Pseudomonas aeruginosa Homoserine lactone activates store-operated cAMP and cystic fibrosis transmembrane regulator-dependent Cl- secretion by human airway epithelia

Protective role of CFTR during fungal infection of cystic fibrosis bronchial epithelial cells with Aspergillus fumigatus

Lung infection with the fungus Aspergillus fumigatus (Af) is a common complication in cystic fibrosis (CF) and is associated with loss of pulmonary function. We established a fungal epithelial co-culture model to examine the impact of Af infection on CF bronchial epithelial barrier function using Af strains 10AF and AF293-GFP, and the CFBE41o- cell line homozygous for the F508del mutation with (CF+CFTR) and without (CF) normal CFTR expression. Following exposure of the epithelial surface to Af conidia, formation of germlings (early stages of fungal growth) was detected after 9-12 hours and hyphae (mature fungal growth) after 12-24 hours. During fungal morphogenesis, bronchial epithelial cells showed signs of damage including rounding, and partial detachment after 24 hours. Fluorescently labeled conidia were internalized after 6 hours and more internalized conidia were observed in CF compared to CF+CFTR cells. Infection of the apical surface with 10AF conidia, germlings, or hyphae was performed to determine growth stage-specific effects on tight junction protein zona occludens protein 1 (ZO-1) expression and transepithelial electrical resistance (TER). In response to infection with conidia or germlings, epithelial barrier function degraded time-dependently (based on ZO-1 immunofluorescence and TER) with a delayed onset in CF+CFTR cell monolayers and required viable fungi and apical application. Infection with hyphae caused an earlier onset and faster rate of decline in TER compared to conidia and germlings. Gliotoxin, a major Af virulence factor, caused a rapid decline in TER and induced a transient chloride secretory response in CF+CFTR but not CF cells. Our findings suggest growth and internalization of Af result in deleterious effects on bronchial epithelial barrier function that occurred more rapidly in the absence of CFTR. Bronchial epithelial barrier breakdown was time-dependent and morphotype-specific and mimicked by acute administration of gliotoxin. Our study also suggests a protective role for CFTR by turning on CFTR-dependent chloride transport in response to gliotoxin, a mechanism that will support mucociliary clearance, and could delay the loss of epithelial integrity during fungal development in vivo.

Environmental, Microbiological, and Immunological Features of Bacterial Biofilms Associated with Implanted Medical Devices

The spread of biofilms on medical implants represents one of the principal triggers of persistent and chronic infections in clinical settings, and it has been the subject of many studies in the past few years, with most of them focused on prosthetic joint infections. We review here recent works on biofilm formation and microbial colonization on a large variety of indwelling devices, ranging from heart valves and pacemakers to urological and breast implants and from biliary stents and endoscopic tubes to contact lenses and neurosurgical implants. We focus on bacterial abundance and distribution across different devices and body sites and on the role of environmental features, such as the presence of fluid flow and properties of the implant surface, as well as on the interplay between bacterial colonization and the response of the human immune system.

cAMP triggers Na+ absorption by distal airway surface epithelium in cystic fibrosis swine

A controversial hypothesis pertaining to cystic fibrosis (CF) lung disease is that the CF transmembrane conductance regulator (CFTR) channel fails to inhibit the epithelial Na⁺ channel (ENaC), yielding increased Na⁺ reabsorption and airway dehydration. We use a non-invasive self-referencing Na⁺-selective microelectrode technique to measure Na⁺ transport across individual folds of distal airway surface epithelium preparations from CFTR^-/- (CF) and wild-type (WT) swine. We show that, under unstimulated control conditions, WT and CF epithelia exhibit similar, low rates of Na⁺ transport that are unaffected by the ENaC blocker amiloride. However, in the presence of the cyclic AMP (cAMP)-elevating agents forskolin+IBMX (isobutylmethylxanthine), folds of WT tissues secrete large amounts of Na⁺, while CFTR^-/- tissues absorb small, but potentially important, amounts of Na⁺. In cAMP-stimulated conditions, amiloride inhibits Na⁺ absorption in CFTR^-/- tissues but does not affect secretion in WT tissues. Our results are consistent with the hypothesis that ENaC-mediated Na⁺ absorption may contribute to dehydration of CF distal airways.

Pharmacology of T2R Mediated Host-Microbe Interactions

Bitter taste receptors (T2Rs) belong to the G protein-coupled receptor superfamily. Humans express 25 T2Rs that are known to detect several bitter compounds including bacterial quorum sensing molecules (QSM). Primarily found to be key receptors for bitter sensation T2Rs are known to play an important role in mediating innate immune responses in oral and extraoral tissues. Several studies have led to identification of Gram-negative and Gram-positive bacterial QSMs as agonists for T2Rs in airway epithelial cells and immune cells. However, the pharmacological characterization for many of the QSM-T2R interactions remains poorly defined. In this chapter, we discuss the extraoral roles including localization of T2Rs in extracellular vesicles, molecular pharmacology of QSM-T2R interactions, role of T2Rs in mediating innate immune responses, and some of the challenges in understanding T2R pharmacology.

