ATP secretion and extracellular protein phosphorylation by CNS neurons in primary culture

AMP-activated protein kinase (AMPK) activator A-769662 increases intracellular calcium and ATP release from astrocytes in an AMPK-independent manner

AIM

To test the hypothesis that, given the role of AMP-activated protein kinase (AMPK) in regulating intracellular ATP levels, AMPK may alter ATP release from astrocytes, the main sources of extracellular ATP (eATP) within the brain.

MATERIALS AND METHODS

Measurements of ATP release were made from human U373 astrocytoma cells, primary mouse hypothalamic (HTAS) and cortical astrocytes (CRTAS) and wild-type and AMPK α1/α2 null mouse embryonic fibroblasts (MEFs). Cells were treated with drugs known to modulate AMPK activity: A-769662, AICAR and metformin, for up to 3 hours. Intracellular calcium was measured using Fluo4 and Fura-2 calcium-sensitive fluorescent dyes.

RESULTS

In U373 cells, A-769662 (100 μM) increased AMPK phosphorylation, whereas AICAR and metformin (1 mM) induced a modest increase or had no effect, respectively. Only A-769662 increased eATP levels, and this was partially blocked by AMPK inhibitor Compound C. A-769662-induced increases in eATP were preserved in AMPK α1/α2 null MEF cells. A-769662 increased intracellular calcium in U373, HTAS and CRTAS cells and chelation of intracellular calcium using BAPTA-AM reduced A-769662-induced eATP levels. A-769662 also increased ATP release from a number of other central and peripheral endocrine cell types.

CONCLUSIONS

AMPK is required to maintain basal eATP levels but is not required for A-769662-induced increases in eATP. A-769662 (>50 μM) enhanced intracellular calcium levels leading to ATP release in an AMPK and purinergic receptor independent pathway.

Purinergic modulation of the excitatory synaptic input onto rat striatal neurons

There is no in situ evidence hitherto for a modulation by ATP of the glutamatergic excitatory transmission onto medium spiny neurons (MSNs) in the rat striatum. In order to resolve this question, we used the patch-clamp technique in brain slice preparations to record excitatory postsynaptic currents (EPSCs) evoked by intrastriatal electrical stimulation and applied N-methyl-d-aspartate (NMDA) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) to activate transmembrane currents of MSNs. In the absence of external Mg(2+), ATP caused a higher maximum inhibition of the EPSCs than adenosine. Only P1 (A(1)), but not P2 receptor antagonists interfered with the effects of both ATP and adenosine. Moreover, A(1) receptor antagonists were less potent in blocking the inhibition by ATP than that by adenosine. Eventually, adenosine deaminase (ADA) almost abolished the adenosine-induced inhibition, but only moderately decreased the ATP-induced inhibition. Antagonists of A(1) receptors (but not of P2 receptors) counteracted the depression by ATP of the current responses to exogenous NMDA, without altering those to AMPA. It is suggested that ATP indirectly, via its degradation product adenosine, stimulates presynaptic inhibitory A(1) receptors situated at glutamatergic nerve terminals of striatal afferents; these nerve terminals are devoid of P2 receptors. However, ATP, in contrast to adenosine, also activates postsynaptic A(1) receptors at the MSN neurons themselves. The resulting negative interaction with NMDA receptors requires localized extracellular catabolism of ATP by ectonucleotidases.

Mapping P2X and P2Y receptor proteins in striatum and substantia nigra: An immunohistological study

