Functional expression of the ecto-ATPase NTPDase2 and of nucleotide receptors by neuronal progenitor cells in the adult murine hippocampus

Impaired olfactory performance and anxiety-like behavior in a rat model of multiple sclerosis are associated with enhanced adenosine signaling in the olfactory bulb via A1R, A2BR, and A3R

The present study shows that animals with experimental autoimmune encephalomyelitis (EAE) exhibit olfactory dysfunction and impaired general cognitive abilities, as well as anxiety-like behavior. Olfactory dysfunction occurs on average at 2 dpi, well before the onset of the first motor signs of EAE (8-10 dpi). After the initial olfactory dysfunction, the EAE animals show a fluctuation in olfactory performance that resembles the relapsing-remitting course of human MS. The study also shows severe neuroinflammation in the olfactory bulb (OB), with numerous infiltrated CD4+ T cells and peripheral macrophages in the superficial OB layers, marked microgliosis, and massive induction of TNF-α, IL-1β, and IL-6. Reduced tyrosine hydroxylase activity in the glomerular layer, pronounced granule cell atrophy, and reduced numbers of type B neuroblasts in the rostral migratory stream also indicate altered plasticity of the neuronal network in the OB. Considering the exceptionally high purinome expression in the OB, the possible involvement of purinergic signaling was also investigated. The study shows that macrophages infiltrating the OB overexpress A3R, while highly reactive microglia overexpress the adenosine-producing enzyme eN/CD73 as well as A2BR, A3R, and P2X4R. Given the simultaneous induction of complement component C3, the results suggest that the microglial cells develop a functional phenotype of phagocytizing microglia. The study also demonstrates transcriptional and translational upregulation of A1R in mitral and tufted cells, which likely influence resting network activity in OB and likely contribute to olfactory dysfunction in EAE. Overall, our study shows that olfactory dysfunction and altered social and cognitive behavior in EAE are associated with increased adenosine signaling via A1R, A2BR, and A3R.

Extracellular Purine Metabolism-Potential Target in Multiple Sclerosis

The purinergic signaling system comprises a complex network of extracellular purines and purine-metabolizing ectoenzymes, nucleotide and nucleoside receptors, ATP release channels, and nucleoside transporters. Because of its immunomodulatory function, this system is critically involved in the pathogenesis of multiple sclerosis (MS) and its best-characterized animal model, experimental autoimmune encephalomyelitis (EAE). MS is a chronic neuroinflammatory demyelinating and neurodegenerative disease with autoimmune etiology and great heterogeneity, mostly affecting young adults and leading to permanent disability. In MS/EAE, alterations were detected in almost all components of the purinergic signaling system in both peripheral immune cells and central nervous system (CNS) glial cells, which play an important role in the pathogenesis of the disease. A decrease in extracellular ATP levels and an increase in its downstream metabolites, particularly adenosine and inosine, were frequently observed at MS, indicating a shift in metabolism toward an anti-inflammatory environment. Accordingly, upregulation of the major ectonucleotidase tandem CD39/CD73 was detected in the blood cells and CNS of relapsing-remitting MS patients. Based on the postulated role of A2A receptors in the transition from acute to chronic neuroinflammation, the association of variants of the adenosine deaminase gene with the severity of MS, and the beneficial effects of inosine treatment in EAE, the adenosinergic system emerged as a promising target in neuroinflammation. More recently, several publications have identified ADP-dependent P2Y12 receptors and the major extracellular ADP producing enzyme nucleoside triphosphate diphosphohydrolase 2 (NTPDase2) as novel potential targets in MS.

Purinergic Signaling and its Role in the Stem Cell Differentiation

Purinergic signaling is a mechanism in which extracellular purines and pyrimidines interact with specialized cell surface receptors known as purinergic receptors. These receptors are divided into two families of P1 and P2 receptors, each responding to different nucleosides and nucleotides. P1 receptors are activated by adenosine, while P2 receptors are activated by pyrimidine and purines. P2X receptors are ligand-gated ion channels, including seven subunits (P2X1-7). However, P2Y receptors are the G-protein coupled receptors comprising eight subtypes (P2Y1/2/4/6/11/12/13/14). The disorder in purinergic signaling leads to various health-related issues and diseases. In various aspects, it influences the activity of non-neuronal cells and neurons. The molecular mechanism of purinergic signaling provides insight into treating various human diseases. On the contrary, stem cells have been investigated for therapeutic applications. Purinergic signaling has shown promising effect in stem cell engraftment. The immune system promotes the autocrine and paracrine mechanisms and releases the significant factors essential for successful stem cell therapy. Each subtype of purinergic receptor exerts a beneficial effect on the damaged tissue. The most common effect caused by purinergic signaling is the proliferation and differentiation that treat different health-related conditions.

P2 receptor-mediated signaling in the physiological and pathological brain: From development to aging and disease

The purinergic pathway mediates both pro-inflammatory and anti-inflammatory responses, whereas the breakdown of adenosine triphosphate (ATP) is in a critical equilibrium. Under physiological conditions, extracellular ATP is maintained at a nanomolar concentration. Whether released into the medium following tissue damage, inflammation, or hypoxia, ATP is considered a clear indicator of cell damage and a marker of pathological conditions. In this overview, we provide an update on the participation of P2 receptor-mediated purinergic signaling in normal and pathological brain development, with special emphasis on neurodevelopmental psychiatric disorders. Since purinergic signaling is ubiquitous, it is not surprising that it plays a prominent role in developmental processes and pathological alterations. The main aim of this review is to conceptualize the time-dependent dynamic changes in the participation of different players in the purinome in shaping the normal and aberrant developmental patterns and diseases of the central nervous system over one's lifespan.

Expression of Ectonucleoside Triphosphate Diphosphohydrolase 2 (NTPDase2) Is Negatively Regulated Under Neuroinflammatory Conditions In Vivo and In Vitro

Ectonucleoside triphosphate diphosphohydrolase 2 (NTPDase2) hydrolyzes extracellular ATP to ADP, which is the ligand for P2Y1,12,13 receptors. The present study describes the distribution of NTPDase2 in adult rat brains in physiological conditions, and in hippocampal neurodegeneration induced by trimethyltin (TMT). The study also describes the regulation of NTPDase2 by inflammatory mediators in primary astrocytes and oligodendroglial cell line OLN93. In physiological conditions, NTPDase2 protein was most abundant in the hippocampus, where it was found in fibrous astrocytes and synaptic endings in the synaptic-rich hippocampal layers. In TMT-induced neurodegeneration, NTPDase2-mRNA acutely decreased at 2-dpi and then gradually recovered to the control level at 7-dpi and 21-dpi. As determined by immunohistochemistry and double immunofluorescence, the decrease was most pronounced in the dentate gyrus (DG), where NTPDase2 withdrew from the synaptic boutons in the polymorphic layer of DG, whereas the recovery of the expression was most profound in the subgranular layer. Concerning the regulation of NTPDase2 gene expression, proinflammatory cytokines IL-6, IL-1β, TNFα, and IFNγ negatively regulated the expression of NTPDase2 in OLN93 cells, while did not altering the expression in primary astrocytes. Different cell-intrinsic stressors, such as depletion of intracellular energy store, oxidative stress, endoplasmic reticulum stress, and activation of protein kinase C, also massively disturbed the expression of the NTPDase2 gene. Together, our results suggest that the expression and the activity of NTPDase2 transiently cease in neurodegeneration and brain injury, most likely as a part of the acute adaptive response designed to promote cell defense, survival, and recovery.

