Effects of purified Pseudomonas rhamnolipids on bioelectric properties of sheep tracheal epithelium

The Role of Pseudomonas aeruginosa Virulence Factors in Cytoskeletal Dysregulation and Lung Barrier Dysfunction

Pseudomonas (P.) aeruginosa is an opportunistic pathogen that causes serious infections and hospital-acquired pneumonia in immunocompromised patients. P. aeruginosa accounts for up to 20% of all cases of hospital-acquired pneumonia, with an attributable mortality rate of ~30-40%. The poor clinical outcome of P. aeruginosa-induced pneumonia is ascribed to its ability to disrupt lung barrier integrity, leading to the development of lung edema and bacteremia. Airway epithelial and endothelial cells are important architecture blocks that protect the lung from invading pathogens. P. aeruginosa produces a number of virulence factors that can modulate barrier function, directly or indirectly, through exploiting cytoskeleton networks and intercellular junctional complexes in eukaryotic cells. This review summarizes the current knowledge on P. aeruginosa virulence factors, their effects on the regulation of the cytoskeletal network and associated components, and molecular mechanisms regulating barrier function in airway epithelial and endothelial cells. A better understanding of these processes will help to lay the foundation for new therapeutic approaches against P. aeruginosa-induced pneumonia.

An Organ System-Based Synopsis of Pseudomonas aeruginosa Virulence

Driven in part by its metabolic versatility, high intrinsic antibiotic resistance, and a large repertoire of virulence factors, Pseudomonas aeruginosa is expertly adapted to thrive in a wide variety of environments, and in the process, making it a notorious opportunistic pathogen. Apart from the extensively studied chronic infection in the lungs of people with cystic fibrosis (CF), P. aeruginosa also causes multiple serious infections encompassing essentially all organs of the human body, among others, lung infection in patients with chronic obstructive pulmonary disease, primary ciliary dyskinesia and ventilator-associated pneumonia; bacteremia and sepsis; soft tissue infection in burns, open wounds and postsurgery patients; urinary tract infection; diabetic foot ulcers; chronic suppurative otitis media and otitis externa; and keratitis associated with extended contact lens use. Although well characterized in the context of CF, pathogenic processes mediated by various P. aeruginosa virulence factors in other organ systems remain poorly understood. In this review, we use an organ system-based approach to provide a synopsis of disease mechanisms exerted by P. aeruginosa virulence determinants that contribute to its success as a versatile pathogen.

Contribution of the Alkylquinolone Quorum-Sensing System to the Interaction of Pseudomonas aeruginosa With Bronchial Epithelial Cells

Pseudomonas aeruginosa causes infections in patients with compromised epithelial barrier function. Multiple virulence factors produced by P. aeruginosa are controlled by quorum sensing (QS) via 2-alkyl-4(1H)-quinolone (AQ) signal molecules. Here, we investigated the impact of AQs on P. aeruginosa PAO1 infection of differentiated human bronchial epithelial cells (HBECs). The pqsA-E operon is responsible for the biosynthesis of AQs including the 2-alkyl-3-hydroxy-4-quinolones, 4-hydroxy-2-alkylquinolines, and 4-hydroxy-2-alkylquinoline N-oxides as exemplified by pseudomonas quinolone signal (PQS), 2-heptyl-4-hydroxyquinoline (HHQ), and 2-heptyl-4-hydroxyquinoline N-oxide (HQNO), respectively. PQS and HHQ both act as QS signal molecules while HQNO is a cytochrome inhibitor. PqsE contributes both to AQ biosynthesis and promotes virulence in a PQS-independent manner. Our results show that PQS, HHQ, and HQNO were produced during PAO1 infection of HBECs, but no differences in growth or cytotoxicity were apparent when PAO1 and an AQ-negative ΔpqsA mutant were compared. Both strains promoted synthesis of inflammatory cytokines TNF-α, interleukin (IL)-6, and IL-17C by HBECs, and the provision of exogenous PQS negatively impacted on this response without affecting bacterial growth. Expression of pqsE and the PQS-independent PqsE-regulated genes mexG and lecA was detected during HBEC infection. Levels were reduced in the ΔpqsA mutant, that is, in the absence of PQS, and increased by exogenous PQS. These results support an AQ-independent role for PqsE during initial infection of HBEC by P. aeruginosa and for PQS as an enhancer of PqsE and PqsE-controlled virulence determinants and as an immunomodulator.

