5'-Nucleotidase immunoreactivity of perineuronal microglia responding to rat facial nerve axotomy

CD73-derived adenosine controls inflammation and neurodegeneration by modulating dopamine signalling

Ectonucleotidase-mediated ATP catabolism provides a powerful mechanism to control the levels of extracellular adenosine. While increased adenosine A2A receptor (A2AR) signaling has been well-documented in both Parkinson's disease models and patients, the source of this enhanced adenosine signalling remains unclear. Here, we show that the ecto-5'-nucleotidase (CD73)-mediated adenosine formation provides an important input to activate A2AR, and upregulated CD73 and A2AR in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinson's disease models coordinatively contribute to the elevated adenosine signalling. Importantly, we demonstrate that CD73-derived adenosine-A2AR signalling modulates microglial immunoresponses and morphological dynamics. CD73 inactivation significantly attenuated lipopolysaccharide-induced pro-inflammatory responses in microglia, but enhanced microglia process extension, movement and morphological transformation in the laser injury and acute MPTP-induced Parkinson's disease models. Limiting CD73-derived adenosine substantially suppressed microglia-mediated neuroinflammation and improved the viability of dopaminergic neurons and motor behaviours in Parkinson's disease models. Moreover, CD73 inactivation suppressed A2AR induction and A2AR-mediated pro-inflammatory responses, whereas replenishment of adenosine analogues restored these effects, suggesting that CD73 produces a self-regulating feed-forward adenosine formation to activate A2AR and promote neuroinflammation. We further provide the first evidence that A2A enhanced inflammation by antagonizing dopamine-mediated anti-inflammation, suggesting that the homeostatic balance between adenosine and dopamine signalling is key to microglia immunoresponses. Our study thus reveals a novel role for CD73-mediated nucleotide metabolism in regulating neuroinflammation and provides the proof-of-principle that targeting nucleotide metabolic pathways to limit adenosine production and neuroinflammation in Parkinson's disease might be a promising therapeutic strategy.

A decade of diverse microglial-neuronal physical interactions in the brain (2008-2018)

Microglia are unique cells of the central nervous system (CNS) with a distinct ontogeny and molecular profile. They are the predominant immune resident cell in the CNS. Recent studies have revealed a diversity of transient and terminal physical interactions between microglia and neurons in the vertebrate brain. In this review, we follow the historical trail of the discovery of these interactions, summarize their notable features, provide implications of these discoveries to CNS function, emphasize emerging themes along the way and peak into the future of what outstanding questions remain to move the field forward.

Microglia derived from the axotomized adult rat facial nucleus uptake glutamate and metabolize it to glutamine in vitro

Microglia in the axotomized adult rat facial nucleus (axoFN) have been shown to highly express a glutamate transporter (GLT-1). The microglia appear to serve as glutamate (Glu) scavengers in the axoFN. However, there is no evidence that the microglia actually have the ability to uptake Glu and convert it to Gln. In this study, we investigated whether axoFN-derived microglia (axoFN-microglia) can uptake Glu and metabolize it to Gln. Microglia obtained by explant culture of axoFN on poly(N-isopropylacrylamide)-grafted dishes were non-invasively sub-cultured onto dishes or wells. Immunoblotting and Glu-uptake experiments revealed that the axoFN-microglia uptake ¹⁴C-Glu mainly by GLT-1 activity. Immunoblotting and immunocytochemical methods clarified that axoFN-microglia express the Gln synthetase (GS) protein in the same manner as newborn rat brain-derived primary microglia (NRB-microglia). Biochemical analysis demonstrated that the specific activity of GS of axoFN-microglia is similar to that of NRB-microglia, suggesting that these microglia play equivalent roles in the metabolic conversion of Glu to Gln. Nuclear magnetic resonance analysis clarified that NRB-microglia metabolize [¹³C]Glu to [¹³C]Gln depending on the incubation time, inferring the similar potential of axoFN-microglia. Taken together, these results demonstrate that axoFN-microglia express functional GLT-1 and GS proteins, and are strongly suggested to serve as Glu scavengers in vivo.

