P2 receptors: intracellular signaling

Coordinate pathways for nucleotide and EGF signaling in cultured adult neural progenitor cells

The adult subventricular zone (SVZ) contains astrocyte-like stem cells capable of generating new neurons for the olfactory bulb. Adult neurogenesis is driven by a variety of signal systems that can induce synergistic or opposing cellular responses. It is therefore important to gain insight into the underlying downstream signaling pathways. We have previously shown that the nucleotides ADPbetaS and UTP induce rapid Ca2+ transients in cultured SVZ-derived adult neural progenitors and augment growth-factor-mediated progenitor cell proliferation. Here, we investigated signaling pathways elicited by ADPbetaS, UTP and epidermal growth factor (EGF). All three agonists elicit ERK1/2 and CREB phosphorylation but the temporal characteristics differ between the nucleotides and EGF. Differentiation of the progenitors alters the receptor profile. Oligodendrocytes and young neurons, but not astrocytes, lose responsiveness to the agonists. Inhibition experiments are indicative of an ADPbetaS-elicited EGF receptor transactivation. Whereas UTP acts via the P2Y2 receptor, ADPbetaS exerts its function via the P2Y1 receptor and the P2Y13 receptor. Our data demonstrate that nucleotides and EGF induce converging, but also differential, intracellular signaling pathways and suggest that they carry the potential to act synergistically in the control of cell proliferation and cell survival in adult neurogenesis.

Uridine-5'-triphosphate protects against hepatic- ischemic/reperfusion injury in mice

BACKGROUND AND AIM

Mitochondrial calcium overload triggers apoptosis and also regulates ATP production. ATP and uridine-5'-triphosphate (UTP) depletion from hepatic tissue after ischemia causes cell death. ATP and UTP binds to cell membranes of the hepatocytes through P2Y receptors. Our aim was to investigate the role of UTP on the hepatic injury induced by ischemia.

METHODS

Isolated mouse livers were randomly divided into five groups: (1) control group; (2) ischemic group (90 min); (3) as group 2, but with the administration of UTP; (4) as group 2, but with the administration of suramin, a P2Y antagonist; and (5) as group 3, but with the simultaneous administration of suramin and UTP.

RESULTS

There was a postischemic significant reduction in the release of liver enzymes in the animals pretreated with UTP, the intrahepatic caspase-3 activity was significantly decreased, and the intrahepatic ATP content increased compared with group 2 (ischemic untreated). UTP prevented intracellular Ca overload after hypoxia in hepatocyte cultures. In the UTP-treated groups, significantly fewer apoptotic hepatocyte cells were noted by weaker activation of caspase-3 and by the transferase-mediated dUTP nick end labeling assay. The administration of suramin prevented the beneficial effect of endogenous ATP. UTP treatment attenuated the degradation of IkappaBalpha (nuclear factor-kappaB inhibitor) by 80% during reperfusion with no effect on c-Jun N terminal kinase phosphorylation.

CONCLUSION

The administration of UTP before induction of ischemia-reperfusion can attenuate hepatic injury. UTP administration decreased cytosolic Ca overload in hypoxic conditions. UTP-mediated protective effects may be regulated through nuclear factor- kappaB inactivation. These findings have important implications for the potential use of UTP in ischemic hepatic injury.

The roles of ASK family proteins in stress responses and diseases

Apoptosis signal-regulating kinase 1 (ASK1) is a member of the mitogen-activated protein kinase kinase kinase family, which activates c-Jun N-terminal kinase and p38 in response to a diverse array of stresses such as oxidative stress, endoplasmic reticulum stress and calcium influx. In the past decade, various regulatory mechanisms of ASK1 have been elucidated, including its oxidative stress-dependent activation. Recently, it has emerged that ASK family proteins play key roles in cancer, cardiovascular diseases and neurodegenerative diseases. In this review, we summarize the recent findings on ASK family proteins and their implications in various diseases.

Expression of purinergic receptors and modulation of P2X7 function by the inflammatory cytokine IFNgamma in human epithelial cells

The cervical epithelial cell line, HeLa, is one of the oldest and most commonly used cell lines in cell biology laboratories. Although a truncated P2X(7) receptor has recently been identified in HeLa cells, the expression of other purinergic receptors or the function of the P2X(7) protein has not been characterized. We here show that HeLa cells express transcripts for most P2X and P2Y purinergic receptors. Treatment of cells with ATP or other P2X(7) agonists does not stimulate cell death, but can induce atypical calcium fluxes and ion currents. Cervical epithelial cells represent an important target for sexually-transmitted pathogens and are commonly exposed to pro-inflammatory cytokines such as IFNgamma. Stimulation of HeLa cells with IFNgamma upregulates expression of P2X(7) mRNA and full-length protein, modifies ATP-dependent calcium fluxes, and renders the cells sensitive to ATP-induced apoptosis, which can be blocked by a P2X(7) antagonist. IFNgamma treatment also increased dramatically the sensitivity of the intestinal epithelial cell line, HCT8, to ATP-induced apoptosis. Significantly, IFNgamma also stimulated P2X(7) expression on human intestinal tissues. Responses to other purinergic receptor ligands suggest that HeLa cells may also express functional P2Y(1), P2Y(2) and P2Y(6) receptors, which could be relevant for modulating ion homeostasis in the cells.

Trophic functions of nucleotides in the central nervous system

In addition to short-term effects, one of the fundamental roles of extracellular nucleotides in the central nervous system involves long-term trophic effects. Physiological outcomes include neurogenesis, neuronal differentiation, glial proliferation, migration, growth arrest and apoptosis. Nucleotides exert these functions via P2-receptor-mediated mechanisms that can also interact with polypeptide-growth-factor-mediated or integrin-mediated signaling pathways. In addition, pathogenic roles for extracellular nucleotides in response to central nervous system injury including trauma and ischemia have been observed after the release of nucleotides by damaged and dying cells and in the development of neuropathic and inflammatory pain. Here, we illuminate the contribution of extracellular nucleotides to the development, growth, cellular plasticity and death of neural cells and the mechanisms regulating these trophic effects.

Mechanism of purinergic activation of endothelial nitric oxide synthase in endothelial cells

BACKGROUND

Decreased endothelial nitric oxide (NO) synthase (eNOS) activity and NO production are critical contributors to the endothelial dysfunction and vascular complications observed in many diseases, including diabetes mellitus. Extracellular nucleotides activate eNOS and increase NO generation; however, the mechanism of this observation is not fully clarified.

METHODS AND RESULTS

To elucidate the signaling pathway(s) leading to nucleotide-mediated eNOS phosphorylation at Ser-1177, human umbilical vein endothelial cells were treated with several nucleotides, including ATP, UTP, and ADP, in the presence or absence of selective inhibitors. These experiments identified P2Y1, P2Y2, and possibly P2Y4 as the purinergic receptors involved in eNOS phosphorylation and demonstrated that this process was adenosine independent. Nucleotide-induced eNOS phosphorylation and activity were inhibited by BAPTA-AM (an intracellular free calcium chelator), rottlerin (a protein kinase Cdelta inhibitor), and protein kinase Cdelta siRNA. In contrast, blockade of AMP-activated protein kinase, calcium/calmodulin-dependent kinase II, calcium/calmodulin-dependent kinase kinase, serine/threonine protein kinase B, protein kinase A, extracellular signal-regulated kinase 1/2, and p38 mitogen-activated protein kinase did not affect nucleotide-mediated eNOS phosphorylation.