Pseudomonas aeruginosa Takes a Multi-Target Approach to Achieve Junction Breach

Pseudomonas aeruginosa is an opportunistic pathogen which uses a number of strategies to cross epithelial and endothelial barriers at cell–cell junctions. In this review, we describe how the coordinated actions of P. aeruginosa's virulence factors trigger various molecular mechanisms to disarm the junctional gate responsible for tissue integrity.

P. aeruginosa LPS stimulates calcium signaling and chloride secretion via CFTR in human bronchial epithelial cells

BACKGROUND

Pseudomonas aeruginosa airway infection is associated with a high mortality rate in cystic fibrosis. Lipopolysaccharide (LPS), a main constituent of the outer membrane of P. aeruginosa, is responsible for activation of innate immune response but its role on airway epithelium ion transport, is not well known. The aim of this study was to determine the role for P. aeruginosa LPS in modulating chloride secretion and intracellular calcium in the human bronchial epithelial cell line, 16HBE14o-.

METHODS

We used intracellular calcium imaging and short-circuit current measurement upon exposure of cells to P. aeruginosa LPS.

RESULTS

Apical LPS stimulated intracellular calcium release and calcium entry and enhanced chloride secretion. This latter effect was significantly inhibited by CFTR(inh)-172 and BAPTA-AM (intracellular Ca(2+) chelator).

CONCLUSIONS

Our data provides evidence for a new role of P. aeruginosa LPS in stimulating calcium entry and release and a subsequent chloride secretion via CFTR in human bronchial epithelium.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.

Rhamnolipids: diversity of structures, microbial origins and roles

Rhamnolipids are glycolipidic biosurfactants produced by various bacterial species. They were initially found as exoproducts of the opportunistic pathogen Pseudomonas aeruginosa and described as a mixture of four congeners: alpha-L-rhamnopyranosyl-alpha-L-rhamnopyranosyl-beta-hydroxydecanoyl-beta-hydroxydecanoate (Rha-Rha-C(10)-C(10)), alpha-L-rhamnopyranosyl-alpha-L-rhamnopyranosyl-beta-hydroxydecanoate (Rha-Rha-C(10)), as well as their mono-rhamnolipid congeners Rha-C(10)-C(10) and Rha-C(10). The development of more sensitive analytical techniques has lead to the further discovery of a wide diversity of rhamnolipid congeners and homologues (about 60) that are produced at different concentrations by various Pseudomonas species and by bacteria belonging to other families, classes, or even phyla. For example, various Burkholderia species have been shown to produce rhamnolipids that have longer alkyl chains than those produced by P. aeruginosa. In P. aeruginosa, three genes, carried on two distinct operons, code for the enzymes responsible for the final steps of rhamnolipid synthesis: one operon carries the rhlAB genes and the other rhlC. Genes highly similar to rhlA, rhlB, and rhlC have also been found in various Burkholderia species but grouped within one putative operon, and they have been shown to be required for rhamnolipid production as well. The exact physiological function of these secondary metabolites is still unclear. Most identified activities are derived from the surface activity, wetting ability, detergency, and other amphipathic-related properties of these molecules. Indeed, rhamnolipids promote the uptake and biodegradation of poorly soluble substrates, act as immune modulators and virulence factors, have antimicrobial activities, and are involved in surface motility and in bacterial biofilm development.