Ectonucleotidases in the hippocampus: Spatial distribution and expression after ovariectomy and estradiol replacement

Extracellular purine nucleotides, such as adenosine 5'-triphosphate (ATP), are important modulators of hippocampal function and plasticity. In the extracellular space, ATP is inherently short-lived molecule, which undergoes rapid enzymatic degradation to adenosine by ectonucleotidases. Given that ectonucleotidases have distinct and overlapping distribution in the hippocampus, and as ovarian hormones participate in a formation, maturation, and a refinement of synaptic contacts, both during development and in adulthood, the present chapter summarizes known data about spatial distribution of selected ecto-enzymes and estradiol-induced effects on ectonucleotidases in the rat hippocampus.

P2Y2 deficiency impacts adult neurogenesis and related forebrain functions

Adult neurogenesis occurs particularly in the subgranular zone (SGZ) of the hippocampus and the subventricular zone (SVZ) of the lateral ventricle. This continuous addition of neurons to pre-existing neuronal networks is essential for intact cognitive and olfactory functions, respectively. Purinergic signaling modulates adult neurogenesis, however, the role of individual purinergic receptor subtypes in this dynamic process and related cognitive performance is poorly understood. In this study, we analyzed the role of P2Y₂ receptor in the neurogenic niches and in related forebrain functions such as spatial working memory and olfaction using mice with a targeted deletion of the P2Y₂ receptor (P2Y2^-/- ). Proliferation, migration, differentiation, and survival of neuronal precursor cells (NPCs) were analyzed by BrdU assay and immunohistochemistry; signal transduction pathway components were analyzed by immunoblot. In P2Y2^-/- mice, proliferation of NPCs in the SGZ and the SVZ was reduced. However, migration, neuronal fate decision, and survival were not affected. Moreover, p-Akt expression was decreased in P2Y2^-/- mice. P2Y2^-/- mice showed an impaired performance in the Y-maze and a higher latency in the hidden food test. These data indicate that the P2Y₂ receptor plays an important role in NPC proliferation as well as in hippocampus-dependent working memory and olfactory function.

Purine Nucleotides Metabolism and Signaling in Huntington's Disease: Search for a Target for Novel Therapies

Huntington’s disease (HD) is a multi-system disorder that is caused by expanded CAG repeats within the exon-1 of the huntingtin (HTT) gene that translate to the polyglutamine stretch in the HTT protein. HTT interacts with the proteins involved in gene transcription, endocytosis, and metabolism. HTT may also directly or indirectly affect purine metabolism and signaling. We aimed to review existing data and discuss the modulation of the purinergic system as a new therapeutic target in HD. Impaired intracellular nucleotide metabolism in the HD affected system (CNS, skeletal muscle and heart) may lead to extracellular accumulation of purine metabolites, its unusual catabolism, and modulation of purinergic signaling. The mechanisms of observed changes might be different in affected systems. Based on collected findings, compounds leading to purine and ATP pool reconstruction as well as purinergic receptor activity modulators, i.e., P2X7 receptor antagonists, may be applied for HD treatment.

Development of Anthraquinone Derivatives as Ectonucleoside Triphosphate Diphosphohydrolase (NTPDase) Inhibitors With Selectivity for NTPDase2 and NTPDase3

Ectonucleoside triphosphate diphosphohydrolases (NTPDases) catalyze the hydrolysis of nucleoside tri- and di-phosphates to mono-phosphates. The products are subsequently hydrolyzed by ecto-5′-nucleotidase (ecto-5′-NT) to nucleosides. NTPDase inhibitors have potential as novel drugs, e.g., for the treatment of inflammation, neurodegenerative diseases, and cancer. In this context, a series of anthraquinone derivatives structurally related to the anthraquinone dye reactive blue-2 (RB-2) was synthesized and evaluated as inhibitors of human NTPDases utilizing a malachite green assay. We identified several potent and selective inhibitors of human NTPDase2 and -3. Among the most potent NTPDase2 inhibitors were 1-amino-4-(9-phenanthrylamino)-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (20, PSB-16131, IC₅₀ of 539 nM) and 1-amino-4-(3-chloro-4-phenylsulfanyl)phenylamino-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (48, PSB-2020, IC₅₀ of 551 nM). The most potent NTPDase3 inhibitors were 1-amino-4-[3-(4,6-dichlorotriazin-2-ylamino)-4-sulfophenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (42, PSB-1011, IC₅₀ of 390 nM) and 1-amino-4-(3-carboxy-4-hydroxyphenylamino)-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (33, PSB-2046, IC₅₀ of 723 nM). The best NTPDase2 inhibitor 20 showed a non-competitive inhibition type, while the NTPDase3 inhibitor 42 behaved as a mixed-type inhibitor. These potent compounds were found to be selective vs. other NTPDases. They will be useful tools for studying the roles of NTPDase2 and -3 in physiology and under pathological conditions.

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Comment on Hippocampal CD 39/ENTPD 1 promotes mouse depression-like behavior through hydrolyzing extracellular ATP by Cui et al.

Purinergic Signaling in Glioma Progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X₇, is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

The ectonucleoside triphosphate diphosphohydrolase-2 (NTPDase2) in human endometrium: a novel marker of basal stroma and mesenchymal stem cells

The human endometrium undergoes repetitive regeneration cycles in order to recover the functional layer, shed during menses. The basal layer, which remains in charge of endometrial regeneration in every cycle, contains adult stem or progenitor cells of epithelial and mesenchymal lineage. Some pathologies such as adenomyosis, in which endometrial tissue develops within the myometrium, originate from this layer. It is well known that the balance between adenosine triphosphate (ATP) and adenosine plays a crucial role in stem/progenitor cell physiology, influencing proliferation, differentiation, and migration. The extracellular levels of nucleotides and nucleosides are regulated by the ectonucleotidases, such as the nucleoside triphosphate diphosphohydrolase 2 (NTPDase2). NTPDase2 is a membrane-expressed enzyme found in cells of mesenchymal origin such as perivascular cells of different tissues and the stem cells of adult neurogenic regions. The aim of this study was to characterize the expression of NTPDase2 in human nonpathological cyclic and postmenopausic endometria and in adenomyosis. We examined proliferative, secretory, and atrophic endometria from women without endometrial pathology and also adenomyotic lesions. Importantly, we identified NTPDase2 as the first marker of basal endometrium since other stromal cell markers such as CD10 label the entire stroma. As expected, NTPDase2 was also found in adenomyotic stroma, thus becoming a convenient tracer of these lesions. We did not record any changes in the expression levels or the localization of NTPDase2 along the cycle, thus suggesting that the enzyme is not influenced by the female sex hormones like other previously studied ectoenzymes. Remarkably, NTPDase2 was expressed by the Sushi Domain containing 2 (SUSD2)⁺ endometrial mesenchymal stem cells (eMSCs) found perivascularly, rendering it useful as a cell marker to improve the isolation of eMSCs needed for regenerative medicine therapies.

Role of Ectonucleotidases in Synapse Formation During Brain Development: Physiological and Pathological Implications

BACKGROUND

Extracellular adenine nucleotides and nucleosides, such as ATP and adenosine, are among the most recently identified and least investigated diffusible signaling factors that contribute to the structural and functional remodeling of the brain, both during embryonic and postnatal development. Their levels in the extracellular milieu are tightly controlled by various ectonucleotidases: ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPP), alkaline phosphatases (AP), ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases) and ecto-5'- nucleotidase (eN).

METHODS

Studies related to the expression patterns of ectonucleotidases and their known features during brain development are reviewed, highlighting involvement of these enzymes in synapse formation and maturation in physiological as well as in pathological states.

RESULTS

During brain development and in adulthood all ectonucleotidases have diverse expression pattern, cell specific localization and function. NPPs are expressed at early embryonic days, but the expression of NPP3 is reduced and restricted to ependymal area in adult brain. NTPDase2 is dominant ectonucleotidase existing in the progenitor cells as well as main astrocytic NTPDase in the adult brain, while NTPDase3 is fully expressed after third postnatal week, almost exclusively on varicose fibers. Specific brain AP is functionally associated with synapse formation and this enzyme is sufficient for adenosine production during neurite growth and peak of synaptogenesis. eN is transiently associated with synapses during synaptogenesis, however in adult brain it is more glial than neuronal enzyme.

