Immunocytochemical localization of ecto-5'-nucleotidase in cultures of cerebellar granule cells

Of adenosine and the blues: The adenosinergic system in the pathophysiology and treatment of major depressive disorder

Major depressive disorder (MDD) is the foremost cause of global disability, being responsible for enormous personal, societal, and economical costs. Importantly, existing pharmacological treatments for MDD are partially or totally ineffective in a large segment of patients. As such, the search for novel antidepressant drug targets, anchored on a clear understanding of the etiological and pathophysiological mechanisms underpinning MDD, becomes of the utmost importance. The adenosinergic system, a highly conserved neuromodulatory system, appears as a promising novel target, given both its regulatory actions over many MDD-affected systems and processes. With this goal in mind, we herein review the evidence concerning the role of adenosine as a potential player in pathophysiology and treatment of MDD, combining data from both human and animal studies. Altogether, evidence supports the assertions that the adenosinergic system is altered in both MDD patients and animal models, and that drugs targeting this system have considerable potential as putative antidepressants. Furthermore, evidence also suggests that modifications in adenosine signaling may have a key role in the effects of several pharmacological and non-pharmacological antidepressant treatments with demonstrated efficacy, such as electroconvulsive shock, sleep deprivation, and deep brain stimulation. Lastly, it becomes clear from the available literature that there is yet much to study regarding the role of the adenosinergic system in the pathophysiology and treatment of MDD, and we suggest several avenues of research that are likely to prove fruitful.

Neonatal hypothyroidism affects the adenine nucleotides metabolism in astrocyte cultures from rat brain

Neonatal hypothyroidism is associated with multiple and severe brain alterations. We recently demonstrated a significant increase in hydrolysis of AMP to adenosine in brain of hypothyroid rats at different ages. However, the origin of this effect was unclear. Considering the effects of adenine nucleotides to brain functions and the harmful effects of neonatal hypothyroidism to normal development of the central nervous system, in this study we investigated the metabolism of adenine nucleotides in hippocampal, cortical and cerebellar astrocyte cultures from rats submitted to neonatal hypothyroidism. ATP and AMP hydrolysis were enhanced by 52 and 210%, respectively, in cerebellar astrocytes from hypothyroid rats. In hippocampus of hypothyroid rats, the 47% increase in AMP hydrolysis was significantly reverted when the astrocytes were treated with T3. Therefore, the imbalance in the ATP and adenosine levels in astrocytes, during brain development, may contribute to some of the effects described in neonatal hypothyroidism.

Extracellular adenine nucleotides metabolism in astrocyte cultures from different brain regions

Primary astrocyte cultures from hippocampus, cortex and cerebellum presented different extracellular pattern of adenine nucleotide hydrolysis. The ATP/ADP hydrolysis ratio was 8:1 for hippocampal and cortical astrocytes and 5:1 for cerebellar astrocytes. The AMP hydrolysis in cerebellar astrocytes was seven-fold higher than in cortical or hippocampal cells. No accumulation of extracellular adenosine in all structures studied was observed. Dipyridamol increased significantly inosine levels in the extracellular medium of hippocampal and cortical, but not in cerebellar astrocytes medium. A higher expression of ecto-5'-nucleotidase was identified by RT-PCR in cerebellum. The differences observed may indicate functional heterogeneity of nucleotides in the brain.

ATP diphosphohydrolase (NTPDase 1) in rat hippocampal slices and effect of glutamate on the enzyme activity in different phases of development

In the present report we describe an NTPDase 1 (ATP diphosphohydrolase; ecto-apyrase; EC 3.6.1.5) in rat hippocampal slices. The effect of glutamate on the ATPase and ADPase activities in rat hippocampal slices of different ages was also studied since adenosine, the final product of an enzymatic chain that includes NTPDase 1 and 5'-nucleotidase, can act upon A1 receptors in turn decreasing the release of glutamate. Hippocampal slices from 7, 14, 20-23 and 60 day-old rats were prepared and ATPase and ADPase activities were measured. The parallelism of ATPase and ADPase activities in all parameters tested indicated the presence of an ATP diphosphohydrolase. In addition, a Chevillard plot indicated that ATP and ADP are hydrolyzed at the same active site on the enzyme. ATPase activity was significantly activated by glutamate in 20-23 and 60 day-old rats, but ADPase activity was not activated. These results could indicate distinct behavior of the ATPase and ADPase activities of NTPDase 1 in relation to glutamate or the simultaneous action of the ecto-ATPase. Activation of ATPase activity by glutamate may constitute an important role in this developmental period, possibly protecting against the neurotoxicity induced by ATP, as well as producing high levels of ADP, by increasing adenosine production, a neuroprotective compound.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

