G protein-coupled receptor 17 is regulated by WNT pathway during oligodendrocyte precursor cell differentiation

G protein-coupled receptor 17 (GPR17) and the WNT pathway are critical players of oligodendrocyte (OL) differentiation acting as essential timers in developing brain to achieve fully-myelinating cells. However, whether and how these two systems are related to each other is still unknown. Of interest, both factors are dysregulated in developing and adult brain diseases, including white matter injury and cancer, making the understanding of their reciprocal interactions of potential importance for identifying new targets and strategies for myelin repair. Here, by a combined pharmacological and biotechnological approach, we examined regulatory mechanisms linking WNT signaling to GPR17 expression in OLs. We first analyzed the relative expression of mRNAs encoding for GPR17 and the T cell factor/Lymphoid enhancer-binding factor-1 (TCF/LEF) transcription factors of the canonical WNT/β-CATENIN pathway, in PDGFRα+ and O4+ OLs during mouse post-natal development. In O4+ cells, Gpr17 mRNA level peaked at post-natal day 14 and then decreased concomitantly to the physiological uprise of WNT tone, as shown by increased Lef1 mRNA level. The link between WNT signaling and GPR17 expression was further reinforced in vitro in primary PDGFRα+ cells and in Oli-neu cells. High WNT tone impaired OL differentiation and drastically reduced GPR17 mRNA and protein levels. In Oli-neu cells, WNT/β-CATENIN activation repressed Gpr17 promoter activity through both putative WNT response elements (WRE) and upregulation of the inhibitor of DNA-binding protein 2 (Id2). We conclude that the WNT pathway influences OL maturation by repressing GPR17, which could have implications in pathologies characterized by dysregulations of the OL lineage including multiple sclerosis and oligodendroglioma.

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.

Rewiring of Glucose and Lipid Metabolism Induced by G Protein-Coupled Receptor 17 Silencing Enables the Transition of Oligodendrocyte Progenitors to Myelinating Cells

In the mature central nervous system (CNS), oligodendrocytes (OLs) provide support and insulation to axons thanks to the production of a myelin sheath. During their maturation to myelinating cells, OLs require energy and building blocks for lipids, which implies a great investment of energy fuels and molecular sources of carbon. The oligodendroglial G protein-coupled receptor 17 (GPR17) has emerged as a key player in OL maturation; it reaches maximal expression in pre-OLs, but then it has to be internalized to allow terminal maturation. In this study, we aim at elucidating the role of physiological GPR17 downregulation in OL metabolism by applying transcriptomics, metabolomics and lipidomics on differentiating OLs. After GPR17 silencing, we found a significant increase in mature OL markers and alteration of several genes involved in glucose metabolism and lipid biosynthesis. We also observed an increased release of lactate, which is partially responsible for the maturation boost induced by GPR17 downregulation. Concomitantly, GPR17 depletion also changed the kinetics of specific myelin lipid classes. Globally, this study unveils a functional link between GPR17 expression, lactate release and myelin composition, and suggests that innovative interventions targeting GPR17 may help to foster endogenous myelination in demyelinating diseases.

Involvement of GPR17 in Neuronal Fibre Outgrowth

Characterization of new pharmacological targets is a promising approach in research of neurorepair mechanisms. The G protein-coupled receptor 17 (GPR17) has recently been proposed as an interesting pharmacological target, e.g., in neuroregenerative processes. Using the well-established ex vivo model of organotypic slice co-cultures of the mesocortical dopaminergic system (prefrontal cortex (PFC) and substantia nigra/ventral tegmental area (SN/VTA) complex), the influence of GPR17 ligands on neurite outgrowth from SN/VTA to the PFC was investigated. The growth-promoting effects of Montelukast (MTK; GPR17- and cysteinyl-leukotriene receptor antagonist), the glial cell line-derived neurotrophic factor (GDNF) and of two potent, selective GPR17 agonists (PSB-16484 and PSB-16282) were characterized. Treatment with MTK resulted in a significant increase in mean neurite density, comparable with the effects of GDNF. The combination of MTK and GPR17 agonist PSB-16484 significantly inhibited neuronal growth. qPCR studies revealed an MTK-induced elevated mRNA-expression of genes relevant for neuronal growth. Immunofluorescence labelling showed a marked expression of GPR17 on NG2-positive glia. Western blot and RT-qPCR analysis of untreated cultures suggest a time-dependent, injury-induced stimulation of GPR17. In conclusion, MTK was identified as a stimulator of neurite fibre outgrowth, mediating its effects through GPR17, highlighting GPR17 as an interesting therapeutic target in neuronal regeneration.

Novel in vitro Experimental Approaches to Study Myelination and Remyelination in the Central Nervous System

Myelin is the lipidic insulating structure enwrapping axons and allowing fast saltatory nerve conduction. In the central nervous system, myelin sheath is the result of the complex packaging of multilamellar extensions of oligodendrocyte (OL) membranes. Before reaching myelinating capabilities, OLs undergo a very precise program of differentiation and maturation that starts from OL precursor cells (OPCs). In the last 20 years, the biology of OPCs and their behavior under pathological conditions have been studied through several experimental models. When co-cultured with neurons, OPCs undergo terminal maturation and produce myelin tracts around axons, allowing to investigate myelination in response to exogenous stimuli in a very simple in vitro system. On the other hand, in vivo models more closely reproducing some of the features of human pathophysiology enabled to assess the consequences of demyelination and the molecular mechanisms of remyelination, and they are often used to validate the effect of pharmacological agents. However, they are very complex, and not suitable for large scale drug discovery screening. Recent advances in cell reprogramming, biophysics and bioengineering have allowed impressive improvements in the methodological approaches to study brain physiology and myelination. Rat and mouse OPCs can be replaced by human OPCs obtained by induced pluripotent stem cells (iPSCs) derived from healthy or diseased individuals, thus offering unprecedented possibilities for personalized disease modeling and treatment. OPCs and neural cells can be also artificially assembled, using 3D-printed culture chambers and biomaterial scaffolds, which allow modeling cell-to-cell interactions in a highly controlled manner. Interestingly, scaffold stiffness can be adopted to reproduce the mechanosensory properties assumed by tissues in physiological or pathological conditions. Moreover, the recent development of iPSC-derived 3D brain cultures, called organoids, has made it possible to study key aspects of embryonic brain development, such as neuronal differentiation, maturation and network formation in temporal dynamics that are inaccessible to traditional in vitro cultures. Despite the huge potential of organoids, their application to myelination studies is still in its infancy. In this review, we shall summarize the novel most relevant experimental approaches and their implications for the identification of remyelinating agents for human diseases such as multiple sclerosis.

Functional genomic analyses highlight a shift in Gpr17-regulated cellular processes in oligodendrocyte progenitor cells and underlying myelin dysregulation in the aged mouse cerebrum

Brain ageing is characterised by a decline in neuronal function and associated cognitive deficits. There is increasing evidence that myelin disruption is an important factor that contributes to the age-related loss of brain plasticity and repair responses. In the brain, myelin is produced by oligodendrocytes, which are generated throughout life by oligodendrocyte progenitor cells (OPCs). Currently, a leading hypothesis points to ageing as a major reason for the ultimate breakdown of remyelination in Multiple Sclerosis (MS). However, an incomplete understanding of the cellular and molecular processes underlying brain ageing hinders the development of regenerative strategies. Here, our combined systems biology and neurobiological approach demonstrate that oligodendroglial and myelin genes are amongst the most altered in the ageing mouse cerebrum. This was underscored by the identification of causal links between signalling pathways and their downstream transcriptional networks that define oligodendroglial disruption in ageing. The results highlighted that the G-protein coupled receptor Gpr17 is central to the disruption of OPCs in ageing and this was confirmed by genetic fate-mapping and cellular analyses. Finally, we used systems biology strategies to identify therapeutic agents that rejuvenate OPCs and restore myelination in age-related neuropathological contexts.