CONCLUSIONS

The present study indicates that extracellular nucleotide-mediated eNOS phosphorylation is calcium and protein kinase Cdelta dependent. This newly identified signaling pathway opens new therapeutic avenues for the treatment of endothelial dysfunction.

Development of a novel transgenic rat overexpressing the P2Y(2) nucleotide receptor using a lentiviral vector

The G protein-coupled P2Y(2) nucleotide receptor (P2Y(2)R) is upregulated in response to stress and tissue injury and has been postulated to play a role in chronic inflammation seen in atherosclerosis, Alzheimer's disease and Sjogren's syndrome. The role of P2Y(2)R upregulation in vivo is poorly understood, in part due to the lack of a P2Y(2)R overexpressing animal model. The P2Y(2)R overexpressing transgenic rat was generated using a lentiviral vector. Rats overexpressing P2Y(2)R showed a significant increase in P2Y(2)R mRNA levels in all tissues screened as compared to nontransgenic rats. Fura 2 imaging of smooth muscle cells (SMCs) isolated from aorta indicated that the percentage of cells exhibiting increases in the intracellular free calcium concentration in response to P2Y(2)R agonists was significantly greater in freshly isolated SMCs from transgenic rats than wild-type controls. Histopathological examination of tissues revealed that P2Y(2)R overexpressing rats develop lymphocytic infiltration in lacrimal glands and kidneys as early as at 3 months of age. These rats show similarities to patients with Sjogren's syndrome who display lymphocyte-mediated tissue damage. This transgenic rat model of P2Y(2)R overexpression may prove useful for linking P2Y(2)R upregulation with chronic inflammatory diseases, neurodegenerative diseases and Sjogren's syndrome.

Extracellular adenosine 5'-triphosphate elicits the expression of brain-derived neurotrophic factor exon IV mRNA in rat astrocytes

A growing body of recent evidence indicates that ATP plays an important role in neuronal-glial communications. In this study, the authors demonstrated that extracellular ATP elicits the gene expression of brain-derived neurotrophic factor (BDNF), especially BDNF exon IV mRNA, in primary cultured rat cortical astrocytes but not in neurons. To investigate the mechanism by which ATP induces BDNF exon IV mRNA expression, the authors used immortalized astrocyte cell line RCG-12. ATP dose-dependently increased the expression of BDNF exon IV mRNA and activated BDNF promoter IV. P2Y receptor agonists (ADP and 2MeS-ADP) but not a P2X receptor agonist (alphabetaMeATP) induced the expression of BDNF exon IV mRNA. Moreover, ATP-induced BDNF exon IV mRNA upregulation was inhibited by a P2Y antagonist (MRS2179) but not by P2X antagonists (TNP-ATP and PPADS). These findings suggest the involvement of P2Y receptors in the ATP-induced transcription of the BDNF gene. Among the signal transduction inhibiters examined in this study, intracellular Ca(2+) chelator (BAPTA-AM) and Ca(2+)/calmodulin-dependent kinase (CaM kinase) inhibitors (KN-93 and W-7) attenuated ATP-induced BDNF exon IV mRNA upregulation. ATP transiently induced the phosphorylation of cAMP-responsive element-binding protein (CREB). ATP-induced CREB phosphorylation was repressed by P2Y antagonists, BAPTA-AM, and CaM kinase inhibitors. Overexpression of dominant negative CREB mutants reduced the activation of BDNF promoter IV and attenuated the upregulation of BDNF exon IV mRNA expression. These results suggest that ATP induces BDNF expression through P2Y receptor followed by the activation of CaM kinase and CREB in astrocytes. These mechanisms are likely to contribute to the enhancement of neuronal-glial networks.

Dinucleotide polyphosphates contribute to purinergic signalling via inhibition of adenylate kinase activity

Dinucleoside polyphosphates are well described as direct vasoconstrictors and as mediators with strong proliferative properties, however, less is known about their effects on nucleotide-converting pathways. Therefore, the present study investigates the effects of Ap(4)A (diadenosine tetraphosphate), Up(4)A (uridine adenosine tetraphosphate) and Ap(5)A (diadenosine pentaphosphate) and the non-selective P2 antagonist suramin on human serum and endothelial nucleotide-converting enzymes. Human serum and HUVECs (human umbilical vein endothelial cells) were pretreated with various concentrations of dinucleotide polyphosphates and suramin. Adenylate kinase and NDP kinase activities were then quantified radiochemically by TLC analysis of the ATP-induced conversion of [(3)H]AMP and [(3)H]ADP into [(3)H]ADP/ATP and [(3)H]ATP respectively. Endothelial NTPDase (nucleoside triphosphate diphosphohydrolase) activity was additionally determined using [(3)H]ADP and [(3)H]ATP as preferred substrates. Dinucleoside polyphosphates and suramin have an inhibitory effect on the serum adenylate kinase [pIC(50) values (-log IC(50)): Ap(4)A, 4.67+/-0.03; Up(4)A, 3.70+/-0.10; Ap(5)A, 6.31+/-0.03; suramin, 3.74+/-0.07], as well as on endothelial adenylate kinase (pIC(50) values: Ap(4)A, 4.17+/-0.07; Up(4)A, 2.94+/-0.02; Ap(5)A, 5.97+/-0.04; suramin, 4.23+/-0.07), but no significant effects on serum NDP kinase, emphasizing the selectivity of these inhibitors. Furthermore, Ap(4)A, Up(4)A, Ap(5)A and suramin progressively inhibited the rates of [(3)H]ADP (pIC(50) values: Ap(4)A, 3.38+/-0.09; Up(4)A, 2.78+/-0.06; Ap(5)A, 4.42+/-0.11; suramin, 4.10+/-0.07) and [(3)H]ATP (pIC(50) values: Ap(4)A, 3.06+/-0.06; Ap(5)A, 3.05+/-0.12; suramin, 4.14+/-0.05) hydrolyses by cultured HUVECs. Up(4)A has no significant effect on the endothelial NTPDase activity. Although the half-lives for Ap(4)A, Up(4)A and Ap(5)A in serum are comparable with the incubation times of the assays used in the present study, secondary effects of the dinucleotide metabolites are not prominent for these inhibitory effects, since the concentration of metabolites formed are relatively insignificant compared with the 800 mumol/l ATP added as a phosphate donor in the adenylate kinase and NDP kinase assays. This comparative competitive study suggests that Ap(4)A and Ap(5)A contribute to the purinergic responses via inhibition of adenylate-kinase-mediated conversion of endogenous ADP, whereas Up(4)A most likely mediates its vasoregulatory effects via direct binding-mediated mechanisms.

