Nucleotides as extracellular signalling molecules

An ecto-ATPase and an ecto-ATP diphosphohydrolase are expressed in rat brain

Extracellular nucleotides acting as signaling molecules are inactivated by hydrolysis catalyzed by ecto-nucleotidases. ATP is sequentially degraded via ADP and AMP to adenosine. Enzymes that can be involved in the extracellular hydrolysis chain are ecto-ATP diphosphohydrolase (ecto-apyrase), ecto-ATPase, ecto-ADPase and 5'-nucleotidase. Mammalian ecto-ATP diphosphohydrolase is a member of a family of apyrases sharing four "apyrase conserved regions" that presumably participate in the formation of the catalytic site. We report the presence of ecto-ATP diphosphohydrolase in rat brain and the primary structure of a new mammalian member of the apyrase family. Expression in CHO cells shows that it represents an ecto-ATPase. As revealed by Northern analysis of rat tissues, the ecto-ATPase is co-expressed with ecto-ATP diphosphohydrolase in heart, kidney, spleen, thymus, lung, skeletal muscle and brain. Signals for both ecto-nucleotidases are very weak in liver. mRNAs for both proteins are present in PC12 cells, suggesting that the two nucleotidases may be co-expressed in the same neural cell. Using computer-aided sequence analysis, primary structure and membrane topography are compared with those of other members of the apyrase family.

Fluorescence spectroscopic studies on interactions between liver annexin VI and nucleotides--a possible role for a tryptophan residue

Annexin VI is a 68-kDa calcium-, phospholipid-, and cytoskeletal-element-binding protein, which has been implicated in various processes, including calcium release and sequestration in calcifying cartilage, in a receptor-mediated endocytosis in human fibroblasts, and in secretion from chromaffin granules. In these processes it was found that, in addition to Ca2+ and annexin, the presence of ATP is also a prerequisite. In the present report we show that annexin VI binds ATP and the binding of nucleotide to protein is accompanied by quenching of an intrinsic fluorescence of annexin VI, which was found to be specific for 2'-(or 3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate, GTP and ATP, and dependent on the annexin conformation. The nucleotide-binding site within an annexin VI molecule is likely to be close to the tryptophan-containing domain of annexin VI. We propose that ATP plays the role of a physiological ligand for annexin VI, and its binding to annexin VI may represent an alternative cellular mechanism for the regulation of annexin-membrane interactions coupled to overall energy transitions in the cell.

Hereditary disorders of purine and pyrimidine metabolism: identification of their biochemical phenotypes in the clinical laboratory

OBJECTIVE

To describe a laboratory approach to the diagnosis of hereditary diseases of purine and pyrimidine metabolism and emphasize clinical situations in which these disorders should be considered in the differential diagnosis.

DESIGN

Disease-specific patterns were identified in random specimens of ultrafiltered urine by using gradient high-performance liquid chromatography with diode-array detection, and reference ranges were established for uric acid, hypoxanthine, xanthine, and uracil expressed per creatinine in random specimens of urine.

MATERIAL AND METHODS

Diagnostically significant purines and pyrimidines were separated with use of a Supelco LC-18-S nucleoside column eluted with 25 mmol/L ammonium acetate buffer and acetonitrile-methanol-water. Biologic fluids were prepared by ultrafiltration after addition of 3-methyluridine as internal standard. We used specimens negative for screening of metabolic disorders to establish reference ranges.

RESULTS

Disease-specific patterns were identified in specimens with purine and pyrimidine disorders and several urea cycle disorders characterized by increased production of pyrimidine.

CONCLUSION

The approach described identified disease-specific patterns of purine and pyrimidine disorders and several urea cycle disorders. We suggest that testing for purine and pyrimidine disorders be done in specimens evaluated in metabolic laboratories for "screening for inborn errors of metabolism."