Our work aimed to provide a topographical analysis of all known ionotropic P2X_1–7 and metabotropic P2Y_{1,2,4,6,11–14} receptors that are present in vivo at the protein level in the basal ganglia nuclei and particularly in rat brain slices from striatum and substantia nigra. By immunohistochemistry-confocal and Western blotting techniques, we show that, with the exception of P2Y_11,13 receptors, all other subtypes are specifically expressed in these areas in different amounts, with ratings of low (P2X_5,6 and P2Y_1,6,14 in striatum), medium (P2X₃ in striatum and substantia nigra, P2X_6,7 and P2Y₁ in substantia nigra) and high. Moreover, we describe that P2 receptors are localized on neurons (colocalizing with neurofilament light, medium and heavy chains) with features that are either dopaminergic (colocalizing with tyrosine hydroxylase) or GABAergic (colocalizing with parvalbumin and calbindin), and they are also present on astrocytes (P2Y_2,4, colocalizing with glial fibrillary acidic protein). In addition, we aimed to investigate the expression of P2 receptors after dopamine denervation, obtained by using unilateral injection of 6-hydroxydopamine as an animal model of Parkinson’s disease. This generates a rearrangement of P2 proteins: most P2X and P2Y receptors are decreased on GABAergic and dopaminergic neurons, in the lesioned striatum and substantia nigra, respectively, as a consequence of dopaminergic denervation and/or neuronal degeneration. Conversely, P2X_1,3,4,6 on GABAergic neurons and P2Y₄ on astrocytes augment their expression exclusively in the lesioned substantia nigra reticulata, probably as a compensatory reaction to dopamine shortage. These results disclose the presence of P2 receptors in the normal and lesioned nigro-striatal circuit, and suggest their potential participation in the mechanisms of Parkinson’s disease.

Modulation by D1 and D2 dopamine receptors of ATP-induced release of intracellular Ca2+ in cultured rat striatal neurons

The aim of the present study was to investigate, whether dopamine D1 and/or D2 receptors are able to interfere with the ATP-induced increase of the intracellular Ca2+ concentration ([Ca2+]i) in cultured striatal neurons identified by their morphological characteristics and their [Ca2+]i transients in response to a high-K+ superfusion medium. ATP appeared to release Ca2+ mostly from an intracellular pool, since its effect was markedly depressed in the presence of cyclopiazonic acid, which is known to deplete such storage sites [Rubini, P., Pinkwart, C., Franke, H., Gerevich, Z., Nörenberg, W., Illes, P., 2006. Regulation of intracellular Ca2+ by P2Y1 receptors may depend on the developmental stage of cultured rat striatal neurons. J. Cell. Physiol. 209, 81-93]. The mixed D1/D2 receptor agonist dopamine increased the ATP-induced [Ca2+]i transients in a subpopulation of neurons. At the same time, dopamine did not alter the responses to K+ in these cells. The selective D1 (SKF 83566) and D2 (sulpiride) receptor antagonists failed to modify the effect of ATP, but unmasked in the previously unresponsive neurons an inhibitory and facilitatory effect of dopamine, respectively. A combination of the two antagonists resulted in a failure of dopamine to modulate the [Ca2+]i responses in any cell investigated. In conclusion, D1 and D2 receptors may modulate in an opposite manner the signalling pathways of P2Y1 receptors in striatal neurons and thereby alter their development/growth or their cellular excitability and/or the release of GABA from their terminals.

Physiology and pathophysiology of purinergic neurotransmission

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

P2X2 and P2Y1 immunofluorescence in rat neostriatal medium-spiny projection neurones and cholinergic interneurones is not linked to respective purinergic receptor function

