Histochemical demonstration of purine nucleoside phosphorylase (PNPase) in microglial and astroglial cells of adult rat brain

PRpnp, a novel dual activity PNP family protein improves plant vigour and confers multiple stress tolerance in Citrus aurantifolia

Under field conditions, plants are often simultaneously exposed to several abiotic and biotic stresses resulting in significant reductions in growth and yield; thus, developing a multi-stress tolerant variety is imperative. Previously, we reported the neofunctionalization of a novel PNP family protein, Putranjiva roxburghii purine nucleoside phosphorylase (PRpnp) to trypsin inhibitor to cater to the needs of plant defence. However, to date, no study has revealed the potential role and mechanism of either member of this protein group in plant defence. Here, we overexpressed PRpnp in Citrus aurantifolia which showed nuclear-cytoplasmic localization, where it functions in maintaining the intracellular purine reservoir. Overexpression of PRpnp significantly enhanced tolerance to salt, oxidative stress, alkaline pH, drought and two pests, Papilio demoleus and Scirtothrips citri in transgenic plants. Global gene expression studies revealed that PRpnp overexpression up-regulated differentially expressed genes (DEGs) related to ABA- and JA-biosynthesis and signalling, plant defence, growth and development. LC-MS/MS analysis validated higher endogenous ABA and JA accumulation in transgenic plants. Taken together, our results suggest that PRpnp functions by enhancing the endogenous ABA and JA, which interact synergistically and it also inhibits trypsin proteases in the insect gut. Also, like other purine salvage genes, PRpnp also regulates CK metabolism and increases the levels of CK-free bases in transgenic Mexican lime. We also suggest that PRpnp can be used as a potential candidate to develop new varieties with improved plant vigour and enhanced multiple stress resistance.

Release of soluble and vesicular purine nucleoside phosphorylase from rat astrocytes and microglia induced by pro-inflammatory stimulation with extracellular ATP via P2X7 receptors

Purine nucleoside phosphorylase (PNP), a crucial enzyme in purine metabolism which converts ribonucleosides into purine bases, has mainly been found inside glial cells. Since we recently demonstrated that PNP is released from rat C6 glioma cells, we then wondered whether this occurs in normal brain cells. Using rat primary cultures of microglia, astrocytes and cerebellar granule neurons, we found that in basal condition all these cells constitutively released a metabolically active PNP with Km values very similar to those measured in C6 glioma cells. However, the enzyme expression/release was greater in microglia or astrocytes that in neurons. Moreover, we exposed primary brain cell cultures to pro-inflammatory agents such as lipopolysaccharide (LPS) or ATP alone or in combination. LPS alone caused an increased interleukin-1β (IL-1β) secretion mainly from microglia and no modification in the PNP release, even from neurons in which it enhanced cell death. In contrast, ATP administered alone to glial cells at high micromolar concentrations significantly stimulated the release of PNP within 1 h, an effect not modified by LPS presence, whereas IL-1β secretion was stimulated by ATP only in cells primed for 2 h with LPS. In both cases ATP effect was mediated by P2X₇ receptor (P2X₇R), since it was mimicked by cell exposure to Bz-ATP, an agonist of P2X₇R, and blocked by cell pre-treatment with the P2X₇R antagonist A438079. Interestingly, ATP-induced PNP release from glial cells partly occurred through the secretion of lysosomal vesicles in the extracellular medium. Thus, during inflammatory cerebral events PNP secretion promoted by extracellular ATP accumulation might concur to control extracellular purine signals. Further studies could elucidate whether, in these conditions, a consensual activity of enzymes downstream of PNP in the purine metabolic cascade avoids accumulation of extracellular purine bases that might concur to brain injury by unusual formation of reactive oxygen species.