Effects of P2 purinergic receptor stimulation in brown adipocytes

Sympathetic stimulation of brown adipocytes plays a major role in body energy homeostasis by activating energy-wasting pathways. Sympathetic neuronal input initiates a variety of metabolic, developmental, and membrane responses in brown fat cells. Many of these actions are mediated by adrenergic pathways mobilized by released norepinephrine. However, since sympathetic stimulation may also release vesicular ATP, we tested brown fat cells for ATP responses. Micromolar concentrations of extracellular ATP had a number of effects on brown adipocytes. We have shown previously that ATP elicits substantial (average of approximately 30%) increases in cell membrane capacitance (P. A. Pappone and S. C. Lee, J. Gen. Physiol. 108: 393-404, 1996). Here, we show that cytosolic calcium levels were increased by ATP, both through release from intracellular stores and through influx, as assessed by fura 2 imaging. In addition, ATP indirectly activated a nonselective cation conductance that was independent of cytosolic calcium levels in patch voltage-clamped brown fat cells. Similar calcium, conductance, and capacitance responses could be activated by 2-methylthio-ATP and ADP, consistent with mediation by a P2 type purinergic receptor. Calorimetric measurements from cell suspensions showed that ATP increased basal heat production of isolated brown fat cells by approximately 40% but had no effect on the greater than fivefold increase in heat production seen with maximal adrenergic stimulation. These myriad responses to extracellular ATP suggest that P2 receptor-mediated signaling is important in brown adipocyte physiology and that sympathetic stimulation may normally activate purinergic as well as adrenergic pathways in brown fat.

ATP induces Ca2+ release from IP3-sensitive Ca2+ stores exclusively in large DRG neurones

PURINORECEPTOR-MEDIATED intracellular Ca2+ release was studied in freshly isolated adult mouse dorsal root ganglia (DRG) neurones. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured using indo-1-based microfluorimetry. The application of 100 microM ATP in Ca(2+)-free solution triggered an increase in [Ca2+]i in 93% of large DRG neurones but in no small ones. The ATP-induced [Ca2+]i transients in large neurones were inhibited by cells incubation with thapsigargin or by intracellular dialysis with heparin-containing solution. The ATP-triggered increase in [Ca2+]i was not mimicked by adenosine and was blocked by suramin, suggesting the involvement of metabotropic (PZY) purinoreceptors. We conclude that large (proprioceptive) DRG neurones express PZY purinoreceptors linked to the inositol 1,4,5-triphosphate-Ca2+ intracellular signal transduction cascade, whereas small (nociceptive) DRG neurones are devoid of such a mechanism.

Diadenosine polyphosphate-stimulated gluconeogenesis in isolated rat proximal tubules

Diadenosine polyphosphates released into the extracellular environment influence a variety of metabolic and other cellular activities in a wide range of target tissues. Here we have studied the impact of these novel nucleotides on gluconeogenesis in isolated rat proximal tubules. Gluconeogenesis was stimulated following exposure of isolated proximal tubules to a range of adenine-containing nucleotides including ADP, ATP, Ap3A, Ap4A, Ap5A and Ap6A. The concentration-dependence of ATP-, Ap3A- and Ap4A-mediated stimulation of gluconeogenesis was similar and was consistent with a role for these agents in the physiological control of renal metabolism. Nucleotide-stimulated gluconeogenesis was diminished in the presence of agents that interfere with phospholipase C activation or intracellular Ca2+ metabolism, indicative of a role for polyphosphoinositide-mediated Ca2+ mobilization in the mechanism of action of ATP, Ap3A and Ap4A. The characteristics of binding of [2-3H]Ap4A to renal plasma-membrane preparations suggest that Ap4A mediates its effects on proximal tubule gluconeogenesis via interaction with P2y-like purinoceptor(s) also recognized by extracellular ATP.