Impact of N-Acyl-Homoserine Lactones, Quorum Sensing Molecules, on Gut Immunity

Among numerous molecules found in the gut ecosystem, quorum sensing (QS) molecules represent an overlooked part that warrants highlighting. QS relies on the release of small molecules (auto-inducers) by bacteria that accumulate in the environment depending on bacterial cell density. These molecules not only are sensed by the microbial community but also interact with host cells and contribute to gut homeostasis. It therefore appears entirely appropriate to highlight the role of these molecules on the immune system in dysbiosis-associated inflammatory conditions where the bacterial populations are imbalanced. Here, we intent to focus on one of the most studied QS molecule family, namely, the type I auto-inducers represented by N-acyl-homoserine lactones (AHL). First described in pathogens such as Pseudomonas aeruginosa, these molecules have also been found in commensals and have been recently described within the complex microbial communities of the mammalian intestinal tract. In this mini-review, we will expound on this emergent field of research. We will first recall evidence on AHL structure, synthesis, receptors, and functions regarding interbacterial communication. Then, we will discuss their interactions with the host and particularly with agents of the innate and adaptive gut mucosa immunity. This will reveal how this new set of molecules, driven by microbial imbalance, can interact with inflammation pathways and could be a potential target in inflammatory bowel disease (IBD). The discovery of the general impact of these compounds on the detection of the bacterial quorum and on the dynamic and immune responses of eukaryotic cells opens up a new field of pathophysiology.

Calcium Regulation of Bacterial Virulence

Calcium (Ca²⁺) is a universal signaling ion, whose major informational role shaped the evolution of signaling pathways, enabling cellular communications and responsiveness to both the intracellular and extracellular environments. Elaborate Ca²⁺ regulatory networks have been well characterized in eukaryotic cells, where Ca²⁺ regulates a number of essential cellular processes, ranging from cell division, transport and motility, to apoptosis and pathogenesis. However, in bacteria, the knowledge on Ca²⁺ signaling is still fragmentary. This is complicated by the large variability of environments that bacteria inhabit with diverse levels of Ca²⁺. Yet another complication arises when bacterial pathogens invade a host and become exposed to different levels of Ca²⁺ that (1) are tightly regulated by the host, (2) control host defenses including immune responses to bacterial infections, and (3) become impaired during diseases. The invading pathogens evolved to recognize and respond to the host Ca²⁺, triggering the molecular mechanisms of adhesion, biofilm formation, host cellular damage, and host-defense resistance, processes enabling the development of persistent infections. In this review, we discuss: (1) Ca²⁺ as a determinant of a host environment for invading bacterial pathogens, (2) the role of Ca²⁺ in regulating main events of host colonization and bacterial virulence, and (3) the molecular mechanisms of Ca²⁺ signaling in bacterial pathogens.

The cAMP Pathway Amplifies Early MyD88-Dependent and Type I Interferon-Independent LPS-Induced Interleukin-10 Expression in Mouse Macrophages

Interleukin-10 (IL-10) is a key anti-inflammatory cytokine, secreted by macrophages and other immune cells to attenuate inflammation. Autocrine type I interferons (IFNs) largely mediate the delayed expression of IL-10 by LPS-stimulated macrophages. We have previously shown that IL-10 is synergistically expressed in macrophages following a costimulus of a TLR agonist and cAMP. We now show that the cAMP pathway directly upregulates IL-10 transcription and plays an important permissive and synergistic role in early, but not late, LPS-stimulated IL-10 mRNA and protein expression in mouse macrophages and in a mouse septic shock model. Our results suggest that the loss of synergism is not due to desensitization of the cAMP inducing signal, and it is not mediated by a positive crosstalk between the cAMP and type I IFN pathways. First, cAMP elevation in LPS-treated cells decreased the secretion of type I IFN. Second, autocrine/paracrine type I IFNs induce IL-10 promoter reporter activity only additively, but not synergistically, with the cAMP pathway. IL-10 promoter reporter activity was synergistically induced by cAMP elevation in macrophages stimulated by an agonist of either TLR4, TLR2/6, or TLR7, receptors which signal via MyD88, but not by an agonist of TLR3 which signals independently of MyD88. Moreover, MyD88 knockout largely reduced the synergistic IL-10 expression, indicating that MyD88 is required for the synergism displayed by LPS with cAMP. This report delineates the temporal regulation of early cAMP-accelerated vs. late type I IFN-dependent IL-10 transcription in LPS-stimulated murine macrophages that can limit inflammation at its onset.

Aspects of Gut Microbiota and Immune System Interactions in Infectious Diseases, Immunopathology, and Cancer

The microbiota consists of a dynamic multispecies community of bacteria, fungi, archaea, and protozoans, bringing to the host organism a dowry of cells and genes more numerous than its own. Among the different non-sterile cavities, the human gut harbors the most complex microbiota, with a strong impact on host homeostasis and immunostasis, being thus essential for maintaining the health condition. In this review, we outline the roles of gut microbiota in immunity, starting with the background information supporting the further presentation of the implications of gut microbiota dysbiosis in host susceptibility to infections, hypersensitivity reactions, autoimmunity, chronic inflammation, and cancer. The role of diet and antibiotics in the occurrence of dysbiosis and its pathological consequences, as well as the potential of probiotics to restore eubiosis is also discussed.

