Mechanisms of release of nucleotides and integration of their action as P2X- and P2Y-receptor activating molecules

Involvement of P2X receptors in the regulation of insulin secretion, proliferation and survival in mouse pancreatic β-cells

In order to clarify the functional role of ionotropic purinergic (P2X) receptors in pancreatic β-cells, we examined the effect of several P2 receptor agonists and antagonists on insulin secretion by mouse pancreatic islets, mouse Beta-TC6 cell proliferation and survival of dispersed islet cells in culture. Reverse transcription-polymerase chain reaction (RT-PCR) analysis showed the expression of mRNAs of P2X(4) receptor in mouse islets and P2X(1), P2X(2), P2X(3), P2X(4), P2X(5) and P2X(7) receptors in Beta-TC6 cells. The presence of P2X(4) receptor proteins in islets and Beta-TC6 cells was confirmed by immunofluorescent staining and Western blot analysis. We have previously found that the functional P2Y(1) receptor but not P2Y(2) and P2Y(4) receptors was present in islets. In this study we found that a nonspecific P2 receptor agonist, ATP (1 μM) stimulated insulin secretion by islets in the presence of high glucose (20 mM) in culture. The effect of ATP was partially inhibited by a P2 receptor antagonist PPADS as well as a P2Y(1) receptor antagonist MRS2179. In addition, a P2X(4) receptor potentiator ivermectin per se augmented glucose-induced insulin secretion and slightly potentiated the effect of ATP. These results suggested the involvement of P2Y(1)and P2X receptors. We also found that ATP inhibited proliferation of Beta-TC6 cells in a concentration-dependent manner during 72 h culture. The inhibitory effect of ATP was completely reversed by PPADS and partially by treating cells with small interfering RNA targeted for P2X(4) receptor mRNA. Furthermore, we found that the phosphorylation of the extracellular signal-regulated kinase 1 and 2 (ERK1/2) was suppressed by treatment with ATP in Beta-TC6 cells. In addition, we found that ATP reduced the cell viability and DNA synthesis of islet cells in culture. The effect of ATP on the cell viability was blocked by PPADS or MRS2179. These results suggested that P2X receptors as well as the P2Y(1) receptor played a role in the modulation of insulin secretion, proliferation and cell viability in mouse pancreatic β-cells.

Regulation of neuronal ion channels via P2Y receptors

Within the last 15 years, at least 8 different G protein-coupled P2Y receptors have been characterized. These mediate slow metabotropic effects of nucleotides in neurons as well as non-neural cells, as opposed to the fast ionotropic effects which are mediated by P2X receptors. One class of effector systems regulated by various G protein-coupled receptors are voltage-gated and ligand-gated ion channels. This review summarizes the current knowledge about the modulation of such neuronal ion channels via P2Y receptors. The regulated proteins include voltage-gated Ca²⁺ and K⁺ channels, as well as N-methyl-d-aspartate, vanilloid, and P2X receptors, and the regulating entities include most of the known P2Y receptor subtypes. The functional consequences of the modulation of ion channels by nucleotides acting at pre- or postsynaptic P2Y receptors are changes in the strength of synaptic transmission. Accordingly, ATP and related nucleotides may act not only as fast transmitters (via P2X receptors) in the nervous system, but also as neuromodulators (via P2Y receptors). Hence, nucleotides are as universal transmitters as, for instance, acetylcholine, glutamate, or γ-aminobutyric acid.

GPR80/99, proposed to be the P2Y(15) receptor activated by adenosine and AMP, is not a P2Y receptor

The orphan receptor GPR80 (also called GPR99) was recently reported to be the P2Y₁₅ receptor activated by AMP and adenosine and coupled to increases in cyclic AMP accumulation and intracellular Ca²⁺ mobilization (Inbe et al. J Biol Chem 2004; 279: 19790–9[12]). However, the cell line (HEK293) used to carry out those studies endogenously expresses A_2A and A_2B adenosine receptors as well as multiple P2Y receptors, which complicates the analysis of a potential P2Y receptor. To determine unambiguously whether GPR80 is a P2Y receptor subtype, HA-tagged GPR80 was either stably expressed in CHO cells or transiently expressed in COS-7 and HEK293 cells, and cell surface expression was verified by radioimmunoassay (RIA). COS-7 cells overexpressing GPR80 showed a consistent twofold increase in basal inositol phosphate accumulation. However, neither adenosine nor AMP was capable of promoting accumulation of either cyclic AMP or inositol phosphates in any of the three GPR80-expressing cells. A recent paper (He et al. Nature 2004; 429: 188–93 [15]) reported that GPR80 is a Gq-coupled receptor activated by the citric acid cycle intermediate, α-ketoglutarate. Consistent with this report, α-ketoglutarate promoted inositol phosphate accumulation in CHO and HEK293 cells expressing GPR80, and pretreatment of GPR80-expressing COS-7 cells with glutamate dehydrogenase, which converts α-ketoglutarate to glutamate, decreased basal levels of inositol phosphates. Taken together, these data demonstrate that GPR80 is not activated by adenosine, AMP or other nucleotides, but instead is activated by α-ketoglutarate. Therefore, GPR80 is not a new member of the P2Y receptor family.

Infection with Leishmania amazonensis upregulates purinergic receptor expression and induces host-cell susceptibility to UTP-mediated apoptosis

Nucleotides are released into the extracellular milieu from infected cells and cells at inflammatory sites. The extracellular nucleotides bind to specific purinergic (P2) receptors and thereby induce a variety of cellular responses including anti-parasitic effects. Here we investigated whether extracellular nucleotides affect leishmanial infection in macrophages, and found that UTP reduces strongly the parasite load in peritoneal macrophages. Ultrastructural analysis of infected cells revealed that UTP induced morphological damage in the intracellular parasites. Uridine nucleotides also induced dose-dependent apoptosis of macrophages and production of ROI and RNI only in infected macrophages. The intracellular calcium measurements of infected cells showed that the response to UTP, but not UDP, increased the sensitivity and amplitude of cytosolic Ca(2+) changes. Infection of macrophages with Leishmania upregulated the expression of P2Y(2) and P2Y(4) receptor mRNA. The data suggest indirectly that Leishmania amazonensis infection induces modulation and heteromerization of P2Y receptors on macrophages. Thus UTP modulates the host response against L. amazonensis infection. UTP and UTP homologues should therefore be considered as novel components of therapeutic strategies against cutaneous leishmaniasis.

Purinergic signalling in epithelial ion transport: regulation of secretion and absorption

Intracellular ATP, the energy source for many reactions, is crucial for the activity of plasma membrane pumps and, thus, for the maintenance of transmembrane ion gradients. Nevertheless, ATP and other nucleotides/nucleosides are also extracellular molecules that regulate diverse cellular functions, including ion transport. In this review, I will first introduce the main components of the extracellular ATP signalling, which have become known as the purinergic signalling system. With more than 50 components or processes, just at cell membranes, it ranks as one of the most versatile signalling systems. This multitude of system components may enable differentiated regulation of diverse epithelial functions. As epithelia probably face the widest variety of potential ATP-releasing stimuli, a special attention will be given to stimuli and mechanisms of ATP release with a focus on exocytosis. Subsequently, I will consider membrane transport of major ions (Cl(-) , HCO(3)(-) , K(+) and Na(+) ) and integrate possible regulatory functions of P2Y2, P2Y4, P2Y6, P2Y11, P2X4, P2X7 and adenosine receptors in some selected epithelia at the cellular level. Some purinergic receptors have noteworthy roles. For example, many studies to date indicate that the P2Y2 receptor is one common denominator in regulating ion channels on both the luminal and basolateral membranes of both secretory and absorptive epithelia. In exocrine glands though, P2X4 and P2X7 receptors act as cation channels and, possibly, as co-regulators of secretion. On an organ level, both receptor types can exert physiological functions and together with other partners in the purinergic signalling, integrated models for epithelial secretion and absorption are emerging.

