Surface protein phosphorylation by ecto-protein kinase is required for the maintenance of hippocampal long-term potentiation

Chemoproteomics reveals immunogenic and tumor-associated cell surface substrates of ectokinase CK2α

New epitopes for immune recognition provide the basis of anticancer immunity. Due to the high concentration of extracellular adenosine triphosphate in the tumor microenvironment, we hypothesized that extracellular kinases (ectokinases) could have dysregulated activity and introduce aberrant phosphorylation sites on cell surface proteins. We engineered a cell-tethered version of the extracellular kinase CK2α, demonstrated it was active on cells under tumor-relevant conditions, and profiled its substrate scope using a chemoproteomic workflow. We then demonstrated that mice developed polyreactive antisera in response to syngeneic tumor cells that had been subjected to surface hyperphosphorylation with CK2α. Interestingly, these mice developed B cell and CD4+ T cell responses in response to these antigens but failed to develop a CD8+ T cell response. This work provides a workflow for probing the extracellular phosphoproteome and demonstrates that extracellular phosphoproteins are immunogenic even in a syngeneic system.

Ecto-protein kinase CK2, the neglected form of CK2

Ecto-protein kinases, including protein kinase CK2 (former name, casein kinase 2), have been the focus of research for more than 30 years. At the beginning of the ecto-kinase research their identification was performed with substrates and inhibitors whose specificity under the current knowledge was rather limited. Since all currently known ecto-kinases, including ecto-CK2, have intracellular counterparts, one has to exclude that an ecto-localization originates from intracellular counterparts after cell damage. Protein kinase CK2 is involved in cellular key processes such as cell cycle progression, inhibition of apoptosis, DNA damage repair, differentiation and many other processes. CK2 is composed of two catalytic CK2α or CK2α' subunits and two non-catalytic CK2β subunits. Progress in the ecto-kinase and in particular ecto-CK2 studies was made with the use of transfected tagged CK2 subunits, which allowed to follow their individual transport and localization on the cell surface after transfection. Furthermore, immunofluorescence studies with antibodies against CK2 subunits as well as affinity chromatography with a binding partner of CK2 subunits have improved ecto-kinase research. The use of new and more specific inhibitors as well as of substrates, which do not cross the plasma membrane, have further improved the specificity for ecto-CK2. From the various substrates of ecto-CK2, it can be concluded that ecto-CK2 plays a role in Alzheimer disease, cell adhesion, platelet aggregation, immune response and cellular signalling. New tools and techniques, to study ecto-CK2 activity, are required to identify new substrates and thereby new functional implications for ecto-CK2.

Extracellular Protein Kinase A Modulates Intracellular Calcium/Calmodulin-Dependent Protein Kinase II, Nitric Oxide Synthase, and the Glutamate-Nitric Oxide-cGMP Pathway in Cerebellum. Differential Effects in Hyperammonemia

Extracellular protein kinases, including cAMP-dependent protein kinase (PKA), modulate neuronal functions including N-methyl-d-aspartate (NMDA) receptor-dependent long-term potentiation. NMDA receptor activation increases calcium, which binds to calmodulin and activates nitric oxide synthase (NOS), increasing nitric oxide (NO), which activates guanylate cyclase, increasing cGMP, which is released to the extracellular fluid, allowing analysis of this glutamate-NO-cGMP pathway in vivo by microdialysis. The function of this pathway is impaired in hyperammonemic rats. The aims of this work were to assess (1) whether the glutamate-NO-cGMP pathway is modulated in cerebellum in vivo by an extracellular PKA, (2) the role of phosphorylation and activity of calcium/calmodulin-dependent protein kinase II (CaMKII) and NOS in the pathway modulation by extracellular PKA, and (3) whether the effects are different in hyperammonemic and control rats. The pathway was analyzed by in vivo microdialysis. The role of extracellular PKA was analyzed by inhibiting it with a membrane-impermeable inhibitor. The mechanisms involved were analyzed in freshly isolated cerebellar slices from control and hyperammonemic rats. In control rats, inhibiting extracellular PKA reduces the glutamate-NO-cGMP pathway function in vivo. This is due to reduction of CaMKII phosphorylation and activity, which reduces NOS phosphorylation at Ser1417 and NOS activity, resulting in reduced guanylate cyclase activation and cGMP formation. In hyperammonemic rats, under basal conditions, CaMKII phosphorylation and activity are increased, increasing NOS phosphorylation at Ser847, which reduces NOS activity, guanylate cyclase activation, and cGMP. Inhibiting extracellular PKA in hyperammonemic rats normalizes CaMKII phosphorylation and activity, NOS phosphorylation, NOS activity, and cGMP, restoring normal function of the pathway.

Ecto-protein kinases and phosphatases: an emerging field for translational medicine

Progress in translational research has led to effective new treatments of a large number of diseases. Despite this progress, diseases including cancer and cardiovascular disorders still are at the top in death statistics and disorders such as osteoporosis and osteoarthritis represent an increasing disease burden in the aging population. Novel strategies in research are needed more than ever to overcome such diseases. The growing field of extracellular protein phosphorylation provides excellent opportunities to make major discoveries of disease mechanisms that can lead to novel therapies. Reversible phosphorylation/dephosphorylation of sites in the extracellular domains of matrix, cell-surface and trans-membrane proteins is emerging as a critical regulatory mechanism in health and disease. Moreover, a new concept is emerging from studies of extracellular protein phosphorylation: in cells where ATP is stored within secretory vesicles and released by exocytosis upon cell-stimulation, phosphorylation of extracellular proteins can operate as a messenger operating uniquely in signaling pathways responsible for long-term cellular adaptation. Here, we highlight new concepts that arise from this research, and discuss translation of the findings into clinical applications such as development of diagnostic disease markers and next-generation drugs.

Targeting synaptic dysfunction in Alzheimer's disease therapy

In the past years, major efforts have been made to understand the genetics and molecular pathogenesis of Alzheimer's disease (AD), which has been translated into extensive experimental approaches aimed at slowing down or halting disease progression. Advances in transgenic (Tg) technologies allowed the engineering of different mouse models of AD recapitulating a range of AD-like features. These Tg models provided excellent opportunities to analyze the bases for the temporal evolution of the disease. Several lines of evidence point to synaptic dysfunction as a cause of AD and that synapse loss is a pathological correlate associated with cognitive decline. Therefore, the phenotypic characterization of these animals has included electrophysiological studies to analyze hippocampal synaptic transmission and long-term potentiation, a widely recognized cellular model for learning and memory. Transgenic mice, along with non-Tg models derived mainly from exogenous application of Aβ, have also been useful experimental tools to test the various therapeutic approaches. As a result, numerous pharmacological interventions have been reported to attenuate synaptic dysfunction and improve behavior in the different AD models. To date, however, very few of these findings have resulted in target validation or successful translation into disease-modifying compounds in humans. Here, we will briefly review the synaptic alterations across the different animal models and we will recapitulate the pharmacological strategies aimed at rescuing hippocampal plasticity phenotypes. Finally, we will highlight intrinsic limitations in the use of experimental systems and related challenges in translating preclinical studies into human clinical trials.