CONCLUSION

Control of extracellular adenine nucleotide levels by ectonucleotidases are important for understanding the role of purinergic signaling in developing tissues and potential targets in developmental disorders such as autism.

Purinergic receptors in neurogenic processes

Neurogenesis is a process of generating functional neurons, which occurs during embryonic and adult stages in mammals. While neurogenesis during development phase is characterized by intensive proliferation activity in all regions of the brain to form the architecture and neural function of the nervous system, adult neurogenesis occurs with less intensity in two brain regions and is involved in the maintenance of neurogenic niches, local repair, memory and cognitive functions in the hippocampus. Taking such differences into account, the understanding of molecular mechanisms involved in cell differentiation in developmental stages and maintenance of the nervous system is an important research target. Although embryonic and adult neurogenesis presents several differences, signaling through purinergic receptors participates in this process throughout life. For instance, while embryonic neurogenesis involves P2X7 receptor down-regulation and calcium waves triggered by P2Y1 receptor stimulation, adult neurogenesis may be enhanced by increased activity of A_2A and P2Y1 receptors and impaired by A₁, P2Y13 and P2X7 receptor stimulation.

Purinergic Signaling Modulates Survival/Proliferation of Human Dental Pulp Stem Cells

Human dental pulp stem cells (hDPSCs) reside in postnatal dental pulp and exhibit the potential to differentiate into odontoblasts as well as neurons. However, the intercellular signaling niches necessary for hDPSC survival and self-renewal remain largely unknown. The objective of this study is to demonstrate the existence of intercellular purinergic signaling in hDPSCs and to assess the impact of purinergic signaling on hDPSC survival and proliferation. hDPSCs were isolated from extracted third molars and cultured in minimum essential medium. To demonstrate responsiveness to ATP application and inhibitions by purinergic receptor antagonists, whole cell patch-clamp recordings of ATP-induced currents were recorded from cultured hDPSCs. Immunofluorescence and enzymatic histochemistry staining were performed to assess purinergic receptor expression and ectonucleotidase activity in hDPSCs, respectively. To determine the effects of purinergic signaling on hDPSC, purinergic receptor antagonists and an ectonucleotidase inhibitor were applied in culture medium, and hDPSC survival and proliferation were assessed with DAPI staining and Ki67 immunofluorescence staining, respectively. We demonstrated that ATP application induced inward currents in hDPSCs. P2X and P2Y receptors are involved in the generation of ATP-induced inward currents. We also detected expression of NTPDase3 and ectonucleotidase activity in hDPSCs. We further demonstrated that purinergic receptors were tonically activated in hDPSCs and that inhibition of ectonucleotidase activity enhanced ATP-induced inward currents. Furthermore, we found that blocking P2Y and P2X receptors reduced-and inhibition of ecto-ATPase activity enhanced-the survival and proliferation of hDPSCs, while blocking P2X receptors alone affected only hDPSC proliferation. Autocrine/paracrine purinergic signaling is essential for hDPSC survival and proliferation. These results reveal potential targets to manipulate hDPSCs to promote tooth/dental pulp repair and regeneration.

Spatial Distribution and Expression of Ectonucleotidases in Rat Hippocampus After Removal of Ovaries and Estradiol Replacement

Purinergic signaling is the main synaptic and non-synaptic signaling system in brain. ATP acts as a fast excitatory transmitter, while adenosine sets a global inhibitory tone within hippocampal neuronal networks. ATP and adenosine are interconnected by ectonucleotidase enzymes, which convert ATP to adenosine. Existing data point to the converging roles of ovarian steroids and purinergic signaling in synapse formation and refinement and synapse activity in the hippocampus. Therefore, in the present study, we have used enzyme histochemistry and expression analysis to obtain data on spatial distribution and expression of ecto-enzymes NTPDase1, NTPDase2, and ecto-5'-nucleotidase (eN) after removal of ovaries (OVX) and estradiol replacement (E2) in female rat hippocampus. The results show that target ectonucleotidases are predominantly localized in synapse-rich hippocampal layers. The most represented NTPDase in the hippocampal tissue is NTPDase2, being at the same time the mostly affected ectonucleotidase by OVX and E2. Specifically, OVX decreases the expression of NTPDase2 and eN, whereas E2 restores their expression to control level. Impact of OVX and E2 on ectonucleotidase expression was also examined in purified synaptosome (SYN) and gliosome (GLIO) fractions. Data reveal that SYN expresses NTPDase1 and NTPDase2, both of which are reduced following OVX and restored with E2. GLIO exhibits NTPDase2-mediated ATP hydrolysis, which falls in OVX, and recovers by E2. These changes in the activity occur without parallel changes in NTPDase2-protein abundance. The same holds for eN. The lack of correlation between NTPDase2 and eN activities and their respective protein abundances suggest a non-genomic mode of E2 action, which is studied further in primary astrocyte culture. Since ovarian steroids shape hippocampal synaptic networks and regulate ectonucleotidase activities, it is possible that cognitive deficits seen after ovary removal may arise from the loss of E2 modulatory actions on ectonucleotidase expression in the hippocampus.

Disruption of the Microglial ADP Receptor P2Y13 Enhances Adult Hippocampal Neurogenesis

In mammalian species, including humans, the hippocampal dentate gyrus (DG) is a primary region of adult neurogenesis. Aberrant adult hippocampal neurogenesis is associated with neurological pathologies. Understanding the cellular mechanisms controlling adult hippocampal neurogenesis is expected to open new therapeutic strategies for mental disorders. Microglia is intimately associated with neural progenitor cells in the hippocampal DG and has been implicated, under varying experimental conditions, in the control of the proliferation, differentiation and survival of neural precursor cells. But the underlying mechanisms remain poorly defined. Using fluorescent in situ hybridization we show that microglia in brain express the ADP-activated P2Y₁₃ receptor under basal conditions and that P2ry13 mRNA is absent from neurons, astrocytes, and neural progenitor cells. Disrupting P2ry13 decreases structural complexity of microglia in the hippocampal subgranular zone (SGZ). But it increases progenitor cell proliferation and new neuron formation. Our data suggest that P2Y₁₃ receptor-activated microglia constitutively attenuate hippocampal neurogenesis. This identifies a signaling pathway whereby microglia, via a nucleotide-mediated mechanism, contribute to the homeostatic control of adult hippocampal neurogenesis. Selective P2Y₁₃R antagonists could boost neurogenesis in pathological conditions associated with impaired hippocampal neurogenesis.

Down-regulation of NTPDase2 and ADP-sensitive P2 Purinoceptors Correlate with Severity of Symptoms during Experimental Autoimmune Encephalomyelitis