Studying bacterial infections through culture-independent approaches

The ability to characterize accurately the cause of infection is fundamental to effective treatment. The impact of any antimicrobial agents used to treat infection will, however, always be constrained by both the appropriateness of their use and our ability to determine their effectiveness. Traditional culture-based diagnostic microbiology is, in many cases, unable to provide this information. Molecular microbiological approaches that assess the content of clinical samples in a culture-independent manner promise to change dramatically the types of data that are obtained routinely from clinical samples. We argue that, in addition to the technical advance that these methodologies offer, a conceptual advance in the way that we reflect on the information generated is also required. Through the development of both of these advances, our understanding of infection, as well as the ways in which infections can be treated, may be improved. In the analysis of the microbiological content of certain clinical samples, such as blood, cerebrospinal fluid, brain and bone biopsy, culture-independent approaches have been well documented. Herein, we discuss how extensions to such studies can shape our understanding of infection at the many sites of the human body where a mixed flora, or in more ecological terms, a community of microbes, is present. To do this, we consider the underlying principles that underpin diagnostic systems, describe the ways in which these systems can be applied to community characterization, and discuss the significance of the data generated. We propose that at all locations within the human body where infection is routinely initiated within the context of a community of microbes, the same principles will apply. To consider this further, we take insights from areas such as the gut, oral cavity and skin. The main focus here is understanding respiratory tract infection, and specifically the infections of the cystic fibrosis lung. The impact that the use of culture-independent, molecular analyses will have on the way we approach the treatment of infections is also considered.

Epithelial sodium channels in the adult lung--important modulators of pulmonary health and disease

Absorption of excess fluid from the airways and alveolar lumen requires active vectorial transepithelial transport of sodium ions (Na+) by alveolar type II and possibly type I cells. The rate-limiting step in this process is the activity of the heterotrimeric apical membrane epithelial Na+ channel (ENaC). Pharmacologic inhibitors and genetic manipulations that disrupt Na+ transport result in fluid accumulation within the lung and failure of gas exchange. The importance of Na+ transport in the lung is also demonstrated in conditions such as ARDS, where abnormal absorption of Na+ contributes to the pathophysiology of pulmonary disease. ENaC expression and function is influenced by diverse factors, such as oxygen tension, glucocorticoids, and cytoskeletal proteins. In addition, ENaC dysfunction has been shown to be induced by purinergic nucleotide activation of P2Y receptors (in paramyxoviral bronchiolitis) and reactive species (in acute lung injury). Finally, beta-adrenergic agonists have been shown experimentally to reverse defects in ENaC function, and improve hypoxemia and pulmonary edema, and may provide a novel therapeutic modality for ARDS, although some viral lung pathogens appear to induce insensitivity to their actions.

Inhibition of airway Na+ transport by respiratory syncytial virus

In previous studies, we have shown that two major respiratory pathogens, influenza virus and parainfluenza virus, produce acute alterations in ion transport upon contacting the apical membrane of the respiratory epithelium. In the present study, we examine the effects on ion transport by the mouse tracheal epithelium of a third major respiratory pathogen, respiratory syncytial virus (RSV). RSV infections are associated with fluid accumulation in the respiratory tract and cause illnesses that range in severity from rhinitis, sinusitis, otitis media, and bronchitis to bronchiolitis and pneumonia. We find that within minutes of RSV contacting the apical membrane; it inhibits amiloride-sensitive Na+ transport by the epithelium. This effect is mediated by protein kinase C and is reproduced by recombinant viral F (fusion) protein. Since this inhibition is not accompanied by any alteration in the epithelial responses to carbachol or to forskolin plus 3-isobutyl-1-methylxanthine (IBMX), it is not due to a nonspecific toxic action of the virus. The inhibition also appears to require Toll-like receptor 4 and the presence of asialogangliosides in the apical membrane. Since the concentration range over which this inhibition is observed (10(2) to 10(5) PFU/ml) is comparable to the viral concentrations observed in clinical and experimental RSV infections, it seems likely that direct inhibition by the virus of epithelial Na+ transport may contribute to the fluid accumulation that is observed in RSV infections.

Interference in adhesion of bacteria and yeasts isolated from explanted voice prostheses to silicone rubber by rhamnolipid biosurfactants

AIMS

The effects and extent of adhesion of four different bacterial and two yeast strains isolated from explanted voice prostheses to silicone rubber with and without an adsorbed rhamnolipid biosurfactant layer obtained from Pseudomonasaeruginosa DS10-129 was studied.