A novel method for the simultaneous enrichment, identification, and quantification of phosphopeptides and sialylated glycopeptides applied to a temporal profile of mouse brain development

We describe a method that combines an optimized titanium dioxide protocol and hydrophilic interaction liquid chromatography to simultaneously enrich, identify and quantify phosphopeptides and formerly N-linked sialylated glycopeptides to monitor changes associated with cell signaling during mouse brain development. We initially applied the method to enriched membrane fractions from HeLa cells, which allowed the identification of 4468 unique phosphopeptides and 1809 formerly N-linked sialylated glycopeptides. We subsequently combined the method with isobaric tagging for relative quantification to compare changes in phosphopeptide and formerly N-linked sialylated glycopeptide abundance in the developing mouse brain. A total of 7682 unique phosphopeptide sequences and 3246 unique formerly sialylated glycopeptides were identified. Moreover 669 phosphopeptides and 300 formerly N-sialylated glycopeptides differentially regulated during mouse brain development were detected. This strategy allowed us to reveal extensive changes in post-translational modifications from postnatal mice from day 0 until maturity at day 80. The results of this study confirm the role of sialylation in organ development and provide the first extensive global view of dynamic changes between N-linked sialylation and phosphorylation.

Extracellular phosphorylation of the amyloid β-peptide promotes formation of toxic aggregates during the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia and associated with progressive deposition of amyloid β-peptides (Aβ) in the brain. Aβ derives by sequential proteolytic processing of the amyloid precursor protein by β- and γ-secretases. Rare mutations that lead to amino-acid substitutions within or close to the Aβ domain promote the formation of neurotoxic Aβ assemblies and can cause early-onset AD. However, mechanisms that increase the aggregation of wild-type Aβ and cause the much more common sporadic forms of AD are largely unknown. Here, we show that extracellular Aβ undergoes phosphorylation by protein kinases at the cell surface and in cerebrospinal fluid of the human brain. Phosphorylation of serine residue 8 promotes formation of oligomeric Aβ assemblies that represent nuclei for fibrillization. Phosphorylated Aβ was detected in the brains of transgenic mice and human AD brains and showed increased toxicity in Drosophila models as compared with non-phosphorylated Aβ. Phosphorylation of Aβ could represent an important molecular mechanism in the pathogenesis of the most common sporadic form of AD.

Immunohistological Determination of Ecto-nucleoside Triphosphate Diphosphohydrolase1 (NTPDase1) and 5′-nucleotidase in Rat Hippocampus Reveals Overlapping Distribution

Distribution of two enzymes involved in the ectonucleotidase enzyme chain, ecto-nucleoside triphosphate diphosphohydrolase1 (NTPDase1) and ecto-5'-nucleotidase, was assessed by immunohistochemistry in the rat hippocampus. Obtained results have shown co-expression of the enzymes in the hippocampal region, as well as wide and strikingly similar cellular distribution. Both enzymes were expressed at the surface of pyramidal neurons in the CA1 and CA2 sections, while cells in the CA3 section were faintly stained. The granule cell layer of the dentate gyrus was moderately stained for NTPDase1, as well as for ecto-5'-nucleotidase. Glial association for ecto-5'-nucleotidase was also observed, and fiber tracts were intensively stained for both enzymes. This is the first comparative study of NTPDase1 and ecto-5'-nucleotidase distribution in the rat hippocampus. Obtained results suggest that the broad overlapping distribution of these enzymes in neurons and glial cells reflects the functional importance of ectonucleotidase actions in the nervous system.

Habituation to an open field alters ecto-nucleotidase activities in rat hippocampal synaptosomes

ATP and adenosine may play a role in the mechanisms of synaptic plasticity and memory formation. Previous studies have shown that ecto-nucleotidase activities are altered during memory consolidation of an aversive task named step-down inhibitory avoidance. Here we investigate ecto-nucleotidase activities in hippocampal synaptosomes of rats submitted to training and test sessions of habituation to open field, which is one of the most elementary forms of learning. There were no significant alterations on ATP, ADP and AMP hydrolysis immediately after the training session. However, immediately after the test session (0min), there was a significant increase of ATP hydrolysis (61%), but not of ADP and AMP hydrolysis. Sixty minutes after the test session, a significant increase of NTPDase (75% and 60.5% for ATP and ADP hydrolysis, respectively) and ecto-5'-nucleotidase (40%) activities was observed. This study reveals the involvement of ecto-nucleotidase activities in different learning paradigms during memory processing.

Increase of Fas-induced apoptosis by inhibition of extracellular phosphorylation of Fas receptor in Jurkat cell line

Apoptosis signalling through the Fas pathway requires several steps of aggregation of the Fas receptor in the membrane, including aggregation that may occur in the absence of Fas ligand. Association of Fas domains is determinant to signal transmission following Fas ligand binding to a specific domain. The domains involved in Fas aggregation are located in its extracellular region and contain three potential protein kinase C-binding motifs. We therefore studied the possibility that phosphorylation of the extracellular region of Fas might be implicated in the regulation of Fas-mediated apoptosis. Inhibition experiments of extracellular phosphorylation were performed in human Jurkat T leukemia cells with K252b, an impermeant protein-kinase inhibitor. Extracellular phosphorylation of Fas receptor was related to ecto-kinase, as assessed by the [gamma-(32)P] ATP labelling of Fas-116 kDa aggregates, suppressed by K252b inhibitor which significantly increased the sensitivity to Fas-mediated apoptosis. Ecto-PKC involvement was demonstrated by bisindolylmaleimide VIII, a selective inhibitor of protein kinase C which significantly increased both Fas aggregation in the membrane and Fas-mediated apoptosis and by the addition of the PKC pseudo-substrate 19-36 which inhibited the phosphorylation of 116 kDa Fas aggregates. These data support a role for Fas phosphorylation in the decreased sensitivity to apoptosis in the Jurkat T leukemia cell line.

Diadenosine polyphosphate analog controls postsynaptic excitation in CA3-CA1 synapses via a nitric oxide-dependent mechanism

Previously, we have described the modulatory effect of diadenosine polyphosphates Ap4A and Ap5A on synaptic transmission in the rat hippocampal slices mediated by presynaptic receptors (Klishin et al., 1994). In contrast, we now describe how nonhydrolyzable Ap4A analog diadenosine-5',5'''-P1,P4-[beta,beta'-methylene]tetraphosphate (AppCH2ppA) at low micromolar concentrations exerts strong nondesensitizing inhibition of orthodromically evoked field potentials (OFPs) without affecting the amplitude of excitatory postsynaptic currents and antidromically evoked field potentials, as recorded in hippocampal CA1 zone. The effects of AppCH2ppA on OFPs are eliminated by a P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) but not mimicked by purinoceptor agonists alpha,beta-methylene-ATP and adenosine 5'-O-(3-thio)-triphosphate, indicating that a P2-like receptor is involved but not one belonging to the conventional P2X/P2Y receptor classes. Diadenosine polyphosphate receptor (P4) antagonist Ip4I (diinosine tetraphosphate) was unable to modulate AppCH2ppA effects. Thus, the PPADS-sensitive P2-like receptor for AppCH2ppA seems to control selectively dendritic excitation of the CA1 neurons. The specific nitric oxide (NO)-scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide is shown to significantly attenuate AppCH2ppA-mediated inhibitory effects, indicating that NO is involved in the cascade of events initiated by AppCH2ppA. Further downstream mediation by adenosine A1 receptors is also demonstrated. Hence, AppCH2ppA-mediated effects involve PPADS-sensitive P2-like receptor activation leading to the production of NO that stimulates intracellular synthesis of adenosine, causing in turn postsynaptic A1 receptor activation and subsequent postsynaptic CA1 dendritic inhibition. Such spatially selective postsynaptic dendritic inhibition may influence dendritic electrogenesis in pyramidal neurons and consequently mediate control of neuronal network activity.

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.