1. The presence of ionotropic P2X receptors, targets of ATP in fast synaptic transmission, as well as metabotropic P2Y receptors, known to activate K(+) currents in cultured neostriatal neurones, was investigated in medium-spiny neurones and cholinergic interneurones contained in neostriatal brain slices from 5-26-day-old rats. 2. In these cells, adenosine-5'-triphosphate (ATP) (100-1000 microm), 2-methylthioadenosine-5'-triphosphate (2MeSATP), alpha,beta-methyleneadenosine-5'-triphosphate (alpha,betameATP, 30-300 microm, each) and adenosine-5'-O-(3-thiotriphosphate (ATPgammaS) (100 microm) failed to evoke P2X receptor currents even when 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 0.1 microm), apyrase (10 U ml(-1)) or intracellular Cs(+) was used to prevent occluding effects of the ATP breakdown product adenosine, desensitisation of P2X receptors by endogenous ATP and an interference with the activation of K(+) channels, respectively. P2X receptor agonists were also ineffective in outside-out patches withdrawn from the brain slice tissue. Muscimol (10 microm) evoked GABA(A) receptor-mediated currents under all these conditions. 3. When used as a control, locus coeruleus neurones responded with P2X receptor-mediated currents to ATP (300 microm), 2MeSATP and alpha,betameATP (100 microm, each). 4. ATP and adenosine-5'-diphosphate (ADP) (100 microm, each) did not activate K(+) currents in the neostriatal neurones. 5. Despite the observed lack of function, P2X(2) and P2Y(1) immunofluorescence was found in roughly 50% of the medium-spiny neurones and cholinergic interneurones. 6. A role of ATP in synaptic transmission to striatal medium-spiny neurones and cholinergic interneurones appears unlikely, however, the otherwise silent P2X and P2Y receptors may gain functionality under certain yet unknown conditions.

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.

Trophic effects of purines in neurons and glial cells

In addition to their well known roles within cells, purine nucleotides such as adenosine 5' triphosphate (ATP) and guanosine 5' triphosphate (GTP), nucleosides such as adenosine and guanosine and bases, such as adenine and guanine and their metabolic products xanthine and hypoxanthine are released into the extracellular space where they act as intercellular signaling molecules. In the nervous system they mediate both immediate effects, such as neurotransmission, and trophic effects which induce changes in cell metabolism, structure and function and therefore have a longer time course. Some trophic effects of purines are mediated via purinergic cell surface receptors, whereas others require uptake of purines by the target cells. Purine nucleosides and nucleotides, especially guanosine, ATP and GTP stimulate incorporation of [3H]thymidine into DNA of astrocytes and microglia and concomitant mitosis in vitro. High concentrations of adenosine also induce apoptosis, through both activation of cell-surface A3 receptors and through a mechanism requiring uptake into the cells. Extracellular purines also stimulate the synthesis and release of protein trophic factors by astrocytes, including bFGF (basic fibroblast growth factor), nerve growth factor (NGF), neurotrophin-3, ciliary neurotrophic factor and S-100beta protein. In vivo infusion into brain of adenosine analogs stimulates reactive gliosis. Purine nucleosides and nucleotides also stimulate the differentiation and process outgrowth from various neurons including primary cultures of hippocampal neurons and pheochromocytoma cells. A tonic release of ATP from neurons, its hydrolysis by ecto-nucleotidases and subsequent re-uptake by axons appears crucial for normal axonal growth. Guanosine and GTP, through apparently different mechanisms, are also potent stimulators of axonal growth in vitro. In vivo the extracellular concentration of purines depends on a balance between the release of purines from cells and their re-uptake and extracellular metabolism. Purine nucleosides and nucleotides are released from neurons by exocytosis and from both neurons and glia by non-exocytotic mechanisms. Nucleosides are principally released through the equilibratory nucleoside transmembrane transporters whereas nucleotides may be transported through the ATP binding cassette family of proteins, including the multidrug resistance protein. The extracellular purine nucleotides are rapidly metabolized by ectonucleotidases. Adenosine is deaminated by adenosine deaminase (ADA) and guanosine is converted to guanine and deaminated by guanase. Nucleosides are also removed from the extracellular space into neurons and glia by transporter systems. Large quantities of purines, particularly guanosine and, to a lesser extent adenosine, are released extracellularly following ischemia or trauma. Thus purines are likely to exert trophic effects in vivo following trauma. The extracellular purine nucleotide GTP enhances the tonic release of adenine nucleotides, whereas the nucleoside guanosine stimulates tonic release of adenosine and its metabolic products. The trophic effects of guanosine and GTP may depend on this process. Guanosine is likely to be an important trophic effector in vivo because high concentrations remain extracellularly for up to a week after focal brain injury. Purine derivatives are now in clinical trials in humans as memory-enhancing agents in Alzheimer's disease. Two of these, propentofylline and AIT-082, are trophic effectors in animals, increasing production of neurotrophic factors in brain and spinal cord. Likely more clinical uses for purine derivatives will be found; purines interact at the level of signal-transduction pathways with other transmitters, for example, glutamate. They can beneficially modify the actions of these other transmitters.