Role of connexin43 hemichannels in mechanical stress-induced ATP release in human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Our previous studies showed that mechanical stress could induce ATP release in human periodontal ligament (HPDL) cells. By signaling through P2 purinergic receptors, ATP increased the expression and the synthesis of osteopontin and RANKL. In this study, the mechanism of stress-induced ATP release was investigated.

MATERIAL AND METHODS

Continuous compressive forces were applied on cultured HPDL cells. The ATP released was measured using luciferin-luciferase bioluminescence. The expression of gap-junction proteins was examined using RT-PCR and western blot analysis. The opening of hemichannels was demonstrated by cellular uptake of a fluorescent dye, 5(6)-carboxyfluorescein, which is known to penetrate hemichannels. Intracellular signal transduction was investigated using inhibitors and antagonists.

RESULTS

Mechanical stress induced the release of ATP into the culture medium, which was attenuated by carbenoxolone, a nonspecific gap-junction inhibitor. Addition of meclofenamic acid sodium salt, a connexin43 inhibitor, inhibited ATP release by mechanical stress. Knockdown of connexin43 expression by small interfering RNA reduced the amount of ATP released by mechanical stress, suggesting the role of connexin43 hemichannels. In addition, intracellular Ca(2+) blockers could also inhibit mechanical stress-induced ATP release and the opening of the gap junction.

CONCLUSION

Our study demonstrated the involvement of gap-junction hemichannels, especially connexin43, in the stress-induced ATP-release mechanism. Furthermore, this mechanism may be regulated by the intracellular Ca(2+) signaling pathway. These results suggest an important role of gap-junction hemichannels in the function and behavior of HPDL cells.

Rho signaling regulates pannexin 1-mediated ATP release from airway epithelia

ATP released from airway epithelial cells promotes purinergic receptor-regulated mucociliary clearance activities necessary for innate lung defense. Cell swelling-induced membrane stretch/strain is a common stimulus that promotes airway epithelial ATP release, but the mechanisms transducing cell swelling into ATP release are incompletely understood. Using knockdown and knockout approaches, we tested the hypothesis that pannexin 1 mediates ATP release from hypotonically swollen airway epithelia and investigated mechanisms regulating this activity. Well differentiated primary cultures of human bronchial epithelial cells subjected to hypotonic challenge exhibited enhanced ATP release, which was paralleled by the uptake of the pannexin probe propidium iodide. Both responses were reduced by pannexin 1 inhibitors and by knocking down pannexin 1. Importantly, hypotonicity-evoked ATP release from freshly excised tracheas and dye uptake in primary tracheal epithelial cells were impaired in pannexin 1 knockout mice. Hypotonicity-promoted ATP release and dye uptake in primary well differentiated human bronchial epithelial cells was accompanied by RhoA activation and myosin light chain phosphorylation and was reduced by the RhoA dominant negative mutant RhoA(T19N) and Rho and myosin light chain kinase inhibitors. ATP release and Rho activation were reduced by highly selective inhibitors of transient receptor potential vanilloid 4 (TRPV4). Lastly, knocking down TRPV4 impaired hypotonicity-evoked airway epithelial ATP release. Our data suggest that TRPV4 and Rho transduce cell membrane stretch/strain into pannexin 1-mediated ATP release in airway epithelia.

Nucleotides control the excitability of sensory neurons via two P2Y receptors and a bifurcated signaling cascade

Nucleotides contribute to the sensation of acute and chronic pain, but it remained enigmatic which G protein-coupled nucleotide (P2Y) receptors and associated signaling cascades are involved. To resolve this issue, nucleotides were applied to dorsal root ganglion neurons under current- and voltage-clamp. Adenosine triphosphate (ATP), adenosine diphosphate (ADP), and uridine triphosphate (UTP), but not uridine diphosphate (UDP), depolarized the neurons and enhanced action potential firing in response to current injections. The P2Y₂ receptor preferring agonist 2-thio-UTP was equipotent to UTP in eliciting these effects. The selective P2Y₁ receptor antagonist MRS2179 largely attenuated the excitatory effects of ADP, but left those of 2-thio-UTP unaltered. Thus, the excitatory effects of the nucleotides were mediated by 2 different P2Y receptors, P2Y₁ and P2Y₂. Activation of each of these 2 receptors by either ADP or 2-thio-UTP inhibited currents through K_V7 channels, on one hand, and facilitated currents through TRPV₁ channels, on the other hand. Both effects were abolished by inhibitors of phospholipase C or Ca²⁺-ATPase and by chelation of intracellular Ca²⁺. The facilitation of TRPV₁, but not the inhibition K_V7 channels, was prevented by a protein kinase C inhibitor. Simultaneous blockage of K_V7 channels and of TRPV₁ channels prevented nucleotide-induced membrane depolarization and action potential firing. Thus, P2Y₁ and P2Y₂ receptors mediate an excitation of dorsal root ganglion neurons by nucleotides through the inhibition of K_V7 channels and the facilitation of TRPV₁ channels via a common bifurcated signaling pathway relying on an increase in intracellular Ca²⁺ and an activation of protein kinase C, respectively.

Transcriptional control mechanisms associated with the nucleotide receptor P2X7, a critical regulator of immunologic, osteogenic, and neurologic functions

The nucleotide receptor P2X(7) is an attractive therapeutic target and potential biomarker for multiple inflammatory and neurologic disorders, and it is expressed in several immune, osteogenic, and neurologic cell types. Aside from its role in the nervous system, it is activated by ATP released at sites of tissue damage, inflammation, and infection. Ligand binding to P2X(7) stimulates many cell responses, including calcium fluxes, MAPK activation, inflammatory mediator release, and apoptosis. Much work has centered on P2X(7) action in cell death and mediator processing (e.g., pro-interleukin-1 cleavage by the inflammasome), but the contribution of P2X(7) to transcriptional regulation is less well defined. This review will focus on the growing evidence for the importance of nucleotide-mediated gene expression, highlight several animal models, human genetic, and clinical studies that support P2X(7) as a therapeutic target, and discuss the latest developments in anti-P2X(7) clinical trials.

Identification of an Arabidopsis plasma membrane-located ATP transporter important for anther development

ATP acts as an extracellular signal molecule in plants. However, the nature of the mechanisms that export this compound into the apoplast are under debate. We identified the protein PM-ANT1 as a candidate transporter able to mediate ATP export. PM-ANT1 joins the mitochondrial carrier family, lacks an N-terminal amino acid extension required for organelle localization, and locates to the plasma membrane. Recombinant PM-ANT1 transports ATP, and the gene is substantially expressed in mature pollen grains. Artificial microRNA (amiRNA) mutants show reduced silique length and less seeds per silique but increased seed weight associated with unchanged pollen viability. Anthers from amiRNA mutants exhibited a normal early development, but stomium breakage is inhibited, leading to impaired anther dehiscence. This results in reduced self-pollination and thus decreased fertilization efficiency. amiRNA pollen grains showed increased intracellular ATP levels but decreased extracellular ATP levels. The latter effects are in line with transport properties of recombinant PM-ANT1, supporting in planta that functional PM-ANT1 resides in the plasma membrane and concur with the PM-ANT1 expression pattern. We assume that PM-ANT1 contributes to ATP export during pollen maturation. ATP export may serve as an extracellular signal required for anther dehiscence and is a novel factor critical for pollination and autogamy.