Regulation of human recombinant P2X3 receptors by ecto-protein kinase C

The whole-cell patch-clamp technique was used to record current responses to nucleotides and nucleosides in human embryonic kidney HEK293 cells transfected with the human purinergic P2X3 receptor. When guanosine 5'-O-(3-thiodiphosphate) was included into the pipette solution, UTP at concentrations that did not alter the holding current facilitated the alpha,beta-methylene ATP (alpha,beta-meATP)-induced current. ATP and GTP, but not UDP or uridine, had an effect similar to that of UTP. Compounds known to activate protein kinase C (PKC) acted like the nucleoside triphosphates investigated, whereas various PKC inhibitors invariably reduced the effects of both PKC activators and UTP. The substitution by Ala of Ser/Thr residues situated within PKC consensus sites of the P2X3 receptor ectodomain either abolished (PKC2 and PKC3; T134A, S178A) or did not alter (PKC4 and PKC6; T196A, S269A) the UTP-induced potentiation of the alpha,beta-meATP current. Both the blockade of ecto-protein kinase C activity and the substitution of Thr-134 or Ser-178 by Ala depressed the maximum of the concentration-response curve for alpha,beta-meATP without altering the EC50 values. Molecular simulation of the P2X3 receptor structure indicated no overlap between assumed nucleotide binding domains and the relevant phosphorylation sites PKC2 and PKC3. alpha,beta-meATP-induced currents through native homomeric P2X3 receptors of rat dorsal root ganglia were also facilitated by UTP. In conclusion, it is suggested that low concentrations of endogenous nucleotides in the extracellular space may prime the sensitivity of P2X3 receptors toward the effect of subsequently applied (released) higher agonistic concentrations. The priming effect of nucleotides might be attributable to a phosphorylation of PKC sites at the ectodomain of P2X3 receptors.

Stabilizing effects of extracellular ATP on synaptic efficacy and plasticity in hippocampal pyramidal neurons

The role of adenosine triphosphate (ATP) as a neurotransmitter and extracellular diffusible messenger has recently received considerable attention because of its possible participation in the regulation of synaptic plasticity. However, the possible contribution of extracellular ATP in maintaining and regulating synaptic efficacy during intracellular ATP depletion is understudied. We tested the effects of extracellular ATP on excitatory postsynaptic currents (EPSCs) evoked in CA1 pyramidal neurons by Schaffer collateral stimulation. In the absence of intracellular ATP, EPSC rundown was neutralized when a low concentration of ATP (1 microm) was added to the extracellular solution. Adenosine and ATP analogues did not prevent the EPSC rundown. The P(2) antagonists piridoxal-5'-phosphate-azophenyl 2',4'-disulphonate (PPADS) and reactive blue-2, and the P(1) adenosine receptor antagonist 8-cyclopentyltheophylline (CPT) had no detectable effects in cells depleted of ATP. However, the protective action of extracellular ATP on synaptic efficacy was blocked by extracellular application of the protein kinase inhibitors K252b and staurosporine. In contrast, K252b and staurosporine per se did not interfere with synaptic transmission in ATP loaded cells. Without intracellular ATP, bath-applied caffeine induced a transient (< 35 min) EPSC potentiation that was transformed into a persistent long-term potentiation (> 80 min) when 1 microm ATP was added extracellularly. An increased probability of transmitter release paralleled the long-term potentiation induced by caffeine, suggesting that it originated presynaptically. Therefore, we conclude that extracellular ATP may operate to maintain and regulate synaptic efficacy and plasticity in conditions of abnormal intracellular ATP depletion by phosphorylation of a surface protein substrate via activation of ecto-protein kinases.

Inhibitory avoidance task reveals differences in ectonucleotidase activities between male and female rats

Studies demonstrated that endogenous levels of estrogen affect the long-term potentiation (LTP) and long-term depression (LTD). ATP and adenosine may play a role in the modulation of LTP. Our laboratory observed in previous studies that inhibitory avoidance task is associated with a decrease in hippocampal ectonucleotidase activities in adult male rats. To explore if ectonucleotidases are modulated in memory formation in female rats, as observed in males, we evaluated the effect of inhibitory avoidance training on synaptosomal NTP Dase and 5'-nucleotidase activities in rat hippocampus from both sexes. The results demonstrated a decrease in ATP, ADP and AMP hydrolysis (37%, 38% and 32%, respectively) immediately after training and a significant inhibition only in ATP hydrolysis (36%) 30 min post-training in male rats. There were no changes in ectonucleotidase activities from female rats. These findings provide support for the view that could exist biochemical differences in ectonucleotidase activities between males and females.

Quantitative proteomics analysis of specific protein expression and oxidative modification in aged senescence-accelerated-prone 8 mice brain

The senescence-accelerated mouse (SAM) is a murine model of accelerated senescence that was established using phenotypic selection. The SAMP series includes nine substrains, each of which exhibits characteristic disorders. SAMP8 is known to exhibit age-dependent learning and memory deficits. In our previous study, we reported that brains from 12-month-old SAMP8 have greater protein oxidation, as well as lipid peroxidation, compared with brains from 4-month-old SAMP8 mice. In order to investigate the relation between age-associated oxidative stress on specific protein oxidation and age-related learning and memory deficits in SAMP8, we used proteomics to identify proteins that are expressed differently and/or modified oxidatively in aged SAMP8 brains. We report here that in 12 month SAMP8 mice brains the expressions of neurofilament triplet L protein, lactate dehydrogenase 2 (LDH-2), heat shock protein 86, and alpha-spectrin are significantly decreased, while the expression of triosephosphate isomerase (TPI) is increased compared with 4-month-old SAMP8 brains. We also report that the specific protein carbonyl levels of LDH-2, dihydropyrimidinase-like protein 2, alpha-spectrin and creatine kinase, are significantly increased in the brain of 12-month-old SAMP8 mice when compared with the 4-month-old SAMP8 brain. These findings are discussed in reference to the effect of specific protein oxidation and changes of expression on potential mechanisms of abnormal alterations in metabolism and neurochemicals, as well as to the learning and memory deficits in aged SAMP8 mice.

Molecular physiology of P2 receptors in the central nervous system

Neurons of the central nervous system (CNS) are endowed with ATP-sensitive receptors belonging to the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. Whereas a number of P2X receptors mediate fast synaptic responses to the transmitter ATP, P2Y receptors mediate either slow changes of the membrane potential in response to non-synaptically released ATP or the interaction with receptors for other transmitters. To date seven P2X and seven P2Y receptors of human origin have been molecularly identified and functionally characterized. P2X subunits may occur as homooligomers or as heterooligomeric assemblies of more than one subunit. P2X(7) subunits do not form heterooligomeric assemblies and are unique in mediating apoptosis and necrosis of glial cells and possibly also of neurons. The P2X(2), P2X(4), P2X(4)/P2X(6) and P2Y(1) receptors appear to be the predominant neuronal types. The localisation of these receptors may be at the somato-dendritic region (postsynaptic) or at the nerve terminals (presynaptic). Postsynaptic P2 receptors appear to be mostly excitatory, while presynaptic P2 receptors may be either excitatory (P2X) or inhibitory (P2Y). Since in the CNS the stimulation of a single neuron may activate multiple networks, a concomitant stimulation of facilitatory and inhibitory circuits as a result of ATP release is also possible. Finally, the enzymatic degradation of ATP may lead to the local generation of adenosine which can modulate via A(1) or A(2A) receptor-activation the ATP effect.