Astrocyte-targeted production of IL-10 induces changes in microglial reactivity and reduces motor neuron death after facial nerve axotomy

Interleukin-10 (IL-10) is a cytokine that plays a crucial role in regulating the inflammatory response and immune reactions. In the central nervous system (CNS), IL-10 is mainly produced by astrocytes and microglia and it is upregulated after various insults, such as experimental autoimmune encephalomyelitis, middle cerebral artery occlusion, excitotoxicity and traumatic brain injury. To better understand the effects of IL-10 in the normal and injured CNS, we generated transgenic mice (termed GFAP-IL-10Tg) that expressed the murine IL-10 gene under the transcriptional control of the glial fibrillary acidic protein (GFAP) promoter. Previous studies demonstrated marked changes in the microglial phenotype in these mice under basal conditions. The objective of the present study was to investigate the effects of local astrocyte-targeted IL-10 production on glial activation, neuronal degeneration and leukocyte recruitment after axotomy. GFAP-IL-10Tg mice had marked changes in the phenotype of activated microglial cells, as well as in the number of microglial clusters and in microglial cell density. These microglial changes are accompanied by a twofold increase in lymphocyte infiltration in GFAP-IL-10Tg mice and around twofold decrease in neuronal cell death at 21 dpi. Altogether, our findings suggested that astrocyte-targeted production of IL-10 impacted the microglial response and lymphocyte recruitment and culminated in a beneficial effect on neuronal survival.

Extracellular ATP differentially modulates Toll-like receptor 4-mediated cell survival and death of microglia

The survival and death rates of inflammatory cells directly control their number and are substantially associated with the degree of inflammation. Microglia, key players in neuroinflammation, often cause excessive reactions implicated in neurological diseases. However, the mechanisms that determine microglial fate under pathological conditions remain to be elucidated. Here, we report that activation by lipopolysaccharide (LPS, a Toll-like receptor 4 ligand), an inflammation inducer, primarily promotes survival of microglia, but as its concentration is increased it induces cell death, resulting in decreased cell number. Moreover, extracellular ATP, which is released upon tissue damage, further enhanced the survival induced by a low LPS concentration and the death induced by a high LPS concentration. The survival-promoting effect of ATP was mimicked by non-hydrolyzable ATP analog, adenosine 5'-O-(3-thiotriphosphate), and also by the P2X(7) receptor agonist, 2'(3')-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate, and was suppressed by the P2X(7) antagonists, Brilliant Blue G and A 438079. On the contrary, the death of LPS-activated microglia was not affected by adenosine 5'-O-(3-thiotriphosphate), but enhanced by adenosine, ATP breakdown product. Thus, extracellular ATP modulates microglial survival and death in different ways involving P2X(7) receptor activation and ATP degradation to adenosine, respectively. Such Toll-like receptor 4/purinergic signaling may provide a fine regulatory system of neuroinflammation through modulating the microglial cell number.

Use of laser microdissection in the investigation of facial motoneuron and neuropil molecular phenotypes after peripheral axotomy

The mechanism underlying axotomy-induced motoneuron loss is not fully understood, but appears to involve molecular changes within the injured motoneuron and the surrounding local microenvironment (neuropil). The mouse facial nucleus consists of six subnuclei which respond differentially to facial nerve transection at the stylomastoid foramen. The ventromedial (VM) subnucleus maintains virtually full facial motoneuron (FMN) survival following axotomy, whereas the ventrolateral (VL) subnucleus results in significant FMN loss with the same nerve injury. We hypothesized that distinct molecular phenotypes of FMN existed within the two subregions, one responsible for maintaining cell survival and the other promoting cell death. In this study, we used laser microdissection to isolate VM and VL facial subnuclear regions for molecular characterization. We discovered that, regardless of neuronal fate after injury, FMN in either subnuclear region respond vigorously to injury with a characteristic "regenerative" profile and additionally, the surviving VL FMN appear to compensate for the significant FMN loss. In contrast, significant differences in the expression of pro-inflammatory cytokine mRNA in the surrounding neuropil response were found between the two subnuclear regions of the facial nucleus that support a causative role for glial and/or immune-derived molecules in directing the contrasting responses of the FMN to axonal transection.

Adenosine A(2A) receptor mediates microglial process retraction

Cell motility drives many biological processes, including immune responses and embryonic development. In the brain, microglia are immune cells that survey and scavenge brain tissue using elaborate and motile cell processes. The motility of these processes is guided by the local release of chemoattractants. However, most microglial processes retract during prolonged brain injury or disease. This hallmark of brain inflammation remains unexplained. We identified a molecular pathway in mouse and human microglia that converted ATP-driven process extension into process retraction during inflammation. This chemotactic reversal was driven by upregulation of the A(2A) adenosine receptor coincident with P2Y(12) downregulation. Thus, A(2A) receptor stimulation by adenosine, a breakdown product of extracellular ATP, caused activated microglia to assume their characteristic amoeboid morphology during brain inflammation. Our results indicate that purine nucleotides provide an opportunity for context-dependent shifts in receptor signaling. Thus, we reveal an unexpected chemotactic switch that generates a hallmark feature of CNS inflammation.