Extracellular ATP through P2 receptors activates AMP-activated protein kinase and suppresses superoxide generation in cultured mouse podocytes

Podocytes are an important constituent of the glomerular filtration barrier. The function of these glomerular cells is affected by extracellular nucleotides through P2 receptors. The activation of P2 receptors may lead to the activation of NAD(P)H oxidase, the key enzyme in oxidative stress, with the intracellular pathways leading to intracellular ATP depletion associated with an increase in the intracellular AMP:ATP ratio. This deregulation of the energy balance activates AMP-activated protein kinase (AMPK) to restore energy homeostasis. We investigated whether P2 receptor activation influences NAD(P)H oxidase-dependent rate of superoxide anion (O(2)(•-)) generation and AMPK activity in cultured mouse podocytes. The rate of O(2)(•-) generation was measured by chemiluminescence and changes in AMPK activity were determined by immunoblotting against AMPKα-Thr(172)-P. The addition of 100 μM ATP induced a rapid and transient decrease in rate of O(2)(•-) generation and increased AMPK phosphorylation with maximal effects in the first minute (2.44±0.09 versus 1.62±0.06 nmol/mg protein/min, P<0.05 and 0.64±0.04 versus 0.97±0.07, P<0.05, respectively). Both parameters returned to control levels at 10 min. Suramin (300 μM, P2 receptor antagonist) and compound C (100μM, AMPK inhibitor) completely, and STO-609 (25 μM, CaMKK-β inhibitor) partially, prevented ATP action in rate of O(2)(•-) generation and AMPK phosphorylation. Various ATP analogues (10 μM) mimicked the effects of ATP on rate of O(2)(•-) generation and AMPK phosphorylation. The data indicate that extracellular ATP, acting through P2 receptors upstream of CaMKK-β, modulates podocyte function through simultaneous effects on AMPK and NAD(P)H oxidase activities. This mechanism may play a role in restoring energy homeostasis after oxidative stress.

Nucleotide receptors as targets in the pharmacological enhancement of dermal wound healing

With a growing interest of the involvement of extracellular nucleotides in both normal physiology and pathology, it has become evident that P2 receptor agonists and antagonists may have therapeutic potential. The P2Y2 receptor agonists (diquafosol tetrasodium and denufosol tetrasodium) are in the phase 3 of clinical trials for dry eye and cystic fibrosis, respectively. The thienopyridine derivatives clopidogrel and ticlopidine (antagonists of the platelet P2Y12 receptor) have been used in cardiovascular medicine for nearly a decade. Purines and pyrimidines may be of therapeutic potential also in wound healing since ATP and UTP have been shown to have many hallmarks of wound healing factors. Recent studies have demonstrated that extracellular nucleotides take part in all phases of wound repair: hemostasis, inflammation, tissue formation, and tissue remodeling. This review is focused on the potent purines and pyrimidines which regulate many physiological processes important for wound healing.

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

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

P2Y(6) agonist uridine 5'-diphosphate promotes host defense against bacterial infection via monocyte chemoattractant protein-1-mediated monocytes/macrophages recruitment

Extracellular nucleotides are important messengers involved in series crucial physiological functions through the activation of P2 purinergic receptors. The detailed function and mechanism of the P2Y family in regulating immune response against invaded pathogens still remains unknown. In this study, the activation of purinoreceptor P2Y(6) by UDP was found to play a crucial role in promoting host defense against invaded bacteria through monocytes/macrophages recruitment. The expression level of P2Y(6) was much higher than other purinoreceptors in RAW264.7 cells, bone marrow macrophages, and peritoneal macrophages determined by real-time PCR. The supernatant of UDP (P2Y(6)-specific agonist)-treated RAW264.7 cells exhibited direct chemotaxis to monocytes/macrophages in vitro through Boyden Chambers assay. Meanwhile, the releasing of MCP-1 (MCP-1/CCL2) was enhanced obviously by UDP both in mRNA and protein level. Furthermore, the activation of P2Y(6) receptor by UDP also promotes ERK phosphorylation and AP-1 activation in a concentration- and time-dependent manner in RAW264.7 cells. This UDP-induced activation could be inhibited by P2Y(6) selectivity antagonist (MRS2578), MEK inhibitor (U0126), and MCP-1 blocking Ab, respectively. Moreover, i.p. injection with UDP resulted in a more efficacious clearance of invaded Escherichia coli and lower mortality in peritonitis mouse model. Together, our studies demonstrate that P2Y(6) receptor could be a novel mediator in upregulating innate immune response against the invaded pathogens through recruiting monocytes/macrophages.

The role of the NLRP3 inflammasome in the pathogenesis of airway disease

The incidences of respiratory diseases like asthma and Chronic Obstructive Pulmonary Disease (COPD) are increasing dramatically. Significantly, there are currently no treatments that can slow or prevent the relentless progression of COPD; and a sub-population of asthmatics are resistant to available therapies. What is more, currently prescribed medication has only minimal effect on the symptoms suffered in these patient groups. There is therefore an urgent need to develop effective drugs to treat these diseases. Whilst asthma and COPD are thought to be distinct diseases, it is currently believed that the pathogenesis of both is driven by the chronic inflammation present in the airways of these patients. It is thus hypothesised that if the inflammation could be attenuated, disease development would be slowed and symptoms reduced. It is therefore paramount to determine the pathways driving/propagating the inflammation. Recently there has been a growing body of evidence to suggest that the multimeric protein complex known as the Inflammasome may play key roles in the inflammation observed in respiratory diseases. The aim of this review is to discuss the role of the NLRP3 Inflammasome, and its associated inflammatory mediators (IL-1β and IL-18), in the pathogenesis of asthma and COPD.

Low dosages: new chemotherapeutic weapons on the battlefield of immune-related disease

Chemotherapeutic drugs eliminate tumor cells at relatively high doses and are considered weapons against tumors in clinics and hospitals. However, despite their ability to induce cellular apoptosis, chemotherapeutic drugs should probably be regarded more as a class of cell regulators than cell killers, if the dosage used and the fact that their targets are involved in basic molecular events are considered. Unfortunately, the regulatory properties of chemotherapeutic drugs are usually hidden or masked by the massive cell death induced by high doses. Recent evidence has begun to suggest that low dosages of chemotherapeutic drugs might profoundly regulate various intracellular aspects of normal cells, especially immune cells. Here, we discuss the immune regulatory roles of three kinds of chemotherapeutic drugs under low-dose conditions and propose low dosages as potential new chemotherapeutic weapons on the battlefield of immune-related disease.

The airway epithelium: more than just a structural barrier

The mammalian airway is lined by a variety of specialized epithelial cells that not only serve as a physical barrier but also respond to environment-induced damage through the release of biologically active factors and constant cellular renewal. The lung epithelium responds to environmental insults such as pathogens, cigarette smoke and pollution by secreting inflammatory mediators and antimicrobial peptides, and by recruiting immune cells to the site of infection or damage. When the epithelium is severely damaged, basal cells and Clara cells that have stem-cell-like properties are capable of self-renewal and proliferation in the affected area, to repair the damage. In order to effectively fight off infections, the epithelium requires the assistance of neutrophils recruited from the peripheral circulation through transendothelial followed by transepithelial migration events. Activated neutrophils migrate across the epithelium through a series of ligand-receptor interactions to the site of injury, where they secrete proteolytic enzymes and oxidative radicals for pathogen destruction. However, chronic activation and recruitment of neutrophils in airway diseases such as chronic obstructive pulmonary disease and asthma has been associated with tissue damage and disease severity. In this paper, we review the current understanding of the airway epithelial response to injury and its interaction with inflammatory cells, in particular the neutrophil.