Pathway-Focused Profiling of Oligodendrocytes Over-Expressing miR-125a-3p Reveals Alteration of Wnt and Cell-to-Cell Signaling

MicroRNAs are small post-transcriptional regulators that modulate gene expression by directly interacting with their target transcripts. Since the interaction between miRNAs and target mRNAs does not require a perfect match, one single miRNA can influence the expression of several genes and lead to a very broad array of functional consequences. Recently, we identified miR-125a-3p as a new regulator of oligodendrocyte development, showing that its over-expression is associated to impaired oligodendrocyte maturation. However, whether and how miR-125a-3p over-expression is causally related to oligodendrocyte maturation is still obscure, as well as the pathways responsible for this effect. To shed light on this issue and to identify the underlying molecular mechanisms, we determined the transcriptomic profile of miR-125a-3p over-expressing oligodendrocytes and, by means of two complementary bioinformatic approaches, we have identified pathways and biological processes consistently modulated by miR-125a-3p alteration. This analysis showed that miR-125a-3p is involved in the regulation of cell-cell interactions and Wnt signaling. By means of pathway-focused PCR arrays, we confirmed that miR-125a-3p induces changes in the expression of several genes encoding for adhesion molecules and gap junctions, which play key roles in oligodendrocytes after exposure to pathological demyelinating stimuli. Moreover, the expression changes of different Wnt targets suggest an over-activation of this pathway. Globally, our studies show that miR-125a-3p over-expression can alter signaling pathways and biological processes essential for myelin formation in oligodendrocytes, suggesting that alteration of miR-125a-3p levels may contribute to impairing oligodendrocyte maturation in demyelinating diseases.

Perspectives on Geoff Burnstock as researcher, teacher and friend

Geoffrey Burnstock, one of the most talented and brilliant scientists of his generation, was born on the 10th of May 1929 in London and died on the 2ndof June 2020, aged 91, in Melbourne (Australia). Geoffrey Burnstock started his research studies with an interest in the nerves controlling the guts of guinea pigs, and discovered a completely unexpected and ubiquitous signalling system mediated via extracellular nucleotides (the "purinergic theory"), which revolutionized our understanding of how cells communicate between each other. He made the highly controversial discovery that ATP (adenosine triphosphate), a molecule well known to biochemists for its role as a source of energy inside cells, could also transmit signals between them. Initially, his somewhat heretical theory, that did not fit conventional views, found considerable resistance in the scientific community. However, he continued to accumulate evidence in favor of his hypothesis, extending it to a variety of organs and systems and demonstrating a role for purinergic signaling in the cardiovascular, respiratory and nervous systems, and in the pathophysiology of pain, blood clotting, cell proliferation and differentiation, and immunity. For his entire life, he struggled to attract scientists to this new field and, finally, in the early 1990s, did evidence emerge that convinced the doubters, due to new molecular biology techniques making it possible to isolate and identify the cell surface receptors for ATP and its breakdown product, adenosine. His death clearly impacted a huge number of scientists who have lost their pioneering leader. In this Review, I will not talk of the many discoveries made by Professor Burnstock, nor of his enormous scientific contributions to the field and of the incredible number of prizes and public recognitions that he has received after his theory was accepted worldwide. Instead, I will share some personal memories on him as a teacher and scientist, and, most of all, as a loyal and reliable friend. Geoff was an extraordinary human being, always eager to collaborate and share data, never jealous of his findings and capable of learning even from young people. He was known for his enthusiasm, empathy and ability to motivate young scientists. I was lucky to meet him when I was still very young, and the collaboration and friendship that we established and maintained across the years has profoundly conditioned my professional and personal life. For me, Geoff was what in Italy we call a "Maestro", one of those leading figures who are fundamental not only for mentoring an individual's career but also their growth as a scientist and as a human being.

Microglial vesicles improve post-stroke recovery by preventing immune cell senescence and favoring oligodendrogenesis

Contrasting myelin damage through the generation of new myelinating oligodendrocytes represents a promising approach to promote functional recovery after stroke. Here, we asked whether activation of microglia and monocyte-derived macrophages affects the regenerative process sustained by G protein-coupled receptor 17 (GPR17)-expressing oligodendrocyte precursor cells (OPCs), a subpopulation of OPCs specifically reacting to ischemic injury. GPR17-iCreER^T2:CAG-eGFP reporter mice were employed to trace the fate of GPR17-expressing OPCs, labeled by the green fluorescent protein (GFP), after permanent middle cerebral artery occlusion. By microglia/macrophages pharmacological depletion studies, we show that innate immune cells favor GFP⁺ OPC reaction and limit myelin damage early after injury, whereas they lose their pro-resolving capacity and acquire a dystrophic "senescent-like" phenotype at later stages. Intracerebral infusion of regenerative microglia-derived extracellular vesicles (EVs) restores protective microglia/macrophages functions, limiting their senescence during the post-stroke phase, and enhances the maturation of GFP⁺ OPCs at lesion borders, resulting in ameliorated neurological functionality. In vitro experiments show that EV-carried transmembrane tumor necrosis factor (tmTNF) mediates the pro-differentiating effects on OPCs, with future implications for regenerative therapies.

The history of the Purine Club: a tribute to Prof. Geoffrey Burnstock

The international purinergic scientific community has lost its pioneer. Geoffrey Burnstock, born on the 10th of May 1929 in London, died on the 2nd of June 2020, aged 91, in Melbourne (Australia). Geoff was one of the most highly regarded scientists of his generation. In the 1960s and 1970s, he developed a radical and somehow heretical new theory and opened an entire new field of science, signalling via extracellular nucleotides (the “purinergic theory”), which revolutionized our understanding of how cells communicate between each other. Initially, his unconventional theory found a lot of resistance in the scientific community. Once, one scientist even threatened to devote his entire life to disproving Burnstock’s theory. Undeterred, Geoff went further on, and continued to accumulate evidence in favour of his hypothesis, and led the field ever since. He struggled to attract new scientists to this new field of research and, in the early 1990s, due to new molecular biology techniques making it possible to isolate and identify cell surface receptors for ATP and its breakdown product adenosine, did evidence emerge that eventually convinced the doubters. The number of spontaneous obituaries and messages honouring Geoff’s memory that have appeared on specialized Journals and in the public press throughout the world since last June indicates that many people are clearly affected by his death. Besides being a rigorous, ethical and extremely brilliant scientist, Geoff was an extraordinary human being, always eager to collaborate and share data, never jealous of his findings and capable of learning things even from young people. He was known for his enthusiasm, empathy and ability to motivate young scientists and promote their careers. After the establishment of the Purine Club back in the 1990s, numerous Purine Club Chapters have been formed around the world with Geoff’s help and encouragement. He has obviously also been the inspirator and founder of our Journal, Purinergic Signalling (PUSI). For this reason, Charles Kennedy, the current Editor of the Journal, and myself thought that it might be nice to invite representatives from all known Purine Clubs to send a few notes to be published in PUSI on the history of their club and how Geoff inspired, aided or supported them. Here, I have collected all their contributions and I share with the entire purinergic community my personal memories on how the Purine Club was born and developed thanks to the invaluable mentoring of Geoffrey Burnstock. I apologize in advance if I am missing some information or forgot to mention somebody, and I strongly encourage all readers to submit memories and additional information that I shall gather for future writing. Keeping alive the history of how the field developed will be the best tribute that we can play to celebrate Geoff’s work along the years.

In Memoriam Geoffrey Burnstock: Creator of Purinergic Signaling

Geoff Burnstock (1929–2020) discovered purinergic signaling in a fastidious research that started in early 1960 and culminated in a concept of purinergic nerves in 1972. Subsequently, Geoff developed the concept of purinergic transmission and demonstrated ATP storage, release, and degradation in the context of cotransmission, which was another fundamental concept developed by him. Purinergic transmission contributes to the most fundamental physiological functions such as sensory transduction, regulation of heart rate, smooth muscle contraction, bile secretion, endocrine regulation, immune responses, as well as to various pathophysiological conditions, including inflammation, cancer, neuropathic pain, diabetes, and kidney failure.