Adenosine receptors and cancer

The A(1), A(2A), A(2B) and A(3) G-protein-coupled cell surface adenosine receptors (ARs) are found to be upregulated in various tumor cells. Activation of the receptors by specific ligands, agonists or antagonists, modulates tumor growth via a range of signaling pathways. The A(1)AR was found to play a role in preventing the development of glioblastomas. This antitumor effect of the A(1)AR is mediated via tumor-associated microglial cells. Activation of the A(2A)AR results in inhibition of the immune response to tumors via suppression of T regulatory cell function and inhibition of natural killer cell cytotoxicity and tumor-specific CD4+/CD8+ activity. Therefore, it is suggested that pharmacological inhibition of A(2A)AR activation by specific antagonists may enhance immunotherapeutics in cancer therapy. Activation of the A(2B)AR plays a role in the development of tumors via upregulation of the expression levels of angiogenic factors in microvascular endothelial cells. In contrast, it was evident that activation of A(2B)AR results in inhibition of ERK1/2 phosphorylation and MAP kinase activity, which are involved in tumor cell growth signals. Finally, A(3)AR was found to be highly expressed in tumor cells and tissues while low expression levels were noted in normal cells or adjacent tissue. Receptor expression in the tumor tissues was directly correlated to disease severity. The high receptor expression in the tumors was attributed to overexpression of NF-kappaB, known to act as an A(3)AR transcription factor. Interestingly, high A(3)AR expression levels were found in peripheral blood mononuclear cells (PBMCs) derived from tumor-bearing animals and cancer patients, reflecting receptor status in the tumors. A(3)AR agonists were found to induce tumor growth inhibition, both in vitro and in vivo, via modulation of the Wnt and the NF-kappaB signaling pathways. Taken together, A(3)ARs that are abundantly expressed in tumor cells may be targeted by specific A(3)AR agonists, leading to tumor growth inhibition. The unique characteristics of these A(3)AR agonists make them attractive as drug candidates.

The ectonucleotidase cd39/ENTPDase1 modulates purinergic-mediated microglial migration

Microglia is activated by brain injury. They migrate in response to ATP and although adenosine alone has no effect on wild type microglial migration, we show that inhibition of adenosine receptors impedes ATP triggered migration. CD39 is the dominant cellular ectonucleotidase that degrades nucleotides to nucleosides, including adenosine. Importantly, ATP fails to stimulate P2 receptor mediated migration in cd39(-/-) microglia. However, the effects of ATP on migration in cd39(-/-) microglia can be restored by co-stimulation with adenosine or by addition of a soluble ectonucleotidase. We also tested the impact of cd39-deletion in a model of ischemia, in an entorhinal cortex lesion and in the facial nucleus after facial nerve lesion. The accumulation of microglia at the pathological sites was markedly decreased in cd39(-/-) animals. We conclude that the co-stimulation of purinergic and adenosine receptors is a requirement for microglial migration and that the expression of cd39 controls the ATP/adenosine balance.

A1 Adenosine Receptors in Microglia Control Glioblastoma-Host Interaction

We report that experimental glioblastoma grow more vigorously in A(1) adenosine receptor (A(1)AR)-deficient mice associated with a strong accumulation of microglial cells at and around the tumors. A(1)ARs were prominently expressed in microglia associated with tumor cells as revealed with immunocytochemistry but low in microglia in the unaffected brain tissue. The A(1)AR could also be detected on microglia from human glioblastoma resections. To study functional interactions between tumor and host cells, we studied glioblastoma growth in organotypical brain slice cultures. A(1)AR agonists suppressed tumor growth. When, however, microglial cells were depleted from the slices, the agonists even stimulated tumor growth. Thus, adenosine attenuates glioblastoma growth acting via A(1)AR in microglia.

Purinergic signaling and microglia

Microglial cells are considered as the pathologic sensors of the brain. In this paper, we review mechanisms of purinergic signaling in microglia. As ATP is not only considered as a physiological signaling substance but is also elevated in pathology, it is not surprising that microglia express a variety of P2X, P2Y and adenosin receptors. As a rapid physiological event, ATP triggers a cationic conductance, increases the potassium conductance and also elicits a calcium response. As a long-term effect, purinergic receptor activation is linked to the movement of microglial processes and, in the context of pathology, to chemotaxis. The purinoreceptors also modulate the release of substances from microglia, such as cytokines, nitric oxide, or superoxide, which are important in the context of a pathologic response.