The present study explores tissue and cellular distribution of ectonucleoside triphosphate diphosphohydrolase 2 (NTPDase2) and the gene and protein expression in rat spinal cord during the course of experimental autoimmune encephalomyelitis (EAE). Given that NTPDase2 hydrolyzes ATP with a transient accumulation of ADP, the expression of ADP-sensitive P2 purinoceptors was analyzed as well. The autoimmune disease was actively induced in Dark Agouti female rats and the changes were analyzed 10, 15 and 29 days after the induction. These selected time points correspond to the onset ( Eo ), peak ( Ep ) and recovery ( Er ) from EAE. In control animals, NTPDase2 was confined in the white matter, in most of the glial fibrillary acidic protein (GFAP)-immunoreactive (ir) astrocytes and in a considerable number of nestin-ir cells, while the other cell types were immunonegative. Immunoreactivity corresponding to NTPDase2 decreased significantly at Eo and Ep and then returned to the baseline levels at Er . The preservation of the proportion of GFAP single-labeled and GFAP/NTPDase2 double-labeled elements along the course of EAE indicated that changes in NTPDase2-ir occurred at fibrous astrocytes that typically express NTPDase2 in normal conditions. Significant downregulation of P2Y₁ and P2Y₁₂ receptor proteins at Eo and several-fold induction of P2Y₁₂ and P2Y₁₃ receptor proteins at Ep and/or Er were observed implying that the pathophysiological process in EAE may be linked to ADP signaling. Cell-surface expression of NTPDase2, NTPDase1/CD39 and ecto-5'-nucleotidase (eN/CD73) was analyzed in CD4⁺ T cells of a draining lymph node by fluorescence-activated cell sorting. The induction of EAE was associated with a transient decrease in a number of CD4⁺ NTPDase2⁺ T cells in a draining lymph node, whereas the recovery was characterized by an increase in NTPDase2⁺ cells in both CD4⁺ and CD4^- cell populations. The opposite was found for NTPDase1/CD39⁺ and eN/CD73⁺ cells, which slightly increased in number with progression of the disease, particularly in CD4^- cells, and then decreased in the recovery. Finally, CD4⁺ NTPDase2⁺ cells were never observed in the spinal cord parenchyma. Taken together, our results suggest that the process of neuroinflammation in EAE may be associated with altered ADP signaling.

Intertwining extracellular nucleotides and their receptors with Ca2+ in determining adult neural stem cell survival, proliferation and final fate

In the central nervous system (CNS), during both brain and spinal cord development, purinergic and pyrimidinergic signalling molecules (ATP, UTP and adenosine) act synergistically with peptidic growth factors in regulating the synchronized proliferation and final specification of multipotent neural stem cells (NSCs) to neurons, astrocytes or oligodendrocytes, the myelin-forming cells. Some NSCs still persist throughout adulthood in both specific 'neurogenic' areas and in brain and spinal cord parenchyma, retaining the potentiality to generate all the three main types of adult CNS cells. Once CNS anatomical structures are defined, purinergic molecules participate in calcium-dependent neuron-to-glia communication and also control the behaviour of adult NSCs. After development, some purinergic mechanisms are silenced, but can be resumed after injury, suggesting a role for purinergic signalling in regeneration and self-repair also via the reactivation of adult NSCs. In this respect, at least three different types of adult NSCs participate in the response of the adult brain and spinal cord to insults: stem-like cells residing in classical neurogenic niches, in particular, in the ventricular-subventricular zone (V-SVZ), parenchymal oligodendrocyte precursor cells (OPCs, also known as NG2-glia) and parenchymal injury-activated astrocytes (reactive astrocytes). Here, we shall review and discuss the purinergic regulation of these three main adult NSCs, with particular focus on how and to what extent modulation of intracellular calcium levels by purinoceptors is mandatory to determine their survival, proliferation and final fate.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Regulation of adult neural progenitor cell functions by purinergic signaling

Extracellular purines are signaling molecules in the neurogenic niches of the brain and spinal cord, where they activate cell surface purinoceptors at embryonic neural stem cells (NSCs) and adult neural progenitor cells (NPCs). Although mRNA and protein are expressed at NSCs/NPCs for almost all subtypes of the nucleotide-sensitive P2X/P2Y, and the nucleoside-sensitive adenosine receptors, only a few of those have acquired functional significance. ATP is sequentially degraded by ecto-nucleotidases to ADP, AMP, and adenosine with agonistic properties for distinct receptor-classes. Nucleotides/nucleosides facilitate or inhibit NSC/NPC proliferation, migration and differentiation. The most ubiquitous effect of all agonists (especially of ATP and ADP) appears to be the facilitation of cell proliferation, usually through P2Y1Rs and sometimes through P2X7Rs. However, usually P2X7R activation causes necrosis/apoptosis of NPCs. Differentiation can be initiated by P2Y2R-activation or P2X7R-blockade. A key element in the transduction mechanism of either receptor is the increase of the intracellular free Ca²⁺ concentration, which may arise due to its release from intracellular storage sites (G protein-coupling; P2Y) or due to its passage through the receptor-channel itself from the extracellular space (ATP-gated ion channel; P2X). Further research is needed to clarify how purinergic signaling controls NSC/NPC fate and how the balance between the quiescent and activated states is established with fine and dynamic regulation. GLIA 2017;65:213-230.

Purine nucleosides in neuroregeneration and neuroprotection

In the present review, we stress the importance of the purine nucleosides, adenosine and guanosine, in protecting the nervous system, both centrally and peripherally, via activation of their receptors and intracellular signalling mechanisms. A most novel part of the review focus on the mechanisms of neuronal regeneration that are targeted by nucleosides, including a recently identified action of adenosine on axonal growth and microtubule dynamics. Discussion on the role of the purine nucleosides transversally with the most established neurotrophic factors, e.g. brain derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), is also focused considering the intimate relationship between some adenosine receptors, as is the case of the A2A receptors, and receptors for neurotrophins. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Purines in neurite growth and astroglia activation

The mammalian nervous system is a complex, functional network of neurons, consisting of local and long-range connections. Neuronal growth is highly coordinated by a variety of extracellular and intracellular signaling molecules. Purines turned out to be an essential component of these processes. Here, we review the current knowledge about the involvement of purinergic signaling in the regulation of neuronal development. We particularly focus on its role in neuritogenesis: the formation and extension of neurites. In the course of maturation mammals generally lose their ability to regenerate the central nervous system (CNS) e.g. after traumatic brain injury; although, spontaneous regeneration still occurs in the peripheral nervous system (PNS). Thus, it is crucial to translate the knowledge about CNS development and PNS regeneration into novel approaches to enable neurons of the mature CNS to regenerate. In this context we give a general overview of growth-inhibitory and growth-stimulatory factors and mechanisms involved in neurite growth. With regard to neuronal growth, astrocytes are an important cell population. They provide structural and metabolic support to neurons and actively participate in brain signaling. Astrocytes respond to injury with beneficial or detrimental reactions with regard to axonal growth. In this review we present the current knowledge of purines in these glial functions. Moreover, we discuss organotypic brain slice co-cultures as a model which retains neuron-glia interactions, and further presents at once a model for CNS development and regeneration. In summary, the purinergic system is a pivotal factor in neuronal development and in the response to injury. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

NTPDase2 and purinergic signaling control progenitor cell proliferation in neurogenic niches of the adult mouse brain

Nerve cells are continuously generated from stem cells in the adult mammalian subventricular zone (SVZ) and hippocampal dentate gyrus. We have previously noted that stem/progenitor cells in the SVZ and the subgranular layer (SGL) of the dentate gyrus express high levels of plasma membrane-bound nucleoside triphosphate diphosphohydrolase 2 (NTPDase2), an ectoenzyme that hydrolyzes extracellular nucleoside diphosphates and triphosphates. We inferred that deletion of NTPDase2 would increase local extracellular nucleoside triphosphate concentrations perturbing purinergic signaling and boosting progenitor cell proliferation and neurogenesis. Using newly generated mice globally null for Entpd2, we demonstrate that NTPDase2 is the major ectonucleotidase in these progenitor cell-rich areas. Using BrdU-labeling protocols, we have measured stem cell proliferation and determined long-term survival of cell progeny under basal conditions. Brains of Entpd2 null mice revealed increased progenitor cell proliferation in both the SVZ and the SGL. However, this occurred without noteworthy alterations in long-term progeny survival. The hippocampal stem cell pool and the pool of the intermediate progenitor type-2 cells clearly expanded. However, substantive proportions of these proliferating cells were lost during expansion at around type-3 stage. Cell loss was paralleled by decreases in cAMP response element-binding protein phosphorylation in the doublecortin-positive progenitor cell population and by an increase in labeling for activated caspase-3 levels. We propose that NTPDase2 has functionality in scavenging mitogenic extracellular nucleoside triphosphates in neurogenic niches of the adult brain, thereby acting as a homeostatic regulator of nucleotide-mediated neural progenitor cell proliferation and expansion.

