P2Y receptor-mediated inhibition of voltage-activated Ca(2+) currents in PC12 cells

To search for inhibitory nucleotide receptors in the sympathoadrenal cell lineage of the rat, voltage-activated Ca(2+) currents were recorded in PC12 cells after differentiation with nerve growth factor. ADP and ATP, but not uridine nucleotides, reduced Ca(2+) current amplitudes and slowed activation kinetics. This effect was mediated by GTP binding proteins, as it was abolished by intracellular GDP beta S and after treatment with pertussis toxin. Furthermore, depolarizations preceding the activation of Ca(2+) currents abolished the ADP-induced slowing of activation kinetics and attenuated its inhibitory action on current amplitudes. The modulatory effect of ADP was neither altered in the presence of adenosine receptor antagonists, nor mimicked by agonists at these receptors. In addition, the action of ADP was antagonized by reactive blue 2, but not by suramin or PPADS. Nucleotides tested for their inhibitory action on Ca(2+) currents displayed the following rank order of potency: 2-methylthio-ADP > or = 2-methylthio-ATP >> ADP beta S > ADP = ATP. When P2X receptors were blocked, the P2X agonists ATP and 2-methylthio-ATP still reduced Ca(2+) currents. The P2Y1 receptor antagonists adenosine-2'-phosphate-5'-phosphate and adenosine-3'-phosphate-5'-phosphate did not alter the inhibitory action of ADP, whereas the Sp-isomer of adenosine-5'-O-(1-thiotriphosphate) and 2'- and 3'-O-(4-benzoylbenzoyl)-ATP showed significant antagonistic activity. These results demonstrate that PC12 cells express an as yet unidentified P2Y receptor with pharmacological characteristics similar to those of P2Y1. As receptor-dependent modulation of Ca(2+) channels is a key event in presynaptic inhibition, this receptor may correspond to previously described presynaptic nucleotide receptors mediating autoinhibition of sympathetic transmitter release.

Identification of the platelet ADP receptor targeted by antithrombotic drugs

Platelets have a crucial role in the maintenance of normal haemostasis, and perturbations of this system can lead to pathological thrombus formation and vascular occlusion, resulting in stroke, myocardial infarction and unstable angina. ADP released from damaged vessels and red blood cells induces platelet aggregation through activation of the integrin GPIIb-IIIa and subsequent binding of fibrinogen. ADP is also secreted from platelets on activation, providing positive feedback that potentiates the actions of many platelet activators. ADP mediates platelet aggregation through its action on two G-protein-coupled receptor subtypes. The P2Y1 receptor couples to Gq and mobilizes intracellular calcium ions to mediate platelet shape change and aggregation. The second ADP receptor required for aggregation (variously called P2Y(ADP), P2Y(AC), P2Ycyc or P2T(AC)) is coupled to the inhibition of adenylyl cyclase through Gi. The molecular identity of the Gi-linked receptor is still elusive, even though it is the target of efficacious antithrombotic agents, such as ticlopidine and clopidogrel and AR-C66096 (ref. 9). Here we describe the cloning of this receptor, designated P2Y12, and provide evidence that a patient with a bleeding disorder has a defect in this gene. Cloning of the P2Y12 receptor should facilitate the development of better antiplatelet agents to treat cardiovascular diseases.

Differential coupling of the human P2Y(11) receptor to phospholipase C and adenylyl cyclase

1. The human P2Y(11) (hP2Y(11)) receptor was stably expressed in two cell lines, 1321N1 human astrocytoma cells (1321N1-hP2Y(11)) and Chinese hamster ovary cells (CHO-hP2Y(11)), and its coupling to phospholipase C and adenylyl cyclase was assessed. 2. In 1321N1-hP2Y(11) cells, ATP promoted inositol phosphate (IP) accumulation with low microM potency (EC(50)=8.5+/-0.1 microM), whereas it was 15 fold less potent (130+/-10 microM) in evoking cyclic AMP production. 3. In CHO-hP2Y(11) cells, ATP promoted IP accumulation with slightly higher potency (EC(50)=3.6+/-1.3 microM) than in 1321N1-hP2Y(11) cells, but it was still 15 fold less potent in promoting cyclic AMP accumulation (EC(50)=62.4+/-15.6 microM) than for IP accumulation. Comparable differences in potencies for promoting the two second messenger responses were observed with other adenosine nucleotide analogues. 4. In 1321N1-hP2Y(11) and CHO-hP2Y(11) cells, down regulation of PKC by chronic treatment with phorbol ester decreased ATP-promoted cyclic AMP accumulation by 60--80% (P<0.001) with no change in its potency. Likewise, chelation of intracellular Ca(2+) decreased ATP-promoted cyclic AMP accumulation by approximately 45% in 1321N1-hP2Y(11) cells, whereas chelation had no effect on either the efficacy or potency of ATP in CHO-hP2Y(11) cells. 5. We conclude that coupling of hP2Y(11) receptors to adenylyl cyclase in these cell lines is much weaker than coupling to phospholipase C, and that activation of PKC and intracellular Ca(2+) mobilization as consequences of inositol lipid hydrolysis potentiates the capacity of ATP to increase cyclic AMP accumulation in both 1321N1-hP2Y(11) and CHO-hP2Y(11) cells.