P2 receptor modulation and cytotoxic function in cultured CNS neurons

In this study we investigate the presence, modulation and biological function of P2 receptors and extracellular ATP in cultured cerebellar granule neurons. As we demonstrate by RT-PCR and western blotting, both P2X and P2Y receptor subtypes are expressed and furthermore regulated as a function of neuronal maturation. In early primary cultures, mRNA for most of the P2 receptor subtypes, except P2X(6), are found, while in older cultures only P2X(3), P2Y(1) and P2Y(6) mRNA persist. In contrast, P2 receptor proteins are more prominent in mature neurons, with the exception of P2Y(1). We also report that extracellular ATP acts as a cell death mediator for fully differentiated and mature granule neurons, for dissociated striatal primary cells and hippocampal organotypic cultures, inducing both apoptotic and necrotic features of degeneration. ATP causes cell death with EC(50) in the 20-50 microM range within few minutes of exposure and with a time lapse of at most two hours. Additional agonists for P2 receptors induce toxic effects, whereas selected antagonists are protective. Cellular swelling, lactic dehydrogenase release and nuclei fragmentation are among the features of ATP-evoked cell death, which also include direct P2 receptor modulation. Comparably to P2 receptor antagonists previously shown preventing glutamate-toxicity, here we report that competitive and non-competitive NMDA receptor antagonists inhibit the detrimental consequences of extracellular ATP. Due to the massive extracellular release of purine nucleotides and nucleosides often occurring during a toxic insult, our data indicate that extracellular ATP can now be included among the potential causes of CNS neurodegenerative events.

Adenosine in the central nervous system: release mechanisms and extracellular concentrations

Adenosine has several functions within the CNS that involve an inhibitory tone of neurotransmission and neuroprotective actions in pathological conditions. The understanding of adenosine production and release in the brain is therefore of fundamental importance and has been extensively studied. Conflicting results are often obtained regarding the cellular source of adenosine, the stimulus that induces release and the mechanism for release, in relation to different experimental approaches used to study adenosine production and release. A neuronal origin of adenosine has been demonstrated through electrophysiological approaches showing that neurones can release significant quantities of adenosine, sufficient to activate adenosine receptors and to modulate synaptic functions. Specific actions of adenosine are mediated by different receptor subtypes (A(1), A(2A), A(2B) and A(3)), which are activated by various ranges of adenosine concentrations. Another important issue is the measurement of adenosine concentrations in the extracellular fluid under different conditions in order to know the degree of receptor stimulation and understand adenosine central actions. For this purpose, several experimental approaches have been used both in vivo and in vitro, which provide an estimation of basal adenosine levels in the range of 50-200 nM. The purpose of this review is to describe pathways of adenosine production and metabolism, and to summarize characteristics of adenosine release in the brain in response to different stimuli. Finally, studies performed to evaluate adenosine concentrations under physiological and hypoxic/ischemic conditions will be described to evaluate the degree of adenosine receptor activation.

Enhanced antisense efficacy of oligonucleotides adsorbed to monomethylaminoethylmethacrylate methylmethacrylate copolymer nanoparticles

The purpose of this study was the investigation of cationic nanoparticles as drug delivery systems for antisense oligonucleotides. Cationic monomethylaminoethylmethacrylate (MMAEMA) copolymer nanoparticles were prepared from N-monomethylaminoethylmethacrylate hydrochloride and methylmethacrylate. Oligonucleotides were adsorbed onto MMAEMA nanoparticles. Cell penetration was investigated in vitro with fluorescently labeled oligonucleotides and nanoparticles. Antisense effects of oligonucleotides adsorbed to MMAEMA nanoparticles were evaluated by sequence specific inhibition of ecto-5'-nucleotidase expression. The amount of enzyme expressed in PC12 cells was detected and quantified by immunocytochemistry using fluorescein isothiocyanate-labeled antibodies. Oligonucleotides were adsorbed to MMAEMA nanoparticles by the formation of ion-pairs between the positively charged secondary amino groups located on the particle surface and the anionic phosphodiester or phosphorothioate backbones of the oligonucleotides. Adsorption to nanoparticles led to an increased cellular uptake of oligonucleotides and to a significantly enhanced antisense efficacy of unmodified phosphodiester oligonucleotides as well as phosphorothioates. The results of the cell penetration and the antisense assay demonstrated that MMAEMA nanoparticles are promising carriers for oligonucleotide administration.