Axotomy-dependent urokinase induction in the rat facial nucleus: possible stimulation of microglia by neurons

The phenomenon in which urokinase-type plasminogen activator (uPA) is induced in the axotomized facial nucleus suggests an interaction between injured motoneurons and microglia. We examined the relation of neurons and microglia to the induction of uPA in vitro. The amount of uPA released from a co-culture of neurons and microglia was much greater than the addition of that from each alone, suggesting the occurrence of an interaction between the two. The analysis of conditioned neuronal medium (CNM)-effects on microglia and conditioned microglial medium (CMM)-effects on neurons revealed that microglia enhance uPA release in response to CNM, rather than vice versa. Characterization of the CNM-effect on microglia demonstrated that CNM enhances not only uPA release but also the specific activity of acid phosphatase and 5'-nucleotidase in microglia. The profile of microglial activation caused by CNM was quite different from that caused by lipopolysaccharide (LPS)-activation. These results suggest that a specific soluble constituent(s) derived from neurons activates microglia by a mechanism different from LPS. As a candidate molecule for the microglial activation, brain-derived neurotrophic factor was detected in the CNM. Thus, uPA induction in the axotomized facial nucleus may be explained by a neuronal stimulus leading to uPA induction in microglia.

Neuropathological imaging: in vivo detection of glial activation as a measure of disease and adaptive change in the brain

Glial cells form a structural and functional network with complex cell-cell communication pathways that enable fast and slow signalling amongst themselves as well as with neurons. They exert regulatory influence on normal synaptic transmission and alter it in disease. It is becoming increasingly clear that an understanding of brain function in disease conditions requires a better account of the highly plastic, disease-associated changes in glial physiology in vivo. Particularly, microglia, the brain's ubiquitous but normally inconspicuous immune effector cell, are prominently involved in many brain diseases. They respond rapidly and in a territorially highly confined way to subtle, acute and chronic pathological stimuli. Detection of microglial activation provides diagnostically useful formal parameters of disease, such as the accurate spatial localisation, disease progression and the secondary neurodegenerative or adaptive changes remote from the primary site of disease. The latter has potential relevance for the understanding of disease-induced brain plasticity. Systematic attempts are now undertaken, using positron emission tomography and a ligand with relative selectivity for activated microglia, to develop generic imaging tools for a cellular in vivo neuropathology.

Brain plasticity and microglia: is transsynaptic glial activation in the thalamus after limb denervation linked to cortical plasticity and central sensitisation?

Microglia are a subset of tissue-macrophages that are ubiquitously distributed throughout the entire CNS. In health, they remain largely dormant until activated by a pathological stimulus. The availability of more sensitive detection techniques has allowed the early measurement of the cell responses of microglia in areas with few signs of active pathology. Subtle neuronal injury can induce microglial activation in retrograde and anterograde projection areas remote from the primary lesion focus. There is also evidence that in cases of long-standing abnormal neuronal activity, such as in patients after limb amputation with chronic pain and phantom sensations, glial activation may occur transsynaptically in the thalamus. Such neuronally driven glial responses may be related to the emergence central sensitisation in chronic pain states or plasticity phenomena in the cerebral cortex. It is suggested, that such persistent low-level microglial activation is not adequately described by the traditional concept of phagocyte-mediated tissue damage that largely evolved from studies of acute brain lesion models or acute human brain pathology. Due to the presence of signal molecules that can act on neurons and microglia alike, the communication between neurons and microglia is likely to be bi-directional. Persistent subtle microglial activity may modulate basal synaptic transmission and thus neuronal functioning either directly or through the interaction with astrocytes. The activation of microglia leads to the emergence of microstructural as well as functional compartments in which neurokines, interleukins and other signalling molecules introduce a qualitatively different, more open mode of cell-cell communication that is normally absent from the healthy adult brain. This 'neo-compartmentalisation', however, occurs along predictable neuronal pathways within which these glial changes are themselves under the modulatory influence of neurons or other glial cells and are subject to the evolving state of the pathology. Depending on the disease state, yet relatively independent of the specific disease cause, fluctuations in the modulatory influence by non-neuronal cells may form the cellular basis for the variability of brain plasticity phenomena, i.e. the plasticity of plasticity.

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.

