Extrinsic purinergic regulation of neural stem/progenitor cells: implications for CNS development and repair

Calcium and Neural Stem Cell Proliferation

Intracellular calcium plays a pivotal role in central nervous system (CNS) development by regulating various processes such as cell proliferation, migration, differentiation, and maturation. However, understanding the involvement of calcium (Ca2+) in these processes during CNS development is challenging due to the dynamic nature of this cation and the evolving cell populations during development. While Ca2+ transient patterns have been observed in specific cell processes and molecules responsible for Ca2+ homeostasis have been identified in excitable and non-excitable cells, further research into Ca2+ dynamics and the underlying mechanisms in neural stem cells (NSCs) is required. This review focuses on molecules involved in Ca2+ entrance expressed in NSCs in vivo and in vitro, which are crucial for Ca2+ dynamics and signaling. It also discusses how these molecules might play a key role in balancing cell proliferation for self-renewal or promoting differentiation. These processes are finely regulated in a time-dependent manner throughout brain development, influenced by extrinsic and intrinsic factors that directly or indirectly modulate Ca2+ dynamics. Furthermore, this review addresses the potential implications of understanding Ca2+ dynamics in NSCs for treating neurological disorders. Despite significant progress in this field, unraveling the elements contributing to Ca2+ intracellular dynamics in cell proliferation remains a challenging puzzle that requires further investigation.

Impairment of adenosine signaling disrupts early embryo development: unveiling the underlying mechanisms

Purinergic signaling has been implicated in many biological functions, including development. In this study, we investigate the functions of extracellular adenosine and adenosine receptors using a mouse embryonic stem cell (ESC) line and morula stages isolated from mouse embryos. Feeder-free mouse ESC was investigated in the absence and presence of the leukemia inhibitory factor (LIF), configuring undifferentiated cells and cells undergoing spontaneous differentiation. High alkaline phosphatase (ALPL) and low CD73 levels resulting in low adenosine (eADO) levels were characteristic for pluripotent cells in the presence of the LIF, while LIF deprivation resulted in augmented adenosine levels and reduced pluripotency marker expression, which indicated differentiation. Tracing ESC proliferation by BrdU labeling revealed that the inhibition of ALPL by levamisole resulted in a decrease in proliferation due to less eADO accumulation. Furthermore, caffeine and levamisole treatment, inhibiting adenosine receptor and eADO accumulation, respectively, reduced ESC migration, similar to that observed in the absence of the LIF. Pharmacological approaches of selective adenosine receptor subtype inhibition triggered specific adenosine receptor activities, thus triggering calcium or MAP kinase pathways leading to differentiation. In line with the in vitro data, mouse embryos at the morula stage were sensitive to treatments with A1 and A3 receptor antagonists, leading to the conclusion that A1 receptor and A3 receptor inhibition impairs proliferation and self-renewal and triggers inappropriate differentiation, respectively. The findings herein define the functions of eADO signaling in early development with implications for developmental disorders, in which adenosine receptors or ectonucleotidase dysfunctions are involved, and which could lead to malformations and miscarriages, due to exposure to caffeine.

Altered Purinergic Signaling in Neurodevelopmental Disorders: Focus on P2 Receptors

With the umbrella term 'neurodevelopmental disorders' (NDDs) we refer to a plethora of congenital pathological conditions generally connected with cognitive, social behavior, and sensory/motor alterations. Among the possible causes, gestational and perinatal insults have been demonstrated to interfere with the physiological processes necessary for the proper development of fetal brain cytoarchitecture and functionality. In recent years, several genetic disorders caused by mutations in key enzymes involved in purine metabolism have been associated with autism-like behavioral outcomes. Further analysis revealed dysregulated purine and pyrimidine levels in the biofluids of subjects with other NDDs. Moreover, the pharmacological blockade of specific purinergic pathways reversed the cognitive and behavioral defects caused by maternal immune activation, a validated and now extensively used rodent model for NDDs. Furthermore, Fragile X and Rett syndrome transgenic animal models as well as models of premature birth, have been successfully utilized to investigate purinergic signaling as a potential pharmacological target for these diseases. In this review, we examine results on the role of the P2 receptor signaling in the etiopathogenesis of NDDs. On this basis, we discuss how this evidence could be exploited to develop more receptor-specific ligands for future therapeutic interventions and novel prognostic markers for the early detection of these conditions.

Microwave-Assisted Synthesis, Structure, and Preliminary Biological Evaluation of Novel 6-Methoxy-5,6-dihydro-5-azapurines

We herein disclose the microwave-assisted synthesis of previously unreported 6-methoxy-5,6-dihydro-5-azapurines, whose purine-like scaffold is promising for drug discovery. The method is simple, fast, and relies on easily accessible reagents such as trimethyl orthoformate, acetic acid, and aminotriazole-derived N,N'-disubstituted formamidines. The preliminary biological evaluation revealed that selected representatives of synthesized 6-methoxy-5,6-dihydro-5-azapurines dose-dependently reduce the viability of HepG2 and A549 cancer cells having little to no influence on five tested purinergic receptors.

Effects of overexpression of Hsp70 in neural stem cells on neurotoxicity and cognitive dysfunction in neonatal mice under sevoflurane exposure

As one of the commonly used inhalation anesthetics in clinical practice, sevoflurane is currently widely applied in surgery for children and the elderly due to its safety and efficacy. However, the neurotoxicity and cognitive impairment induced by sevoflurane exposure cannot be ignored. A recombinant adenovirus with green fluorescent protein-labeled heat shock protein 70 (Hsp70) was constructed and used to infect neural stem cells (NSCs) separated from neonatal mice. Quantitative real-time PCR and Western blot assays were used to evaluate the expression of certain genes. 5‑Ethynyl‑2'‑deoxyuridine staining and cell counting kit assay were used to detect the proliferation and differentiation ability of NSCs. The Morris water maze experiment was used to test the cognitive abilities of mice. Adv-Hsp70 induced the overexpression of Hsp70 in mouse NSCs. Upregulation of Hsp70 promoted the proliferation ability and differentiation of mouse NSCs. NSCs that overexpressed Hsp70 attenuated sevoflurane-induced neurotoxicity and protected cognitive dysfunction in mice under sevoflurane exposure. In summary, our findings demonstrate the potential of overexpression of Hsp70 in NSCs against sevoflurane-induced impairments.