Activity of adenosine diphosphates and triphosphates on a P2Y(T) -type receptor in brain capillary endothelial cells

1. A P2Y (nucleotide) receptor activity in a clonal population (B10) of rat brain capillary endothelial cells is coupled to inhibition of adenylyl cyclase and has functional similarities to the P2Y(T) (previously designated 'P2T') receptor for ADP of blood platelets. However, the only P2Y receptor which was detectable in a previous study of B10 cells by mRNA analysis was the P2Y(1) receptor, which elsewhere shows no transduction via cyclic nucleotides. We have sought here to clarify these issues. 2. The inhibition of forskolin-stimulated adenylyl cyclase induced by purified nucleotides was measured on B10 cells. The EC(50) value for 2-methylthioADP (2-MeSADP) was 2.2 nM and, surprisingly, 2-MeSATP was an almost equally strong agonist (EC(50)=3.5 nM). ATP and 2-ClATP were weak partial agonists (EC(50)=26 microM and 10 microM respectively) and under appropriate conditions could antagonise the activity on 2-MeSADP. 3. A known selective antagonist of the platelet P2Y(T) receptor, 2-propylthioadenosine-5'-(beta,gamma)-difluoromethylene) triphosphonate (AR-C 66096), was a competitive antagonist of this B10 cell receptor, with pK(B)=7.6. That ligand is inactive at the P2Y(1) receptor in the same cells. Conversely, the competitive P2Y(1) receptor antagonists, the 3', 5'- and 2', 5'-adenosine bis-monophosphates, are, instead, weak agonists at the adenylyl cyclase-inhibitory receptor. 4. The inhibition of adenylyl cyclase by 2-MeSADP was completely abolished by pertussis toxin. 5. In summary, these brain endothelial cells possess a P2Y(T)-type receptor in addition to the P2Y(1) receptor. The two have similarities in agonist profiles but are clearly distinguishable by antagonists and by their second messenger activations. The possible relationships between the B10 and platelet P2Y(T) receptors are discussed.

P2 purinoceptor-mediated control of rat cerebral (pial) microvasculature; contribution of P2X and P2Y receptors

Purine and pyrimidine nucleotides evoke changes in the vascular tone of medium to large cerebral vessels through the activation of P2 purinoceptors. We have applied P2 receptor drugs to rat pial arterioles and measured changes in arteriole diameter (o.d. 40-84 micrometer at rest), and recorded currents from arteriolar smooth muscle cells using patch-clamp techniques. Transient vasoconstrictions and rapidly inactivating currents were evoked by alpha,beta-methylene ATP (0.1-30 micrometer) and were sensitive to the P2 receptor antagonists suramin and iso-PPADS. UTP and UDP (0.1-1000 micrometer) evoked sustained suramin-sensitive vasoconstrictions. ATP (0.1-1000 micrometer) and 2-methylthioATP (2MeSATP, 300 micrometer) evoked transient vasoconstrictions followed by sustained vasodilatations. ADP application resulted in only vasodilatation (EC50 approximately 4 micrometer). Vasodilator responses to ATP, 2MeSATP or ADP were unaffected by suramin (100 micrometer). RT-PCR analysis indicated that P2X1-7 and P2Y1,2,6 RNA can be amplified from the pial sheet. Our results provide direct evidence for the presence of functional P2X receptors with a phenotype resembling the P2X1 receptor subtype on cerebral resistance arterioles. The pharmacological properties of the pyrimidine-evoked responses suggest that a combination of P2Y2- and P2Y6-like receptors are responsible for the sustained vasoconstrictions. It is therefore likely that the nucleotides and their associated receptors are involved in a complicated regulatory system to control cerebral blood pressure.

