Pharmacological profiles of cloned mammalian P2Y-receptor subtypes

Nucleotide-mediated airway clearance

A thin layer of airway surface liquid (ASL) lines the entire surface of the lung and is the first point of contact between the lung and the environment. Surfactants contained within this layer are secreted in the alveolar region and are required to maintain a low surface tension and to prevent alveolar collapse. Mucins are secreted into the ASL throughout the respiratory tract and serve to intercept inhaled pathogens, allergens and toxins. Their removal by mucociliary clearance (MCC) is facilitated by cilia beating and hydration of the ASL by active ion transport. Throughout the lung, secretion, ion transport and cilia beating are under purinergic control. Pulmonary epithelia release ATP into the ASL which acts in an autocrine fashion on P2Y(2) (ATP) receptors. The enzymatic network describes in Chap. 2 then mounts a secondary wave of signaling by surface conversion of ATP into adenosine (ADO), which induces A(2B) (ADO) receptor-mediated responses. This chapter offers a comprehensive description of MCC and the extensive ramifications of the purinergic signaling network on pulmonary surfaces.

Purinergic receptor-mediated Ca signaling in the olfactory bulb and the neurogenic area of the lateral ventricles

Like in other vertebrates, the anterior part of the telencephalon of amphibians mainly consists of the olfactory bulb (OB), but different from higher vertebrates, the lateral telencephalic ventricles of larval Xenopus laevis expand deep into the anterior telencephalon. The neurogenic periventricular zone (PVZ) of the lateral ventricles generates new OB neurons throughout the animal's lifetime. We investigated the ultrastructural organization of the PVZ and found that within a time period of 24 h, 42.54 ± 6.65% of all PVZ cells were actively proliferating. Functional purinergic receptors are widespread in the central nervous system and their activation has been associated with many critical physiological processes, including the regulation of cell proliferation. In the present study we identified and characterized the purinergic system of the OB and the PVZ. ATP and 2MeSATP induced strong [Ca(2+)](i) increases in cells of both regions, which could be attenuated by purinergic antagonists. However, a more thorough pharmacological investigation revealed clear differences between the two brain regions. Cells of the OB almost exclusively express ionotropic P2X purinergic receptor subtypes, whereas PVZ cells express both ionotropic P2X and metabotropic P1 and P2Y receptor subtypes. The P2X receptors expressed in the OB are evidently not involved in the immediate processing of olfactory information.

A₂B receptors mediate the induction of early genes and inhibition of arterial smooth muscle cell proliferation via Epac

AIMS

Extracellular adenosine and adenine nucleotides play important roles in the regulation of the blood vessel tonus and platelet aggregation. Less is known about the effects of these extracellular signalling molecules on gene expression in vascular smooth muscle cells involved in long-term vascular effects. In the present study, we therefore searched for adenosine-induced changes in the expression of early genes in cultured human coronary artery smooth muscle cells (HCASMCs).

METHODS AND RESULTS

Whole-genome DNA array hybridization revealed that adenosine induced a set of early genes including the nuclear receptor subfamily 4, group A, member 1 (NR4A1/Nur77/TR3). The pattern of the effects of adenosine on gene expression resembles the change in expression induced by the direct activator of adenylate cyclase forskolin. Real-time reverse-transcriptase PCR confirmed that adenosine and its analogue N-ethyl-carboxamidoadenosine elicited a strong induction of NR4A1. These effects were markedly attenuated by A(2B) receptor antagonists including 8-[4-(4-benzylpiperazide-1-sulfonyl)phenyl]-1-propylxanthine (PSB-601) and were mimicked by a cyclic AMP (cAMP) analogue [8-(4-chlorophenylthio)-2'-O-methyl-cAMP, 8CPT] acting on the exchange protein activated by cAMP (Epac). Long-term experiments over 5 days showed that 2-chloroadenosine decreased cell proliferation in the presence of platelet-derived growth factor. This effect of 2-chloroadenosine was also attenuated by PSB-601 and mimicked by 8CPT. Treatment with small interfering RNA directed against NR4A1 attenuated the inhibitory effect of 8CPT on proliferation.

CONCLUSIONS

In summary, our results demonstrate the operation of adenosine A₂(B) receptors mediating an early induction of NR4A1 and a decrease in cell proliferation via the cAMP/Epac pathway in HCASMCs.

Electrophysiological classification of P2X7 receptors in rat cultured neocortical astroglia

BACKGROUND AND PURPOSE

P2X7 receptors are ATP-gated cation channels mediating important functions in microglial cells, such as the release of cytokines and phagocytosis. Electrophysiological evidence that these receptors also occur in CNS astroglia is rare and rather incomplete.

EXPERIMENTAL APPROACH

We used whole-cell patch-clamp recordings to search for P2X7 receptors in astroglial-neuronal co-cultures prepared from the cerebral cortex of rats.

KEY RESULTS

All the astroglial cells investigated responded to ATP with membrane currents, reversing around 0 mV. These currents could be also detected in isolated outside-out patch vesicles. The results of the experiments with the P2X [alpha,beta-methylene ATP and 2'-3'-O-(4-benzoyl) ATP] and P2Y receptor agonists [adenosine 5'-O-(2-thiodiphosphate), uridine 5'-diphosphate, uridine 5'-triphosphate (UTP) and UDP-glucose] suggested the involvement of P2X receptors in this response. The potentiation of ATP responses in a low divalent cation or alkaline bath, but not by ivermectin, made it likely that a P2X7 receptor is operational. Blockade of the ATP effect by the P2X7 antagonists Brilliant Blue G, calmidazolium and oxidized ATP corroborated this assumption.

CONCLUSIONS AND IMPLICATIONS

Rat cultured cortical astroglia possesses functional P2X7 receptors. It is suggested that astrocytic P2X7 receptors respond to high local ATP concentrations during neuronal injury.

Microglial calcium signal acts as a rapid sensor of single neuron damage in vivo

In the healthy adult brain microglia, the main immune-competent cells of the CNS, have a distinct (so-called resting or surveying) phenotype. Resting microglia can only be studied in vivo since any isolation of brain tissue inevitably triggers microglial activation. Here we used in vivo two-photon imaging to obtain a first insight into Ca(2+) signaling in resting cortical microglia. The majority (80%) of microglial cells showed no spontaneous Ca(2+) transients at rest and in conditions of strong neuronal activity. However, they reliably responded with large, generalized Ca(2+) transients to damage of an individual neuron. These damage-induced responses had a short latency (0.4-4s) and were localized to the immediate vicinity of the damaged neuron (< 50 μm cell body-to-cell body distance). They were occluded by the application of ATPγS as well as UDP and 2-MeSADP, the agonists of metabotropic P2Y receptors, and they required Ca(2+) release from the intracellular Ca(2+) stores. Thus, our in vivo data suggest that microglial Ca(2+) signals occur mostly under pathological conditions and identify a Ca(2+) store-operated signal, which represents a very sensitive, rapid, and highly localized response of microglial cells to brain damage. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

The mechanism of the negative chronotropic and dromotropic actions of adenosine 5'-triphosphate in the heart: an update

Adenosine 5'-triphosphate (ATP) plays a critical role in intracellular metabolism and energetics. Extracellular ATP is rapidly degraded to adenosine by ectoenzymes. Both ATP and adenosine suppress cardiac pacemakers' automaticity and atrioventricular nodal conduction, albeit via the different mechanism of actions. This historical update summarizes the current knowledge regarding the negative chronotropic and dromotropic actions of ATP and discusses the clinical implications regarding the utility of ATP as a diagnostic and therapeutic agent in the management of neutrally mediated syncope and paroxysmal supra ventricular tachycardia.

Molecular mechanisms of nucleoside recycling in the brain

A major role of plasma membrane bound ectonucleotidases is the modulation of ATP, ADP, adenosine (the purinergic agonists), UTP, and UDP (the pyrimidinergic agonists) availability in the extracellular space at their respective receptors. We have recently shown that an ATP driven uridine-UTP cycle is operative in the brain, based on the strictly compartmentalized processes of uridine salvage to UTP and uridine generation from UTP, in which uptaken uridine is anabolized to UTP in the cytosol, and converted back to uridine in the extracellular space by the action of ectonucleotidases (Ipata et al. Int J Biochem Cell Biol 2010;42:932-7). In this paper we show that a similar cytidine-CTP cycle exists in rat brain. Since (i) brain relies on imported preformed nucleosides for the synthesis of nucleotides, RNA, nuclear and mitochondrial DNA, coenzymes, pyrimidine sugar- and lipid-conjugates and (ii) no specific pyrimidinergic receptors have been identified for cytidine and their nucleotides, our results, taken together with previous studies on the intra- and extracellular metabolic network of ATP, GTP, UTP, and their nucleosides in the brain (Barsotti and Ipata. Int J Biochem Cell Biol 2004;36:2214-25; Balestri et al. Neurochem Int 2007;50:517-23), strongly suggest that, apart from the modulation of ligand availability, ectonucleotidases may serve the process of local nucleoside recycling in the brain.