Chemical strategies to unravel bacterial-eukaryotic signaling

The common language of bacteria and higher life forms is a lexicon of small molecules that the research community is only beginning to decipher. While many new signaling molecules have been discovered in recent years, the identification of their targets is mostly lagging. This review will focus on the latest chemical-probe based research aimed at understanding how bacteria interact chemically with mammals and plants. In general, chemical biology strategies remain under-utilized in this complex field of research, with a few key exceptions, and we hope that this review encourages others to implement these techniques in their research. Specifically, we highlight the chemical biology techniques used in recent studies, especially activity-based protein profiling, that have been applied to unravel the chemical mechanisms of interkingdom interactions.

N-(3-Oxo-acyl)-homoserine lactone induces apoptosis primarily through a mitochondrial pathway in fibroblasts

N-(3-Oxododecanoyl)-l-homoserine lactone (C12) is produced by Pseudomonas aeruginosa to function as a quorum-sensing molecule for bacteria-bacteria communication. C12 is also known to influence many aspects of human host cell physiology, including induction of cell death. However, the signalling pathway(s) leading to C12-triggered cell death is (are) still not completely known. To clarify cell death signalling induced by C12, we examined mouse embryonic fibroblasts deficient in "initiator" caspases or "effector" caspases. Our data indicate that C12 selectively induces the mitochondria-dependent intrinsic apoptotic pathway by quickly triggering mitochondrial outer membrane permeabilisation. Importantly, the activities of C12 to permeabilise mitochondria are independent of activation of both "initiator" and "effector" caspases. Furthermore, C12 directly induces mitochondrial outer membrane permeabilisation in vitro. Overall, our study suggests a mitochondrial apoptotic signalling pathway triggered by C12, in which C12 or its metabolite(s) acts on mitochondria to permeabilise mitochondria, leading to activation of apoptosis.

The Cystic Fibrosis Transmembrane Conductance Regulator Potentiator Ivacaftor Augments Mucociliary Clearance Abrogating Cystic Fibrosis Transmembrane Conductance Regulator Inhibition by Cigarette Smoke

Acquired cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction may contribute to chronic obstructive pulmonary disease pathogenesis and is a potential therapeutic target. We sought to determine the acute effects of cigarette smoke on ion transport and the mucociliary transport apparatus, their mechanistic basis, and whether deleterious effects could be reversed with the CFTR potentiator ivacaftor (VX-770). Primary human bronchial epithelial (HBE) cells and human bronchi were exposed to cigarette smoke extract (CSE) and/or ivacaftor. CFTR function and expression were measured in Ussing chambers and by surface biotinylation. CSE-derived acrolein modifications on CFTR were determined by mass spectroscopic analysis of purified protein, and the functional microanatomy of the airway epithelia was measured by 1-μm resolution optical coherence tomography. CSE reduced CFTR-dependent current in HBE cells (P < 0.05) and human bronchi (P < 0.05) within minutes of exposure. The mechanism involved CSE-induced reduction of CFTR gating, decreasing CFTR open-channel probability by approximately 75% immediately after exposure (P < 0.05), whereas surface CFTR expression was partially reduced with chronic exposure, but was stable acutely. CSE treatment of purified CFTR resulted in acrolein modifications on lysine and cysteine residues that likely disrupt CFTR gating. In primary HBE cells, CSE reduced airway surface liquid depth (P < 0.05) and ciliary beat frequency (P < 0.05) within 60 minutes that was restored by coadministration with ivacaftor (P < 0.005). Cigarette smoking transmits acute reductions in CFTR activity, adversely affecting the airway surface. These effects are reversible by a CFTR potentiator in vitro, representing a potential therapeutic strategy in patients with chronic obstructive pulmonary disease with chronic bronchitis.

Host Responses to Biofilm

From birth to death the human host immune system interacts with bacterial cells. Biofilms are communities of microbes embedded in matrices composed of extracellular polymeric substance (EPS), and have been implicated in both the healthy microbiome and disease states. The immune system recognizes many different bacterial patterns, molecules, and antigens, but these components can be camouflaged in the biofilm mode of growth. Instead, immune cells come into contact with components of the EPS matrix, a diverse, hydrated mixture of extracellular DNA (bacterial and host), proteins, polysaccharides, and lipids. As bacterial cells transition from planktonic to biofilm-associated they produce small molecules, which can increase inflammation, induce cell death, and even cause necrosis. To survive, invading bacteria must overcome the epithelial barrier, host microbiome, complement, and a variety of leukocytes. If bacteria can evade these initial cell populations they have an increased chance at surviving and causing ongoing disease in the host. Planktonic cells are readily cleared, but biofilms reduce the effectiveness of both polymorphonuclear neutrophils and macrophages. In addition, in the presence of these cells, biofilm formation is actively enhanced, and components of host immune cells are assimilated into the EPS matrix. While pathogenic biofilms contribute to states of chronic inflammation, probiotic Lactobacillus biofilms cause a negligible immune response and, in states of inflammation, exhibit robust antiinflammatory properties. These probiotic biofilms colonize and protect the gut and vagina, and have been implicated in improved healing of damaged skin. Overall, biofilms stimulate a unique immune response that we are only beginning to understand.