Nucleotides in neuroregeneration and neuroprotection

Brain injury generates the release of a multitude of factors including extracellular nucleotides, which exhibit bi-functional properties and contribute to both detrimental actions in the acute phase and also protective and reparative actions in the later recovery phase to allow neuroregeneration. A promising strategy toward restoration of neuronal function is based on activation of endogenous adult neural stem/progenitor cells. The implication of purinergic signaling in stem cell biology, including regulation of proliferation, differentiation, and cell death has become evident in the last decade. In this regard, current strategies of acute transplantation of ependymal stem/progenitor cells after spinal cord injury restore altered expression of P2X4 and P2X7 receptors and improve functional locomotor recovery. The expression of both receptors is transcriptionally regulated by Sp1 factor, which plays a key role in the startup of the transcription machinery to induce regeneration-associated genes expression. Finally, general signaling pathways triggered by nucleotide receptors in neuronal populations converge on several intracellular kinases, such as PI3K/Akt, GSK3 and ERK1,2, as well as the Nrf-2/heme oxigenase-1 axis, which specifically link them to neuroprotection. In this regard, regulation of dual specificity protein phosphatases can become novel mechanism of actions for nucleotide receptors that associate them to cell homeostasis regulation. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Mesenchymal stem cells from different murine tissues have differential capacity to metabolize extracellular nucleotides

Mesenchymal stem cells (MSCs) have shown a great potential for cell-based therapy and many different therapeutic purposes. Despite the recent advances in the knowledge of MSCs biology, their biochemical and molecular properties are still poorly defined. Ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases) and ecto-5'-nucleotidase (eNT/CD73) are widely expressed enzymes that hydrolyze extracellular nucleotides, generating an important cellular signaling cascade. Currently, studies have evidenced the relationship between the purinergic system and the development, maintenance, and differentiation of stem cells. The objective of this study is to identify the NTPDases and eNT/CD73 and compare the levels of nucleotide hydrolysis on MSCs isolated from different murine tissues (bone marrow, lung, vena cava, kidney, pancreas, spleen, skin, and adipose tissue). MSCs from all tissues investigated expressed the ectoenzymes at different levels. In MSCs from pancreas and adipose tissue, the hydrolysis of triphosphonucleosides was significantly higher when compared to the other cells. The diphosphonucleosides were hydrolyzed at a higher rate by MSC from pancreas when compared to MSC from other tissues. The differential nucleotide hydrolysis activity and enzyme expression in these cells suggests that MSCs play different roles in regulating the purinergic system in these tissues. Overall MSCs are an attractive adult-derived cell population for therapies, however, the fact that ecto-nucleotide metabolism can affect the microenvironment, modulating important events, such as immune response, makes the assessment of this metabolism an important part of the characterization of MSCs to be applied therapeutically.

Expression of ectonucleotidases in the prosencephalon of melatonin-proficient C3H and melatonin-deficient C57Bl mice: spatial distribution and time-dependent changes

Extracellular purines (ATP, ADP, AMP and adenosine) are important signaling molecules in the CNS. Levels of extracellular purines are regulated by enzymes located at the cell surface referred to as ectonucleotidases. Time-dependent changes in their expression could profoundly influence the availability of extracellular purines and thereby purinergic signaling. Using radioactive in situ hybridization, we analyzed the mRNA distribution of the enzymes NTPDase1, -2 and -3 and ecto-5'-nucleotidase in the prosencephalon of two mouse strains: melatonin-proficient C3H and melatonin-deficient C57Bl. The mRNAs of these enzymes were localized to specific brain regions, such as hippocampus, striatum, medial habenula and ventromedial hypothalamus. NTPDase3 expression was more widely distributed than previously thought. All ectonucleotidases investigated revealed a prominent time-dependent expression pattern. In C3H, the mRNA expression of all four enzymes gradually increased during the day and peaked during the night. In contrast, in C57Bl, ecto-5'-nucleotidase expression peaked at the beginning of the day and gradually decreased to trough levels at night. Recording of locomotor activity revealed higher daytime activity of C57Bl than of C3H. Our results indicate that the expression of ectonucleotidases varies according to time and genotype and suggest that melatonin exerts modulatory effects associated with different regulations of purinergic signaling in the brain. These findings provide an important basis for further examination of the complexity of the purinergic system in the brain.

The interactions of TGF-beta signalling pathway and Jagged2/Notch1 pathway induce acanthosis in lingual epithelia

The aims of this study were to distinguish between the primary and secondary effects of TGF-β signalling disruption by Dox treatment in NTPDase2+ cells; and to investigate the interactions between TGF-β signalling and Jagged2/Notch1 pathway in regulating the expansion of tongue epithelia stem cells.Transgenic mice expressing rtTA from the mouse NTPDase2 promoter or K14 promoter were used to generate an inducible dominant negative TGF-β receptor type II (Tgfbr2) mutant model.Disruption of TGF-β signalling in NTPDase2+ cells initially inhibited the formation of filiform papillae but led to their regeneration over time. In contrast, disruption of TGF-β signalling induced proliferation of lingual epithelia in the middle tongue. We also observed the proliferation of lingual epithelia in the posterior tongue near the circumvallate papillae. Interactions among the TGF-β signalling pathways, Jagged2/Notch1 signalling pathways and epigenetic modifications regulate the expansion of lingual epithelial stem cells. Different molecular mechanisms are involved in the developmental regulation of lingual epithelia and filiform papillae, dependent on the location along the whole tongue. The fluctuating phenotype of tongue epithelia, over time, may be the combined effects of signalling pathways and epigenetic modifications.

Purinergic Receptors in Spinal Cord-Derived Ependymal Stem/Progenitor Cells and Their Potential Role in Cell-Based Therapy for Spinal Cord Injury

Spinal cord injury (SCI) is a major cause of paralysis with no current therapies. Following SCI, large amounts of ATP and other nucleotides are released by the traumatized tissue leading to the activation of purinergic receptors that, in coordination with growth factors, induce lesion remodeling and repair. We found that adult mammalian ependymal spinal cord-derived stem/progenitor cells (epSPCs) are capable of responding to ATP and other nucleotidic compounds, mainly through the activation of the ionotropic P2X4, P2X7, and the metabotropic P2Y1 and P2Y4 purinergic receptors. A comparative study between epSPCs from healthy rats versus epSPCis, obtained after SCI, shows a downregulation of P2Y1 receptor together with an upregulation of P2Y4 receptor in epSPCis. Moreover, spinal cord after severe traumatic contusion shows early and persistent increases in the expression of P2X4 and P2X7 receptors around the injury, which are completely reversed when epSPCis were ectopically transplanted. Since epSPCi transplantation significantly rescues neurological function after SCI in parallel to inhibition of the induced P2 ionotropic receptors, a potential avenue is open for therapeutic alternatives in SCI treatments based on purinergic receptors and the endogenous reparative modulation.

Crystal structure of NTPDase2 in complex with the sulfoanthraquinone inhibitor PSB-071

In many vertebrate tissues CD39-like ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) act in concert with ecto-5'-nucleotidase (e5NT, CD73) to convert extracellular ATP to adenosine. Extracellular ATP is a cytotoxic, pro-inflammatory signalling molecule whereas its product adenosine constitutes a universal and potent immune suppressor. Interference with these ectonucleotidases by use of small molecule inhibitors or inhibitory antibodies appears to be an effective strategy to enhance anti-tumour immunity and suppress neoangiogenesis. Here we present the first crystal structures of an NTPDase catalytic ectodomain in complex with the Reactive Blue 2 (RB2)-derived inhibitor PSB-071. In both of the two crystal forms presented the inhibitor binds as a sandwich of two molecules at the nucleoside binding site. One of the molecules is well defined in its orientation. Specific hydrogen bonds are formed between the sulfonyl group and the nucleoside binding loop. The methylphenyl side chain functionality that improved NTPDase2-specificity is sandwiched between R245 and R394, the latter of which is exclusively found in NTPDase2. The second molecule exhibits great in-plane rotational freedom and could not be modelled in a specific orientation. In addition to this structural insight into NTPDase inhibition, the observation of the putative membrane interaction loop (MIL) in two different conformations related by a 10° rotation identifies the MIL as a dynamic section of NTPDases that is potentially involved in regulation of catalysis.

