A fusion protein of the human P2Y(1) receptor and NTPDase1 exhibits functional activities of the native receptor and ectoenzyme and reduced signaling responses to endogenously released nucleotides

Ecto-nucleoside triphosphate diphosphohydrolase 2 modulates local ATP-induced calcium signaling in human HaCaT keratinocytes

Keratinocytes are the major building blocks of the human epidermis. In many physiological and pathophysiological conditions, keratinocytes release adenosine triphosphate (ATP) as an autocrine/paracrine mediator that regulates cell proliferation, differentiation, and migration. ATP receptors have been identified in various epidermal cell types; therefore, extracellular ATP homeostasis likely determines its long-term, trophic effects on skin health. We investigated the possibility that human keratinocytes express surface-located enzymes that modulate ATP concentration, as well as the corresponding receptor activation, in the pericellular microenvironment. We observed that the human keratinocyte cell line HaCaT released ATP and hydrolyzed extracellular ATP. Interestingly, ATP hydrolysis resulted in adenosine diphosphate (ADP) accumulation in the extracellular space. Pharmacological inhibition by ARL 67156 or gene silencing of the endogenous ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) isoform 2 resulted in a 25% reduction in both ATP hydrolysis and ADP formation. Using intracellular calcium as a reporter, we found that although NTPDase2 hydrolyzed ATP and generated sustainable ADP levels, only ATP contributed to increased intracellular calcium via P2Y2 receptor activation. Furthermore, knocking down NTPDase2 potentiated the nanomolar ATP-induced intracellular calcium increase, suggesting that NTPDase2 globally attenuates nucleotide concentration in the pericellular microenvironment as well as locally shields receptors in the vicinity from being activated by extracellular ATP. Our findings reveal an important role of human keratinocyte NTPDase2 in modulating nucleotide signaling in the extracellular milieu of human epidermis.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

The Arabidopsis apyrase AtAPY1 is localized in the Golgi instead of the extracellular space

BACKGROUND

The two highly similar Arabidopsis apyrases AtAPY1 and AtAPY2 were previously shown to be involved in plant growth and development, evidently by regulating extracellular ATP signals. The subcellular localization of AtAPY1 was investigated to corroborate an extracellular function.

RESULTS

Transgenic Arabidopsis lines expressing AtAPY1 fused to the SNAP-(O(6)-alkylguanine-DNA alkyltransferase)-tag were used for indirect immunofluorescence and AtAPY1 was detected in punctate structures within the cell. The same signal pattern was found in seedlings stably overexpressing AtAPY1-GFP by indirect immunofluorescence and live imaging. In order to identify the nature of the AtAPY1-positive structures, AtAPY1-GFP expressing seedlings were treated with the endocytic marker stain FM4-64 (N-(3-triethylammoniumpropyl)-4-(p-diethylaminophenyl-hexatrienyl)-pyridinium dibromide) and crossed with a transgenic line expressing the trans-Golgi marker Rab E1d. Neither FM4-64 nor Rab E1d co-localized with AtAPY1. However, live imaging of transgenic Arabidopsis lines expressing AtAPY1-GFP and either the fluorescent protein-tagged Golgi marker Membrin 12, Syntaxin of plants 32 or Golgi transport 1 protein homolog showed co-localization. The Golgi localization was confirmed by immunogold labeling of AtAPY1-GFP. There was no indication of extracellular AtAPY1 by indirect immunofluorescence using antibodies against SNAP and GFP, live imaging of AtAPY1-GFP and immunogold labeling of AtAPY1-GFP. Activity assays with AtAPY1-GFP revealed GDP, UDP and IDP as substrates, but neither ATP nor ADP. To determine if AtAPY1 is a soluble or membrane protein, microsomal membranes were isolated and treated with various solubilizing agents. Only SDS and urea (not alkaline or high salt conditions) were able to release the AtAPY1 protein from microsomal membranes.