Extracellular adenosine triphosphate affects neural cell adhesion molecule (NCAM)-mediated cell adhesion and neurite outgrowth

The neural cell adhesion molecule (NCAM) plays an important role in synaptic plasticity in embryonic and adult brain. Recently, it has been demonstrated that NCAM is capable of binding and hydrolyzing extracellular ATP. The purpose of the present study was to evaluate the role of extracellular ATP in NCAM-mediated cellular adhesion and neurite outgrowth. We here show that extracellularly added adenosine triphosphate (ATP) and its structural analogues, adenosine-5'-O-(3-thiothiophosphate), beta, gamma-methylenadenosine-5'-triphosphate, beta, gamma-imidoadenosine-5-triphosphate, and UTP, in varying degrees inhibited aggregation of hippocampal neurons. Rat glial BT4Cn cells are unable to aggregate when grown on agar but acquire this capacity when transfected with NCAM. However, addition of extracellular ATP to NCAM-transfected BT4Cn cells inhibited aggregation. Furthermore, neurite outgrowth from hippocampal neurons in cultures allowing NCAM-homophilic interactions was inhibited by addition of extracellular nucleotides. These findings indicate that NCAM-mediated adhesion may be modulated by extracellular ATP. Moreover, extracellularly added ATP stimulated neurite outgrowth from hippocampal neurons under conditions non-permissive for NCAM-homophilic interactions, and neurite outgrowth stimulated by extracellular ATP could be inhibited by a synthetic peptide corresponding to the so-called cell adhesion molecule homology domain (CHD) of the fibroblast growth factor receptor (FGFR) and by FGFR antibodies binding to this domain. Antibodies against the fibronectin type-III homology modules of NCAM, in which a putative site for ATP binding and hydrolysis is located, also abolished the neurite outgrowth-promoting effect of ATP. The non-hydrolyzable analogues of ATP all strongly inhibited neurite outgrowth. Our results indicate that extracellular ATP may be involved in synaptic plasticity through a modulation of NCAM-mediated adhesion and neurite outgrowth.

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.

ATP acts as fast neurotransmitter in rat habenula: neurochemical and enzymecytochemical evidence

The release of ATP and ADP, the putative central neurotransmitters, from the isolated habenula preparation was investigated in the rat, at rest and during electrical stimulation, using the luciferin-luciferase assay and the creatine phosphokinase assay. Electrical field stimulation (2 Hz, 360 pulses) released a considerable amount of ATP (2450 +/- 280 pmol/g wet tissue) from the tissue; inhibition of the voltage Na+ entry by tetrodotoxin (1 microM) reduced significantly the evoked release (by 66.25 +/- 6.65%), but not the resting release of ATP. Endogenous ADP also appeared in the effluent, but its amount differed during resting condition and after stimulation from that of ATP, suggesting that the majority of the released compound is ATP in response to stimulation. When ATP was added to the tissue, it readily decomposed to ADP and AMP (Km = 811.6 +/- 68.88 microM, vmax = 23.1 +/- 2.75 nmol/min per prep., measured by high-performance liquid chromatography combined with ultraviolet detection), indicating that the habenula contains ectoATPases. In addition, the inactivation of extracellular ATP by the ectoATPase enzyme was also visualized by electron microscopic enzyme cytochemistry. The ectoATPase enzyme was present on the membranes of the dendrites and nerve terminals and in the synapses of the habenula. Taking into account the fact that ATP is ubiquitous in excitable cells (storage) and the findings published by Edwards et al. in 1992 ("ATP receptor-mediated synaptic currents in the central nervous system", Nature, Vol. 359, pp. 144-147), our data provides evidence for the release by axonal stimulation and extracellular decomposition of ATP, all needed for an endogenous substance qualified as a transmitter.