Neurotrophins regulate the function of cultured microglia

Although the physiological role of neurotrophins in neuronal development and survival has been extensively investigated, their role in glial cell physiology remains to be elucidated. In the present study, we investigated the effects of neurotrophins on cultured microglia from newborn rat brain. All of the neurotrophins tested nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4), increased the secretion of plasminogen and urokinase type-plasminogen activator and specific activity of acid phosphatase, but suppressed the release of constitutively-produced and lipopolysaccharide-stimulated nitric oxide (NO) from microglia. The reverse transcription-polymerase chain reaction, immunocytochemical staining, and Western blotting revealed that cultured microglia express Trk A, B, and C, and low-affinity NGF receptor, LNGFRp75. Neurotrophin was found to phosphorylate Trk A and B, and the neurotrophin-induced enhancement of plasminogen-secretion was suppressed by protein kinase inhibitor, K252a. Furthermore, neurotrophins caused an activation of transcription factor, NF-kappaB. These results indicate that the neurotrophin family regulate the function of microglia through Trk and/or LNGFRp75-mediated signal transduction.

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.

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.

Central neuron-glial and glial-glial interactions following axon injury

Axon injury rapidly activates microglial and astroglial cells close to the axotomized neurons. Following motor axon injury, astrocytes upregulate within hour(s) the gap junction protein connexin-43, and within one day glial fibrillary acidic protein (GFAP). Concomitantly, microglial cells proliferate and migrate towards the axotomized neuron perikarya. Analogous responses occur in central termination territories of peripherally injured sensory ganglion cells. The activated microglia express a number of inflammatory and immune mediators. When neuron degeneration occurs, microglia act as phagocytes. This is uncommon after peripheral nerve injury in the adult mammal, however, and the functional implications of the glial cell responses in this situation are unclear. When central axons are injured, the glial cell responses around the affected neuron perikarya appears to be minimal or absent, unless neuron degeneration occurs. Microglia proliferate, and astrocytes upregulate GFAP along central axons undergoing anterograde, Wallerian, degeneration. Although microglia develop into phagocytes, they eliminate the disintegrating myelin very slowly, presumably because they fail to release molecules which facilitate phagocytosis. During later stages of Wallerian degeneration, oligodendrocytes express clusterin, a glycoprotein implicated in several conditions of cell degeneration. A hypothetical scheme for glial cell activation following axon injury is discussed, implying the injured neurons initially interact with adjacent astrocytes. Subsequently, neighbouring resting microglia are activated. These glial reactions are amplified by paracrine and autocrine mechanisms, in which cytokines appear to be important mediators. The specific functional properties of the activated glial cells will determine their influence on neuronal survival, axon regeneration, and synaptic plasticity. The control of the induction and progression of these responses are therefore likely to be critical for the outcome of, for example, neurotrauma, brain ischemia and chronic neurodegenerative diseases.

Association of ecto-5'-nucleotidase with specific cell types in the adult and developing rat olfactory organ

A unique feature of the olfactory epithelium is its ability to give rise to new sensory neurons throughout life and also following injury. Cells at the basal side of the epithelium serve as neurogenic progenitor cells. The enzyme ecto-5'-nucleotidase is expressed at the surface of developing nerve cells and is regarded as a marker of neural development. To study the expression pattern of the enzyme, we analyzed its distribution in the adult and developing rat olfactory organ. Labeling is restricted to specific cell types and varies between the epithelia investigated. At the basal side of the olfactory epithelium, activity of 5'-nucleotidase is associated specifically with the dark/horizontal basal cells. Neither the light/globose basal cells, which are the immediate precursors of the sensory receptor cells, nor subsets of potentially immature olfactory receptor cells are labeled. On the other hand, microvillar cells dispersed at the lumenal side of the epithelium contain 5'-nucleotidase activity. The enzyme is also present at the inner lining of the ducts of Bowman's glands as they traverse the epithelium. Within the respiratory epithelium, activity of 5'-nucleotidase is associated with basal cells as well as with the epithelial surface. During development, 5'-nucleotidase is initially limited to the respiratory epithelium, including its basal cells. Dark/horizontal basal cells of the olfactory epithelium, which are positive for 5'-nucleotidase, first appear at the border of the respiratory epithelium, suggesting that they might originate from immigrating basal cells of the respiratory epithelium. Within the vomeronasal organ, labeling is largely restricted to the receptor-free epithelium. Although the functional role of 5'-nucleotidase in the olfactory system needs to be further defined, the distribution of the enzyme can be used successfully as a marker for defined cell types.

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.