International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and classification of adenosine receptors--an update

In the 10 years since our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors, no developments have led to major changes in the recommendations. However, there have been so many other developments that an update is needed. The fact that the structure of one of the adenosine receptors has recently been solved has already led to new ways of in silico screening of ligands. The evidence that adenosine receptors can form homo- and heteromultimers has accumulated, but the functional significance of such complexes remains unclear. The availability of mice with genetic modification of all the adenosine receptors has led to a clarification of the functional roles of adenosine, and to excellent means to study the specificity of drugs. There are also interesting associations between disease and structural variants in one or more of the adenosine receptors. Several new selective agonists and antagonists have become available. They provide improved possibilities for receptor classification. There are also developments hinting at the usefulness of allosteric modulators. Many drugs targeting adenosine receptors are in clinical trials, but the established therapeutic use is still very limited.

The danger signal, extracellular ATP, is a sensor for an airborne allergen and triggers IL-33 release and innate Th2-type responses

The molecular mechanisms underlying the initiation of innate and adaptive proallergic Th2-type responses in the airways are not well understood. IL-33 is a new member of the IL-1 family of molecules that is implicated in Th2-type responses. Airway exposure of naive mice to a common environmental aeroallergen, the fungus Alternaria alternata, induces rapid release of IL-33 into the airway lumen, followed by innate Th2-type responses. Biologically active IL-33 is constitutively stored in the nuclei of human airway epithelial cells. Exposing these epithelial cells to A. alternata releases IL-33 extracellularly in vitro. Allergen exposure also induces acute extracellular accumulation of a danger signal, ATP; autocrine ATP sustains increases in intracellular Ca(2+) concentration and releases IL-33 through activation of P2 purinergic receptors. Pharmacological inhibitors of purinergic receptors or deficiency in the P2Y2 gene abrogate IL-33 release and Th2-type responses in the Alternaria-induced airway inflammation model in naive mice, emphasizing the essential roles for ATP and the P2Y(2) receptor. Thus, ATP and purinergic signaling in the respiratory epithelium are critical sensors for airway exposure to airborne allergens, and they may provide novel opportunities to dampen the hypersensitivity response in Th2-type airway diseases such as asthma.

Extracellular ATP signaling during differentiation of C2C12 skeletal muscle cells: role in proliferation

Evidence shows that extracellular ATP signals influence myogenesis, regeneration and physiology of skeletal muscle. Present work was aimed at characterizing the extracellular ATP signaling system of skeletal muscle C2C12 cells during differentiation. We show that mechanical and electrical stimulation produces substantial release of ATP from differentiated myotubes, but not from proliferating myoblasts. Extracellular ATP-hydrolyzing activity is low in myoblasts and high in myotubes, consistent with the increased expression of extracellular enzymes during differentiation. Stimulation of cells with extracellular nucleotides produces substantial Ca(2+) transients, whose amplitude and shape changed during differentiation. Consistently, C2C12 cells express several P2X and P2Y receptors, whose level changes along with maturation stages. Supplementation with either ATP or UTP stimulates proliferation of C2C12 myoblasts, whereas excessive doses were cytotoxic. The data indicate that skeletal muscle development is accompanied by major functional changes in extracellular ATP signaling.

Regulatory volume decrease and P receptor signaling in fish cells: mechanisms, physiology, and modeling approaches

For animal cell plasma membranes, the permeability of water is much higher than that of ions and other solutes, and exposure to hyposmotic conditions almost invariably causes rapid water influx and cell swelling. In this situation, cells deploy regulatory mechanisms to preserve membrane integrity and avoid lysis. The phenomenon of regulatory volume decrease, the partial or full restoration of cell volume following cell swelling, is well-studied in mammals, with uncountable investigations yielding details on the signaling network and the effector mechanisms involved in the process. In comparison, cells from other vertebrates and from invertebrates received little attention, despite of the fact that e.g. fish cells could present rewarding model systems given the diversity in ecology and lifestyle of this animal group that may be reflected by an equal diversity of physiological adaptive mechanisms, including those related to cell volume regulation. In this review, we therefore present an overview on the most relevant aspects known on hypotonic volume regulation presently known in fish, summarizing transporters and signaling pathways described so far, and then focus on an aspect we have particularly studied over the past years using fish cell models, i.e. the role of extracellular nucleotides in mediating cell volume recovery of swollen cells. We, furthermore, present diverse modeling approaches developed on the basis of data derived from studies with fish and other models and discuss their potential use for gaining insight into the theoretical framework of volume regulation.

Extracellular nucleosides and nucleotides regulate liver functions via a complex system of membrane proteins

Nucleosides and nucleotides are now considered as extracellular signalling molecules, like neurotransmitters and hormones. Hepatic cells, amongst other cells, ubiquitously express specific transmembrane receptors that transduce the physiological signals induced by extracellular nucleosides and nucleotides, as well as various cell surface enzymes that regulate the levels of these mediators in the extracellular medium. Here, we cover various aspects of the signalling pathways initiated by extracellular nucleosides and nucleotides in the liver, and discuss their overall impact on hepatic physiology.

Purinergic signaling in wound healing and airway remodeling

Airway epithelia are continuously damaged by airborne pollutants, pathogens and allergens, and they rely on intrinsic mechanisms to restore barrier integrity. Epithelial repair is a multi-step process including cell migration into the wounded area, proliferation, differentiation and matrix deposition. Each step requires the secretion of various molecules, including growth factors, integrins and matrix metalloproteinases. Evidence is emerging that purinergic signaling promotes repair in human airway epithelia. An injury induces ATP release, which binds P2Y(2) receptors (P2Y(2)Rs) to initiate protein kinase C (PKC)-dependent oxidative activation of TNFα-converting enzyme (TACE), which then releases the membrane-bound ligands of the epidermal growth factor receptor (EGFR). The P2Y(2)R- and EGFR-dependent signaling cascades converge to induce mediator release, whereas the latter also induces cytoskeletal rearrangement for cell migration and proliferation. Similar roles for purinergic signaling are reported in pulmonary endothelial cells, smooth muscle cells and fibroblasts. In chronic airway diseases, the aberrant regulation of extracellular purines is implicated in the development of airway remodeling by mucus cell metaplasia and hypersecretion, excess collagen deposition, fibrosis and neovascularization. This chapter describes the crosstalk between these signaling cascades and discusses the impact of deregulated purinergic signaling in chronic lung diseases.

Purinergic modulation of glucose uptake into cultured rat podocytes: effect of diabetic milieu

Extracellular purines act via P1 and P2 receptors on podocytes and may influence on their function. This action may be modified under various (patho)physiological conditions leading to development of podocytopathy. Aim of study was to investigate effects of diabetic milieu, represented by high glucose concentration (HG, 30 mM glucose) on purinergic-induced changes of 2-deoxy-D-glucose (2-DG) uptake and on extracellular purines metabolism in cultured rat podocytes. Basal 2-DG uptake was 2.7-fold enhanced in HG compared to normal glucose concentration, NG (1271 ± 86 vs. 477 ± 37 nmol/h/mg protein, P<0.001). ATP stimulated 2-DG uptake by 44 ± 4% and 29 ± 5% in NG and HG, respectively. ATP analogues, β, γ-methylene ATP and 2-methylthio ATP stimulated 2-DG uptake in range of 18-34% in NG and 16-17% in HG. Benzoylbenzoyl ATP increased 2-DG uptake about 24 ± 2% in NG however, its effect in HG reached 50 ± 1%. The antagonists of P2 receptors (suramin, reactive blue 2, PPADS) decreased basal 2-DG uptake in NG and HG; suramin and reactive blue 2 at average of 15 ± 4% in NG but in HG the effect was in following order: suramin 28 ± 3%; PPADS 20 ± 3% and RB-2 9 ± 0.9%. Extracellular adenosine concentration was higher in HG than in NG (0.48 ± 0.01 vs. 5.05 ± 0.39 μM, P < 0.05), however intracellular ATP content and extracellular ATP concentration were not affected. Neither ecto-ATPase nor ecto-5'-nucleotidase activities were affected in HG. In conclusion, diabetic milieu affects purinergic modulation of glucose transport into podocytes which may play a role in development of diabetic podocytopathy.