The stable pyrimidines UDPbetaS and UTPgammaS discriminate between the P2 receptors that mediate vascular contraction and relaxation of the rat mesenteric artery

The contractile and relaxant effects of the different P2 receptors were characterized in the rat isolated mesenteric artery by use of extracellular nucleotides, including the stable pyrimidines uridine 5'-O-thiodiphosphate (UDPbetaS) and uridine 5'-O-3-thiotriphosphate (UTPgammaS). The selective P2X receptor agonist, alphabeta-methylene-adenosine triphosphate (alphabeta-MeATP) stimulated a potent (pEC(50)=6.0) but relatively weak contraction (E:(max)=57% of 60 mM K(+)). The contractile concentration-response curve of adenosine triphosphate (ATP) was biphasic when added in single concentrations. The first part of the response could be desensitized by alphabeta-MeATP, indicating involvement of P2X receptors, while the second part might be mediated by P2Y receptors. The contractile P2Y receptors were further characterized after P2X receptor desensitization with 10 microM alphabeta-MeATP. Uridine diphosphate (UDP), uridine triphosphate (UTP) and ATP stimulated contraction only in high concentrations (1 - 10 mM). The selective P2Y(6) agonist, UDPbetaS, and the P2Y(2)/P2Y(4)-receptor agonists UTPgammaS and adenosine 5'-O-3-thiotriphosphate (ATPgammaS) were considerably more potent and efficacious (E:(max) approximately 250% of 60 mM K(+)). Adenosine 5'-O-thiodiphosphate (ADPbetaS) was inactive, excluding contractile P2Y(1) receptors. After precontraction with 1 microM noradrenaline, UTP, ADP and ATP induced relaxations with similar potencies (pEC(50) approximately 5.0). UTPgammaS, ADPbetaS and ATPgammaS were approximately one log unit more potent indicating the presence of endothelial P2Y(1) and P2Y(2)/P2Y(4) receptors. The P2Y(6) receptor agonist, UDPbetaS, had no effect. UDPbetaS and UTPgammaS are useful tools when studying P2 receptors in tissue preparations with ectonucleotidase activity. Contractile responses can be elicited by stimulation of P2Y(6) and, slightly less potently, P2Y(2)/P2Y(4) receptors. The P2X response was relatively weak, and there was no P2Y(1) response. Stimulation of P2Y(1) and P2Y(2)/P2Y(4) receptors elicited relaxation, while P2Y(6) did not contribute.

ADP is not an agonist at P2X(1) receptors: evidence for separate receptors stimulated by ATP and ADP on human platelets

ADP, an important agonist in thrombosis and haemostasis, has been reported to activate platelets via three receptors, P2X(1), P2Y(1) and P2T(AC). Given the low potency of ADP at P2X(1) receptors and recognized contamination of commercial samples of adenosine nucleotides, we have re-examined the activation of P2X(1) receptors by ADP following HPLC and enzymatic purification. Native P2X(1) receptor currents in megakaryocytes were activated by alpha, beta-meATP (10 microM) and commercial samples of ADP (10 microM), but not by purified ADP (10 - 100 microM). Purified ADP (up to 1 mM) was also inactive at recombinant human P2X(1) receptors expressed in Xenopus oocytes. Purification did not modify the ability of ADP to activate P2Y receptors coupled to Ca(2+) mobilization in rat megakaryocytes. In human platelets, P2X(1) and P2Y receptor-mediated [Ca(2+)](i) responses were distinguished by their different kinetics at 13 degrees C. In 1 mM Ca(2+) saline, alpha,beta-meATP (10 microM) and commercial ADP (40 microM) activated a rapid [Ca(2+)](i) increase (lag time < or =0.5 s) through the activation of P2X(1) receptors. Hexokinase treatment of ADP shifted the lag time by approximately 2 s, indicating loss of the P2X(1) receptor-mediated response. A revised scheme is proposed for physiological activation of P2 receptors in human platelets. ATP stimulates P2X(1) receptors, whereas ADP is a selective agonist at metabotropic (P2Y(1) and P2T(AC)) receptors.

The discovery and development of P2 receptor subtypes

Extracellular purine and pyrimidine nucleotides modulate cellular activity by acting at P2 receptors. The first receptor to be identified was the P(2)-purinoceptor, which was characterised and named in 1978. In the 1980s this site was subdivided into P(2X) and P(2Y) purinoceptors on the basis of pharmacological criteria in functional studies on native receptors. Subsequently, a similar approach led to the characterisation of the P(2T), P(2Z), P(2U) and P(2D) purinoceptors. In the 1990s a molecular biological approach has led to the cloning and functional expression of at least 12 mammalian P2 receptor subtypes. The challenge now is to relate these recombinant receptors to native receptors present within a wide range of tissues.