5'-nucleotidase enzyme cytochemistry as a tool for revealing activated glial cells and malleable synapses in CNS development and regeneration

The demonstration of 5'-nucleotidase in neural tissue is achieved at both the light and electron microscopic levels by means of an enzyme cytochemical lead method, which is specific, sensitive and fast. By its activity this adenosine-producing ecto-enzyme (EC 3.1.3.5) outlines cellular surface membranes at the ultrastructural level. It is classically known as a marker of myelin and of astrocytes as well as (activated) microglial cells in the mature nervous system. In recent years, we discovered that 5'-nucleotidase is transiently active within synaptic clefts under conditions of development and regeneration. The enzyme is also seen at terminals in the mature retina and olfactory bulb, where spontaneous synaptic turnover occurs at adulthood. Thus, 5'-nucleotidase cytochemistry is useful in revealing sites of glial reactions and synaptic plasticity in CNS development and repair. It is assumed that the molecule affects terminal formation and cell motility due to dual functions in adenosine production and cell adhesion. Finally, at the light microscopic level, 5'-nucleotidase activity displays a dense neuropil staining which identifies topographic sub-units of certain parts of the nervous system, such as the striosomes of the basal ganglia, ocular dominance columns of the visual cortex and parasagittal bands of the cerebellum.

Development of 5'-nucleotidase staining in the olfactory bulbs of normal and naris-occluded rats

The distribution of the adenosine-producing ecto-enzyme 5'-nucleotidase was investigated histochemically in the developing rat olfactory bulb. Rat pups underwent either unilateral surgical occlusion of the right external naris or sham surgery on postnatal day 1. At 10, 20, or 30 days postpartum, horizontal sections of the olfactory bulb were reacted histochemically to reveal the locus and intensity of 5'-nucleotidase activity. Relative staining levels were determined by optical densitometry in standardized bulb regions. A marked, age-related increase in staining density was observed. Reaction product was found primarily in neuropil areas. The P10 and P20 control animals did not exhibit right/left differences in bulb staining; however, some laterality was observed in P30 animals. Inter-glomerular and regional variations were observed throughout the developmental period, including (1) differences between neighboring glomeruli; (2) a gradient in the dorsal-ventral axis of the bulb; and (3) a higher staining density in the medial-caudal portion of the bulb. In subjects with occluded nares, asymmetries in right/left bulb 5'-nucleotidase staining patterns were detected throughout development. Bulbs ipsilateral to the blocked nares exhibited increased staining density, suggesting that the procedure enhanced enzymatic activity. Understanding these variations in 5'-nucleotidase staining may be important for a complete understanding of the mechanisms of olfactory bulb maturation and may give insight into the possible role of this enzyme in synaptic malleability during nervous system development and regeneration.

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.

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

The distribution of ecto-5'-nucleotidase, a glycosyl phosphatidylinositol anchored membrane protein capable of hydrolysing extracellular nucleoside monophosphates, was investigated by immunocytochemistry in cultures of rat cerebellar cells obtained at postnatal days 6 and 8. The enzyme was expressed at the surface of granule cells including their neurites as well as on other neurons in the culture. The distribution of 5'-nucleotidase matched that of the synaptic vesicle protein 2. Oligodendroglial cells were identified by their immunoreactivity for 2',3'-cyclic nucleotide 3'-phosphodiesterase. Their entire surface was labelled for 5'-nucleotidase. In contrast, only a subset of astrocytes immunopositive for the glial fibrillary acidic protein revealed surface-located immunoreactivity for 5'-nucleotidase. Antibody-binding of the labelled-astrocytes was enhanced at restricted surface domains. Endothelial cells that avidly bind Lycopersicon esculentum lectin were the most intensely anti-5'-nucleotidase-labelled cell type of the culture. Double labelling revealed an exact match of surface-located antibody binding sites for 5'-nucleotidase and laminin. Immunoreactivity for 5'-nucleotidase was essentially absent from fibroblasts that could be identified by their immunoreactivity for fibronectin. All cell types that carried surface-bound 5'-nucleotidase also revealed a cytoplasmic pool of the enzyme. Our results provide the first immunocytochemical demonstration of the surface-location of 5'-nucleotidase in neurons. They suggest that the broad distribution of the enzyme at the surface of neurons and other cells types from neonatal brain reflects its functional importance in the differentiation of the nervous system.

Glial cytokines in Alzheimer's disease: review and pathogenic implications

The roles of activated glia and of glial cytokines in the pathogenesis of Alzheimer's disease are reviewed. Interleukin-1 (IL-1), a microglia-derived acute phase cytokine, activates astrocytes and induces expression of the astrocyte-derived cytokine, S100 beta, which stimulates neurite growth (and thus has been implicated in neuritic plaque formation) and increases intracellular free calcium levels. Interleukin-1 also upregulates expression and processing of beta-amyloid precursor proteins (beta-APPs) (thus favoring beta-amyloid deposition) and induces expression of alpha 1-antichymotrypsin, thromboplastin, the complement protein C3, and apolipoprotein E, all of which are present in neuritic plaques. These cytokines, and the molecular and cellular events that they engender, form a complex of interactions that may be capable of self-propagation, leading to chronic overexpression of glial cytokines with neurodegenerative consequences. Self-propagation may be facilitated by means of several reinforcing feedback loops. beta-Amyloid, for instance, directly activates microglia, thus inducing further IL-1 production, and activates the complement system, which also leads to microglial activation with IL-1 expression. Self-propagation also could result when S100 beta-induced increases in intraneuronal free calcium levels lead to neuronal injury and death with consequent microglial activation. Such chronic, self-propagating, cytokine-mediated molecular and cellular reactions would explain the progressive neurodegeneration and dementia of Alzheimer's disease.