Modulation of Host Biology by Pseudomonas aeruginosa Quorum Sensing Signal Molecules: Messengers or Traitors

Bacterial cells sense their population density and respond accordingly by producing various signal molecules to the surrounding environments thereby trigger a plethora of gene expression. This regulatory pathway is termed quorum sensing (QS). Plenty of bacterial virulence factors are controlled by QS or QS-mediated regulatory systems and QS signal molecules (QSSMs) play crucial roles in bacterial signaling transduction. Moreover, bacterial QSSMs were shown to interfere with host cell signaling and modulate host immune responses. QSSMs not only regulate the expression of bacterial virulence factors but themselves act in the modulation of host biology that can be potential therapeutic targets.

Airway Epithelial Cell Integrity Protects from Cytotoxicity of Pseudomonas aeruginosa Quorum-Sensing Signals

Cell-to-cell communication via gap junctions regulates airway epithelial cell homeostasis and maintains the epithelium host defense. Quorum-sensing molecules produced by Pseudomonas aeruginosa coordinate the expression of virulence factors by this respiratory pathogen. These bacterial signals may also incidentally modulate mammalian airway epithelial cell responses to the pathogen, a process called interkingdom signaling. We investigated the interactions between the P. aeruginosa N-3-oxo-dodecanoyl-L-homoserine lactone (C12) quorum-sensing molecule and human airway epithelial cell gap junctional intercellular communication (GJIC). C12 degradation and its effects on cells were monitored in various airway epithelial cell models grown under nonpolarized and polarized conditions. Its concentration was further monitored in daily tracheal aspirates of colonized intubated patients. C12 rapidly altered epithelial integrity and decreased GJIC in nonpolarized airway epithelial cells, whereas other quorum-sensing molecules had no effect. The effects of C12 were dependent on [Ca(2+)]i and could be prevented by inhibitors of Src tyrosine family and Rho-associated protein kinases. In contrast, polarized airway cells grown on Transwell filters were protected from C12 except when undergoing repair after wounding. In vivo during colonization of intubated patients, C12 did not accumulate, but it paralleled bacterial densities. In vitro C12 degradation, a reaction catalyzed by intracellular paraoxonase 2 (PON2), was impaired in nonpolarized cells, whereas PON2 expression was increased during epithelial polarization. The cytotoxicity of C12 on nonpolarized epithelial cells, combined with its impaired degradation allowing its accumulation, provides an additional pathogenic mechanism for P. aeruginosa infections.

"Store-operated" cAMP signaling contributes to Ca2+-activated Cl- secretion in T84 colonic cells

Apical cAMP-dependent CFTR Cl(-) channels are essential for efficient vectorial movement of ions and fluid into the lumen of the colon. It is well known that Ca(2+)-mobilizing agonists also stimulate colonic anion secretion. However, CFTR is apparently not activated directly by Ca(2+), and the existence of apical Ca(2+)-dependent Cl(-) channels in the native colonic epithelium is controversial, leaving the identity of the Ca(2+)-activated component unresolved. We recently showed that decreasing free Ca(2+) concentration ([Ca(2+)]) within the endoplasmic reticulum (ER) lumen elicits a rise in intracellular cAMP. This process, which we termed "store-operated cAMP signaling" (SOcAMPS), requires the luminal ER Ca(2+) sensor STIM1 and does not depend on changes in cytosolic Ca(2+). Here we assessed the degree to which SOcAMPS participates in Ca(2+)-activated Cl(-) transport as measured by transepithelial short-circuit current (Isc) in polarized T84 monolayers in parallel with imaging of cAMP and PKA activity using fluorescence resonance energy transfer (FRET)-based reporters in single cells. In Ca(2+)-free conditions, the Ca(2+)-releasing agonist carbachol and Ca(2+) ionophore increased Isc, cAMP, and PKA activity. These responses persisted in cells loaded with the Ca(2+) chelator BAPTA-AM. The effect on Isc was enhanced in the presence of the phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), inhibited by the CFTR inhibitor CFTRinh-172 and the PKA inhibitor H-89, and unaffected by Ba(2+) or flufenamic acid. We propose that a discrete component of the "Ca(2+)-dependent" secretory activity in the colon derives from cAMP generated through SOcAMPS. This alternative mode of cAMP production could contribute to the actions of diverse xenobiotic agents that disrupt ER Ca(2+) homeostasis, leading to diarrhea.

Deleterious impact of Pseudomonas aeruginosa on cystic fibrosis transmembrane conductance regulator function and rescue in airway epithelial cells

The epithelial response to bacterial airway infection, a common feature of lung diseases such as chronic obstructive pulmonary disease and cystic fibrosis, has been extensively studied. However, its impact on cystic fibrosis transmembrane conductance regulator (CFTR) channel function is not clearly defined. Our aims were, therefore, to evaluate the effect of Pseudomonas aeruginosa on CFTR function and expression in non-cystic fibrosis airway epithelial cells, and to investigate its impact on ΔF508-CFTR rescue by the VRT-325 corrector in cystic fibrosis cells. CFTR expression/maturation was evaluated by immunoblotting and its function by short-circuit current measurements. A 24-h exposure to P. aeruginosa diffusible material (PsaDM) reduced CFTR currents as well as total and membrane protein expression of the wildtype (wt) CFTR protein in CFBE-wt cells. In CFBE-ΔF508 cells, PsaDM severely reduced CFTR maturation and current rescue induced by VRT-325. We also confirmed a deleterious impact of PsaDM on wt-CFTR currents in non-cystic fibrosis primary airway cells as well as on the rescue of ΔF508-CFTR function induced by VRT-325 in primary cystic fibrosis cells. These findings show that CFTR function could be impaired in non-cystic fibrosis patients infected by P. aeruginosa. Our data also suggest that CFTR corrector efficiency may be affected by infectious components, which should be taken into account in screening assays of correctors.