Imagination and reality in the search for the P2Y receptors

A great body of evidence based on tissue and organ physiology and pharmacology led to the recognition, widespread by about 1990, that there must be cell membrane receptors for extracellular nucleotides to transduce their effects. This evidence was provided by the pioneering work of Geoffrey Burnstock and those who worked with him, or was developed by others starting from that information. This article will review how we could start from that foundation to clone the first known gene for such a receptor, P2Y(1). Some unusual properties of that receptor were revealed. I will consider further the P2Y receptors as a class - its definition, now that many such genes have become known. Imagination and reality have been intertwined in this saga.

Recombinant P2Y receptors: the UCL experience

The beginning of the last decade heralded three important and sequential developments in our understanding of cell-to-cell signalling by extracellular ATP via its cell surface receptors, the P2 purinoceptors. One major development in ATP signalling culminated in a timely review in 1991, when it was established in the clearest of terms that ATP receptors exploited discrete signal transduction pathways (Dubyak, G.R., 1991. Signal transduction by P2-purinergic receptors for extracellular ATP. Am. J. Respir. Cell. Mol. Biol. 4, 295-300; and later in Dubyak, G.R., El-Moatassim, C., 1993. Signal transduction via P2-purinergic receptors for extracellular ATP and other nucleotides. Am. J. Physiol. 265, C577-C606). Henceforth, it was universally acknowledged that some P2 purinoceptors interacted with heterotrimeric G-proteins to activate intracellular signalling cascades (metabotropic ATP receptors), whereas others contained intrinsic ion-channels (ionotropic ATP receptors). A second key development can be traced to 1992, from the discovery that ATP receptors were involved in excitatory neurotransmission in the CNS and PNS (Edwards, F.A., Gibb, A.J., Colquhoun, D., 1992. ATP receptor-mediated synaptic currents in the central nervous system. Nature 359, 144-147; Evans, R.J., Derkach, V., Surprenant, A., 1992. ATP mediates fast synaptic transmission in mammalian neurons. Nature 357, 503-505; Silinsky, E.M., Gerzanich, V., Vanner, S.M., 1992. ATP mediates excitatory synaptic transmission in mammalian neurones. Br. J. Pharmacol., 106, 762-763). Thereafter, it was accepted that ATP could play a neurotransmitter and/or modulatory role throughout the entire nervous system. The third key development stemmed from the isolation of a cDNA, from chick brain, encoding a metabotropic ATP receptor (Webb, T.E., Simon, J., Krishek, B.J., Bateson, A.N., Smart, T.G., King, B.F., Burnstock, G., Barnard, E.A., 1993. Cloning and functional expression of a brain G-protein-coupled ATP receptor. FEBS Lett. 324, 219-225). The cloning of a membrane protein serving as an ATP receptor ignited a widespread international interest in purinergic signalling. Investigators at University College London (UCL) - colleagues and associates of Geoffrey Burnstock - were at the forefront of this rapid phase of discovery. In this review, we highlight the UCL experience when the fields of molecular biology, physiology and cell biology converged to help advance our understanding of ATP as an extracellular signalling molecule.

Adventures in the pharmacological analysis of P2 receptors

The pharmacological classification of P2 receptors owes its origin to the pioneering efforts of Geoff Burnstock and those who followed him, research that was conducted primarily in physiological experimental systems. Over recent years, the techniques of molecular biology have been increasingly applied in the study of P2 receptors while, at the same time, advances in their pharmacological analysis have been limited by a lack of potent and selective agonist or antagonist ligands. This has resulted in a classification scheme which is largely structural in nature, with relatively little contribution from pharmacology. Our endeavours in this area have been directed towards the discovery of ligands with which the pharmacological analysis and definition of P2 receptors could be advanced, the ultimate goal being the design of therapeutic agents. This article will describe some of our experiences in this challenging but rewarding area.