In vivo silencing of miR-125a-3p promotes myelin repair in models of white matter demyelination

In the last decade, microRNAs have been increasingly recognized as key modulators of glial development. Recently, we identified miR-125a-3p as a new player in oligodendrocyte physiology, regulating in vitro differentiation of oligodendrocyte precursor cells (OPCs). Here, we show that miR-125a-3p is upregulated in active lesions of multiple sclerosis (MS) patients and in OPCs isolated from the spinal cord of chronic experimental autoimmune encephalomyelitis (EAE) mice, but not in those isolated from the spontaneously remyelinating corpus callosum of lysolecithin-treated mice. To test whether a sustained expression of miR-125a-3p in OPCs contribute to defective remyelination, we modulated miR-125a-3p expression in vivo and ex vivo after lysolecithin-induced demyelination. We found that lentiviral over-expression of miR-125a-3p impaired OPC maturation, whereas its downregulation accelerated remyelination. Transcriptome analysis and luciferase reporter assay revealed that these effects are partly mediated by the direct interaction of miR-125a-3p with Slc8a3, a sodium-calcium membrane transporter, and identified novel candidate targets, such as Gas7, that we demonstrated necessary to correctly address oligodendrocytes to terminal maturation. These findings show that miR-125a-3p upregulation negatively affects OPC maturation in vivo, suggest its role in the pathogenesis of demyelinating diseases and unveil new targets for future promyelinating protective interventions.

Regulation and signaling of the GPR17 receptor in oligodendroglial cells

Remyelination, namely, the formation of new myelin sheaths around denuded axons, counteracts axonal degeneration and restores neuronal function. Considerable advances have been made in understanding this regenerative process that often fails in diseases like multiple sclerosis, leaving axons demyelinated and vulnerable to damage, thus contributing to disease progression. The identification of the membrane receptor GPR17 on a subset of oligodendrocyte precursor cells (OPCs), which mediate remyelination in the adult central nervous system (CNS), has led to a huge amount of evidence that validated this receptor as a new attractive target for remyelinating therapies. Here, we summarize the role of GPR17 in OPC function, myelination and remyelination, describing its atypical pharmacology, its downstream signaling, and the genetic and epigenetic factors modulating its activity. We also highlight crucial insights into GPR17 pathophysiology coming from the demonstration that oligodendrocyte injury, associated with inflammation in chronic neurodegenerative conditions, is invariably characterized by abnormal and persistent GPR17 upregulation, which, in turn, is accompanied by a block of OPCs at immature premyelinating stages. Finally, we discuss the current literature in light of the potential exploitment of GPR17 as a therapeutic target to promote remyelination.

Dipeptidyl peptidase-4 inhibitors and sulfonylureas prevent the progressive impairment of the nigrostriatal dopaminergic system induced by diabetes during aging

The nigrostriatal dopaminergic system (NDS) controls motor activity, and its impairment during type 2 diabetes (T2D) progression could increase Parkinson's disease risk in diabetics. If so, whether glycemia regulation prevents this impairment needs to be addressed. We investigated whether T2D impairs the NDS and whether dipeptidyl peptidase-4 inhibition (DPP-4i; a clinical strategy against T2D but also neuroprotective in animal models) prevents this effect, in middle-aged mice. Neither T2D (induced by 12 months of high-fat diet) nor aging (14 months) changed striatal dopamine content assessed by high-performance liquid chromatography. However, T2D reduced basal and amphetamine-stimulated striatal extracellular dopamine, assessed by microdialysis. Both the DPP-4i linagliptin and the sulfonylurea glimepiride (an antidiabetic comparator unrelated to DPP-4i) counteracted these effects. The functional T2D-induced effects did not correlate with NDS neuronal/glial alterations. However, aging itself affected striatal neurons/glia, and the glia effects were counteracted mainly by DPP-4i. These findings show NDS functional pathophysiology in T2D and suggest the preventive use of two unrelated anti-T2D drugs. Moreover, DPP-4i counteracted striatal age-related glial alterations suggesting striatal rejuvenation properties.

Surface Plasmon Resonance as a Tool for Ligand Binding Investigation of Engineered GPR17 Receptor, a G Protein Coupled Receptor Involved in Myelination

The aim of this study was to investigate the potential of surface plasmon resonance (SPR) spectroscopy for the measurement of real-time ligand-binding affinities and kinetic parameters for GPR17, a G protein-coupled receptor (GPCR) of major interest in medicinal chemistry as potential target in demyelinating diseases. The receptor was directly captured, in a single-step, from solubilized membrane extracts on the sensor chip through a covalently bound anti-6x-His-antibody and retained its ligand binding activity for over 24 h. Furthermore, our experimental setup made possible, after a mild regeneration step, to remove the bound receptor without damaging the antibody, and thus to reuse many times the same chip. Two engineered variants of GPR17, designed for crystallographic studies, were expressed in insect cells, extracted from crude membranes and analyzed for their binding with two high affinity ligands: the antagonist Cangrelor and the agonist Asinex 1. The calculated kinetic parameters and binding constants of ligands were in good agreement with those reported from activity assays and highlighted a possible functional role of the N-terminal residues of the receptor in ligand recognition and binding. Validation of SPR results was obtained by docking and molecular dynamics of GPR17-ligands interactions and by functional in vitro studies. The latter allowed us to confirm that Asinex 1 behaves as GPR17 receptor agonist, inhibits forskolin-stimulated adenylyl cyclase pathway and promotes oligodendrocyte precursor cell maturation and myelinating ability.

Adenosine Signaling in Glioma Cells

Purines and pyrimidines are fundamental signaling molecules in controlling the survival and proliferation of astrocytes, as well as in mediating cell-to-cell communication between glial cells and neurons in the healthy brain. The malignant transformation of astrocytes towards progressively more aggressive brain tumours (from astrocytoma to anaplastic glioblastoma) leads to modifications in both the survival and cell death pathways which overall confer a growth advantage to malignant cells and resistance to many cytotoxic stimuli. It has been demonstrated, however, that, in astrocytomas, several purinergic (in particular adenosinergic) pathways controlling cell survival and death are still effective and, in some cases, even enhanced, providing invaluable targets for purine-based chemotherapy, that still represents an appropriate pharmacological approach to brain tumours. In this chapter, the current knowledge on both receptor-mediated and receptor-independent adenosine pathways in astrocytomas will be reviewed, with a particular emphasis on the most promising targets which could be translated from in vitro studies to in vivo pharmacology. Additionally, we have included new original data from our laboratory demonstrating a key involvement of MAP kinases in the cytostastic and cytotoxic effects exerted by an adenosine analogue, 2-CdA, which with the name of Cladribine is already clinically utilized in haematological malignancies. Here we show that 2-CdA can activate multiple intracellular pathways leading to cell cycle block and cell death by apoptosis of a human astrocytoma cell line that bears several pro-survival genetic mutations. Although in vivo data are still lacking, our results suggest that adenosine analogues could therefore be exploited to overcome resistance to chemotherapy of brain tumours.

Basal astrocyte and microglia activation in the central nervous system of Familial Hemiplegic Migraine Type I mice

Background

Gain-of-function missense mutations in the α1A subunit of neuronal CaV2.1 channels, which define Familial Hemiplegic Migraine Type 1 (FHM1), result in enhanced cortical glutamatergic transmission and a higher susceptibility to cortical spreading depolarization. It is now well established that neurons signal to surrounding glial cells, namely astrocytes and microglia, in the central nervous system, which in turn become activated and in pathological conditions can sustain neuroinflammation. We and others previously demonstrated an increased activation of pro-algogenic pathways, paralleled by augmented macrophage infiltration, in both isolated trigeminal ganglia and mixed trigeminal ganglion neuron-satellite glial cell cultures of FHM1 mutant mice. Hence, we hypothesize that astrocyte and microglia activation may occur in parallel in the central nervous system.