BM88/Cend1 regulates stimuli-induced intracellular calcium mobilization

In neurogenesis, little is known about signal transduction pathways upstream of gene expression however, mounting evidence suggests that calcium release from internal stores plays a critical role. We have previously demonstrated that BM88 is a neuronal lineage-specific regulator of cell cycle exit and differentiation; we now report a link between BM88 and calcium signaling. Calcium imaging experiments revealed that P2Y-induced calcium mobilization is diminished in mouse neuroblastoma Neuro 2a cells stably transfected with BM88 (N2A-BM88 cells) as compared with N2A cells or N2A cells differentiated with retinoic acid. This effect is not restricted to N2A cells but is also observed in HeLa cells that are transiently transfected with BM88, indicating that cells of both neural and non-neural origin respond similarly. Further, activation of P2Y1 but not purinergic P2X receptors induces proliferation of N2A and to a lesser extent of N2A-BM88 cells. Conversely, knockdown of BM88 facilitates N2A cell proliferation both under stimulating and non-stimulating conditions. Importantly, N2A-BM88 cells are less susceptible to apoptosis triggered by C2-ceramide and exhibit reduced C2-ceramide-induced intracellular calcium release. Higher calcium uptake from mitochondria and/or lower calcium levels inside the endoplasmic reticulum may explain the reduced calcium mobilization in response to BM88. Overall, our data reveal a novel signaling mechanism by which BM88 interferes with calcium release from inositol 1,4,5-trisphosphate-sensitive stores and exerts anti-proliferative and anti-apoptotic functions.

Purinergic signalling in the nervous system: an overview

Purinergic receptors, represented by several families, are arguably the most abundant receptors in living organisms and appeared early in evolution. After slow acceptance, purinergic signalling in both peripheral and central nervous systems is a rapidly expanding field. Here, we emphasize purinergic co-transmission, mechanisms of release and breakdown of ATP, ion channel and G-protein-coupled-receptor subtypes for purines and pyrimidines, the role of purines and pyrimidines in neuron-glial communication and interactions of this system with other transmitter systems. We also highlight recent data involving purinergic signalling in pathological conditions, including pain, trauma, ischaemia, epilepsy, migraine, psychiatric disorders and drug addiction, which we expect will lead to the development of therapeutic strategies for these disorders with novel mechanisms of action.

ATP-induced apoptosis involves a Ca2+-independent phospholipase A2 and 5-lipoxygenase in macrophages

Macrophages express P2X(7) and other nucleotide (P2) receptors, and display the phenomena of extracellular ATP (ATP(e))-induced P2X(7)-dependent membrane permeabilization and cell death by apoptosis and necrosis. P2X(7) receptors also cooperate with toll-like receptors (TLRs) to induce inflammasome activation and IL-1beta secretion. We investigated signaling pathways involved in the induction of cell death by ATP(e) in intraperitoneal murine macrophages. Apoptosis (hypodiploid nuclei) and necrosis (LDH release) were detected 6h after an induction period of 20 min in the presence of ATP. Apoptosis was blocked by caspase 3 and caspase 9 inhibitors and by cyclosporin A. The MAPK inhibitors PD-98059, SB-203580 and SB-202190 provoked no significant effect on apoptosis, but SB-203580 blocked LDH release. Neither apoptosis nor necrosis was inhibited when both intra- and extracellular Ca(2+) were chelated during the induction period. Mepacrine, a generic PLA(2) inhibitor and BEL, an inhibitor of Ca(2+)-independent PLA(2) (iPLA(2)) blocked apoptosis, while pBPB and AACOOPF(3), inhibitors of secretory and Ca(2+)-dependent PLA(2) respectively, had no significant effect. Cycloxygenase inhibitors had no effect on apoptosis, while the inhibitors of lipoxygenase (LOX) and leukotriene biosynthesis nordihydroguaiaretic acid (NDGA), zileuton, AA-861, and MK-886 significantly decreased apoptosis. Neither NDGA nor MK-886 blocked apoptosis of 5-LOX(-/-) macrophages. CP-105696 and MK-571, antagonists of leukotriene receptors, had no significant effect on apoptosis. None of the inhibitors of PLA(2) and LOX/leukotriene pathway had a significant inhibitory effect on LDH release. Our results indicate that a Ca(2+)-independent step involving an iPLA(2) and 5-LOX are involved in the triggering of apoptosis but not necrosis by P2X(7) in macrophages.

The proto-oncoprotein SYT (SS18) controls ATP release and regulates cyst formation by polarized MDCK cells

The SYT proto-oncoprotein (also known as SS18) is a gene expression regulator conserved across species. Although its biological function is still unknown, the importance of SYT as a housekeeping protein is illustrated by the lethal phenotype of SYT-null embryos. Notably, SYT is a component of the synovial sarcoma-associated translocation product, the SYT-SSX oncogene. SYT was previously reported as a mediator of cell adhesion. In the present study we show that SYT possesses distinct domains that control MDCK cyst formation in three-dimensional collagen cultures. While the carboxy-half of SYT, the QPGY domain, is required for cyst growth, the amino-terminal region appears to exert on this process a regulatory effect. Further analysis suggested that the purinergic G protein-coupled P2Y receptor signaling is involved in SYT-induced cystogenesis. Activation of this cascade is due to facilitation of ATP release in the extracellular space of polarized MDCK cells by SYT. These studies allow us to begin to understand the vital role of SYT in controlling epithelial morphogenesis and might explain the lethality of its loss in the developing embryo.

ATP-induced P2X7-associated uptake of large molecules involves distinct mechanisms for cations and anions in macrophages

Macrophages express the P2X(7) receptor and other nucleotide (P2) receptors, and display the phenomenon of extracellular ATP (ATP(e))-induced P2X(7)-dependent membrane permeabilization, which occurs through a poorly understood mechanism. We used patch-clamp recordings, cytoplasmic Ca(2+) measurements and fluorescent dye uptake assays to compare P2X(7)-associated transport phenomena of macrophages and HEK-293 cells transfected with P2X(7) receptors (HEK-P2X(7) cells). Both cell types showed inward currents, increase of free cytoplasmic Ca(2+) concentration and the uptake of cationic dyes upon exposure to ATP(e), as previously described. However, in contrast to the macrophages, HEK-P2X(7) cells did not take up anionic dyes and did not display the 440 pS channels (Z pores) under cell-attached patch-clamping conditions. In addition, the transport mechanism of anionic dyes displayed by macrophages was also able to support dye efflux and, once activated at 37 degrees C, it remained active at 4 degrees C, whereas uptake of cationic dyes was temperature-dependent and unidirectional. Our results indicate that the mechanism of ATP(e)-induced dye uptake, usually called a ;permeabilization phenomenon' and associated with a ;permeabilization pore' can be ascribed to at least two distinct mechanisms in macrophages: a diffusional pathway, possibly associated with the 440 pS Z pores, and a cation uptake mechanism that is not diffusional and should be ascribed to an, as yet, unidentified transport mechanism.

Regulation of the pH sensitivity of human P2X receptors by N-linked glycosylation

The human (h) P2X(3) receptor and its mutants deficient in one out of four N-glycosylation sites were expressed in HEK293 cells. Concentration-response curves were generated by whole-cell recordings of alpha,beta-methylene ATP (alpha,beta-meATP)-induced currents. A gradual change of external pH from the alkaline 8.0 to the acidic 5.0 successively decreased the maximum current amplitude (E(max)) without affecting the EC(50) value. The replacement of Asn-139 and -170 by Asp (N139D, N170D) abolished the pH sensitivity of the wild-type (WT) hP2X(3) receptor. In the case of N194D, the E(max) was again the highest at the alkaline pH value with no change from 7.4 to 6.5, whereas in the case of N290D, there was an inverse pH sensitivity, with an increase of E(max) in the acidic range. However, this effect appeared to be due to enhanced protonation by the insertion of Asp into the receptor, because replacement of Asn by the neutral Thr resulted in a comparable potency of alpha,beta-meATP at any of the pH values investigated. In accordance with the reported finding that His-206 is involved in the modulation of WT P2X(3) receptors by protons, we showed that the normal change of E(max) by an acidic, but not alkaline pH was abolished after substitution of this His by Ala. However, the double mutant H206A + N290D did not react to acidification or alkalinization with any change in E(max). In conclusion, only fully N-glycosylated P2X(3) receptors recognize external pH with a modified sensitivity towards alpha,beta-meATP.

