Release of ATP during host cell killing by enteropathogenic E. coli and its role as a secretory mediator

Attaching and effacing pathogens modulate host mitochondrial structure and function

Enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) are human enteric pathogens that contribute significantly to morbidity and mortality worldwide. These extracellular pathogens attach intimately to intestinal epithelial cells and cause signature lesions by effacing the brush border microvilli, a property they share with other "attaching and effacing" (A/E) bacteria, including the murine pathogen Citrobacter rodentium. A/E pathogens use a specialized apparatus called a type III secretion system (T3SS) to deliver specific proteins directly into the host cytosol and modify host cell behavior. The T3SS is essential for colonization and pathogenesis, and mutants lacking this apparatus fail to cause disease. Thus, deciphering effector-induced host cell modifications is critical for understanding A/E bacterial pathogenesis. Several of the ∼20-45 effector proteins delivered into the host cell modify disparate mitochondrial properties, some via direct interactions with the mitochondria and/or mitochondrial proteins. In vitro studies have uncovered the mechanistic basis for the actions of some of these effectors, including their mitochondrial targeting, interaction partners, and consequent impacts on mitochondrial morphology, oxidative phosphorylation and ROS production, disruption of membrane potential, and intrinsic apoptosis. In vivo studies, mostly relying on the C. rodentium/mouse model, have been used to validate a subset of the in vitro observations; additionally, animal studies reveal broad changes to intestinal physiology that are likely accompanied by mitochondrial alterations, but the mechanistic underpinnings remain undefined. This chapter provides an overview of A/E pathogen-induced host alterations and pathogenesis, specifically focusing on mitochondria-targeted effects.

I'm Infected, Eat Me! Innate Immunity Mediated by Live, Infected Cells Signaling To Be Phagocytosed

Innate immunity against pathogens is known to be mediated by barriers to pathogen invasion, activation of complement, recruitment of immune cells, immune cell phagocytosis of pathogens, death of infected cells, and activation of the adaptive immunity via antigen presentation. Here, we propose and review evidence for a novel mode of innate immunity whereby live, infected host cells induce phagocytes to phagocytose the infected cell, thereby potentially reducing infection. We discuss evidence that host cells, infected by virus, bacteria, or other intracellular pathogens (i) release nucleotides and chemokines as find-me signals, (ii) expose on their surface phosphatidylserine and calreticulin as eat-me signals, (iii) release and bind opsonins to induce phagocytosis, and (iv) downregulate don't-eat-me signals CD47, major histocompatibility complex class I (MHC1), and sialic acid. As long as the pathogens of the host cell are destroyed within the phagocyte, then infection can be curtailed; if antigens from the pathogens are cross-presented by the phagocyte, then an adaptive response would also be induced. Phagocytosis of live infected cells may thereby mediate innate immunity.

Zinc: Multidimensional Effects on Living Organisms

Zinc is a redox-inert trace element that is second only to iron in abundance in biological systems. In cells, zinc is typically buffered and bound to metalloproteins, but it may also exist in a labile or chelatable (free ion) form. Zinc plays a critical role in prokaryotes and eukaryotes, ranging from structural to catalytic to replication to demise. This review discusses the influential properties of zinc on various mechanisms of bacterial proliferation and synergistic action as an antimicrobial element. We also touch upon the significance of zinc among eukaryotic cells and how it may modulate their survival and death through its inhibitory or modulatory effect on certain receptors, enzymes, and signaling proteins. A brief discussion on zinc chelators is also presented, and chelating agents may be used with or against zinc to affect therapeutics against human diseases. Overall, the multidimensional effects of zinc in cells attest to the growing number of scientific research that reveal the consequential prominence of this remarkable transition metal in human health and disease.

Role and Function of Adenosine and its Receptors in Inflammation, Neuroinflammation, IBS, Autoimmune Inflammatory Disorders, Rheumatoid Arthritis and Psoriasis

The physiological effects of endogenous adenosine on various organ systems are very complex and numerous which are elicited upon activation of any of the four G-protein-coupled receptors (GPCRs) denoted as A1, A2A, A2B and A3 adenosine receptors (ARs). Several fused heterocyclic and non-xanthine derivatives are reported as a possible target for these receptors due to physiological problems and lack of selectivity of xanthine derivatives. In the present review, we have discussed the development of various new chemical entities as a target for these receptors. In addition, compounds acting on adenosine receptors can be utilized in treating diseases like inflammation, neuroinflammation, autoimmune and related diseases.

Approaches for designing and discovering purinergic drugs for gastrointestinal diseases

INTRODUCTION

Purines finely modulate physiological motor, secretory, and sensory functions in the gastrointestinal tract. Their activity is mediated by the purinergic signaling machinery, including receptors and enzymes regulating their synthesis, release, and degradation. Several gastrointestinal dysfunctions are characterized by alterations affecting the purinergic system.

AREAS COVERED

The authors provide an overview on the purinergic receptor signaling machinery, the molecules and proteins involved, and a summary of medicinal chemistry efforts aimed at developing novel compounds able to modulate the activity of each player involved in this machinery. The involvement of purinergic signaling in gastrointestinal motor, secretory, and sensory functions and dysfunctions, and the potential therapeutic applications of purinergic signaling modulators, are then described.

EXPERT OPINION

A number of preclinical and clinical studies demonstrate that the pharmacological manipulation of purinergic signaling represents a viable way to counteract several gastrointestinal diseases. At present, the paucity of purinergic therapies is related to the lack of receptor-subtype-specific agonists and antagonists that are effective in vivo. In this regard, the development of novel therapeutic strategies should be focused to include tools able to control the P1 and P2 receptor expression as well as modulators of the breakdown or transport of purines.

Cyclic nucleotides, gut physiology and inflammation

Misregulation of gut function and homeostasis impinges on the overall well-being of the entire organism. Diarrheal disease is the second leading cause of death in children under 5 years of age, and globally, 1.7 billion cases of childhood diarrhea are reported every year. Accompanying diarrheal episodes are a number of secondary effects in gut physiology and structure, such as erosion of the mucosal barrier that lines the gut, facilitating further inflammation of the gut in response to the normal microbiome. Here, we focus on pathogenic bacteria-mediated diarrhea, emphasizing the role of cyclic adenosine 3',5'-monophosphate and cyclic guanosine 3',5'-monophosphate in driving signaling outputs that result in the secretion of water and ions from the epithelial cells of the gut. We also speculate on how this aberrant efflux and influx of ions could modulate inflammasome signaling, and therefore cell survival and maintenance of gut architecture and function.

Tinkering with targeting nucleotide signaling for control of intracellular Leishmania parasites

Nucleotides are one of the most primitive extracellular signalling molecules across all phyla and regulate a multitude of responses. The biological effects of extracellular nucleotides/sides are mediated via the specific purinergic receptors present on the cell surface. In mammalian system, adenine nucleotides are the predominant nucleotides found in the extracellular milieu and mediate a constellation of physiological functions. In the context of host-pathogen interaction, extracellular ATP is recognized as a danger signal and potentiates the release of pro-inflammatory mediators from activated immune cells, on the other hand, its breakdown product adenosine exerts potential anti-inflammatory and immunosuppressive actions. Therefore, it is increasingly apparent that the interplay between extracellular ATP/adenosine ratios has a significant role in coordinating the regulation of the immune system in health and diseases. Several pathogens express ectonucleotidases on their surface and exploit the purinergic signalling as one of the mechanisms to modulate the host immune response. Leishmania pathogens are one of the most successful intracellular pathogens which survive within host macrophages and manipulate protective Th1 response into disease promoting Th2 response. In this review, we discuss the regulation of extracellular ATP and adenosine levels, the role of ATP/adenosine counter signalling in regulating the inflammation and immune responses during infection and how Leishmania parasites exploit the purinergic signalling to manipulate host response. We also discuss the challenges and opportunities in targeting purinergic signalling and the future prospects.