Regional and cellular distribution of the P2Y(1) purinergic receptor in the human brain: striking neuronal localisation

The biological actions of extracellular nucleotides are exerted via two families of P2 receptors, P2X and P2Y. The metabotropic P2Y receptors comprise at least 7 distinct subtypes, which have been cloned from a number of species. However, none of the P2Y receptor proteins have been visualised yet in human brain. In the present study, the regional and cellular distribution of the P2Y(1) receptor was investigated in the human brain by using immunohistochemistry. Polyclonal antibodies were raised against a synthetic peptide from the C-terminus of the P2Y(1) protein. Immunoblot analysis demonstrated that P2Y(1) antiserum specifically recognised a 63-kDa band in human and rat brain membranes. Similarly, the antiserum specifically detected the human P2Y(1) receptor in transfected 1321N1 cells. Immunohistochemical analysis on perfusion-fixed human brain tissue showed a widespread distribution for this receptor throughout the brain. At the cellular level, the P2Y(1) receptor was strikingly localised to neuronal structures of the cerebral cortex, cerebellar cortex, hippocampus, caudate nucleus, putamen, globus pallidus, subthalamic nucleus, red nucleus, and midbrain. Expression of the P2Y(1) receptor was not detected in other non-neuronal cell types. These results provide the first characterisation of the cellular distribution of a P2Y receptor in the human brain. The widespread and abundant distribution of the P2Y(1) receptor suggests its involvement in a number of important functions within the human brain. The neuronal localisation of this receptor points towards a possible role in neurotransmission, and also highlights a major role for extracellular nucleotides as signaling molecules within the brain.

Advances in signalling by extracellular nucleotides. the role and transduction mechanisms of P2Y receptors

Nucleotides are ubiquitous intercellular messengers whose actions are mediated by specific receptors. Since the first clonings in 1993, it is known that nucleotide receptors belong to two families: the ionotropic P2X receptors and the metabotropic P2Y receptors. Five human P2Y receptor subtypes have been cloned so far and a sixth one must still be isolated. In this review we will show that they differ by their preference for adenine versus uracil nucleotides and triphospho versus diphospho nucleotides, as well as by their transduction mechanisms and cell expression.

Evidence that 2-methylthioATP and 2-methylthioADP are both agonists at the rat hepatocyte P2Y(1) receptor

In the absence of selective antagonists, pharmacological characterization of P2Y receptor subtypes has relied heavily upon their distinct agonist profiles. 2-methylthioADP (2-MeSADP) is a selective agonist for the P2Y(1) receptor. The agonist action of 2-MeSATP at the P2Y(1) receptor has recently been questioned. The effects of both 2-MeSADP and 2-MeSATP have been studied on rat hepatocytes injected with the bioluminescent Ca(2+) indicator, aequorin. Single hepatocytes generate series of repetitive transients in cytosolic free calcium concentration ([Ca(2+)](i)) when stimulated with agonists acting through the phosphoinositide signalling pathway. The transients induced by 2-MeSADP and 2-MeSATP in the same cell were indistinguishable, indicating that they act at a common receptor. In contrast the transients evoked by ATP and UTP had very different profiles. Treatment of 2-MeSATP with an ATP-regenerating system to remove contaminating 2-MeSADP did not abolish its agonist activity. Application of the P2Y(1) antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P) inhibited the transients induced by both 2-MeSADP and 2-MeSATP. In contrast the transients induced by ATP and UTP were enhanced by the addition of A3P5P. These results indicate that both 2-MeSADP and 2-MeSATP are agonists at the rat hepatocyte P2Y(1) receptor.

Adenosine triphosphate is full antagonist at human P2Y(1) purinoceptors

Both agonistic and antagonistic effects have been reported for ATP at P2Y(1) purinoceptors at micromolar ligand concentrations. These conflicting data hamper specification of the true pharmacological profile as well as structural requirements for antagonistic ligands of this receptor. In this report the type of ATP activity at human P2Y(1) receptors in hP2Y(1)-1321N1 cells was revisited. In parallel, kinetics of degradation of ATP in the assay mixture was analysed. It was found that transformation of this ligand to ADP was responsible for initiation of synthesis of inositol phosphates, observed in the presence of ATP in hP2Y(1)-1321N1 cells. This agonistic effect was abolished in the presence of the triphosphate regeneration system (CP/CPK). On the other hand, if the agonistic effect caused by degradation product of ATP was taken into consideration, this ligand behaved as a full antagonist at P2Y(1) receptors and was characterized by the apparent inhibitory constant 5 microM.