Methods

We have evaluated signs of reactive glia in brains from naïve FHM1 mutant mice in comparison with wild type animals by immunohistochemistry and Western blotting.

Results

Here we show for the first time signs of reactive astrogliosis and microglia activation in the naïve FHM1 mutant mouse brain.

Conclusions

Our data reinforce the involvement of glial cells in migraine, and suggest that modulating such activation may represent an innovative approach to reduce pathology.

Improvement of fiber connectivity and functional recovery after stroke by montelukast, an available and safe anti-asthmatic drug

Stroke is one of the main causes of death, neurological dysfunctions or disability in elderly. Neuroprotective drugs have been proposed to improve long-term recovery after stroke, but failed to reach clinical effectiveness. Hence, recent studies suggested that restorative therapies should combine neuroprotection and remyelination. Montelukast, an anti-asthmatic drug, was shown to exert neuroprotection in animal models of CNS injuries, but its ability to affect oligodendrocytes, restoring fiber connectivity, remains to be determined. In this study, we evaluated whether montelukast induces long-term repair by promoting fiber connectivity up to 8 weeks after middle cerebral artery occlusion (MCAo), using different experimental approaches such as in vivo diffusion magnetic resonance imaging (MRI), electrophysiological techniques, ex vivo diffusion tensor imaging (DTI)-based fiber tracking and immunohistochemistry. We found that, in parallel with a reduced evolution of ischemic lesion and atrophy, montelukast increased the DTI-derived axial diffusivity and number of myelin fibers, the density of myelin binding protein (MBP) and the number of GSTpi⁺ mature oligodendrocytes. Together with the rescue of MCAo-induced impairments of local field potentials in ischemic cortex, the data suggest that montelukast may improve fibers reorganization. Thus, to ascertain whether this effect involved changes of oligodendrocyte precursor cells (OPCs) activation and maturation, we used the reporter GPR17iCreERT2:CAG-eGreen florescent protein (GFP) mice that allowed us to trace the fate of OPCs throughout animal's life. Our results showed that montelukast enhanced the OPC recruitment and proliferation at acute phase, and increased their differentiation to mature oligodendrocytes at chronic phase after MCAo. Considering the crosstalk between OPCs and microglia has been widely reported in the context of demyelinating insults, we also assessed microglia activation. We observed that montelukast influenced the phenotype of microglial cells, increasing the number of M2 polarized microglia/macrophages, over the M1 phenotype, at acute phase after MCAo. In conclusion, we demonstrated that montelukast improves fiber re-organization and long-term functional recovery after brain ischemia, enhancing recruitment and maturation of OPCs. The present data suggest that montelukast, an already approved drug, could be "repositioned "as a protective drug in stroke acting also on fiber re-organization.

Purple Corn Extract as Anti-allodynic Treatment for Trigeminal Pain: Role of Microglia

Natural products have attracted interest in the search for new and effective analgesics and coadjuvant approaches to several types of pain. It is in fact well known that many of their active ingredients, such as anthocyanins (ACNs) and polyphenols, can exert potent anti-inflammatory actions. Nevertheless, their potential beneficial effects in orofacial painful syndromes have not been assessed yet. Here, we have evaluated the preventive effect of an ACN-enriched purple corn extract against the development of orofacial allodynia, in comparison with isogenic yellow corn extract containing only polyphenols. Orofacial allodynia developed following induction of temporomandibular joint (TMJ) inflammation in male rats, due to the injection of Complete Freund’s Adjuvant (CFA), and was evaluated by von Frey filaments. Animals drank purple or yellow corn extracts or water starting from 11 days before induction of inflammation and up to the end of the experiment 3 days later. To highlight possible additive and/or synergic actions, some animals also received the anti-inflammatory drug acetyl salicylic acid (ASA). In parallel with the evaluation of allodynia, we have focused our attention on the activation of microglia cells in the central nervous system (CNS), as it is well-known that they significantly contribute to neuronal sensitization and pain. Our data demonstrate that purple corn extract is as effective as ASA in preventing the development of orofacial allodynia, and only partial additive effect is observed when the two agents are co-administered. Yellow corn exerted no effect. Multiple mechanisms are possibly involved in the action of purple corn, including reduction of trigeminal macrophage infiltration and the shift of microglia cell polarization to an anti-inflammatory phenotype. In fact, in rats receiving yellow corn or water microglia cells show thick, short cell processes typical of activated cells. Conversely, thinner and longer microglia cell processes are observed in the brainstem of animals drinking purple corn extract; shape changes are accompanied by a reduction in the expression of pro-inflammatory molecules and increased production of anti-inflammatory mediators. Administration of purple corn extracts therefore represents a possible low-cost and easy way to reduce trigeminal-associated pain in various pathological conditions also thanks to the modulation of microglia reactivity.

Pharmacological Properties and Biological Functions of the GPR17 Receptor, a Potential Target for Neuro-Regenerative Medicine

In 2006, cells heterologously expressing the "orphan" receptor GPR17 were shown to acquire responses to both uracil nucleotides and cysteinyl-leukotrienes, two families of signaling molecules accumulating in brain or heart as a result of hypoxic/traumatic injuries. In subsequent years, evidence of GPR17 key role in oligodendrogenesis and myelination has highlighted it as a "model receptor" for new therapies in demyelinating and neurodegenerative diseases. The apparently contrasting evidence in the literature about the role of GPR17 in promoting or inhibiting myelination can be due to its transient expression in the intermediate stages of differentiation, exerting a pro-differentiating function in early oligodendrocyte precursor cells (OPCs), and an inhibitory role in late stage maturing cells. Meanwhile, several papers extended the initial data on GPR17 pharmacology, highlighting a "promiscuous" behavior of this receptor; indeed, GPR17 is able to respond to other emergency signals like oxysterols or the pro-inflammatory cytokine SDF-1, underlying GPR17 ability to adapt its responses to changes of the surrounding extracellular milieu, including damage conditions. Here, we analyze the available literature on GPR17, in an attempt to summarize its emerging biological roles and pharmacological properties.

Differential local tissue permissiveness influences the final fate of GPR17-expressing oligodendrocyte precursors in two distinct models of demyelination

Promoting remyelination is recognized as a novel strategy to foster repair in neurodegenerative demyelinating diseases, such as multiple sclerosis. In this respect, the receptor GPR17, recently emerged as a new target for remyelination, is expressed by early oligodendrocyte precursors (OPCs) and after a certain differentiation stage it has to be downregulated to allow progression to mature myelinating oligodendrocytes. Here, we took advantage of the first inducible GPR17 reporter mouse line (GPR17‐iCreER^T2xCAG‐eGFP mice) allowing to follow the final fate of GPR17⁺ cells by tamoxifen‐induced GFP‐labeling to unveil the destiny of these cells in two demyelination models: experimental autoimmune encephalomyelitis (EAE), characterized by marked immune cell activation and inflammation, and cuprizone induced demyelination, where myelin dysfunction is achieved by a toxic insult. In both models, demyelination induced a strong increase of fluorescent GFP⁺ cells at damaged areas. However, only in the cuprizone model reacting GFP⁺ cells terminally differentiated to mature oligodendrocytes, thus contributing to remyelination. In EAE, GFP⁺ cells were blocked at immature stages and never became myelinating oligodendrocytes. We suggest these strikingly distinct fates be due to different permissiveness of the local CNS environment. Based on previously reported GPR17 activation by emergency signals (e.g., Stromal Derived Factor‐1), we propose that a marked inflammatory milieu, such as that reproduced in EAE, induces GPR17 overactivation resulting in impaired downregulation, untimely and prolonged permanence in OPCs, leading, in turn, to differentiation blockade. Combined treatments with remyelinating agents and anti‐inflammatory drugs may represent new potential adequate strategies to halt neurodegeneration and foster recovery.

