatty acids, including eicosapentaenoic acid, on surfactant secretion. Surfactant secretion was expressed as the percent of [3H]choline-derived phospholipids released into culture medium by type II pneumocytes of adult rats. Consistent with the earlier findings, arachidonic acid stimulated secretion in a concentration-dependent fashion (3.5-21 microM), doubling baseline secretion at 21 microM. Eicosapentaenoic acid was found to be equally effective as arachidonic acid in stimulating secretion. A comparison with palmitic, oleic and linoleic acids revealed that highly unsaturated fatty acids stimulated secretion to the greatest extent. This was supported by a positive correlation between degree of unsaturation (i.e., 0, 1, 2, 4 and 5 double bonds) and stimulation of surfactant secretion. In the present study, exogenous leukotriene E4 (10(-13)-10(-6) did not stimulate surfactant secretion. Neither nordihydroguairetic acid (0.1 microM) nor indomethacin (0.1 microM) affected arachidonic acid-stimulated secretion. The stimulatory effects of arachidonic acid and eicosapentaenoic acid on surfactant secretion were related to the highly unsaturated nature of the fatty acids and were not mediated by increased levels of cyclic adenosine monophosphate or calcium.

Low density lipoprotein- and high density lipoprotein-mediated signal transduction and exocytosis in alveolar type II cells

Low density lipoproteins (LDL) and high density lipoproteins (HDL) from serum stimulate signal-transduction pathways and exocytosis in rat alveolar type II cells. Both LDL and HDL stimulated primary cultures of type II cells to secrete phosphatidylcholine (PtdCho), the major phospholipid component of pulmonary surfactant. The effects on secretion were preceded temporally by stimulation of inositol phospholipid catabolism, calcium mobilization, and translocation of protein kinase C from cytosolic to membrane compartments. Heparin, which blocks the binding of ligands to the LDL receptor, completely inhibited the effects of LDL on signal transduction and PtdCho secretion but did not inhibit the effects of HDL. Unilamellar PtdCho liposomes the size of native LDL had no effect on type II cells; however, PtdCho complexes containing either apolipoproteins E or A-I stimulated both signal transduction and PtdCho secretion. LDL receptors were present in type II cell membranes by immunoblotting. In contrast to findings with hepatic membranes, type II cells exhibited two major bands of 130 kDa and 120 kDa and a minor band at 230 kDa that also was present under reducing conditions. These results are consistent with our hypothesis that the LDL-receptor pathway functions in vivo to deliver cholesterol to type II cells and that this process is coupled to surfactant assembly and secretion via signal-transduction pathway(s). HDL elicits similar responses independent of the LDL receptor, suggesting that type II cells may use the selective uptake pathway to obtain cholesterol or that HDL triggers signal transduction by mechanisms unrelated to lipid delivery.

The effects of Bay K 8644 and nifedipine on the responses of rat urinary bladder to electrical field stimulation, beta,gamma-methylene ATP and acetylcholine

1. Bay K 8644 (0.33 nM to 1 microM) greatly increased the contractions of rat urinary bladder detrusor muscle induced by beta, gamma-methylene ATP (beta, gamma-MeATP, 10 microM) and by electrical field stimulation of the purinergic component (the cholinergic response was blocked by atropine). 2. The contractions induced by acetylcholine (ACh, 10 microM) and by electrical field stimulation of the cholinergic component (the purinergic response was blocked following desensitization by alpha, beta-MeATP) were also potentiated by Bay K 8644, although to a lesser extent than the purinergic responses. 3. Nifedipine (1 nM to 3.3 microM) inhibited all the contractions induced by beta, gamma-MeATP, ACh and electrical field stimulation. However, while the responses to beta, gamma-MeATP and electrical field stimulation of the purinergic component were almost abolished, a substantial proportion of the responses to ACh and electrical field stimulation of the cholinergic component were nifedipine resistant. 4. The concentration-effect curves for the potentiation by Bay K 8644 of the responses to beta, gamma-MeATP, ACh and electrical field stimulation were shifted to the right by nifedipine (10 nM). At concentrations greater than 1 microM, Bay K 8644 inhibited contraction. 5. It is concluded that voltage-sensitive calcium channels play an important role in the excitatory mechanical action of P2X-purinoceptor-mediated purinergic responses in the rat urinary bladder, while cholinergic-mediated responses are less dependent on such channels.

P2 purinoceptor-mediated inositol phosphate formation in relation to cytoplasmic calcium in DDT1 MF-2 smooth muscle cells

The effect of P2 purinoceptor stimulation on inositol phosphate (InsP) formation in relation to the intracellular Ca2+ concentration was measured in vas deferens DDT1 MF-2 smooth muscle cells. The different [3H]myo-inositol-labelled InsP fractions were analyzed by high performance liquid chromatography and intracellular Ca2+ was determined by measuring fluorescence using Indo-1 as indicator. Stimulation with ATP (10(-4) M) resulted in an enhanced formation of inositol mono-, bis-, tris- and tetrakisphosphate (InsP1, InsP2, InsP3 and InsP4), but no changes occurred in the formation of inositol pentakis- and hexakisphosphate (InsP5 and InsP6). The putative second messenger Ins(1,3,4,5)P4 rapidly increased after addition of the agonist, reaching a maximum after about 2 min. The isomer Ins(1,4,5)P3 showed a delayed rise starting after about 2 min. The formation of Ins(1,3,4,5)P4 in the presence of ATP (2 min) was concentration-dependent, reaching a half maximal value at about 50 microM of the agonist. The intracellular Ca2+ concentration showed an initial increase after P2 purinoceptor stimulation, reaching a plateau after 2 min. Both the top of the initial phase and the plateau value of the response reached a half maximal value at an ATP concentration of about 7 microM. This Ca2+ response could be evoked repeatedly by ATP and was not affected by diltiazem (10(-5) M). In the absence of external Ca2+, the internal Ca2+ concentration increased transiently in the presence of ATP without showing the plateau phase. This response could be evoked only once under Ca2(+)-free conditions.(ABSTRACT TRUNCATED AT 250 WORDS)