Lack of the purinergic receptor P2X(7) results in resistance to contact hypersensitivity

Engagement of P2X₇ on mouse dendritic cells, presumably by ATP released in response to contact allergen, is needed for IL-1β production and the sensitization phase of contact hypersensitivity.

Sensitization to contact allergens requires activation of the innate immune system by endogenous danger signals. However, the mechanisms through which contact allergens activate innate signaling pathways are incompletely understood. In this study, we demonstrate that mice lacking the adenosine triphosphate (ATP) receptor P2X₇ are resistant to contact hypersensitivity (CHS). P2X₇-deficient dendritic cells fail to induce sensitization to contact allergens and do not release IL-1β in response to lipopolysaccharide (LPS) and ATP. These defects are restored by pretreatment with LPS and alum in an NLRP3- and ASC-dependent manner. Whereas pretreatment of wild-type mice with P2X₇ antagonists, the ATP-degrading enzyme apyrase or IL-1 receptor antagonist, prevents CHS, IL-1β injection restores CHS in P2X₇-deficient mice. Thus, P2X₇ is a crucial receptor for extracellular ATP released in skin in response to contact allergens. The lack of P2X₇ triggering prevents IL-1β release, which is an essential step in the sensitization process. Interference with P2X₇ signaling may be a promising strategy for the prevention of allergic contact dermatitis.

A method for isolation of rat lymphocyte-rich mononuclear cells from lung tissue useful for determination of nucleoside triphosphate diphosphohydrolase activity

Methods for the isolation of peripheral blood mononuclear cells (PBMCs) and human lung mononuclear cells (LMCs) have been proposed previously. This study describes a method that allows the separation of lymphocyte-rich LMCs from rats. Trypan blue was applied to determine cell viability. White blood cell and differential cell counts were also performed. Relationships between nucleoside triphosphate diphosphohydrolase (NTPDase, EC 3.6.1.5) activities expressed in milligrams of protein, millions of cells, and millions of viable cells were examined as linear correlations. The lung tissue yielded 82.46% lymphocytes, 8.6% macrophages, 2.20% monocytes, and 1.27% polymorphonuclear cells (PMNs). In LMCs, a very strong correlation was observed as follows: between NTPDase activity, as determined using ATP or ADP as a substrate, expressed in milligrams of protein and that expressed in millions of cells (r ≥ 0.91), between that expressed in milligrams of protein and that expressed in millions of viable cells (r ≥ 0.91), and between that expressed in millions of cells and that expressed in millions of viable cells (r ≥ 0.98). Based on our results, we affirm that NTPDase activity could be expressed in millions of viable cells, millions of cells, or milligrams of protein.

Ebselen is a potent non-competitive inhibitor of extracellular nucleoside diphosphokinase

Nucleoside di- and triphosphates and adenosine regulate several components of the mucocilairy clearance process (MCC) that protects the lung against infections, via activation of epithelial purinergic receptors. However, assessing the contribution of individual nucleotides to MCC functions remains difficult due to the complexity of the mechanisms of nucleotide release and metabolism. Enzymatic activities involved in the metabolism of extracellular nucleotides include ecto-ATPases and secreted nucleoside diphosphokinase (NDPK) and adenyl kinase, but potent and selective inhibitors of these activities are sparse. In the present study, we discovered that ebselen markedly reduced NDPK activity while having negligible effect on ecto-ATPase and adenyl kinase activities. Addition of radiotracer [γ(32)P]ATP to human bronchial epithelial (HBE) cells resulted in rapid and robust accumulation of [(32)P]-inorganic phosphate ((32)Pi). Inclusion of UDP in the incubation medium resulted in conversion of [γ(32)P]ATP to [(32)P]UTP, while inclusion of AMP resulted in conversion of [γ(32)P]ATP to [(32)P]ADP. Ebselen markedly reduced [(32)P]UTP formation but displayed negligible effect on (32)Pi or [(32)P]ADP accumulations. Incubation of HBE cells with unlabeled UTP and ADP resulted in robust ebselen-sensitive formation of ATP (IC(50) = 6.9 ± 2 μM). This NDPK activity was largely recovered in HBE cell secretions and supernatants from lung epithelial A549 cells. Kinetic analysis of NDPK activity indicated that ebselen reduced the V(max) of the reaction (K(i) = 7.6 ± 3 μM), having negligible effect on K(M) values. Our study demonstrates that ebselen is a potent non-competitive inhibitor of extracellular NDPK.

Activation of the transcription factor FosB/activating protein-1 (AP-1) is a prominent downstream signal of the extracellular nucleotide receptor P2RX7 in monocytic and osteoblastic cells

Activation of the ionotropic P2RX7 nucleotide receptor by extracellular ATP has been implicated in modulating inflammatory disease progression. Continuous exposure of P2RX7 to ligand can result in apoptosis in many cell types, including monocytic cells, whereas transient activation of P2RX7 is linked to inflammatory mediator production and the promotion of cell growth. Given the rapid hydrolysis of ATP in the circulation and interstitial space, transient activation of P2RX7 appears critically important for its action, yet its effects on gene expression are unclear. The present study demonstrates that short-term stimulation of human and mouse monocytic cells as well as mouse osteoblasts with P2RX7 agonists substantially induces the expression of several activating protein-1 (AP-1) members, particularly FosB. The potent activation of FosB after P2RX7 stimulation is especially noteworthy considering that little is known concerning the role of FosB in immunological regulation. Interestingly, the magnitude of FosB activation induced by P2RX7 stimulation appears greater than that observed with other known inducers of FosB expression. In addition, we have identified a previously unrecognized role for FosB in osteoblasts with respect to nucleotide-induced expression of cyclooxygenase-2 (COX-2), which is the rate-limiting enzyme in prostaglandin biosynthesis from arachidonic acid and is critical for osteoblastic differentiation and immune behavior. The present studies are the first to link P2RX7 action to FosB/AP-1 regulation in multiple cell types, including a role in nucleotide-induced COX-2 expression, and support a role for FosB in the control of immune and osteogenic function by P2RX7.

Extracellular ATP signaling in plants

Extracellular adenosine-5'-triphosphate (ATP) induces a number of cellular responses in plants and animals. Some of the molecular components for purinergic signaling in animal cells appear to be lacking in plant cells, although some cellular responses are similar in both systems [e.g. increased levels of cytosolic free calcium, nitric oxide (NO), and reactive oxygen species (ROS)]. The purpose of this review is to compare and contrast purinergic signaling mechanisms in animal and plant cells. This comparison will aid our overall understanding of plant physiology and also provide details of the general fundamentals of extracellular ATP signaling in eukaryotes.

Extracellular high dosages of adenosine triphosphate induce inflammatory response and insulin resistance in rat adipocytes

Adenosine triphosphate (ATP), an important signaling molecule, participates in various pathophysiological processes via the activation of purinergic-receptors. Recent studies have shown that the expression and function of purinergic-receptors (P2-receptors) could be altered in diabetic or hyperinsulinemia conditions. To characterize the effect of ATP on insulin signaling, we treated primary rat adipocytes with varied concentrations of ATP. The pre-treatment led to impaired insulin signaling, i.e., blunted phosphorylation in Insulin Receptor Substrate-1 (IRS-1) tyrosine and Protein Kinase B (PKB) Ser473 in response to insulin treatment, when ATP concentration reached 1mM. We then observed that ATP dose-dependently reduced the level of IκB, a negative regulator of inflammatory response. Consistently, IRS-1 Ser307 phosphorylation in response to insulin treatment, a site for inflammatory pathway to interfere insulin signaling, was enhanced by ATP. Furthermore, effects of ATP on insulin signaling and IκB content were blocked by P2-receptor inhibition. Finally, insulin-stimulated glucose uptake was impaired by ATP in adipocytes but not in the L6 muscle cells. This study therefore shows for the first time the involvement of ATP-evoked P2-receptor activation in mediating the inflammatory response and the generation of insulin resistance in adipocytes.