Characterization of the P2 receptors in rabbit pulmonary artery

1. We have identified the P2 receptors mediating vasomotor responses in the rabbit pulmonary artery. 2. Neither ATP nor UTP contracted intact or endothelium-denuded rings. However, both relaxed intact rings of rabbit pulmonary artery that had been preconstricted with phenylephrine (pD2 5.2 and 5.6, respectively). 3. The vasodilator effect of UTP was endothelium-dependent and abolished by the nitric oxide synthase inhibitor NG-nitro-L-arginine (L-NOARG). 4. The vasodilator effect of ATP was only partially inhibited by removal of endothelium or addition of L-NOARG, suggesting an additional direct effect on vascular smooth muscle. 5. The endothelium-dependent vasodilator responses to UTP and ATP were competitively antagonized by suramin. 6. Preconstricted, endothelium-denuded rings were also relaxed by 2-methylthio ATP (pD2 6.6), a P2Y receptor agonist. 7. Ca(2+)-mobilizing P2U receptors were identified on smooth muscle cells on the basis of single cell responses to ATP (pD2 7.8) and UTP (pD2 7.9; 6.7 in the presence of 100 microM suramin). 8. There was no evidence of a Ca(2+)-mobilizing P2Y receptor in these cultured cells. 9. The data suggest the presence of (i) a suramin-sensitive P2U receptor on endothelial cells that induces vasorelaxation through NO release, (ii) a suramin-sensitive P2U receptor on cultured smooth muscle cells that mobilizes Ca2+ but is not coupled to vasomotor responses and (iii) a putative P2Y receptor on vascular smooth muscle cells that induces relaxation via a Ca(2+)-independent signal transduction pathway.

Autotaxin is an exoenzyme possessing 5'-nucleotide phosphodiesterase/ATP pyrophosphatase and ATPase activities

Autotaxin (ATX) is an extracellular enzyme and an autocrine motility factor that stimulates pertussis toxin-sensitive chemotaxis in human melanoma cells at picomolar to nanomolar concentrations. This 125-kDa glycoprotein contains a peptide sequence identified as the catalytic site in type I alkaline phosphodiesterases (PDEs), and it possesses 5'-nucleotide PDE (EC 3.1.4.1) activity (Stracke, M. L., Krutzsch, H. C., Unsworth, E. J., Arestad, A., Cioce, V., Schiffmann, E., and Liotta, L. (1992) J. Biol. Chem. 267, 2524-2529; Murata, J., Lee, H. Y., Clair, T., Krutsch, H. C., Arestad, A. A., Sobel, M. E., Liotta, L. A., and Stracke, M. L. (1994) J. Biol. Chem. 269, 30479-30484). ATX binds ATP and is phosphorylated only on threonine. Thr210 at the PDE active site of ATX is required for phosphorylation, 5'-nucleotide PDE, and motility-stimulating activities (Lee, H. Y., Clair, T., Mulvaney, P. T., Woodhouse, E. C., Aznavoorian, S., Liotta, L. A., and Stracke, M. L. (1996) J. Biol. Chem. 271, 24408-24412). In this article we report that the phosphorylation of ATX is a transient event, being stable at 0 degrees C but unstable at 37 degrees C, and that ATX has adenosine-5'-triphosphatase (ATPase; EC 3.6.1.3) and ATP pyrophosphatase (EC 3.6.1.8) activities. Thus ATX catalyzes the hydrolysis of the phosphodiester bond on either side of the beta-phosphate of ATP. ATX also catalyzes the hydrolysis of GTP to GDP and GMP, of either AMP or PPi to Pi, and the hydrolysis of NAD to AMP, and each of these substrates can serve as a phosphate donor in the phosphorylation of ATX. ATX possesses no detectable protein kinase activity toward histone, myelin basic protein, or casein. These results lead to the proposal that ATX is capable of at least two alternative reaction mechanisms, threonine (T-type) ATPase and 5'-nucleotide PDE/ATP pyrophosphatase, with a common site (Thr210) for the formation of covalently bound reaction intermediates threonine phosphate and threonine adenylate, respectively.

Characterization of recombinant human P2X4 receptor reveals pharmacological differences to the rat homologue