P2Y1 and P2Y13 purinergic receptors mediate Ca2+ signaling and proliferative responses in pulmonary artery vasa vasorum endothelial cells

Extracellular ATP and ADP have been shown to exhibit potent angiogenic effects on pulmonary artery adventitial vasa vasorum endothelial cells (VVEC). However, the molecular signaling mechanisms of extracellular nucleotide-mediated angiogenesis remain not fully elucidated. Since elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) is required for cell proliferation and occurs in response to extracellular nucleotides, this study was undertaken to delineate the purinergic receptor subtypes involved in Ca(2+) signaling and extracellular nucleotide-mediated mitogenic responses in VVEC. Our data indicate that stimulation of VVEC with extracellular ATP resulted in the elevation of [Ca(2+)](i) via Ca(2+) influx through plasma membrane channels as well as Ca(2+) mobilization from intracellular stores. Moreover, extracellular ATP induced simultaneous Ca(2+) responses in both cytosolic and nuclear compartments. An increase in [Ca(2+)](i) was observed in response to a wide range of purinergic receptor agonists, including ATP, ADP, ATPγS, ADPβS, UTP, UDP, 2-methylthio-ATP (MeSATP), 2-methylthio-ADP (MeSADP), and BzATP, but not adenosine, AMP, diadenosine tetraphosphate, αβMeATP, and βγMeATP. Using RT-PCR, we identified mRNA for the P2Y1, P2Y2, P2Y4, P2Y13, P2Y14, P2X2, P2X5, P2X7, A1, A2b, and A3 purinergic receptors in VVEC. Preincubation of VVEC with the P2Y1 selective antagonist MRS2179 and the P2Y13 selective antagonist MRS2211, as well as with pertussis toxin, attenuated at varying degrees agonist-induced intracellular Ca(2+) responses and activation of ERK1/2, Akt, and S6 ribosomal protein, indicating that P2Y1 and P2Y13 receptors play a major role in VVEC growth responses. Considering the broad physiological implications of purinergic signaling in the regulation of angiogenesis and vascular homeostasis, our findings suggest that P2Y1 and P2Y13 receptors may represent novel and specific targets for treatment of pathological vascular remodeling involving vasa vasorum expansion.

A new function for ATP: activating cardiac sympathetic afferents during myocardial ischemia

Myocardial ischemia activates cardiac sympathetic afferents leading to chest pain and reflex cardiovascular responses. Brief myocardial ischemia leads to ATP release in the interstitial space. Furthermore, exogenous ATP and α,β-methylene ATP (α,β-meATP), a P2X receptor agonist, stimulate cutaneous group III and IV sensory nerve fibers. The present study tested the hypothesis that endogenous ATP excites cardiac afferents during ischemia through activation of P2 receptors. Nerve activity of single unit cardiac sympathetic afferents was recorded from the left sympathetic chain or rami communicates (T(2)-T(5)) in anesthetized cats. Single fields of 45 afferents (conduction velocities = 0.25-4.92 m/s) were identified in the left ventricle with a stimulating electrode. Five minutes of myocardial ischemia stimulated 39 of 45 cardiac afferents (8 Aδ, 37 C fibers). Epicardial application of ATP (1-4 μmol) stimulated six ischemically sensitive cardiac afferents in a dose-dependent manner. Additionally, epicardial ATP (2 μmol), ADP (2 μmol), a P2Y agonist, and α,β-meATP (0.5 μmol) significantly activated eight other ischemically sensitive afferents. Third, pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid, a P2 receptor antagonist, abolished the responses of six afferents to epicardial ATP (2 μmol) and attenuated the ischemia-related increase in activity of seven other afferents by 37%. In the absence of P2 receptor blockade, cardiac afferents responded consistently to repeated application of ATP (n = 6) and to recurrent myocardial ischemia (n = 6). Finally, six ischemia-insensitive cardiac spinal afferents did not respond to epicardial ATP (2-4 μmol), although these afferents did respond to epicardial bradykinin. Taken together, these data indicate that, during ischemia, endogenously released ATP activates ischemia-sensitive, but not ischemia-insensitive, cardiac spinal afferents through stimulation of P2 receptors likely located on the cardiac sensory neurites.

G protein-coupled receptor kinase 2 and arrestin2 regulate arterial smooth muscle P2Y-purinoceptor signalling

AIMS

prolonged P2Y-receptor signalling can cause vasoconstriction leading to hypertension, vascular smooth muscle hypertrophy, and hyperplasia. G protein-coupled receptor signalling is negatively regulated by G protein-coupled receptor kinases (GRKs) and arrestin proteins, preventing prolonged or inappropriate signalling. This study investigates whether GRKs and arrestins regulate uridine 5'-triphosphate (UTP)-stimulated contractile signalling in adult Wistar rat mesenteric arterial smooth muscle cells (MSMCs).

METHODS AND RESULTS

mesenteric arteries contracted in response to UTP challenge: When an EC(50) UTP concentration (30 µM, 5 min) was added 5 min before (R(1)) and after (R(2)) the addition of a maximal UTP concentration (R(max): 100 µM, 5 min), R(2) responses were decreased relative to R(1), indicating desensitization. UTP-induced P2Y-receptor desensitization of phospholipase C signalling was studied in isolated MSMCs transfected with an inositol 1,4,5-trisphosphate biosensor and/or loaded with Ca(2+)-sensitive dyes. A similar protocol (R(1)/R(2) = 10 µM; R(max) = 100 µM, applied for 30 s) revealed markedly reduced R(2) when compared with R(1) responses. MSMCs were transfected with dominant-negative GRKs or siRNAs targeting specific GRK/arrestins to probe their respective roles in P2Y-receptor desensitization. GRK2 inhibition, but not GRK3, GRK5, or GRK6, attenuated P2Y-receptor desensitization. siRNA-mediated knockdown of arrestin2 attenuated UTP-stimulated P2Y-receptor desensitization, whereas arrestin3 depletion did not. Specific siRNA knockdown of the P2Y(2)-receptor almost completely abolished UTP-stimulated IP(3)/Ca(2+) signalling, strongly suggesting that our study is specifically characterizing this purinoceptor subtype.

CONCLUSION

these new data highlight roles of GRK2 and arrestin2 as important regulators of UTP-stimulated P2Y(2)-receptor responsiveness in resistance arteries, emphasizing their potential importance in regulating vasoconstrictor signalling pathways implicated in vascular disease.

FAD mutations in amyloid precursor protein do not directly perturb intracellular calcium homeostasis

Disturbances in intracellular calcium homeostasis are likely prominent and causative factors leading to neuronal cell death in Alzheimer's disease (AD). Familial AD (FAD) is early-onset and exhibits autosomal dominant inheritance. FAD-linked mutations have been found in the genes encoding the presenilins and amyloid precursor protein (APP). Several studies have shown that mutated presenilin proteins can directly affect calcium release from intracellular stores independently of Abeta production. Although less well established, there is also evidence that APP may directly modulate intracellular calcium homeostasis. Here, we directly examined whether overexpression of FAD-linked APP mutants alters intracellular calcium dynamics. In contrast to previous studies, we found that overexpression of mutant APP has no effects on basal cytosolic calcium, ER calcium store size or agonist-induced calcium release and subsequent entry. Thus, we conclude that mutated APP associated with FAD has no direct effect on intracellular calcium homeostasis independently of Abeta production.

P2Y1 receptors mediate an activation of neuronal calcium-dependent K+ channels

Molecularly defined P2Y receptor subtypes are known to regulate the functions of neurons through an inhibition of K(V)7 K(+) and Ca(V)2 Ca(2+) channels and via an activation or inhibition of Kir3 channels. Here, we searched for additional neuronal ion channels as targets for P2Y receptors. Rat P2Y(1) receptors were expressed in PC12 cells via an inducible expression system, and the effects of nucleotides on membrane currents and intracellular Ca(2+) were investigated. At a membrane potential of 30 mV, ADP induced transient outward currents in a concentration-dependent manner with half-maximal effects at 4 μm. These currents had reversal potentials close to the K(+) equilibrium potential and changed direction when extracellular Na(+) was largely replaced by K(+), but remained unaltered when extracellular Cl() was changed. Currents were abolished by P2Y(1) antagonists and by blockade of phospholipase C. ADP also caused rises in intracellular Ca(2+), and ADP-evoked currents were abolished when inositol trisphosphate-sensitive Ca(2+) stores were depleted. Blockers of K(Ca)2, but not those of K(Ca)1.1 or K(Ca)3.1, channels largely reduced ADP-evoked currents. In hippocampal neurons, ADP also triggered outward currents at 30 mV which were attenuated by P2Y(1) antagonists, depletion of Ca(2+) stores, or a blocker of K(Ca)2 channels. These results demonstrate that activation of neuronal P2Y(1) receptors may gate Ca(2+)-dependent K(+) (K(Ca)2) channels via phospholipase C-dependent increases in intracellular Ca(2+) and thereby define an additional class of neuronal ion channels as novel effectors for P2Y receptors. This mechanism may form the basis for the control of synaptic plasticity via P2Y(1) receptors.