Extracellular nucleotides elicit cytosolic free calcium oscillations in Arabidopsis

Extracellular ATP induces a rise in the level of cytosolic free calcium ([Ca(2+)](cyt)) in plant cells. To expand our knowledge about the function of extracellular nucleotides in plants, the effects of several nucleotide analogs and pharmacological agents on [Ca(2+)](cyt) changes were studied using transgenic Arabidopsis (Arabidopsis thaliana) expressing aequorin or the fluorescence resonance energy transfer-based Ca(2+) sensor Yellow Cameleon 3.6. Exogenously applied CTP caused elevations in [Ca(2+)](cyt) that displayed distinct time- and dose-dependent kinetics compared with the purine nucleotides ATP and GTP. The inhibitory effects of antagonists of mammalian P2 receptors and calcium influx inhibitors on nucleotide-induced [Ca(2+)](cyt) elevations were distinct between CTP and purine nucleotides. These results suggest that distinct recognition systems may exist for the respective types of nucleotides. Interestingly, a mutant lacking the heterotrimeric G protein Gβ-subunit exhibited a remarkably higher [Ca(2+)](cyt) elevation in response to all tested nucleotides in comparison with the wild type. These data suggest a role for Gβ in negatively regulating extracellular nucleotide signaling and point to an important role for heterotrimeric G proteins in modulating the cellular effects of extracellular nucleotides. The addition of extracellular nucleotides induced multiple temporal [Ca(2+)](cyt) oscillations, which could be localized to specific root cells. The oscillations were attenuated by a vesicle-trafficking inhibitor, indicating that the oscillations likely require ATP release via exocytotic secretion. The results reveal new molecular details concerning extracellular nucleotide signaling in plants and the importance of fine control of extracellular nucleotide levels to mediate specific plant cell responses.

Real-time monitoring of suspension cell-cell communication using an integrated microfluidics

For the first time, we have developed a microfluidic device for on-chip monitoring of suspension cell-cell communication from stimulated to recipient HL-60 cells. A deformable PDMS membrane was developed as a compressive component to perform cell entrapment and exert different modes of mechanical stimulation. The number of cells trapped by this component could be modulated by flushing excessive cells towards the device outlet. The trapped cells could be triggered to release mediators by mechanical stimulation. Sandbag microstructures were used to immobilize recipient cells at well-defined positions. These recipient cells were evoked by mediators released from mechanically stimulated cells trapped in the compressive component. Normally closed microvalves were integrated to provide continuous-flow and static environment. We studied cell-cell communication between stimulated (in compressive component) and recipient (in sandbag structures) cells. Calcium oscillations were observed in some recipient cells only when a low number of cells were stimulated. Different mechanical stimulation and flow environment were also employed to study their impact on the behavior of cell-cell communication. We observed that both the duration and intensity of intracellular calcium responses increased in persistent stimulation and decreased in flowing environment. This microdevice may open up new avenues for real-time monitoring of suspension cell-cell communication, which propagates via gap-junction independent mechanism, with multiple variables under control.

Bothrops snake myotoxins induce a large efflux of ATP and potassium with spreading of cell damage and pain

Myotoxins play a major role in the pathogenesis of the envenomations caused by snake bites in large parts of the world where this is a very relevant public health problem. We show here that two myotoxins that are major constituents of the venom of Bothrops asper, a deadly snake present in Latin America, induce the release of large amounts of K(+) and ATP from skeletal muscle. We also show that the released ATP amplifies the effect of the myotoxins, acting as a "danger signal," which spreads and causes further damage by acting on purinergic receptors. In addition, the release of ATP and K(+) well accounts for the pain reaction characteristic of these envenomations. As Bothrops asper myotoxins are representative of a large family of snake myotoxins with phospholipase A(2) structure, these findings are expected to be of general significance for snake bite envenomation. Moreover, they suggest potential therapeutic approaches for limiting the extent of muscle tissue damage based on antipurinergic drugs.

AVP-stimulated nucleotide secretion in perfused mouse medullary thick ascending limb and cortical collecting duct

Extracellular nucleotides are local, short-lived signaling molecules that inhibit renal tubular transport via both luminal and basolateral P2 receptors (1, 2). Apparently, the renal epithelium itself is able to release nucleotides (3, 4). The mechanism and circumstances under which epithelia nucleotide release is stimulated remains elusive (5, 6). Here, we investigate the phenomenon of nucleotide secretion in intact perfused mouse medullary thick ascending limb (mTAL) and cortical collecting duct (CCD). The nucleotide secretion was monitored by a biosensor cell placed to register nucleotides in the tubular out-flow. [Ca(2+)](i) was measured simultaneously in the biosensor cells and the renal tubule with fluo-4. We were able to identify spontaneous tubular nucleotide secretion in resting perfused mTAL. This was seen as lively [Ca(2+)](i) oscillations in the nucleotide biosensor cells when the tubular outflow fluid engulfed the sensing cells. In mouse mTAL 10 nM AVP and dDAVP induced robust [Ca(2+)](i) oscillations, whereas AVP in the CCD induced large, slow and transient [Ca(2+)](i) elevations. Importantly, we identify that AVP/dDAVP triggers tubular secretion of nucleotides in mTAL. After addition of AVP/dDAVP the biosensor cells registered bursts of nucleotides originating from the tubular perfusate. The approximated tubular nucleotide concentration reached peak values of approximately 0.2-0.3 microM. A very similar response was observed after AVP stimulation of CCDs. Thus, AVP stimulated tubular secretion of nucleotides in a burst like pattern with peak tubular nucleotide concentrations in the low micromolar range. Luminal nucleotides are prone to activate luminal P2 receptors which in turn are well described to inhibit AVP-augmented aquaporin-2-dependent water absorption or ENaC-mediated Na(+) transport (8). Therefore, we speculate that local nucleotide signaling is an intrinsic feed-back element of hormonal control of renal tubular transport.

Selective depletion of CD4+CD25+Foxp3+ regulatory T cells by low-dose cyclophosphamide is explained by reduced intracellular ATP levels

CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells have been shown to play important roles in mediating cancer development. Although cyclophosphamide (CY) has shown promise as a drug to selectively target Treg cells with low-dose in vivo, the underlying molecular mechanism remains unclear. In this report, we provide evidence that ATP, the energy molecule and signal element, accounts for the selective depletion of Treg cells by low-dose CY. Relative to conventional T cells or other cell types, ATP levels were much lower in Treg cells. This was due to Treg cells that downregulate one microRNA, miR-142-3p, and upregulate ecto-nucleoside triphosphate diphosphohydrolase CD39. The transfection of miR-142-3p or the blockade of CD39 could increase intracellular ATP levels of Treg cells, consequently decreasing the sensitivity of Treg cells to low-dose CY. On the other hand, the transfection of miR-142-3p inhibitor or the addition of soluble CD39 to the cultured CD4(+)CD25(-) T cells resulted in the decrease of intracellular ATP levels and increase of sensitivity of conventional T cells to low-dose CY. Furthermore, we found that the low levels of ATP attenuated the synthesis of glutathione, leading to the decrease of CY detoxification, thus increasing the sensitivity of Treg cells to low-dose CY. Therefore, we here identify a molecular pathway through which low-dose CY selectively ablates Treg cells. Our findings also imply that low levels of ATP are probably related to Treg cell function.