Nucleotides as extracellular signalling molecules

There is now wide acceptance that ATP and other nucleotides are ubiquitous extracellular chemical messengers. ATP and diadenosine polyphosphates can be released from synaptosomes. They act on a large and diverse family of P2 purinoceptors, four of which have been cloned. This receptor family can be divided into two distinct classes: ligand-gated ion channels for P2X receptors and G protein-coupled receptors for P2Y, P2U, P2T and P2D receptors. The P2Y, P2U and P2D receptors have a fairly wide tissue distribution, while the P2X receptor is mainly found in neurons and muscles and the P2T and P2Z receptors confined to platelets and immune cells, respectively. Inositol phosphate and calcium signalling appear to be the predominant mechanisms for transducing the G-protein linked P2 receptor signals. Multiple P2 receptors are expressed by neurons and glia in the CNS and also in neuroendocrine cells. ATP and other nucleotides may therefore have important roles not only as a neurotransmitter but also as a neuroendocrine regulatory messenger.

Activation of specific ATP receptors induces a rapid increase in intracellular calcium ions in rat hypothalamic neurons

We have used real-time dynamic video imaging of Fura-2 fluorescence to study the acute effects of external ATP on [Ca2+]i in cultured rat hypothalamic neurons. The addition of ATP at microM concentrations, but not adenosine, AMP, ADP or GTP, produced a rapid, dose-dependent increase in cytosolic Ca2+. The hydrolysis-resistant ATP analogues 3-thio-ATP and beta,gamma-imido-ATP produced a similar response but alpha,beta-methylene ATP had much lower efficacy. The ATP response was inhibited by 10 microM nifedipine, abolished by 50 microM cadmium and by the absence of extracellular Ca2+, but was unaffected by ryanodine or omega-conotoxin GVIA. The P2-purinoceptor antagonist suramin reversibly and selectively inhibited the ATP response but had no effect on other neurotransmitter-induced Cai2+ responses. Antagonists to muscarinic, nicotinic, NMDA, non-NMDA, GABA, 5-HT and adenosine receptors had no effect on the ATP response. Thus the Ca2+ response of hypothalamic neurons to ATP is mediated by specific suramin-sensitive ATP-receptors, activation of which is independent of ATP hydrolysis and results in an influx of extracellular Ca2+ largely through high voltage-gated Ca2+ channels. These findings support the assertion that ATP acts in the CNS as an excitatory neurotransmitter.

A splice variant of the N-methyl-D-aspartate (NMDAR1) receptor

A splice variant of the NMDA receptor (NMDAR1) was discovered containing a deletion of 37 amino acids near the carboxyl tail and has been designated NMDAR1b. The 111 nucleotides corresponding to the deleted amino acid sequence were found in a separate exon bounded by consensus intron/exon junction sequences in rat genomic DNA. A partial restriction map of genomic DNA bounding this region placed the deleted exon approximately 600 base pairs (bp) downstream of the upstream exon. RT/PCR analysis of RNA from different brain regions showed that the deletion variant is more abundantly expressed in the brain stem and cerebellum while the full-length form is expressed more abundantly in the olfactory bulb, striatum, hippocampus, and cortex. Northern analysis of poly(A)+ RNA from different brain regions with probes specific for the deleted exon (i.e., full-length form) and for the splice junction (deletion form) indicated approximately 4.4 kb transcripts. The probe for the deleted exon hybridized to transcripts in olfactory bulb, cortex, striatum, and hippocampus while the splice junction probe hybridized most strongly to transcripts in cerebellum. The results suggest an interesting rostral to caudal shift in the expression of splice variants of the NMDAR1 which may signify important functional differences in native forms of NMDA receptors.

