Characterization of [35S]-ATP alpha S and [3H]-alpha, beta-MeATP binding sites in rat brain cortical synaptosomes: regulation of ligand binding by divalent cations

Binding thermodynamic characterization of human P2X1 and P2X3 purinergic receptors

The present study was designed to perform binding and thermodynamic characterization of human P2X1 and P2X3 purinergic receptors expressed in HEK 293 cells. The thermodynamic parameters DeltaG degrees , DeltaH degrees and DeltaS degrees (standard free energy, enthalpy and entropy) of the binding equilibrium of well-known purinergic agonists and antagonists at P2X1 and P2X3 receptors were determined. Saturation binding experiments, performed in the temperature range 4-30 degrees C by using the high affinity purinergic agonist [3H]alphabetameATP, revealed a single class of binding sites with an affinity value in the nanomolar range in both cell lines examined. The affinity changed with the temperature whereas receptor density was essentially independent of it. van't Hoff plots of the purinergic receptors were linear in the range 4-30 degrees C for agonists and antagonists. The thermodynamic parameters of the P2X1 or P2X3 purinergic receptors were in the ranges -31 kJ mol(-1) < or =DeltaH degrees < or =-19 kJ mol(-1) and 17 J K(-1) mol(-1)< or =DeltaS degrees < or =51 J K(-1)mol(-1) or -26 kJ mol(-1)< or =DeltaH degrees < or =36 kJ mol(-1) and 59< or =DeltaS degrees < or =249 JK(-1) mol(-1), respectively. The results of these parameters showed that P2X1 receptors are not thermodynamically discriminated and that the binding of agonists and antagonists was both enthalpy and entropy-driven. P2X3 receptors were thermodynamically discriminated and purinergic agonist binding was enthalpy and entropy-driven while antagonist binding was totally entropy-driven. The analysis of such thermodynamic data makes it possible to obtain additional information on the nature of the forces driving the purinergic binding interaction. These data could be interesting in drug discovery programs aimed at development of novel and potent P2X1 and P2X3 purinergic ligands.

Biochemical analysis of ecto-nucleotide pyrophosphatase phosphodiesterase activity in brain membranes indicates involvement of NPP1 isoenzyme in extracellular hydrolysis of diadenosine polyphosphates in central nervous system

Synaptosomes and plasma membranes obtained from rat brain display ectoenzymatic hydrolytic activity responsible for hydrolysis of the neurotransmitter/neuroregulatory nucleotides diadenosine polyphosphates. Intact synaptosomes and plasma and synaptic membranes isolated by sucrose-gradient ultracentrifugation from several brain regions (hypothalamus, hippocampus, temporal cortex, frontal cortex striatum and cerebellum) degraded the fluorogenic substrates diethenoadenosine polyphosphates up to ethenoadenosine as by-product. Purified ectoenzyme cleaved substrates always releasing the mononucleotide moieties ethenoadenosine 5'-monophosphate and the corresponding ethenoadenosine (n-1) 5'-phosphate. Ectoenzymatic hydrolysis reached maximal activity at pH 9.0 (pH range 6.5-9.0) and was activated by Ca(2+) and Mg(2+) ions, with maximal effects around 2.0 mM cation. EDTA drastically reduced activity and Zn(2+) was required for enzyme reactivation. Hydrolysis of substrates followed hyperbolic kinetics with K(m) values in the 3-10 microM range. Diadenosine polyphosphates and heparin behaved as competitive inhibitors in the enzymatic hydrolysis of diethenoadenosine polyphosphates and AMP, ATP, alpha,beta-methyleneADP, ADPbetaS ATPgammaS, beta,gamma-methyleneATP, suramin and diethyl pyrocarbonate were also inhibitors. Ectoenzymatic activity shared the typical characteristics of members of the ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family and inhibition data suggest that NPP1 ectoenzyme is involved in the cleavage of extracellular diadenosine polyphosphates in brain. Synaptic membranes from cerebellum, hypothalamus and hippocampus presented the highest activities and no activity differences were observed between young and aged animals. However, plasma membranes showed a more homogeneous distribution of ectoenzymatic activity but a general increase was detected in aged animals. Enhancement of ectoenzymatic diadenosine polyphosphate cleaving activity found in plasma membranes from old animals could play a deleterious role in aged brain by limiting neuroprotective effects reported for extracellular diadenosine tetraphosphate.