Differential expression of P2Y receptors in the rat cochlea during development

Purinergic signaling has broad physiological significance to the hearing organ, involving signal transduction via ionotropic P2X receptors and metabotropic G-protein-coupled P2Y and P1 (adenosine), alongside conversion of nucleotides and nucleosides by ecto-nucleotidases and ecto-nucleoside diphosphokinase. In addition, ATP release is modulated by acoustic overstimulation or stress and involves feedback regulation. Many of these principal elements of the purinergic signaling complex have been well characterized in the cochlea, while the characterization of P2Y receptor expression is emerging. The present study used immunohistochemistry to evaluate the expression of five P2Y receptors, P2Y(1), P2Y(2), P2Y(4), P2Y(6), and P2Y(12), during development of the rat cochlea. Commencing in the late embryonic period, the P2Y receptors studied were found in the cells lining the cochlear partition, associated with establishment of the electrochemical environment which provides the driving force for sound transduction. In addition, early postnatal P2Y(2) and P2Y(4) protein expression in the greater epithelial ridge, part of the developing hearing organ, supports the view that initiation and regulation of spontaneous activity in the hair cells prior to hearing onset is mediated by purinergic signaling. Sub-cellular compartmentalization of P2Y receptor expression in sensory hair cells, and diversity of receptor expression in the spiral ganglion neurons and their satellite cells, indicates roles for P2Y receptor-mediated Ca(2+)-signaling in sound transduction and auditory neuron excitability. Overall, the dynamics of P2Y receptor expression during development of the cochlea complement the other elements of the purinergic signaling complex and reinforce the significance of extracellular nucleotide and nucleoside signaling to hearing.

Structural, metabolic and developmental evaluation of ovulated rabbit oocytes before and after cryopreservation by vitrification and slow freezing

The cryopreservation of oocytes is valuable for the preservation of women's fertility and might also be an interesting tool to preserve animal genetic biodiversity but it is not often used because of the very poor fertility recovered after thawing, especially in rabbit species. The objective of our study was to evaluate the effect of slow-freezing and vitrification on the structural integrity of ovulated rabbit oocytes, their ATP contents, and their developmental competence. Results show that, whatever the method is used, cryopreservation has a dramatic effect on the metabolic integrity, the structural integrity, and the developmental ability of the oocytes. Vitrification and slow freezing both impair the rabbit oocytes viability after thawing but the processes act differently. Further studies are needed to improve the cryopreservation techniques in rabbit species. Moreover, we underlined that morphology and maintenance of the structural integrity of the oocytes are not suitable enough to assess the potential for further development of cryopreserved M(II) oocytes. The assessment of ATP metabolism allows efficient evaluation of the viability of the frozen or vitrified oocytes. It should be used in addition to parthenogenesis to better assess the potential for further development.

Extracellular ATP is a danger signal activating P2X7 receptor in lung inflammation and fibrosis

RATIONALE

Pulmonary fibrosis is a devastating as yet untreatable disease. We previously investigated the endogenous mediators released on lung injury and showed that uric acid is a danger signal activating Nod-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome in lung inflammation and fibrosis (Gasse et al., Am J Respir Crit Care Med 2009;179:903-913).

OBJECTIVES

Here we address the role of extracellular adenosine triphosphate (eATP) in pulmonary inflammation and fibrosis.

METHODS

ATP was quantified in bronchoalveolar lavage fluid (BALF) of control subjects and patients with idiopathic pulmonary fibrosis. The contribution of eATP as a danger signal was assessed in a murine model of lung fibrosis induced by airway-administered bleomycin (BLM), an intercalating agent that causes DNA strand breaks.

MEASUREMENTS AND MAIN RESULTS

Fibrotic patients have elevated ATP content in BALF in comparison with control individuals. In mice, we report an early increase in eATP levels in BALF on BLM administration. Modulation of eATP levels with the ATP-degrading enzyme apyrase greatly reduced BLM-induced inflammatory cell recruitment, lung IL-1β, and tissue inhibitor of metalloproteinase (TIMP)-1 production, while administration of ATP-γS, a stable ATP derivative, enhanced inflammation. P2X(7) receptor-deficient mice presented dramatically reduced lung inflammation, with reduced fibrosis markers such as lung collagen content and matrix-remodeling proteins TIMP-1 and matrix metalloproteinase-9. The acute inflammation depends on a functional pannexin-1 hemichannel protein. In vitro, ATP is released by pulmonary epithelial cells on BLM-induced stress and this is partly dependent on the presence of functional P2X(7) receptor and pannexin-1 hemichannel.

CONCLUSIONS

ATP released from BLM-injured lung cells constitutes a major endogenous danger signal that engages the P2X(7) receptor/pannexin-1 axis, leading to IL-1β maturation and lung fibrosis.

P2Y nucleotide receptors: promise of therapeutic applications

Extracellular nucleotides, such as ATP and UTP, have distinct signaling roles through a class of G-protein-coupled receptors, termed P2Y. The receptor ligands are typically charged molecules of low bioavailability and stability in vivo. Recent progress in the development of selective agonists and antagonists for P2Y receptors and study of knockout mice have led to new drug concepts based on these receptors. The rapidly accelerating progress in this field has already resulted in drug candidates for cystic fibrosis, dry eye disease and thrombosis. On the horizon are novel treatments for cardiovascular diseases, inflammatory diseases and neurodegeneration.

Purinergic receptor inhibition prevents the development of smoke-induced lung injury and emphysema

Extracellular ATP acts as a "danger signal" and can induce inflammation by binding to purinergic receptors. Chronic obstructive pulmonary disease is one of the most common inflammatory diseases associated with cigarette smoke inhalation, but the underlying mechanisms are incompletely understood. In this study, we show that endogenous pulmonary ATP levels are increased in a mouse model of smoke-induced acute lung inflammation and emphysema. ATP neutralization or nonspecific P2R-blockade markedly reduced smoke-induced lung inflammation and emphysema. We detected an upregulation the purinergic receptors subtypes on neutrophils (e.g., P2Y2R), macrophages, and lung tissue from animals with smoke-induced lung inflammation. By using P2Y(2)R deficient ((-/-)) animals, we show that ATP induces the recruitment of blood neutrophils to the lungs via P2Y(2)R. Moreover, P2Y(2)R deficient animals had a reduced pulmonary inflammation following acute smoke-exposure. A series of experiments with P2Y(2)R(-/-) and wild type chimera animals revealed that P2Y(2)R expression on hematopoietic cell plays the pivotal role in the observed effect. We demonstrate, for the first time, that endogenous ATP contributes to smoke-induced lung inflammation and then development of emphysema via activation of the purinergic receptor subtypes, such as P2Y(2)R.

Signalling and pharmacological properties of the P2Y receptor

The P2Y(14) receptor is a relatively broadly expressed G protein-coupled receptor that is prominently associated with immune and inflammatory cells as well as with many epithelia. This receptor historically was thought to be activated selectively by UDP-glucose and other UDP-sugars. However, UDP is also a very potent agonist of this receptor, and may prove to be one of its most important cognate activators.

Crystal structure of a Legionella pneumophila ecto -triphosphate diphosphohydrolase, a structural and functional homolog of the eukaryotic NTPDases

Many pathogenic bacteria have sophisticated mechanisms to interfere with the mammalian immune response. These include the disruption of host extracellular ATP levels that, in humans, is tightly regulated by the nucleoside triphosphate diphosphohydrolase family (NTPDases). NTPDases are found almost exclusively in eukaryotes, the notable exception being their presence in some pathogenic prokaryotes. To address the function of bacterial NTPDases, we describe the structures of an NTPDase from the pathogen Legionella pneumophila (Lpg1905/Lp1NTPDase) in its apo state and in complex with the ATP analog AMPPNP and the subtype-specific NTPDase inhibitor ARL 67156. Lp1NTPDase is structurally and catalytically related to eukaryotic NTPDases and the structure provides a basis for NTPDase-specific inhibition. Furthermore, we demonstrate that the activity of Lp1NTPDase correlates directly with intracellular replication of Legionella within macrophages. Collectively, these findings provide insight into the mechanism of this enzyme and highlight its role in host-pathogen interactions.