Purinergic P2 receptors trigger adenosine release leading to adenosine A2A receptor activation and facilitation of long-term potentiation in rat hippocampal slices

Electrophysiological recordings were used to investigate the effects of ATP analogues on theta-burst-induced long-term potentiation (LTP) in rat hippocampal slices. alpha,beta-Methylene ATP (alpha,beta-MeATP; 20 microM) decreased LTP from 36+/-9% to 17+/-5%, an effect prevented by adenosine A(1) receptor blockade in accordance with the localised catabolism of ATP analogues into adenosine, leading to adenosine A(1) receptor activation. Thus, to probe the role of extracellular ATP, all experiments were performed with the A(1) receptor selective antagonist, 1,3-dipropyl-8-cyclopentylxanthine (50 nM). In these conditions, alpha,beta-MeATP or 5'-adenylylimido-diphosphate (beta,gamma-ImATP; 20 microM) facilitated LTP by 120%, an effect prevented by the P2 receptor antagonists, pyridoxalphosphate-6-azophenyl-2'-4'-disulphonic acid (PPADS; 20 microM) or suramin (75 microM), as well as by the P2X(1/3)-selective antagonist 8-(benzamido)naphthalene-1,3,5-trisulfonate (10 microM). The facilitations of LTP by either alpha,beta-MeATP or beta,gamma-ImATP (20 microM) were also prevented by both 4-(2-[7-amino-2-(2-furyl(1,2,4)-triazolo(2,3a)-(1,3,5)triazin-5-yl-amino]ethyl)phenol (50 nM) or 7-2(-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c] pyrimidine (50 nM), antagonists of facilitatory adenosine A(2A) receptors, were occluded by the A(2A) receptor agonist, CGS 21680 (10 nM) and were prevented by the protein kinase C inhibitor, chelerythrine (6 microM) and unaffected by the protein kinase A inhibitor, H89 (1 microM). Furthermore, beta,gamma-ImATP (20 microM) enhanced [(3)H]adenosine outflow from rat hippocampal slices by nearly 150%, an effect prevented by PPADS (20 microM) or suramin (75 microM). The adenosine transport inhibitors, nitrobenzylthioinosine (5 microM) and dipyridamole (10 microM) also prevented beta,gamma-ImATP (20 microM)-induced [(3)H]adenosine outflow and facilitation of LTP. These results suggest that ATP analogues facilitate LTP through P2 receptor activation that mainly triggers adenosine release leading to the activation of adenosine A(2A) receptors.

Dual effects of ATP on rat hippocampal synaptic plasticity

Although ATP is reported to modulate synaptic plasticity, the mechanism of action of ATP on synaptic transmission is not fully understood. Here we show that ATP enhances long-term potentiation (LTP), and P2X receptor antagonists inhibit this ATP effect, but do not affect paired pulse facilitation (PPF) in rat hippocampal slices. ATP rapidly increases intracellular calcium, and P2X receptor antagonists inhibit this increase in cultured dissociated neurons. These results indicate that ATP enhances LTP via activation of postsynaptic P2X receptors. A pertussis toxin-sensitive G-protein inhibitor significantly attenuates PPF, although it does not affect LTP, indicating that presynaptic P2Y receptors also play an important role in neuronal plasticity. We conclude that ATP modulates synaptic plasticity via dual effects on pre- and post-synaptic mechanisms.

Caffeine-mediated presynaptic long-term potentiation in hippocampal CA1 pyramidal neurons

We report a new form of long-term potentiation (LTP) in Schaffer collateral (SC)-CA1 pyramidal neuron synapses that originates presynaptically and does not require N-methyl-d-aspartate (NMDA) receptor activation nor increases in postsynaptic-free Ca2+. Using rat hippocampal slices, application of a brief "pulse" of caffeine in the bath evoked a nondecremental LTP (CAFLTP) of SC excitatory postsynaptic currents. An increased probability of transmitter release paralleled the CAFLTP, suggesting that it originated presynaptically. The P1 adenosine receptor antagonist 8-cyclopentyltheophylline and the P2 purinoreceptor antagonists suramin and piridoxal-5'-phosphate-azophenyl 2',4'-disulphonate blocked the CAFLTP. Inhibition of Ca2+ release from caffeine/ryanodine stores by bath-applied ryanodine inhibited the CAFLTP, but ryanodine in the pipette solution was ineffective, suggesting a presynaptic effect of ryanodine. Previous induction of the "classical" LTP did not prevent the CAFLTP, suggesting that the LTP and the CAFLTP have different underlying cellular mechanisms. The CAFLTP is insensitive to the block of NMDA receptors by 2-amino-5-phosphonopentanoic acid and to Ca2+ chelation with intracellular 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid, indicating that neither postsynaptic NMDA receptors nor increases in cytosolic-free Ca2+ participate in the CAFLTP. We conclude that the CAFLTP requires the interaction of caffeine with presynaptic P1, P2 purinoreceptors, and ryanodine receptors and is caused by an increased probability of glutamate release at SC terminals.

Cooperativity between extracellular adenosine 5'-triphosphate and activation of N-methyl-D-aspartate receptors in long-term potentiation induction in hippocampal CA1 neurons

The mechanism of ATP-induced long-term potentiation (LTP) was studied pharmacologically using guinea-pig hippocampal slices. LTP, induced in CA1 neurons by 10 min application of 10 microM ATP, was blocked by co-application of the N-methyl-D-aspartate (NMDA) receptor antagonist, D,L-2-amino-5-phosphonovalerate (5 or 50 microM). In ATP-induced LTP, the delivery of test synaptic inputs (once every 20 s) to CA1 neurons could be replaced by co-application of NMDA (100 nM) during ATP perfusion. These results suggest that, in CA1 neurons, a co-operative effect between extracellular ATP and activation of NMDA receptors is required to trigger the process involved in ATP-induced LTP. In addition, ATP-induced LTP was blocked by co-application of an ecto-protein kinase inhibitor, K-252b (40 or 200 nM), whereas a P2X purinoceptor antagonist, pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid 4-sodium (50 microM), or a P2Y purinoceptor antagonist, basilen blue (10 microM), had no effect.The results of the present study, therefore, indicate that the mechanisms of ATP-induced LTP involve the modulation of NMDA receptors/Ca(2+) channels and the phosphorylation of extracellular domains of synaptic membrane proteins, one of which could be the NMDA receptor/Ca(2+) channel.

Phosphorylation of neuroglycan C, a brain-specific transmembrane chondroitin sulfate proteoglycan, and its localization in the lipid rafts

Neuroglycan C (NGC) is a brain-specific transmembrane chondroitin sulfate proteoglycan. In the present study, we examined whether NGC could be phosphorylated in neural cells. On metabolic labeling of cultured cerebral cortical cells from the rat fetus with (32)P(i), serine residues in NGC were radiolabeled. Some NGC became detectable in the raft fraction from the rat cerebrum, a signaling microdomain of the plasma membrane, with cerebral development. NGC from the non-raft fraction, not the raft fraction, could be phosphorylated by an in vitro kinase reaction. The phosphorylation of NGC was inhibited by adding to the reaction mixture a recombinant peptide representing the ectodomain of NGC, but not by adding a peptide representing its cytoplasmic domain. NGC could be labeled by an in vitro kinase reaction using [gamma-(32)P]GTP as well as [gamma-(32)P]ATP, and this kinase activity was partially inhibited by 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole, a selective inhibitor of casein kinase II. In addition to the intracellular phosphorylation, NGC was also phosphorylated at the cell surface by an ectoprotein kinase. This is the first report to demonstrate that NGC can be phosphorylated both intracellularly and pericellularly, and our findings suggest that a kinase with a specificity similar to that of casein kinase II is responsible for the NGC ectodomain phosphorylation.