Evidence for purine nucleoside phosphorylase (PNP) release from rat C6 glioma cells

Intracellular purine turnover is mainly oriented to preserving the level of triphosphate nucleotides, fundamental molecules in vital cell functions that, when released outside cells, act as receptor signals. Conversely, high levels of purine bases and uric acid are found in the extracellular milieu, even in resting conditions. These compounds could derive from nucleosides/bases that, having escaped to cell reuptake, are metabolized by extracellular enzymes similar to the cytosolic ones. Focusing on purine nucleoside phosphorylase (PNP) that catalyzes the reversible phosphorolysis of purine (deoxy)-nucleosides/bases, we found that it is constitutively released from cultured rat C6 glioma cells into the medium, and has a molecular weight and enzyme activity similar to the cytosolic enzyme. Cell exposure to 10 μM ATP or guanosine triphosphate (GTP) increased the extracellular amount of all corresponding purines without modifying the levels/activity of released PNP, whereas selective activation of ATP P2Y₁ or adenosine A_2A metabotropic receptors increased PNP release and purine base formation. The reduction to 1% in oxygen supply (2 h) to cells decreased the levels of released PNP, leading to an increased presence of extracellular nucleosides and to a reduced formation of xanthine and uric acid. Conversely, 2 h cell re-oxygenation enhanced the extracellular amounts of both PNP and purine bases. Thus, hypoxia and re-oxygenation modulated in opposite manner the PNP release/activity and, thereby, the extracellular formation of purine metabolism end-products. In conclusion, extracellular PNP and likely other enzymes deputed to purine base metabolism are released from cells, contributing to the purinergic system homeostasis and exhibiting an important pathophysiological role.

Tomato lectin histochemistry for microglial visualization

The use of different lectins for the study of microglial cells in the central nervous system (CNS) is a valuable tool that has been extensively used in the last years for the selective staining of this glial cell population, not only in normal physiological conditions, but also in a wide range of pathological situations where the normal homeostasis of the parenchyma is disturbed. In this chapter we accurately describe the methodology for the selective labelling of microglial cells by using the tomato lectin (TL), a protein lectin obtained from Lycopersicum esculentum with specific affinity for poly-N-acetyl lactosamine sugar residues which are found on the plasma membrane and in the cytoplasm of microglia. Here we describe how to perform this technique on vibratome, frozen, and paraffin sections for optical microscopy, as well as for transmission electron microscopy (TEM) studies. Using this methodology it is possible to visualize amoeboid microglia in the developing brain, ramified microglia in the adult, and activated/reactive microglia in the experimentally damaged brain. In addition, as TL also recognized sugar residues in endothelial cells, this technique is very useful for the study of the relationship established between microglia and the CNS vasculature.

Adenosine produced by neurons is metabolized to hypoxanthine by astrocytes

Adenosine (ADO) is an important neuromodulator in brain. During pathophysiological events such as stroke or brain trauma, ADO levels can increase up to 100-fold. We tested the hypothesis that astrocytes are important for the removal of ADO produced by neurons and for the metabolism of ADO to inosine (INO) and hypoxanthine (HX). We used four different cell culture preparations: cortical neurons, cortical astrocytes, cocultures of neurons and astrocytes, and neurons transiently cocultured with astrocytes on transwell filters. These cultures were treated with N-methyl-D-aspartate (NMDA), because NMDA receptor activation is a common factor among many causes of neurotoxicity. NMDA significantly increased extracellular ADO, INO, and HX levels from cultured cortical neurons by 3-, 3.5-, and 2-fold, respectively. In cocultures, NMDA significantly increased INO, by 4.5-fold, and HX, by 3-fold, but did not increase ADO levels. There was no NMDA-evoked purine production from astrocytes. Inhibition of purine nucleoside phosphorylase (PNP) significantly decreased HX production from both neurons and cocultures to less than 30% of control levels. The transient addition of astrocytes to neurons during NMDA treatment significantly increased HX and decreased ADO levels compared with neurons alone. In addition, increasing the number of astrocytes was directly correlated with an increased capacity of ADO metabolism to INO and HX. In conclusion, NMDA evoked the production of ADO, INO, and HX from neurons. In the presence of astrocytes, there was significantly less ADO and more HX produced. Thus, ADO produced by neurons is subject to metabolism by astrocytes, a process that may limit its neuromodulatory actions.