METHODS AND RESULTS

The ability of rhamnolipid biosurfactant to inhibit adhesion of micro-organisms to silicone rubber was investigated in a parallel-plate flow chamber. The anti-adhesive activity of the biosurfactant at different concentrations was significant against all the strains and depended on the micro-organism tested. The results showed an effective reduction in the initial deposition rates, and the number of bacterial cells adhering after 4 h, for all micro-organisms tested at the 4 g l(-1) undiluted rhamnolipid solution. Maximum initial reduction of adhesion rate (an average of 66%) occurred for Streptococcus salivarius GB 24/9 and Candida tropicalis GB 9/9. The number of cells adhering after 4 h on silicone rubber conditioned with biosurfactant was reduced to 48% for Staphylococcus epidermidis GB 9/6, Strep. salivarius GB 24/9, Staphylococcus aureus GB 2/1 and C. tropicalis GB 9/9 in comparison to controls. Perfusing the flow chamber with biosurfactant containing solution followed by the passage of a liquid-air interface, to investigate detachment of micro-organisms adhering to silicone rubber, produced high detachment (96%) of adhered cells for all micro-organisms studied, except for Staph. aureus GB 2/1 (67%).

SIGNIFICANCE AND IMPACT OF THE STUDY

It is concluded that biosurfactant represent suitable compounds that should be considered in developing future strategies to prevent the microbial colonization of silicone rubber voice prostheses.

Rhamnolipids are virulence factors that promote early infiltration of primary human airway epithelia by Pseudomonas aeruginosa

The opportunistic bacterium Pseudomonas aeruginosa causes chronic respiratory infections in cystic fibrosis and immunocompromised individuals. Bacterial adherence to the basolateral domain of the host cells and internalization are thought to participate in P. aeruginosa pathogenicity. However, the mechanism by which the pathogen initially modulates the paracellular permeability of polarized respiratory epithelia remains to be understood. To investigate this mechanism, we have searched for virulence factors secreted by P. aeruginosa that affect the structure of human airway epithelium in the early stages of infection. We have found that only bacterial strains secreting rhamnolipids were efficient in modulating the barrier function of an in vitro-reconstituted human respiratory epithelium, irrespective of their release of elastase and lipopolysaccharide. In contrast to previous reports, we document that P. aeruginosa was not internalized by epithelial cells. We further report that purified rhamnolipids, applied on the surfaces of the epithelia, were sufficient to functionally disrupt the epithelia and to promote the paracellular invasion of rhamnolipid-deficient P. aeruginosa. The mechanism involves the incorporation of rhamnolipids within the host cell membrane, leading to tight-junction alterations. The study provides direct evidence for a hitherto unknown mechanism whereby the junction-dependent barrier of the respiratory epithelium is selectively altered by rhamnolipids.

Flagellin of Pseudomonas aeruginosa inhibits Na+ transport in airway epithelia

Pseudomonas aeruginosa causes severe life-threatening airway infections that are a frequent cause for hospitalization of cystic fibrosis (CF) patients. These Gram-negative pathogens possess flagella that contain the protein flagellin as a major structural component. Flagellin binds to the host cell glycolipid asialoGM1 (ASGM1), which appears enriched in luminal membranes of respiratory epithelial cells. We demonstrate that in mouse airways, luminal exposure to flagellin leads to inhibition of Na+ absorption by the epithelial Na+ channel ENaC, but does not directly induce a secretory response. Inhibition of ENaC was observed in tracheas of wild-type mice and was attenuated in mice homozygous for the frequent cystic fibrosis conductance regulator (CFTR) mutation G551D. Similar to flagellin, anti-ASGM1 antibody also inhibited ENaC. The inhibitory effects of flagellin on ENaC were attenuated by blockers of the purinergic signaling pathway, although an increase in the intracellular Ca2+ concentration by recombinant or purified flagellin or whole flagella was not observed. Because an inhibitor of the mitogen-activated protein kinase (MAPK) pathway also attenuated the effects of flagellin on Na+ absorption, we conclude that flagellin exclusively inhibits ENaC, probably due to release of ATP and activation of purinergic receptors of the P2Y subtype. Stimulation of these receptors activates the MAPK pathway, thereby leading to inhibition of ENaC. Thus, P. aeruginosa reduces Na+ absorption, which could enhance local mucociliary clearance, a mechanism that seem to be attenuated in CF.