Unfolding Role of a Danger Molecule Adenosine Signaling in Modulation of Microbial Infection and Host Cell Response

Ectonucleotidases CD39 and CD73, specific nucleotide metabolizing enzymes located on the surface of the host, can convert a pro-inflammatory environment driven by a danger molecule extracellular-ATP to an adenosine-mediated anti-inflammatory milieu. Accordingly, CD39/CD73 signaling have has strongly implicated in modulating the intensity, duration, and composition of purinergic danger signals delivered to host. Recent studies have eluted potential roles for CD39 and CD73 in selective triggering of a variety of host immune cells and molecules in the presence of pathogenic microorganisms or microbial virulence molecules. Growing evidence also suggests that CD39 and CD73 present complimentary, but likely differential, actions against pathogens to shape the course and severity of microbial infection as well as the associated immune response. Similarly, adenosine receptors A2A and A2B have been proposed to be major immunomodulators of adenosine signaling during chronic inflammatory conditions induced by opportunistic pathogens, such as oral colonizer Porphyromonas gingivalis. Therefore, we here review the recent studies that demonstrate how complex network of molecules in the extracellular adenosine signaling machinery and their interactions can reshape immune responses and may also be targeted by opportunistic pathogens to establish successful colonization in human mucosal tissues and modulate the host immune response.

Adenosine A2B Receptors: An Optional Target for the Management of Irritable Bowel Syndrome with Diarrhea?

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder, with the characteristic symptoms of chronic abdominal pain and altered bowel habits (diarrhea, constipation, or both). IBS is a highly prevalent condition, which negatively affects quality of life and is a significant burden on global healthcare costs. Although many pharmacological medicines have been proposed to treat IBS, including those targeting receptors, channels, and chemical mediators related to visceral hypersensitivity, successful pharmacotherapy for the disease has not been established. Visceral hypersensitivity plays an important role in IBS pathogenesis. Immune activation is observed in diarrhea-predominant patients with IBS and contributes to the development of visceral hypersensitivity. Adenosine is a chemical mediator that regulates many physiological processes, including inflammation and nociception. Among its receptors, the adenosine A_2B receptor regulates intestinal secretion, motor function, and the immune response. We recently demonstrated that the adenosine A_2B receptor is involved in visceral hypersensitivity in animal models of IBS. In this review, we discuss the possibility of the adenosine A_2B receptor as a novel therapeutic target for IBS.

Intestinal Epithelial Ecto-5'-Nucleotidase (CD73) Regulates Intestinal Colonization and Infection by Nontyphoidal Salmonella

Ecto-5'-nucleotidase (CD73) is expressed abundantly on the apical surface of intestinal epithelial cells (IECs) and functions as the terminal enzyme in the generation of extracellular adenosine. Previous work demonstrated that adenosine signaling in IECs results in a number of tissue-protective effects during inflammation; however, a rationale for its apical expression has been lacking. We hypothesized that the highly polarized expression of CD73 is indicative of an important role for extracellular adenosine as a mediator of host-microbe interactions. We show that adenosine harbors bacteriostatic activity against Salmonella enterica serovar Typhimurium that is not shared by the related purine metabolite 5'-AMP, inosine, or hypoxanthine. Analysis of Salmonella colonization in IEC-specific CD73 knockout mice (CD73^f/fVillin^Cre ) revealed a nearly 10-fold increase in colonization compared to that in controls. Despite the increased luminal colonization by Salmonella, CD73^f/fVillin^Cre mice were protected against Salmonella colitis and showed reduced Salmonella burdens in viscera, suggesting that adenosine promotes dissemination. The knockdown of CD73 expression in cultured IECs resulted in dramatic defects in intraepithelial localization and replication as well as defective transepithelial translocation by Salmonella In conclusion, we define a novel antimicrobial activity of adenosine in the gastrointestinal tract and unveil an important role for adenosine as a regulator of host-microbe interactions. These findings have broad implications for the development of new therapeutic agents for infectious disease.

Dynamic regulation of extracellular ATP in Escherichia coli

We studied the kinetics of extracellular ATP (ATPe) in Escherichia coli and their outer membrane vesicles (OMVs) stimulated with amphipatic peptides melittin (MEL) and mastoparan 7 (MST7). Real-time luminometry was used to measure ATPe kinetics, ATP release, and ATPase activity. The latter was also determined by following [³²P]Pi released from [γ-³²P]ATP. E. coli was studied alone, co-incubated with Caco-2 cells, or in rat jejunum segments. In E. coli, the addition of [γ-³²P]ATP led to the uptake and subsequent hydrolysis of ATPe. Exposure to peptides caused an acute 3-fold (MST7) and 7-fold (MEL) increase in [ATPe]. In OMVs, ATPase activity increased linearly with [ATPe] (0.1-1 µM). Exposure to MST7 and MEL enhanced ATP release by 3-7 fold, with similar kinetics to that of bacteria. In Caco-2 cells, the addition of ATP to the apical domain led to a steep [ATPe] increase to a maximum, with subsequent ATPase activity. The addition of bacterial suspensions led to a 6-7 fold increase in [ATPe], followed by an acute decrease. In perfused jejunum segments, exposure to E. coli increased luminal ATP 2 fold. ATPe regulation of E. coli depends on the balance between ATPase activity and ATP release. This balance can be altered by OMVs, which display their own capacity to regulate ATPe. E. coli can activate ATP release from Caco-2 cells and intestinal segments, a response which in vivo might lead to intestinal release of ATP from the gut lumen.

Biological Activities of Uric Acid in Infection Due to Enteropathogenic and Shiga-Toxigenic Escherichia coli

In previous work, we identified xanthine oxidase (XO) as an important enzyme in the interaction between the host and enteropathogenic Escherichia coli(EPEC) and Shiga-toxigenic E. coli(STEC). Many of the biological effects of XO were due to the hydrogen peroxide produced by the enzyme. We wondered, however, if uric acid generated by XO also had biological effects in the gastrointestinal tract. Uric acid triggered inflammatory responses in the gut, including increased submucosal edema and release of extracellular DNA from host cells. While uric acid alone was unable to trigger a chloride secretory response in intestinal monolayers, it did potentiate the secretory response to cyclic AMP agonists. Uric acid crystals were formed in vivo in the lumen of the gut in response to EPEC and STEC infections. While trying to visualize uric acid crystals formed during EPEC and STEC infections, we noticed that uric acid crystals became enmeshed in the neutrophilic extracellular traps (NETs) produced from host cells in response to bacteria in cultured cell systems and in the intestine in vivo Uric acid levels in the gut lumen increased in response to exogenous DNA, and these increases were enhanced by the actions of DNase I. Interestingly, addition of DNase I reduced the numbers of EPEC bacteria recovered after a 20-h infection and protected against EPEC-induced histologic damage.