Rat retinal microglial cells under normal conditions, after optic nerve section, and after optic nerve section and intravitreal injection of trophic factors or macrophage inhibitory factor

Retinal microglial cells may have a role in both degeneration and neuroprotection of retinal ganglion cells (RGC) after optic nerve (ON) section. We have used NDPase enzymohistochemistry to label adult rat retinal microglial cells and have studied these cells under normal conditions, after left ON section, and after left ON section and eye puncture or intravitreal injection of different substances: vehicle, brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin 3 (NT3), or macrophage inhibitory factor (MIF). Resident microglial cells are present in four layers in the adult rat retina: the nerve fiber layer (NFL), ganglion cell layer (GCL), inner plexiform layer (IPL), and outer plexiform layer (OPL). Left ON section induces microglial activation in the ipsilateral and contralateral retina as manifested by stronger staining intensity in both retinas and increased microglial cell densities in the NFL, IPL, and GCL of the ipsilateral retina. Left ON section followed by left eye puncture or intravitreal injection increases microglial cell density in both retinas and induces changes in the microglial cells of the ipsilateral retina that vary depending on the substance injected: BDNF injections delay microglial activation, possibly through retinal ganglion cell neuroprotection, whereas NT3 partially inhibits microglial activation in the NFL; MIF injections have no clear effects on microglial activation. In conclusion, retinal microglial cells become activated after an ON section and react more intensely when the eye is also punctured or injected, and this response may be altered by using neurotrophic factors, although the effects of MIF are less clear.

Gene expression for enzymes and transporters involved in regulating adenosine and inosine levels in rat forebrain neurons, astrocytes and C6 glioma cells

In brain, levels of adenosine increase up to 100-fold during cerebral ischemia. Based on in vitro studies, both astrocytes and neurons contribute to this adenosine release. Neurons release adenosine per se whereas astrocytes release adenine nucleotides that are metabolized to adenosine extracellularly. In contrast, inosine is released from both cell types via a nucleoside transporter. C6 glioma cells, which are derived from astrocytes, release inosine but not adenosine. The present study investigated the relative expression of purine metabolizing enzymes and transporters in neurons, astrocytes and C6 glioma cells by real-time PCR analysis. In agreement with the extracellular formation of adenosine and intracellular formation of inosine by astrocytes, the present study showed high expression of ecto 5'-nucleotidase and AMP deaminase type 3 in astrocytes. The lack of adenosine release from C6 glioma cells was consistent with the absence of expression of the AMP-preferring cytosolic 5'-nucleotidase in these cells. The predominance of nitrobenzylthioinosine (NBMPR) insensitive equilibrative nucleoside transport (ENT2) in all three cell types was consistent with the greater activity of this isoform in comparison to NBMPR-sensitive ENT1 in these rat cells. Thus, cell type differences in adenosine formation and release are primarily a function of differences in expression of purinergic enzymes and transporters.

Stimulus- and cell-type-specific release of purines in cultured rat forebrain astrocytes and neurons

Adenosine is formed during conditions that deplete ATP, such as ischemia. Adenosine deaminase converts adenosine into inosine, and both adenosine and inosine can be beneficial for postischemic recovery. This study investigated adenosine and inosine release from astrocytes and neurons during chemical hypoxia or oxygen-glucose deprivation. In both cell types, 2-deoxyglucose was the most effective stimulus for depleting cellular ATP and for evoking inosine release; in contrast, oxygen-glucose deprivation evoked the greatest adenosine release. alpha,beta-Methylene ADP, an inhibitor of ecto-5'nucleotidase, significantly reduced adenosine release from astrocytes but not neurons. Dipyridamole, an inhibitor of equilibrative nucleoside transporters, inhibited both adenosine and inosine release from neurons. Erythro-9-(2-hydroxy-3-nonyl)adenine, an inhibitor of adenosine deaminase, reduced neuronal inosine release evoked by oxygen-glucose deprivation but not by 2-deoxyglucose treatment. These data indicate that (1). astrocytes release adenine nucleotides that are hydrolyzed extracellularly to adenosine, whereas neurons release adenosine per se, (2). inosine is formed intracellularly and released via nucleoside transporters, and (3). inosine is formed by an adenosine deaminase-dependent pathway during oxygen-glucose deprivation but not during 2-deoxyglucose treatment. In summary, the metabolic pathways for adenosine formation and release were cell-type dependent whereas the pathways for inosine formation were stimulus dependent.