Activation of P2Y receptors by ATP and by its analogue, ADPbetaS, triggers two calcium signal pathways in the longitudinal muscle of mouse distal colon

Our previous research showed that ATP and adenosine 5'-O-2-thiodiphosphate (ADPbetaS) induce contractile effects in the longitudinal muscle of mouse distal colon via activation of P2Y receptors which are not P2Y(1) or P2Y(12) subtypes. This study investigated the nature of the P2Y receptor subtype(s) and the mechanisms leading to the intracellular calcium concentration increase necessary to trigger muscular contraction. Motor responses of mouse colonic longitudinal muscle to P2Y receptor agonists were examined in vitro as changes in isometric tension. ATP or ADPbetaS induced muscular contraction, which was not affected by P2Y(11) or P2Y(13) selective antagonists. Calcium-free solution or the calcium channel blocker, nifedipine, failed to modify the contractile responses to ATP or ADPbetaS, which were virtually abolished by depletion of calcium intracellular stores after repetitive addition of carbachol in calcium-free medium with addition of cyclopiazonic acid. Neomycin or U-73122, phospholipase C inhibitors, or 2-aminoethoxy-diphenylborate (2-APB), membrane-permeant IP(3) receptor inhibitor reduced the response to ATP, whilst ryanodine or ruthenium red, inhibiting calcium release from ryanodine-sensitive stores, abolished the response to ADPbetaS. Responses to maximally effective concentrations of ATP and ADPbetaS were not fully additive. Desensitisation with ADPbetaS antagonized the contractile effects of ATP, as desensitisation with ATP antagonized the response to ADPbetaS. In the longitudinal muscle of mouse distal colon, ATP and ADPbetaS induce muscular contraction via a P2Y receptor, coupled to differential signal pathways leading to intracellular calcium increase.

Purinergic (ATP) signaling stimulates JNK1 but not JNK2 MAPK in osteoblast-like cells: contribution of intracellular Ca2+ release, stress activated and L-voltage-dependent calcium influx, PKC and Src kinases

This work shows that ATP activates JNK1, but not JNK2, in rat osteoblasts and ROS-A 17/2.8 osteoblast-like cells. In ROS-A 17/2.8 cells ATP induced JNK1 phosphorylation in a dose- and time-dependent manner. JNK1 phosphorylation also increased after osteoblast stimulation with ATPgammaS and UTP, but not with ADPbetaS. RT-PCR studies supported the expression of P2Y(2) receptor subtype. ATP-induced JNK1 activation was reduced by PI-PLC, IP(3) receptor, PKC and Src inhibitors and by gadolinium, nifedipine and verapamil or a Ca(2+)-free medium. ERK 1/2 or p38 MAPK inhibitors diminished JNK1 activation by ATP, suggesting a cross-talk between these pathways. ATP stimulated osteoblast-like cell proliferation consistent with the participation of P2Y(2) receptors. These results show that P2Y(2) receptor stimulation by ATP induces JNK1 phosphorylation in ROS-A 17/2.8 cells in a way dependent on PI-PLC/IP(3)/intracellular Ca(2+) release and Ca(2+) influx through stress activated and L-type voltage-dependent calcium channels and involves PKC and Src kinases.

P2X1 and P2X5 subunits form the functional P2X receptor in mouse cortical astrocytes

ATP plays an important role in signal transduction between neuronal and glial circuits and within glial networks. Here we describe currents activated by ATP in astrocytes acutely isolated from cortical brain slices by non-enzymatic mechanical dissociation. Brain slices were prepared from transgenic mice that express enhanced green fluorescent protein under the control of the human glial fibrillary acidic protein promoter. Astrocytes were studied by whole-cell voltage clamp. Exogenous ATP evoked inward currents in 75 of 81 astrocytes. In the majority ( approximately 65%) of cells, ATP-induced responses comprising a fast and delayed component; in the remaining subpopulation of astrocytes, ATP triggered a smoother response with rapid peak and slowly decaying plateau phase. The fast component of the response was sensitive to low concentrations of ATP (with EC(50) of approximately 40 nm). All ATP-induced currents were blocked by pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS); they were insensitive to ivermectin. Quantitative real-time PCR demonstrated strong expression of P2X(1) and P2X(5) receptor subunits and some expression of P2X(2) subunit mRNAs. The main properties of the ATP-induced response in cortical astrocytes (high sensitivity to ATP, biphasic kinetics, and sensitivity to PPADS) were very similar to those reported for P2X(1/5) heteromeric receptors studied previously in heterologous expression systems.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Purinergic signaling in the lumen of a normal nephron and in remodeled PKD encapsulated cysts

The nephron is the functional unit of the kidney. Blood and plasma are continually filtered within the glomeruli that begin each nephron. Adenosine 5' triphosphate (ATP) and its metabolites are freely filtered by each glomerulus and enter the lumen of each nephron beginning at the proximal convoluted tubule (PCT). Flow rate, osmolality, and other mechanical or chemical stimuli for ATP secretion are present in each nephron segment. These ATP-release stimuli are also different in each nephron segment due to water or salt permeability or impermeability along different luminal membranes of the cells that line each nephron segment. Each of the above stimuli can trigger additional ATP release into the lumen of a nephron segment. Each nephron-lining epithelial cell is a potential source of secreted ATP. Together with filtered ATP and its metabolites derived from the glomerulus, secreted ATP and adenosine derived from cells along the nephron are likely the principal two of several nucleotide and nucleoside candidates for renal autocrine and paracrine ligands within the tubular fluid of the nephron. This minireview discusses the first principles of purinergic signaling as they relate to the nephron and the urinary bladder. The review discusses how the lumen of a renal tubule presents an ideal purinergic signaling microenvironment. The review also illustrates how remodeled and encapsulated cysts in autosomal dominant polycystic kidney disease (ADPKD) and remodeled pseudocysts in autosomal recessive PKD (ARPKD) of the renal collecting duct likely create an even more ideal microenvironment for purinergic signaling. Once trapped in these closed microenvironments, purinergic signaling becomes chronic and likely plays a significant epigenetic and detrimental role in the secondary progression of PKD, once the remodeling of the renal tissue has begun. In PKD cystic microenvironments, we argue that normal purinergic signaling within the lumen of the nephron provides detrimental acceleration of ADPKD once remodeling is complete.