Aldh2 is a lineage-specific metabolic gatekeeper in melanocyte stem cells

Melanocyte stem cells (McSCs) in zebrafish serve as an on-demand source of melanocytes during growth and regeneration, but metabolic programs associated with their activation and regenerative processes are not well known. Here, using live imaging coupled with scRNA-sequencing, we discovered that, during regeneration, quiescent McSCs activate a dormant embryonic neural crest transcriptional program followed by an aldehyde dehydrogenase (Aldh) 2 metabolic switch to generate progeny. Unexpectedly, although ALDH2 is well known for its aldehyde-clearing mechanisms, we find that, in regenerating McSCs, Aldh2 activity is required to generate formate – the one-carbon (1C) building block for nucleotide biosynthesis – through formaldehyde metabolism. Consequently, we find that disrupting the 1C cycle with low doses of methotrexate causes melanocyte regeneration defects. In the absence of Aldh2, we find that purines are the metabolic end product sufficient for activated McSCs to generate progeny. Together, our work reveals McSCs undergo a two-step cell state transition during regeneration, and that the reaction products of Aldh2 enzymes have tissue-specific stem cell functions that meet metabolic demands in regeneration.

Summary: In zebrafish melanocyte regeneration, quiescent McSCs respond by re-expressing a neural crest identity, followed by an Aldh2-dependent metabolic switch to generate progeny.

ATP and spontaneous calcium oscillations control neural stem cell fate determination in Huntington's disease: a novel approach for cell clock research

Calcium, the most versatile second messenger, regulates essential biology including crucial cellular events in embryogenesis. We investigated impacts of calcium channels and purinoceptors on neuronal differentiation of normal mouse embryonic stem cells (ESCs), with outcomes being compared to those of in vitro models of Huntington's disease (HD). Intracellular calcium oscillations tracked via real-time fluorescence and luminescence microscopy revealed a significant correlation between calcium transient activity and rhythmic proneuronal transcription factor expression in ESCs stably expressing ASCL-1 or neurogenin-2 promoters fused to luciferase reporter genes. We uncovered that pharmacological manipulation of L-type voltage-gated calcium channels (VGCCs) and purinoceptors induced a two-step process of neuronal differentiation. Specifically, L-type calcium channel-mediated augmentation of spike-like calcium oscillations first promoted stable expression of ASCL-1 in differentiating ESCs, which following P2Y2 purinoceptor activation matured into GABAergic neurons. By contrast, there was neither spike-like calcium oscillations nor responsive P2Y2 receptors in HD-modeling stem cells in vitro. The data shed new light on mechanisms underlying neurogenesis of inhibitory neurons. Moreover, our approach may be tailored to identify pathogenic triggers of other developmental neurological disorders for devising targeted therapies.

Purinergic Receptor Blockade with Suramin Increases Survival of Postnatal Neural Progenitor Cells In Vitro

Neural progenitor cells (NPCs) are self-renewing and multipotent cells that persist in the postnatal and adult brain in the subventricular zone and the hippocampus. NPCs can be expanded in vitro to be used in cell therapy. However, expansion is limited, since the survival and proliferation of adult NPCs decrease with serial passages. Many signaling pathways control NPC survival and renewal. Among these, purinergic receptor activation exerts differential effects on the biology of adult NPCs depending on the cellular context. In this study, we sought to analyze the effect of a general blockade of purinergic receptors with suramin on the proliferation and survival of NPCs isolated from the subventricular zone of postnatal rats, which are cultured as neurospheres. Treatment of neurospheres with suramin induced a significant increase in neurosphere diameter and in NPC number attributed to a decrease in apoptosis. Proliferation and multipotency were not affected. Suramin also induced an increase in the gap junction protein connexin43 and in vascular endothelial growth factor, which might be involved in the anti-apoptotic effect. Our results offer a valuable tool for increasing NPC survival before implantation in the lesioned brain and open the possibility of using this drug as adjunctive therapy to NPC transplantation.

Membrane Transporters and Channels in Melanoma

Recent research has revealed that ion channels and transporters can be important players in tumor development, progression, and therapy resistance in melanoma. For example, members of the ABC family were shown to support cancer stemness-like features in melanoma cells, while several members of the TRP channel family were reported to act as tumor suppressors.Also, many transporter proteins support tumor cell viability and thus suppress apoptosis induction by anticancer therapy. Due to the high number of ion channels and transporters and the resulting high complexity of the field, progress in understanding is often focused on single molecules and is in total rather slow. In this review, we aim at giving an overview about a broad subset of ion transporters, also illustrating some aspects of the field, which have not been addressed in detail in melanoma. In context with the other chapters in this special issue on "Transportome Malfunctions in the Cancer Spectrum," a comparison between melanoma and these tumors will be possible.

Aluminum-induced alterations of purinergic signalling in embryonic neural progenitor cells

The ubiquitous presence of aluminum in the environment leads to a high likelihood of human exposure. Neurotoxicity of the trivalent cationic form of this metal (Al3+) occurs in the central nervous system via accumulation of Al in cells of neural origin, including neural progenitor cells (NPCs). NPCs play a key role in the development and regeneration of the brain throughout life; therefore, this metal may contribute to neuropathological conditions. Here, we evaluated the effects of different Al³⁺ concentrations (0-50 μM) on the purinergic system of NPCs isolated from embryonic telencephalons, cultured as neurospheres. Al³⁺ adhered to the cell surface of neurospheres reducing extracellular ATP release, as well as ATP, ADP, and AMP hydrolysis by NTPDase and 5'-nucleotidase, respectively. In addition, impaired nucleotide release by Al³⁺ reduced P2Y1 and adenosine A2A receptors expression in differentiated neurospheres. These receptors are crucial for NPC proliferation during brain development and self-repair against external stimuli, such as metal exposure. Thus, Al³⁺ represents an environmental agent linked to neurodegeneration through alterations in the ATP-signalling pathway, proving to be a potential mechanism associated with NPC proliferation and brain degeneration.

A Selective P2Y Purinergic Receptor Agonist 2-MesADP Enhances Locomotor Recovery after Acute Spinal Cord Injury

INTRODUCTION

Spinal cord injury (SCI) causes most severe motor and sensory dysfunctions. In Chinese traditional medicine, the agonist of a purinergic receptor is believed to have a positive effect on SCIs, and 2-Methylthio-adenosine-5'-diphosphate (2-MesADP) is a selective agonist of the P2Y purinergic receptor.

METHODS

To investigate its therapeutic function and molecular mechanism in SCI, transcriptome analysis associated with weighted gene co-expression network analysis (WGCNA) was carried out at various time points after T9 crush injury.

RESULTS

2-MesADP demonstrated recovery of limb motor function at the 6 weeks after injury, accompanied by neuronal regeneration and axon remyelination at 2 and 6 weeks. Furthermore, gene profiling revealed alternated gene expression with the treatment of 2-MesADP. These genes were assigned to a total of 38 modules, followed by gene ontology analysis; of these, 18 represented neuronal apoptosis and regeneration, immune response, synaptic transmission, cell cycle, and angiogenesis. In the neuronal apoptosis and regeneration module, Nefh, NeuroD6, and Dcx in the 2-MesADP group were noticed due to their interesting expression pattern. The gene expression patterns of Mag, Mog, and Cnp, which played key roles in myelination, were significantly changed with the treatment of 2-MesADP. Wnt signal pathway was the most important pathway in 2-MesADP treatment for acute SCI.