Paraoxonase 2 serves a proapopotic function in mouse and human cells in response to the Pseudomonas aeruginosa quorum-sensing molecule N-(3-Oxododecanoyl)-homoserine lactone

Pseudomonas aeruginosa use quorum-sensing molecules, including N-(3-oxododecanoyl)-homoserine lactone (C12), for intercellular communication. C12 activated apoptosis in mouse embryo fibroblasts (MEF) from both wild type (WT) and Bax/Bak double knock-out mice (WT MEF and DKO MEF that were responsive to C12, DKOR MEF): nuclei fragmented; mitochondrial membrane potential (Δψmito) depolarized; Ca(2+) was released from the endoplasmic reticulum (ER), increasing cytosolic [Ca(2+)] (Cacyto); and caspase 3/7 was activated. DKOR MEF had been isolated from a nonclonal pool of DKO MEF that were non-responsive to C12 (DKONR MEF). RNAseq analysis, quantitative PCR, and Western blots showed that WT and DKOR MEF both expressed genes associated with cancer, including paraoxonase 2 (PON2), whereas DKONR MEF expressed little PON2. Adenovirus-mediated expression of human PON2 in DKONR MEF rendered them responsive to C12: Δψmito depolarized, Cacyto increased, and caspase 3/7 activated. Human embryonic kidney 293T (HEK293T) cells expressed low levels of endogenous PON2, and these cells were also less responsive to C12. Overexpression of PON2, but not PON2-H114Q (no lactonase activity) in HEK293T cells caused them to become sensitive to C12. Because [C12] may reach high levels in biofilms in lungs of cystic fibrosis (CF) patients, PON2 lactonase activity may control Δψmito, Ca(2+) release from the ER, and apoptosis in CF airway epithelia. Coupled with previous data, these results also indicate that PON2 uses its lactonase activity to prevent Bax- and Bak-dependent apoptosis in response to common proapoptotic drugs like doxorubicin and staurosporine, but activates Bax- and Bak-independent apoptosis in response to C12.

Pseudomonas aeruginosa quorum-sensing molecule homoserine lactone modulates inflammatory signaling through PERK and eI-F2α

Pseudomonas aeruginosa secrete N-(3-oxododecanoyl)-homoserine lactone (HSL-C12) as a quorum-sensing molecule to regulate bacterial gene expression. Because HSL-C12 is membrane permeant, multiple cell types in P. aeruginosa-infected airways may be exposed to HSL-C12, especially adjacent to biofilms where local (HSL-C12) may be high. Previous reports showed that HSL-C12 causes both pro- and anti-inflammatory effects. To characterize HSL-C12's pro- and anti-inflammatory effects in host cells, we measured protein synthesis, NF-κB activation, and KC (mouse IL-8) and IL-6 mRNA and protein secretion in wild-type mouse embryonic fibroblasts (MEF). To test the role of the endoplasmic reticulum stress inducer, PERK we compared these responses in PERK(-/-) and PERK-corrected PERK(-/-) MEF. During 4-h treatments of wild-type MEF, HSL-C12 potentially activated NF-κB p65 by preventing the resynthesis of IκB and increased transcription of KC and IL-6 genes (quantitative PCR). HSL-C12 also inhibited secretion of KC and/or IL-6 into the media (ELISA) both in control conditions and also during stimulation by TNF-α. HSL-C12 also activated PERK (as shown by increased phosphorylation of eI-F2α) and inhibited protein synthesis (as measured by incorporation of [(35)S]methionine by MEF). Comparisons of PERK(-/-) and PERK-corrected MEF showed that HSL-C12's effects were explained in part by activation of PERK→phosphorylation of eI-F2α→inhibition of protein synthesis→reduced IκBα production→activation of NF-κB→increased transcription of the KC gene but reduced translation and secretion of KC. HSL-C12 may be an important modulator of early (up to 4 h) inflammatory signaling in P. aeruginosa infections.

Thapsigargin blocks Pseudomonas aeruginosa homoserine lactone-induced apoptosis in airway epithelia