Neuronal and glial purinergic receptors functions in neuron development and brain disease

Brain development requires the interaction of complex signaling pathways, involving different cell types and molecules. For a long time, most attention has focused on neurons in a neuronocentric conceptualization of central nervous system development, these cells fulfilling an intrinsic program that establishes the brain’s morphology and function. By contrast, glia have mainly been studied as support cells, offering guidance or as the cells that react to brain injury. However, new evidence is appearing that demonstrates a more fundamental role of glial cells in the control of different aspects of neuronal development and function, events in which the influence of neurons is at best weak. Moreover, it is becoming clear that the function and organization of the nervous system depends heavily on reciprocal neuron–glia interactions. During development, neurons are often generated far from their final destination and while intrinsic mechanisms are responsible for neuronal migration and growth, they need support and regulatory influences from glial cells in order to migrate correctly. Similarly, the axons emitted by neurons often have to reach faraway targets and in this sense, glia help define the way that axons grow. Moreover, oligodendrocytes and Schwann cells ultimately envelop axons, contributing to the generation of nodes of Ranvier. Finally, recent publications show that astrocytes contribute to the modulation of synaptic transmission. In this sense, purinergic receptors are expressed widely by glial cells and neurons, and recent evidence points to multiple roles of purines and purinergic receptors in neuronal development and function, from neurogenesis to axon growth and functional axonal maturation, as well as in pathological conditions in the brain. This review will focus on the role of glial and neuronal secreted purines, and on the purinergic receptors, fundamentally in the control of neuronal development and function, as well as in diseases of the nervous system.

Neurochemical Changes in the Mouse Hippocampus Underlying the Antidepressant Effect of Genetic Deletion of P2X7 Receptors

Recent investigations have revealed that the genetic deletion of P2X7 receptors (P2rx7) results in an antidepressant phenotype in mice. However, the link between the deficiency of P2rx7 and changes in behavior has not yet been explored. In the present study, we studied the effect of genetic deletion of P2rx7 on neurochemical changes in the hippocampus that might underlie the antidepressant phenotype. P2X7 receptor deficient mice (P2rx7−/−) displayed decreased immobility in the tail suspension test (TST) and an attenuated anhedonia response in the sucrose preference test (SPT) following bacterial endotoxin (LPS) challenge. The attenuated anhedonia was reproduced through systemic treatments with P2rx7 antagonists. The activation of P2rx7 resulted in the concentration-dependent release of [³H]glutamate in P2rx7+/+ but not P2rx7−/− mice, and the NR2B subunit mRNA and protein was upregulated in the hippocampus of P2rx7−/− mice. The brain-derived neurotrophic factor (BDNF) expression was higher in saline but not LPS-treated P2rx7−/− mice; the P2rx7 antagonist Brilliant blue G elevated and the P2rx7 agonist benzoylbenzoyl ATP (BzATP) reduced BDNF level. This effect was dependent on the activation of NMDA and non-NMDA receptors but not on Group I metabotropic glutamate receptors (mGluR_1,5). An increased 5-bromo-2-deoxyuridine (BrdU) incorporation was also observed in the dentate gyrus derived from P2rx7−/− mice. Basal level of 5-HT was increased, whereas the 5HIAA/5-HT ratio was lower in the hippocampus of P2rx7−/− mice, which accompanied the increased uptake of [³H]5-HT and an elevated number of [³H]citalopram binding sites. The LPS-induced elevation of 5-HT level was absent in P2rx7−/− mice. In conclusion there are several potential mechanisms for the antidepressant phenotype of P2rx7−/− mice, such as the absence of P2rx7-mediated glutamate release, elevated basal BDNF production, enhanced neurogenesis and increased 5-HT bioavailability in the hippocampus.

P2X7 receptors at adult neural progenitor cells of the mouse subventricular zone

Neurogenesis requires the balance between the proliferation of newly formed progenitor cells and subsequent death of surplus cells. RT-PCR and immunocytochemistry demonstrated the presence of P2X7 receptor mRNA and immunoreactivity in cultured neural progenitor cells (NPCs) prepared from the adult mouse subventricular zone (SVZ). Whole-cell patch-clamp recordings showed a marked potentiation of the inward current responses both to ATP and the prototypic P2X7 receptor agonist dibenzoyl-ATP (Bz-ATP) at low Ca(2+) and zero Mg(2+) concentrations in the bath medium. The Bz-ATP-induced currents reversed their polarity near 0 mV; in NPCs prepared from P2X7(-/-) mice, Bz-ATP failed to elicit membrane currents. The general P2X/P2Y receptor antagonist PPADS and the P2X7 selective antagonists Brilliant Blue G and A-438079 strongly depressed the effect of Bz-ATP. Long-lasting application of Bz-ATP induced an initial current, which slowly increased to a steady-state response. In combination with the determination of YO-PRO uptake, these experiments suggest the dilation of a receptor-channel and/or the recruitment of a dye-uptake pathway. Ca(2+)-imaging by means of Fura-2 revealed that in a Mg(2+)-deficient bath medium Bz-ATP causes [Ca(2+)](i) transients fully depending on the presence of external Ca(2+). The MTT test indicated a concentration-dependent decrease in cell viability by Bz-ATP treatment. Correspondingly, Bz-ATP led to an increase in active caspase 3 immunoreactivity, indicating a P2X7-controlled apoptosis. In acute SVZ brain slices of transgenic Tg(nestin/EGFP) mice, patch-clamp recordings identified P2X7 receptors at NPCs with pharmacological properties identical to those of their cultured counterparts. We suggest that the apoptotic/necrotic P2X7 receptors at NPCs may be of particular relevance during pathological conditions which lead to increased ATP release and thus could counterbalance the ensuing excessive cell proliferation.

Implications of purinergic receptor-mediated intracellular calcium transients in neural differentiation

Purinergic receptors participate, in almost every cell type, in controlling metabolic activities and many physiological functions including signal transmission, proliferation and differentiation. While most of P2Y receptors induce transient elevations of intracellular calcium concentration by activation of intracellular calcium pools and forward these signals as waves which can also be transmitted into neighboring cells, P2X receptors produce calcium spikes which also include activation of voltage-operating calcium channels. P2Y and P2X receptors induce calcium transients that activate transcription factors responsible for the progress of differentiation through mediators including calmodulin and calcineurin. Expression of P2X2 as well as of P2X7 receptors increases in differentiating neurons and glial cells, respectively. Gene expression silencing assays indicate that these receptors are important for the progress of differentiation and neuronal or glial fate determination. Metabotropic receptors, mostly P2Y1 and P2Y2 subtypes, act on embryonic cells or cells at the neural progenitor stage by inducing proliferation as well as by regulation of neural differentiation through NFAT translocation. The scope of this review is to discuss the roles of purinergic receptor-induced calcium spike and wave activity and its codification in neurodevelopmental and neurodifferentiation processes.