Differences between rat primary cortical neurons and astrocytes in purine release evoked by ischemic conditions

In the brain, the levels of adenosine increase up to 100-fold during cerebral ischernia; however, the roles of specific cell types, enzymatic pathways and membrane transport processes in regulating intra- and extracellular concentrations of adenosine are poorly characterized. Rat primary cortical neurons and astrocytes were incubated with [(3)H]adenine for 30 min to radiolabel intracellular ATP. Cells were then treated with buffer, glucose deprivation (GD), oxygen-glucose deprivation (OGD), 100 micro M sodium cyanide (NaCN) or 500 micro M iodoacetate (IAA) for 1 h to stimulate the metabolism of ATP and cellular release of [(3)H]purines. The nucleoside transport inhibitor dipyridamole (DPR) (10 micro M), the adenosine kinase inhibitor iodotubercidin (ITU) (1 micro M), the adenosine deaminase inhibitor EHNA (1 micro M) and the purine nucleoside phosphorylase inhibitor BCX-34 (10 micro M) were tested to investigate the contribution of specific enzymes and transporters in the metabolism and release of purines from each cell type. Our results indicate that (a). under basal conditions astrocytes released significantly more [(3)H]adenine nucleotides and [(3)H]adenosine than neurons, (b). OGD, NaCN and IAA conditions produced significant increases in [(3)H]adenosine release from neurons but not astrocytes, and (c) DPR blocked [(3)H]inosine release from both astrocytes and neurons but only blocked [(3)H]adenosine release from neurons. These data suggest that, in these experimental conditions, adenosine was formed by an intracellular pathway in neurons and then released via a nucleoside transporter. In contrast, adenine nucleotide release and extracellular metabolism to adenosine appeared to predominate in astrocytes.

Microglial cells in the retina of Carassius auratus: effects of optic nerve crush

To study the morphology and distribution of the retinal microglial cells of the goldfish retina in normal conditions and after optic nerve crush, we have used the nucleoside diphosphatase (NDPase) technique, applied to whole-mounts or sections, for light and electron microscopy. In normal retinas, two populations of NDPase-positive cells were identified: compact cells associated with the retinal vessels on the vitreal surface of the retina and microglial cells in various retinal layers. The microglial cells had a bipolar or multipolar morphology. Bipolar cells were observed in the nerve fibre layer, and multipolar cells were visualised in the ganglion cell layer (GCL), inner plexiform layer (IPL), and outer plexiform layer. The highest densities of multipolar cells were observed in the IPL layer, where they adopted a regular mosaic-like arrangement in which the occasional spaces were occupied by cells of the GCL. After optic nerve crush, we observed an increase in the number of compact cells associated with the vessels and changes in NDPase activity, morphology, and distribution of the retinal microglial cells. These cells showed an increase in NDPase activity in all retinal layers from day 1 to day 15 after axotomy, and retraction of their processes from day 1 to day 7. In addition, the densities of microglial cells increased in the GCL between 2 and 15 days after axotomy, and decreased in the IPL by day 4 after axotomy. These microglial changes resemble those observed in other regenerating and nonregenerating neuronal systems and may reflect a general response of microglia directed to help the regeneration process.

Inosine and guanosine preserve neuronal and glial cell viability in mouse spinal cord cultures during chemical hypoxia

Murine spinal cord primary mixed cultures were treated with the respiratory inhibitor, rotenone, to mimic hypoxic conditions. Under these conditions neurons rapidly underwent oncosis (necrosis) with a complete loss in viability occurring within 260 min; however, astrocytes, which accounted for most of the cell population, died more slowly with 50% viability occurring at 565 min. Inosine preserved both total cell and neuronal viability in a concentration-dependent manner. The time of inosine addition relative to hypoxic insult was critical with the most effective protection occurring when inosine was added just prior to or within 5 min after insult. Inosine was ineffective when added 30 min after hypoxic insult. The effect of guanosine was similar to that of inosine. Treatment of cultures with BCX-34, a purine nucleoside phosphorylase inhibitor, prevented protection by inosine or guanosine, suggesting involvement of a purine nucleoside phosphorylase in the nucleoside protective effect.