P2X receptors and synaptic plasticity

Adenosine triphosphate (ATP) is released in many synapses in the CNS either together with other neurotransmitters, such as glutamate and GABA, or on its own. Postsynaptic action of ATP is mediated through metabotropic P2Y and ionotropic P2X receptors abundantly expressed in neural cells. Activation of P2X receptors induces fast excitatory postsynaptic currents in synapses located in various brain regions, including medial habenula, hippocampus and cortex. P2X receptors display relatively high Ca2+ permeability and can mediate substantial Ca2+ influx at resting membrane potential. P2X receptors can dynamically interact with other neurotransmitter receptors, including N-methyl-D-aspartate (NMDA) receptors, GABA(A) receptors and nicotinic acetylcholine (ACh) receptors. Activation of P2X receptors has multiple modulatory effects on synaptic plasticity, either inhibiting or facilitating the long-term changes of synaptic strength depending on physiological context. At the same time precise mechanisms of P2X-dependent regulation of synaptic plasticity remain elusive. Further understanding of the role of P2X receptors in regulation of synaptic transmission in the CNS requires dissection of P2X-mediated effects on pre-synaptic terminals, postsynaptic membrane and glial cells.

Volume-sensitive NADPH oxidase activity and taurine efflux in NIH3T3 mouse fibroblasts

Reactive oxygen species (ROS) are produced in NIH3T3 fibroblasts during hypotonic stress, and H(2)O(2) potentiates the concomitant release of the organic osmolyte taurine (Lambert IH. J Membr Biol 192: 19-32, 2003). The increase in ROS production [5-(and-6)-carboxy-2', 7'-dichlorodihydrofluorescein diacetate fluorescence] is detectable after a reduction in the extracellular osmolarity from 335 mosM (isotonic) to 300 mosM and reaches a maximal value after a reduction to 260 mosM. The swelling-induced ROS production is reduced by the flavoprotein inhibitor diphenylene iodonium chloride (25 microM) but is unaffected by the nitric oxide synthase inhibitor N omega-nitro-l-arginine methyl ester, indicating that the volume-sensitive ROS production is NADPH oxidase dependent. NIH3T3 cells express the NADPH oxidase components: p22 phox, a NOX4 isotype; p47 phox; and p67 phox (real-time PCR). Exposure to the Ca2+-mobilizing agonist ATP (10 microM) potentiates the release of taurine but has no effect on ROS production under hypotonic conditions. On the other hand, addition of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA, 100 nM) or the lipid messenger lysophosphatidic acid (LPA, 10 nM) potentiates the swelling-induced taurine release as well as the ROS production. Overexpression of Rac1 or p47 phox or p47 phox knockdown [small interfering (si)RNA] had no effect on the swelling-induced ROS production or taurine release. NOX4 knockdown (siRNA) impairs the increase in the ROS production and the concomitant taurine release following osmotic exposure. It is suggested that a NOX4 isotype plus p22 phox account for the swelling-induced increase in the ROS production in NIH3T3 cells and that the oxidase activity is potentiated by PKC and LPA but not by Ca2+.

Signaling and cross-talk by C5a and UDP in macrophages selectively use PLCbeta3 to regulate intracellular free calcium

Studies in fibroblasts, neurons, and platelets have demonstrated the integration of signals from different G protein-coupled receptors (GPCRs) in raising intracellular free Ca(2+). To study signal integration in macrophages, we screened RAW264.7 cells and bone marrow-derived macrophages (BMDM) for their Ca(2+) response to GPCR ligands. We found a synergistic response to complement component 5a (C5a) in combination with uridine 5'-diphosphate (UDP), platelet activating factor (PAF), or lysophosphatidic acid (LPA). The C5a response was Galpha(i)-dependent, whereas the UDP, PAF, and LPA responses were Galpha(q)-dependent. Synergy between C5a and UDP, mediated by the C5a and P2Y6 receptors, required dual receptor occupancy, and affected the initial release of Ca(2+) from intracellular stores as well as sustained Ca(2+) levels. C5a and UDP synergized in generating inositol 1,4,5-trisphosphate, suggesting synergy in activating phospholipase C (PLC) beta. Macrophages expressed transcripts for three PLCbeta isoforms (PLCbeta2, PLCbeta3, and PLCbeta4), but GPCR ligands selectively used these isoforms in Ca(2+) signaling. C5a predominantly used PLCbeta3, whereas UDP used PLCbeta3 but also PLCbeta4. Neither ligand required PLCbeta2. Synergy between C5a and UDP likewise depended primarily on PLCbeta3. Importantly, the Ca(2+) signaling deficiency observed in PLCbeta3-deficient BMDM was reversed by re-constitution with PLCbeta3. Neither phosphatidylinositol (PI) 3-kinase nor protein kinase C was required for synergy. In contrast to Ca(2+), PI 3-kinase activation by C5a was inhibited by UDP, as was macropinocytosis, which depends on PI 3-kinase. PLCbeta3 may thus provide a selective target for inhibiting Ca(2+) responses to mediators of inflammation, including C5a, UDP, PAF, and LPA.

Extracellular ATP activates the PLC/PKC/ERK signaling pathway through the P2Y2 purinergic receptor leading to the induction of early growth response 1 expression and the inhibition of viability in human endometrial stromal cells

ATP is an extracellular signaling molecule that activates specific G protein-coupled P2Y receptors in most cell types to mediate diverse biological effects. ATP has been shown to activate the phospholipase C (PLC)/diacylglycerol/protein kinase C (PKC) pathway in various systems. However, little is known about the signaling events in human endometrial stromal cells (hESCs). The objective of this study was to examine the presence of the P2Y2 receptor and the effects of exogenous ATP on the intracellular mitogen-activated protein kinases (MAPKs) signaling pathway, immediate early genes expression, and cell viability in hESCs. Western blot analysis, gene array analysis, and MTT assay for cell viability were performed. The current study demonstrated the existence of the P2Y2 purinergic receptor in hESCs. UTP and ATP activated MAPK in a dose- and time-dependent manner. Suramin (a P2-purinoceptor antagonist), neomycin (a PLC inhibitor), staurosporin (a PKC inhibitor), and PD98059 (a MEK inhibitor) significantly attenuated the ATP-induced activation of MAPK. ATP activated ERK1/2 and induced translocation of activated ERK1/2 to the nucleus. The gene array for 23 genes associated with members of the mitogenic pathway cascade and immediate early genes revealed that the expression of early growth response 1 was increased. In addition, MTT assay revealed an inhibition effect of ATP on cell viability. ATP activated MAPKs through the P2Y2 purinoceptor/PLC/PKC/ERK signaling pathway and induced translocation of ERK1/2 into the nucleus. Further, ATP induced the expression of early growth response 1 and inhibited cell viability in hESCs.

Purinergic receptors in the endocrine and exocrine pancreas

The pancreas is a complex gland performing both endocrine and exocrine functions. In recent years there has been increasing evidence that both endocrine and exocrine cells possess purinergic receptors, which influence processes such as insulin secretion and epithelial ion transport. Most commonly, these processes have been viewed separately. In beta cells, stimulation of P2Y(1) receptors amplifies secretion of insulin in the presence of glucose. Nucleotides released from secretory granules could also contribute to autocrine/paracrine regulation in pancreatic islets. In addition to P2Y(1) receptors, there is also evidence for other P2 and adenosine receptors in beta cells (P2Y(2), P2Y(4), P2Y(6), P2X subtypes and A(1) receptors) and in glucagon-secreting alpha cells (P2X(7), A(2) receptors). In the exocrine pancreas, acini release ATP and ATP-hydrolysing and ATP-generating enzymes. P2 receptors are prominent in pancreatic ducts, and several studies indicate that P2Y(2), P2Y(4), P2Y(11), P2X(4) and P2X(7) receptors could regulate secretion, primarily by affecting Cl(-) and K(+) channels and intracellular Ca(2+) signalling. In order to understand the physiology of the whole organ, it is necessary to consider the full complement of purinergic receptors on different cells as well as the structural and functional relation between various cells within the whole organ. In addition to the possible physiological function of purinergic receptors, this review analyses whether the receptors could be potential therapeutic targets for drug design aimed at treatment of pancreatic diseases.