Pathophysiological role of extracellular purinergic mediators in the control of intestinal inflammation

Purinergic mediators such as adenosine 5′-triphosphate (ATP) are released into the extracellular compartment from damaged tissues and activated immune cells. They are then recognized by multiple purinergic P2X and P2Y receptors. Release and recognition of extracellular ATP are associated with both the development and the resolution of inflammation and infection. Accumulating evidence has recently suggested the potential of purinergic receptors as novel targets for drugs for treating intestinal disorders, including intestinal inflammation and irritable bowel syndrome. In this review, we highlight recent findings regarding the pathophysiological role of purinergic mediators in the development of intestinal inflammation.

Toll-like receptor-triggered calcium mobilization protects mice against bacterial infection through extracellular ATP release

Extracellular ATP (eATP), released as a "danger signal" by injured or stressed cells, plays an important role in the regulation of immune responses, but the relationship between ATP release and innate immune responses is still uncertain. In this study, we demonstrated that ATP was released through Toll-like receptor (TLR)-associated signaling in both Escherichia coli-infected mice and lipopolysaccharide (LPS)- or Pam3CSK4-treated macrophages. This ATP release could be blocked completely only by N-ethylmaleimide (NEM), not by carbenoxolone (CBX), flufenamic acid (FFA), or probenecid, suggesting the key role of exocytosis in this process. Furthermore, LPS-induced ATP release could also be reduced dramatically through suppressing calcium mobilization by use of U73122, caffeine, and thapsigargin (TG). In addition, the secretion of interleukin-1β (IL-1β) and CCL-2 was enhanced significantly by ATP, in a time- and dose-dependent manner. Meanwhile, macrophage-mediated phagocytosis of bacteria was also promoted significantly by ATP stimulation. Furthermore, extracellular ATP reduced the number of invading bacteria and protected mice from peritonitis by activating purinergic receptors. Mechanistically, phosphorylation of AKT and ERK was overtly increased by ATP in antibacterial immune responses. Accordingly, if we blocked the P2X- and P2Y-associated signaling pathway by using suramin and pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid), tetrasodium salt (PPADS), the ATP-enhanced immune response was restrained significantly. Taken together, our findings reveal an internal relationship between danger signals and TLR signaling in innate immune responses, which suggests a potential therapeutic significance of calcium mobilization-mediated ATP release in infectious diseases.

Activation of A1-adenosine receptors promotes leukocyte recruitment to the lung and attenuates acute lung injury in mice infected with influenza A/WSN/33 (H1N1) virus

We have shown that bronchoalveolar epithelial A1-adenosine receptors (A1-AdoR) are activated in influenza A virus-infected mice. Alveolar macrophages and neutrophils also express A1-AdoRs, and we hypothesized that activation of A1-AdoRs on these cells will promote macrophage and neutrophil chemotaxis and activation and thereby play a role in the pathogenesis of influenza virus-induced acute lung injury. Wild-type (WT) C57BL/6 mice, congenic A1-AdoR knockout (A1-KO) mice, and mice that had undergone reciprocal bone marrow transfer were inoculated intranasally with 10,000 PFU/mouse influenza A/WSN/33 (H1N1) virus. Alternatively, WT mice underwent daily treatment with the A1-AdoR antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) from 1 day prior to inoculation. Infection increased bronchoalveolar lining fluid (BALF) adenosine comparably in WT and A1-KO mice. Infection of WT mice resulted in reduced carotid arterial O2 saturation (hypoxemia), lung pathology, pulmonary edema, reduced lung compliance, increased basal airway resistance, and hyperresponsiveness to methacholine. These effects were absent or significantly attenuated in A1-KO mice. Levels of BALF leukocytes, gamma interferon (IFN-γ), and interleukin 10 (IL-10) were significantly reduced in infected A1-KO mice, but levels of KC, IP-10, and MCP-1 were increased. Reciprocal bone marrow transfer resulted in WT-like lung injury severity, but BALF leukocyte levels increased only in WT and A1-KO mice with WT bone barrow. Hypoxemia, pulmonary edema, and levels of BALF alveolar macrophages, neutrophils, IFN-γ, and IL-10 were reduced in DPCPX-treated WT mice. Levels of viral replication did not differ between mouse strains or treatment groups. These findings indicate that adenosine activation of leukocyte A1-AdoRs plays a significant role in their recruitment to the infected lung and contributes to influenza pathogenesis. A1-AdoR inhibitor therapy may therefore be beneficial in patients with influenza virus-induced lung injury.

IMPORTANCE

Because antiviral drugs are of limited efficacy in patients hospitalized for influenza virus-induced respiratory failure, there is an urgent need for new therapeutics that can limit the progression of lung injury and reduce influenza death rates. We show that influenza A virus infection results in increased production of the nucleoside adenosine in the mouse lung and that activation of A1-subtype adenosine receptors by adenosine contributes significantly to both recruitment of innate immune cells to the lung and development of acute lung injury following influenza virus infection. We also show that treatment with an A1-adenosine receptor antagonist reduces the severity of lung injury in influenza virus-infected mice. Our findings indicate that adenosine plays an important and previously unrecognized role in the innate immune response to influenza virus infection and suggest that drugs which can inhibit either generation of adenosine or activation of A1-adenosine receptors may be beneficial in treating influenza patients hospitalized for respiratory failure.

Connexins in respiratory and gastrointestinal mucosal immunity

The mucosal lining forms the physical and chemical barrier that protects against pathogens and hostile particles and harbors its own population of bacteria, fungi and archea, known as the microbiota. The immune system controls tolerance of this population of microorganisms that have proven to be beneficial for its host. Keeping its physical integrity and a correct balance with the microbiota, the mucosa preserves its homeostasis and its protective function and maintains host's health. However, in some conditions, pathogens may succeed in breaching mucosal homeostasis and successfully infecting the host. In this review we will discuss the role the mucosa plays in the defense against bacterial pathogens by considering the gap junction protein connexins. We will detail their implication in mucosal homeostasis and upon infection with bacteria in the respiratory and the gastrointestinal tracts.

Pro-inflammatory effects of uric acid in the gastrointestinal tract

Uric acid can be generated in the gastrointestinal (GI) tract from the breakdown of nucleotides ingested in the diet or from purines released from host cells as a result of pathogen-induced cell damage. Xanthine oxidase (XO) is the enzyme that converts hypoxanthine or xanthine into uric acid, a reaction that also generates hydrogen peroxide. It has been assumed that the product of XO responsible for the pro-inflammatory effects of this enzyme is hydrogen peroxide. Recent literature on uric acid, however, has indicated that uric acid itself may have biological effects. We tested whether uric acid itself has detectable pro-inflammatory effects using an in vivo model using ligated rabbit intestinal segments ("loops") as well as in vitro assays using cultured cells. Addition of exogenous uric acid increased the influx of heterophils into rabbit intestinal loops, as measured by myeloperoxidase activity. In addition, white blood cells adhered avidly to uric acid crystals, forming large aggregates of cells. Uric acid acts as a leukocyte chemoattractant in the GI tract. The role of uric acid in enteric infections and in non-infectious disorders of the GI tract deserves more attention.