Purinergic signaling in glioma progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X(7), is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

A ketogenic diet did not prevent effects on the ectonucleotidases pathway promoted by lithium-pilocarpine-induced status epilepticus in rat hippocampus

A Ketogenic Diet (KD) mimics the anticonvulsant effects of fasting, which are known to suppress seizures. The purinergic system has been investigated in the matter of epilepsy development, especially the nucleoside adenosine, which has been considered a natural brain anticonvulsant. During epileptic seizures, extracellular adenosine concentration rises rapidly to micromolar levels. Adenosine can exert its anticonvulsant functions, after its release by nucleoside bidirectional transport, or by production through the sequential catabolism of ATP by ectonucleotidases, such as E-NTPDases (ectonucleoside triphosphate diphosphohydrolases) and ecto-5'-nucleotidase. Here, we have investigated the effect of a ketogenic diet on the nucleotide hydrolysis and NTPDases expression in the lithium-pilocarpine (Li-Pilo) model of epilepsy. For the induction of Status Epileticus (SE), 21-day-old female Wistar rats received an i.p. injection of lithium chloride (127 mg/kg) and 18-19 h later an i.p. injection of pilocarpine hydrochloride (60 mg/kg). The control groups received an injection of saline. After induction of SE, the control and Li-Pilo groups received standard or ketogenic diets for 6 weeks. The lithium-pilocarpine exposure affected the ATP (a decrease of between 8 % and 16 %) and ADP (an increase of between 18 % and 22 %) hydrolysis in both groups whereas the diet did not impact the nucleotide hydrolysis. NTPDase2 and 3 mRNA expressions decreased in the Li-Pilo group (41 % and 42 %). This data highlights the participation of the purinergic system in the pathophysiology of this model of epilepsy, since nucleotide hydrolysis and NTPDase expressions were altered by Li-Pilo exposure, with no significant effects of the ketogenic diet. However, the interaction between purinergic signaling and a ketogenic diet on epilepsy still needs to be better elucidated.

NTPDase2+ Cells Generate Lingual Epithelia and Papillae

The tongue epithelium is one of the most rapidly self-renewing tissues in adult mammals. Multiple stem cell populations are currently believed to exist in tongue epithelia. Keratin 14 (K14) positive cells differentiate into either lingual epithelia or lingual papillae, while ecto-nucleoside triphosphate diphosphohydrolase 2 (NTPDase2) is associated with neural stem cells and astrocyte-like cells ensheathing the migrating neuroblasts. Here, using a transgenic mouse expressing rtTA from the mouse NTPDase2 promoter, we generated an inducible model by treatment with Doxycycline. By immunohistochemical analysis and in situ hybridization, we found exclusive expression of NTPDase2 in lingual epithelia and lingual papillae. Using inducible genetic cell fate mapping, we further showed that the NTPDase2+ cells generated lingual papillae and epithelia in the adult tongue. Finally, building on our previously proposed paradigm of cell migration stream, a model is further described here for lingual epithelia cell genesis. In short, the current results not only extend our understanding of the cell migration stream in lingual epithelia and lingual papillae, but they also support the concept of multiple stem cell populations in lingual epithelia and papillae.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

The medial habenula contains a specific nonstellate subtype of astrocyte expressing the ectonucleotidase NTPDase2

ATP-mediated synaptic transmission represents the only transmitter-gated Ca(2+)-entry pathway in neurons of the rodent medial habenula. In addition to direct purinergic receptor-mediated synaptic inputs, the medial habenula contains purinergic systems that modulate synaptic transmission. Purinergic signaling is modulated or terminated by ectonucleotidase, nucleotide-hydrolyzing enzymes of the cell surface. Here we identify the major ectonucleotidase responsible for the hydrolysis of extracellular ATP in the mouse medial habenula as ectonucleoside triphosphate diphosphohydrolase 2 (NTPDase2), using immunostaining and enzyme histochemistry. Double labeling experiments reveal that the enzyme is expressed by astrocytes enwrapping the densely packed neurons and also the myelinated fiber bundles of the stria medullaris. NTPDase2 immunoreactivity is absent from the lateral habenula. The analysis of mice expressing enhanced green fluorescent protein under the promoter of glial fibrillary acidic protein revealed that the medial habenula harbors a highly polar type of astrocytes with very long laminar cellular processes, untypical for grey matter astrocytes. Its morphology strongly differs from that of the stellate astrocytes in the adjacent lateral habenula. Our results suggest that the mouse medial habenula contains a specific perineuronal nonstellate subtype of astrocyte that expresses the ectonucleotidase NTPDase2 and is in a strategic position to modulate purinergic transmission in this subnucleus.

G-protein-coupled receptors in adult neurogenesis

The importance of adult neurogenesis has only recently been accepted, resulting in a completely new field of investigation within stem cell biology. The regulation and functional significance of adult neurogenesis is currently an area of highly active research. G-protein-coupled receptors (GPCRs) have emerged as potential modulators of adult neurogenesis. GPCRs represent a class of proteins with significant clinical importance, because approximately 30% of all modern therapeutic treatments target these receptors. GPCRs bind to a large class of neurotransmitters and neuromodulators such as norepinephrine, dopamine, and serotonin. Besides their typical role in cellular communication, GPCRs are expressed on adult neural stem cells and their progenitors that relay specific signals to regulate the neurogenic process. This review summarizes the field of adult neurogenesis and its methods and specifies the roles of various GPCRs and their signal transduction pathways that are involved in the regulation of adult neural stem cells and their progenitors. Current evidence supporting adult neurogenesis as a model for self-repair in neuropathologic conditions, adult neural stem cell therapeutic strategies, and potential avenues for GPCR-based therapeutics are also discussed.

Expression of ecto-ATPase NTPDase2 in human dental pulp

Dental pulpal nerve fibers express ionotropic adenosine triphosphate (ATP) receptors, suggesting that ATP signaling participates in the process of dental nociception. In this study, we investigated if the principal enzymes responsible for extracellular ATP hydrolysis, namely, nucleoside triphosphate diphosphohydrolases (NTPDases), are present in human dental pulp. Immunohistochemical and immunofluorescence experiments showed that NTPDase2 was predominantly expressed in pulpal nerve bundles, Raschkow's nerve plexus, and in the odontoblast layer. NTPDase2 was expressed in pulpal Schwann cells, with processes accompanying the nerve fibers and projecting into the odontoblast layer. Odontoblasts expressed the gap junction protein, connexin43, which can form transmembrane hemichannels for ATP release. NTPDase2 was localized close to connexin43 within the odontoblast layer. These findings provide evidence for the existence of an apparatus for ATP release and degradation in human dental pulp, consistent with the involvement of ATP signaling in the process of dentin sensitivity and dental pain.

Perspectives of purinergic signaling in stem cell differentiation and tissue regeneration

Replacement of lost or dysfunctional tissues by stem cells has recently raised many investigations on therapeutic applications. Purinergic signaling has been shown to regulate proliferation, differentiation, cell death, and successful engraftment of stem cells originated from diverse origins. Adenosine triphosphate release occurs in a controlled way by exocytosis, transporters, and lysosomes or in large amounts from damaged cells, which is then subsequently degraded into adenosine. Paracrine and autocrine mechanisms induced by immune responses present critical factors for the success of stem cell therapy. While P1 receptors generally exert beneficial effects including anti-inflammatory activity, P2 receptor-mediated actions depend on the subtype of stimulated receptors and localization of tissue repair. Pro-inflammatory actions and excitatory tissue damages mainly result from P2X7 receptor activation, while other purinergic receptor subtypes participate in proliferation and differentiation, thereby providing adequate niches for stem cell engraftment and novel mechanisms for cell therapy and endogenous tissue repair. Therapeutic applications based on regulation of purinergic signaling are foreseen for kidney and heart muscle regeneration, Clara-like cell replacement for pulmonary and bronchial epithelial cells as well as for induction of neurogenesis in case of neurodegenerative diseases.