Potentiation of the glutamatergic synaptic input to rat locus coeruleus neurons by P2X7 receptors

Locus coeruleus (LC) neurons in a rat brain slice preparation were superfused with a Mg(2+)-free and bicuculline-containing external medium. Under these conditions, glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) were recorded by means of the whole-cell patch-clamp method. ATP, as well as its structural analogue 2-methylthio ATP (2-MeSATP), both caused transient inward currents, which were outlasted by an increase in the frequency but not the amplitude of the sEPSCs. PPADS, but not suramin or reactive blue 2 counteracted both effects of 2-MeSATP. By contrast, α,β-methylene ATP (α,β-meATP), UTP and BzATP did not cause an inward current response. Of these latter agonists, only BzATP slightly facilitated the sEPSC amplitude and strongly potentiated its frequency. PPADS and Brilliant Blue G, as well as fluorocitric acid and aminoadipic acid prevented the activity of BzATP. Furthermore, BzATP caused a similar facilitation of the miniature (m)EPSC (recorded in the presence of tetrodotoxin) and sEPSC frequencies (recorded in its absence). Eventually, capsaicin augmented the frequency of the sEPSCs in a capsazepine-, but not PPADS-antagonizable, manner. In conclusion, the stimulation of astrocytic P2X7 receptors appears to lead to the outflow of a signalling molecule, which presynaptically increases the spontaneous release of glutamate onto LC neurons from their afferent fibre tracts. It is suggested, that the two algogenic compounds ATP and capsaicin utilise separate receptor systems to potentiate the release of glutamate and in consequence to increase the excitability of LC neurons.

P2X(7) receptor-mediated release of cathepsins from macrophages is a cytokine-independent mechanism potentially involved in joint diseases

The ATP-gated P2X(7) receptor (P2X(7)R) is a promising therapeutic target in chronic inflammatory diseases with highly specific antagonists currently under clinical trials for rheumatoid arthritis. Anti-inflammatory actions of P2X(7)R antagonists are considered to result from inhibition of P2X(7)R-induced release of proinflammatory cytokines from activated macrophages. However, P2X(7)Rs are also expressed in resting macrophages, suggesting that P2X(7)R may also signal via cytokine-independent mechanisms involved in joint disease. In this study, we examined P2X(7)R function in resting human lung macrophages and mouse bone marrow-derived macrophages and found that ATP induced rapid release of the lysosomal cysteine proteases cathepsin B, K, L, and S and that was independent of the presence of the proinflammatory cytokines IL-1beta and IL-18. Cathepsins released into the medium were effective to degrade collagen extracellular matrix. ATP-induced cathepsin release was abolished by P2X(7)R antagonists, absent from P2X(7)R(-/-) mouse macrophages, and not associated with cell death. Our results suggest P2X(7)R activation may play a novel and direct role in tissue damage through release of cathepsins independently of its proinflammatory actions via IL-1 cytokines.

Calcium-activated K(+) channel (K(Ca)3.1) activity during Ca(2+) store depletion and store-operated Ca(2+) entry in human macrophages

STIM1 'senses' decreases in endoplasmic reticular (ER) luminal Ca(2+) and induces store-operated Ca(2+) (SOC) entry through plasma membrane Orai channels. The Ca(2+)/calmodulin-activated K(+) channel K(Ca)3.1 (previously known as SK4) has been implicated as an 'amplifier' of the Ca(2+)-release activated Ca(2+) (CRAC) current, especially in T lymphocytes. We have previously shown that human macrophages express K(Ca)3.1, and here we used the whole-cell patch-clamp technique to investigate the activity of these channels during Ca(2+) store depletion and store-operated Ca(2+) influx. Using RT-PCR, we found that macrophages express the elementary CRAC channel components Orai1 and STIM1, as well as Orai2, Orai3 and STIM2, but not the putatively STIM1-activated channels TRPC1, TRPC3-7 or TRPV6. In whole-cell configuration, a robust Ca(2+)-induced outwardly rectifying K(+) current inhibited by clotrimazole and augmented by DC-EBIO could be detected, consistent with K(Ca)3.1 channel current (also known as intermediate-conductance IK1). Introduction of extracellular Ca(2+) following Ca(2+) store depletion via P2Y(2) receptors induced a robust charybdotoxin (CTX)- and 2-APB-sensitive outward K(+) current and hyperpolarization. We also found that SOC entry induced by thapsigargin treatment induced CTX-sensitive K(+) current in HEK293 cells transiently expressing K(Ca)3.1. Our data suggest that SOC and K(Ca)3.1 channels are tightly coupled, such that a small Ca(2+) influx current induces a much large K(Ca)3.1 channel current and hyperpolarization, providing the necessary electrochemical driving force for prolonged Ca(2+) signaling and store repletion.

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.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Nucleotides and epidermal growth factor induce parallel cytoskeletal rearrangements and migration in cultured adult murine neural stem cells

AIM

The adult subventricular zone (SVZ) contains neural stem cells that generate neuroblasts migrating to the olfactory bulb (OB) and differentiating into interneurones. The molecular cues controlling essential functions within the neurogenesis pathway such as proliferation, short and long distance migration, functional integration and cell survival are poorly understood. We have previously shown that cultured adult neural stem cells express a considerable variety of nucleotide receptors and that nucleotides and epidermal growth factor (EGF) induce converging intracellular signalling pathways that carry potential for synergism in the control of neural stem cell proliferation and cell survival. Here we investigate the role of EGF and the nucleotides ATP, ADPbetaS and UTP in neural stem cell migration.

METHODS

Neural stem cells were prepared from adult mice and subjected to adherent culture. Labelling of F-actin was performed with tetramethylrhodamine isothiocyanate-phalloidin. Images were processed for quantitative evaluation of fluorescence labelling. Agonist-induced phosphorylation of AKT and focal adhesion kinase was analysed by quantitative Western blotting. Agonist-dependent cell migration was assayed using 48-well microchemotaxis chambers.

RESULTS

Nucleotides and EGF induce the formation of stress fibres, an increase in the cortical actin cytoskeleton and in cell spreading. This is associated with increased phosphorylation of AKT and focal adhesion kinase. Using microchemotaxis chambers we demonstrate a parallel increase in cell migration.

CONCLUSION

Our results suggest that nucleotides and EGF acting as paracrine or autocrine signalling substances can be of relevance for structuring and maintaining the cytoarchitecture of the SVZ and the stream of neuroblasts migrating to the OB.

Copper inhibits P2Y(2)-dependent Ca(2+) signaling through the effects on thapsigargin-sensitive Ca(2+) stores in HTC hepatoma cells

Purinergic P2Y(2) G-protein coupled receptors play a key role in the regulation of hepatic Ca(2+) signaling by extracellular ATP. The concentration of copper in serum is about 20muM. Since copper accumulates in the liver in certain disease states, the purpose of these studies was to assess the effects of copper on P2Y(2) receptors in a model liver cell line. Exposure to a P2Y(2) agonist UTP increased [Ca(2+)](i) by stimulating Ca(2+) release from thapsigargin-sensitive Ca(2+) stores. Pretreatment of HTC cells for several minutes with copper did not affect cell viability, but potently inhibited increases in [Ca(2+)](i) evoked by UTP and thapsigargin. During this pretreatment, copper was not transported into the cytosol, and inhibited P2Y(2) receptors in a concentration-dependent manner with the IC(50) of about 15muM. These results suggest that copper inhibits P2Y(2) receptors through the effects on thapsigargin-sensitive Ca(2+) stores by acting from an extracellular side. Further experiments indicated that these effect of copper may lead to inhibition of regulatory volume decrease (RVD) evoked by hypotonic solution. Thus, copper may contribute to defective regulation of purinergic signaling and liver cell volume in diseases associated with the increased serum copper concentration.

ATP stimulates the proliferation of MCF-7 cells through the PI3K/Akt signaling pathway

We studied the modulation of the PI3K/Akt signaling pathway by ATP in MCF-7 cells. Western blot analysis showed that ATP stimulated the phosphorylation of Akt in a dose- and time-dependent manner. Akt phosphorylation in response to nucleotides followed the potency order ATP=UTP=ATPgammaS>>ADP=UDP>ADPbetaS=adenosine, suggesting participation of P2Y(2/4) receptors. Inhibitors of PI3K, PLC, PKC and Src or Src antisense oligonucleotides prevented ATP-induced phosphorylation of Akt. Incubation of cells with 2-APB or in a nominally Ca(2+)-free medium plus EGTA showed that Akt phosphorylation by ATP depends on intracellular calcium release but is independent of calcium influx. The PI3K inhibitor was not effective in reducing MAPKs phosphorylation by ATP. ATP and UTP stimulated MCF-7 cell proliferation, effect that was inhibited by PI3K, PLC, PKC, Src and MAPKs inhibitors. These findings suggest that ATP modulation of P2Y(2/4) receptors increases MCF-7 cell proliferation by activation of the PI3K/Akt signaling pathway through PLC/IP(3)/Ca(2+), PKC and Src.