Effects of the mono- and tetrasialogangliosides GM1 and GQ1b on ATP-induced long-term potentiation in hippocampal CA1 neurons

The effects of the mono- and tetrasialogangliosides, GM1 and GQ1b, on ATP-induced long-term potentiation (LTP) were studied in CA1 neurons of guinea pig hippocampal slices. Application of 5 or 10 microM ATP for 10 min resulted in a transient depression followed by a slow augmentation of synaptic transmission, leading to LTP. LTP induced by treatment with 5 microM ATP was facilitated in hippocampal slices prepared from animals treated for 6 days with a ceramide analog, L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propranol, which stimulates ganglioside biosynthesis. In addition, LTP induced by 5 microM ATP was significantly enhanced when naive slices were incubated with GQ1b but not with GM1. These results suggest that a cooperative effect between extracellular ATP and GQ1b enhances ATP-induced LTP in hippocampal CA1 neurons. In addition, the LTP induced by 10 microM ATP was blocked by coapplication of the NMDA antagonist AP5 (5 microM or 50 microM), and this effect was partially inhibited by GQ1b pretreatment of the slices, suggesting that in hippocampal CA1 neurons, the enhancing effect of GQ1b on ATP-induced LTP is mediated by modulation of NMDA receptors/Ca(2+) channels.

Kinetic characterization and immunodetection of ecto-ATP diphosphohydrolase (EC 3.6.1.5) in cultured hippocampal neurons

Extracellular ATP and adenosine modulate synaptic transmission in hippocampal neurons. ATP released from neural cells is hydrolyzed to adenosine by a chain of ecto-nucleotidases. ATP diphosphohydrolase hydrolyses ATP and ADP nucleotides to AMP and 5'-nucleotidase hydrolyses AMP to adenosine. In this work, we investigated the ATPase and ADPase activities of ATP diphosphohydrolase in cultured hippocampal neurons. The apparent Michaelis-Menten constant (K(m)) was 233.9 +/- 14.6 and 221.8 +/- 63.6 microM, with a calculated maximal velocity (V(max), approximately) of 49.2 +/- 10.7 and 10.9 +/- 5.2 nmol Pi/mg protein/min for ATP and ADP, respectively. The horizontal straight line obtained in the competition plot indicated that only one active site is able to hydrolyze both substrates. Furthermore, we detected the presence of this enzyme using anti-CD39 antibody, which strongly stained the soma of pyramidal and bipolar neurons, but the neurites connecting the cell clusters were also immunopositive. This antibody recognized three bands with a molecular mass close to 95, 80 and 60kDa in immunoblotting analysis. The present results show, for the first time, the kinetic and immunocytochemical characterization of an ATP diphosphohydrolase in cultured hippocampal neurons. Probably, the widespread distribution of this enzyme on the surface of neurons in culture could reflect its functional importance in studies of synaptic plasticity hippocampal.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Selective secretion of protein kinase C isozymes by thrombin-stimulated human platelets

The protein kinase C (PKC) was secreted from thrombin-stimulated human platelets in a time- and dose-dependent manner. The PKC specific inhibitors Ro31-8220 (0.05 microM) and GF 109203X (0.5 microM) totally inhibited the secreted kinase activity. Western blot analysis of the secretory components showed reactivity to PKCalpha, PKCbetaII, and PKCdelta antibodies, but not to PKCbetaI, and p42/44 MAPK, although they were present in lysed platelets. The fractionation of platelets secreted components showed that PKC activity increased in both soluble and microparticle fractions after thrombin treatments. This is the first report demonstrating that activated human platelets selectively secrete protein kinase C isozymes. Protein kinase C secreted by platelets in this unique manner may have an extracellular role in the plasma, and may regulate cellular functions, including remodeling of vascular endothelial cells.

Characterisation of ATP-induced facilitation of transmission in rat hippocampus

Superfusion of rat hippocampal slices with ATP induces a form of facilitation that has been poorly characterised. The present study has confirmed that at low concentrations of ATP (10 microM or less), an initial depression of evoked potential size is followed by a rebound facilitation which is not reproduced by alphabeta-methyleneATP, betagamma-methyleneATP, or the dinucleotide P1,P6-diadenosine hexaphosphate. The post-ATP facilitation could be prevented by the adenosine A1 receptor antagonists 8-phenyltheophylline or 1,3-dipropyl-8-cyclopentyltheophylline (50 nM), or superfusion of adenosine deaminase. The adenosine A2A receptor antagonist 8-(chlorostyryl)-caffeine did not affect the inhibition but prevented the post-ATP facilitation. The NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid prevented the establishment of post-ATP facilitation. The post-ATP facilitation was also blocked by suramin at a concentration (50 microM) that does not block glutamate receptors. Suramin prevented the induction but not the maintenance phase of the post-ATP facilitation. The repeated induction of post-ATP facilitation by bursts of electrical stimulation designed to saturate the normal mechanisms of long-term potentiation prevented the induction of post-ATP facilitation. However, repeated applications of ATP to achieve saturation of its receptor did not prevent the subsequent induction of electrically evoked long-term potentiation. It is concluded that ATP can induce a form of synaptic facilitation which resembles only partially that induced by electrical stimulation and which may require the simultaneous activation of P1 and P2 receptors.

Stanniocalcin 1 and 2 are secreted as phosphoproteins from human fibrosarcoma cells

Stanniocalcin 1 (STC1) and stanniocalcin 2 (STC2) are two recently identified mammalian peptide hormones. STC1 plays a role in calcium and phosphate homoeostasis, while the role of STC2 is unknown. We examined a human fibrosarcoma cell line, HT1080, that has high steady-state STC1 and STC2 mRNA levels, to determine whether these proteins are secreted. Following incubation of HT1080 cells with (32)P, labelled STC1 and STC2 were found to be secreted into the medium. STC1 was phosphorylated in vitro by protein kinase C (PKC). In vitro and in vivo phosphorylation both occurred exclusively on serine and the phosphopeptide maps were similar, suggesting that PKC might be the in vivo kinase. STC2 was phosphorylated in vitro by casein kinase II (CK2), in vitro and in vivo phosphorylation were exclusively on serine and the phosphopeptide maps were indistinguishable. Phosphorylation of STC2 in intact cells resulted from the action of an ecto-protein kinase, since exogenous STC2 was phosphorylated by HT1080 cells and no phosphorylated STC2 was detectable inside the cells. The ectokinase activity was abolished by heparin and GTP could substitute for ATP as the phosphate donor, indicative of an ecto-CK2-like activity. The in vitro CK2 phosphorylation site was shown by matrix-assisted laser-desorption ionization-time-of-flight MS to be a single serine located between Ser-285 and Ser-298 in the C-terminal region of STC2. This is the first report of the secretion of STC1 or STC2 from mammalian cells. We conclude that these human fibrosarcoma cells express both STC1 and STC2 as secreted phosphoproteins in vivo, with STC2 being phosphorylated by an ecto-CK2-like enzyme.

Phosphorylation of the beta-amyloid precursor protein at the cell surface by ectocasein kinases 1 and 2

The beta-amyloid precursor protein (betaAPP) is one of the rare proteins known to be phosphorylated within its ectodomain. We have shown previously that betaAPP can be phosphorylated within secretory vesicles and at the cell surface (Walter, J., Capell, A., Hung, A. Y. , Langen, H., Schnölzer, M., Thinakaran, G., Sisodia, S. S., Selkoe, D. J., and Haass, C. (1997) J. Biol. Chem. 272, 1896-1903). We have now specifically characterized the phosphorylation of cell surface-located betaAPP and identified two ectoprotein kinases that phosphorylate betaAPP at the outer face of the plasma membrane. By using selective protein kinase inhibitors and by investigating the usage of ATP and GTP as cosubstrates, we demonstrate that membrane-bound betaAPP as well as secreted forms of betaAPP can be phosphorylated by casein kinase (CK) 1- and CK2-like ectoprotein kinases. The ectodomain of betaAPP was also phosphorylated by purified CK1 and CK2 in vitro, but not by protein kinases A and C. Phosphorylation of betaAPP by ectoprotein kinases and by purified CK1 and CK2 occurred within an acidic domain in the N-terminal half of the protein. Heparin strongly inhibited the phosphorylation of cell-surface betaAPP by ecto-CK1 and ecto-CK2, indicating a regulatory role of this extracellular matrix component in betaAPP phosphorylation.