Oxygen-glucose deprivation increases the enzymatic activity and the microvesicle-mediated release of ectonucleotidases in the cells composing the blood-brain barrier

The blood-brain barrier (BBB), the dynamic interface between the nervous tissue and the blood, is composed by endothelial cells, pericytes and astrocytes. Extracellular nucleotides and nucleosides and their receptors (the purinergic system) constitute a widely diffused signaling system involved in many pathophysiological processes. However, the role of this system in controlling BBB functions is still largely unknown. By using cultures of these three cell types grown separately and a BBB in vitro model consisting of triple co-cultures, we studied for the first time the expression and distribution of the ecto-enzymes nucleoside triphosphate diphosphohydrolases (NTPDases, the enzymes which hydrolyze extracellular nucleotides) under control and ischemic (oxygen-glucose deprivation in vitro; OGD) conditions. NTPDase1 was detected in all three cell types, whereas NTPDase2 was expressed by astrocytes and pericytes and, to a lesser extent, by endothelial cells. Endothelial cells were extremely susceptible to cell death when OGD was applied to mimic in vitro the cytotoxicity induced by ischemia, whereas astrocytes and pericytes were more resistant. A semi-quantitative assay highlighted markedly increased e-ATPase activity following exposure to OGD in all three cell types, either when grown separately or when co-cultured together to resemble the composition of the BBB. Moreover, electron microscopy analysis showed that both endothelial cells and astrocytes shed microvesicles containing NTPDases from their membrane, which may suggest a novel mechanism to increase the breakdown of ATP released to toxic levels by damaged BBB cells. We hypothesize that this phenomenon could have a protective and/or modulatory effect for brain parenchymal cells. This in vitro model is therefore useful to study the role of extracellular nucleotides in modulating BBB responses to ischemic events, and to develop new effective purinergic-based approaches for brain ischemia.

Purinergic signaling in embryonic and stem cell development

Nucleotides are of crucial importance as carriers of energy in all organisms. However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted. Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals. Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells. Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system. In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells. Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors. Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.

Adenosine receptors and brain diseases: neuroprotection and neurodegeneration

Adenosine acts in parallel as a neuromodulator and as a homeostatic modulator in the central nervous system. Its neuromodulatory role relies on a balanced activation of inhibitory A(1) receptors (A1R) and facilitatory A(2A) receptors (A2AR), mostly controlling excitatory glutamatergic synapses: A1R impose a tonic brake on excitatory transmission, whereas A2AR are selectively engaged to promote synaptic plasticity phenomena. This neuromodulatory role of adenosine is strikingly similar to the role of adenosine in the control of brain disorders; thus, A1R mostly act as a hurdle that needs to be overcame to begin neurodegeneration and, accordingly, A1R only effectively control neurodegeneration if activated in the temporal vicinity of brain insults; in contrast, the blockade of A2AR alleviates the long-term burden of brain disorders in different neurodegenerative conditions such as ischemia, epilepsy, Parkinson's or Alzheimer's disease and also seem to afford benefits in some psychiatric conditions. In spite of this qualitative agreement between neuromodulation and neuroprotection by A1R and A2AR, it is still unclear if the role of A1R and A2AR in the control of neuroprotection is mostly due to the control of glutamatergic transmission, or if it is instead due to the different homeostatic roles of these receptors related with the control of metabolism, of neuron-glia communication, of neuroinflammation, of neurogenesis or of the control of action of growth factors. In spite of this current mechanistic uncertainty, it seems evident that targeting adenosine receptors might indeed constitute a novel strategy to control the demise of different neurological and psychiatric disorders.

Purinergic receptor-mediated Ca signaling in the olfactory bulb and the neurogenic area of the lateral ventricles

Like in other vertebrates, the anterior part of the telencephalon of amphibians mainly consists of the olfactory bulb (OB), but different from higher vertebrates, the lateral telencephalic ventricles of larval Xenopus laevis expand deep into the anterior telencephalon. The neurogenic periventricular zone (PVZ) of the lateral ventricles generates new OB neurons throughout the animal's lifetime. We investigated the ultrastructural organization of the PVZ and found that within a time period of 24 h, 42.54 ± 6.65% of all PVZ cells were actively proliferating. Functional purinergic receptors are widespread in the central nervous system and their activation has been associated with many critical physiological processes, including the regulation of cell proliferation. In the present study we identified and characterized the purinergic system of the OB and the PVZ. ATP and 2MeSATP induced strong [Ca(2+)](i) increases in cells of both regions, which could be attenuated by purinergic antagonists. However, a more thorough pharmacological investigation revealed clear differences between the two brain regions. Cells of the OB almost exclusively express ionotropic P2X purinergic receptor subtypes, whereas PVZ cells express both ionotropic P2X and metabotropic P1 and P2Y receptor subtypes. The P2X receptors expressed in the OB are evidently not involved in the immediate processing of olfactory information.

Phosphorylation of p27Kip1 at Thr187 by cyclin-dependent kinase 5 modulates neural stem cell differentiation

Cdk5 plays a role in nervous system development; its role in the initial stages of neural differentiation is poorly understood. We isolated neural stem cells from E13 Cdk5 WT and KO mouse and observed them as they switched from proliferating stage to neural differentiation. We show that Cdk5 phosphorylation of p27^kip1 at Thr187 is crucial to neural differentiation.

Cyclin-dependent kinase 5 (Cdk5) plays a key role in the development of the mammalian nervous system; it phosphorylates a number of targeted proteins involved in neuronal migration during development to synaptic activity in the mature nervous system. Its role in the initial stages of neuronal commitment and differentiation of neural stem cells (NSCs), however, is poorly understood. In this study, we show that Cdk5 phosphorylation of p27^Kip1 at Thr187 is crucial to neural differentiation because 1) neurogenesis is specifically suppressed by transfection of p27^Kip1 siRNA into Cdk5^+/+ NSCs; 2) reduced neuronal differentiation in Cdk5^−/− compared with Cdk5^+/+ NSCs; 3) Cdk5^+/+ NSCs, whose differentiation is inhibited by a nonphosphorylatable mutant, p27/Thr187A, are rescued by cotransfection of a phosphorylation-mimicking mutant, p27/Thr187D; and 4) transfection of mutant p27^Kip1 (p27/187A) into Cdk5^+/+ NSCs inhibits differentiation. These data suggest that Cdk5 regulates the neural differentiation of NSCs by phosphorylation of p27^Kip1 at theThr187 site. Additional experiments exploring the role of Ser10 phosphorylation by Cdk5 suggest that together with Thr187 phosphorylation, Ser10 phosphorylation by Cdk5 promotes neurite outgrowth as neurons differentiate. Cdk5 phosphorylation of p27^Kip1, a modular molecule, may regulate the progress of neuronal differentiation from cell cycle arrest through differentiation, neurite outgrowth, and migration.

Purinergic signaling regulates cell proliferation of olfactory epithelium progenitors

In the olfactory epithelium (OE) continuous neurogenesis is maintained throughout life. The OE is in direct contact with the external environment, and its cells are constantly exposed to pathogens and noxious substances. To maintain a functional sense of smell the OE has evolved the ability to permanently replenish olfactory receptor neurons and sustentacular cells lost during natural turnover. A cell population residing in the most basal part of the OE, the so-called basal cells (BCs), keep up this highly regulated genesis of new cells. The population of BCs is thought to include both the stem cells of the OE and various progenitor cells. In recent years a number of regulatory factors that positively and/or negatively regulate the proliferation within the OE have been identified, but a thorough comprehension of the complex interplay of these regulatory factors and the role of the different epithelial cell types is still illusive. Combining labeling techniques, immunohistochemistry, electron microscopy, functional calcium imaging, and a bromo-2'-deoxyuridine incorporation assay, we show for the first time that purinergic receptors are expressed in BCs of the OE of larval Xenopus laevis and that nucleotide-induced Ca(2+) signaling in these cells is involved in the regulation of the cell turnover in the OE. Our data contribute to a better understanding of the regulation of the cell turnover in the OE in particular and also of how the proliferation of neuronal progenitor cells is regulated in general.