CONCLUSION

2-MesADP enhanced locomotor recovery in mouse SCI by altering the expression of neuronal apoptosis and remyelination-related genes and Wnt signaling pathways.

Adenosine-5'-triphosphate suppresses proliferation and migration capacity of human endometrial stem cells

Extracellular ATP through the activation of the P2X and P2Y purinergic receptors affects the migration, proliferation and differentiation of many types of cells, including stem cells. High plasticity, low immunogenicity and immunomodulation ability of mesenchymal stem cells derived from human endometrium (eMSCs) allow them to be considered a prominent tool for regenerative medicine. Here, we examined the role of ATP in the proliferation and migration of human eMSCs. Using a wound healing assay, we showed that ATP-induced activation of purinergic receptors suppressed the migration ability of eMSCs. We found the expression of one of the ATP receptors, the P2X₇ receptor in eMSCs. In spite of this, cell activation with specific P2X₇ receptor agonist, BzATP did not significantly affect the cell migration. The allosteric P2X₇ receptor inhibitor, AZ10606120 also did not prevent ATP-induced inhibition of cell migration, confirming that inhibition occurs without P2X₇ receptor involvement. Flow cytometry analysis showed that high concentrations of ATP did not have a cytotoxic effect on eMSCs. At the same time, ATP induced the cell cycle arrest, suppressed the proliferative and migration capacity of eMSCs and therefore could affect the regenerative potential of these cells.

Effects of single-dose antipurinergic therapy on behavioral and molecular alterations in the valproic acid-induced animal model of autism

Autism spectrum disorder (ASD) is characterized by deficits in communication and social interaction, restricted interests, and stereotyped behavior. Environmental factors, such as prenatal exposure to valproic acid (VPA), may contribute to the increased risk of ASD. Since disturbed functioning of the purinergic signaling system has been associated with the onset of ASD and used as a potential therapeutic target for ASD in both clinical and preclinical studies, we analyzed the effects of suramin, a non-selective purinergic antagonist, on behavioral, molecular and immunological in an animal model of autism induced by prenatal exposure to VPA. Treatment with suramin (20 mg/kg, intraperitoneal) restored sociability in the three-chamber apparatus and decreased anxiety measured by elevated plus maze apparatus, but had no impact on decreased reciprocal social interactions or higher nociceptive threshold in VPA rats. Suramin treatment did not affect VPA-induced upregulation of P2X4 and P2Y2 receptor expression in the hippocampus, and P2X4 receptor expression in the medial prefrontal cortex, but normalized an increased level of interleukin 6 (IL-6). Our results suggest an important role of purinergic signaling modulation in behavioral, molecular, and immunological aberrations described in VPA model, and indicate that the purinergic signaling system might be a potential target for pharmacotherapy in preclinical studies of ASD.

LPS-Induced Systemic Neonatal Inflammation: Blockage of P2X7R by BBG Decreases Mortality on Rat Pups and Oxidative Stress in Hippocampus of Adult Rats

Neonatal lipopolysaccharide (LPS) exposure-induced brain inflammation has been associated to neuronal injury and facilitates the development of models of neurological disorders in adult rats. The P2X7 receptor (P2X7R) plays a fundamental role in the onset and maintenance of the inflammatory cascade. Brilliant blue G (BBG), a P2X7R antagonist, has been shown to effectively promote neuroinflammatory protection. Here, we have investigated the long-term effects of the neonatal systemic inflammation on hippocampal oxidative stress, anxiety behavior and pain sensitivity in adulthood. We hypothesized that P2X7R blockade is able to modulate the effects of inflammation on these variables. Male and female rat pups received LPS and/or BBG solution intraperitoneally on the 1st, 3rd, 5th and 7th postnatal days. The survival rate and body weight were evaluated during the experimental procedures. The animals were submitted to behavioral tests for anxiety (elevated plus maze, EPM) and nociception (hot-plate and tail-flick) and the oxidative stress was measured by superoxide production in the dentate gyrus of the hippocampus using dihydroethidium (DHE) probe. BBG increased the survival rate in LPS-treated rats. No significant differences were found regarding anxiety behavior and pain sensitivity between the experimental groups. Systemic neonatal inflammation leads to a higher production of superoxide anion in the dentate gyrus of the hippocampus in adulthood and BBG inhibited that effect. Our data suggest that blocking the activation of the P2X7R during neonatal systemic inflammation may have a potential neuroprotective effect in adulthood.

P2X7 receptor signaling during adult hippocampal neurogenesis

Neurogenesis is a persistent and essential feature of the adult mammalian hippocampus. Granular neurons generated from resident pools of stem or progenitor cells provide a mechanism for the formation and consolidation of new memories. Regulation of hippocampal neurogenesis is complex and multifaceted, and numerous signaling pathways converge to modulate cell proliferation, apoptosis, and clearance of cellular debris, as well as synaptic integration of newborn immature neurons. The expression of functional P2X7 receptors in the central nervous system has attracted much interest and the regulatory role of this purinergic receptor during adult neurogenesis has only recently begun to be explored. P2X7 receptors are exceptionally versatile: in their canonical role they act as adenosine triphosphate-gated calcium channels and facilitate calcium-signaling cascades exerting control over the cell via calcium-encoded sensory proteins and transcription factor activation. P2X7 also mediates transmembrane pore formation to regulate cytokine release and facilitate extracellular communication, and when persistently stimulated by high extracellular adenosine triphosphate levels large P2X7 pores form, which induce apoptotic cell death through cytosolic ion dysregulation. Lastly, as a scavenger receptor P2X7 directly facilitates phagocytosis of the cellular debris that arises during neurogenesis, as well as during some disease states. Understanding how P2X7 receptors regulate the physiology of stem and progenitor cells in the adult hippocampus is an important step towards developing useful therapeutic models for regenerative medicine. This review considers the relevant aspects of adult hippocampal neurogenesis and explores how P2X7 receptor activity may influence the molecular physiology of the hippocampus, and neural stem and progenitor cells.

Temporal variations of nucleosides and nucleotides in rabbit milk

Nucleotides and nucleosides have a preeminent role in physiological and biochemical processes for newborns, the major source of these during early development is the breast milk. Different biomolecules exhibit daily fluctuations in maternal milk that could transfer temporal information that synchronize newborn circadian system. As a first approach, we characterized the diurnal profile of nucleotides and nucleosides contained in maternal milk of rabbits during the first week of lactation. It is possible that some nucleosides, such as adenosine, play a relevant role in setting up the emerging circadian rhythmicity, whereas uridine and guanosine could participate in the maintenance of rhythmicity.