Human eosinophils recognize endogenous danger signal crystalline uric acid and produce proinflammatory cytokines mediated by autocrine ATP

Eosinophils are multifunctional leukocytes involved in various inflammatory processes, as well as tissue remodeling and immunoregulation. During inflammation and infection, injured cells and damaged tissues release uric acid and monosodium urate (MSU) crystals as important endogenous danger signals. Uric acid is also implicated in the immunogenic effects of an authentic Th2 adjuvant, aluminum hydroxide. Eosinophils often localize at sites of Th2-type chronic inflammation; therefore, we hypothesized that eosinophils may react to endogenous danger signals. We found that human eosinophils migrate toward soluble uric acid and MSU crystals in a gradient-dependent manner. Eosinophils incubated with MSU crystals, but not those incubated with uric acid solution, produced elevated levels of IL-6 and IL-8/CXCL8. Other cytokines and chemokines, including IL-1beta, IL-10, IL-17, IFN-gamma, CCL2, CCL3, CCL4, TNF-alpha, G-CSF, GM-CSF, fibroblast growth factor, vascular endothelial growth factor, and TGF-beta, were also produced by eosinophils incubated with MSU crystals. Eosinophils exposed to MSU crystals rapidly (i.e., within 1 min of exposure) released ATP into the extracellular milieu. Importantly, this autocrine ATP was necessary for eosinophils to produce cytokines in response to MSU crystals, and P2 nucleotide receptors, in particular P2Y(2), are likely involved in this positive feedback loop. Finally, at higher concentrations, MSU crystals promoted P2R-dependent release of a granule protein (eosinophil-derived neurotoxin) and cell death. Thus, human eosinophils may respond to particulate damage-associated endogenous danger signals. These responses by eosinophils to tissue damage may explain the self-perpetuating nature of chronic inflammation in certain human diseases, such as asthma.

Regulation of the epithelial Na+ channel and airway surface liquid volume by serine proteases

Mammalian airways are protected from infection by a thin film of airway surface liquid (ASL) which covers airway epithelial surfaces and acts as a lubricant to keep mucus from adhering to the epithelial surface. Precise regulation of ASL volume is essential for efficient mucus clearance and too great a reduction in ASL volume causes mucus dehydration and mucus stasis which contributes to chronic airway infection. The epithelial Na(+) channel (ENaC) is the rate-limiting step that governs Na(+) absorption in the airways. Recent in vitro and in vivo data have demonstrated that ENaC is a critical determinant of ASL volume and hence mucus clearance. ENaC must be cleaved by either intracellular furin-type proteases or extracellular serine proteases to be active and conduct Na(+), and this process can be inhibited by protease inhibitors. ENaC can be regulated by multiple pathways, and once proteolytically cleaved ENaC may then be inhibited by intracellular second messengers such as cAMP and PIP(2). In the airways, however, regulation of ENaC by proteases seems to be the predominant mode of regulation since knockdown of either endogenous serine proteases such as prostasin, or inhibitors of ENaC proteolysis such as SPLUNC1, has large effects on ENaC activity in airway epithelia. In this review, we shall discuss how ENaC is proteolytically cleaved, how this process can regulate ASL volume, and how its failure to operate correctly may contribute to chronic airway disease.

Assessment and characterization of purinergic contractions and relaxations in the rat urinary bladder

The aim of the present study was to assess the purinoceptor functional responses of the urinary bladder by using isolated rat urinary bladder strip preparations. ATP elicited a transient bladder contraction followed by a sustained relaxation and ADP, UDP and UTP generated predominantly potent relaxations (relaxatory potencies: ADP = ATP > UDP = UTP). The ATP contractions were desensitized with the P2X(1/3) purinoceptor agonist/desensitizer alpha,beta-meATP and reduced by the P2 purinoceptor antagonist PPADS but unaffected by the P2 purinoceptor antagonist suramin. Electrical field stimulation (1-60 Hz) evoked frequency-dependent bladder contractions that were decreased by incubation with alpha,beta-meATP but not further decreased by PPADS. Suramin antagonized relaxations generated by UDP but not those by ADP, ATP or UTP. PPADS antagonized and tended to antagonize UTP and UDP relaxations, respectively, but did neither affect ADP nor ATP relaxations. ADP relaxations were insensitive to the P2Y(1) purinoceptor antagonist MRS 2179 and the ATP-sensitive potassium channel antagonist glibenclamide. The ATP relaxations were inhibited by the P1 purinoceptor antagonist 8-p-sulfophenyltheophylline but unaffected by the A2A adenosine receptor antagonist 8-(3-chlorostyryl)caffeine and glibenclamide. Adenosine evoked relaxations that were antagonized by the A2B adenosine receptor antagonist PSB 1115. Thus, in the rat urinary bladder purinergic contractions are elicited predominantly by stimulation of the P2X(1) purinoceptors, while UDP/UTP-sensitive P2Y purinoceptor(s) and P1 purinoceptors of the A2B adenosine receptor subtype are involved in bladder relaxation.

P2Y4-mediated regulation of Na+ absorption in the Reissner's membrane of the cochlea

The epithelial cells of Reissner's membrane (RM) are capable of transporting Na(+) out of endolymph via epithelial Na(+) channel (ENaC). However, much remains to be known as to mechanism of regulation of Na(+) absorption in RM. We investigated P2Y signaling as a possible regulatory mechanism of ENaC in gerbil RM using voltage-sensitive vibrating probe technique and immunohistochemistry. Results showed that UTP induced partial inhibition of the amiloride-sensitive short-circuit current but did not change short-circuit current when applied in the presence of amiloride. The inhibitory effect of UTP was not completely reversible in minutes. The response to UTP was inhibited by reactive blue-2 and 2',3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate but not by suramin or pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid, which indicates this P2Y receptor as the P2Y(4) subtype. The phospholipase C (PLC) inhibitors 1-[6[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione and 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine markedly inhibited the effect of UTP on ENaC. In contrast, neither modulation of protein kinase C nor application of 2-aminoehoxydiphenyl borate affected P2Y(4)-mediated inhibition of ENaC. Immunoreactive staining for P2Y(4) was observed in the RM, apical membrane of stria vascularis, spiral ligament, and organ of Corti, including outer hair cell, inner hair cell, outer pillar cell, Deiters' cell, and Hensen cell. These results suggest that the physiological role of P2Y(4) receptor in RM is likely to regulate Na(+) homeostasis in the endolymph. The acute inhibition of ENaC activity by activation of P2Y(4) receptor is possibly mediated by decrease of phosphatidylinositol 4,5-biphosphate in the plasma membrane through PLC activation.

P2X2, P2X4 and P2Y1 receptors elevate intracellular Ca2+ in mouse embryonic stem cell-derived GABAergic neurons

BACKGROUND AND PURPOSE

Neurons derived from mouse embryonic stem cells (mESCs) are a valuable resource for basic pharmacological research. With the exception of cardiomyocytes, there is relatively little understanding of the pharmacology of stem cell-derived differentiated cells. In this study we investigate P2 receptor agonist effects on GABAergic neurons derived from mESCs.

EXPERIMENTAL APPROACH

mESCs were differentiated into GABAergic neurons in the presence of N2B27 culture medium. At day 24 of differentiation GABAergic neuronal responsiveness to purinergic agonists was investigated using calcium imaging and [3H]-GABA release studies.

KEY RESULTS

Sub-populations of GABAergic neurons responded to some or all of the adenine and uracil nucleotides ATP, ADP, UTP and UDP (all 100 microM) with elevations of intracellular Ca2+ ([Ca2+]i). The number of neurons responding to ATP was reduced by suramin (100 microM), PPADS (10 microM) and MRS2179 (10 microM), but not by NF023 (10 microM). The response to ATP was modulated by extracellular Zn2+ and pH. Neurons also responded to ATP (100 microM) with the release of [3H]-GABA, an effect completely inhibited by tetrodotoxin (100 nM). Ap4A and 2-methylthioATP both elicited significant [3H]-GABA release. Reverse transcriptase PCR showed the presence of P2X1,2,3,4,5,6 and P2X7, and P2Y1,2 and P2Y6 receptors. mESCs expressed P2X2,5 and P2X7 and P2Y1,2 and P2Y6 receptors.

CONCLUSIONS AND IMPLICATIONS

GABAergic neurons derived from stem cells elevate [Ca2+]i predominantly via the activation of P2X2, P2X4 and P2Y1 receptors. This study shows that mESCs generate good models of neuronal function for in vitro pharmacological investigation.