Pseudomonas aeruginosa secretes N-(3-oxododecanoyl)-homoserine lactone (C12) as a quorum-sensing molecule to regulate gene expression. Micromolar concentrations are found in the airway surface liquid of infected lungs. Exposure of the airway surface to C12 caused a loss of transepithelial resistance within 1 h that was accompanied by disassembly of tight junctions, as indicated by relocation of the tight junction protein zonula occludens 1 from the apical to the basolateral pole and into the cytosol of polarized human airway epithelial cell cultures (Calu-3 and primary tracheal epithelial cells). These effects were blocked by carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone, a pan-caspase blocker, indicating that tight junction disassembly was an early event in C12-triggered apoptosis. Short-duration (10 min) pretreatment of airway epithelial (Calu-3 and JME) cells with 1 μM thapsigargin (Tg), an inhibitor of Ca(2+) uptake into the endoplasmic reticulum (ER), was found to be protective against the C12-induced airway epithelial barrier breakdown and also against other apoptosis-related effects, including shrinkage and fragmentation of nuclei, activation of caspase 3/7 (the executioner caspase in apoptosis), release of ER-targeted redox-sensitive green fluorescent protein into the cytosol, and depolarization of mitochondrial membrane potential. Pretreatment of Calu-3 airway cell monolayers with BAPTA-AM [to buffer cytosolic Ca(2+) concentration (Cacyto)] or Ca(2+)-free solution + BAPTA-AM reduced C12 activation of apoptotic events, suggesting that C12-triggered apoptosis may involve Ca(2+). Because C12 and Tg reduced Ca(2+) concentration in the ER and increased Cacyto, while Tg increased mitochondrial Ca(2+) concentration (Camito) and C12 reduced Camito, it is proposed that Tg may reduce C12-induced apoptosis in host cells not by raising Cacyto, but by preventing C12-induced decreases in Camito.

Pseudomonas aeruginosa homoserine lactone triggers apoptosis and Bak/Bax-independent release of mitochondrial cytochrome C in fibroblasts

Pseudomonas aeruginosa use N-(3-oxododecanoyl)-homoserine lactone (C12) as a quorum-sensing molecule to regulate gene expression in the bacteria. It is expected that in patients with chronic infections with P. aeruginosa, especially as biofilms, local [C12] will be high and, since C12 is lipid soluble, diffuse from the airways into the epithelium and underlying fibroblasts, capillary endothelia and white blood cells. Previous work showed that C12 has multiple effects in human host cells, including activation of apoptosis. The present work tested the involvement of Bak and Bax in C12-triggered apoptosis in mouse embryo fibroblasts (MEF) by comparing MEF isolated from embryos of wild-type (WT) and Bax(-/-) /Bak(-/-) (DKO) mice. In WT MEF C12 rapidly triggered (minutes to 2 h): activation of caspases 3/7 and 8, depolarization of mitochondrial membrane potential (Δψmito ), release of cytochrome C from mitochondria into the cytosol, blebbing of plasma membranes, shrinkage/condensation of cells and nuclei and, subsequently, cell killing. A DKO MEF line that was relatively unaffected by the Bak/Bax-dependent proapoptotic stimulants staurosporine and etoposide responded to C12 similarly to WT MEF: activation of caspase 3/7, depolarization of Δψmito and release of cytochrome C and cell death. Re-expression of Bax or Bak in DKO MEF did not alter the WT-like responses to C12 in DKO MEF. These data showed that C12 triggers novel, rapid proapoptotic Bak/Bax-independent responses that include events commonly associated with activation of both the intrinsic pathway (depolarization of Δψmito and release of cytochrome C from mitochondria into the cytosol) and the extrinsic pathway (activation of caspase 8). Unlike the proapoptotic agonists staurosporine and etoposide that release cytochrome C from mitochondria, C12's effects do not require participation of either Bak or Bax.

Termination and activation of store-operated cyclic AMP production

Diverse pathophysiological processes (e.g. obesity, lifespan determination, addiction and male fertility) have been linked to the expression of specific isoforms of the adenylyl cyclases (AC1-AC10), the enzymes that generate cyclic AMP (cAMP). Our laboratory recently discovered a new mode of cAMP production, prominent in certain cell types, that is stimulated by any manoeuvre causing reduction of free [Ca²⁺] within the lumen of the endoplasmic reticulum (ER) calcium store. Activation of this ‘store-operated’ pathway requires the ER Ca²⁺ sensor, STIM1, but the identity of the enzymes responsible for cAMP production and how this process is regulated is unknown. Here, we used sensitive FRET-based sensors for cAMP in single cells combined with silencing and overexpression approaches to show that store-operated cAMP production occurred preferentially via the isoform AC3 in NCM460 colonic epithelial cells. Ca²⁺ entry via the plasma membrane Ca²⁺ channel, Orai1, suppressed cAMP production, independent of store refilling. These findings are an important first step towards defining the functional significance and to identify the protein composition of this novel Ca²⁺/cAMP crosstalk system.

Small molecule screen yields inhibitors of Pseudomonas homoserine lactone-induced host responses

Pseudomonas aeruginosa infections are commonly associated with cystic fibrosis, pneumonias, neutropenia and burns. The P. aeruginosa quorum sensing molecule N-(3-oxo-dodecanoyl) homoserine lactone (C12) cause multiple deleterious host responses, including repression of NF-κB transcriptional activity and apoptosis. Inhibition of C12-mediated host responses is predicted to reduce P. aeruginosa virulence. We report here a novel, host-targeted approach for potential adjunctive anti-Pseudomonal therapy based on inhibition of C12-mediated host responses. A high-throughput screen was developed to identify C12 inhibitors that restore NF-κB activity in C12-treated, lipopolysaccharide (LPS)-stimulated cells. Triazolo[4,3-a]quinolines with nanomolar potency were identified as C12-inhibitors that restore NF-κB-dependent luciferase expression in LPS- and TNF-stimulated cell lines. In primary macrophages and fibroblasts, triazolo[4,3-a]quinolines inhibited C12 action to restore cytokine secretion in LPS-stimulated cells. Serendipitously, in the absence of an inflammatory stimulus, triazolo[4,3-a]quinolines prevented C12-mediated responses, including cytotoxicity, elevation of cytoplasmic calcium, and p38 MAPK phosphorylation. In vivo efficacy was demonstrated in a murine model of dermal inflammation involving intradermalC12 administration. The discovery of triazolo[4,3-a]quinolines provides a pharmacological tool to investigate C12-mediated host responses, and a potential host-targeted anti-Pseudomonal therapy.