P2 receptors and neuronal injury

Extracellular adenosine 5'-triphosphate (ATP) was proposed to be an activity-dependent signaling molecule that regulates glia-glia and glia-neuron communications. ATP is a neurotransmitter of its own right and, in addition, a cotransmitter of other classical transmitters such as glutamate or GABA. The effects of ATP are mediated by two receptor families belonging either to the P2X (ligand-gated cationic channels) or P2Y (G protein-coupled receptors) types. P2X receptors are responsible for rapid synaptic responses, whereas P2Y receptors mediate slow synaptic responses and other types of purinergic signaling involved in neuronal damage/regeneration. ATP may act at pre- and postsynaptic sites and therefore, it may participate in the phenomena of long-term potentiation and long-term depression of excitatory synaptic transmission. The release of ATP into the extracellular space, e.g., by exocytosis, membrane transporters, and connexin hemichannels, is a widespread physiological process. However, ATP may also leave cells through their plasma membrane damaged by inflammation, ischemia, and mechanical injury. Functional responses to the activation of multiple P2 receptors were found in neurons and glial cells under normal and pathophysiological conditions. P2 receptor-activation could either be a cause or a consequence of neuronal cell death/glial activation and may be related to detrimental and/or beneficial effects. The present review aims at demonstrating that purinergic mechanisms correlate with the etiopathology of brain insults, especially because of the massive extracellular release of ATP, adenosine, and other neurotransmitters after brain injury. We will focus in this review on the most important P2 receptor-mediated neurodegenerative and neuroprotective processes and their beneficial modulation by possible therapeutic manipulations.

Dual presynaptic control by ATP of glutamate release via facilitatory P2X1, P2X2/3, and P2X3 and inhibitory P2Y1, P2Y2, and/or P2Y4 receptors in the rat hippocampus

ATP is released in a vesicular manner from nerve terminals mainly at higher stimulation frequencies. There is a robust expression of ATP (P2) receptors in the brain, but their role is primarily unknown. We report that ATP analogs biphasically modulate the evoked release of glutamate from purified nerve terminals of the rat hippocampus, the facilitation being mediated by P2X1, P2X2/3, and P2X3 [antagonized by 8-(benzamido)naphthalene-1,3,5-trisulfonate and 2',3'-O-(2,4,6-trinitrophenyl)-ATP] and the inhibition by P2Y1, P2Y2, and/or P2Y4 [antagonized by reactive blue 2 and 2'deoxy-N6-methyladenosine-3',5'-bisphosphate and mimicked by P1-(urinine 5'-),P4-(inosine 5'-) tetraphosphate and 2-methylthio-ADP] receptors. The combination of single-cell PCR analysis of rat hippocampal pyramidal neurons, Western blot analysis of purified presynaptic active zone fraction, and immunocytochemical analysis of hippocampal glutamatergic terminals revealed that the P2 receptors expressed in glutamatergic neurons, located in the active zone and in glutamatergic terminals, were precisely P2X1, P2X2, and P2X3 subunits and P2Y1, P2Y2, and P2Y4 receptors. This provides coincident functional and molecular evidence that P2 receptors are present and act presynaptically as a modulatory system controlling hippocampal glutamate release.

Kinetics of [35S]dATPalphaS interaction with P2Y1 purinoceptor in rat brain membranes