Extracellular beta-nicotinamide adenine dinucleotide (beta-NAD) promotes the endothelial cell barrier integrity via PKA- and EPAC1/Rac1-dependent actin cytoskeleton rearrangement

Extracellular beta-NAD is known to elevate intracellular levels of calcium ions, inositol 1,4,5-trisphate and cAMP. Recently, beta-NAD was identified as an agonist for P2Y1 and P2Y11 purinergic receptors. Since beta-NAD can be released extracellularly from endothelial cells (EC), we have proposed its involvement in the regulation of EC permeability. Here we show, for the first time, that endothelial integrity can be enhanced in EC endogenously expressing beta-NAD-activated purinergic receptors upon beta-NAD stimulation. Our data demonstrate that extracellular beta-NAD increases the transendothelial electrical resistance (TER) of human pulmonary artery EC (HPAEC) monolayers in a concentration-dependent manner indicating endothelial barrier enhancement. Importantly, beta-NAD significantly attenuated thrombin-induced EC permeability as well as the barrier-compromising effects of Gram-negative and Gram-positive bacterial toxins representing the barrier-protective function of beta-NAD. Immunofluorescence microscopy reveals more pronounced staining of cell-cell junctional protein VE-cadherin at the cellular periphery signifying increased tightness of the cell-cell contacts after beta-NAD stimulation. Interestingly, inhibitory analysis (pharmacological antagonists and receptor sequence specific siRNAs) indicates the participation of both P2Y1 and P2Y11 receptors in beta-NAD-induced TER increase. beta-NAD-treatment attenuates the lipopolysaccharide (LPS)-induced phosphorylation of myosin light chain (MLC) indicating its involvement in barrier protection. Our studies also show the involvement of cAMP-dependent protein kinase A and EPAC1 pathways as well as small GTPase Rac1 in beta-NAD-induced EC barrier enhancement. With these results, we conclude that beta-NAD regulates the pulmonary EC barrier integrity via small GTPase Rac1- and MLCP- dependent signaling pathways.

The P2Y2 receptor sensitizes mouse bladder sensory neurons and facilitates purinergic currents

Sensitization of bladder afferents is an underlying contributor to the development and maintenance of painful bladder syndrome/interstitial cystitis. Extracellular purines and pyrimidines (e.g., ATP and UTP), released during bladder distension or from damaged cells after tissue insult, are thought to play an important role in bladder physiological and pathological states by actions at ionotropic P2X and metabotropic P2Y receptors. In the present study, we examined the ability of P2Y receptors to sensitize and modulate P2X-mediated responses in mouse bladder sensory neurons. UTP (a P2Y(2) and P2Y(4) agonist) increased excitability of bladder neurons by depolarizing resting membrane potential, increasing action potential firing, and facilitating responses to suprathreshold current injection as well as to P2X agonist application. These effects of UTP on bladder neuron excitability were blocked by the P2Y(2) receptor antagonist suramin. UTP also facilitated bladder neuron homomeric P2X(2) sustained currents and homomeric P2X(3) fast currents. The facilitatory effect of UTP on P2X(2) sustained currents was mediated by a G-protein-coupled P2Y(2) receptor/PKC pathway, whereas the effect of UTP on P2X(3) fast currents was G-protein independent. We also examined P2X and P2Y receptor expression in bladder neurons. P2Y(2) and P2Y(4) transcripts were detected in approximately 50 and approximately 20% of bladder neurons, respectively. Approximately 50% of P2X(2)- and P2X(3)-positive bladder neurons expressed P2Y(2) transcripts, whereas < or =25% of the same bladder neurons expressed P2Y(4) transcripts. These results support involvement of P2Y(2) receptors in bladder sensation, suggesting an important contribution to bladder neuron excitability and hypersensitivity.

Pannexin-I/P2X 7 purinergic receptor channels mediate the release of cardioprotectants induced by ischemic pre- and postconditioning

Ischemic pre- and postconditioning protect ex vivo rat hearts from ischemia/reperfusion injury by promoting the release of cardioprotective agents by an unknown mechanism. Because P2X( 7) purinergic receptors are known to combine with pannexin-1 to form channels that allow adenosine triphosphate (ATP) release from cells, we hypothesized that these channels have a role in the release of multiple cardioprotectants during ischemic preconditioning (IPC). Addition of either a pannexin-1 hemichannel blocker (5 micromol/L carbenoxolone [CBX] or 0.4 micromol/L mefloquine [MF]) or a selective antagonist of the rat P2X(7) purinergic receptor (2 micromol/L brilliant blue G [BBG]) blocked IPC. These antagonists also blocked ischemic postconditioning. Preconditioning by exogenous addition of either sphingosine-1-phosphate or adenosine was not blocked by either CBX or BBG, indicating that they only affected the release of endogenous mediators, not any subsequent steps. To determine if only ATP release was mediated by pannexin-1/P2X(7) channels, we added an extra cycle of IPC to release sufficient quantities of additional cardioprotectants to eliminate the dependence on adenosine derivatives. This did not override the inhibition of IPC by CBX or MF, suggesting that the channel mediates the release of multiple cardioprotectants. Inhibitors of other P2X receptors, P2Y receptors, or connexins did not affect IPC. We conclude that a pannexin-1/P2X(7) channel is responsible for the release of cardioprotectants induced by ischemic pre- and postconditioning.

P2Y(1) receptors mediate inhibitory neuromuscular transmission in the rat colon

BACKGROUND AND PURPOSE

Inhibitory junction potentials (IJP) are responsible for smooth muscle relaxation in the gastrointestinal tract. The aim of this study was to pharmacologically characterize the neurotransmitters [nitric oxide (NO) and adenosine triphosphate (ATP)] and receptors involved at the inhibitory neuromuscular junctions in the rat colon using newly available P2Y(1) antagonists.

EXPERIMENTAL APPROACH

Organ bath and microelectrode recordings were used to evaluate the effect of drugs on spontaneous mechanical activity and resting membrane potential. IJP and mechanical relaxation were studied using electrical field stimulation (EFS).

KEY RESULTS

N(omega)-nitro-L-arginine (L-NNA) inhibited the slow component of the IJP and partially inhibited the mechanical relaxation induced by EFS. MRS2179, MRS2500 and MRS2279, all selective P2Y(1) receptor antagonists, inhibited the fast component of the IJP without having a major effect on the relaxation induced by EFS. The combination of both L-NNA and P2Y(1) antagonists inhibited the fast and the slow components of the IJP and completely blocked the mechanical relaxation induced by EFS. Sodium nitroprusside caused smooth muscle hyperpolarization and cessation of spontaneous motility that was prevented by oxadiazolo[4,3-alpha]quinoxalin-1-one. Adenosine 5'-O-2-thiodiphosphate, a preferential P2Y agonist, hyperpolarized smooth muscle cells and decreased spontaneous motility. This effect was inhibited by P2Y(1) antagonists.

CONCLUSIONS AND IMPLICATIONS

The co-transmission process in the rat colon involves ATP and NO. P2Y(1) receptors mediate the fast IJP and NO the slow IJP. The rank order of potency of the P2Y(1) receptor antagonists is MRS2500 greater than MRS2279 greater than MRS2179. P2Y(1) receptors might be potential pharmacological targets for the regulation of gastrointestinal motility.

NTPDase1 (CD39) controls nucleotide-dependent vasoconstriction in mouse

AIMS

Extracellular nucleotides are vasoactive molecules. The concentrations of these molecules are regulated by ectonucleotidases. In this study, we investigated the role of the blood vessel ectonucleotidase NTPDase1, in the vasoconstrictor effect of nucleotides using Entpd1(-/-) mice.

METHODS AND RESULTS

Immunofluorescence, enzyme histochemistry, and HPLC analysis were used to evaluate both NTPDase expression and activity in arteries and isolated vascular smooth muscle cells (VSMCs). Vascular reactivity was evaluated in vitro and mean arterial blood pressure was recorded in anesthetized mice after nucleotide i.v. infusion. Expression of nucleotide receptors in VSMCs was determined by RT-PCR. Entpd1(-/-) mice displayed a dramatic deficit of nucleotidase activity in blood vessel wall in situ and in VSMCs in comparison to control mice. In aortic rings from Entpd1(-/-) mice, UDP and UTP induced a potent and long-lasting constriction contrasting with the weak response obtained in wild-type rings. This constriction occurred through activation of P2Y(6) receptor and was independent of other uracil nucleotide-responding receptors (P2Y(2) and P2Y(4)). UDP infusion in vivo increased blood pressure and this effect was potentiated in Entpd1(-/-) mice. In addition, pressurized mesenteric arteries from Entpd1(-/-) mice displayed an enhanced myogenic response, consistent with higher local concentrations of endogenously released nucleotides. This effect was inhibited by the P2 receptor antagonist RB-2.

CONCLUSION

NTPDase1 is the major enzyme regulating nucleotide metabolism at the surface of VSMCs and thus contributes to the local regulation of vascular tone by nucleotides.

Heparin and suramin alter plitidepsin uptake via inhibition of GPCR coupled signaling

Plitidepsin (Aplidin) is a novel antitumor agent, derived from the mediterranean tunicate Aplidium albicans, and is currently in phase ii clinical trials with evidence of activity in heavily pretreated multiple myeloma, renal cell carcinoma, melanoma and neuroblastoma patients. As compared to its parental compound didemnin B, plitidepsin has shown a better therapeutic index with less bone marrow toxicity, cardiotoxicity and neurotoxicity in patients and a more potent cytotoxic effect in several tumor cell lines. As sensitivity to the drug varies between cell lines and fresh leukemia samples, we performed studies on transport of plitidepsin in leukemia and lymphoma cell lines to determine the mechanism of uptake. The drug is taken up by an active transport process, i.e. the process is temperature and energy dependent, and has a high-affinity binding site with Kt =212 nM and Vmax = 15 pmoles/min. Importantly, once inside the cell, efflux of plitidepsin is minimum, suggesting that the drug is bound to intracellular macromolecules. Further work showed that plitidepsin binds to G-Protein Coupled Receptors (GPCRs), since GPCR and GRK (GPCR kinases) inhibitors suramin and heparin respectively, markedly reduce the drug uptake and its cytotoxic activity. Signaling via Jak/Stat pathway is inhibited by pharmacological concentrations of plitidepsin, further confirming the relationship between plitidepsin and GPCRs.