5'-nucleotidase activates and an inhibitory antibody prevents neuritic differentiation of PC12 cells

Ecto-5'-nucleotidase catalyses the hydrolysis of AMP at the surface of a variety of cells whereas it is absent from others. In addition to its catalytic activity, a function in neural development and also its interaction with extracellular matrix proteins has been reported. In order to further elucidate the biological function of ecto-5'-nucleotidase we have investigated the effect of 5'-nucleotidase on nerve growth factor-induced differentiation of PC12 cells. Furthermore, we compared the effect of an inhibitory versus a non-inhibitory monospecific antibody against the enzyme on neuritic differentiation and survival of PC12 cells that constitutively express the enzyme. When coverslips are coated with the soluble form of ecto-5'-nucleotidase in addition to collagen, there is a considerable increase in nerve growth factor-induced neurite length during the first 24 h of culture. Addition of an antibody to a culture medium that inhibits 5'-nucleotidase activity to 33% of control values dramatically reduces the number of neurites per cell within 3 days of culture. The cells round up, cluster and eventually die. On the contrary, another antibody that had no significant effect on enzyme activity affected neither nerve growth factor-induced neurite formation nor survival of PC12 cells. Addition of adenosine (200 nM, 10 or 20 microM) to the culture medium did not influence PC12 cell differentiation. The effects induced by the inhibitory antibody could be only partially prevented by simultaneous application of adenosine. Our results suggest that 5'-nucleotidase is essential for nerve growth factor-induced neurite outgrowth and survival of PC12 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that 5'-nucleotidase is associated with malleable synapses--an enzyme cytochemical investigation of the olfactory bulb of adult rats

The adenosine-producing ecto-enzyme 5'-nucleotidase has recently been assigned to malleable axon terminals in both the developing and regenerating adult nervous system, but is otherwise only glia-bound. Using a cytochemical lead method, we now show that 5'-nucleotidase activity is localized predominantly at glomerular and mitral synapses within the main olfactory bulb of normal, adult rats. As these terminals are prone to synaptic turnover even at maturity, the present findings favour the view that this enzyme constitutes a marker molecule for plastic synapses. It is suggested that functions of 5'-nucleotidase in purinergic neuromodulation and cell adhesion are unique to the olfactory bulb, and implied in synaptic arrangements and information processing.

Differential response of neutral endopeptidase 24.11 ("enkephalinase"), and cholinergic and opioidergic markers to hypoglossal axotomy

Neutral endopeptidase 24.11 (NEP; "enkephalinase") may inactivate a number of centrally active neuropeptides including the enkephalins and substance P. In most areas of the central nervous system, the cell types which express NEP activity are not known. The hypoglossal nucleus (N.XII) was selected as a model system to characterize the cytochemical localization of NEP. The effect of hypoglossal nerve axotomy upon the distribution of NEP activity in the hypoglossal nucleus was compared to the effect upon cholinergic markers, the mu opiate receptor, and the enkephalins. By use of a fluorescence histochemical method, NEP was localized at all levels of N.XII to the soma and proximal processes of the majority of the apparent motor neurons in the nucleus. Fluorescent double-labeling studies revealed the presence of numerous enkephalinergic varicosities which localized to the neuropil surrounding NEP-stained motor neurons. To determine whether NEP was synthesized by these motor neurons, 18 rats received a unilateral transection of the hypoglossal nerve. A pronounced decrease in NEP staining in N.XII was observed on the operated side as early as 3 days following axotomy. This decrease persisted at all levels of the nucleus for about 5 weeks. By 7 weeks, the staining between the control and operated sides was indistinguishable. By contrast, there was no apparent change in the density or distribution of enkephalin-immunoreactive varicosities in five animals examined 6 to 32 days following axotomy. Radioligand binding of [3H]DAMGO to the mu-opiate receptor in N.XII was studied in 20 animals by quantitative autoradiography at 2, 6, and 11 days after axotomy. No significant changes in the level of radioligand binding to the mu-receptor were detected in response to axotomy. In contrast to the opiate system, the cholinergic enzymes choline acetyltransferase, acetylcholinesterase, and pseudocholinesterase showed a coordinate decrease in motor neuron-associated staining on the operated side of N.XII at 3, 6, and 11 days following axotomy which paralleled the decrease in NEP staining. By contrast, the lysosomal enzyme marker, acid phosphatase, showed a pronounced increase in staining on the operated side. The results of this study are consistent with the synthesis of NEP by cholinergic N.XII motor neurons and indicates that the enkephalins and NEP in N.XII are closely associated, but derive from separate neuronal populations. The widespread overlap in the distribution of NEP-stained motor neurons and enkephalinergic varicosities in N.XII provides additional anatomical support for a potential role for NEP in the inactivation of centrally active enkephalins.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional roles of microglia in the brain