Kinetics of [35S]dATPalphaS (2'deoxyadenosine-5'-[alpha-35S]-thiotriphosphate) interaction with rat brain membrane fragments was studied at 25 degrees C and at radioligand concentrations from 2 to 250 nM. At least two different ways of [35S]dATPalphaS interaction with the membranes were distinguished on the basis of radioligand on-rate. Firstly, the binding sites characterized by 'fast' on-rate can be observed. Secondly, the 'slow' binding sites were kinetically identified and quantified. As in both cases the bound radioligand could be displaced by excess of ATP, all these binding sites can be defined as 'specific sites'. In the 'slow' binding sites isomerization of the receptor-ligand complex was observed, as is typical for interaction of antagonists with G-protein coupled receptors, and the kinetic parameters for this interaction were similar with the appropriate data for the hP2Y1 receptors expressed in 1321N1 astrocytoma cells Therefore these sites could be assigned to the same receptor subtype in brain membranes while the 'fast' binding sites belong to other membrane-bound proteins, also interacting with ATP and its analogues. The kinetic properties of the latter sites were not analysed in detail.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Diadenosine polyphosphate receptors. from rat and guinea-pig brain to human nervous system

Diadenosine polyphosphates are a family of naturally occurring nucleotidic compounds present in secretory vesicles together with other chemical messengers. The exocytotic release of these compounds permits them to stimulate receptors termed "purinoceptors" or "ATP receptors." Purinoceptors for nucleotides are named P2 in contrast with those sensitive to nucleosides (P1). P2 receptors are further subdivided into metabotropic P2Y receptors, further divided into 5 subtypes, and ionotropic P2X receptors, with 7 different subtypes. Diadenosine polyphosphates can activate recombinant P2Y(1), P2Y(2), and P2Y(4) and recombinant homomeric P2X(1), P2X(2), P2X(3), P2X(4), and P2X(6). Heteromeric P2X receptors change their sensitivity to diadenosine polyphosphates when co-assembly between different subunits occurs. Diadenosine polyphosphates can activate specific receptors termed dinucleotide receptors or P4 receptors, which are insensitive to other nucleosides or nucleotides. The P4 receptor is a receptor-operated Ca(2)+ channel present in rat brain synaptic terminals, stimulated by diadenosine pentaphosphate and diadenosine tetraphosphate. This receptor is strongly modulated by protein kinases A and C and protein phosphatases. The dinucleotide receptor is present in different brain areas, such as midbrain (in rat and guinea-pig), cerebellum (in guinea-pig), and cortex (in human).

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.

In human and rat lung membranes [35S]GTPgammaS binding is a tool for pharmacological characterization of G protein-coupled dinucleotide receptors

The P2Y receptor family is activated by extracellular nucleotides such as ATP and UTP. P2Y receptors regulate physiological functions in numerous cell types. In lung, the P2Y2 receptor subtype plays a role in controlling Cl- and fluid transport. Besides ATP or UTP, also diadenosine tetraphosphate (Ap4A), a stable nucleotide, seems to be of physiological importance. In membrane preparations from human and rat lung we applied several diadenosine polyphosphates to investigate whether they act as agonists for G protein-coupled receptors. We assessed this by determining the stimulation of [35S]GTPgammaS binding. Stimulation of [35S]GTPgammaS binding to G proteins has already been successfully applied to elucidate agonist binding to various G protein-coupled receptors. Ap(n)A (n = 2 to 6) enhanced [35S]GTPgammaS binding similarly in human and rat lung membranes, an indication of the existence of G protein-coupled receptor binding sites specific for diadenosine polyphosphates. Moreover, in both human and rat lung membranes comparable pharmacological properties were found for a diadenosine polyphosphate ([3H]Ap4A) binding site. The affinity for Ap2A, Ap3A, Ap4A, Ap5A, and Ap6A was also comparable. 8-Diazido-Ap4A and ATP were less potent, whereas the pyrimidine nucleotide UTP showed hardly any affinity. Thus, we present evidence that different diadenosine polyphosphates bind to a common G protein-coupled receptor binding site in membranes derived either from human or rat lung.