Identification of ecto-PKC on surface of human platelets: role in maintenance of latent fibrinogen receptors

Human platelets express a protein phosphorylation system on their surface. A specific protein kinase C (PKC) antibody, monoclonal antibody (MAb) 1.9, which binds to the catalytic domain of PKC and inhibits its activity, causes the aggregation of intact platelets while inhibiting the phosphorylation of platelet surface proteins. Photoaffinity labeling with 100 nM 8-azido-[alpha(32)P]ATP identified this ecto-PKC as a single surface protein of 43 kDa sensitive to proteolysis by extracellular 0.0005% trypsin. Inhibition of the binding of 8-azido-[alpha(32)P]ATP to the 43-kDa surface protein by MAb 1.9 identified this site as the active domain of ecto-PKC. Covalent binding of the azido-ATP molecule to the 43-kDa surface protein inhibited the phosphorylative activity of the platelet ecto-PKC. Furthermore, PKC pseudosubstrate inhibitory peptides directly induced the aggregation of platelets and inhibited azido-ATP binding to the 43-kDa protein. Platelet aggregation induced by MAb 1.9 and by PKC inhibitory peptides required the presence of fibrinogen and resulted in an increase in the level of intracellular free calcium concentration. This increase in intracellular free calcium concentration induced by MAb 1.9 was found to be dependent on the binding of fibrinogen to activated GPIIb/IIIa integrins, suggesting that MAb 1.9 causes Ca(2+) flux through the fibrinogen receptor complex. We conclude that a decrease in the state of phosphorylation of platelet surface proteins caused by inhibition of ecto-PKC results in membrane rearrangements that can induce the activation of latent fibrinogen receptors, leading to platelet aggregation. Accordingly, the maintenance of a physiological steady state of phosphorylation of proteins on the platelet surface by ecto-PKC activity appears to be one of the homeostatic mechanisms that maintain fibrinogen receptors of circulating platelets in a latent state that cannot bind fibrinogen.

Long-term potentiation induction--a synaptic catch mechanism released by extracellular phosphorylation

The best understood form of long-term potentiation in hippocampal CA1 neurons is induced by activation of the N-methyl-D-aspartate receptor complex and subsequent activation of the intracellular second messenger systems. In addition to this intracellular mechanism, long-term potentiation can also be induced by an extracellular mechanism involving phosphorylation by ATP-ecto-protein kinase. In the present study, we hypothesize that a putative blocking molecule of the formation of long-term potentiation exists on the synaptic membrane, and examine whether ecto-protein kinases play a role in the block of long-term potentiation by phosphorylating the ecto-domains of this molecule in CA1 neurons of guinea-pig hippocampal slices. Long-term potentiation was induced by theta burst stimulation whether or not the test input was delivered to the CA1 neurons following burst stimulation. However, 5 microM K-252b, an ecto-protein kinase inhibitor, only blocked the induction of long-term potentiation when the test input was delivered during a 30-min period following burst stimulation. The results suggest that the process of formation of long-term potentiation continues independently of test synaptic input, while the block of long-term potentiation results from a combination of an interruption of the ATP-ecto-protein kinase-dependent processes and continued test synaptic input after burst stimulation. This block of long-term potentiation, which should be released by activation of ATP-ecto-protein kinase, is suggested to act as a "safety catch" for synaptic plasticity in hippocampal CA1 neurons.

N-Acetylcysteine delays age-associated memory impairment in mice: role in synaptic mitochondria

Mitochondrial oxidative damage is implicated in brain aging and in age-related neurodegenerative diseases. Since N-acetylcysteine (NAC) has recently been shown to prevent apoptotic death in neuronal cells and protect synaptic mitochondria proteins from oxidative damage in aged mice, we have investigated whether dietary administration of this thiolic antioxidant retards age-related memory loss. At 48 weeks of age, a control female OF-1 mice group was fed standard food pellets and another group received pellets containing 0.3% (w/w) of NAC. After 23 weeks of this diet, the NAC had partially restored the memory deficit associated with aging in mice. Moreover, the lipid peroxide and protein carbonyl contents of the synaptic mitochondria were significantly decreased in the NAC-supplemented animals in comparison with their age-matched controls. The antioxidant properties and probable action on mitochondrial bioenergetic ability in the synaptic terminals may explain, at least partially, the beneficial action of NAC administration.

Learning-specific decrease in synaptosomal ATP diphosphohydrolase activity from hippocampus and entorhinal cortex of adult rats

Considering the involvement of extracellular ATP in the memory formation, we analyzed the effect of inhibitory avoidance training on ectonucleotidase activities in synaptosomes from hippocampus, entorhinal cortex and parietal cortex. ATP diphosphohydrolase activity presented a decrease (33%) in hippocampal synaptosomes of rats sacrificed 180 min after training. Our results also showed a decrease in synaptosomal ATP diphosphohydrolase (30% and 42% for ATP and ADP, respectively) in entorhinal cortex immediately after training. These findings suggest an integrated action of ATP diphosphohydrolase from hippocampus and entorhinal cortex in the formation of inhibitory avoidance memory.

Ecto-protein kinases: ecto-domain phosphorylation as a novel target for pharmacological manipulation?

An increasing number of studies document the presence of protein kinases facing outwards at the cell surface of a diverse array of cells. These ecto-protein kinases phosphorylate cell-surface proteins and soluble extracellular substrates, and thus could affect many physiological processes involving cell-cell contacts, cellular differentiation and proliferation, ion fluxes and cellular activation. To date, only limited attention has been paid to exploring ecto-protein kinases as possible pharmacological targets. Here, the identification and physiological role of ecto-protein kinases in different biological systems is described; it is suggested that ecto-protein kinases are attractive and novel candidates for pharmacological manipulation under various (patho)physiological conditions.

Functional expression of a cDNA encoding a human ecto-ATPase

1 The metabolism of extracellular nucleotides plays an important role in nucleotide signalling mediated by P2 receptors. The nucleotide sequence encoding a putative human ecto-ATPase named CD39L1 was reported recently. However, the biological activity of this protein has not been established. 2 Based on the sequence of CD39L1 we isolated from mRNA from human ECV-304 cells a sequence encoding a 495 amino acid protein that is identical to CD39L1, with the exception that this sequence contains a 23 amino acid stretch in the putative extracellular loop that is missing in CD39L1. Partial sequence of a genomic DNA clone indicates that the CD39L1 gene corresponds to an alternative spliced form of the human ecto-ATPase. 3 Stable expression of isolated sequence in NIH-3T3 mouse fibroblasts conferred a marked nucleotide hydrolytic activity consistent with the activity of an ecto-ATPase. 4 The human ecto-ATPase hydrolyzed all naturally occurring nucleoside triphosphates in a Ca(2+)- or Mg(2+)-dependent manner. Nucleoside diphosphates were hydrolyzed at a rate approximately 5% of that of the corresponding triphosphates. The apparent Km and Vmax values were: 394+/-62 microM and 107+/-7 nmol Pi min-1 10(6) cells-1 for the hydrolysis of ATP, and 102+/-33 microM and 4+/-0.4 nmol Pi min-1 10(6) cells-1 for the hydrolysis of ADP, respectively. 5 In conclusion, we report here the cloning and functional expression of a human ecto-ATPase. The study of the biochemical properties and the regulatory mechanisms of ecto-ATPases of defined sequence will be valuable in the definition of their role in nucleotide signalling.