Expression and function of the P2X(7) receptor in rat C6 glioma cells

Our results demonstrate the first findings of expression and function of the purinergic P2X7 receptor (P2X7R) in rat C6 glioma cells. P2X7R mRNA and protein were present in unstimulated C6 cells and were up-regulated by cell exposure to the P2X7R agonist, 2',3'-(benzoyl-4-benzoyl)-ATP (BzATP). Activation of P2X7R in C6 in response to BzATP led to increased mobilization of intracellular calcium [Ca2+]i and formation of large pores. Chronic exposure of C6 cells to BzATP enhanced the expression of pro-inflammatory factors including MCP-1, IL-8 and VEGF. In a scratch-wound migration assay, the P2X7R was shown to regulate cell mobility. The overall results suggest that P2X7R activation in C6 is linked with increased pro-inflammatory factors and tumor cell migration.

P2 receptors in the regulation of renal transport mechanisms

Extracellular nucleotides (e.g., ATP) regulate physiological and pathophysiological processes through activation of nucleotide P2 receptors in the plasma membrane. Examples include such diverse processes as communication from taste buds to gustatory nerves, platelet aggregation, nociception, or neutrophil chemotaxis. Over approximately the last 15 years, evidence has also accumulated that cells in renal epithelia release nucleotides in response to physiological stimuli and that these nucleotides act in a paracrine and autocrine way to activate P2 receptors and play a significant role in the regulation of transport mechanisms and cell volume regulation. This review discusses potential stimuli and mechanisms involved in nucleotide release in renal epithelia and summarizes the available data on the expression and function of nucleotide P2 receptors along the native mammalian tubular and collecting duct system. Using established agonist profiles for P2 receptor subtypes, significant insights have been gained particularly into a potential role for P2Y(2)-like receptors in the regulation of transport mechanisms in the collecting duct. Due to the lack of receptor subtype-specific antagonists, however, the in vivo relevance of P2 receptor subtypes is unclear. Studies in gene knockout mice provided first insights including an antihypertensive activity of P2Y(2) receptors that is linked to an inhibitory influence on renal Na(+) and water reabsorption. We are only beginning to unravel the important roles of extracellular nucleotides and P2 receptors in the regulation of the diverse transport mechanisms of the kidney.

P2Y nucleotide receptor signaling through MAPK/ERK is regulated by extracellular matrix: involvement of beta3 integrins

Extracellular matrix influences cell behavior through receptors such as integrins and through transmission of mechanical forces. Nucleotides are released in response to mechanical stimuli and bind to P2 nucleotide receptors. As chondrocytes are subjected to frequent mechanical stimulation within a rich extracellular matrix, they are an excellent model for studying integration of signals induced by matrix and nucleotides. We investigated signaling of G protein-coupled P2Y receptors to MAPK/ERK and how this is influenced by matrix. Rat articular chondrocytes expressed transcripts for P2Y1, P2Y2, P2Y4, and P2Y6 receptors and responded to extracellular nucleotides by transient elevation of cytosolic calcium and MAPK/ERK phosphorylation. ERK1/2 activation was suppressed by the protein kinase C (PKC) inhibitors bisindolylmaleimide I and rottlerin, and by the phospholipase D inhibitor 1-butanol. Thus, nucleotides stimulate P2Y receptors to activate ERK1/2 through a mechanism dependent on PKC and phospholipase D. We next examined the involvement of integrins. Both an RGD-containing pentapeptide and a beta3 integrin blocking antibody, but not a beta1 integrin blocking antibody, abolished nucleotide-induced ERK1/2 phosphorylation. Moreover, chondrocytes adhering to fibronectin (which binds to beta1 and beta3 containing integrins in an RGD-dependent manner) displayed prolonged ERK1/2 signaling compared to cells grown on type I or II collagen (which bind to beta1-containing integrins in an RGD-independent manner). In conclusion, P2Y receptor signaling through ERK1/2 is gated selectively by matrix proteins. Thus, nucleotides released in response to mechanical stimulation will have differing effects on cell function due to changes in the composition of the extracellular matrix during development and disease.

Do ATP and NO interact in the CNS?

Enzymatically derived NO and extracellular ATP are receiving greater attention due to their role as messengers in the CNS during different physiological and pathological processes. Ionotropic (P2XR) and metabotropic (P2YR) purinergic receptors mediate ATP effects and are present throughout the body. Particularly P2XR are crucial for brain plasticity mechanisms, and are involved in the pathogenesis of different CNS illnesses. NO does not have a specific receptor and its actions are directly dependent on the production on demand by different nitric oxide synthase isoforms. NO synthesizing enzymes are present virtually in all tissues, and NO influences multifarious physiological and pathological functions. Interestingly, various are the tissue and organs modulated by both ATP and NO, such as the immune, brain and vascular systems. Moreover, direct interactions between purinergic and nitrergic mechanisms outside the CNS are well documented, with several studies also indicating that ATP and NO do participate to the same CNS functions. In the past few years, further experimental evidence supported the physiological and pathological relevance of ATP and NO direct interactions in the CNS. The aim of the present review is to provide an account of the available information on the interplay between purinergic and nitrergic systems, focussing on the CNS. The already established relevance of ATP and NO in different pathological processes would predict that the knowledge of ATP/NO cross-talk mechanisms would support pharmacological approaches toward the development of novel ATP/NO combined pharmacological agents.

Ca(2+) influx through P2X receptors induces actin cytoskeleton reorganization by the formation of cofilin rods in neurites

In physiological and pathological events, extracellular ATP plays an important role by controlling several types of purinergic receptors and changing cytoskeleton dynamics. To know the process of ATP-dependent cytoskeleton remodeling, we focused on cofilin, a key regulator of actin cytoskeleton, and investigated the dynamics of cofilin in PC12 cells through fluorescent protein-labeled cofilin and actin, Ca(2+) imaging, and fluorescence resonance energy transfer (FRET) techniques. As a result, ATP induced intracellular Ca(2+) increase, following cofilin rods' formation. ATP-induced cofilin rods' formation was not observed in cells expressing unphosphorylatable variant of cofilin. A P2X receptor agonist, but not P2Y, induced the formation of cofilin rods, whereas calmodulin and calcineurin inhibitors suppressed it. These results indicate that Ca(2+) influx through P2X receptors induces the formation of cofilin rods via calcineurin-dependent dephosphorylation of cofilin. This pathway might be one candidate to explain the effects of ATP on neuronal development and injury.