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.

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'.

The role of oligodendrocyte precursor cells expressing the GPR17 receptor in brain remodeling after stroke

Following stroke-induced neuronal damage, quiescent oligodendrocyte precursors (OPCs) are activated to proliferate and later to differentiate to myelin-producing cells. GPR17, a receptor transiently expressed on early OPCs, has emerged as a target to implement stroke repair through stimulation of OPC maturation. However, being GPR17 completely downregulated in myelin-producing oligodendrocytes, its actual role in determining the final fate of OPCs after cerebral ischemia is still uncertain. Here, to univocally define the spatiotemporal changes and final fate of GPR17-expressing OPCs, we induced ischemia by middle cerebral artery occlusion (MCAo) in reporter GPR17iCreER^T2:CAG-eGreen florescent protein (GFP) mice, in which, upon tamoxifen treatment, cells expressing GPR17 become green and traceable for their entire life. Starting from 3 days and up to 2 weeks after MCAo, GFP⁺ cells markedly accumulated in regions surrounding the ischemic lesion; several of them proliferated, as shown by co-labeling of the DNA synthesis marker 5-Bromo-2′-deoxyuridine (BrdU). Almost all GFP⁺/BrdU⁺ cells expressed the OPC early marker neural/glial antigen 2 (NG2), indicating that they were still precursors. Accumulation of GFP⁺ cells was also because of OPC recruitment from surrounding areas, as suggested in vivo by acquisition of typical features of migrating OPCs, shown in vitro in presence of the chemoattractant PDGF-AA and confirmed by transplantation of GFP⁺-OPCs in wild-type MCAo mice. Eight weeks after MCAo, only some of these precociously recruited cells had undergone maturation as shown by NG2 loss and acquisition of mature myelinating markers like GSTpi. A pool of recruited GFP⁺-OPCs was kept at a precursor stage to likely make it available for further insults. Thus, very early after ischemia, GFP⁺-OPCs proliferate and migrate toward the lesion; however, most of these cells remain undifferentiated, suggesting functional roles other than myelination.

MiR-125a-3p timely inhibits oligodendroglial maturation and is pathologically up-regulated in human multiple sclerosis

In the mature central nervous system (CNS), oligodendrocytes provide support and insulation to axons thanks to the production of a myelin sheath. During their maturation to myelinating cells, oligodendroglial precursors (OPCs) follow a very precise differentiation program, which is finely orchestrated by transcription factors, epigenetic factors and microRNAs (miRNAs), a class of small non-coding RNAs involved in post-transcriptional regulation. Any alterations in this program can potentially contribute to dysregulated myelination, impaired remyelination and neurodegenerative conditions, as it happens in multiple sclerosis (MS). Here, we identify miR-125a-3p, a developmentally regulated miRNA, as a new actor of oligodendroglial maturation, that, in the mammalian CNS regulates the expression of myelin genes by simultaneously acting on several of its already validated targets. In cultured OPCs, over-expression of miR-125a-3p by mimic treatment impairs while its inhibition with an antago-miR stimulates oligodendroglial maturation. Moreover, we show that miR-125a-3p levels are abnormally high in the cerebrospinal fluid of MS patients bearing active demyelinating lesions, suggesting that its pathological upregulation may contribute to MS development, at least in part by blockade of OPC differentiation leading to impaired repair of demyelinated lesions.

A promiscuous recognition mechanism between GPR17 and SDF-1: Molecular insights

Recent data and publications suggest a promiscuous behaviour for GPR17, a class-A GPCR operated by different classes of ligands, such as uracil nucleotides, cysteinyl-leukotrienes and oxysterols. This observation, together with the ability of several class-A GPCRs to form homo- and hetero-dimers, is likely to unveil new pathophysiological roles and novel emerging pharmacological properties for some of these GPCRs, including GPR17. This receptor shares structural, phylogenetic and functional properties with some chemokine receptors, CXCRs. Both GPR17 and CXCR2 are operated by oxysterols, and both GPR17 and CXCR ligands have been demonstrated to have a role in orchestrating inflammatory responses and oligodendrocyte precursor cell differentiation to myelinating cells in acute and chronic diseases of the central nervous system. Here, by combining in silico modelling data with in vitro validation in (i) a classical reference pharmacological assay for GPCR activity and (ii) a model of maturation of primary oligodendrocyte precursor cells, we demonstrate that GPR17 can be activated by SDF-1, a ligand of chemokine receptors CXCR4 and CXCR7, and investigate the underlying molecular recognition mechanism. We also demonstrate that cangrelor, a GPR17 orthosteric antagonist, can block the SDF-1-mediated activation of GPR17 in a concentration-dependent manner. The ability of GPR17 to respond to different classes of GPCR ligands suggests that this receptor modifies its function depending on the extracellular mileu changes occurring under specific pathophysiological conditions and advocates it as a strategic target for neurodegenerative diseases with an inflammatory/immune component.

Structural and functional rejuvenation of the aged brain by an approved anti-asthmatic drug

As human life expectancy has improved rapidly in industrialized societies, age-related cognitive impairment presents an increasing challenge. Targeting histopathological processes that correlate with age-related cognitive declines, such as neuroinflammation, low levels of neurogenesis, disrupted blood–brain barrier and altered neuronal activity, might lead to structural and functional rejuvenation of the aged brain. Here we show that a 6-week treatment of young (4 months) and old (20 months) rats with montelukast, a marketed anti-asthmatic drug antagonizing leukotriene receptors, reduces neuroinflammation, elevates hippocampal neurogenesis and improves learning and memory in old animals. By using gene knockdown and knockout approaches, we demonstrate that the effect is mediated through inhibition of the GPR17 receptor. This work illustrates that inhibition of leukotriene receptor signalling might represent a safe and druggable target to restore cognitive functions in old individuals and paves the way for future clinical translation of leukotriene receptor inhibition for the treatment of dementias.

The leukotriene receptor antagonist montelukast is an anti-asthmatic drug. Here, the authors show that montelukast reduces neuroinflammation, promotes hippocampal neurogenesis and restores learning and memory in old rats suffering from ageing-associated cognitive dysfunction.

GPR17 expressing NG2-Glia: Oligodendrocyte progenitors serving as a reserve pool after injury

In the adult brain NG2-glia continuously generate mature, myelinating oligodendrocytes. To which extent the differentiation process is common to all NG2-glia and whether distinct pools are recruited for repair under physiological and pathological conditions still needs clarification. Here, we aimed at investigating the differentiation potential of adult NG2-glia that specifically express the G-protein coupled receptor 17 (GPR17), a membrane receptor that regulates the differentiation of these cells at postnatal stages. To this aim, we generated the first BAC transgenic GPR17-iCreER(T2) mouse line for fate mapping studies. In these mice, under physiological conditions, GPR17(+) cells--in contrast to GPR17(-) NG2-glia--did not differentiate within 3 months, a peculiarity that was overcome after cerebral damage induced by acute injury or ischemia. After these insults, GPR17(+) NG2-glia rapidly reacted to the damage and underwent maturation, suggesting that they represent a 'reserve pool' of adult progenitors maintained for repair purposes.

The ubiquitin ligase Mdm2 controls oligodendrocyte maturation by intertwining mTOR with G protein-coupled receptor kinase 2 in the regulation of GPR17 receptor desensitization

During oligodendrocyte precursor cell (OPC) differentiation, defective control of the membrane receptor GPR17 has been suggested to block cell maturation and impair remyelination under demyelinating conditions. After the immature oligodendrocyte stage, to enable cells to complete maturation, GPR17 is physiologically down-regulated via phosphorylation/desensitization by G protein-coupled receptor kinases (GRKs); conversely, GRKs are regulated by the "mammalian target of rapamycin" mTOR. However, how GRKs and mTOR are connected to each other in modulating GPR17 function and oligodendrogenesis has remained elusive. Here we show, for the first time, a role for Murine double minute 2 (Mdm2), a ligase previously involved in ubiquitination/degradation of the onco-suppressor p53 protein. In maturing OPCs, both rapamycin and Nutlin-3, a small molecule inhibitor of Mdm2-p53 interactions, increased GRK2 sequestration by Mdm2, leading to impaired GPR17 down-regulation and OPC maturation block. Thus, Mdm2 intertwines mTOR with GRK2 in regulating GPR17 and oligodendrogenesis and represents a novel actor in myelination.