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.

Role of ecto-kinase in phorbol ester-enhanced growth hormone-binding protein release from human IM-9 cells

Previously we reported that a phorbol ester, phorbol 12, 13-dibutyrate (PDBu), increased the release of human growth hormone-binding protein (hGH-BP) in IM-9 cells, and that this phorbol ester-enhanced release was mediated by protein kinase Ca (PKCalpha). In the present study, the mechanisms of the phorbol ester-enhanced hGH-BP release were further investigated. Treatment of IM-9 cells with PDBu did not increase hGH-BPs (55-60 kDa) in the intracellular soluble fraction. When the cells were treated with trypsin to remove human growth hormone receptors (hGHRs) on the cell surface after stimulation, no hGH-BPs were detected in the culture supernatants, nor did treatment with bafilomycin A1 or chloroquine affect the PDBu-enhanced hGH-BP release. These results suggest that hGH-BPs released by PDBu stimulation are derived from cell surface hGHRs and not generated within the cells. Protein kinase inhibitors with broad specificities, K-252a and K-252b, inhibited the PDBu-enhanced release with almost the same dose-dependency, although only a trace amount of K-252b was incorporated into IM-9 cells than K-252a, suggesting that K-252b probably inhibits an ecto-kinase extracellularly. PDBu actually enhanced the phosphorylation of several extracellular proteins, and this enhanced phosphorylation was completely inhibited by K-252b treatment. Moreover, the PKCalpha-specific inhibitor bisindolylmaleimide III which inhibits PDBu enhanced hGH-BP release inhibited the PDBu-enhanced phosphorylation of extracellular proteins. On the other hand, the impermeable PKC inhibitor PKC inhibitor peptide 19-31 did not inhibit PDBu-enhanced release, suggesting that the target PKCalpha for PDBu is not present on the extracellular surface. Taken together, these results suggest that, in addition to intracellular PKCalpha, activation of an undefined ecto-kinase may also be involved in the PDBu-enhanced hGH-BP release.

Involvement of P2 purinoceptors and the nitric oxide pathway in [3H]purine outflow evoked by short-term hypoxia and hypoglycemia in rat hippocampal slices

The objective of this study was to study how the outflow of [3H]purines is altered during a brief period of ischemic-like conditions in superfused hippocampal slices and to show whether it is regulated by P2 purinoceptors and the nitric oxide (NO) pathway. The outflow of [3H]purines increased in response to 5 min of combined hypoxia/hypoglycemia. High performance liquid chromatography analysis verified the efflux of [3H]adenosine-triphosphate, [3H]adenosine-diphosphate, [3H]adenosine-monophosphate, [3H]adenosine, [3H]inosine, and [3H]hypoxanthine in response to ischemic-like conditions. The P2 receptor antagonists suramin and pyridoxal-phosphate-6-azophenyl-2'-4'-disulphonic-acid-tetrasodium (PPADS) reduced significantly the [3H]purine efflux evoked by ischemic-like conditions, showing that P2 purinoceptors are involved in the initiation of purine outflow. The NO synthase inhibitor N-nitro-l-arginine-methyl-ester (l-NAME) attenuated significantly the [3H]purine outflow, evoked by ischemic-like conditions, while 7-nitroindazole (7-NI) caused only a mild decrease in the outflow. The NO donor sodium nitroprusside increased significantly the basal efflux of [3H]purines. In summary, a brief period of combined hypoxia/hypoglycemia induced the efflux of ATP in addition to the outflow of other purines. Since P2 receptor antagonists decreased the [3H]purine outflow evoked by ischemic-like conditions we propose that ATP, acting on P2 purinoceptors, is responsible for further efflux of purines after ischemic-like period. It seems likely that NO is also involved in the regulation of purine outflow, since inhibition of NO production attenuated the [3H]purine outflow, evoked by ischemic-like conditions, while exogenous NO facilitated the basal outflow.

Extracellular ATP-induced apoptosis in PC12 cells

Studies in our laboratory indicate that extracellular ATP (ATP)o may induce cell death by reactive oxygen insults. We have also shown that the Ca(2+)-induced oxidative stress as elicited by ATP may lead to an activation of a specific AP-1 activity. Since early impairment of mitochondria constitutes a critical event of the apoptotic cell death, we have examined whether (ATP)o will affect mitochondrial damage and cell injury by using mitochondrial specific probes, dihydrorhodamine and 3-(4,5-dimethylthiazo-2-yl)-2,5-diphenyl tetrazolium bromide (MTT). We have found that (ATP)o induced cell death in a concentration dependent manner by MTT assay. The (ATP)o induced cell death correlated well with the reactive oxygen species (ROS) generation in mitochondria, since (ATP)o enhanced both cell death and ROS production and antioxidant blocked both of these processes. We found (ATP)o treatment led to apoptotic cell death by examining DNA laddering and the TUNEL assay. Interestingly, vitamin C and vitamin E combined treatment appeared to attenuate the (ATP)o-induced apoptosis. Results indicated that (ATP)o may cause oxidative damage of mitochondria leading to apoptotic cell death. Antioxidants may be useful in preventing apoptosis by preventing ROS formation in mitochondria.

An ecto-nucleotide pyrophosphatase is one of the main enzymes involved in the extracellular metabolism of ATP in rat C6 glioma

The presence of a nucleotide pyrophosphatase (EC 3.6.1.9) on the plasma membrane of rat C6 glioma has been demonstrated by analysis of the hydrolysis of ATP labeled in the base and in the alpha- and gamma-phosphates. The enzyme degraded ATP into AMP and PPi and, depending on the ATP concentration, accounted for approximately 50-75% of the extracellular degradation of ATP. The association of the enzyme with the plasma membrane was confirmed by ATP hydrolysis in the presence of a varying concentration of pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), a membrane-impermeable inhibitor of the enzyme. PPADS concentration above 20 microM abolished the degradation of ATP into AMP and PPi. The nucleotide pyrophosphatase has an alkaline pH optimum and a Km for ATP of 17 +/- 5 microM. The enzyme has a broad substrate specificity and hydrolyzes nucleoside triphosphates, nucleoside diphosphates, dinucleoside polyphosphates, and nucleoside monophosphate esters but is inhibited by nucleoside monophosphates, adenosine 3',5'-bisphosphate, and PPADS. The substrate specificity characterizes the enzyme as a nucleotide pyrophosphatase/phosphodiesterase I (PD-I). Immunoblotting and autoadenylylation identified the enzyme as a plasma cell differentiation antigen-related protein. Hydrolysis of ATP terminates the autophosphorylation of a nucleoside diphosphate kinase (NDPK/nm23) detected in the conditioned medium of C6 cultures. A function of the pyrophosphatase/PD-I and NDPK in the purinergic and pyrimidinergic signal transduction in C6 is discussed.