We isolated a cDNA from human brain encoding a purinergic receptor that shows a high degree of homology to the rat P2X4 receptor (87% identity). By fluorescence in situ hybridization, the human P2X4 gene has been mapped to region q24.32 of chromosome 12. Tissue distribution analysis of human P2X4 transcripts demonstrates a broad expression pattern in that the mRNA was detected not only in brain but also in all tissues tested. Heterologous expression of the human P2X4 receptor in Xenopus laevis oocytes and human embryonic kidney 293 cells evoked an ATP-activated channel. Simultaneous whole-cell current and Fura-2 fluorescence measurements in human embronic kidney 293 cells transfected with human P2X4 cDNA allowed us to determine the fraction of the current carried by Ca2: this was approximately 8%, demonstrating a high Ca2+ permeability. Low extracellular Zn2+ concentrations (5-10 microM) increase the apparent gating efficiency of human P2X4 by ATP without affecting the maximal response. However, raising the concentration of the divalent cation (> 100 microM) inhibits the ATP-evoked current in a non-voltage-dependent manner. The human P2X4 receptor displays a very similar agonist potency profile to that of rat P2X4 (ATP > > 2-methylthio-ATP > or = CTP > alpha, beta-methylene-ATP > dATP) but has a notably higher sensitivity for the antagonists suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid, and bromphenol blue. Chimeric constructs between human and rat isoforms as well as single-point mutations were engineered to map the regions responsible for the different sensitivity to suramin and pyridoxal-phosphate-6-azophenyl-2'4'-disulfonic acid.

Diadenosine polyphosphates induce intracellular Ca2+ mobilization in human neutrophils via a pertussis toxin sensitive G-protein

The diadenosine polyphosphates diadenosine 5',5"'-P1,P3-triphosphate (Ap3A), diadenosine 5',5"'-P1,P4-tetraphosphate (Ap4A), diadenosine 5',5"'-P1,P5-pentaphosphate (Ap5A) and diadenosine 5',5"'-P1,P6-hexaphosphate (Ap6A) all stimulated increases in intracellular Ca2+ in human neutrophils. Maximal increases in intracellular Ca2+ of 650 nM were obtained at dinucleotide concentrations of 500-700 microM. These increases in intracellular, Ca2+ were completely abolished by pre-treatment of the neutrophils with pertussis toxin and were hardly affected when the extracellular buffer was devoid of Ca2+. On the other hand, adenosine triphosphate (ATP) could stimulate much greater increases in intracellular Ca2+ (up to 1.1 microM) at much lower concentrations (half maximal responses obtained at around 5 microM ATP). Receptor de-sensitization experiments indicate that human neutrophils may possess two types of P2-purinoceptors. The first of these may bind ATP (but not the dinucleotides) with high affinity whilst the second may bind the dinucleotides with lower affinity and also bind ATP.

Neuropharmacology: a part for purines in pattern generation

Purinergic transmission has been found to play a key role in the neural control of rhythmic swimming behaviour in Xenopus embryos: it may have similar importance in other vertebrate motor behaviours.

C-terminal deletion of human tissue transglutaminase enhances magnesium-dependent GTP/ATPase activity

Tissue transglutaminase (tTG) exhibits a magnesium-dependent GTP/ATPase activity that is involved in the regulation of the cell cycle and cell receptor signaling. The portion of the molecule involved in GTP/ATP hydrolysis is unknown. We expressed and purified a series of C-terminal truncation mutants of human tTG as glutathione S-transferase fusion proteins (DeltaS538, DeltaE447, DeltaP345, DeltaC290, DeltaV228, and DeltaF185) to determine the effect on GTP/ATPase activity. The truncation of the C terminus did not change significantly the apparent Km value for either GTP or ATP. In contrast, the Kcat value for GTP was increased by 4.6- and 3-fold for the DeltaS538 and DeltaE447 mutants, respectively. The DeltaP345 mutant had the highest hydrolysis activity with a 34-fold increase. The hydrolysis activity then declined to 8.1-, 8.7-, and 1. 9-fold for the DeltaC290, DeltaV228, and DeltaF185 mutants, respectively. The Kcat for ATP changed in parallel with the GTPase results. Thin layer chromatography analysis of the hydrolysis reaction products revealed that ATP was rapidly converted to ADP followed by a much slower conversion of ADP to AMP when incubated with wild type tTG or the DeltaP345 mutant. There was a substantial decrease in the calcium-dependent TGase activity when the last 149 amino acid residues were deleted from the C terminus. Less than 5% of the TGase activity was detected for the DeltaS538 and DeltaE447 mutants. In conclusion, we have located the ATP and GTP hydrolytic domain to amino acid residues 1-185. The C terminus functions to inhibit the expression of endogenous GTP/ATPase activity of tTG, and the potential role of the C terminus in modulating this activity is discussed.