P2X7 Receptors Regulate Phagocytosis and Proliferation in Adult Hippocampal and SVZ Neural Progenitor Cells: Implications for Inflammation in Neurogenesis

Identifying the signaling mechanisms that regulate adult neurogenesis is essential to understanding how the brain may respond to neuro‐inflammatory events. P2X7 receptors can regulate pro‐inflammatory responses, and in addition to their role as cation channels they can trigger cell death and mediate phagocytosis. How P2X7 receptors may regulate adult neurogenesis is currently unclear. Here, neural progenitor cells (NPCs) derived from adult murine hippocampal subgranular (SGZ) and cerebral subventricular (SVZ) zones were utilized to characterize the roles of P2X7 in adult neurogenesis, and assess the effects of high extracellular ATP, characteristic of inflammation, on NPCs. Immunocytochemistry found NPCs in vivo and in vitro expressed P2X7, and the activity of P2X7 in culture was demonstrated using calcium influx and pore formation assays. Live cell and confocal microscopy, in conjunction with flow cytometry, revealed P2X7⁺ NPCs were able to phagocytose fluorescent beads, and this was inhibited by ATP, indicative of P2X7 involvement. Furthermore, P2X7 receptors were activated with ATP or BzATP, and 5‐ethynyl‐2′‐deoxyuridine (EdU) used to observe a dose‐dependent decrease in NPC proliferation. A role for P2X7 in decreased NPC proliferation was confirmed using chemical inhibition and NPCs from P2X7^−/− mice. Together, these data present three distinct roles for P2X7 during adult neurogenesis, depending on extracellular ATP concentrations: (a) P2X7 receptors can form transmembrane pores leading to cell death, (b) P2X7 receptors can regulate rates of proliferation, likely via calcium signaling, and (c) P2X7 can function as scavenger receptors in the absence of ATP, allowing NPCs to phagocytose apoptotic NPCs during neurogenesis. stem cells2018;36:1764–1777

Special Issue for Purinergic Receptors, Particularly P2X7 Receptor, in the Eye

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Differentially expressed genes in response to amitraz treatment suggests a proposed model of resistance to amitraz in R. decoloratus ticks

The widespread geographical distribution of Rhipicephalus decoloratus in southern Africa and its ability to transmit the pathogens causing redwater, gallsickness and spirochaetosis in cattle makes this hematophagous ectoparasite of economic importance. In South Africa, the most commonly used chemical acaricides to control tick populations are pyrethroids and amitraz. The current amitraz resistance mechanism described in R. microplus, from South Africa and Australia, involves mutations in the octopamine receptor, but it is unlikely that this will be the only contributing factor to mediate resistance. Therefore, in this study we aimed to gain insight into the more complex mechanism(s) underlying amitraz resistance in R. decoloratus using RNA-sequencing. Differentially expressed genes (DEGs) were identified when comparing amitraz susceptible and resistant ticks in the presence of amitraz while fed on bovine hosts. The most significant DEGs were further analysed using several annotation tools. The predicted annotations from these genes, as well as KEGG pathways potentially point towards a relationship between the α-adrenergic-like octopamine receptor and ionotropic glutamate receptors in establishing amitraz resistance. All genes with KEGG pathway annotations were further validated using RT-qPCR across all life stages of the tick. In susceptible ticks, the proposed model is that in the presence of amitraz, there is inhibition of Ca²⁺ entry into cells and subsequent membrane hyperpolarization which prevents the release of neurotransmitters. In resistant ticks, we hypothesize that this is overcome by ionotropic glutamate receptors (NMDA and AMPA) to enhance synaptic transmission and plasticity in the presence of neurosteroids. Activation of NMDA receptors initiates long term potentiation (LTP) which may allow the ticks to respond more rapidly and with less stimulus when exposed to amitraz in future. Overactivation of the NMDA receptor and excitotoxicity is attenuated by the estrone, NAD⁺ and ATP hydrolysing enzymes. This proposed pathway paves the way to future studies on understanding amitraz resistance and should be validated using in vivo activity assays (through the use of inhibitors or antagonists) in combination with metabolome analyses.

Pathophysiological Role of Purines and Pyrimidines in Neurodevelopment: Unveiling New Pharmacological Approaches to Congenital Brain Diseases

In recent years, a substantial body of evidence has emerged demonstrating that purine and pyrimidine synthesis and metabolism play major roles in controlling embryonic and fetal development and organogenesis. Dynamic and time-dependent changes in the expression of purine metabolizing enzymes (such as ectonucleotidases and adenosine deaminase) represent a key checkpoint for the correct sequential generation of the different signaling molecules, that in turn activate their specific membrane receptors. In neurodevelopment, Ca²⁺ release from radial glia mediated by P2Y₁ purinergic receptors is fundamental to allow neuroblast migration along radial glia processes, and their correct positioning in the different layers of the developing neocortex. Moreover, ATP is involved in the development of synaptic transmission and contributes to the establishment of functional neuronal networks in the developing brain. Additionally, several purinergic receptors (spanning from adenosine to P2X and P2Y receptor subtypes) are differentially expressed by neural stem cells, depending on their maturation stage, and their activation tightly regulates cell proliferation and differentiation to either neurons or glial cells, as well as their correct colonization of the developing telencephalon. The purinergic control of neurodevelopment is not limited to prenatal life, but is maintained in postnatal life, when it plays fundamental roles in controlling oligodendrocyte maturation from precursors and their terminal differentiation to fully myelinating cells. Based on the above-mentioned and other literature evidence, it is now increasingly clear that any defect altering the tight regulation of purinergic transmission and of purine and pyrimidine metabolism during pre- and post-natal brain development may translate into functional deficits, which could be at the basis of severe pathologies characterized by mental retardation or other disturbances. This can occur either at the level of the recruitment and/or signaling of specific nucleotide or nucleoside receptors or through genetic alterations in key steps of the purine salvage pathway. In this review, we have provided a critical analysis of what is currently known on the pathophysiological role of purines and pyrimidines during brain development with the aim of unveiling new future strategies for pharmacological intervention in different neurodevelopmental disorders.