Expression of purine metabolism-related enzymes by microglial cells in the developing rat brain

The nucleoside triphosphatase (NTPase), nucleoside diphosphatase (NDPase), 5'-nucleotidase (5'-Nase), and purine nucleoside phosphorylase (PNPase) activity has been examined in the cerebral cortex, subcortical white matter, and hippocampus from embryonic day (E)16 to postnatal day (P)18. Microglia display all four purine-related enzymatic activities, but the expression of these enzymatic activities differed depending on the distinct microglial typologies observed during brain development. We have identified three main morphologic typologies during the process of microglial differentiation: ameboid microglia (parenchymatic precursors), primitive ramified microglia (intermediate forms), and resting microglia (differentiated cells). Ameboid microglia, which were encountered from E16 to P12, displayed the four enzymatic activities. However, some ameboid microglial cells lacked 5'-Nase activity in gray matter, and some were PNPase-negative in both gray and white matter. Primitive ramified microglia were already observed in the embryonic period but mostly distributed during the first 2 postnatal weeks. These cells expressed NTPase, NDPase, 5'-Nase, and PNPase. Similar to ameboid microglia, we found primitive ramified microglia lacking the 5'-Nase and PNPase activities. Resting microglia, which were mostly distinguishable from the third postnatal week, expressed NTPase and NDPase, but they lacked or displayed very low levels of 5'-Nase activity, and only a subpopulation of resting microglia was PNPase-positive. Apart from cells of the microglial lineage, GFAP-positive astrocytes and radial glia cells were also labeled by the PNPase histochemistry. As shown by our results, the differentiation process from cell precursors into mature microglia is accompanied by changes in the expression of purine-related enzymes. We suggest that the enzymatic profile and levels of the different purine-related enzymes may depend not only on the differentiation stage but also on the nature of the cells. The use of purine-related histoenzymatic techniques as a microglial markers and the possible involvement of microglia in the control of extracellular purine levels during development are also discussed.

Genetic influences on cellular reactions to brain injury: activation of microglia in denervated neuropil in mice carrying a mutation (Wld(S)) that causes delayed Wallerian degeneration

This study examines the relationship between the appearance of degenerative changes in synaptic terminals and axons and the activation of microglia in denervated neuropil regions of normal mice of the C57BL/6 strain and mutant mice (Wld(S)), in which Wallerian degeneration is substantially delayed. The time course of degenerative changes in synaptic terminals and axons was assessed using selective silver staining. Microglial cells were identified by immunostaining for Mac-1, a monoclonal antibody to the CR3 complement receptor, and by histochemical staining for nucleoside diphosphatase (NDPase). Increased argyrophilia, indicative of degenerative changes, was evident as early as 1 day postlesion in normal mice, but was not seen until 6-8 days in mice with the Wld(S) mutation. Microglial activation in normal C57BL/6 mice was evident by 24 hours postlesion, as evidenced by increased immunostaining for Mac-1, increased histochemical staining for NDPase, and morphological changes indicative of an activated phenotype (short, thick processes). Quantitative evaluation of immunostaining for Mac-1 revealed that peak activation occurred between 2 and 6 days postlesion with a return to a quiescent phenotype by 12 days. In contrast, the microglial response was significantly delayed and prolonged in mice bearing the Wld(S) mutation. Activated microglia were not seen within the deafferented area until 6 to 8 days postlesion and peak activation occurred between 12 and 20 days postlesion. These data suggest that the response of microglia in denervated neuropil zones is triggered by the same types of degenerative changes that cause increased argyrophilia as detected by selective silver staining methods.