P2Y2 receptor antagonists as anti-allodynic agents in acute and sub-chronic trigeminal sensitization: role of satellite glial cells

Trigeminal (TG) pain often lacks a satisfactory pharmacological control. A better understanding of the molecular cross-talk between TG neurons and surrounding satellite glial cells (SGCs) could help identifying innovative targets for the development of more effective analgesics. We have previously demonstrated that neuronal pro-algogenic mediators upregulate G protein-coupled nucleotide P2Y receptors (P2YRs) expressed by TG SGCs in vitro. Here, we have identified the specific P2YR subtypes involved (i.e., the ADP-sensitive P2Y1 R and the UTP-responsive P2Y2 R subtypes), and demonstrated the contribution of neuron-derived prostaglandins to their upregulation. Next, we have translated these data to an in vivo model of TG pain (namely, rats injected with Complete Freund's adjuvant in the temporomandibular joint), by demonstrating activation of SGCs and upregulation of P2Y1 R and P2Y2 R in the ipsi-lateral TG. To unequivocally link P2YRs to the development of facial allodynia, we treated animals with various purinergic antagonists. The selective P2Y2 R antagonist AR-C118925 completely inhibited SGCs activation, exerted a potent anti-allodynic effect that lasted over time, and was still effective when administration was started 6-days post induction of allodynia, i.e. under subchronic pain conditions. Conversely, the selective P2Y1 R antagonist MRS2179 was completely ineffective. Moreover, similarly to the anti-inflammatory drug acetylsalicylic acid and the known anti-migraine agent sumatriptan, the P2X/P2Y nonselective antagonist PPADS was only partially effective, and completely lost its activity under sub-chronic conditions. Taken together, our results highlight glial P2Y2 Rs as potential "druggable" targets for the successful management of TG-related pain.

Early phenotypic asymmetry of sister oligodendrocyte progenitor cells after mitosis and its modulation by aging and extrinsic factors

Oligodendrocyte progenitor cells (OPCs) persist in the adult central nervous system and guarantee oligodendrocyte turnover throughout life. It remains obscure how OPCs avoid exhaustion during adulthood. Similar to stem cells, OPCs could self-maintain by undergoing asymmetric divisions generating a mixed progeny either keeping a progenitor phenotype or proceeding to differentiation. To address this issue, we examined the distribution of stage-specific markers in sister OPCs during mitosis and later after cell birth, and assessed its correlation with distinct short-term fates. In both the adult and juvenile cerebral cortex a fraction of dividing OPCs gives rise to sister cells with diverse immunophenotypic profiles and short-term behaviors. Such heterogeneity appears as cells exit cytokinesis, but does not derive from the asymmetric segregation of molecules such as NG2 or PDGFRa expressed in the mother cell. Rather, rapid downregulation of OPC markers and upregulation of molecules associated with lineage progression contributes to generate early sister OPC asymmetry. Analyses during aging and upon exposure to physiological (i.e., increased motor activity) and pathological (i.e., trauma or demyelination) stimuli showed that both intrinsic and environmental factors contribute to determine the fraction of symmetric and asymmetric OPC pairs and the phenotype of the OPC progeny as soon as cells exit mitosis.

Expression of dual nucleotides/cysteinyl-leukotrienes receptor GPR17 in early trafficking of cardiac stromal cells after myocardial infarction

GPR17 is a G_i-coupled dual receptor activated by uracil-nucleotides and cysteinyl-leukotrienes. These mediators are massively released into hypoxic tissues. In the normal heart, GPR17 expression has been reported. By contrast, its role in myocardial ischaemia has not yet been assessed. In the present report, the expression of GPR17 was investigated in mice before and at early stages after myocardial infarction by using immunofluorescence, flow cytometry and RT-PCR. Before induction of ischaemia, results indicated the presence of the receptor in a population of stromal cells expressing the stem-cell antigen-1 (Sca-1). At early stages after ligation of the coronary artery, the receptor was expressed in Sca-1⁺ cells, and cells stained with Isolectin-B4 and anti-CD45 antibody. GPR17⁺ cells also expressed mesenchymal marker CD44. GPR17 function was investigated in vitro in a Sca-1⁺/CD31⁻ cell line derived from normal hearts. These experiments showed a migratory function of the receptor by treatment with UDP-glucose and leukotriene LTD4, two GPR17 pharmacological agonists. The GPR17 function was finally assessed in vivo by treating infarcted mice with Cangrelor, a pharmacological receptor antagonist, which, at least in part, inhibited early recruitment of GPR17⁺ and CD45⁺ cells. These findings suggest a regulation of heart-resident mesenchymal cells and blood-borne cellular species recruitment following myocardial infarction, orchestrated by GPR17.

Microglia is a key player in the reduction of stroke damage promoted by the new antithrombotic agent ticagrelor

The ADP-responsive P2Y12 receptor is expressed on both platelets and microglia. Clinical data show that ticagrelor, a direct-acting, reversibly binding P2Y12-receptor antagonist, reduces total cardiovascular events, including stroke. In our present study, we investigated the expression of P2Y12 receptors and the effects of ticagrelor on brain injury in Sprague-Dawley rats subjected to a permanent middle cerebral artery occlusion (MCAo). Rats were treated per os with ticagrelor 3 mg/kg or vehicle at 10 minutes, 22, and 36 hours after MCAo and killed after 48 hours. Immunofluorescence analysis showed an ischemia-related modulation of the P2Y12 receptor, which is constitutively expressed in Iba1(+) resting microglia. After MCAo, activated microglia was mainly concentrated around the lesion, with fewer cells present inside the ischemic core. Ticagrelor significantly attenuated the evolution of ischemic damage-evaluated by magnetic resonance imaging (MRI) at 2, 24, and 48 hours after MCAo-, the number of infiltrating cells expressing the microglia/monocyte marker ED-1, the cerebral expression of proinflammatory mediators (interleukin 1 (IL-1), monocyte chemoattractant protein 1 (MCP-1), nitric oxide synthase (iNOS)) and the associated neurologic impairment. In transgenic fluorescent reporter CX3CR1-green fluorescent protein (GFP) mice, 72 hours after MCAo, ticagrelor markedly reduced GFP(+) microglia and both early and late infiltrating blood-borne cells. Finally, in primary cultured microglia, ticagrelor fully inhibited ADP-induced chemotaxis (P<0.01). Our results show that ticagrelor is protective against ischemia-induced cerebral injury and this effect is mediated, at least partly, by inhibition of P2Y12-mediated microglia activation and chemotaxis.

A rapid and efficient immunoenzymatic assay to detect receptor protein interactions: G protein-coupled receptors

G protein-coupled receptors (GPCRs) represent one of the largest families of cell surface receptors, and are the target of at least one-third of the current therapeutic drugs on the market. Along their life cycle, GPCRs are accompanied by a range of specialized GPCR-interacting proteins (GIPs), which take part in receptor proper folding, targeting to the appropriate subcellular compartments and in receptor signaling tasks, and also in receptor regulation processes, such as desensitization and internalization. The direction of protein-protein interactions and multi-protein complexes formation is crucial in understanding protein function and their implication in pathological events. Although several methods have been already developed to assay protein complexes, some of them are quite laborious, expensive, and, more important, they do not generate fully quantitative results. Herein, we show a rapid immunoenzymatic assay to quantify GPCR interactionswith its signaling proteins. The recently de-orphanized GPCR, GPR17, was chosen as a GPCR prototype to optimize the assay. In a GPR17 transfected cell line and primary oligodendrocyte precursor cells, GPR17 interaction with proteins involved in the typical GPCR regulation, such as desensitization and internalization machinery, was investigated. The obtained results were validated by co-immunoprecipitation experiments, confirming this new method as a rapid and quantitative assay to study protein-protein interactions.