Human P2Y(14) receptor agonists: truncation of the hexose moiety of uridine-5'-diphosphoglucose and its replacement with alkyl and aryl groups

Uridine-5'-diphosphoglucose (UDPG) activates the P2Y(14) receptor, a neuroimmune system GPCR. P2Y(14) receptor tolerates glucose substitution with small alkyl or aryl groups or its truncation to uridine 5'-diphosphate (UDP), a full agonist at the human P2Y(14) receptor expressed in HEK-293 cells. 2-Thiouracil derivatives displayed selectivity for activation of the human P2Y(14) vs the P2Y(6) receptor, such as 2-thio-UDP 4 (EC(50) = 1.92 nM at P2Y(14), 224-fold selectivity vs P2Y(6)) and its beta-propyloxy ester 18. EC(50) values of the beta-methyl ester of UDP and its 2-thio analogue were 2730 and 56 nM, respectively. beta-tert-Butyl ester of 4 was 11-fold more potent than UDPG, but beta-aryloxy or larger, branched beta-alkyl esters, such as cyclohexyl, were less potent. Ribose replacement of UDP with a rigid North or South methanocarba (bicyclo[3.1.0]hexane) group abolished P2Y(14) receptor agonist activity. alpha,beta-Methylene and difluoromethylene groups were well tolerated at the P2Y(14) receptor and are expected to provide enhanced stability in biological systems. alpha,beta-Methylene-2-thio-UDP 11 (EC(50) = 0.92 nM) was 2160-fold selective versus P2Y(6). Thus, these nucleotides and their congeners may serve as important pharmacological probes for the detection and characterization of the P2Y(14) receptor.

P2Y1 receptors mediate apamin-sensitive and -insensitive inhibitory junction potentials in murine colonic circular smooth muscle

Purinergic inhibitory neuromuscular transmission plays an important role in the control of intestinal motility. In most tissues this neurotransmission is apamin-sensitive, but recent studies in human colonic circular smooth muscle (CSM) suggest the presence of apamin-insensitive purinergic inhibitory junction potentials (IJPs). The current studies used conventional intracellular recordings on colonic CSM strips to characterize the purinergic IJPs in murine colonic CSM. P2Y1 receptor expression was examined by using reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemistry. The IJP induced by nerve stimulation (NS) of one and four pulses in neuronal nitric-oxide synthase knockout mice consists of an apamin-sensitive and a dominant apamin-resistant component. These are identical to the IJPs in wild-type and CD1 mice in the presence of N(omega)-nitro-l-arginine methyl ester (200 microM) and were significantly inhibited by alpha,beta-methylene ATP (50 microM), an analog of ATP. IJPs were not affected by the P2X receptor antagonist 2',3'-o-(2,4,6-trinitrophenyl)-ATP (10 microM). Furthermore, apamin-resistant IJPs induced by single-pulse NS were abolished by pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (100 microM), a P2 receptor antagonist; 2'-deoxy-N6-methyl adenosine 3,5-diphosphate (MRS-2179; 10 microM), a selective P2Y1 receptor antagonist; and tetrodotoxin (1 microM). Aboral NS induced apamin-sensitive purinergic IJPs, whereas oral and circumferential NS produced apamin-sensitive and -resistant IJPs, with the latter predominating. RT-PCR and immunohistochemistry confirmed the presence of P2Y1 receptors on smooth muscle and in the myenteric plexus. These data suggest that, depending on stimulus location, activation of P2Y1 receptors produces both apamin-sensitive and apamin-resistant IJPs in murine colonic CSM.

Purinergic signaling regulates cell proliferation of olfactory epithelium progenitors

In the olfactory epithelium (OE) continuous neurogenesis is maintained throughout life. The OE is in direct contact with the external environment, and its cells are constantly exposed to pathogens and noxious substances. To maintain a functional sense of smell the OE has evolved the ability to permanently replenish olfactory receptor neurons and sustentacular cells lost during natural turnover. A cell population residing in the most basal part of the OE, the so-called basal cells (BCs), keep up this highly regulated genesis of new cells. The population of BCs is thought to include both the stem cells of the OE and various progenitor cells. In recent years a number of regulatory factors that positively and/or negatively regulate the proliferation within the OE have been identified, but a thorough comprehension of the complex interplay of these regulatory factors and the role of the different epithelial cell types is still illusive. Combining labeling techniques, immunohistochemistry, electron microscopy, functional calcium imaging, and a bromo-2'-deoxyuridine incorporation assay, we show for the first time that purinergic receptors are expressed in BCs of the OE of larval Xenopus laevis and that nucleotide-induced Ca(2+) signaling in these cells is involved in the regulation of the cell turnover in the OE. Our data contribute to a better understanding of the regulation of the cell turnover in the OE in particular and also of how the proliferation of neuronal progenitor cells is regulated in general.

Tripartite purinergic modulation of central respiratory networks during perinatal development: the influence of ATP, ectonucleotidases, and ATP metabolites

ATP released during hypoxia from the ventrolateral medulla activates purinergic receptors (P2Rs) to attenuate the secondary hypoxic depression of breathing by a mechanism that likely involves a P2Y(1)R-mediated excitation of preBötzinger complex (preBötC) inspiratory rhythm-generating networks. In this study, we used rhythmically active in vitro preparations from embryonic and postnatal rats and ATP microinjection into the rostral ventral respiratory group (rVRG)/preBötC to reveal that these networks are sensitive to ATP when rhythm emerges at embryonic day 17 (E17). The peak frequency elicited by ATP at E19 and postnatally was the same ( approximately 45 bursts/min), but relative sensitivity was threefold greater at E19, reflecting a lower baseline frequency (5.6 +/- 0.9 vs 19.0 +/- 1.3 bursts/min). Combining microinjection techniques with ATP biosensors revealed that ATP concentration in the rVRG/preBötC falls rapidly as a result of active processes and closely correlates with inspiratory frequency. A phosphate assay established that preBötC-containing tissue punches degrade ATP at rates that increase perinatally. Thus, the agonist profile [ATP/ADP/adenosine (ADO)] produced after ATP release in the rVRG/preBötC will change perinatally. Electrophysiology further established that the ATP metabolite ADP is excitatory and that, in fetal but not postnatal animals, ADO at A(1) receptors exerts a tonic depressive action on rhythm, whereas A(1) antagonists extend the excitatory action of ATP on inspiratory rhythm. These data demonstrate that ATP is a potent excitatory modulator of the rVRG/preBötC inspiratory network from the time it becomes active and that ATP actions are determined by a dynamic interaction between the actions of ATP at P2 receptors, ectonucleotidases that degrade ATP, and ATP metabolites on P2Y and P1 receptors.

Activation of distinct P2Y receptor subtypes stimulates insulin secretion in MIN6 mouse pancreatic beta cells

Extracellular nucleotides and their receptor antagonists have therapeutic potential in disorders such as inflammation, brain disorders, and cardiovascular diseases. Pancreatic beta cells express several purinergic receptors, and reported nucleotide effects on insulin secretion are contradictory. We studied the effect of P2Y receptors on insulin secretion and cell death in MIN6, mouse pancreatic beta cells. Expression of P2Y(1) and P2Y(6) receptors was revealed by total mRNA analysis using RT-PCR. MIN6 cells were stimulated in the presence of 16.7 mM glucose with or without P2Y(1) and P2Y(6) agonists, 2-MeSADP and Up(3)U, respectively. Both the agonists increased insulin secretion with EC(50) values of 44.6+/-7.0 nM and 30.7+/-12.7 nM respectively. The insulin secretion by P2Y(1) and P2Y(6) agonists was blocked by their selective antagonists MRS2179 and MRS2578, respectively. Binding of the selective P2Y(1) receptor antagonist radioligand [125I]MRS2500 in MIN6 cell membranes was saturable (K(D) 4.74+/-0.47 nM), and known P2Y(1) ligands competed with high affinities. Inflammation and glucose toxicity lead to pancreatic beta cell death in diabetes. Flow cytometric analysis revealed that Up(3)U but not 2-MeSADP protected MIN6 cells against TNF-alpha induced apoptosis. Overall, the results demonstrate that selective stimulation of P2Y(1) and P2Y(6) receptors increases insulin secretion that accompanies intracellular calcium release, suggesting potential application of P2Y receptor ligands in the treatment of diabetes.