Ecto-protein kinase and surface protein phosphorylation in PC12 cells: interactions with nerve growth factor

The phosphorylation of surface proteins by ecto-protein kinase has been proposed to play a role in mechanisms underlying neuronal differentiation and their responsiveness to nerve growth factor (NGF). PC12 clones represent an optimal model for investigating the mode of action of NGF in a homogeneous cell population. In the present study we obtained evidence that PC12 cells possess ecto-protein kinase and characterized the endogenous phosphorylation of its surface protein substrates. PC12 cells maintained in a chemically defined medium exhibited phosphorylation of proteins by [gamma-32P]ATP added to the medium at time points preceding the intracellular phosphorylation of proteins in cells labeled with 32Pi. This activity was abolished by adding apyrase or trypsin to the medium but was not sensitive to addition of an excess of unlabeled Pi. As also expected from ecto-protein kinase activity, PC12 cells catalyzed the phosphorylation of an exogenous protein substrate added to the medium, dephospho-alpha-casein, and this activity competed with the endogenous phosphorylation for extracellular ATP. Based on these criteria, three protein components migrating in sodium dodecyl sulfate gels with apparent molecular weights of 105K, 39K, and 20K were identified as exclusive substrates of ecto-protein kinase in PC12 cells. Of the phosphate incorporated into these proteins from extracellular ATP, 75-87% was found in phosphothreonine. The phosphorylation of the 39K protein by ecto-protein kinase did not require Mg2+, implicating this activity in the previously demonstrated regulation of Ca(2+)-dependent, high-affinity norepinephrine uptake in PC12 cells by extracellular ATP. The protein kinase inhibitor K-252a inhibited both intra- and extracellular protein phosphorylation in intact PC12 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Production of adenosine from extracellular ATP at the striatal cholinergic synapse

The components of the ectonucleotidase pathway at the immunoaffinity-purified striatal cholinergic synapse have been studied. The ecto-ATPase (EC 3.6.1.15) had a Km of 131 microM, whereas the ecto-ADPase (EC 3.6.1.6) had a Km of 58 microM, was Ca(2+)-dependent, and was inhibited by the ATP analogue 5'-adenylylimidodiphosphate (AMPPNP). The ecto-5'-nucleotidase (EC 3.1.3.5) had a Km of 21 microM, was inhibited by AMPPNP and alpha,beta-methylene ADP, and by a specific antiserum. The Vmax values of the ATPase, ADPase, and 5'-nucleotidase enzymes present at this synapse were in a ratio of 30:14:1. Very little ecto-adenylate kinase activity was detected on these purified synapses. The intraterminal 5'-nucleotidase enzyme, which amounted to 40% of the total 5'-nucleotidase activity, was inhibited by AMPPNP, alpha,beta-methylene ADP, and the antiserum, and also had the same kinetic properties as the ectoenzyme. The time course of ATP degradation to adenosine outside the nerve terminals showed a delay, followed by a period of sustained adenosine production. The delay in adenosine production was proportional to the initial ATP concentration, was a consequence of feedforward inhibition of the ADPase and 5'-nucleotidase, and was inversely proportional to the ecto-5'-nucleotidase activity. The function and characteristics of this pathway and the central role of 5'-nucleotidase in the regulation of extraterminal adenosine concentrations are discussed.

Ecto-phosphorylation on aortic endothelial cells. Exquisite sensitivity to staurosporine