Diadenosine polyphosphates as antagonists of the endogenous P2Y(1) receptor in rat brain capillary endothelial cells of the B7 and B10 clones

1. Diadenosine polyphosphates (Ap(n)As, n=2 - 7) are considered as stress mediators in the cardiovascular system. They act both via identified P2 purinoceptors and via yet to be characterized receptors. This study analyses the actions of Ap(n)As in clones of rat brain capillary endothelial cells that express P2Y(1) receptors (B10 cells) or both P2Y(1) and P2Y(2) receptors (B7 cells). 2. B10 cells responded to Ap(3)A with rises in intracellular Ca(2+) concentration ([Ca(2+)](i)). This response was prevented by adenosine-3'-phosphate-5'-phosphate, an antagonist of P2Y(1) receptors. It was largely suppressed by a treatment with apyrase VII or with creatine phosphokinase/creatine phosphate to degrade contaminating ADP. 3. Ap(n)As inhibited ADP induced increases in [Ca(2+)](i) mediated by P2Y(1) receptors by shifting ADP concentration-response curves to larger concentrations. Apparent K(i) values were estimated to be 6 microM for Ap(4)A, 10 microM for Ap(5)A and 47 microM for Ap(6)A. Ap(2)A and Ap(3)A were much less active. 4. Ap(n)As were neither agonists nor antagonists of the endogenous P2Y(2) receptor in B7 cells. 5. Ap(n)As are neither agonists nor antagonists of the G(i)-coupled, ADP receptor in B10 cells. 6. The results suggest that most actions of Ap(n)As in B7 and B10 cells can be accounted for by endogenous P2Y(1) receptors. Ap(4)A, Ap(5)A and Ap(6)A are specific antagonists of endogenous Ca(2+)-coupled P2Y(1) receptors.

A chimeric rat brain P2Y1 receptor tagged with green-fluorescent protein: high-affinity ligand recognition of adenosine diphosphates and triphosphates and selectivity identical to that of the wild-type receptor

We tested how the green fluorescent protein (GFP) tag affects signaling of the nucleotide-activated P2Y1 receptor. Therefore, we generated stably transfected human embryonic kidney 293 cells expressing the rat P2Y1 wild-type receptor (rP2Y1-wt) or the receptor tagged at the C-terminus with the enhanced GFP (rP2Y1-eGFP). The chimeric rP2Y1-eGFP receptor is localized mainly to the plasma membrane as revealed by Western blotting of subcellular fractions. Both receptors were analyzed by measuring Ca2+ responses to short pulses of the agonists in single cells by continuous superfusion with medium. The rP2Y1-eGFP receptor was coupled to Ca2+ release as was the rP2Y1-wt receptor. 2-Methylthio adenosine 5'-diphosphate and -triphosphate (2-MeSATP and 2-MeSADP) were the most potent agonists at the heterologously expressed receptors, with EC50 values of 50 to 70 nM for rP2Y1-eGFP and 0.06 to 0.4 nM for rP2Y1-wt. These potencies of the two P2Y-selective agonists at rP2Y1-wt receptor-expressing cells are the highest values reported so far. This increase is probably due to a receptor reserve. In both rP2Y1-wt- and in rP2Y1-eGFP-expressing cells, the effect of 2-MeSATP was inhibited equally by the antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid. We established that ATP as well as adenosine 5'-O-(1-thiotriphosphate) (ATPalphaS) are full agonists at the rP2Y1 receptor at both transfected cell lines. The rP2Y1-eGFP receptor has the same ligand selectivity as the rP2Y1-wt receptor (2-MeSADP = 2-MeSATP > ADP > ATPalphaS, ATP >> UTP). Thus, the GFP-tagged P2Y1 receptor is fully active and shows regular signal transduction coupling. It provides the means for biochemical characterization, since it can be solubilized and is a tool for further physiological analysis.

Synthesis, biological activity, and molecular modeling of ribose-modified deoxyadenosine bisphosphate analogues as P2Y(1) receptor ligands