CONCLUSIONS

AtAPY1 is an integral Golgi protein with the substrate specificity typical for Golgi apyrases. It is therefore not likely to regulate extracellular nucleotide signals as previously thought. We propose instead that AtAPY1 exerts its growth and developmental effects by possibly regulating glycosylation reactions in the Golgi.

The GDA1_CD39 superfamily: NTPDases with diverse functions

The first comprehensive review of the ubiquitous "ecto-ATPases" by Plesner was published in 1995. A year later, a lymphoid cell activation antigen, CD39, that had been cloned previously, was shown to be an ecto-ATPase. A family of proteins, related to CD39 and a yeast GDPase, all containing the canonical apyrase conserved regions in their polypeptides, soon started to expand. They are now recognized as members of the GDA1_CD39 protein family. Because proteins in this family hydrolyze nucleoside triphosphates and diphosphates, a unifying nomenclature, nucleoside triphosphate diphopshohydrolases (NTPDases), was established in 2000. Membrane-bound NTPDases are either located on the cell surface or membranes of intracellular organelles. Soluble NTPDases exist in the cytosol and may be secreted. In the last 15 years, molecular cloning and functional expression have facilitated biochemical characterization of NTPDases of many organisms, culminating in the recent structural determination of the ecto-domain of a mammalian cell surface NTPDase and a bacterial NTPDase. The first goal of this review is to summarize the biochemical, mutagenesis, and structural studies of the NTPDases. Because of their ability in hydrolyzing extracellular nucleotides, the mammalian cell surface NTPDases (the ecto-NTPDases) which regulate purinergic signaling have received the most attention. Less appreciated are the functions of intracellular NTPDases and NTPDases of other organisms, e.g., bacteria, parasites, Drosophila, plants, etc. The second goal of this review is to summarize recent findings which demonstrate the involvement of the NTPDases in multiple and diverse physiological processes: pathogen-host interaction, plant growth, eukaryote cell protein and lipid glycosylation, eye development, and oncogenesis.

A membrane network of receptors and enzymes for adenine nucleotides and nucleosides

Most cells express more than one receptor plus degrading enzymes for adenine nucleotides or nucleosides, and cellular responses to purines are rarely compatible with the actions of single receptors. Therefore, these receptors are viewed as components of a combinatorial receptor web rather than self-dependent entities, but it remained unclear to what extent they can associate with each other to form signalling units. P2Y(1), P2Y(2), P2Y(12), P2Y(13), P2X(2), A(1), A(2A) receptors and NTPDase1 and -2 were expressed as fluorescent fusion proteins which were targeted to membranes and signalled like the unlabelled counterparts. When tested by FRET microscopy, all the G protein-coupled receptors proved able to form heterooligomers with each other, and P2Y(1), P2Y(12), P2Y(13), A(1), A(2A), and P2X(2) receptors also formed homooligomers. P2Y receptors did not associate with P2X, but G protein-coupled receptors formed heterooligomers with NTPDase1, but not NTPDase2. The specificity of prototypic interactions (P2Y(1)/P2Y(1), A(2A)/P2Y(1), A(2A)/P2Y(12)) was corroborated by FRET competition or co-immunoprecipitation. These results demonstrate that G protein-coupled purine receptors associate with each other and with NTPDase1 in a highly promiscuous manner. Thus, purinergic signalling is not only determined by the expression of receptors and enzymes but also by their direct interaction within a previously unrecognized multifarious membrane network.