ATP and acetylcholine, equal brethren

Acetylcholine was the first neurotransmitter identified and ATP is the hitherto final compound added to the list of small molecule neurotransmitters. Despite the wealth of evidence assigning a signaling role to extracellular ATP and other nucleotides in neural and non-neural tissues, the significance of this signaling pathway was accepted very reluctantly. In view of this, this short commentary contrasts the principal molecular and functional components of the cholinergic signaling pathway with those of ATP and other nucleotides. It highlights pathways of their discovery and analyses tissue distribution, synthesis, uptake, vesicular storage, receptors, release, extracellular hydrolysis as well as pathophysiological significance. There are differences but also striking similarities. Comparable to ACh, ATP is taken up and stored in synaptic vesicles, released in a Ca(2+)-dependent manner, acts on nearby ligand-gated or metabotropic receptors and is hydrolyzed extracellularly. ATP and acetylcholine are also costored and coreleased. In addition, ATP is coreleased from biogenic amine storing nerve terminals as well as from at least subpopulations of glutamatergic and GABAergic terminals. Both ACh and ATP fulfill the criteria postulated for neurotransmitters. More recent evidence reveals that the two messengers are not confined to neural functions, exerting a considerable variety of non-neural functions in non-innervated tissues. While it has long been known that a substantial number of pathologies originate from malfunctions of the cholinergic system there is now ample evidence that numerous pathological conditions have a purinergic component.

The ectonucleotidases alkaline phosphatase and nucleoside triphosphate diphosphohydrolase 2 are associated with subsets of progenitor cell populations in the mouse embryonic, postnatal and adult neurogenic zones

Subventricular zone (SVZ)-derived adult neurospheres express two ectonucleotidases, nucleoside triphosphate diphosphohydrolase 2 (NTPDase2) and tissue non-specific alkaline phosphatase (TNAP). Agonists of the nucleotide receptors P2Y(1) and P2Y(2) as well as adenosine augment growth factor-mediated progenitor cell proliferation. NTPDase2 converts ATP and UTP to ADP and UDP, respectively, which are all P2Y receptor agonists. TNAP hydrolyzes nucleoside triphosphates and diphosphates and produces the P1 receptor agonist adenosine. In the SVZ, NTPDase2 is specifically expressed by type B cells. In order to further scrutinize the association of key molecules of the purinergic signaling pathway with neurogenic regions, we analyzed the expression of TNAP at the lateral ventricles of the adult and developing mouse brain. In the adult brain, TNAP was expressed by type B, type A and at least subsets of type C cells of the SVZ and throughout the rostral migratory stream. Almost 100% of the proliferating, Ki-67-positive cells of the adult SVZ stained for TNAP, supporting the notion of a ubiquitous association of TNAP with SVZ progenitors. In contrast, NTPDase2-positive progenitors of the dentate gyrus were TNAP-negative. Essentially all cells of the telencephalic vesicle at embryonic day (E) 14 revealed TNAP activity, including doublecortin-positive neuroblasts. During further embryonic development, enhanced TNAP activity became restricted to cells of the ventricular and SVZ. In contrast to TNAP, NTPDase2 was first expressed in the SVZ perinatally, in association with TNAP-positive SVZ border cells. During later development, NTPDase2-positive cells disappeared from the ventricular surface and began to form sheaths around clusters of subventricular doublecortin-positive cells, apparently transforming into type B cells. Our results identify TNAP and NTPDase2 as novel markers for subsets of progenitors in the adult and developing mouse brain. They further support the notion that signaling via extracellular nucleotides and nucleosides contributes to embryonic and adult neurogenesis.

The vascular ectonucleotidase ENTPD1 is a novel renoprotective factor in diabetic nephropathy

Ectonucleoside triphosphate diphosphohydrolase 1 (ENTPD1) (also known as CD39) is the dominant vascular ectonucleotidase. By hydrolyzing ATP and ADP to AMP, ENTPD1 regulates ligand availability to a large family of P2 (purinergic) receptors. Modulation of extracellular nucleotide metabolism is an important factor in several acute and subacute models of vascular injury. We hypothesized that aberrant nucleotide signaling would promote chronic glomerular injury in diabetic nephropathy. Inducing diabetes in ENTPD1-null mice with streptozotocin resulted in increased proteinuria and more severe glomerular sclerosis compared with matched diabetic wild-type mice. Diabetic ENTPD1-null mice also had more glomerular fibrin deposition and glomerular plasminogen activator inhibitor-1 (PAI-1) staining than wild-type controls. In addition, ENTPD1-null mice showed increased glomerular inflammation, in association with higher levels of monocyte chemoattractant protein-1 (MCP-1) expression. Mesangial cell PAI-1 and MCP-1 mRNA expression were upregulated by ATP and UTP but not ADP or adenosine in vitro. The stable nucleotide analog ATPgammaS stimulated sustained expression of PAI-1 and MCP-1 in vitro, whereas the stable adenosine analog NECA [5'-(N-ethylcarboxamido)adenosine] downregulated expression of both genes. Extracellular nucleotide-stimulated upregulation of MCP-1 is, at least in part, protein kinase C dependent. We conclude that ENTPD1 is a vascular protective factor in diabetic nephropathy that modulates glomerular inflammation and thromboregulation.

P2X3 receptor involvement in pain states

The understanding of how pain is processed at each stage in the peripheral and central nervous system is the precondition to develop new therapies for the selective treatment of pain. In the periphery, ATP can be released from various cells as a consequence of tissue injury or visceral distension and may stimulate the local nociceptors. The highly selective distribution of P2X(3) and P2X(2/3) receptors within the nociceptive system has inspired a variety of approaches to elucidate the potential role of ATP as a pain mediator. Depolarization by ATP of neurons in pain-relevant neuronal structures such as trigeminal ganglion, dorsal root ganglion, and spinal cord dorsal horn neurons are well investigated. P2X receptor-mediated afferent activation appears to have been implicated in visceral and neuropathic pain and even in migraine and cancer pain. This article reviews recently published research describing the role that ATP and P2X receptors may play in pain perception, highlighting the importance of the P2X(3) receptor in different states of pain.

G alpha(q/11)-coupled P2Y2 nucleotide receptor inhibits human keratinocyte spreading and migration

Reepithelialization is a critical step in wound healing. It is initiated by keratinocyte migration at the wound edges. After wounding, extracellular nucleotides are released by keratinocytes and other skin cells. Here, we report that activation of P2Y2 nucleotide receptor by ATP/UTP inhibits keratinocyte cell spreading and induces lamellipodium withdrawal. Kymography analysis demonstrates that these effects correlate with a durable decrease of lamellipodium dynamics. P2Y2 receptor activation also induces a dramatic dismantling of the actin network, the loss of alpha3 integrin expression at the cell periphery, and the dissolution of focal contacts as indicated by the alteration of alpha(v) integrins and focal contact protein distribution. In addition, activation of P2Y2R prevents growth factor-induced phosphorylation of Erk(1,2) and Akt/PkB. The use of a specific pharmacological inhibitor (YM-254890), the depletion of G alpha(q/11) by siRNA, or the expression of a constitutively active G alpha(q/11) mutant (Q209L) show that activation of G alpha(q/11) is responsible for these ATP/UTP-induced effects. Finally, we report that ATP delays growth factor-induced wound healing of keratinocyte monolayers. Collectively, these findings provide evidence for a unique and important role for extracellular nucleotides as efficient autocrine/paracrine regulators of keratinocyte shape and migration during wound healing.

Extracellular ATP and ATPgammaS suppress the proliferation of human periodontal ligament cells by different mechanisms

BACKGROUND

Periodontitis, similar to many other known inflammatory diseases, is thought to increase adenosine triphosphate (ATP) levels in extracellular spaces. Extracellular ATP acts on specific receptors to modulate the proliferation of various cell types. Platelet-rich plasma (PRP) contains high levels of ectonucleotidase activity capable of degrading ATP. The aim of this study was to investigate the effects of ATP on the proliferation of human periodontal ligament (PDL) cells and how these effects are altered by ectonucleotidases.