The modulation of ecto-nucleotidase activities by glutamate in cultured cerebellar granule cells

Several enzymes hydrolyze ATP, producing ADP which is hydrolyzed to AMP. Ecto-5'-nucleotidase produces adenosine from AMP. Glutamate (Glu) is an excitatory neurotransmitter and increases extracellular adenosine levels, which is considered an important inhibitory neuromodulator. Here we show that Glu activates ADP and AMP hydrolysis. NMDA and kainic acid (KA) also increased these enzymatic activities, but 1-aminocyclopentane-1S,3R-dicarboxylic acid (ACPD) had no effect. Dihydrokainate (DHK), an inhibitor of glutamate uptake, also blocked glutamate-evoked activation of ecto-nucleotidases, suggesting that this activation was also Glu transporters dependent. Therefore, we suggest that the Glu-evoked stimulation of ecto-nucleotidases might contribute to the increase of adenosine in extracellular space induced by Glu.

Endocytic vacuoles formed following a short pulse of K+ -stimulation contain a plethora of presynaptic membrane proteins

It is now well established that the membrane of synaptic vesicles is recycled following exocytosis. However, little is known concerning the identity of the primary or secondary endocytic structures and their molecular composition. Using cultured rat cerebellar granule cells we combined uptake of horseradish peroxidase as a fluid phase marker and immunogold labeling for a variety of presynaptic proteins to assess the molecular identity of the stimulation-induced endocytic compartments. Short periods (5 or 30 s) of stimulation with 50 mM KCl were followed by periods of recovery for up to 30 min. Stimulation resulted in the formation of horseradish-peroxidase-filled vacuoles in the axonal varicosities as the apparent primary endocytic compartment. Horseradish peroxidase-filled synaptic vesicles were formed when stimulated cells were allowed to recover in horseradish peroxidase-free culture medium. Horseradish peroxidase-filled vacuoles as wells as vesicles contained the synaptic vesicle membrane proteins VAMP II, synaptotagmin, SV2, and synaptophysin, the vesicle-associated proteins rab 3A and synapsin I, and in addition SNAP-25. No incorporation of vesicle proteins into the plasma membrane was observed. Horseradish peroxidase-filled vesicles and vacuoles generated on incubation of unstimulated granule cells with horseradish peroxidase for prolonged periods of time were equally immunolabeled. Renewed stimulation of prestimulated granule cells with either 100 mM KCl or 30 microM Ca2+ ionophore A23187 resulted in a reduction of horseradish peroxidase-filled vacuoles suggesting that the vacuolar membrane compartment was exocytosis-competent. Our results suggest that varicosities of cultured cerebellar granule cells possess a fast stimulation-induced pathway for recycling the entire synaptic vesicle membrane compartment. The primary endocytic compartment represents not a synaptic vesicle but a somewhat larger vesicle protein-containing vacuolar entity from which smaller vesicles of identical protein composition may be regenerated. Endocytic vacuoles and synaptic vesicles share membrane and membrane-associated proteins and presumably also major functional properties.