The role of purinergic pathways in the pathophysiology of gut diseases: pharmacological modulation and potential therapeutic applications

Gut homeostasis results from complex neuro-immune interactions aimed at triggering stereotypical and specific programs of coordinated mucosal secretion and powerful motor propulsion. A prominent role in the regulation of this highly integrated network, comprising a variety of immune/inflammatory cells and the enteric nervous system, is played by purinergic mediators. The cells of the digestive tract are literally plunged into a "biological sea" of functionally active nucleotides and nucleosides, which carry out the critical task of driving regulatory interventions on cellular functions through the activation of P1 and P2 receptors. Intensive research efforts are being made to achieve an integrated view of the purinergic system, since it is emerging that the various components of purinergic pathways (i.e., enzymes, transporters, mediators and receptors) are mutually linked entities, deputed to finely modulating the magnitude and the duration of purinergic signaling, and that alterations occurring in this balanced network could be intimately involved in the pathophysiology of several gut disorders. This review article intends to provide a critical appraisal of current knowledge on the purinergic system role in the regulation of gastrointestinal functions, considering these pathways as a whole integrated network, which is capable of finely controlling the levels of bioactive nucleotides and nucleosides in the biophase of their respective receptors. Special attention is paid to the mechanisms through which alterations in the various compartments of the purinergic system could contribute to the pathophysiology of gut disorders, and to the possibility of counteracting such dysfunctions by means of pharmacological interventions on purinergic molecular targets.

Apical targeting of the P2Y(4) receptor is directed by hydrophobic and basic residues in the cytoplasmic tail

The P2Y(4) receptor is selectively targeted to the apical membrane in polarized epithelial cell lines and has been shown to play a key role in intestinal chloride secretion. In this study, we delimit a 23 amino acid sequence within the P2Y(4) receptor C-tail that directs its apical targeting. Using a mutagenesis approach, we found that four hydrophobic residues near the COOH-terminal end of the signal are necessary for apical sorting, whereas two basic residues near the NH(2)-terminal end of the signal are involved to a lesser extent. Interestingly, mutation of the key hydrophobic residues results in a basolateral enrichment of the receptor construct, suggesting that the apical targeting sequence may prevent insertion or disrupt stability of the receptor at the basolateral membrane. The signal is not sequence specific, as an inversion of the 23 amino acid sequence does not disrupt apical targeting. We also show that the apical targeting sequence is an autonomous signal and is capable of redistributing the normally basolateral P2Y(12) receptor, suggesting that the apical signal is dominant over the basolateral signal in the main body of the P2Y(12) receptor. The targeting sequence is unique to the P2Y(4) receptor, and sequence alignments of the COOH-terminal tail of mammalian orthologs reveal that the hydrophobic residues in the targeting signal are highly conserved. These data define the novel apical sorting signal of the P2Y(4) receptor, which may represent a common mechanism for trafficking of epithelial transmembrane proteins.

Role of host xanthine oxidase in infection due to enteropathogenic and Shiga-toxigenic Escherichia coli

Xanthine oxidase (XO), also known as xanthine oxidoreductase, has long been considered an important host defense molecule in the intestine and in breastfed infants. Here, we present evidence that XO is released from and active in intestinal tissues and fluids in response to infection with enteropathogenic Escherichia coli (EPEC) and Shiga-toxigenic E. coli (STEC), also known as enterohemorrhagic E. coli (EHEC). XO is released into intestinal fluids in EPEC and STEC infection in a rabbit animal model. XO activity results in the generation of surprisingly high concentrations of uric acid in both cultured cell and animal models of infection. Hydrogen peroxide (H(2)O(2)) generated by XO activity triggered a chloride secretory response in intestinal cell monolayers within minutes but decreased transepithelial electrical resistance at 6 to 22 h. H(2)O(2) generated by XO activity was effective at killing laboratory strains of E. coli, commensal microbiotas, and anaerobes, but wild-type EPEC and STEC strains were 100 to 1,000 times more resistant to killing or growth inhibition by this pathway. Instead of killing pathogenic bacteria, physiologic concentrations of XO increased virulence by inducing the production of Shiga toxins from STEC strains. In vivo, exogenous XO plus the substrate hypoxanthine did not protect and instead worsened the outcome of STEC infection in the rabbit ligated intestinal loop model of infection. XO released during EPEC and STEC infection may serve as a virulence-inducing signal to the pathogen and not solely as a protective host defense.

Regulation of macrophage function by adenosine

Following its release into the extracellular space in response to metabolic disturbances, the endogenous nucleoside adenosine exerts a range of immunomodulatory effects and cells of the mononuclear phagocyte system are among its major targets. Adenosine governs mononuclear phagocyte functions via 4 G-protein-coupled cell membrane receptors, which are denoted A(1), A(2A), A(2B), and A(3) receptors. Adenosine promotes osteoclast differentiation via A(1) receptors and alters monocyte to dendritic cell differentiation through A(2B) receptors. Adenosine downregulates classical macrophage activation mainly through A(2A) receptors. In contrast A(2B) receptor activation upregulates alternative macrophage activation. Adenosine promotes angiogenesis, which is mediated by inducing the production of vascular endothelial growth factor by mononuclear phagocytes through A(2A), A(2B), and A(3) receptors. By regulating mononuclear phagocyte function adenosine dictates the course of inflammatory and vascular diseases and cancer.

Interkingdom adenosine signal reduces Pseudomonas aeruginosa pathogenicity

Pseudomonas aeruginosa is becoming recognized as an important pathogen in the gastrointestinal (GI) tract. Here we demonstrate that adenosine, derived from hydrolysis of ATP from the eucaryotic host, is a potent interkingdom signal in the GI tract for this pathogen. The addition of adenosine nearly abolished P. aeruginosa biofilm formation and abolished swarming by preventing production of rhamnolipids. Since the adenosine metabolite inosine did not affect biofilm formation and since a mutant unable to metabolize adenosine behaved like the wild-type strain, adenosine metabolism is not required to reduce pathogenicity. Adenosine also reduces production of the virulence factors pyocyanin, elastase, extracellular polysaccharide, siderophores and the Pseudomonas quinolone signal which led to reduced virulence with Caenorhabditis elegans. To provide insights into how adenosine reduces the virulence of P. aeruginosa, a whole-transcriptome analysis was conducted which revealed that adenosine addition represses genes similar to an iron-replete condition; however, adenosine did not directly bind Fur. Therefore, adenosine decreases P. aeruginosa pathogenicity as an interkingdom signal by causing genes related to iron acquisition to be repressed.

Escherichia coli-induced epithelial hyporesponsiveness to secretagogues is associated with altered CFTR localization

Both pathogenic and commensal strains of Escherichia coli colonize the human intestinal tract. Pathogenic strains differ only in the expression of virulence factors, many of which comprise a type III secretion system (TTSS). Little is known regarding the effect of E. coli on the intestinal epithelial response to the secretagogues that drive ion secretion, despite its importance in causing clinically significant diarrhoea. Using Ussing chambers to measure electrogenic ion transport of T84 intestinal epithelial cell monolayers, we found that all strains of E. coli tested (pathogenic, commensal, probiotic and lab strain) significantly reduced cAMP-dependent ion secretion after 4-8 h exposure. Enteropathogenic E. coli mutants lacking a functional TTSS caused similar hyposecretion while not causing significant apoptosis (as shown by caspase-3 cleavage) or necrosis (lactate dehydrogenase release), as did the commensal strain F18, indicating that epithelial cell death was not the cause of hyposecretion. Enteropathogenic E. coli and the TTSS mutant significantly reduced cell surface expression of the apical anion channel, cystic fibrosis transmembrane conductance regulator, which is likely the mechanism behind the pathogen-induced hyposecretion. However, F18 did not cause cystic fibrosis transmembrane conductance regulator mislocalization and the commensal-induced mechanism remains unclear.