NADH oxidase activity of soybean plasma membranes inhibited by submicromolar concentrations of ATP

The activity of an auxin-stimulated NADH oxidase activity from soybean hypocotyls was inhibited by submicromolar concentrations of ATP. Auxins are plant growth regulators that increase the rate of cell enlargement in plant stems. A synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D), was used. The inhibition was half maximal at 1 nM ATP and was not observed with other nucleotides and nucleosides. The inhibition was the result of an increase in the Km for NADH from about 60 microM to > 100 microM and was noncompetitive. The decrease in Km due to ATP was enhanced by the addition of 1 microM 2,4-D. The Vmax of the plasma membrane NADH oxidase was approximately doubled (1.5-2.8-fold) by ATP and by 1 microM 2,4-D. No further increase in the Vmax was observed by the combination of 1 nM to 0.1 mM ATP in the presence of 1 microM 2,4-D. The results demonstrate a response of the NADH oxidase activity of isolated vesicles of soybean plasma membranes to ATP distinct from that observed previously with other nucleotide di- and triphosphates. The results are suggestive either of control of the cell surface NADH oxidase by phosphorylation or a direct response to ATP binding at nanomolar concentrations of ATP.

Inhibitory avoidance learning inhibits ectonucleotidases activities in hippocampal synaptosomes of adult rats

Several lines of evidence indicate that ATP may play an important role in Long-Term Potentiation. In this investigation we evaluated the effect of a memory task (step-down inhibitory avoidance) on the synaptosomal ecto-enzymes (ATP diphosphohydrolase and 5'-nucleotidase) involved in the degradation of ATP to adenosine. After the training session, a decrease in the ATPase (40%) and ADPase (29%) activities of ATP diphosphohydrolase as well as was a decrease in 5'-nucleotidase activity (31%) was observed in hippocampal synaptosomes of rats trained and killed immediately after training. In synaptosomes of rats killed 30 minutes after training, a decrease in ATPase activity (28%) was observed. In the test session, no significant changes were observed in the enzyme activities studied. These results provide new information about the activity of ecto-enzymes involved in nucleotide degradation and their possible participation in mechanisms of acquisition and modulation of memory processing.

The CD39-like gene family: identification of three new human members (CD39L2, CD39L3, and CD39L4), their murine homologues, and a member of the gene family from Drosophila melanogaster

The human lymphoid cell activation antigen CD39 is a known E-type apyrase that hydrolyzes extracellular ATP and ADP, a function important in homotypic adhesion, platelet aggregation, and removal by activated lymphocytes of the lytic effect of ATP. The recently identified putative rat homologue of CD39L1 has been shown to have E-type ecto-ATPase activity, by hydrolyzing extracellular ATP. We have characterized three novel CD39-like transcripts, CD39L2, CD39L3, and CD39L4, which share extensive amino acid homology with other nucleotide triphosphatases in vertebrates, invertebrates, and plants, suggesting that these genes also encode proteins with ecto-nucleotidase activity. Isolation and sequencing of full-length cDNA clones for each gene identified putative proteins of 485, 529, and 429 amino acids. The expression pattern of all five human members of the gene family was analyzed. CD39L2, CD39L3, and CD39L4 were mapped on the human genome, and the murine homologues identified with the putative map locations were assigned on the basis of regions of conserved gene order between human and mouse chromosomes. The map location of mcd39l4 places the gene within a region associated with audiogenic seizure susceptibility in mouse. This disorder is characterized by convulsions induced by loud high-frequency sound and has been shown to be associated with increased nucleotide triphosphatase activity.

Ecto-phosphodiesterase/pyrophosphatase of lymphocytes and non-lymphoid cells: structure and function of the PC-1 family

Many developmentally regulated membrane proteins of lymphocytes are ecto-enzymes, with their active sites on the external surface of the cell. These enzymes commonly have peptidase, phosphodiesterase or nucleotidase activity. Their biological roles are just beginning to be discovered. Although their expression is usually associated with particular stages of lymphoid differentiation, the same gene products are often expressed on the surface of certain non-lymphoid cell types outside the immune system, indicating that their functions cannot be unique to lymphocytes, nor can they be ubiquitous. The plasma cell membrane protein PC-1 (phosphodiesterase I; EC 3.1.4.1/nucleotide pyrophosphatase; EC 3.6.1.9), which was one of the first serological markers for lymphocyte subsets to be discovered, is a typical example. Within the immune system, PC-1 is confined to plasma cells, which represent about 0.1% of lymphocytes. However, PC-1 is also expressed on cells of the distal convoluted tubule of the kidney, chondrocytes, osteoblasts, epididymis and hepatocytes. Recent work has shown that PC-1 is a member of a multigene family of ecto-phosphodiesterases that currently has two other members, PD-1 alpha (autotaxin) and PD-1 beta (B10). Within this family, the extracellular domains are highly conserved, especially around the active site. In contrast, the transmembrane and cytoplasmic domains are highly divergent. Individual members of the eco-phosphodiesterase family have distinct patterns of distribution in different cell types, and even within the same cell. For example, PC-1 is present only on the basolateral surface of hepatocytes, while B10 (PD-1 beta) is confined to the apical surface. Analysis of conservation and differences in the sequence of their cytoplasmic tails may illuminate intracellular targetting signals. Ecto-phosphodiesterases may play a part in diverse activities in different tissues, including recycling of nucleotides. They may also regulate the concentration of pharmacologically active extracellular compounds such as adenosine or its derivatives and cell motility. Some members may modulate local concentrations of pyrophosphate, and hence influence calcification in bone and cartilage.

Postsynaptic inhibitors of calcium/calmodulin-dependent protein kinase type II block induction but not maintenance of pairing-induced long-term potentiation

The role of postsynaptic kinases in the induction and maintenance of long-term potentiation (LTP) was studied in the CA1 region of the rat hippocampal slice. A peptide inhibitor for the catalytic domain of calcium/calmodulin-dependent protein kinase type II (CaM-kinase) was applied through a perfused patch pipette. The inhibitor completely blocked both the short-term potentiation and LTP induced by a pairing protocol. This indicates that the kinase or kinases affected by the peptide are downstream from depolarization in the LTP cascade. The ability to block LTP required that measures be taken to interfere with degradation of the peptide kinase inhibitor by endogenous proteases; either addition of protease inhibitors or modifications of the peptide itself greatly enhanced the effectiveness of the peptide. Protease inhibitors by themselves or control peptide did not block LTP induction. To study the effect of kinase inhibitor on LTP maintenance, we induced LTP in one pathway. Subsequent introduction of the kinase inhibitor blocked the induction of LTP in a second pathway, but it did not affect maintenance of LTP in the first. The implications for the role of kinases in LTP maintenance are discussed.

Postsynaptic inhibitors of calcium/calmodulin-dependent protein kinase type II block induction but not maintenance of pairing-induced long-term potentiation

The role of postsynaptic kinases in the induction and maintenance of long-term potentiation (LTP) was studied in the CA1 region of the rat hippocampal slice. A peptide inhibitor for the catalytic domain of calcium/calmodulin-dependent protein kinase type II (CaM-kinase) was applied through a perfused patch pipette. The inhibitor completely blocked both the short-term potentiation and LTP induced by a pairing protocol. This indicates that the kinase or kinases affected by the peptide are downstream from depolarization in the LTP cascade. The ability to block LTP required that measures be taken to interfere with degradation of the peptide kinase inhibitor by endogenous proteases; either addition of protease inhibitors or modifications of the peptide itself greatly enhanced the effectiveness of the peptide. Protease inhibitors by themselves or control peptide did not block LTP induction. To study the effect of kinase inhibitor on LTP maintenance, we induced LTP in one pathway. Subsequent introduction of the kinase inhibitor blocked the induction of LTP in a second pathway, but it did not affect maintenance of LTP in the first. The implications for the role of kinases in LTP maintenance are discussed.