The structure-activity relationships of adenosine-3', 5'-bisphosphates as P2Y(1) receptor antagonists have been explored, revealing the potency-enhancing effects of the N(6)-methyl group and the ability to substitute the ribose moiety (Nandanan et al. J. Med. Chem. 1999, 42, 1625-1638). We have introduced constrained carbocyclic rings (to explore the role of sugar puckering), non-glycosyl bonds to the adenine moiety, and a phosphate group shift. The biological activity of each analogue at P2Y(1) receptors was characterized by measuring its capacity to stimulate phospholipase C in turkey erythrocyte membranes (agonist effect) and to inhibit its stimulation elicited by 30 nM 2-methylthioadenosine-5'-diphosphate (antagonist effect). Addition of the N(6)-methyl group in several cases converted pure agonists to antagonists. A carbocyclic N(6)-methyl-2'-deoxyadenosine bisphosphate analogue was a pure P2Y(1) receptor antagonist and equipotent to the ribose analogue (MRS 2179). In the series of ring-constrained methanocarba derivatives where a fused cyclopropane moiety constrained the pseudosugar ring of the nucleoside to either a Northern (N) or Southern (S) conformation, as defined in the pseudorotational cycle, the 6-NH(2) (N)-analogue was a pure agonist of EC(50) 155 nM and 86-fold more potent than the corresponding (S)-isomer. The 2-chloro-N(6)-methyl-(N)-methanocarba analogue was an antagonist of IC(50) 51.6 nM. Thus, the ribose ring (N)-conformation appeared to be favored in recognition at P2Y(1) receptors. A cyclobutyl analogue was an antagonist with IC(50) of 805 nM, while morpholine ring-containing analogues were nearly inactive. Anhydrohexitol ring-modified bisphosphate derivatives displayed micromolar potency as agonists (6-NH(2)) or antagonists (N(6)-methyl). A molecular model of the energy-minimized structures of the potent antagonists suggested that the two phosphate groups may occupy common regions. The (N)- and (S)-methanocarba agonist analogues were docked into the putative binding site of the previously reported P2Y(1) receptor model.

The P2Y(1) receptor closes the N-type Ca(2+) channel in neurones, with both adenosine triphosphates and diphosphates as potent agonists

The rat P2Y(1) nucleotide receptor, the P2Y subtype abundant in the brain, was heterologously expressed in rat superior cervical ganglion neurones by micro-injection of the receptor cRNA or cDNA. ADP inhibited the N-type Ca(2+) current by 64%, with EC(50) 8.2 nM, an action blocked competitively by the P2Y(1) receptor antagonist adenosine 3', 5'-bis-phosphate (K(i) 0.7 microM). 2-Methylthio-ADP inhibited the Ca(2+) current likewise, but with EC(50) 0.57 nM, giving the highest potency reported therewith for P2Y(1). Significantly, ATP and 2-methylthio-ATP were also agonists, the latter again at a very high potency (EC(50) 2.5 nM). We propose that this neuronal receptor, when present in brain at a high density as at synapses, can respond to very low concentrations of ATP and ADP as agonists, and that this would result in inhibition of N-type Ca(2+) currents and hence can reduce transmitter release or increase neuronal excitability.

P(2Y) purinoceptor subtypes recruit different mek activators in astrocytes

Extracellular ATP can function as a glial trophic factor as well as a neuronal transmitter. In astrocytes, mitogenic signalling by ATP is mediated by metabotropic P(2Y) receptors that are linked to the extracellular signal regulated protein kinase (Erk) cascade, but the types of P(2Y) receptors expressed in astrocytes have not been defined and it is not known whether all P(2Y) receptor subtypes are coupled to Erk by identical or distinct signalling pathways. We found that the P(2Y) receptor agonists ATP, ADP, UTP and 2-methylthioATP (2MeSATP) activated Erk and its upstream activator MAP/Erk kinase (Mek). cRaf-1, the first kinase in the Erk cascade, was activated by 2MeSATP, ADP and UTP but, surprisingly, cRaf-1 was not stimulated by ATP. Furthermore, ATP did not activate B-Raf, the major isoform of Raf in the brain, nor other Mek activators such as Mek kinase 1 (MekK1) and MekK2/3. Reverse transcriptase-polymerase chain reaction (RT - PCR) studies using primer pairs for cloned rat P(2Y) receptors revealed that rat cortical astrocytes express P(2Y(1)), a receptor subtype stimulated by ATP and ADP and their 2MeS analogues, as well as P(2Y(2)) and P(2Y(4)), subtypes in rats for which ATP and UTP are equipotent. Transcripts for P(2Y(6)), a pyrimidine-preferring receptor, were not detected. ATP did not increase cyclic AMP levels, suggesting that P(2Y(11)), an ATP-preferring receptor, is not expressed or is not linked to adenylyl cyclase in rat cortical astrocytes. These signal transduction and RT - PCR experiments reveal differences in the activation of cRaf-1 by P(2Y) receptor agonists that are inconsistent with properties of the P(2Y(1)), P(2Y(2)) and P(2Y(4)) receptors shown to be expressed in astrocytes, i.e. ATP=UTP; ATP=2MeSATP, ADP. This suggests that the properties of the native P(2Y) receptors coupled to the Erk cascade differ from the recombinant P(2Y) receptors or that astrocytes express novel purine-preferring and pyrimidine-preferring receptors coupled to the ERK cascade.