METHODS

PDL cells were derived from healthy young volunteers. ATP content and DNA synthesis were quantified by a bioluminescence and an enzyme-linked immunosorbent assay, respectively. CD39 and p21(WAF1/cip1) expression was analyzed by Western blot. Apoptosis was evaluated by caspase-3/7 activity, terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate nick end-labeling (TUNEL) activity, annexin-V-binding, and DNA fragmentation.

RESULTS

CD39 and ectonucleotidase-like activity were found in PDL cells and serum, respectively. Because less CD39 is expressed in freshly plated cells, both exogenous ATP and ATPgammaS, a slowly hydrolysable analog, inhibited cell proliferation under low serum conditions. ATP upregulated p21(WAF1/cip1), an inhibitor of cell-cycle progression, whereas ATPgammaS induced caspase-dependent apoptosis. Either upregulation of CD39 or added serum rescued cells from the cytostatic actions of exogenous ATP.

CONCLUSIONS

In PDL cells expressing low CD39 levels, both ATP and ATPgammaS inhibited proliferation but by different mechanisms. ATP-induced growth arrest suggests that periodontal tissue regeneration is often suppressed at the site of injury. Furthermore, added ectonucleotidases protected PDL cells from ATP's cytostatic actions, suggesting that ectonucleotidase-rich PRP augments the regenerative actions of its constituent growth factors by protecting against exogenous ATP at clinical sites.

Mice lacking P2Y2 receptors have salt-resistant hypertension and facilitated renal Na+ and water reabsorption

Extracellular nucleotides (e.g., ATP) regulate many physiological and pathophysiological processes through activation of nucleotide (P2) receptors in the plasma membrane. Here we report that gene-targeted (knockout) mice that lack P2Y2 receptors have salt-resistant arterial hypertension in association with an inverse relationship between salt intake and heart rate, indicating intact baroreceptor function. Knockout mice have multiple alterations in their handling of salt and water: these include suppressed plasma renin and aldosterone concentrations, lower renal expression of the aldosterone-induced epithelial sodium channel alpha-ENaC, greater medullary expression of the Na-K-2Cl-cotransporter NKCC2, and greater furosemide-sensitive Na+ reabsorption in association with greater renal medullary expression of aquaporin-2 and vasopressin-dependent renal cAMP formation and water reabsorption despite similar vasopressin levels compared with wild type. Of note, smaller increases in plasma aldosterone were required to adapt renal Na+ excretion to restricted intake in knockout mice, suggesting a facilitation in renal Na+ retention. The results thus identify a previously unrecognized role for P2Y2 receptors in blood pressure regulation that is linked to an inhibitory influence on renal Na+ and water reabsorption. Based on these findings in knockout mice, we propose that a blunting in P2Y2 receptor expression or activity is a new mechanism for salt-resistant arterial hypertension.

Differential role of extracellular histidines in copper, zinc, magnesium and proton modulation of the P2X7 purinergic receptor

The P2X7 receptor is a non-selective cationic channel activated by extracellular ATP, belonging to the P2X receptor family. To assess the role of extracellular histidines on the allosteric modulation of the rat P2X7 receptor by divalent metals (copper, zinc and magnesium) and protons, these amino acid residues were singly substituted for corresponding alanines. Wild-type and mutated receptors were injected to Xenopus laevis oocytes; metal-related effects were evaluated by the two-electrode voltage-clamp technique. Copper inhibited the ATP-gated currents with a median inhibitory concentration of 4.4 +/- 1.0 micromol/L. The inhibition was non-competitive and time-dependent; copper was 60-fold more potent than zinc. The mutant H267A, resulted in a copper resistant receptor; mutants H201A and H130A were less sensitive to copper inhibition (p < 0.05). The rest of the mutants examined, H62A, H85A, and H219A, conserved the copper-induced inhibition. Only mutants H267A and H219A were less sensitive to the modulator action of zinc. Moreover, the magnesium-induced inhibition was abolished exclusively on the H130A and H201A mutants, suggesting that this metal may act at a novel cationic modulator site. Media acidification inhibited the ATP-gated current 87 +/- 3%; out of the six mutants examined, only H130A was significantly less sensitive to the change in pH, suggesting that His-130 could be involved as a pH sensor. In conclusion, while His-267 is critically involved in the copper or zinc allosteric modulation, the magnesium inhibitory effects is related to His-130 and His-201, His-130 is involved in proton sensing, highlighting the role of defined extracellular histidines in rat P2X7 receptor allosteric modulation.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

P1,P4-diadenosine tetraphosphate (Ap4A) inhibits proximal tubular reabsorption of sodium in rats

BACKGROUND/AIMS

P1,P4-diadenosine tetraphosphate (Ap4A) is a vasoactive dinucleotide possessing natriuretic activity. It is unclear, however, which part of the nephron is the target site of action for Ap4A.

METHODS

We evaluated the tubular sites of Ap4A action using the lithium clearance technique.

RESULTS

Ap4A at a priming dose of 2 micromol/kg with subsequent infusion at 20 nmol/kg/min increased fractional water and sodium excretion 2.5- and 5.6-fold, respectively. Moreover, Ap4A increased lithium clearance 1.9-fold and fractional lithium excretion 2.8-fold. Fractional water and sodium excretion from distal nephron segments was not significantly affected by Ap4A.

CONCLUSION

These results suggest that Ap4A induces natriuresis mainly through inhibition of proximal tubular reabsorption of sodium.

Purinergic pathway suppresses the release of .NO and stimulates proteoglycan synthesis in chondrocyte/agarose constructs subjected to dynamic compression

Mechanical loading plays a fundamental role in the physiological and pathological processes of articular cartilage. The application of dynamic compression to chondrocytes cultured in agarose, downregulates the release of nitric oxide (NO) and enhances cell proliferation and proteoglycan synthesis. We hypothesize that the observed metabolic changes in response to dynamic compression involve a purinergic signaling pathway. Chondrocyte/agarose constructs were subjected to dynamic compression (15%, 1 Hz, 48 h) in the presence of antagonists for the purinergic pathway. Gadolinium was used as a putative inhibitor of stretch-activated calcium ion channels including adenosine 5'-triphosphate (ATP) release channels; suramin was employed as a P2 receptor antagonist and apyrase was used to catalyze the hydrolysis of extracellular ATP. The data presented demonstrate that in the absence of the inhibitor, dynamic compression suppressed .NO release. Treatment with gadolinium and suramin caused a compression-induced upregulation of .NO release, a response abolished with apyrase. Compression-induced stimulation of cell proliferation was reversed with gadolinium, suramin, or apyrase. By contrast, compression-induced stimulation of proteoglycan synthesis was abolished under all treatment conditions. Thus, the purinergic pathway is important in suppressing the release of .NO and stimulation of proteoglycan synthesis. Indeed, high levels of .NO could trigger a downstream catabolic response and mediate the compression-induced inhibition of cell proliferation. The current study demonstrates for the first time the importance of a purinergic pathway in mediating the metabolic response to dynamic compression and suppressing an inflammatory effect.