Reactive species mediate inhibition of alveolar type II sodium transport during mycoplasma infection

RATIONALE

Mycoplasma pneumoniae is a significant cause of pneumonia in humans.

OBJECTIVES

To determine the impact of mycoplasma infection and the host inflammatory response on alveolar type II (ATII) cell ion transport in vivo and in vitro.

METHODS

Mice were infected with M. pulmonis for measurements of alveolar fluid clearance (AFC) in vivo and isolation of ATII cells. ATII cells were infected in vivo for determination of epithelial Na+ channel (ENaC) total and cell surface protein levels by biotinylation and Western blot and in vitro for whole cell patch clamp recording and measurement of nitric oxide (NO) production by chemiluminescence.

RESULTS

Mycoplasma infection significantly inhibited AFC at 24 h and total and amiloride-sensitive AFC by 48 h postinfection (pi). In contrast, infected myeloperoxidase-deficient mice had similar basal and amiloride-sensitive AFC values to uninfected control mice at 48 h pi. Addition of forskolin restored total and amiloride-sensitive AFC to control values at 48 h pi. ATII cells isolated from infected mice demonstrated normal alpha, beta, and gamma ENaC total protein levels; however, infected whole-lung cell-surface levels of gamma ENaC were significantly decreased. Patch-clamp recordings demonstrated a significant decrease in total and amiloride-sensitive Na+ currents at 24 h pi. ATII cells demonstrated a significant increase in the production of NO at 24 h pi and inhibition of NO by ATII cells before infection reversed the decrease in total Na+ currents.

CONCLUSIONS

These data indicate that mycoplasma infection results in decreased AFC and functional ENaC via the production of reactive oxygen nitrogen intermediates.

Pseudomonas aeruginosa inhibits endocytic recycling of CFTR in polarized human airway epithelial cells

The most common mutation in the CFTR gene in individuals with cystic fibrosis (CF), DeltaF508, leads to the absence of CFTR Cl(-) channels in the apical plasma membrane, which in turn results in impairment of mucociliary clearance, the first line of defense against inhaled bacteria. Pseudomonas aeruginosa is particularly successful at colonizing and chronically infecting the lungs and is responsible for the majority of morbidity and mortality in patients with CF. Rescue of DeltaF508-CFTR by reduced temperature or chemical means reveals that the protein is at least partially functional as a Cl(-) channel. Thus current research efforts have focused on identification of drugs that restore the presence of CFTR in the apical membrane to alleviate the symptoms of CF. Because little is known about the effects of P. aeruginosa on CFTR in the apical membrane, whether P. aeruginosa will affect the efficacy of new drugs designed to restore the plasma membrane expression of CFTR is unknown. Accordingly, the objective of the present study was to determine whether P. aeruginosa affects CFTR-mediated Cl(-) secretion in polarized human airway epithelial cells. We report herein that a cell-free filtrate of P. aeruginosa reduced CFTR-mediated transepithelial Cl(-) secretion by inhibiting the endocytic recycling of CFTR and thus the number of WT-CFTR and DeltaF508-CFTR Cl(-) channels in the apical membrane in polarized human airway epithelial cells. These data suggest that chronic infection with P. aeruginosa may interfere with therapeutic strategies aimed at increasing the apical membrane expression of DeltaF508-CFTR.

Modulation of lung epithelial functions by Pseudomonas aeruginosa

Microorganisms gain access to the airways and respiratory epithelial surface during normal breathing. Most inhaled microbes are trapped on the mucous layer coating the nasal epithelium and upper respiratory tract, and are cleared by ciliary motion. Microorganisms reaching the alveolar spaces are deposited on the pulmonary epithelium. This contact initiates complex offensive and defensive strategies by both parties. Here, we briefly outline how the pulmonary pathogen Pseudomonas aeruginosa uses multi-pronged strategies that include cell surface appendages, and secreted and injected virulence determinants to switch from an unobtrusive soil bacterium to a pathogen for lung epithelium colonization. Understanding the complex interactions between the lung epithelium and P. aeruginosa might enable more effective therapeutic strategies against infection in cystic fibrosis and immuno-compromised individuals.