Quorum quenching quandary: resistance to antivirulence compounds

Quorum sensing (QS) is the regulation of gene expression in response to the concentration of small signal molecules, and its inactivation has been suggested to have great potential to attenuate microbial virulence. It is assumed that unlike antimicrobials, inhibition of QS should cause less Darwinian selection pressure for bacterial resistance. Using the opportunistic pathogen Pseudomonas aeruginosa, we demonstrate here that bacterial resistance arises rapidly to the best-characterized compound that inhibits QS (brominated furanone C-30) due to mutations that increase the efflux of C-30. Critically, the C-30-resistant mutant mexR was more pathogenic to Caenorhabditis elegans in the presence of C-30, and the same mutation arises in bacteria responsible for chronic cystic fibrosis infections. Therefore, bacteria may evolve resistance to many new pharmaceuticals thought impervious to resistance.

Apoptosis-inducing factor contributes to epithelial cell apoptosis induced by enteropathogenic Escherichia coli

The mechanisms by which enteropathogenic Escherichia coli (EPEC) causes intestinal epithelial cell apoptosis remain unclear. We tested the hypothesis that apoptosis-inducing factor (AIF) is involved in apoptosis induced by EPEC. Infection of intestinal epithelial cells in vitro with EPEC led to the mitochondrial and cytosolic accumulation of AIF. This effect was partially dependent on caspase activity. Knockdown of AIF with siRNA blocked cellular apoptosis in response to EPEC infection, as assessed by poly(ADP-ribose) polymerase cleavage and oligonucleosome formation. Taken together, these data suggest that caspase-dependent mobilization of AIF contributes to EPEC-induced epithelial cell apoptosis.

Discovery of LAS101057: A Potent, Selective, and Orally Efficacious A2B Adenosine Receptor Antagonist

The structure-activity relationships for a series of pyrazine-based A2B adenosine receptor antagonists are described. From this work, LAS101057 (17), a potent, selective, and orally efficacious A2B receptor antagonist, was identified as a clinical development candidate. LAS101057 inhibits agonist-induced IL-6 production in human fibroblasts and is active in an ovalbumin (OVA)-sensitized mouse model after oral administration, reducing airway hyperresponsiveness to methacholine, Th2 cytokine production, and OVA-specific IgE levels.

The unexpected link between infection-induced apoptosis and a TH17 immune response

Microbial pathogens can initiate MOMP in host cells and as such, initiate the mitochondrial pathway of apoptosis. Innate immune recognition of cells dying in this way by infection-induced apoptosis would involve recognition of ligands derived from the apoptotic host cell simultaneously with those derived from the infecting pathogen. The resultant signal transduction pathways engaged direct DCs to concomitantly synthesize TGF-β and IL-6, two cytokines that subsequently favor the differentiation of naïve CD4 T cells into T(h)17 cells. Citrobacter rodentium is one rodent pathogen that targets mitochondria and induces apoptosis, and blockade of apoptosis during enteric Citrobacter infection impairs the characteristic T(h)17 response in the intestinal LP. Here, we review these original findings. We discuss microbial infections other than Citrobacter that have been shown to induce T(h)17 responses, and we examine what is known about the ability of those pathogens to induce apoptosis. We also consider types of cell death other than apoptosis that can be triggered by microbial infection, and we highlight how little we know about the impact of various forms of cell death on the ensuing adaptive immune response.

Responding to infection and apoptosis--a task for TH17 cells

Two of the critical cytokines required for the differentiation of T helper 17 (T(H)17) cells from naive CD4 T cells are transforming growth factor-beta (TGF-β) and interleukin-6 (IL-6). Innate recognition of apoptotic cells in the presence of Toll-like receptor engagement directs the simultaneous synthesis of these cytokines by antigen-presenting cells (APCs), and as such provides a cytokine milieu that favors T(H)17 cell induction. In this situation, APCs are activated in response to ligands derived from apoptotic cells, but also to those from the infecting pathogen. Induction of a T(H)17 response against Citrobacter rodentium infection was dependent on the ability of Citrobacter to induce apoptosis of intestinal epithelial cells. In this review, we will discuss how simultaneous activation of inflammatory and noninflammatory pattern recognition receptors on APCs impacts T helper cell differentiation, and what relevance this effect has on the immune response generated against bacterial infections that cause host cell apoptosis.

Extracellular ATP and P2Y receptor activation induce a proinflammatory host response in the human urinary tract

Extracellular ATP can be released by many cell types under conditions of cellular stress and signals through activation of purinergic receptors. Bladder uroepithelial cells grown in vitro have previously been shown to release ATP in response to stretch. In the present study, we investigated ATP release from uroepithelial cells infected with bacteria and the effect of ATP on the host cell proinflammatory interleukin 8 (IL-8) response. The human kidney epithelial cell line A498 and the human uroepithelial cell line UROtsa were grown in culture and stimulated by the uropathogenic Escherichia coli (UPEC) IA2 strain or the stable ATP analogue ATP-gamma-S. ATP and IL-8 levels were measured in cell culture medium with a luciferin-luciferase assay and enzyme-linked immunosorbent assay (ELISA), respectively. The results showed that UPEC infection of uroepithelial cells for 1 h significantly increased (P < 0.01) the extracellular ATP levels. ATP-gamma-S (10 and 100 microM) stimulated release of IL-8 from UROtsa and A498 cells after 6 and 24 h. Experiments with different purinoceptor agonists suggested that P2Y receptors, and not P2X receptors, were responsible for the ATP-gamma-S-induced IL-8 release. The potency profile further suggested involvement of P2Y(1), P2Y(2), and/or P2Y(11) receptors, and reverse transcription-PCR (RT-PCR) studies confirmed that the cells expressed these receptors. The amount of IL-8 released increased 12-fold in UPEC-infected cells, and apyrase, an enzyme that degrades ATP, reduced this increase by approximately 50%. The present study suggests that enhanced ATP release and P2Y receptor activation during urinary tract infection may represent a novel, non-TLR4-mediated mechanism for production of proinflammatory IL-8 in human urinary tract epithelial cells.

The role of epithelial malfunction in the pathogenesis of enteropathogenic E. coli-induced diarrhea

The homeostatic balance of the gastrointestinal tract relies on a single layer of epithelial cells, which assumes both digestive and protective functions. Enteric pathogens, including enteropathogenic Escherichia coli (EPEC), have evolved numerous mechanisms to disrupt basic intestinal epithelial functions, promoting the development of gastrointestinal disorders. Despite its non-invasive nature, EPEC inflicts severe damage to the intestinal mucosa, including the dysregulation of water and solute transport and the disruption of epithelial barrier structure and function. Despite the high prevalence and morbidity of disease caused by EPEC infections, the etiology of its pathogenesis remains incompletely understood. This review integrates the newest findings on EPEC-epithelial interactions with established mechanisms of disease in an attempt to give a comprehensive understanding of the cellular processes whereby this common pathogen may cause diarrheal illness.

