Modulation of astroglial cell proliferation by analogues of adenosine and ATP in primary cultures of rat striatum

Effects of ATP on the Physicochemical Properties and Cytocompatibility of Calcium Sulfate/Calcium Citrate Composite Cement

Adenosine triphosphate (ATP), acting as a source of energy, has effects on cellular activities, such as adhesion, proliferation, and differentiation. In this study, ATP-loaded calcium sulfate hemihydrate/calcium citrate tetrahydrate cement (ATP/CSH/CCT) was successfully prepared for the first time. The effect of different contents of ATP on the structure and physicochemical properties of ATP/CSH/CCT was also studied in detail. The results indicated that incorporating ATP into the cement did not significantly alter their structures. However, the addition ratio of ATP directly impacted the mechanical properties and in vitro degradation properties of the composite bone cement. The compressive strength of ATP/CSH/CCT gradually decreased with an increasing ATP content. The degradation rate of ATP/CSH/CCT did not significantly change at low concentrations of ATP, but it increased with a higher ATP content. The composite cement induced the deposition of a Ca-P layer in a phosphate buffer solution (PBS, pH = 7.4). Additionally, the release of ATP from the composite cement was controlled. The ATP was controlled releasing at the 0.5% and 1% ATP in cement by the diffusion of ATP and the degradation of the cement, whereas it was controlled by the diffusion process merely at the 0.1% ATP in cement. Furthermore, ATP/CSH/CCT demonstrated good cytoactivity with the addition of ATP and is expected to be used for the repair and regeneration of bone tissue.

Role of P2Y receptors in astrocyte physiology and pathophysiology

Astrocytes are active constituents of the brain that manage ion homeostasis and metabolic support of neurons and directly tune synaptic transmission and plasticity. Astrocytes express all known P2Y receptors. These regulate a multitude of physiological functions such as cell proliferation, Ca²⁺ signalling, gliotransmitter release and neurovascular coupling. In addition, P2Y receptors are fundamental in the transition of astrocytes into reactive astrocytes, as occurring in many brain disorders such as neurodegenerative diseases, neuroinflammation and epilepsy. This review summarizes the current literature addressing the function of P2Y receptors in astrocytes in the healthy brain as well as in brain 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.

From astrocytes to satellite glial cells and back: A 25 year-long journey through the purinergic modulation of glial functions in pain and more

Fundamental progresses have been made in pain research with a comprehensive understanding of the neuronal pathways which convey painful sensations from the periphery and viscera to the central nervous system and of the descending modulating pathways. Nevertheless, many patients still suffer from various painful conditions, which are often associated to other primary pathologies, and get no or poor relief from available painkillers. Thus, the interest of many researchers has concentrated on new and promising cellular targets and biochemical pathways. This is the case of glia cells, both in the peripheral and in the central nervous system, and of purinergic receptors. Starting from many intuitions and hypotheses raised by Prof. Geoffrey Burnstock, data have accumulated which clearly highlight the fundamental role exerted by several nucleotide and nucleoside receptors in the modulation of glial cell reaction to pain triggers and of their cross-talk with sensory neurons which significantly contributes to the transition from acute to chronic pain. The purinergic system has therefore become an appealing pharmacological target in pain research, also based on the quite unexpected discovery that purines are involved in ancient analgesic techniques such as acupuncture. A more in-depth understanding of the complex and intricated purine-orchestrated scenario in pain conditions will hopefully lead to the identification and clinical development of new and effective analgesics.

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.

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'.

Short- and long-term (trophic) purinergic signalling

There is long-term (trophic) purinergic signalling involving cell proliferation, differentiation, motility and death in the development and regeneration of most systems of the body, in addition to fast purinergic signalling in neurotransmission, neuromodulation and secretion. It is not always easy to distinguish between short- and long-term signalling. For example, adenosine triphosphate (ATP) can sometimes act as a short-term trigger for long-term trophic events that become evident days or even weeks after the original challenge. Examples of short-term purinergic signalling during sympathetic, parasympathetic and enteric neuromuscular transmission and in synaptic transmission in ganglia and in the central nervous system are described, as well as in neuromodulation and secretion. Long-term trophic signalling is described in the immune/defence system, stratified epithelia in visceral organs and skin, embryological development, bone formation and resorption and in cancer. It is likely that the increase in intracellular Ca(2+) in response to both P2X and P2Y purinoceptor activation participates in many short- and long-term physiological effects.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Defect-Related Luminescent Hydroxyapatite-Enhanced Osteogenic Differentiation of Bone Mesenchymal Stem Cells Via an ATP-Induced cAMP/PKA Pathway