First Encounter: How Pathogens Compromise Epithelial Transport

Pathogenic organisms trigger numerous signaling pathways that ultimately lead to drastic changes in physiological functions. Apart from altering structure and function of the epithelial tight junction barrier and activating inflammatory cascades, they induce changes in fluid and electrolyte transport. Pathogens do so by activating or by inhibiting ion channels and transporters, and the result might be to their benefit or to their disadvantage.

Acute effects of parainfluenza virus on epithelial electrolyte transport

Parainfluenza viruses are important causes of respiratory disease in both children and adults. In particular, they are the major cause of the serious childhood illness croup (laryngotracheobronchitis). The infections produced by parainfluenza viruses are associated with the accumulation of ions and fluid in the respiratory tract. It is not known, however, whether this accumulation is because of a direct effect of the viruses on ion and fluid transport by the respiratory epithelium. Here we show that a model parainfluenza virus (the Sendai virus), in concentrations observed during respiratory infections, activates Cl- secretion and inhibits Na+ absorption across the tracheal epithelium. It does so by binding to a neuraminidase-insensitive glycolipid, possibly asialo-GM1, triggering the release of ATP, which then acts in an autocrine fashion on apical P2Y receptors to produce the observed changes in ion transport. These findings indicate that fluid accumulation in the respiratory tract associated with parainfluenza virus infection is attributable, at least in part, to direct effects of the virus on ion transport by the respiratory epithelium.

Nucleotide-mediated inhibition of alveolar fluid clearance in BALB/c mice after respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract disease in infants and children worldwide. Intranasal infection of BALB/c mice with RSV strain A2, but not ultraviolet-inactivated RSV, for 2 or 4 days reduced basal alveolar fluid clearance (AFC), a seminal function of bronchoalveolar epithelium, and caused loss of AFC sensitivity to amiloride inhibition. Reduced AFC was temporally associated with increased lung water content but was not a consequence of increased epithelial permeability or cell death. Reduced AFC was also not due to decreased transcription of epithelial Na+ channel subunit genes in lung tissue. RSV-mediated inhibition of AFC 2 days after infection was rapidly prevented by addition to the instillate of P2Y receptor antagonists (suramin and XAMR-0721) or enzymes that degrade UTP, but not those that degrade ATP. After UTP degradation, AFC returned to control levels but was no longer sensitive to amiloride. UTP at nanomolar concentrations recapitulated the AFC inhibitory effect of RSV in normal mice and mice infected with RSV for 6 days, indicating that normalization of AFC at this time point is a consequence of cessation of UTP release, rather than P2Y receptor desensitization. We conclude that RSV infection of the bronchoalveolar epithelium results in reduced AFC as a consequence of autocrine feedback inhibition mediated by UTP. These studies are the first to demonstrate AFC inhibition by an important pulmonary viral pathogen. Reduced AFC may result in formation of an increased volume of fluid mucus, airway congestion, and rhinorrhea, all features of severe RSV disease.

General overview of mineralocorticoid hormone action

The adrenal cortex elaborates two major groups of steroids that have been arbitrarily classified as glucocorticoids and mineralocorticoids, despite the fact that carbohydrate metabolism is intimately linked to mineral balance in mammals. In fact, glucocorticoids assured both of these functions in all living cells, animal and photosynthetic, prior to the appearance of aldosterone in teleosts at the dawn of terrestrial colonization. The evolutionary drive for a hormone specifically designed for hydromineral regulation led to zonation for the conversion of 18-hydroxycorticosterone into aldosterone through the catalytic action of a synthase in the secluded compartment of the adrenal zona glomerulosa. Corticoid hormones exert their physiological action by binding to receptors that belong to a transcription factor superfamily, which also includes some of the proteins regulating steroid synthesis. Steroids stimulate sodium absorption by the activation and/or de novo synthesis of the ion-gated, amiloride-sensitive sodium channel in the apical membrane and that of the Na+/K+-ATPase in the basolateral membrane. Receptors, channels, and pumps apparently are linked to the cytoskeleton and are further regulated variously by methylation, phosphorylation, ubiquination, and glycosylation, suggesting a complex system of control at multiple checkpoints. Mutations in genes for many of these different proteins have been described and are known to cause clinical disease.