Pharmacological profiles of cloned mammalian P2Y-receptor subtypes

Membrane-bound P2-receptors mediate the actions of extracellular nucleotides in cell-to-cell signalling. P2X-receptors are ligand-gated ion channels, whereas P2Y-receptors belong to the superfamily of G-protein-coupled receptors (GPCRs). So far, the P2Y family is composed out of 8 human subtypes that have been cloned and functionally defined; species orthologues have been found in many vertebrates. P2Y1-, P2Y2-, P2Y4-, P2Y6-, and P2Y11-receptors all couple to stimulation of phospholipase C. The P2Y11-receptor mediates in addition a stimulation of adenylate cyclase. In contrast, activation of the P2Y12-, P2Y13-, and P2Y14-receptors causes an inhibition of adenylate cyclase activity. The expression of P2Y1-receptors is widespread. The receptor is involved in blood platelet aggregation, vasodilatation and neuromodulation. It is activated by ADP and ADP analogues including 2-methylthio-ADP (2-MeSADP). 2'-Deoxy-N6-methyladenosine-3',5'-bisphosphate (MRS2179) and 2-chloro-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate (MRS2279) are potent and selective antagonists. P2Y2 transcripts are abundantly distributed. One important example for its functional role is the control of chloride ion fluxes in airway epithelia. The P2Y2-receptor is activated by UTP and ATP and blocked by suramin. The P2Y2-agonist diquafosol is used for the treatment of the dry eye disease. P2Y4-receptors are expressed in the placenta and in epithelia. The human P2Y4-receptor has a strong preference for UTP as agonist, whereas the rat P2Y4-receptor is activated about equally by UTP and ATP. The P2Y4-receptor is not blocked by suramin. The P2Y6-receptor has a widespread distribution including heart, blood vessels, and brain. The receptor prefers UDP as agonist and is selectively blocked by 1,2-di-(4-isothiocyanatophenyl)ethane (MRS2567). The P2Y11-receptor may play a role in the differentiation of immunocytes. The human P2Y11-receptor is activated by ATP as naturally occurring agonist and it is blocked by suramin and reactive blue 2 (RB2). The P2Y12-receptor plays a crucial role in platelet aggregation as well as in inhibition of neuronal cells. It is activated by ADP and very potently by 2-methylthio-ADP. Nucleotide antagonists including N6-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP (=cangrelor; AR-C69931MX), the nucleoside analogue AZD6140, as well as active metabolites of the thienopyridine compounds clopidogrel and prasugrel block the receptor. These P2Y12-antagonists are used in pharmacotherapy to inhibit platelet aggregation. The P2Y13-receptor is expressed in immunocytes and neuronal cells and is again activated by ADP and 2-methylthio-ADP. The 2-chloro-5-nitro pyridoxal-phosphate analogue 6-(2'-chloro-5'-nitro-azophenyl)-pyridoxal-alpha5-phosphate (MRS2211) is a selective antagonist. mRNA encoding for the human P2Y14-receptor is found in many tissues. However, a physiological role of the receptor has not yet been established. UDP-glucose and related analogues act as agonists; antagonists are not known. Finally, UDP has been reported to act on receptors for cysteinyl leukotrienes as an additional agonist--indicating a dual agonist specificity of these receptors.

Functional expression of single-chain heterodimeric G-protein-coupled receptors for adenosine and dopamine

The direct homo- and heteromeric association between G-protein-coupled receptors (GPCRs), adenosine A2A receptor (A(2A)R) and dopamine D2 receptor (D2R), occurs although little is known about the selectivity of their formation (A(2A)R/A(2A)R vs. A(2A)R/D2R). In order to stimulate the heteromerization of A(2A)R and D2R, we have designed a single-polypeptide-chain heterodimeric A(2A)R/D2R complex by fusing the C-terminus of the A(2A)R via transmembrane (TM) of a type II TM protein with the N-terminus of D2R in tandem. This was successfully expressed on the cell surface as a full-length protein with specific binding to the respective ligands and functional coupling to G-proteins comparable to wild-type receptors, suggesting the possible creation of physiologically relevant heteromeric A(2A)R/D2R. This expression system would be useful to exclusively clarify the properties of heteromeric GPCRs irrespective of homomeric receptors.