X-box binding protein 1 (XBP1s) is a critical determinant of Pseudomonas aeruginosa homoserine lactone-mediated apoptosis

Pseudomonas aeruginosa infections are associated with high mortality rates and occur in diverse conditions including pneumonias, cystic fibrosis and neutropenia. Quorum sensing, mediated by small molecules including N-(3-oxo-dodecanoyl) homoserine lactone (C12), regulates P. aeruginosa growth and virulence. In addition, host cell recognition of C12 initiates multiple signalling responses including cell death. To gain insight into mechanisms of C12-mediated cytotoxicity, we studied the role of endoplasmic reticulum stress in host cell responses to C12. Dramatic protection against C12-mediated cell death was observed in cells that do not produce the X-box binding protein 1 transcription factor (XBP1s). The leucine zipper and transcriptional activation motifs of XBP1s were sufficient to restore C12-induced caspase activation in XBP1s-deficient cells, although this polypeptide was not transcriptionally active. The XBP1s polypeptide also regulated caspase activation in cells stimulated with N-(3-oxo-tetradecanoyl) homoserine lactone (C14), produced by Yersinia enterolitica and Burkholderia pseudomallei, and enhanced homoserine lactone-mediated caspase activation in the presence of endogenous XBP1s. In C12-tolerant cells, responses to C12 including phosphorylation of p38 MAPK and eukaryotic initiation factor 2α were conserved, suggesting that C12 cytotoxicity is not heavily dependent on these pathways. In summary, this study reveals a novel and unconventional role for XBP1s in regulating host cell cytotoxic responses to bacterial acyl homoserine lactones.

Chronic and acute infections associated with P. aeruginosa constitute a major healthcare burden. Antimicrobial approaches are currently used against P. aeruginosa; however, infections are typically refractory to treatment and drug resistant strains have been isolated. As such, there is urgent need to understand mechanisms of P. aeruginosa virulence and for new strategies to fight infections. The P. aeruginosa-derived quorum-sensing molecule C12 is recognized by host cells and initiates stress responses including cytotoxicity. In this study, the X-box binding protein 1 transcription factor (XBP1s) was identified as a host factor critical for apoptotic responses initiated by C12 and other similar quorum sensing molecules. Additional C12-initiated host responses, including phosphorylation of p38 MAPK and eIF2α were found to be of lesser importance for C12-initiated cytotoxicity. These studies have broad implications for our understanding of bacterial virulence mechanisms and for development of potential new strategies to combat infections.

The bacterial quorum-sensing signal molecule N-3-oxo-dodecanoyl-L-homoserine lactone reciprocally modulates pro- and anti-inflammatory cytokines in activated macrophages

The bacterial molecule N-3-oxo-dodecanoyl-l-homoserine lactone (C12) has critical roles in both interbacterial communication and interkingdom signaling. The ability of C12 to downregulate production of the key proinflammatory cytokine TNF-α in stimulated macrophages was suggested to contribute to the establishment of chronic infections by opportunistic Gram-negative bacteria, such as Pseudomonas aeruginosa. We show that, in contrast to TNF-α suppression, C12 amplifies production of the major anti-inflammatory cytokine IL-10 in LPS-stimulated murine RAW264.7 macrophages, as well as peritoneal macrophages. Furthermore, C12 increased IL-10 mRNA levels and IL-10 promoter reporter activity in LPS-stimulated RAW264.7 macrophages, indicating that C12 modulates IL-10 expression at the transcriptional level. Finally, C12 substantially potentiated LPS-stimulated NF-κB DNA-binding levels and prolonged p38 MAPK phosphorylation in RAW264.7 macrophages, suggesting that increased transcriptional activity of NF-κB and/or p38-activated transcription factors serves to upregulate IL-10 production in macrophages exposed to both LPS and C12. These findings reveal another part of the complex array of host transitions through which opportunistic bacteria downregulate immune responses to flourish and establish a chronic infection.

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.

Pseudomonas aeruginosa biofilm-associated homoserine lactone C12 rapidly activates apoptosis in airway epithelia