One- and two-dimensional gel electrophoresis of proteins from bovine aortic endothelial cells (BAEC) incubated with [gamma-32P]ATP revealed the preferential labelling of a cell-associated 21 kDa substrate. The labelling of this band was detectable within 30 s, increased up to 30 min and was stable for at least 3 h following the wash-out of the ATP. This protein was also labelled after incubation of the cells with [gamma-35S]ATP. Incorporation of radioactivity into the 21 kDa band did not occur if the endothelial cells were treated with low concentrations of trypsin (0.01%) before or after the labelling period. The pattern of BAEC protein phosphorylation by [gamma-32P]ATP was completely different from that of the fetal calf serum used for the cell culture. The presence of serum during the incubation of BAEC with [gamma-32P]ATP did not modify qualitatively the labelling pattern and, in particular, did not enhance the phosphorylation of the 21 kDa substrate; this suggests that neither the kinase nor the 21 kDa substrate are adsorbed serum proteins. Staurosporine, a protein kinase inhibitor with low specificity, decreased the labelling of the 21 kDa protein with an IC50 of 2 nM. In contrast, at 100 nM, staurosporine did not decrease the accumulation of inositol phosphates induced by ATP via the activation of P2y receptors. These data indicate the presence of aortic endothelial cells of an ecto-kinase which uses extracellular ATP to produce the selective and long-lived phosphorylation of a 21 kDa endothelial substrate. Ecto-phosphorylation of this protein might play a role in the modulation of endothelial cell functions by ATP, in addition to the P2y receptors [Boeynaems & Pearson (1990) Trends Pharmacol. Sci. 11, 34-37]. The exquisite sensitivity of ecto-phosphorylation to inhibition by staurosporine and its specific inhibition by some isoquinolinesulphonamide compounds provide potential pharmacological tools to investigate this hypothesis.

Calcium ion mobilization in neuronal cells induced by PAF

We have reported previously that platelet-activating factor (PAF) interacts with the neuronal cell line NG108-15 (neuroblastoma X glioma hybrid) and the pheochromocytoma cell line, PC12. PAF acts on these cells by raising levels of intracellular free calcium ions. In the present report, we extend these studies. PAF induced the vesicular release of adenosine 5'-triphosphate (ATP) from PC12 cells in a dose-dependent manner. The PAF-induced ATP release was inhibited by the PAF antagonists, CV-3988 and CV-6209, and the calcium antagonist prenylamine. The relevance of the interaction of PAF with neuronal cells was investigated further by using brain synaptosomal preparations and primary cortical and neostriatal cells. Nanomolar concentrations of PAF induced calcium transients in aequorin-loaded synaptosomal preparations, and cortical and neostriatal cells were sensitive to the action of PAF. The possible physiological and pathophysiological roles of PAF in brain function are discussed.

Ependymin, a brain extracellular glycoprotein, and CNS plasticity

Ependymin, a glycoprotein of the brain ECF, has been implicated in the neurochemistry of memory and neuronal regeneration. Three behavioral experiments (swimming with a float, avoidance conditioning, and classical conditioning) in the goldfish and one in the mouse (T-maze learning) indicate that ependymin has a role in the synaptic changes that take place in the consolidation step of memory formation and the activity-dependent phase of sharpening of goldfish retinotectal connections during neuronal regeneration. The ECF concentration of the protein was found to decrease after the goldfish learned to associate a light stimulus (CS) with the subsequent arrival of a shock (US): paired CS-US gave changes whereas an unpaired presentation of CS-US gave no changes relative to the unstimulated controls. Ependymin is present in ECF as a mixture of three disulfide-linked dimers of two acidic (alpha and beta) polypeptide chains (37 kDa and 31 kDa). Upon removal of its N-linked glycan fragment by N-glycosidase F, the beta chain yields gamma-ependymin (26 kDa). Determinations of the amino acid sequence of gamma-ependymin indicate that it is a unique protein with no long sequence homologies to any known polypeptide. There are, however, small segments (5-7 amino acids long) with homologies to fibronectin, laminin, and tubulin. Ependymin has the capacity to polymerize into FIP (after activation by phosphorylation) in response to events that deplete ECF calcium. FIP is insoluble in 2% SDS in 6 M urea, 10 mM Ca2+Ac2, 100% acetic acid, chloroform/methanol (2/1), saturated KCNS, and even 100% trifluoroacetic acid. FIP was found to be present in goldfish brain and to be formed as a labeled product in vivo. Ependymin's FIP-forming property was used to propose a molecular hypothesis for generating synaptic changes in response to local extracellular depletions of calcium at sites of "associating inputs." The model assumes that, following NMDA receptor stimulation, the translocated PKC that is generated activates extracellular ependymin by converting it to its phosphorylated form using presynaptically released ATP. The hypothesis was tested in studies of LTP of rat hippocampal slices at CA1. After LTP, new sites that stained with antisera to ependymin, visible at 100x, were obtained in its potentiated radiatum in the CA1 region but not in the unpotentiated CA3. Electron microscopic studies showed that the horseradish peroxidase reaction product obtained was localized at synaptic clefts and postsynaptic regions. The results suggest that FIP may be formed at extracellular and postsynaptic loci where multiple associating inputs interact at CA1.