Intrarenal purinergic signaling in the control of renal tubular transport

Renal tubular epithelial cells receive hormonal input that regulates volume and electrolyte homeostasis. In addition, numerous intrarenal, local signaling agonists have appeared on the stage of renal physiology. One such system is that of intrarenal purinergic signaling. This system involves all the elements necessary for agonist-mediated intercellular communication. ATP is released from epithelial cells, which activates P2 receptors in the apical and basolateral membrane and thereby modulates tubular transport. Termination of the signal is conducted via the breakdown of ATP to adenosine. Recent far-reaching advances indicate that ATP is often used as a local transmitter for classical sensory transduction. This transmission apparently also applies to sensory functions in the kidney. Locally released ATP is involved in sensing of renal tubular flow or in detecting the distal tubular load of NaCl at the macula densa. This review describes the relevant aspects of local, intrarenal purinergic signaling and outlines its integrative concepts.

[Identification of novel molecules regulating differentiation and hormone secretion and clarification of their functional mechanisms in pancreatic endocrine cells]

In order to find novel bioactive molecules regulating differentiation and hormone secretion of pancreatic endocrine cells, the effects of various substances including purinergic receptor agonists and inhibitors of polyamine biosynthesis were examined in pancreatic islets and several pancreatic cell lines. The nicotinic alpha3beta4 receptor was found to be present and capable of increasing cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) and insulin secretion in mouse pancreatic Beta-TC6 cells. Activation of both nicotinic and muscarinic M(3)/M(4) receptors resulted in reduction of insulin release when compared with stimulation of muscarinic receptor alone in Beta-TC6 cells. In mouse islets, purinergic P2Y(1) and P2Y(6) receptors, which are coupled to Gq proteins, were expressed and appeared to regulate insulin secretion through Ca(2+) mobilization from intracellular stores. Similar results were observed in Beta-TC6 cells. Spermidine, one of polyamines, was found to modulate insulin synthesis and [Ca(2+)](i) in Beta-TC6 cells by use of a specific spermidine synthesis inhibitor, trans-4-methylcyclohexylamine (MCHA). Antizyme, which binds to ornithine decarboxylase (ODC) and thereby reduces the cellular polyamine level, was found to be necessary for conversion of ASPC-1 cells, a pancreatic ductal tumor cell line, into alpha-cells forming the islet-like structure and expressing glucagon gene. These findings help advance our understanding of the complex mechanisms involved in the regulation of pancreatic endocrine cell function and develop new therapeutic agents in diabetes mellitus.

Farnesyl pyrophosphate is a novel pain-producing molecule via specific activation of TRPV3

Temperature-sensitive transient receptor potential ion channels (thermoTRPs) expressed in epidermal keratinocytes and sensory afferents play an important role as peripheral pain detectors for our body. Many natural and synthetic compounds have been found to act on the thermoTRPs leading to altered nociception, but little is known about endogenous painful molecules activating TRPV3. Here, we show that farnesyl pyrophosphate (FPP), an intermediate metabolite in the mevalonate pathway, specifically activates TRPV3 among six thermoTRPs using Ca(2+) imaging and electrophysiology with cultured keratinocytes and TRPV3-overexpressing cells. Agonistic potencies of related compounds in the FPP metabolism were ignorable. Voltage-dependence of TRPV3 was shifted by FPP, which appears to be the activation mechanism. An intraplantar injection of FPP acutely elicits nociceptive behaviors in inflamed animals, indicating that FPP is a novel endogenous pain-producing substance via TRPV3 activation. Co-culture experiments demonstrated that this FPP-evoked signal in the keratinocytes is transmitted to sensory neurons. In addition, FPP reduced TRPV3 heat threshold resulting in heightened behavioral sensitivity to noxious heat. Taken together, our data suggest that FPP is the firstly identified endogenous TRPV3 activator that causes nociception. Our results may provide useful chemical information to elucidate TRPV3 physiology and novel pain-related metabolisms.

Negative feedback of extracellular ADP on ATP release in goldfish hepatocytes: a theoretical study

A mathematical model was built to account for the kinetic of extracellular ATP (ATPe) and extracellular ADP (ADPe) concentrations from goldfish hepatocytes exposed to hypotonicity. The model was based on previous experimental results on the time course of ATPe accumulation, ectoATPase activity, and cell viability [Pafundo et al., 2008]. The kinetic of ATPe is controlled by a lytic ATP flux, a non-lytic ATP flux, and ecto-ATPase activity, whereas ADPe kinetic is governed by a lytic ADP flux and both ecto-ATPase and ecto-ADPase activities. Non-lytic ATPe efflux was included as a diffusion equation modulated by ATPe activation (positive feedback) and ADPe inhibition (negative feedback). The model yielded physically meaningful and stable steady-state solutions, was able to fit the experimental time evolution of ATPe and simulated the concomitant kinetic of ADPe. According to the model during the first minute of hypotonicity the concentration of ATPe is mainly governed by both lytic and non-lytic ATP efflux, with almost no contribution from ecto-ATPase activity. Later on, ecto-ATPase activity becomes important in defining the time dependent decay of ATPe levels. ADPe inhibition of the non-lytic ATP efflux was strong, whereas ATPe activation was minimal. Finally, the model was able to predict the consequences of partial inhibition of ecto-ATPase activity on the ATPe kinetic, thus emulating the exposure of goldfish cells to hypotonic medium in the presence of the ATP analog AMP-PCP. The model predicts this analog to both inhibit ectoATPase activity and increase non-lytic ATP release.

Regulation of vascular smooth muscle cell calcification by extracellular pyrophosphate homeostasis: synergistic modulation by cyclic AMP and hyperphosphatemia

Vascular calcification is a multifaceted process involving gain of calcification inducers and loss of calcification inhibitors. One such inhibitor is inorganic pyrophosphate (PP(i)), and regulated generation and homeostasis of extracellular PP(i) is a critical determinant of soft-tissue mineralization. We recently described an autocrine mechanism of extracellular PP(i) generation in cultured rat aortic vascular smooth muscle cells (VSMC) that involves both ATP release coupled to the ectophosphodiesterase/pyrophosphatase ENPP1 and efflux of intracellular PP(i) mediated or regulated by the plasma membrane protein ANK. We now report that increased cAMP signaling and elevated extracellular inorganic phosphate (P(i)) act synergistically to induce calcification of these VSMC that is correlated with progressive reduction in ability to accumulate extracellular PP(i). Attenuated PP(i) accumulation was mediated in part by cAMP-dependent decrease in ANK expression coordinated with cAMP-dependent increase in expression of TNAP, the tissue nonselective alkaline phosphatase that degrades PP(i). Stimulation of cAMP signaling did not alter ATP release or ENPP1 expression, and the cAMP-induced changes in ANK and TNAP expression were not sufficient to induce calcification. Elevated extracellular P(i) alone elicited only minor calcification and no significant changes in ANK, TNAP, or ENPP1. In contrast, combined with a cAMP stimulus, elevated P(i) induced decreases in the ATP release pathway(s) that supports ENPP1 activity; this resulted in markedly reduced rates of PP(i) accumulation that facilitated robust calcification. Calcified VSMC were characterized by maintained expression of multiple SMC differentiation marker proteins including smooth muscle (SM) alpha-actin, SM22alpha, and calponin. Notably, addition of exogenous ATP (or PP(i) per se) rescued cAMP + phosphate-treated VSMC cultures from progression to the calcified state. These observations support a model in which extracellular PP(i) generation mediated by both ANK- and ATP release-dependent mechanisms serves as a critical regulator of VSMC calcification.