Feedback effects of host-derived adenosine on enteropathogenic Escherichia coli

Enteropathogenic E. coli (EPEC) is a common cause of diarrhea in children in developing countries. After adhering to intestinal cells, EPEC secretes effector proteins into host cells, causing cell damage and eventually death. We previously showed that EPEC infection triggers the release of ATP from host cells and that ATP is broken down to ADP, AMP, and adenosine. Adenosine produced from the breakdown of extracellular ATP triggers fluid secretion in intestinal monolayers and may be an important mediator of EPEC-induced diarrhea. Here we examined whether adenosine has any effects on EPEC bacteria. Adenosine stimulated EPEC growth in several types of media in vitro. Adenosine also altered the pattern of EPEC adherence to cultured cells from a localized adherence pattern to a more diffuse pattern. Adenosine changed the expression of virulence factors in EPEC, inhibiting the expression of the bundle-forming pilus (BFP) and enhancing expression of the EPEC secreted proteins (Esps). In vivo, experimental manipulations of adenosine levels had strong effects on the outcome of EPEC infection in rabbit intestinal loops. In addition to its previously reported effects on host tissues, adenosine has strong effects on EPEC bacteria, stimulating EPEC growth, altering its adherence pattern, and changing the expression of several important virulence genes. Adenosine, like noradrenaline, is a small, host-derived molecule that is utilized as a signal by EPEC.

Purinergic regulation of the epithelial Na+ channel

1. The epithelial Na(+) channel (ENaC) is a major conductive pathway that transports Na(+) across the apical membrane of the distal nephron, the respiratory tract, the distal colon and the ducts of exocrine glands. The ENaC is regulated by hormonal and humoral factors, including extracellular nucleotides that are available from the epithelial cells themselves. 2. Extracellular nucleotides, via the P2Y2 receptors (P2Y2Rs) at the basolateral and apical membrane of the epithelia, trigger signalling systems that inhibit the activity of the ENaC and activate Ca(2+) -dependent Cl(-) secretion. 3. Recent data from our laboratory suggest that stimulation of the P2Y2Rs at the basolateral membrane inhibits ENaC activity by a signalling mechanism that involves G beta gamma subunits freed from a pertussis toxin (PTX)-sensitive G-protein and phospholipase C (PLC) beta 4. A similar signalling mechanism is also partially responsible for inhibition of the ENaC during activation of apical P2Y2Rs. 4. Stimulation of apical P2Y2Rs also activates an additional signalling mechanism that inhibits the ENaC and involves the activated Galpha subunit of a PTX-insensitive G-protein and activation of an unidentified PLC. The effect of this PTX-insensitive system requires the activity of the basolateral Na(+)/K(+)/2Cl(-) cotransporter.

Enteric infection meets intestinal function: how bacterial pathogens cause diarrhoea

Infectious diarrhoea is a significant contributor to morbidity and mortality worldwide. In bacterium-induced diarrhoea, rapid loss of fluids and electrolytes results from inhibition of the normal absorptive function of the intestine as well as the activation of secretory processes. Advances in the past 10 years in the fields of gastrointestinal physiology, innate immunity and enteric bacterial virulence mechanisms highlight the multifactorial nature of infectious diarrhoea. This review explores the various mechanisms that contribute to loss of fluids and electrolytes following bacterial infections, and attempts to link these events to specific virulence factors and toxins.

Regulator of G-protein signaling 4 suppresses LPS-induced MUC5AC overproduction in the airway

Mucus overproduction and airway obstruction are common features in airway mucosal inflammation. The mechanism by which LPS induces MUC5AC overexpression, however, has not been fully explored. The aims of this study were twofold: first, to examine the ATP-dependent mechanism by which LPS induces MUC5AC gene expression, and second, to identify specific molecules that could suppress LPS-induced MUC5AC expression at a G-protein-coupled receptor level. Here, we suggest that LPS from Pseudomonas aeruginosa induces MUC5AC overproduction by both an ATP-dependent pathway and an ATP-independent pathway. In addition, we showed that Regulator of G-protein signaling (RGS) 4 plays as a suppressor for ATP-induced MUC5AC expression by interacting with G alpha q in a GTP-dependent manner in vivo. These results give additional insights into the molecular mechanism of negative regulation of mucin overproduction and enhance our understanding of mucus hypersecretion during airway mucosal inflammation.

Heat-labile enterotoxin promotes Escherichia coli adherence to intestinal epithelial cells

Given recent evidence suggesting that the heat-labile enterotoxin (LT) provides a colonization advantage for enterotoxigenic Escherichia coli (ETEC) in vivo, we hypothesized that LT preconditions the host intestinal epithelium for ETEC adherence. To test this hypothesis, we used an in vitro model of ETEC adherence to examine the role of LT in promoting bacterium-host interactions. We present data demonstrating that elaboration of LT promotes a significant increase in E. coli adherence. This phenotype is primarily dependent on the inherent ADP-ribosylation activity of this toxin, with a secondary role observed for the receptor-binding LT-B subunit. Rp-3',5'-cyclic AMP (cAMP), an inhibitor of protein kinase A, was sufficient to abrogate LT's ability to promote subsequent bacterial adherence. Increased adherence was not due to changes in the surface expression of the host receptor for the K88ac adhesin. Evidence is also presented for a role for bacterial sensing of host-derived cAMP in promoting adherence to host cells.

Regulation of enteric functions by adenosine: pathophysiological and pharmacological implications

The wide distribution of ATP and adenosine receptors as well as enzymes for purine metabolism in different gut regions suggests a complex role for these mediators in the regulation of gastrointestinal functions. Studies in rodents have shown a significant involvement of adenosine in the control of intestinal secretion, motility and sensation, via activation of A1, A2A, A2B or A3 purinergic receptors, as well as the participation of ATP in the regulation of enteric functions, through the recruitment of P2X and P2Y receptors. Increasing interest is being focused on the involvement of ATP and adenosine in the pathophysiology of intestinal disorders, with particular regard for inflammatory bowel diseases (IBDs), intestinal ischemia, post-operative ileus and related dysfunctions, such as gut dysmotility, diarrhoea and abdominal discomfort/pain. Current knowledge suggests that adenosine contributes to the modulation of enteric immune and inflammatory responses, leading to anti-inflammatory actions. There is evidence supporting a role of adenosine in the alterations of enteric motor and secretory activity associated with bowel inflammation. In particular, several studies have highlighted the importance of adenosine in diarrhoea, since this nucleoside participates actively in the cross-talk between immune and epithelial cells in the presence of diarrhoeogenic stimuli. In addition, adenosine exerts complex regulatory actions on pain transmission at peripheral and spinal sites. The present review illustrates current information on the role played by adenosine in the regulation of enteric functions, under normal or pathological conditions, and discusses pharmacological interventions on adenosine pathways as novel therapeutic options for the management of gut disorders and related abdominal symptoms.

Diarrhea as a cause of mortality in a mouse model of infectious colitis

Analysis of gene expression in the colons of Citrobacter rodentium-infected susceptible and resistant mice suggests that mortality is associated with impaired intestinal ion transport.

Background

Comparative characterization of genome-wide transcriptional changes during infection can help elucidate the mechanisms underlying host susceptibility. In this study, transcriptional profiling of the mouse colon was carried out in two cognate lines of mice that differ in their response to Citrobacter rodentium infection; susceptible inbred FVB/N and resistant outbred Swiss Webster mice. Gene expression in the distal colon was determined prior to infection, and at four and nine days post-inoculation using a whole mouse genome Affymetrix array.