P2Y receptors and atherosclerosis in apolipoprotein E-deficient mice

BACKGROUND AND PURPOSE

P2Y nucleotide receptors are involved in the regulation of vascular tone, smooth muscle cell (SMC) proliferation and inflammatory responses. The present study investigated whether they are involved in atherosclerosis.

EXPERIMENTAL APPROACH

mRNA of P2Y receptors was quantified (RT-PCR) in atherosclerotic and plaque-free aorta segments of apolipoprotein E-deficient (apoE(-/-)) mice. Macrophage activation was assessed in J774 macrophages, and effects of non-selective purinoceptor antagonists on atherosclerosis were evaluated in cholesterol-fed apoE(-/-) mice.

KEY RESULTS

P2Y(6) receptor mRNA was consistently elevated in segments with atherosclerosis, whereas P2Y(2) receptor expression remained unchanged. Expression of P2Y(1) or P2Y(4) receptor mRNA was low or undetectable, and not influenced by atherosclerosis. P2Y(6) mRNA expression was higher in cultured J774 macrophages than in cultured aortic SMCs. Furthermore, immunohistochemical staining of plaques demonstrated P2Y(6)-positive macrophages, but few SMCs, suggesting that macrophage recruitment accounted for the increase in P2Y(6) receptor mRNA during atherosclerosis. In contrast to ATP, the P2Y(6)-selective agonist UDP increased mRNA expression and activity of inducible nitric oxide synthase and interleukin-6 in J774 macrophages; this effect was blocked by suramin (100-300 microM) or pyridoxal-phosphate-6-azophenyl-2'-4'-disulphonic acid (PPADS, 10-30 microM). Finally, 4-week treatment of cholesterol-fed apoE(-/-) mice with suramin or PPADS (50 and 25 mg.kg(-1).day(-1) respectively) reduced plaque size, without changing plaque composition (relative SMC and macrophage content) or cell replication.

CONCLUSIONS AND IMPLICATIONS

These results suggest involvement of nucleotide receptors, particularly P2Y(6) receptors, during atherosclerosis, and warrant further research with selective purinoceptor antagonists or P2Y(6) receptor-deficient mice.

ATP stimulates chemokine production via a store-operated calcium entry pathway in C6 glioma cells

Background

Glioma present as one of the most challenging cancers to treat, however, understanding of tumor cell biology is not well understood. Extracellular adenosine triphosphate (ATP) could serve as a critical signaling molecule regulating tumor development. This study has examined pharmacological modulation of calcium (Ca²⁺) entry through store-operated channels (SOC) on cellular expression and production of immune-cell mobilizing chemokines in ATP-stimulated C6 glioma cells.

Methods

Calcium spectrofluorometry was carried out to measure mobilization of intracellular Ca²⁺ [Ca²⁺]i following ATP stimulation of rat C6 glioma cells. Pretreatment with two inhibitors of SOC, SKF96365 or gadolinium, was used to examine for effects on [Ca²⁺]i. RT-PCR was performed to determine effects of purinergic stimulation on C6 cell expression of metabotropic P2Y receptors (P2YR) and the chemokines, monocyte chemoattractant protein-1 (MCP-1) and interleukin-8 (IL-8). ELISA was carried out to measure production of MCP-1 and IL-8 with ATP stimulation of glioma cells.

Results

Application of ATP (at 100 μM) to C6 glioma induced an increase in [Ca²⁺]i with the response exhibiting two components of decay. In the presence of the SOC inhibitors, SKF96365 or gadolinium, or with Ca²⁺-free solution, ATP responses lacked a slow phase suggesting the secondary component was due to SOC-mediated influx of Ca²⁺. RT-PCR confirmed expression of purinergic P2Y-subtype receptors in C6 cells which would serve as a precursor to activation of SOC. In addition, ATP-stimulated C6 cells showed enhanced expression of the chemokines, MCP-1 and IL-8, with SKF96365 or gadolinium effective in reducing chemokine expression. Gadolinium treatment of ATP-stimulated C6 cells was also found to inhibit the production of MCP-1 and IL-8.

Conclusion

These results suggest ATP-induced Ca²⁺ entry, mediated by activation of SOC in C6 glioma, as a mechanism leading to increased cellular expression and release of chemokines. Elevated levels of MCP-1 and IL-8 are predicted to enhance the mobility of tumor cells and promote recruitment of microglia into developing tumors thereby supporting tumor growth.

Endogenous purinergic signaling is required for osmotic volume regulation of retinal glial cells

Intense neuronal activity in the sensory retina is associated with a volume increase of neuronal cells (Uckermann et al., J. Neurosci. 2004, 24:10149) and a decrease in the osmolarity of the extracellular space fluid (Dmitriev et al., Vis. Neurosci. 1999, 16:1157). Here, we show the existence of an endogenous purinergic mechanism that prevents hypoosmotic swelling of retinal glial (Müller) cells in mice. In contrast to the cells from wild-type mice, hypoosmotic stress induced rapid swelling of glial cell somata in retinal slices from mice deficient in P2Y(1), adenosine A(1) receptors, or ecto-5'-nucleotidase (CD73). Consistently, glial cell bodies in retinal slices from wild-type mice displayed osmotic swelling when P2Y(1) or A(1) receptors, or CD73, were pharmacologically blocked. Exogenous ATP, UTP, and UDP inhibited glial swelling in retinal slices, while the swelling of isolated glial cells was prevented by ATP but not by UTP or UDP, suggesting that uracil nucleotides indirectly regulate the glial cell volume via activation of neuronal P2Y(4/6) and neuron-to-glia signaling. It is suggested that autocrine/paracrine activation of purinergic receptors and enzymes is crucially involved in the regulation of the glial cell volume.

NTPDase family in zebrafish: Nucleotide hydrolysis, molecular identification and gene expression profiles in brain, liver and heart

The nucleoside triphosphate diphosphohydrolase (NTPDase) family cleaves tri- and diphosphonucleosides to monophosphonucleosides and is responsible for terminating purinergic transmission. Since the NTPDase family in zebrafish is poorly understood, here we evaluated the nucleotide hydrolysis in three tissues of adult zebrafish (brain, liver, and heart), confirmed the presence of distinct NTPDase members by a phylogenetic analysis and verified their relative gene expression profiles in the respective tissues. A different profile of ATP and ADP hydrolysis in the brain, liver, and heart as a function of time and protein concentration was observed. Sodium azide (20mM), ARL 67156 (300 microM) and Suramin (300 microM) differently altered the nucleotide hydrolysis in zebrafish tissues, suggesting the contribution of distinct NTPDase activities. Homology-based searches identified the presence of NTPDase1-6 and NTPDase8 orthologs and the phylogeny also grouped three NTPDase2 and two NTPDase5 paralogs. The deduced amino acid sequences share the apyrase conserved regions, conserved cysteine residues, putative N-glycosylation, phosphorylation, N-acetylation sites, and different numbers of transmembrane domains. RT-PCR experiments revealed the existence of a distinct relative entpd1-6 and entpd8 expression profile in brain, liver, and heart. Taken together, these results indicate that several NTPDase members might contribute to a tight regulation of nucleotide hydrolysis in zebrafish tissues.

ATP modulates transcription factors through P2Y2 and P2Y4 receptors via PKC/MAPKs and PKC/Src pathways in MCF-7 cells

In this work, we studied the involvement of PKC and Src in the phosphorylation of ERK1/2, p38 and JNK1 MAPKs and in the modulation of ATF-1, c-Fos, c-Jun and Jun D transcription factors by ATP in MCF-7 breast cancer cells. RT-PCR studies and nucleotide sequence analysis confirmed first the expression of P2Y(2)- and P2Y(4)-receptor subtypes. The use of specific inhibitors and Src antisense oligonucleotides showed that PKC, but not Src, plays a role in the phosphorylation of MAPKs by ATP. ATP stimulated the expression of c-Fos and the phosphorylation c-Jun, Jun D and ATF-1. PKC and Src only participated in c-Fos induction and in ATF-1 phosphorylation. Pharmacological inhibition of MAPKs demonstrated that c-Fos induction and phosphorylation of c-Jun and Jun D, but not of ATF-1, depend on MAPK activation. These results suggest that stimulation of P2Y(2) and P2Y(4) receptors by ATP modulates transcription factors through PKC/MAPKs and PKC/Src pathways in MCF-7 cells.