Immature chemodifferentiation of Purkinje cell synapses revealed by 5'-nucleotidase ecto-enzyme activity in the cerebellum of the reeler mouse

During postnatal development of the rodent cerebellum, a transient enzyme activity of ecto-5'-nucleotidase has been shown in the asymmetrical synapses of Purkinje cells. The alterations of the afferent circuitry and microenvironment of the ectopic Purkinje cells present in the cerebellum of the reeler mutant mouse could enlighten parameters that influence the synaptic 5'-nucleotidase activity of these cells. Ecto-enzyme cytochemistry reveals intense 5'-nucleotidase activity in 43% of synapses of the Purkinje cells throughout the cortex and the core of the reeler cerebellar vermis, although the molecular layer displays large areas with less than 1% of labelled synapses. However, enzymatic labelling is found in considerably more Purkinje cells synapses (73%) throughout the granular layer and the subcortical mass. Climbing fiber synapses of monoinnervated Purkinje cells are labelled by 5'-nucleotidase activity in the molecular layer, as well as asymmetrical synapses made on the subjacent ectopic Purkinje cells by the multiple climbing fibers and by the heterologous afferences. The non-innervated dendritic spines of these cells are also labelled, suggesting that 5'-nucleotidase activity at postsynaptic sites of reeler Purkinje cells does not depend on the presynaptic innervation. Rather, 5'-nucleotidase enzyme activity is enhanced at theses sites when the Purkinje cells have not achieved chemodifferentiation but have conserved immature wiring, i.e., low parallel fiber and multiple climbing fiber inputs.

New insights into molecular structure and function of ectonucleotidases in the nervous system

Nucleotides can be released as signaling substances in the nervous system from both neural and glial cells. Their function is terminated by ecto-nucleotidases and sequential extracellular metabolism to the nucleoside. Recently considerable progress has been made in unraveling the molecular structure of an ecto-ATPase and an ecto-ATP diphosphohydrolase, two closely related ectoenzymes. Molecular structure, tissue distribution and functional properties of the ecto-nucleotidases are discussed with particular emphasis on the nervous system.

Focal cerebral ischemia enhances glial expression of ecto-5'-nucleotidase

The effect of ischemia on the reactive expression of ecto-5'-nucleotidase in rat brain was studied 6 h and 1, 2 and 7 days after permanent middle cerebral artery occlusion (MCAO). The distribution of 5'-nucleotidase in the infarcted brain was compared to markers for astrocytes (glial fibrillary acidic protein (GFAP)) and microglia (complement receptor type 3, antibody OX42) using histological staining or immunohistochemistry. 5'-Nucleotidase could be associated with reactive astrocytes by immunohistochemistry and with reactive microglia by enzyme histochemistry. In the untreated control 5'-nucleotidase was associated with astrocytes only in the hippocampus and the submeningeal space. After ischemia the enzyme was expressed on reactive astrocytes in the tissue surrounding the volume of infarction. Individual reactive astrocytes were observed 6 h after MCAO and the astrocytic expression became continuously enhanced during the following days. An enzyme histochemical analysis of 5'-nucleotidase activity revealed a postischemic increase in reaction product around the infarcted tissue. Seven days after MCAO a discrete band (0.2-0.4 mm) of reaction product characterized the rim of the infarcted area. This band of activity of 5'-nucleotidase colocalized with a band of immunoreactivity for OX42, indicative of an intense accumulation of 5'-nucleotidase expressing microglia. Our results suggest that ischemia following permanent MCAO results in an upregulation of the capacity for the hydrolysis of nucleotides within the tissue adjacent to the infarcted volume. Nucleotides released from the damaged cells can be hydrolyzed and the adenosine eventually formed may exert neuroprotective functions limiting the extent of damage.

Extracellular metabolism of nucleotides in the nervous system

1. A variety of surface-located enzymes are involved in the metabolism of extracellular nucleotides. The biochemical properties of some of these are briefly discussed. 2. The molecular identity of ecto-diadenosine polyphosphate hydrolase has not yet been revealed. On neural cells the enzyme catalyses the hydrolysis of ApnA to Apn-1 and AMP. 3. The molecular structure of ATP-diphosphohydrolase has recently been identified. The enzyme occurs in essentially all tissues where it catalyses the extracellular hydrolysis of ATP and ADP with the formation of AMP. 4. Ecto-5'-nucleotidase is a GPI-anchored glycoprotein and catalyses the formation of AMP to adenosine. In the adult brain, and as revealed by immunocytochemistry, the enzyme is mainly associated with astrocytes. It is associated with developing nerve cells and cultured neural cells. In vitro its inhibition or suppression of its synthesis result in the inhibition of neurite formation and long-time survival of neural cells. Continued extracellular hydrolysis of AMP and formation of adenosine thus appear to be essential for neural differentiation and survival.

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.