Pseudomonas aeruginosa (PA) forms biofilms in lungs of cystic fibrosis (CF) patients, a process regulated by quorum-sensing molecules including N-(3-oxododecanoyl)-l-homoserine lactone (C12). C12 (10-100 µM) rapidly triggered events commonly associated with the intrinsic apoptotic pathway in JME (CF ΔF508CFTR, nasal surface) epithelial cells: depolarization of mitochondrial (mito) membrane potential (Δψ(mito)) and release of cytochrome C (cytoC) from mitos into cytosol and activation of caspases 3/7, 8 and 9. C12 also had novel effects on the endoplasmic reticulum (release of both Ca(2+) and ER-targeted GFP and oxidized contents into the cytosol). Effects began within 5 min and were complete in 1-2 h. C12 caused similar activation of caspases and release of cytoC from mitos in Calu-3 (wtCFTR, bronchial gland) cells, showing that C12-triggered responses occurred similarly in different airway epithelial types. C12 had nearly identical effects on three key aspects of the apoptosis response (caspase 3/7, depolarization of Δψ(mito) and reduction of redox potential in the ER) in JME and CFTR-corrected JME cells (adenoviral expression), showing that CFTR was likely not an important regulator of C12-triggered apoptosis in airway epithelia. Exposure of airway cultures to biofilms from PAO1wt caused depolarization of Δψ(mito) and increases in Ca(cyto) like 10-50 µM C12. In contrast, biofilms from PAO1ΔlasI (C12 deficient) had no effect, suggesting that C12 from P. aeruginosa biofilms may contribute to accumulation of apoptotic cells that cannot be cleared from CF lungs. A model to explain the effects of C12 is proposed.

CFTR and Ca Signaling in Cystic Fibrosis

Among the diverse physiological functions exerted by calcium signaling in living cells, its role in the regulation of protein biogenesis and trafficking remains incompletely understood. In cystic fibrosis (CF) disease the most common CF transmembrane conductance regulator (CFTR) mutation, F508del-CFTR generates a misprocessed protein that is abnormally retained in the endoplasmic reticulum (ER) compartment, rapidly degraded by the ubiquitin/proteasome pathway and hence absent at the plasma membrane of CF epithelial cells. Recent studies have demonstrated that intracellular calcium signals consequent to activation of apical G-protein-coupled receptors by different agonists are increased in CF airway epithelia. Moreover, the regulation of various intracellular calcium storage compartments, such as ER is also abnormal in CF cells. Although the molecular mechanism at the origin of this increase remains puzzling in epithelial cells, the F508del-CFTR mutation is proposed to be the onset of abnormal Ca²⁺ influx linking the calcium signaling to CFTR pathobiology. This article reviews the relationships between CFTR and calcium signaling in the context of the genetic disease CF.

N-Acylhomoserine lactones are potent neutrophil chemoattractants that act via calcium mobilization and actin remodeling

In gram-negative bacteria, cell-cell communication based on HSL QS molecules is known to coordinate the production of virulence factors and biofilms. These bacterial signals can also modulate human immune cell behavior. Using a Transwell migration assay, we found that human primary neutrophils are strongly stimulated by 3O-C(12)-HSL and -C(10)-HSL but not C(4)-HSL in a concentration-dependent manner. Moreover, 3O-C(12)-HSL and -C(10)-HSL activate PLCγ1 but not -γ2, mobilize intracellular calcium, and up-regulate IP(3)R. These changes were paralleled by F-actin accumulation, primarily in the leading edge of neutrophils, as evidenced by phalloidin staining and confocal microscopy. F- and G-actin isolation and quantification by immunoblotting revealed that the F/G-actin ratio was increased significantly after treatment with all three HSLs. Furthemore, 3O-C(12)-HSL- and 3O-C(10)-HSL treatment resulted in phosphorylation of Rac1 and Cdc42. In contrast, C(4)-HSL had negligible influence on the phosphorylation status of PLC and Rac1/Cdc42 and failed to attract neutrophils and induce calcium release. The calcium inhibitor thapsigargin, which blocks ER calcium uptake, strongly prevented neutrophil migration toward 3O-C(12)-HSL and -C(10)-HSL. These findings show that the bacterial QS molecules 3O-C(12)-HSL and -C(10)-HSL may attract human neutrophils to the sites of bacterial infection and developing biofilms. Indeed, recognition of HSL QS signals by neutrophils may play a critical role in their recruitment during infections.

The Pseudomonas aeruginosa autoinducer 3O-C12 homoserine lactone provokes hyperinflammatory responses from cystic fibrosis airway epithelial cells

The discovery of novel antiinflammatory targets to treat inflammation in the cystic fibrosis (CF) lung stands to benefit patient populations suffering with this disease. The Pseudomonas aeruginosa quorum sensing autoinducer N-3-oxododecanoyl homoserine lactone (3O-C12) is an important bacterial virulence factor that has been reported to induce proinflammatory cytokine production from a variety of cell types. The goal of this study was to examine the ability of 3O-C12 to induce proinflammatory cytokine production in normal and CF bronchial epithelial cells, and better understand the cellular mechanisms by which this cytokine induction occurs. 3O-C12 was found to induce higher levels of IL-6 production in the CF cell lines IB3-1 and CuFi, compared to their corresponding control cell lines C38 and NuLi. Systems biology and network analysis revealed a high predominance of over-represented innate immune pathways bridged together by calcium-dependant transcription factors governing the transcriptional responses of A549 airway cells to stimulation with 3O-C12. Using calcium-flux assays, 3O-C12 was found to induce larger and more sustained increases in intracellular calcium in IB3-1 cells compared to C38, and blocking this calcium flux with BAPTA-AM reduced the production of IL-6 by IB3-1 to the levels produced by C38. These data suggest that 3O-C12 induces proinflammatory cytokine production in airway epithelial cells in a calcium-dependent manner, and that dysregulated calcium storage or signalling in CF cells results in an increased production of proinflammatory cytokines.