Injury-induced purinergic signalling molecules upregulate pluripotency gene expression and mitotic activity of progenitor cells in the zebrafish retina

Damage in fish activates retina repair that restores sight. The purinergic signalling system serves multiple homeostatic functions and has been implicated in cell cycle control of progenitor cells in the developing retina. We examined whether changes in the expression of purinergic molecules were instrumental in the proliferative phase after injury of adult zebrafish retinas with ouabain. P2RY1 messenger RNA (mRNA) increased early after injury and showed maximal levels at the time of peak progenitor cell proliferation. Extracellular nucleotides, mainly ADP, regulate P2RY1 transcriptional and protein expression. The injury-induced upregulation of P2RY1 is mediated by an autoregulated mechanism. After injury, the transcriptional expression of ecto-nucleotidases and ecto-ATPases also increased and ecto-ATPase activity inhibitors decreased Müller glia-derived progenitor cell amplification. Inhibition of P2RY1 endogenous activation prevented progenitor cell proliferation at two intervals after injury: one in which progenitor Müller glia mitotically activates and the second one in which Müller glia-derived progenitor cells amplify. ADPβS induced the expression of lin28a and ascl1a genes in mature regions of uninjured retinas. The expression of these genes, which regulate multipotent Müller glia reprogramming, was significantly inhibited by blocking the endogenous activation of P2RY1 early after injury. We consistently observed that the number of glial fibrillary acidic protein-BrdU-positive Müller cells after injury was larger in the absence than in the presence of the P2RY1 antagonist. Ecto-ATPase activity inhibitors or P2RY1-specific antagonists did not modify apoptotic cell death at the time of peak progenitor cell proliferation. The results suggested that ouabain injury upregulates specific purinergic signals which stimulates multipotent progenitor cell response.

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'.

ATP-induced Ca2+-signalling mechanisms in the regulation of mesenchymal stem cell migration

The ability of cells to migrate to the destined tissues or lesions is crucial for physiological processes from tissue morphogenesis, homeostasis and immune responses, and also for stem cell-based regenerative medicines. Cytosolic Ca²⁺ is a primary second messenger in the control and regulation of a wide range of cell functions including cell migration. Extracellular ATP, together with the cognate receptors on the cell surface, ligand-gated ion channel P2X receptors and a subset of G-protein-coupled P2Y receptors, represents common autocrine and/or paracrine Ca²⁺ signalling mechanisms. The P2X receptor ion channels mediate extracellular Ca²⁺ influx, whereas stimulation of the P2Y receptors triggers intracellular Ca²⁺ release from the endoplasmic reticulum (ER), and activation of both type of receptors thus can elevate the cytosolic Ca²⁺ concentration ([Ca²⁺]_c), albeit with different kinetics and capacity. Reduction in the ER Ca²⁺ level following the P2Y receptor activation can further induce store-operated Ca²⁺ entry as a distinct Ca²⁺ influx pathway that contributes in ATP-induced increase in the [Ca²⁺]_c. Mesenchymal stem cells (MSC) are a group of multipotent stem cells that grow from adult tissues and hold promising applications in tissue engineering and cell-based therapies treating a great and diverse number of diseases. There is increasing evidence to show constitutive or evoked ATP release from stem cells themselves or mature cells in the close vicinity. In this review, we discuss the mechanisms for ATP release and clearance, the receptors and ion channels participating in ATP-induced Ca²⁺ signalling and the roles of such signalling mechanisms in mediating ATP-induced regulation of MSC migration.

Purinergic signaling and the functioning of the nervous system cells

Purinergic signaling in the nervous system has been the focus of a considerable number of studies since the 1970s. The P2X and P2Y receptors are involved in the initiation of purinergic signaling. They are very abundant in the central and peripheral nervous systems, where they are expressed on the surface of neurons and glial cells--microglia, astrocytes, oligodendrocytes and Schwann cells and the precursors of the latter two. Their ligands--extracellular nucleotides--are released in the physiological state by astrocytes and neurons forming synaptic connections, and are essential for the proper functioning of nervous system cells. Purinergic signaling plays a crucial role in neuromodulation, neurotransmission, myelination in the CNS and PNS, intercellular communication, the regulation of ramified microglia activity, the induction of the response to damaging agents, the modulation of synaptic activity and other glial cells by astrocytes, and the induction of astrogliosis. Understanding these mechanisms and the fact that P2 receptors and their ligands are involved in the pathogenesis of diseases of the nervous system may help in the design of drugs with different and more effective mechanisms of action.

Effects of FTY720 (Fingolimod) on Proliferation, Differentiation, and Migration of Brain-Derived Neural Stem Cells

Insufficient proliferation, differentiation, and migration are the main pitfalls of neural stem cells (NSCs) in reparative therapeutics for the central nervous system (CNS) diseases. The potent lipid mediator sphingosine-1-phosphate (S1P) regulates cells' biological behavior broadly in the CNS. However, the effects of activating S1P on NSCs are not quite clear. In the current study, FTY720 (Fingolimod), an analog of S1P, was employed to induce the proliferation, differentiation, and migration of cultured brain-derived NSCs. The results indicated that proliferation and migration ability of NSCs were promoted by FTY720. Though we observed no obvious neuron prefers differentiation of NSCs, there were more protoplasmic astrocytes developed in the presence of certain concentration of FTY720. This work gives more comprehensive understanding of how FTY720 affects NSCs.

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.

Adenine Nucleotides Control Proliferation In Vivo of Rat Retinal Progenitors by P2Y1 Receptor

Previous studies demonstrated that exogenous ATP is able to regulate proliferation of retinal progenitor cells (RPCs) in vitro possibly via P2Y₁ receptor, a G protein-coupled receptor. Here, we evaluated the function of adenine nucleotides in vivo during retinal development of newborn rats. Intravitreal injection of apyrase, an enzyme that hydrolyzes nucleotides, reduced cell proliferation in retinas at postnatal day 2 (P2). This decrease was reversed when retinas were treated together with ATPγ-S or ADPβ-S, two hydrolysis-resistant analogs of ATP and ADP, respectively. During early postnatal days (P0 to P5), an increase in ectonucleotidase (E-NTPDase) activity was observed in the retina, suggesting a decrease in the availability of adenine nucleotides, coinciding with the end of proliferation. Interestingly, intravitreal injection of the E-NTPDase inhibitor ARL67156 increased proliferation by around 60 % at P5 rats. Furthermore, immunolabeling against P2Y₁ receptor was observed overall in retina layers from P2 rats, including proliferating Ki-67-positive cells in the neuroblastic layer (NBL), suggesting that this receptor could be responsible for the action of adenine nucleotides upon proliferation of RPCs. Accordingly, intravitreal injection of MRS2179, a selective antagonist of P2Y₁ receptors, reduced cell proliferation by approximately 20 % in P2 rats. Moreover, treatment with MRS 2179 caused an increase in p57^KIP2 and cyclin D1 expression, a reduction in cyclin E and Rb phosphorylated expression and in BrdU-positive cell number. These data suggest that the adenine nucleotides modulate the proliferation of rat RPCs via activation of P2Y₁ receptors regulating transition from G1 to S phase of the cell cycle.