Multiple P2Y receptors mediate contraction in guinea pig mesenteric vein

Vasoconstrictor responses to exogenous adenine and pyrimidine nucleotides were measured in endothelium-denuded segments of guinea pig mesenteric vein and compared with responses in mesenteric artery. The rank order of potency for nucleotides in veins was: 2-MeSADP = 2-MeSATP > UTP > ATPgammaS = alpha,betaMeATP > UDP = ATP > ADP >> beta,gamma-D-MeATP = beta,gamma-L-MeATP. In contrast 2-MeSADP, UTP, and UDP were inactive in arteries, and the rank order of potency of other nucleotides differed; that is, alpha,betaMeATP > beta, gamma-D-MeATP > beta,gamma-L-MeATP = ATPgammaS = 2-MeSATP > ATP > ADP. In veins, UTP, ATP, and 2-MeSATP were more efficacious contractile agents than alpha,beta MeATP. In addition, the ability to desensitize responses to these nucleotides and inhibit them with various blockers differed. The response to alpha,betaMeATP in veins exhibited rapid desensitization and was inhibited by pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS) and suramin. The response to 2-MeSATP in veins did not desensitize; nor was it inhibited by prior alpha,betaMeATP desensitization, but it was inhibited by PPADS, suramin, and the selective P2Y(1) receptor antagonist adenosine 3',5'-bisphosphate (ABP, 10-100 microM). Responses to ATP and UTP in veins did not desensitize and were not inhibited by PPADS, suramin, ABP, or alpha, betaMeATP desensitization. In conclusion, our results suggest that venous contraction to a variety of nucleotides is mediated in large part by P2Y receptors including P2Y(1) receptors and an UTP-preferring P2Y receptor. A small component of contraction also appears to be mediated by P2X(1) receptors. This receptor profile differs markedly from that of mesenteric arteries in which P2X(1) receptors predominate.

Effect of loss of P2Y(2) receptor gene expression on nucleotide regulation of murine epithelial Cl(-) transport

Extracellular nucleotides are believed to be important regulators of ion transport in epithelial tissues as a result of their ability to activate cell surface receptors. Although numerous receptors that bind nucleotides have been identified, the complexity of this receptor family, combined with the lack of pharmacological agents specific for these receptors, has made the assignment of particular receptors and ligands to physiological responses difficult. Because ATP and UTP appear equipotent and equieffective in regulating ion transport in many epithelia, we tested the hypothesis that the P2Y(2) receptor (P2Y(2)-R) subtype mediates these responses in mouse epithelia, with gene targeting techniques. Mice with the P2Y(2)-R locus targeted and inactivated (P2Y(2)-R(-/-)) were generated, airways (trachea), gallbladder, and intestines (jejunum) excised, and Cl(-) secretory responses to luminal nucleotide additions measured in Ussing chambers. Comparison of P2Y(2)-R(+/+) with P2Y(2)-R(-/-) mice revealed that P2Y(2)-R mediated most (>85-95%) nucleotide-stimulated Cl(-) secretion in trachea, about 50% of nucleotide responses in the gallbladder, and none of the responses in the jejunum. Dose-effect relationships for nucleotides in tissues from P2Y(2)-R(-/-) mice suggest that the P2Y(6)-R regulates ion transport in gallbladder and to a lesser extent trachea, whereas P2Y(4) and/or unidentified receptor(s) regulate ion transport in jejunum. We conclude that the P2Y(2) receptor is the dominant P2Y purinoceptor that regulates airway epithelial ion transport, whereas other P2Y receptor subtypes are relatively more important in other nonrespiratory epithelia.

Only pyrimidinoceptors are functionally expressed in mouse neuroblastoma cell lines

The ability of UTP, UDP, ATP, and ADP to influence inositol phospholipid hydrolysis in neuroblastoma origin cell lines was assessed. The mouse neuroblastoma lines N1E 115, Neuro 2a, and NB4 1A3 and the rat glioma/mouse neuroblastoma hybrid line NG108-15 gave robust responses to both UTP and UDP, which were essentially equipotent. Thus a range of cell lines of mouse neuroblastoma origin express a pyrimidine-selective P2Y receptor. The NG108-15 cells were the only cell type tested at which ATP and ADP displayed activity with EC50 values of greater than 100 microM, compared with values of 0.58 and 1.25 microM for UTP and UDP, respectively. In contrast to the cell lines derived from mouse neuroblastoma, the human neuroblastoma lines SH-SY5Y and SK-N-SH did not respond to any nucleotides, although both responded well to carbachol.