Evidence for a G protein-coupled diadenosine-5',5'''-P1,P4-tetraphosphate (Ap4A) receptor binding site in lung membranes from rat

Nucleotide receptors are of considerable importance in the treatment of lung diseases, such as cystic fibrosis. Because diadenosine polyphosphates may also be of significance as signalling molecules in lung, as they are in a variety of tissues, in the present work we investigated the binding sites for [3H]diadenosine-5',5'''-P1,P4-tetraphosphate (Ap4A) in plasma membranes from rat lung and studied their possible coupling to G proteins. We present evidence for a single high-affinity binding site for [3H]Ap4A with similar affinity for other diadenosine polyphosphates ApnA (n = 2 to 6). Displacement studies with different nucleotides revealed that the [3H]Ap4A binding site was different from P2X and P2Y2 receptor binding sites. Pretreatment of lung membranes with GTPgammaS or GTP in the presence of Mg2+ increased the Ki for Ap4A from 91 nM to 5.1 microM, which is indicative of G protein coupling. The putative coupling to G proteins was further confirmed by the enhancement of [35S]GTPgammaS binding (to Galpha proteins) to lung membranes by Ap4A (63% increase over basal) in a concentration-dependent manner. Therefore, our data for the first time provide evidence of a G protein-coupled Ap4A binding site in lung membranes.

Characterization of ATP and P2 agonists binding to the cardiac plasma membrane P1,P4-diadenosine 5'-tetraphosphate receptor

P1,P4-Diadenosine 5'-tetraphosphate (Ap4A) acts as an extracellular modulator through its interaction with purinoceptors. Our laboratory has demonstrated the presence of an Ap4A receptor in cardiac tissue [1,2]. Due to the rapid hydrolysis of ATP by cardiac membranes the relationship of ATP and Ap4A binding to purinoceptors on cardiac membranes has not been characterized. In this communication we used two approaches to determine the relationship of ATP to the Ap4A receptor. Radioligand binding carried out with [alpha-32P]Ap4A and adenosine 5'-O-¿3-thiotriphosphate¿ ([gamma-35S]ATPgammaS) demonstrates the presence of a single high affinity binding site for Ap4A and the presence of two binding sites for ATPgammaS. The second approach utilized immunoaffinity purified Ap4A receptor that was shown to be free of ATPase and Ap4Aase activities. Non-radiolabeled Ap4A and ATPgammaS effectively inhibited photocrosslinking of [alpha-32P]8-N3Ap4A to the receptor polypeptide while ATP was a much less effective inhibitor. Furthermore, on plasma membranes [alpha-32P]8-N3Ap4A photocrosslinked to only a 50 kDa polypeptide. These data are consistent with Ap4A interacting with a homogeneous population of receptors on cardiac plasma membranes but with ATP having a low affinity for the receptor.

Identification of rat brain p42(IP4), a high-affinity inositol(1,3,4, 5)tetrakisphosphate/phosphatidylinositol(3,4,5)trisphosphate binding protein

Inositol(1,3,4,5)tetrakisphosphate (InsP4) and phosphatidylinositol(3,4,5)trisphosphate (PtdInsP3) are two potential second messengers with a still largely unknown mode of action. We recently cloned the 42 kDa protein p42IP4 previously purified from pig cerebellum, which binds InsP4 (Kd approximately 2 nM) and PtdInsP3 with comparable affinities (Stricker et al., FEBS Lett. 405 (1997) 229). The protein p42IP4 (pig) is highly homologous to centaurin-alpha, a larger protein of 46 kDa, derived from a rat brain cDNA library clone (Hammonds-Odie et al., J. Biol. Chem. 271 (1996) 18859). Here we investigated whether also p42IP4 is expressed in rat brain and how it might be related to centaurin-alpha. When we carried out RT-PCR using mRNA from brain of rats of different ages we obtained several clones corresponding to p42IP4, but not to centaurin-alpha. The existence of p42IP4 in rat brain is supported by the following findings: (1) biochemical analysis of the purified rat brain protein shows inositol phosphate ligand affinities identical to those of the protein from other species; (2) Western blot analysis of rat brain membrane fractions using a peptide-specific antiserum revealed only the 42 kDa protein (p42IP4), but did not give evidence for the occurrence of a larger 46 kDa centaurin-alpha-like protein in rat brain; and (3) the amino acid sequences deduced from p42IP4 cDNA are highly homologous in several species and are confirmed by protein fragment microsequences. Thus, p42IP4 from rat brain which has two pleckstrin homology domains is a protein largely conserved between different species and most likely has an important function in inositol phosphate or inositol lipid signal transduction.