A new role for the P2Y-like GPR17 receptor in the modulation of multipotency of oligodendrocyte precursor cells in vitro

Oligodendrocyte precursor cells (OPCs, also called NG2 cells) are scattered throughout brain parenchyma, where they function as a reservoir to replace lost or damaged oligodendrocytes, the myelin-forming cells. The hypothesis that, under some circumstances, OPCs can actually behave as multipotent cells, thus generating astrocytes and neurons as well, has arisen from some in vitro and in vivo evidence, but the molecular pathways controlling this alternative fate of OPCs are not fully understood. Their identification would open new opportunities for neuronal replace strategies, by fostering the intrinsic ability of the brain to regenerate. Here, we show that the anti-epileptic epigenetic modulator valproic acid (VPA) can promote the generation of new neurons from NG2⁺ OPCs under neurogenic protocols in vitro, through their initial de-differentiation to a stem cell-like phenotype that then evolves to "hybrid" cell population, showing OPC morphology but expressing the neuronal marker βIII-tubulin and the GPR17 receptor, a key determinant in driving OPC transition towards myelinating oligodendrocytes. Under these conditions, the pharmacological blockade of the P2Y-like receptor GPR17 by cangrelor, a drug recently approved for human use, partially mimics the effects mediated by VPA thus accelerating cells' neurogenic conversion. These data show a co-localization between neuronal markers and GPR17 in vitro, and suggest that, besides its involvement in oligodendrogenesis, GPR17 can drive the fate of neural precursor cells by instructing precursors towards the neuronal lineage. Being a membrane receptor, GPR17 represents an ideal "druggable" target to be exploited for innovative regenerative approaches to acute and chronic brain diseases.

Adenosine A1 receptor inhibits postnatal neurogenesis and sustains astrogliogenesis from the subventricular zone

We previously demonstrated that activation of ATP P2X receptors during oxygen and glucose deprivation inhibits neuroblast migration and in vitro neurogenesis from the subventricular zone (SVZ). Here, we have studied the effects of adenosine, the natural end-product of ATP hydrolysis, in modulating neurogenesis and gliogenesis from the SVZ. We provide immunochemical, molecular and pharmacological evidence that adenosine via A1 receptors reduces neuronal differentiation of neurosphere cultures generated from postnatal SVZ. Furthermore, activation of A1 receptors induces downregulation of genes related to neurogenesis as demonstrated by gene expression analysis. Specifically, we found that A1 receptors trigger a signaling cascade that, through the release of IL10, turns on the Bmp2/SMAD pathway. Furthermore, activating A1 receptors in SVZ-neural progenitor cells inhibits neurogenesis and stimulates astrogliogenesis as assayed in vitro in neurosphere cultures and in vivo in the olfactory bulb. Together, these data indicate that adenosine acting at A1 receptors negatively regulates adult neurogenesis while promoting astrogliogenesis, and that this feature may be relevant to pathological conditions whereby purines are profusely released. GLIA 2016;64:1465-1478.

Sulfated polysaccharide isolated from the sea cucumber Stichopus japonicas promotes the SDF-1α/CXCR4 axis-induced NSC migration via the PI3K/Akt/FOXO3a, ERK/MAPK, and NF-κB signaling pathways

The present study describes a positive regulatory loop between SJP and the SDF-1α/CXCR4 axis in NSC migration. The treatment of NSCs with SJP and SDF-1α increases the cell migration capacity and promotes cell migration from the neurospheres. These effects are accompanied by the up-regulation of Nestin, N-cadherin, TLR4, TNF-α, Cyclin D1, EGFR, Alpha 6 integrin, MMP-2, MMP-9, and iNOS, including SDF-1α and CXCR4 themselves. However, these effects are blocked by AMD3100, LY294002, U0126, and PDTC. SJP enhances the SDF-1α/CXCR4 axis-induced MMP-2 and MMP-9 secretion and NO release. Results demonstrate that interaction of SJP with the SDF-1α/CXCR4 axis regulates NSC migration via the PI3K/Akt/FOXO3a, ERK-MAPK, and NF-κB signaling pathways.

ZDHHC16 modulates FGF/ERK dependent proliferation of neural stem/progenitor cells in the zebrafish telencephalon

In vertebrates, neural stem/progenitor cells (NSPCs) maintenance is critical for nervous system development and homeostasis. However, the molecular mechanisms underlying the maintenance of NSPCs have not been fully elucidated. Here, we demonstrated that zebrafish ZDHHC16, a DHHC encoding protein, which was related to protein palmitoylation after translation, was expressed in the developing forebrain, and especially in the telencephalon. Loss- and gain-of-function studies showed that ZDHHC16 played a crucial role in the regualtion of NSPCs proliferation during zebrafish telencephalic development, via a mechanism dependent on its palmitoyltransferase activity. Further analyses showed that the inhibition of ZDHHC16 led to inactivation of the FGF/ERK signaling pathway during telencephalic NSPCs proliferation and maintenance. Taken together, our results suggest that ZDHHC16 activity is essential for early NSPCs proliferation where it acts to activate the FGF/ERK network, allowing for the initiation of proliferation -regulated gene expression programs. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1014-1028, 2016.

High Risk of Depressive Disorders in Patients With Gout: A Nationwide Population-Based Cohort Study

Metabolic abnormalities are common in patients with depressive disorders. However, the relationship between gout and depression is unclear. We explored the causal relationship among gout, antigout medication, and the associated risk of incidental depressive disorders.In this nationwide cohort study, we sampled data from the National Health Insurance Research Database to recruit 34,050 patients with gout as the gout cohort and 68,100 controls (without gout) as the nongout cohort. Our primary endpoint was the diagnosis of depressive disorders during follow-up. The overall study population was followed up until depression diagnosis, withdrawal from the NHI program, or the end of the study. The differences in demographic and clinical characteristics between both cohorts were determined using the Chi-square test for categorical variables and the t-test for continuous variables. Cox proportional hazard regression models were used to examine the effect of gout on the risk of depression, represented using the hazard ratio with the 95% confidence interval.Patients with gout exhibited a higher risk of depressive disorders than controls did. The risk of depressive disorders increased with age and was higher in female patients and those with hypertension, stroke, and coronary artery disease. Nonsteroidal antiinflammatory drug and prednisolone use was associated with a reduced risk of depression. Patients with gout who had received antigout medication exhibited a reduced risk of depressive disorders compared with nongout patients.Our findings support that gout increases the risk of depressive disorders, and that antigout medication use reduces the risk.

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'.