Results

Computational analysis identified 462 probe sets more than 2-fold differentially expressed between uninoculated resistant and susceptible mice. In response to C. rodentium infection, 5,123 probe sets were differentially expressed in one or both lines of mice. Microarray data were validated by quantitative real-time RT-PCR for 35 selected genes and were found to have a 94% concordance rate. Transcripts represented by 1,547 probe sets were differentially expressed between susceptible and resistant mice regardless of infection status, a host effect. Genes associated with transport were over-represented to a greater extent than even immune response-related genes. Electrolyte analysis revealed reduction in serum levels of chloride and sodium in susceptible animals.

Conclusion

The results support the hypothesis that mortality in C. rodentium-infected susceptible mice is associated with impaired intestinal ion transport and development of fatal fluid loss and dehydration. These studies contribute to our understanding of the pathogenesis of C. rodentium and suggest novel strategies for the prevention and treatment of diarrhea associated with intestinal bacterial infections.

Extracellular purines are biomarkers of neutrophilic airway inflammation

Purinergic signalling regulates airway defence mechanisms, suggesting that extracellular purines could serve as airway inflammation biomarkers in cystic fibrosis (CF). The purines adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP) and adenosine were measured in sputum from 21 adults (spontaneously expectorated from seven CF patients, induced from 14 healthy controls) to assess normal values and CF-associated changes. Subsequently, purine levels were measured in bronchoalveolar lavage fluid (BALF) from 37 children (25 CF patients, 12 disease controls) and compared with neutrophil counts, presence of airway infection and lung function. To noninvasively assess airway purines, ATP levels were measured using luminometry in exhaled breath condensate (EBC) from 14 children with CF and 14 healthy controls, then 14 CF children during a pulmonary exacerbation. Both ATP and AMP were elevated in sputum and BALF from CF subjects compared with controls. In BALF, ATP and AMP levels were inversely related to lung function and strongly correlated with neutrophil counts. In EBC, ATP levels were increased in CF relative to controls and decreased after treatment of CF pulmonary exacerbation. The purines adenosine triphosphate and adenosine monophosphate are candidate biomarkers of neutrophilic airways inflammation. Measurement of purines in sputum or exhaled breath condensate may provide a relatively simple and noninvasive method to track this inflammation.

Effect of zinc in enteropathogenic Escherichia coli infection

Enteropathogenic Escherichia coli (EPEC) infection triggers the release of ATP from host intestinal cells, and the ATP is broken down to ADP, AMP, and adenosine in the lumen of the intestine. Ecto-5'-nucleotidase (CD73) is the main enzyme responsible for the conversion of 5'-AMP to adenosine, which triggers fluid secretion from host intestinal cells and also has growth-promoting effects on EPEC bacteria. In a recent study, we examined the role of the host enzyme CD73 in EPEC infection by testing the effect of ecto-5'-nucleotidase inhibitors. Zinc was a less potent inhibitor of ecto-5'-nucleotidase in vitro than the nucleotide analog alpha,beta-methylene-ADP, but in vivo, zinc was much more efficacious in preventing EPEC-induced fluid secretion in rabbit ileal loops than alpha,beta-methylene-ADP. This discrepancy between the in vitro and in vivo potencies of the two inhibitors prompted us to search for potential targets of zinc other than ecto-5'-nucleotidase. Zinc, at concentrations that produced little or no inhibition of EPEC growth, caused a decrease in the expression of EPEC protein virulence factors, such as bundle-forming pilus (BFP), EPEC secreted protein A, and other EPEC secreted proteins, and reduced EPEC adherence to cells in tissue culture. The effects of zinc were not mimicked by other transition metals, such as manganese, iron, copper, or nickel, and the effects were not reversed by an excess of iron. Quantitative real-time PCR showed that zinc reduced the abundance of the RNAs encoded by the bfp gene, by the plasmid-encoded regulator (per) gene, by the locus for the enterocyte effacement (LEE)-encoded regulator (ler) gene, and by several of the esp genes. In vivo, zinc reduced EPEC-induced fluid secretion into ligated rabbit ileal loops, decreased the adherence of EPEC to rabbit ileum, and reduced histopathological damage such as villus blunting. Some of the beneficial effects of zinc on EPEC infection appear to be due to the action of the metal on EPEC bacteria as well as on the host.

Ecto-5'-nucleotidase and intestinal ion secretion by enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) triggers a large release of adenosine triphosphate (ATP) from host intestinal cells and the extracellular ATP is broken down to adenosine diphosphate (ADP), AMP, and adenosine. Adenosine is a potent secretagogue in the small and large intestine. We suspected that ecto-5'-nucleotidase (CD73, an intestinal enzyme) was a critical enzyme involved in the conversion of AMP to adenosine and in the pathogenesis of EPEC diarrhea. We developed a nonradioactive method for measuring ecto-5'-nucleotidase in cultured T84 cell monolayers based on the detection of phosphate release from 5'-AMP. EPEC infection triggered a release of ecto-5'-nucleotidase from the cell surface into the supernatant medium. EPEC-induced 5'-nucleotidase release was not correlated with host cell death but instead with activation of phosphatidylinositol-specific phospholipase C (PI-PLC). Ecto-5'-nucleotidase was susceptible to inhibition by zinc acetate and by alpha,beta-methylene-adenosine diphosphate (alpha,beta-methylene-ADP). In the Ussing chamber, these inhibitors could reverse the chloride secretory responses triggered by 5'-AMP. In addition, alpha,beta-methylene-ADP and zinc blocked the ability of 5'-AMP to stimulate EPEC growth under nutrient-limited conditions in vitro. Ecto-5'-nucleotidase appears to be the major enzyme responsible for generation of adenosine from adenine nucleotides in the T84 cell line, and inhibitors of ecto-5'-nucleotidase, such as alpha,beta-methylene-ADP and zinc, might be useful for treatment of the watery diarrhea produced by EPEC infection.

Distal colonic Na(+) absorption inhibited by luminal P2Y(2) receptors

Luminal P2 receptors are ubiquitously expressed in transporting epithelia. In steroid-sensitive epithelia (e.g., lung, distal nephron) epithelial Na(+) channel (ENaC)-mediated Na(+) absorption is inhibited via luminal P2 receptors. In distal mouse colon, we have identified that both, a luminal P2Y(2) and a luminal P2Y(4) receptor, stimulate K(+) secretion. In this study, we investigate the effect of luminal adenosine triphosphate/uridine triphosphate (ATP/UTP) on electrogenic Na(+) absorption in distal colonic mucosa of mice treated on a low Na(+) diet for more than 2 weeks. Transepithelial electrical parameters were recorded in an Ussing chamber. Baseline parameters: transepithelial voltage (V (te)): -13.7 +/- 1.9 mV (lumen negative), transepithelial resistance (R (te)): 24.1 +/- 1.8 Omega cm(2), equivalent short circuit current (I (sc)): -563.9 +/- 63.8 microA/cm(2) (n = 21). Amiloride completely inhibited I (sc) to -0.5 +/- 8.5 microA/cm(2). Luminal ATP induced a slowly on-setting and persistent inhibition of the amiloride-sensitive I (sc) by 160.7 +/- 29.7 microA/cm(2) (n = 12, NMRI mice). Luminal ATP and UTP were almost equipotent with IC(50) values of 10 microM and 3 microM respectively. In P2Y(2) knock-out (KO) mice, the effect of luminal UTP on amiloride-sensitve Na(+) absorption was absent. In contrast, in P2Y(4) KO mice the inhibitory effect of luminal UTP on Na(+) absorption remained present. Semiquantitative polymerase chain reaction did not indicate regulation of the P2Y receptors under low Na(+) diet, but it revealed a pronounced axial expression of both receptors with highest abundance in surface epithelia. Thus, luminal P2Y(2) and P2Y(4) receptors and ENaC channels co-localize in surface epithelium. Intriguingly, only the stimulation of the P2Y(2) receptor mediates inhibition of electrogenic Na(+) absorption.