Pseudomonas aeruginosa induces changes in fluid transport across airway surface epithelia

Fluid transport across cultures of bovine tracheal epithelium was measured with a capacitance probe technique. Baseline fluid absorption (Jv) across bovine cells of 3.2 microliter. cm-2. h-1 was inhibited by approximately 78% after 1 h of exposure to suspensions of Pseudomonas aeruginosa, with a concomitant decrease in transepithelial potential (TEP) and increase in transepithelial resistance (Rt). Effects of P. aeruginosa were blocked by amiloride, which decreased Jv by 112% from baseline of 2.35 +/- 1.25 microliter. cm-2. h-1, increased Rt by 101% from baseline of 610 +/- 257 Omega. cm2, and decreased TEP by 91% from baseline of -55 +/- 18.5 mV. Microelectrode studies suggested that effects of P. aeruginosa on amiloride-sensitive Na absorption were due in part to a block of basolateral membrane K channels. In the presence of Cl transport inhibitors [5-nitro-2-(3-phenylpropylamino)-benzoic acid, H2-DIDS, and bumetanide], P. aeruginosa induced a fluid secretion of approximately 2.5 +/- 0.4 microliter. cm-2. h-1 and decreased Rt without changing TEP. However, these changes were abolished when the transport inhibitors were used in a medium in which Cl was replaced by an impermeant organic anion. Filtrates of P. aeruginosa suspensions had no effect on Jv, TEP, or Rt. Mutants lacking exotoxin A or rhamnolipids or with defective lipopolysaccharide still inhibited fluid absorption and altered bioelectrical properties. By contrast, mutations in the rpoN gene encoding a sigma factor of RNA polymerase abolished actions of P. aeruginosa. In vivo, changes in transepithelial salt and water transport induced by P. aeruginosa may alter viscosity and ionic composition of airway secretions so as to foster further bacterial colonization.

Mycoplasma pulmonis inhibits electrogenic ion transport across murine tracheal epithelial cell monolayers

Murine chronic respiratory disease is characterized by persistent colonization of tracheal and bronchial epithelial cell surfaces by Mycoplasma pulmonis, submucosal and intraluminal immune and inflammatory cells, and altered airway activity. To determine the direct effect of M. pulmonis upon transepithelial ion transport in the absence of immune and inflammatory cell responses, primary mouse tracheal epithelial cell monolayers (MTEs) were apically infected and assayed in Ussing chambers. M. pulmonis-infected MTEs, but not those infected with a nonmurine mycoplasma, demonstrated reductions in amiloride-sensitive Na+ absorption, cyclic AMP, and cholinergic-stimulated Cl- secretion and transepithelial resistance. These effects were shown to require interaction of viable organisms with the apical surface of the monolayer and to be dependent upon organism number and duration of infection. Altered transport due to M. pulmonis was not merely a result of epithelial cell death as evidenced by the following: (i) active transport of Na+ and Cl-, albeit at reduced rates; (ii) normal cell morphology, including intact tight junctions, as demonstrated by electron microscopy; (iii) maintenance of a mean transepithelial resistance of 440 omega/cm2; and (iv) lack of leakage of fluid from the basolateral to the apical surface of the monolayer. Alteration in epithelial ion transport in vitro is consistent with impaired pulmonary clearance and altered airway function in M. pulmonis-infected animals. Furthermore, the ability of M. pulmonis to alter transport without killing the host cell may explain its successful parasitism and long-term persistence in the host. Further study of the MTE-M. pulmonis model should elucidate the molecular mechanisms which mediate this reduction in transepithelial ion transport.