Purinergic receptors in embryonic and adult neurogenesis

ATP (adenosine 5'-triphosphate), one of the most ancient neurotransmitters, exerts essential functions in the brain, including neurotransmission and modulation of synaptic activity. Moreover, this nucleotide has been attributed with trophic properties and experimental evidence points to the participation of ATP-activated P2X and P2Y purinergic receptors in embryonic brain development as well as in adult neurogenesis for maintenance of normal brain functions and neuroregeneration upon brain injury. We discuss here the available data on purinergic P2 receptor expression and function during brain development and in the neurogenic zones of the adult brain, as well as the insights based on the use of in vitro stem cell cultures. While several P2 receptor subtypes were shown to be expressed during in vitro and in vivo neurogenesis, specific functions have been proposed for P2Y1, P2Y2 metabotropic as well as P2X2 ionotropic receptors to promote neurogenesis. Further, the P2X7 receptor is suggested to function in the maintenance of pools of neural stem and progenitor cells through induction of proliferation or cell death, depending on the microenvironment. Pathophysiological actions have been proposed for this receptor in worsening damage in brain disease. The P2X7 receptor and possibly additional P2 receptor subtypes have been implicated in pathophysiology of neurological diseases including Parkinson's disease, Alzheimer's disease and epilepsy. New strategies in cell therapy could involve modulation of purinergic signaling, either in the achievement of more effective protocols to obtain viable and homogeneous cell populations or in the process of functional engraftment of transplanted cells into the damaged brain. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

P2X7 receptors mediate innate phagocytosis by human neural precursor cells and neuroblasts

During early human neurogenesis there is overproduction of neuroblasts and neurons accompanied by widespread programmed cell death (PCD). While it is understood that CD68(+) microglia and astrocytes mediate phagocytosis during target-dependent PCD, little is known of the cell identity or the scavenger molecules used to remove apoptotic corpses during the earliest stages of human neurogenesis. Using a combination of multiple-marker immunohistochemical staining, functional blocking antibodies and antagonists, we showed that human neural precursor cells (hNPCs) and neuroblasts express functional P2X7 receptors. Furthermore, using live-cell imaging, flow cytometry, phagocytic assays, and siRNA knockdown, we showed that in a serum-free environment, doublecortin(+) (DCX) neuroblasts and hNPCs can clear apoptotic cells by innate phagocytosis mediated via P2X7. We found that both P2X7(high) DCX(low) hNPCs and P2X7(high) DCX(high) neuroblasts, derived from primary cultures of human fetal telencephalon, phagocytosed targets including latex beads, apoptotic ReNcells, and apoptotic hNPC/neuroblasts. Pretreatment of neuroblasts and hNPCs with 1 mM adenosine triphosphate (ATP), 100 µM OxATP (P2X7 antagonist), or siRNA knockdown of P2X7 inhibited phagocytosis of these targets. Our results show that P2X7 functions as a scavenger receptor under serum-free conditions resembling those in early neurogenesis. This is the first demonstration that hNPCs and neuroblasts may participate in clearance of apoptotic corpses during pre target-dependent neurogenesis and mediate phagocytosis using P2X7 as a scavenger receptor.

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'.

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

An introduction to the roles of purinergic signalling in neurodegeneration, neuroprotection and neuroregeneration

Purinergic signalling appears to play important roles in neurodegeneration, neuroprotection and neuroregeneration. Initially there is a brief summary of the background of purinergic signalling, including release of purines and pyrimidines from neural and non-neural cells and their ectoenzymatic degradation, and the current characterisation of P1 (adenosine), and P2X (ion channel) and P2Y (G protein-coupled) nucleotide receptor subtypes. There is also coverage of the localization and roles of purinoceptors in the healthy central nervous system. The focus is then on the roles of purinergic signalling in trauma, ischaemia, stroke and in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's diseases, as well as multiple sclerosis and amyotrophic lateral sclerosis. Neuroprotective mechanisms involving purinergic signalling are considered and its involvement in neuroregeneration, including the role of adult neural stem/progenitor cells. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Purinergic signaling: a common pathway for neural and mesenchymal stem cell maintenance and differentiation

Extracellular ATP, related nucleotides and adenosine are among the earliest signaling molecules, operating in virtually all tissues and cells. Through their specific receptors, namely purinergic P1 for nucleosides and P2 for nucleotides, they are involved in a wide array of physiological effects ranging from neurotransmission and muscle contraction to endocrine secretion, vasodilation, immune response, and fertility. The purinergic system also participates in the proliferation and differentiation of stem cells from different niches. In particular, both mesenchymal stem cells (MSCs) and neural stem cells are endowed with several purinergic receptors and ecto-nucleotide metabolizing enzymes, and release extracellular purines that mediate autocrine and paracrine growth/proliferation, pro- or anti-apoptotic processes, differentiation-promoting effects and immunomodulatory actions. Here, we discuss the often opposing roles played by ATP and adenosine in adult neurogenesis in both physiological and pathological conditions, as well as in adipogenic and osteogenic MSC differentiation. We also focus on how purinergic ligands produced and released by transplanted stem cells can be regarded as ideal candidates to mediate the crosstalk with resident stem cell niches, promoting cell growth and survival, regulating inflammation and, therefore, contributing to local tissue homeostasis and repair.

Purinergic system dysfunction in mood disorders: a key target for developing improved therapeutics

Uric acid and purines (such as adenosine) regulate mood, sleep, activity, appetite, cognition, memory, convulsive threshold, social interaction, drive, and impulsivity. A link between purinergic dysfunction and mood disorders was first proposed a century ago. Interestingly, a recent nationwide population-based study showed elevated risk of gout in subjects with bipolar disorder (BD), and a recent meta-analysis and systematic review of placebo-controlled trials of adjuvant purinergic modulators confirmed their benefits in bipolar mania. Uric acid may modulate energy and activity levels, with higher levels associated with higher energy and BD spectrum. Several recent genetic studies suggest that the purinergic system - particularly the modulation of P1 and P2 receptor subtypes - plays a role in mood disorders, lending credence to this model. Nucleotide concentrations can be measured using brain spectroscopy, and ligands for in vivo positron emission tomography (PET) imaging of adenosine (P1) receptors have been developed, thus allowing potential target engagement studies. This review discusses the key role of the purinergic system in the pathophysiology of mood disorders. Focusing on this promising therapeutic target may lead to the development of therapies with antidepressant, mood stabilization, and cognitive effects.

mESC-based in vitro differentiation models to study vascular response and functionality following genotoxic insults