Extracellular metabolism of nucleotides in the nervous system

1. A variety of surface-located enzymes are involved in the metabolism of extracellular nucleotides. The biochemical properties of some of these are briefly discussed. 2. The molecular identity of ecto-diadenosine polyphosphate hydrolase has not yet been revealed. On neural cells the enzyme catalyses the hydrolysis of ApnA to Apn-1 and AMP. 3. The molecular structure of ATP-diphosphohydrolase has recently been identified. The enzyme occurs in essentially all tissues where it catalyses the extracellular hydrolysis of ATP and ADP with the formation of AMP. 4. Ecto-5'-nucleotidase is a GPI-anchored glycoprotein and catalyses the formation of AMP to adenosine. In the adult brain, and as revealed by immunocytochemistry, the enzyme is mainly associated with astrocytes. It is associated with developing nerve cells and cultured neural cells. In vitro its inhibition or suppression of its synthesis result in the inhibition of neurite formation and long-time survival of neural cells. Continued extracellular hydrolysis of AMP and formation of adenosine thus appear to be essential for neural differentiation and survival.

Stimulation of tumor cell motility linked to phosphodiesterase catalytic site of autotaxin

A family of extracellular type I phosphodiesterases has recently been isolated by cDNA cloning, but a physiological function linked to the phosphodiesterase active site has remained unknown. We now present evidence that the phosphodiesterase catalytic site, 201YMRPVYPTKTFPN213, is essential for the motility stimulating activity of autotaxin (ATX), one member of the exophosphodiesterase family. Native ATX possesses phosphodiesterase activity at neutral and alkaline pH, binds ATP noncovalently, and undergoes threonine phosphorylation. Homogeneously purified recombinant ATX, based on the teratocarcinoma sequence, retains these same activities. A single amino acid in the phosphodiesterase catalytic site, Thr210, is found to be necessary for motility stimulation, phosphodiesterase activity, and phosphorylation. Two mutant recombinant proteins, Ala210- and Asp210-ATX, lack motility stimulation and lack both enzymatic activities; Ser210-ATX possesses intermediate activities. Another mutation, with the adjacent lysine (Lys209) changed to Leu209-ATX, possesses normal motility stimulation with sustained phosphodiesterase activity but exhibits no detectable phosphorylation. This mutation eliminates the phosphorylation reaction and indicates that the dephosphorylated state is an active motility-stimulating form of the ATX molecule. By demonstrating that the phosphodiesterase enzymatic site is linked to motility stimulation, these data reveal a novel role for this family of exo/ecto-enzymes and open up the possibility of extracellular enzymatic cascades as a regulatory mechanism for cellular motility.

Expression of purinergic receptor channels and their role in calcium signaling and hormone release in pituitary gonadotrophs. Integration of P2 channels in plasma membrane- and endoplasmic reticulum-derived calcium oscillations

The role of ATP as a positive feedback element in Ca2+ signaling and secretion was examined in female rat pituitary gonadotrophs. ATP and ADP, but not AMP or adenosine, induced a dose- and extracellular Ca2+-dependent rise in [Ca2+]i in identified gonadotrophs in a Mg2+- and suramin-sensitive manner. ATP, adenosine-5'-O-(3-thiotriphosphate), adenosine-5'-O-(1-thiotriphosphate), 2-methylthio-ATP, and 3'-O-(4-benzoyl)benzoyl-ATP were roughly equipotent in rising [Ca2+]i in gonadotrophs, while ADP was effective only at submillimolar concentration range, and none of these compounds permeabilized the cells. On the other hand, alpha,beta-methylene-ATP, beta,gamma-methylene-ATP, and UTP were unable to induce any rise in [Ca2+]i. This pharmacological profile is consistent with expression of P2X2 and/or P2X5 purinergic receptor channels. Patch-clamp experiments showed that ATP induced an inward depolarizing current in gonadotrophs clamped at -90 mV, associated with an increase in [Ca2+]i. The ATP-induced [Ca2+]i response was partially inhibited by nifedipine, a blocker of voltage-sensitive Ca2+ channels (VSCC), but was not affected by tetrodotoxin, a blocker of voltage-sensitive Na+ channels. Thus, the P2-depolarizing current itself drives Ca2+ into the cell, but also activates Ca2+ entry through VSCC. In accord with this, low [ATP] induced plasma membrane-dependent [Ca2+]i oscillations in quiescent cells, and increased the frequency of spiking in spontaneously active cells. ATP-induced Ca2+ influx also affected agonist-induced and InsP3-dependent [Ca2+]i oscillations by increasing the frequency, base line, and duration of Ca2+ spiking. In addition, ATP stimulated gonadotropin secretion and enhanced agonist-induced gonadotropin release. ATP was found to be secreted by pituitary cells during agonist stimulation and was promptly degraded by ectonucleotidase to adenosine. These observations indicate that ATP represents a paracrine/autocrine factor in the regulation of Ca2+ signaling and secretion in gonadotrophs, and that these actions are mediated by P2 receptor channels.