Morphology and distribution of microglial cells in the young and adult mouse cerebellum

The morphology and distribution of microglial cells were studied in the normal cerebellum of young and adult mice using the histochemical demonstration of nucleoside diphosphatase as a specific microglial marker. Our results showed that microglial cells were present in all cerebellular lobules of both young and adult mice, but their distribution and morphology were not homogeneous throughout the cerebellum. Heterogeneity in microglial cell distribution was exclusively related to their location in the different histological layers, and no significant differences were found either between the different cerebellar lobules or between young and adult mice. Microglial density was higher in the cerebellar nuclei than in the cortex; within the cortex, the molecular layer was less densely populated by microglial cells than the granular layer and the white matter. The morphological study revealed that microglial cells were ramified in all cerebellar lobules of both young and adult mice but showed different sizes and ramification patterns as a function of their specific location in the different histological layers. Several typologies of microglial cells were described on the basis of observations in both horizontal and coronal sections. The specific layer-related pattern of microglial distribution and morphology in mouse cerebellum strongly suggests a physical and functional adaptation of these cells to the characteristics of their microenvironment.

Stroke in purine nucleoside phosphorylase deficiency

The first documented case of cerebrovascular disease occurring in a 13-year-old girl with purine nucleoside phosphorylase deficiency is reported. This patient, the oldest known survivor with purine nucleoside phosphorylase deficiency, had previously experienced multiple sequential neurologic problems. She presented with episodes of transient left hemiparesis, followed shortly thereafter by dense left hemiplegia. Magnetic resonance imaging revealed a right internal capsule infarct; cerebral angiography revealed vasculopathy of the proximal vessels. Proposed mechanisms for neurologic dysfunction and cerebrovascular disease in purine nucleoside phosphorylase deficiency are discussed.

An enhancer in the first intron of the human purine nucleoside phosphorylase-encoding gene

In previous studies characterizing intron-dependent expression of the human purine nucleoside phosphorylase-encoding gene (PNP), we identified a putative enhancer sequence in the first intron which was capable of mediating increased cat reporter gene expression in transfected murine NIH 3T3 cells in a position- and orientation-independent manner. In order to further characterize this enhancer activity, the nucleotide sequence was determined for the region of intron 1 to which this activity was originally ascribed. The sequence was analyzed for the presence of binding sites for known transcription factors, but none were identified. A 444-bp downstream portion of the intron-1 sequence enhanced cat expression either in conjunction with a human PNP promoter sequence or with a 105-bp heterologous herpes simplex virus thymidine kinase (TK) promoter. Nested deletions of the downstream intron-1 sequence fused to a TK::cat fusion gene localized the enhancer activity to a 170-bp sequence in intron 1. A 154-bp HgiAI fragment (bp 424 to 577 of intron 1) excised from this region contained enhancer activity which varied directly with the number of fragments inserted upstream from the TK::cat fusion gene. However, inversion of the HgiAI fragment in a PNP abbreviated gene, or relocation of the HgiAI fragment from intron 1 to a position upstream from the PNP promoter, reduced or eliminated PNP expression. The effect of the intron-1 enhancer element on PNP expression is thus maximized in a position- and orientation-dependent manner.

Identification and distribution of microglial cells in the cerebral cortex of the lizard: a histochemical study

The histochemical demonstration of nucleoside diphosphatase as a specific microglial marker was used to study the distribution of this glial cell type in the cerebral cortex of Podarcis muralis and Podarcis hispanica. Our results showed that in both species, NDPase staining was specific for the microglial cell population and that microglial cells displayed a specific localization pattern in the different cortical areas. In the medal cortex, microglial cells were principally found in the outer and inner plexiform layers in the strata adjacent to the granular layer. Moreover, some microglial cells were found near the ependymal layer, but no microglial cells were normally present near the brain surface and never in the deep inner plexiform layer. In the dorsomedial cortex, microglial cells were found near the brain surface in the outer plexiform layer, in the upper part of the granular layer, and near the ependymal layer. No microglial cells were found, however, in the outer and inner plexiform layers adjacent to the granular layer. Finally, in the dorsolateral cortex, microglial cells were located in the upper part of the outer plexiform layer, in and bordering the granular layer, and scattered in the inner plexiform layer. This layered-pattern distribution of microglial cell population in the cerebral cortex of the lizard differs from the apparently homogeneous distribution of microglia in the brain of mammals.