Because of high exposure to systemic noxae, vascular endothelial cells (EC) have to ensure distinct damage defense and regenerative mechanisms to guarantee vascular health. For meaningful toxicological drug assessments employing embryonic stem cell (ESC)-based in vitro models, functional competence of differentiated progeny and detailed knowledge regarding damage defense mechanisms are essential. Here, mouse ESCs (mESC) were differentiated into functionally competent vascular cells (EC and smooth muscle cells [SMC]). mESC, EC, and SMC were comparatively analyzed regarding DNA repair and DNA damage response (DDR). Differentiation was accompanied by both congruent and unique alterations in repair and DDR characteristics. EC and SMC shared the downregulation of genes involved cell cycle regulation and repair of DNA double-strand breaks (DSBs) and mismatches, whereas genes associated with nucleotide excision repair (NER), apoptosis, and autophagy were upregulated when compared with mESC. Expression of genes involved in base excision repair (BER) was particularly low in SMC. IR-induced formation of DSBs, as detected by nuclear γH2AX foci formation, was most efficient in SMC, the repair of DSBs was fastest in EC. Together with substantial differences in IR-induced phosphorylation of p53, Chk1, and Kap1, the data demonstrate complex alterations in DDR capacity going along with the loss of pluripotency and gain of EC- and SMC-specific functions. Notably, IR exposure of early vascular progenitors did not impair differentiation into functionally competent EC and SMC. Summarizing, mESC-based vascular differentiation models are informative to study the impact of environmental stressors on differentiation and function of vascular cells.

The role of purinergic receptors in stem cell differentiation

A major challenge modern society has to face is the increasing need for tissue regeneration due to degenerative diseases or tumors, but also accidents or warlike conflicts. There is great hope that stem cell-based therapies might improve current treatments of cardiovascular diseases, osteochondral defects or nerve injury due to the unique properties of stem cells such as their self-renewal and differentiation potential. Since embryonic stem cells raise severe ethical concerns and are prone to teratoma formation, adult stem cells are still in the focus of research. Emphasis is placed on cellular signaling within these cells and in between them for a better understanding of the complex processes regulating stem cell fate. One of the oldest signaling systems is based on nucleotides as ligands for purinergic receptors playing an important role in a huge variety of cellular processes such as proliferation, migration and differentiation. Besides their natural ligands, several artificial agonists and antagonists have been identified for P1 and P2 receptors and are already used as drugs. This review outlines purinergic receptor expression and signaling in stem cells metabolism. We will briefly describe current findings in embryonic and induced pluripotent stem cells as well as in cancer-, hematopoietic-, and neural crest-derived stem cells. The major focus will be placed on recent findings of purinergic signaling in mesenchymal stem cells addressed in in vitro and in vivo studies, since stem cell fate might be manipulated by this system guiding differentiation towards the desired lineage in the future.

The two-hit hypothesis for neuroinflammation: role of exogenous ATP in modulating inflammation in the brain

Brain inflammation is a common occurrence following responses to varied insults such as bacterial infections, stroke, traumatic brain injury and neurodegenerative disorders. A common mediator for these varied inflammatory responses is prostaglandin E₂ (PGE₂), produced by the enzymatic activity of cyclooxygenases (COX) 1 and 2. Previous attempts to reduce neuronal inflammation through COX inhibition, by use of nonsteroidal anti-inflammatory drugs (NSAIDs), have met with limited success. We are proposing the two-hit model for neuronal injury—an initial localized inflammation mediated by PGE₂ (first hit) and the simultaneous release of adenosine triphosphate (ATP) by injured cells (second hit), which significantly enhances the inflammatory response through increased synthesis of PGE₂. Several evidences on the role of exogenous ATP in inflammation have been reported, including contrary instances where extracellular ATP reduces inflammatory events. In this review, we will examine the current literature on the role of P2 receptors, to which ATP binds, in modulating inflammatory reactions during neurodegeneration. Targeting the P2 receptors, therefore, provides a therapeutic alternative to reduce inflammation in the brain. P2 receptor-based anti-inflammatory drugs (PBAIDs) will retain the activities of essential COX enzymes, yet will significantly reduce neuroinflammation by decreasing the enhanced production of PGE₂ by extracellular ATP.

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.

Purinergic signalling: pathophysiology and therapeutic potential

The article begins with a review of the main conceptual steps involved in the development of our understanding of purinergic signalling, including non-adrenergic, non-cholinergic (NANC) neurotransmission; identification of ATP as a NANC transmitter; purinergic cotransmission; recognition of two families of purinoceptors [P1 (adenosine) and P2 (ATP/ADP)]; and, later, cloning and characterisation of P1 (G protein-coupled), P2X (ion channel) and P2Y (G protein-coupled) receptor subtypes. Further studies have established the involvement of ATP in synaptic neurotransmission in both ganglia and in the central nervous system; long-term (trophic) purinergic signalling in cell proliferation, differentiation and death occurring in development and regeneration; and short-term purinergic signalling in neurotransmission, neuromodulation and secretion. ATP is released from most cell types in response to gentle mechanical stimulation and is rapidly degraded to adenosine by ecto-nucleotidases. This review then focuses on the pathophysiology of purinergic signalling in a wide variety of systems, including urinogenital, cardiovascular, airway, musculoskeletal and gastrointestinal. Consideration is also given to the involvement of purinoceptors in pain, cancer and diseases of the central nervous system. Purinergic therapeutic approaches for the treatment of some of these diseases are discussed.

Stem cell characteristics in glioblastoma are maintained by the ecto-nucleotidase E-NPP1

Glioblastomas are highly aggressive brain tumours and are characterised by substantial cellular heterogeneity within a single tumour. A sub-population of glioblastoma stem-like cells (GSCs) that shares properties with neural precursor cells has been described, exhibiting resistance to therapy and therefore being considered responsible for the high recurrence rate in glioblastoma. To elucidate the underlying cellular processes we investigated the role of phosphatases in the GSC phenotype, using an in vitro phosphatome-wide RNA interference screen. We identified a set of genes, the knockdown of which induces a significant decrease in the glioma stem cell marker CD133, indicating a role in the glioblastoma stem-like phenotype. Among these genes, the ecto-nucleotidase ENPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) was found to be highly expressed in GSCs compared with normal brain and neural stem cells. Knockdown of ENPP1 in cultured GSCs resulted in an overall downregulation of stem cell-associated genes, induction of differentiation into astrocytic cell lineage, impairment of sphere formation, in addition to increased cell death, accumulation of cells in G1/G0 cell cycle phase and sensitisation to chemotherapeutic treatment. Genome-wide gene expression analysis and nucleoside and nucleotide profiling revealed that knockdown of ENPP1 affects purine and pyrimidine metabolism, suggesting a link between ENPP1 expression and a balanced nucleoside-nucleotide pool in GSCs. The phenotypic changes in E-NPP1-deficient GSCs are assumed to be a consequence of decreased transcriptional function of E2F1. Together, these results reveal that E-NPP1, by acting upstream of E2F1, is indispensable for the maintenance of GSCs in vitro and hence required to keep GSCs in an undifferentiated, proliferative state.