Chronic inflammatory diseases: do immunological patterns drive the choice of biotechnology drugs? A critical review

Chronic inflammatory diseases represent a heterogeneous group of conditions that can affect practically any organ or system. An increasing number of biologic agents have been developed to selectively target the cell populations and signaling pathways involved in chronic inflammation, including cytokines, monoclonal antibodies and engineered receptors. This approach has been remarkably successful in alleviating some of the signs and symptoms of refractory autoimmune diseases. The use of this therapeutic strategy is likely to increase with the introduction of biosimilar agents. The different nature of these biological products makes the comparison of their pharmaceutical and clinical characteristics difficult, including safety and potency and these issues may be particularly relevant in the case of biosimilars. In addition, the heterogeneity of autoimmune diseases and of autoimmune patients, further adds to the complexity of choosing the right drug for each patient and predicting efficacy and safety of the treatment. In this review, we summarize actual knowledge about current biological agents and their use in autoimmune diseases, with a special emphasis for rheumatoid arthritis, inflammatory bowel diseases and psoriasis. The purpose of this analysis is to address the most critical issues raised by the rapid advancements in this field over recent years, and to acknowledge the potentially valuable gains brought about by the increasing availability of these new biologic agents.

Purines regulate adult brain subventricular zone cell functions: contribution of reactive astrocytes

Brain injuries modulate activation of neural stem cells (NSCs) in the adult brain. In pathological conditions, the concentrations of extracellular nucleotides (eNTs) raise several folds, contribute to reactive gliosis, and possibly directly affect subventricular zone (SVZ) cell functioning. Among eNTs and derived metabolites, the P2Y1 receptor agonist ADP strongly promotes astrogliosis and might also influence SVZ progenitor activity. Here, we tested the ability of the stable P2Y1 agonist adenosine 5'-O-(2-thiodiphosphate) (ADPβS) to control adult NSC functions both in vitro and in vivo, with a focus on the possible effects exerted by reactive astrocytes. In the absence of growth factors, ADPβS promoted proliferation and differentiation of SVZ progenitors. Moreover, ADPβS-activated astrocytes markedly changed the pattern of released cytokines and chemokines, and strongly modulated neurosphere-forming capacity of SVZ progenitors. Notably, a significant enhancement in proliferation was observed when SVZ cells, initially grown in the supernatant of astrocytes exposed to ADPβS, were shifted to normal medium. In vivo, ADPβS administration in the lateral ventricle of adult mice by osmotic minipumps caused diffused reactive astrogliosis, and a strong response of SVZ progenitors. Indeed, proliferation of glial fibrillary acidic protein-positive NSCs increased and led to a significant expansion of SVZ transit-amplifying progenitors and neuroblasts. Lineage tracing experiments performed in the GLAST::CreERT2;Rosa-YFP transgenic mice further demonstrated that ADPβS promoted proliferation of glutamate/aspartate transporter-positive progenitors and sustained their progression toward the generation of rapidly dividing progenitors. Altogether, our results show that the purinergic system crucially affects SVZ progenitor activities both directly and through the involvement of reactive astrocytes.

Changes of the GPR17 receptor, a new target for neurorepair, in neurons and glial cells in patients with traumatic brain injury

Unveiling the mechanisms participating in the damage and repair of traumatic brain injury (TBI) is fundamental to develop new therapies. The P2Y-like GPR17 receptor has recently emerged as a sensor of damage and a key actor in lesion remodeling/repair in the rodent brain, but its role in humans is totally unknown. Here, we characterized GPR17 expression in brain specimens from seven intensive care unit TBI patients undergoing neurosurgery for contusion removal and from 28 autoptic TBI cases (and 10 control subjects of matched age and gender) of two university hospitals. In both neurosurgery and autoptic samples, GPR17 expression was strong inside the contused core and progressively declined distally according to a spatio-temporal gradient. Inside and around the core, GPR17 labeled dying neurons, reactive astrocytes, and activated microglia/macrophages. In peri-contused parenchyma, GPR17 decorated oligodendrocyte precursor cells (OPCs) some of which had proliferated, indicating re-myelination attempts. In autoptic cases, GPR17 expression positively correlated with death for intracranial complications and negatively correlated with patients' post-traumatic survival. Data indicate lesion-specific sequential involvement of GPR17 in the (a) death of irreversibly damaged neurons, (b) activation of microglia/macrophages remodeling the lesion, and (c) activation/proliferation of multipotent parenchymal progenitors (both reactive astrocytes and OPCs) starting repair processes. Data validate GPR17 as a target for neurorepair and are particularly relevant to setting up new therapies for TBI patients.

UDP-glucose enhances outward K(+) currents necessary for cell differentiation and stimulates cell migration by activating the GPR17 receptor in oligodendrocyte precursors

In the developing and mature central nervous system, NG2 expressing cells comprise a population of cycling oligodendrocyte progenitor cells (OPCs) that differentiate into mature, myelinating oligodendrocytes (OLGs). OPCs are also characterized by high motility and respond to injury by migrating into the lesioned area to support remyelination. K(+) currents in OPCs are developmentally regulated during differentiation. However, the mechanisms regulating these currents at different stages of oligodendrocyte lineage are poorly understood. Here we show that, in cultured primary OPCs, the purinergic G-protein coupled receptor GPR17, that has recently emerged as a key player in oligodendrogliogenesis, crucially regulates K(+) currents. Specifically, receptor stimulation by its agonist UDP-glucose enhances delayed rectifier K(+) currents without affecting transient K(+) conductances. This effect was observed in a subpopulation of OPCs and immature pre-OLGs whereas it was absent in mature OLGs, in line with GPR17 expression, that peaks at intermediate phases of oligodendrocyte differentiation and is thereafter downregulated to allow terminal maturation. The effect of UDP-glucose on K(+) currents is concentration-dependent, blocked by the GPR17 antagonists MRS2179 and cangrelor, and sensitive to the K(+) channel blocker tetraethyl-ammonium, which also inhibits oligodendrocyte maturation. We propose that stimulation of K(+) currents is responsible for GPR17-induced oligodendrocyte differentiation. Moreover, we demonstrate, for the first time, that GPR17 activation stimulates OPC migration, suggesting an important role for this receptor after brain injury. Our data indicate that modulation of GPR17 may represent a strategy to potentiate the post-traumatic response of OPCs under demyelinating conditions, such as multiple sclerosis, stroke, and brain trauma.

The regulated expression, intracellular trafficking, and membrane recycling of the P2Y-like receptor GPR17 in Oli-neu oligodendroglial cells

GPR17 is a G-protein-coupled receptor that is activated by two classes of molecules: uracil-nucleotides and cysteinyl-leukotrienes. GPR17 is required for initiating the differentiation of oligodendrocyte precursors but has to be down-regulated to allow cells to undergo terminal maturation. Although a great deal has been learned about GPR17 expression and signaling, no information is currently available about the trafficking of native receptors after the exposure of differentiating oligodendrocytes to endogenous agonists. Here, we demonstrate that neuron-conditioned medium induces the transcriptionally mediated, time-regulated expression of GPR17 in Oli-neu, an oligodendrocyte precursor cell line, making these cells suitable for studying the endocytic traffic of the native receptor. Agonist-induced internalization, intracellular trafficking, and membrane recycling of GPR17 were analyzed by biochemical and immunofluorescence assays using an ad hoc-developed antibody against the extracellular N-terminal of GPR17. Both UDP-glucose and LTD(4) increased GPR17 internalization, although with different efficiency. At early time points, internalized GPR17 co-localized with transferrin receptor, whereas at later times it partially co-localized with the lysosomal marker Lamp1, suggesting that a portion of GPR17 is targeted to lysosomes upon ligand binding. An analysis of receptor recycling and degradation demonstrated that a significant aliquot of GPR17 is recycled to the cell surface. Furthermore, internalized GPR17 displayed a co-localization with the marker of the "short loop" recycling endosomes, Rab4, while showing very minor co-localization with the "long loop" recycling marker, Rab11. Our results provide the first data on the agonist-induced trafficking of native GPR17 in oligodendroglial cells and may have implications for both physiological and pathological myelination.