Regulation of P2Y1 receptor-mediated signaling by the ectonucleoside triphosphate diphosphohydrolase isozymes NTPDase1 and NTPDase2

Ectonucleoside triphosphate diphosphohydrolases (NTPDases) control the concentration of released extracellular nucleotides, but the precise physiological roles played by these isozymes in modulation of P2 receptor signaling remain unclear. Activation of the human P2Y(1) receptor was studied in the presence of NTPDase1 or NTPDase2 expressed either in the same cell as the receptor or in P2Y(1) receptor-expressing cells cocultured with NTPDaseexpressing cells. Coexpression of NTPDase1 with the P2Y(1) receptor resulted in increases in the EC(50) for 2'-methylthioadenosine 5'-diphosphate (2MeSADP; 12-fold), ADP (50-fold), and ATP (10-fold) for activation of phospholipase C. Similar effects were observed when the P2Y(1) receptor and NTPDase1 were expressed on different cells. These results are explained by the capacity of NTPDase1 to hydrolyze both nucleoside triphosphates and diphosphates. NTPDase2 preferentially hydrolyzes nucleoside triphosphates, and the presence of NTPDase2 under either coexpression or coculture conditions did not change the EC(50) of 2MeSADP, ADP, or adenosine 5'-O-(2-thiodiphosphate) for activation of the P2Y(1) receptor. However, the EC(50) for ATP was 15-fold lower in the presence of NTPDase2 than in cells expressing the P2Y(1) receptor alone. Whereas expression of NTPDase1 decreased basal activity of the P2Y(1) receptor, the presence of the NTPDase2 resulted in P2Y(1) receptor-dependent increases in basal activity. These results suggest that basal activity of the P2Y(1) receptor is maintained by paracrine or autocrine release of receptor agonists and that the biological and/or pharmacological response mediated by P2Y receptors in target tissues is highly dependent on the types of ectonucleotidases expressed in the vicinity of the receptor.

Quantitation of the P2Y(1) receptor with a high affinity radiolabeled antagonist

2-Chloro-N(6)-methyl-(N )-methanocarba-2'-deoxyadenosine-3',5'- bisphosphate (MRS2279) was developed previously as a selective high-affinity, non-nucleotide P2Y(1) receptor (P2Y1-R) antagonist (J Med Chem 43:829-842, 2002; Br J Pharmacol 135:2004-2010, 2002). We have taken advantage of the N(6)-methyl substitution in the adenine base to incorporate [(3)H]methylamine into the synthesis of [(3)H]MRS2279 to high (89 Ci/mmol) specific radioactivity and have used this molecule as a radioligand for the P2Y1-R. [(3)H]MRS2279 bound to membranes from Sf9 insect cells expressing recombinant human P2Y1-R but not to membranes from wild-type Sf9 cells or Sf9 cells expressing high levels of recombinant P2Y(2) or P2Y(12) receptors. Equilibrium binding of [(3)H]MRS2279 to P2Y1-R expressed in Sf9 membranes was with a high affinity (K(d) = 8 nM) essentially identical to the apparent affinity of MRS2279 determined previously in studies of P2Y1-R-promoted inositol phosphate accumulation or platelet aggregation. A kinetically derived K(d) calculated from independent determinations of the rate constants of association (7.15 x 10(7) M(-1) min(-1)) and dissociation (0.72 min(-1)) of [(3)H]MRS2279 also was in good agreement with the K(d) derived from equilibrium binding studies. Competition binding assays with [(3)H]MRS2279 and P2Y1-R expressing Sf9 cell membranes revealed K(i) values for the P2Y1-R antagonists MRS2279 (K(i) = 13 nM), N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS2179; K(i) = 84 nM), adenosine-3', 5'-bisphosphate (K(i)=900 nM), and pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (K(i) = 6 microM) that were in good agreement with antagonist activities of these molecules previously determined at the P2Y1-R in intact tissues. Moreover, [(3)H]MRS2279 also bound with high affinity (K(d) = 4-8 nM) to Chinese hamster ovary (CHO) or 1321N1 human astrocytoma cells stably expressing the human P2Y1-R, but specific binding was not observed in wild-type CHO or 1321N1 cells. [(3)H]MRS2279 bound with high affinity (K(d) = 16 nM) to a binding site on out-dated human platelets (5-35 receptors/platelet) and rat brain membranes (210 fmol/mg protein) that fit the expected drug selectivity of a P2Y1-R. Taken together, these results indicate that [(3)H]MRS2279 is the first broadly applicable antagonist radioligand for a P2Y receptor.