It has been suggested that microglia, a type of glial cells in the central nervous system, play various important roles in normal and pathologic brains. In this article, we discussed the association or roles of microglia in injury and in brain diseases such as Alzheimer's disease, AIDS dementia complex, multiple sclerosis and ischemia. Furthermore, microglia-derived cytotoxic products and other secretory factors were summarized. In addition to the pathological aspects, secretory factors that showed neurotrophic effects were described with special reference to their physiological significance in the neuronal growth, neuronal function and regeneration processes. Accumulated evidence suggests that microglia are associated with not only brain pathology but also normal physiology in the brain.

Species-specific patterns of glycoprotein expression in the developing rodent caudoputamen: association of 5'-nucleotidase activity with dopamine islands and striosomes in rat, but with extrastriosomal matrix in mouse

The glycoprotein 5'-nucleotidase is a cell surface phosphatase and represents a new marker for striosomes in the adult rat caudoputamen. We report here on its developmental expression in the rat and mouse striatum, and show an unexpected converse 5'-nucleotidase chemoarchitecture of the caudoputamen in these closely related species. In the rat, 5'-nucleotidase activity was first visible as neuropil staining in tyrosine hydroxylase-positive dopamine islands of the midstriatum on postnatal day 1, and by the end of the first postnatal week, 5'-nucleotidase-positive dopamine islands also appeared rostrally. This compartmental pattern persisted thereafter, so that in adult animals, in all but the caudal caudoputamen, zones of enhanced 5'-nucleotidase staining were restricted to calbindin-D28k-poor striosomes. Weak 5'-nucleotidase activity also emerged in the matrix. In striking contrast, in the mouse striatum, enhanced 5'-nucleotidase activity was preferentially associated with extrastriosomal tissue. Enzymatic reaction first appeared on embryonic day 18, and developed over the first postnatal week into a mosaic pattern in which the matrix was stained but the dopamine islands were unstained. The matrix staining itself was heterogeneous. After the second postnatal week, most of the caudoputamen was stained, and in adult mice only rostral striosomes expressed low 5'-nucleotidase activity. We conclude that in rats, 5'-nucleotidase represents one of the few substances that maintains a preferential dopamine island/striosome distribution during striatal development. In mice, 5'-nucleotidase activity is expressed preferentially in the matrix during development, and its compartmental pattern is gradually lost with maturation, except very rostrally. These findings do not suggest an instructive role of the enzyme in striatal compartment formation in either species, but do suggest the possibility that 5'-nucleotidase contributes to the differentiation of striatal compartments during development.

Cytochemical redistribution of 5'-nucleotidase in the developing cat visual cortex

The adenosine-producing ectoenzyme 5'-nucleotidase has recently been shown to undergo a marked redistribution during development of the cat visual cortex and to be involved in the remodelling of ocular dominance columns (Schoen et al., J. Comp. Neurol., 296, 379-392, 1990). Using an enzyme-cytochemical technique, we now investigate the developmental redistribution of 5'-nucleotidase activity in area 17 of kittens at the ultrastructural level. Between postnatal days 35 and 42, when 5'-nucleotidase is concentrated in layer IV, enzyme reaction product occupies the clefts of asymmetrical synapses within the neuropil. During later development (9th and 13th postnatal weeks), when 5'-nucleotidase spreads over all cortical laminae, the enzyme disappears from its synaptic localization and becomes increasingly associated with astrocytic membranes. The transient appearance of 5'-nucleotidase at synapses parallels the time-course and laminar profile of the synaptic remodelling which takes place during the critical period of visual cortex development. This suggests that synapse-bound 5'-nucleotidase activity plays a role in synaptic malleability, whereas its later association with glial profiles is likely to reflect other functions of the enzyme.