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

There has been tremendous progress in understanding neural stem cell (NSC) biology, with genetic and cell biological methods identifying sequential gene expression and molecular interactions guiding NSC specification into distinct neuronal and glial populations during development. Data has emerged on the possible exploitation of NSC-based strategies to repair adult diseased brain. However, despite increased information on lineage specific transcription factors, cell-cycle regulators and epigenetic factors involved in the fate and plasticity of NSCs, understanding of extracellular cues driving the behavior of embryonic and adult NSCs is still very limited. Knowledge of factors regulating brain development is crucial in understanding the pathogenetic mechanisms of brain dysfunction. Since injury-activated repair mechanisms in adult brain often recapitulate ontogenetic events, the identification of these players will also reveal novel regenerative strategies. Here, we highlight the purinergic system as a key emerging player in the endogenous control of NSCs. Purinergic signalling molecules (ATP, UTP and adenosine) act with growth factors in regulating the synchronized proliferation, migration, differentiation and death of NSCs during brain and spinal cord development. At early stages of development, transient and time-specific release of ATP is critical for initiating eye formation; once anatomical CNS structures are defined, purinergic molecules participate in calcium-dependent neuron-glia communication controlling NSC behaviour. When development is complete, some purinergic mechanisms are silenced, but can be re-activated in adult brain after injury, suggesting a role in regeneration and self-repair. Targeting the purinergic system to develop new strategies for neurodevelopmental disorders and neurodegenerative diseases will be also discussed.

Cardiomyocyte death induced by ischaemic/hypoxic stress is differentially affected by distinct purinergic P2 receptors

Blood levels of extracellular nucleotides (e.g. ATP) are greatly increased during heart ischaemia, but, despite the presence of their specific receptors on cardiomyocytes (both P2X and P2Y subtypes), their effects on the subsequent myocardial damage are still unknown. In this study, we aimed at investigating the role of ATP and specific P2 receptors in the appearance of cell injury in a cardiac model of ischaemic/hypoxic stress. Cells were maintained in a modular incubator chamber in a controlled humidified atmosphere of 95% N₂ for 16 hrs in a glucose-free medium. In this condition, we detected an early increase in the release of ATP in the culture medium, which was followed by a massive increase in the release of cytoplasmic histone-associated-DNA-fragments, a marker of apoptosis. Addition of either apyrase, which degrades extracellular ATP, or various inhibitors of ATP release via connexin hemichannels fully abolished ischaemic/hypoxic stress-associated apoptosis. To dissect the role of specific P2 receptor subtypes, we used a combined approach: (i) non-selective and, when available, subtype-selective P2 antagonists, were added to cardiomyocytes before ischaemic/hypoxic stress; (ii) selected P2 receptors genes were silenced via specific small interfering RNAs. Both approaches indicated that the P2Y₂ and P2χ₇ receptor subtypes are directly involved in the induction of cell death during ischaemic/hypoxic stress, whereas the P2Y₄ receptor has a protective effect. Overall, these findings indicate a role for ATP and its receptors in modulating cardiomyocyte damage during ischaemic/hypoxic stress.

Purinergic trophic signalling in glial cells: functional effects and modulation of cell proliferation, differentiation, and death

In the last decades, the discovery that glial cells do not only fill in the empty space among neurons or furnish them with trophic support but are rather essential participants to the various activities of the central and peripheral nervous system has fostered the search for the signalling pathways controlling their functions. Since the early 1990s, purines were foreseen as some of the most promising candidate molecules. Originally just a hypothesis, this has become a certainty as experimental evidence accumulated over years, as demonstrated by the exponentially growing number of articles related to the role of extracellular nucleotides and nucleosides in controlling glial cell functions. Indeed, as new functions for already known glial cells (for example, the ability of parenchymal astrocytes to behave as stem cells) or new subtypes of glial cells (for example, NG2(+) cells, also called polydendrocytes) are discovered also, new actions and new targets for the purinergic system are identified. Thus, glial purinergic receptors have emerged as new possible pharmacological targets for various acute and chronic pathologies, such as stroke, traumatic brain and spinal cord injury, demyelinating diseases, trigeminal pain and migraine, and retinopathies. In this article, we will summarize the most important and promising actions mediated by extracellular purines and pyrimidines in controlling the functions, survival, and differentiation of the various "classical" types of glial cells (i.e., astrocytes, oligodendrocytes, microglial cells, Müller cells, satellite glial cells, and enteric glial cells) but also of some rather new members of the family (e.g., polydendrocytes) and of other cells somehow related to glial cells (e.g., pericytes and spinal cord ependymal cells).

The GPR17 receptor in NG2 expressing cells: focus on in vivo cell maturation and participation in acute trauma and chronic damage

NG2-expressing cells comprise a population of cycling precursors that can exit the cell cycle and differentiate into mature oligodendrocytes. As a whole, they display heterogeneous properties and behaviors that remain unresolved at the molecular level, although partly interpretable as distinct maturation stages. To address this issue, we analyzed the expression of the GPR17 receptor, recently shown to decorate NG2-expressing cells and to operate as an early sensor of brain damage, in immature and adult oligodendrocyte progenitors in the intact brain and after injury. In both the early postnatal and adult cerebral cortex, distinct GPR17 protein localizations and expression levels define different stages of oligodendroglial maturation, ranging from the precursor phase to the premyelinating phenotype. As soon as cells exit mitosis, a fraction of NG2-expressing cells displays accumulation of GPR17 protein in the Golgi apparatus. GPR17 expression is subsequently upregulated and distributed to processes of cells that stop dividing, progressively lose NG2 positivity and assume premyelinating features. Absence of colabeling with mature markers or myelin proteins indicates that GPR17 is downregulated when cells complete their final maturation. BrdU-based fate-mapping demonstrated that a significant fraction of newly generated oligodendrocyte progenitors transiently upregulates GPR17 during maturation. Importantly, we also found that GPR17 does not participate to the early reaction of NG2-expressing cells to damage, while it is induced at postacute stages after injury. These findings identify GPR17 as a marker for progenitor progression within the oligodendroglial lineage and highlight its participation to postacute reactivity of NG2 cells in different injury paradigms.

Purinergic signalling: from normal behaviour to pathological brain function

Purinergic neurotransmission, involving release of ATP as an efferent neurotransmitter was first proposed in 1972. Later, ATP was recognised as a cotransmitter in peripheral nerves and more recently as a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the CNS. Both ATP, together with some of its enzymatic breakdown products (ADP and adenosine) and uracil nucleotides are now recognised to act via P2X ion channels and P1 and P2Y G protein-coupled receptors, which are widely expressed in the brain. They mediate both fast signalling in neurotransmission and neuromodulation and long-term (trophic) signalling in cell proliferation, differentiation and death. Purinergic signalling is prominent in neurone-glial cell interactions. In this review we discuss first the evidence implicating purinergic signalling in normal behaviour, including learning and memory, sleep and arousal, locomotor activity and exploration, feeding behaviour and mood and motivation. Then we turn to the involvement of P1 and P2 receptors in pathological brain function; firstly in trauma, ischemia and stroke, then in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's, as well as multiple sclerosis and amyotrophic lateral sclerosis. Finally, the role of purinergic signalling in neuropsychiatric diseases (including schizophrenia), epilepsy, migraine, cognitive impairment and neuropathic pain will be considered.