Subepithelial fibroblasts in intestinal villi: roles in intercellular communication

Ingestion of food and water induces chemical and mechanical signals that trigger peristaltic reflexes in the gut. Intestinal villi are motile, equipped with chemosensors and mechanosensors, and transduce signaling to sensory neurons, but the exact mechanisms have not yet been elucidated. Subepithelial fibroblasts located under the villous epithelium form contractile cellular networks via gap junctions. The networks ensheathe lamina propria and are in close contact with epithelium, neural and capillary networks, smooth muscles, and immune cells. Unique characteristics of subepithelial fibroblasts have been revealed by primary cultures isolated from rat duodenal villi. They include rapid reversal changes in cell shape by cAMP reagents and endothelins, cell shape-dependent mechanosensitivity that induces ATP release as a paracrine mediator, contractile ability, and expression of various receptors for vasoactive and neuroactive substances. Herein, we review these characteristics that play a key role in the villi. They serve as a barrier/sieve, flexible mechanical frame, mechanosensor, and signal transduction machinery in the intestinal villi, which are regulated locally and dynamically by rapid cell shape conversion.

Mutual enhancement of virulence by enterotoxigenic and enteropathogenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) and enteropathogenic E. coli (EPEC) are common causes of diarrhea in children in developing countries. Dual infections with both pathogens have been noted fairly frequently in studies of diarrhea around the world. In previous laboratory work, we noted that cholera toxin and forskolin markedly potentiated EPEC-induced ATP release from the host cell, and this potentiated release was found to be mediated by the cystic fibrosis transmembrane conductance regulator. In this study, we examined whether the ETEC heat-labile toxin (LT) or the heat-stable toxin (STa, also known as ST) potentiated EPEC-induced ATP release. We found that crude ETEC culture filtrates, as well as purified ETEC toxins, did potentiate EPEC-induced ATP release in cultured T84 cells. Coinfection of T84 cells with live ETEC plus EPEC bacteria also resulted in enhanced ATP release compared to EPEC alone. In Ussing chamber studies of chloride secretion, adenine nucleotides released from the host by EPEC also significantly enhanced the chloride secretory responses that were triggered by crude ETEC filtrates, purified STa, and the peptide hormone guanylin. In addition, adenosine and LT had additive or synergistic effects in inducing vacuole formation in T84 cells. Therefore, ETEC toxins and EPEC-induced damage to the host cell both enhance the virulence of the other type of E. coli. Our in vitro data demonstrate a molecular basis for a microbial interaction, which could result in increased severity of disease in vivo in individuals who are coinfected with ETEC and EPEC.

Physiological roles for ecto-5'-nucleotidase (CD73)

Nucleotides and nucleosides influence nearly every aspect of physiology and pathophysiology. Extracellular nucleotides are metabolized through regulated phosphohydrolysis by a series of ecto-nucleotidases. The formation of extracellular adenosine from adenosine 5’-monophosphate is accomplished primarily through ecto-5’-nucleotidase (CD73), a glycosyl phosphatidylinositol-linked membrane protein found on the surface of a variety of cell types. Recent in vivo studies implicating CD73 in a number of tissue protective mechanisms have provided new insight into its regulation and function and have generated considerable interest. Here, we review contributions of CD73 to cell and tissue stress responses, with a particular emphasis on physiologic responses to regulated CD73 expression and function, as well as new findings utilizing Cd73-deficient animals.

The role of epithelial P2Y2 and P2Y4 receptors in the regulation of intestinal chloride secretion

UTP-induced chloride secretion by the intestinal mucosa mounted in Ussing chambers was assessed by measurement of the short-circuit current (I(sc)) in the presence of phloridzin in the case of jejunum or amiloride in the case of colon to eliminate any contribution of electrogenic Na(+) movement to the net ionic transport. Since we have previously demonstrated the absence of chloride-secretory response to apical UTP in the jejunum from P2Y(4)-null mice, in the present study we studied the response to basolateral UTP in the jejunum and to either apical or basolateral UTP in the colon, in both P2Y(2)- and P2Y(4)-deficient mice. In the jejunum, the chloride-secretory response to basolateral UTP was partially reduced in both P2Y(2)- (40%) and P2Y(4)- (60%) null mice. In the colon, both apical or basolateral UTP increased the I(sc). That response was abolished in a chloride-free medium. The colonic chloride-secretory response to either basolateral or apical UTP was abolished in P2Y(4)-deficient mice, but not significantly affected in P2Y(2)-deficient mice. The chloride-secretory response to forskolin was potentiated by prior basolateral addition of UTP and this potentiation was abolished in P2Y(4)-null mice. The jejunum of mice homozygous for the DeltaF508 mutation of cystic fibrosis transmembrane conductance regulator was responsive to UTP, but the magnitude of that response was smaller than in the wild-type littermates. In conclusion, the P2Y(4) receptor fully mediates the chloride-secretory response to UTP in both small and large intestines, except at the basolateral side of the jejunum, where both P2Y(2) and P2Y(4) receptors are involved.

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.

Salmonella and Enteropathogenic Escherichia coli Interactions with Host Cells: Signaling Pathways

The host-pathogen interaction involves a myriad of initiations and responses from both sides. Bacterial pathogens such as enteropathogenic Escherichia coli (EPEC) and Salmonella enterica have numerous virulence factors that interact with and alter signaling components of the host cell to initiate responses that are beneficial to pathogen survival and persistence. The study of Salmonella and EPEC infection reveals intricate connections between host signal transduction, cytoskeletal architecture, membrane trafficking, and cytokine gene expression. The emerging picture includes elements of molecular mimicry by bacterial effectors and bacterial subversion of typical host events, with the result that EPEC is able to survive and persist in an extracellular milieu, while Salmonella establishes an intracellular niche and is able to spread systemically throughout the host. This review focuses on recent advances in our understanding of the signaling events stemming from the host-pathogen interactions specific to Salmonella and EPEC.

Human neutrophil peptides induce interleukin-8 production through the P2Y6 signaling pathway

Antimicrobial human neutrophil peptides (HNPs) play a pivotal role in innate host defense against a broad spectrum of prokaryotic pathogens. In addition, HNPs modulate cellular immune responses by producing the chemokine interleukin-8 (IL-8) in myeloid and epithelial cells and by exerting chemotaxis to T cells, immature dendritic cells, and monocytes. However, the mechanisms by which HNPs modulate the immune responses in the eukaryotic cells remain unclear. We demonstrated that, as with adenosine triphosphate (ATP) and uridine diphosphate (UDP), HNP stimulation of human lung epithelial cells selectively induced IL-8 production in 10 pro- and anti-inflammatory cytokines examined. HNP-induced IL-8 release was inhibited by treatment with the nucleotide receptor antagonists suramin and reactive blue. Transfection of lung epithelial cells with antisense oligonucleotides targeting specific purinergic P2Y receptors revealed that the P2Y6 (ligand of UDP) signaling pathway plays a predominant role in mediating HNP-induced IL-8 production.