Increase of intracellular Ca2+ by P2Y but not P2X receptors in cultured cortical multipolar neurons of the rat

The expression and functionality of P2X/P2Y receptor subtypes in multipolar nonpyramidal neurons of mixed cortical cell cultures were investigated by means of immunocytochemistry and fura-2 microfluorimetry. The morphological studies revealed that most of the neurons are immunoreactive for GABA and express a range of P2X/P2Y receptors, predominantly of the P2X(2,4,6) and P2Y(1,2) subtypes. P2X(1) and P2X(7) receptor immunoreactivity (IR) was found on thin axon-like processes and presynaptic structures, respectively. Application of ATP caused a small concentration-dependent increase in intracellular Ca2+ concentration ([Ca2+]i) in most investigated neurons, whereas only about the half of these cells responded to 2',3'-O-(benzoyl-4-benzoyl)-ATP (BzATP), ADPbetaS, 2MeSADP, or 2MeSATP and even fewer cells to UTP. In contrast, alpha,beta-meATP, UDP, and UDP-glucose failed to produce any [Ca2+]i signaling. The response to ATP itself was inhibited by pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), Reactive Blue 2, 2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate (MRS2179), and suramin (300 microM) as well as by a cyclopiazonic acid-induced depletion of intracellular Ca2+ stores. A Ca2+-free external medium tended to decrease the ATP-induced [Ca2+]i transients, although this action did not reach statistical significance. Various blockers of voltage-sensitive Ca2+ channels and the gap junction inhibitor carbenoxolone did not interfere with the effect of ATP, whereas a combination of the ionotropic glutamate receptor antagonists D(-)-2-amino-5-phosphonopentanoic acid (AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) decreased it. Cross-desensitization experiments between ADPbetaS or UTP and ATP suggested that ATP acts on the one hand via P2Y(1,2) receptors and on the other hand by additional signaling mechanisms. These mechanisms may involve the release of glutamate (which in consequence activates ionotropic glutamate receptors) and the entry of Ca2+ via store-operated Ca2+ channels. Evidence for the presence of functional P2X receptors, in particular P2X(7), remains elusive.

Extracellular ATP activates MAP kinase cascades through a P2Y purinergic receptor in the human intestinal Caco-2 cell line

BACKGROUND

ATP exerts diverse effects on various cell types via specific purinergic P2Y receptors. Intracellular signaling cascades are the main routes of communication between P2Y receptors and regulatory targets in the cell.

METHODS AND RESULTS

We examined the role of ATP in the modulation of ERK1/2, JNK1/2, and p38 MAP kinases (MAPKs) in human colon cancer Caco-2 cells. Immunoblot analysis showed that ATP induces the phosphorylation of MAPKs in a time- and dose-dependent manner, peaking at 5 min at 10 microM ATP. Moreover, ATPgammaS, UTP, and UDP but not ADP or ADPbetaS increased phosphorylation of MAPKs, indicating the involvement of, at least, P2Y2/P2Y4 and P2Y6 receptor subtypes. RT-PCR studies and PCR product sequencing supported the expression of P2Y2 and P2Y4 receptors in this cell line. Spectrofluorimetric measurements showed that cell stimulation with ATP induced transient elevations in intracellular calcium concentration. In addition, ATP-induced phosphorylation of MAPKs in Caco-2 cells was dependent on Src family tyrosine kinases, calcium influx, and intracellular Ca2+ release and was partially dependent on the cAMP/PKA and PKC pathways and the EGFR.

GENERAL SIGNIFICANCE

These findings provide new molecular basis for further understanding the mechanisms involved in ATP functions, as a signal transducer and activator of MAP kinase cascades, in colon adenocarcinoma Caco-2 cells.

Extracellular NAD(+) induces a rise in [Ca(2+)](i) in activated human monocytes via engagement of P2Y(1) and P2Y(11) receptors

Extracellular nicotinamide adenine dinucleotide (NAD(+)) is known to increase the intracellular calcium concentration [Ca(2+)](i) in different cell types and by various mechanisms. Here we show that NAD(+) triggers a transient rise in [Ca(2+)](i) in human monocytes activated with lipopolysaccharide (LPS), which is caused by a release of Ca(2+) from IP(3)-responsive intracellular stores and an influx of extracellular Ca(2+). By the use of P2 receptor-selective agonists and antagonists we demonstrate that P2 receptors play a role in the NAD(+)-induced calcium response in activated monocytes. Of the two subclasses of P2 receptors (P2X and P2Y) the P2Y receptors were considered the most likely candidates, since they share calcium signaling properties with NAD(+). The identification of P2Y(1) and P2Y(11) as receptor subtypes responsible for the NAD(+)-triggered increase in [Ca(2+)](i) was supported by several lines of evidence. First, specific P2Y(1) and P2Y(11) receptor antagonists inhibited the NAD(+)-induced increase in [Ca(2+)](i). Second, NAD(+) was shown to potently induce calcium signals in cells transfected with either subtype, whereas untransfected cells were unresponsive. Third, NAD(+) caused an increase in [cAMP](i), prevented by the P2Y(11) receptor-specific antagonist NF157.

beta-Arrestin-2 interaction and internalization of the human P2Y1 receptor are dependent on C-terminal phosphorylation sites

The nucleotide receptor P2Y(1) regulates a variety of physiological processes and is involved in platelet aggregation. Using human P2Y(1)-receptors C-terminally fused with a fluorescent protein, we studied the role of potential receptor phosphorylation sites in receptor internalization and beta-arrestin-2 translocation by means of confocal microscopy. Three receptor constructs were generated that lacked potential phosphorylation sites in the third intracellular loop, the proximal C terminus, or the distal C terminus. The corresponding receptor constructs were expressed in human embryonic kidney (HEK)-293 cells and stimulated with 100 muM ADP. Rapid receptor internalization was observed for the wild-type receptor and from those constructs mutated in the third intracellular loop and the proximal C terminus. However, the construct lacking phosphorylation sites at the distal C terminus did not show receptor internalization upon stimulation. The microscopic data were validated by HA-tagged receptor constructs using a cell surface enzyme-linked immunosorbent assay. P2Y(1)-receptor stimulated beta-arrestin-2-yellow fluorescent protein (YFP) translocation followed the same pattern as receptor internalization. Hence, no beta-arrestin-2-YFP translocation was observed when the distal C-terminal phosphorylation sites were mutated. Individual mutations indicate that residues Ser352 and Thr358 are essential for receptor internalization and beta-arrestin-2-YFP translocation. In contrast, protein kinase C (PKC)-mediated receptor desensitization was not affected by mutation of potential phosphorylation sites in the distal C terminus but was prevented by mutation of potential phosphorylation sites in the proximal C terminus. P2Y(1)-receptor internalization in HEK-293 cells was not blocked by inhibitors of PKC and calmodulin-dependent protein kinase. Thus, we conclude that P2Y(1)-receptor desensitization and internalization are mediated by different phosphorylation sites and kinases.

P2 receptor-mediated stimulation of the PI3-K/Akt-pathway in vivo

ATP acts as a growth factor as well as a toxic agent by stimulating P2 receptors. The P2 receptor-activated signaling cascades mediating cellular growth and cell survival after injury are only incompletely understood. Therefore, the aim of the present study was to identify the role of the phosphoinositide 3 kinase (PI3-K/Akt) and the mitogen-activated protein kinase/extracellular signal regulated protein kinase (MAPK/ERK) pathways in P2Y receptor-mediated astrogliosis after traumatic injury and after microinfusion of ADP beta S (P2Y(1,12,13) receptor agonist) into the rat nucleus accumbens (NAc). Mechanical damage and even more the concomitant treatment with ADP beta S, enhanced P2Y(1) receptor-expression in the NAc, which could be reduced by pretreatment with the P2X/Y receptor antagonist PPADS. Quantitative Western blot analysis indicated a significant increase in phosphorylated (p)Akt and pERK1/2 2 h after ADP beta S-microinjection. Pretreatment with PPADS or wortmannin abolished the up-regulation of pAkt by injury alone or ADP beta S-treatment. The ADP beta S-enhanced expression of the early apoptosis marker active caspase 3 was reduced by PPADS and PD98059, but not by wortmannin. Multiple immunofluorescence labeling indicated a time-dependent expression of pAkt and pMAPK on astrocytes and neurons and additionally the colocalization of pAkt, pMAPK, and active caspase 3 with the P2Y(1) receptor especially at astrocytes. In conclusion, the data show for the first time the involvement of PI3-K/Akt-pathway in processes of injury-induced astroglial proliferation and anti-apoptosis via activation of P2Y(1) receptors in vivo, suggesting specific roles of P2 receptors in glial cell pathophysiology in neurodegenerative diseases.

Myogenic tone in mouse mesenteric arteries: evidence for P2Y receptor-mediated, Na(+), K (+), 2Cl (-) cotransport-dependent signaling

This study examines the action of agonists and antagonists of P2 receptors on mouse mesenteric artery contractions and the possible involvement of these signaling pathways in myogenic tone (MT) evoked by elevated intraluminal pressure. Both ATP and its non-hydrolyzed analog alpha,beta-ATP triggered transient contractions that were sharply decreased in the presence of NF023, a potent antagonist of P2X(1) receptors. In contrast, UTP and UDP elicited sustained contractions which were suppressed by MRS2567, a selective antagonist of P2Y(6) receptors. Inhibition of Na(+), K(+), 2Cl(-) cotransport (NKCC) with bumetanide led to attenuation of contractions in UTP- but not ATP-treated arteries. Both UTP-induced contractions and MT were suppressed by MRS2567 and bumetanide but were insensitive to NF023. These data implicate a P2Y(6)-mediated, NKCC-dependent mechanism in MT of mesenteric arteries. The action of heightened intraluminal pressure on UTP release from mesenteric arteries and its role in the triggering of P2Y(6)-mediated signaling should be examined further.