Biochemistry, localization and functional roles of ecto-nucleotidases in the nervous system

Nucleotides such as ATP, ADP, UTP or the diadenosine polyphosphates and possibly even NAD+ are extracellular signaling substances in the brain and in other tissues. Enzymes located on the cell surface catalyze the hydrolysis of these compounds and thus limit their spatio-temporal activity. As a final hydrolysis product they generate the nucleoside and phosphate. The paper discusses the biochemical properties, cellular localization and functional properties of surface-located enzymes that hydrolyse nucleotides released from nervous tissue. This is preceded by a brief discussion of nucleotide receptors, cellular storage and mechanisms of nucleotide release. In nervous tissue nucleoside 5'-triphosphates are hydrolysed by ecto-ATP-diphosphohydrolase and possibly in addition also by ecto-nucleoside triphosphatase and ecto-nucleoside diphosphatase. The molecular identity of the ATP-diphosphohydrolase has now been revealed. The hydrolysis of nucleoside 5'-monophosphates is catalysed by 5'-nucleotidase whose biochemical properties and molecular structure have been studied in detail. Little is known about the molecular properties of the diadenosine polyphosphatases. Surface located enzymes for the extracellular hydrolysis of NAD+ and also ecto-protein kinases are discussed briefly. The cellular localization of the ecto-nucleotidases is only partly defined. Whereas in adult mammalian brain activity for hydrolysis of ATP and ADP may be associated with nerve cells or glial cells 5'-nucleotidase appears to have a preferential glial allocation in the adult mammal. The extracellular hydrolysis of the nucleotides is of functional importance not only during synaptic transmission where it functions in signal elimination. It plays a crucial role also for the survival and differentiation of neural cells in vitro and presumably during neuronal development in vivo.

P2X4: an ATP-activated ionotropic receptor cloned from rat brain

Extracellular ATP exerts pronounced biological actions in virtually every organ or tissue that has been studied. In the central and peripheral nervous system, ATP acts as a fast excitatory transmitter in certain synaptic pathways [Evans, R.J., Derkach, V. & Surprenant, A. (1992) Nature (London) 357, 503-505; Edwards, F.A., Gigg, A.J. & Colquhoun, D. (1992) Nature (London) 359, 144-147]. Here, we report the cloning and characterization of complementary DNA from rat brain, encoding an additional member (P2X4) of the emerging multigenic family of ligand-gated ATP channels, the P2X receptors. Expression in Xenopus oocytes gives an ATP-activated cation-selective channel that is highly permeable to Ca2+ and whose sensitivity is modulated by extracellular Zn2+. Surprisingly, the current elicited by ATP is almost insensitive to the common P2X antagonist suramin. In situ hybridization reveals the expression of P2X4 mRNA in central nervous system neurons. Northern blot and reverse transcription-PCR (RT-PCR) analysis demonstrate a wide distribution of P2X4 transcripts in various tissues, including blood vessels and leukocytes. This suggests that the P2X4 receptor might mediate not only ATP-dependent synaptic transmission in the central nervous system but also a wide repertoire of biological responses in diverse tissues.