Ectoprotein kinase in the regulation of cellular responsiveness to extracellular ATP

The reversible phosphorylation of intracellular proteins has been established as a key regulatory mechanism in numerous cellular functions. In this process the enzyme protein kinase transfers the gamma-phosphate of ATP to form a covalent bond with specific proteins. Another line of investigation has demonstrated that extracellular ATP is a potent physiological regulator in various cellular systems. Although many of the physiological effects of extracellular ATP were shown to be mediated by the action of purinergic receptors, it is possible that extracellular protein phosphorylation systems are also implicated in the mechanisms underlying the responsiveness of cells to extracellular ATP. The identification of ectoprotein kinase at the surface of various cells has provided evidence for the existence of such mechanisms, and revealed how the regulatory powers of protein phosphorylation systems can extend to the extracellular environment. The versatile roles that extracellular protein phosphorylation activity may play in the regulation of cellular functions is underscored by the presence of multiple protein substrates for this activity at the cell surface. Each such surface phosphoprotein may have a unique function. FIGURE 5 depicts the hypothetical relationships between the extracellular ATP secreted by exocytosis and the specific physiological function of these secreting neurons. Based on findings described in this article, we propose that extracellular ATP can be utilized by two types of extracellular protein kinase: a membrane-bound ectoprotein kinase, and a soluble exoprotein kinase. The exoprotein kinase can originate by detachment of an ectokinase from the cell surface, or be an intravesicular protein that is coreleased with ATP by exocytosis from stimulated cells. Phosphorylation of specific proteins at the surface of a secreting cell may have an important feedback control over its own presynaptic activity. The ectoprotein kinase could exert this feedback regulation by phosphorylating ion channels involved in secretion, and/or by phosphorylating transporters that carry out the reuptake of released transmitter molecules. Phosphorylation of receptors can regulate intercellular communication, and phosphorylation of integrins could regulate the interaction of the cell surface with components of the extracellular matrix. Although most of the relationships suggested in FIGURE 5 are still hypothetical, it should be possible to test them experimentally in a direct manner by raising antibodies against the phosphorylated sites of specific surface phosphoproteins. The ability of such antibodies to inhibit protein phosphorylation without penetrating the cells provides an experimental paradigm for the direct testing of potential physiological function of ecto- and exoprotein kinase activities in a variety of cells.

Neuroregulatory and neuropathological actions of the ether-phospholipid platelet-activating factor

Platelet-activating factor (PAF) is a naturally occurring phospholipid that serves as a critical mediator in diverse biological and pathophysiological processes. In this study of the interactions of PAF with neuronal cells, it was found that PAF increased the intracellular levels of free calcium ions in cells of the clones NG108-15 and PC12. The increase was dependent on extracellular calcium and was inhibited by the antagonistic PAF analog CV-3988 and by the calcium-influx blockers prenylamine and diltiazem. A functional consequence of this interaction was revealed by measuring a PAF-elicited, Ca2+-dependent secretion of adenosine triphosphate from PC12 cells. Exposure of NG108-15 cells for 3 to 4 days to low concentrations of PAF induced neuronal differentiation; higher concentrations were neurotoxic. Thus, by influencing Ca2+ fluxes, PAF may play a physiological role in neuronal development and a pathophysiological role in the degeneration that occurs when neurons are exposed to circulatory factors as a result of trauma, stroke, or spinal cord injury.