Novel defect-related hydroxyapatite (DHAP), which combines the advantages of HAP and defect-related luminescence, has the potential application in tissue engineering and biomedical area, because of its excellent capability of monitoring the osteogenic differentiation and material biodegradation. Although the extracellular mechanism of DHAP minerals and PO4(3-) functioning in osteogenic differentiation has been widely studied, the intracellular molecular mechanism through which PO4(3-) mediates osteogenesis of bone mesenchymal stem cells (BMSCs) is not clear. We examined a previously unknown molecular mechanism through which PO4(3-) promoted osteogenesis of BMSCs with an emphasis on adenosine-triphosphate (ATP)-induced cAMP/PKA pathway. Our studies showed that DHAP could be uptaken into lysosome, in which PO4(3-) was released from DHAP, because of the acid environment of lysosome. The released PO4(3-) interacted with ADP to form ATP, and then degraded into adenosine, an ATP metabolite, which interacted with A2b adenosine receptor to activate the cAMP/PKA pathway, resulting in the high expression of osteogenesis-related genes, such as Runx2, BMP-2, and OCN. These findings first revealed the function of ATP-metabolism in bone physiological homeostasis, which may be developed to cure bone metabolic diseases.

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'.

P2X7 receptors and Fyn kinase mediate ATP-induced oligodendrocyte progenitor cell migration

Recruitment of oligodendrocyte precursor cells (OPCs) to the lesions is the most important event for remyelination after central nervous system (CNS) injury or in demyelinating diseases. However, the underlying molecular mechanism is not fully understood. In the present study, we found high concentrations of ATP could increase the number of migrating OPCs in vitro, while after pretreatment with oxidized ATP (a P2X7 receptor antagonist), the promotive effect was attenuated. The promotive effect of 2'(3')-O-(4-benzoylbenzoyl) adenosine 5'-triphosphate (BzATP) (a P2X7 receptor agonist) was more potent than ATP. After incubation with BzATP, the activity of Fyn, one member of the Src family of kinases, was enhanced. Moreover, the interaction between P2X7 and Fyn was identified by co-immunoprecipitation. After blocking the activity of Fyn or down-regulating the expression of Fyn, the migration of OPCs induced by BzATP was inhibited. These data indicate that P2X7 receptors/Fyn may mediate ATP-induced OPC migration under pathological conditions.

Pathological potential of astroglial purinergic receptors

Acute brain injury and neurodegenerative disorders may result in astroglial activation. Astrocytes are able to determine the progression and outcome of these neuropathologies in a beneficial or detrimental way. Nucleotides, e.g. adenosine 5'-triphosphate (ATP), released after acute or chronic neuronal injury, are important mediators of glial activation and astrogliosis.Acute injury may cause significant changes in ATP balance, resulting in (1) a decline of intracellular ATP levels and (2) an increase in extracellular ATP concentrations via efflux from the intracellular space. The released ATP may have trophic effects, but can also act as a proinflammatory mediator or cytotoxic factor, inducing necrosis/apoptosis as a universal "danger" signal. Furthermore, ATP, primarily released from astrocytes, is a means of communication between neurons, glial cells, and intracerebral blood vessels.Astrocytes express a heterogeneous battery of purinergic ionotropic and metabotropic receptors (P2XRs and P2YRs, respectively) to respond to extracellular nucleotides.In this chapter, we summarize the contemporary knowledge on the pathological potential of P2Rs in relation to changes of astrocytic functions, determined by distinct molecular signaling cascades, in a variety of diseases. We discuss specific aspects of reactive astrogliosis, with respect to the involvement of prominent receptor subtypes, such as the P2X7 and P2Y1/2Rs. Examples of purinergic signaling of microglia, oligodendrocytes, and blood vessels under pathophysiological conditions will also be presented.The understanding of the pathological potential of purinergic signaling in "controlling and fine-tuning" of astrocytic responses is important for identifying possible therapeutic principles to treat acute and chronic central nervous system diseases.

Nucleotide signaling in astrogliosis

Acute and chronic damage to the central nervous system (CNS) releases large quantities of ATP. Whereas the ATP concentration in the extracellular space is normally in the micromolar range, under these conditions it increases to millimolar levels. A number of ligand-gated cationic channels termed P2X receptors (7 mammalian subtypes), and G protein-coupled P2Y receptors (8 mammalian subtypes) are located at astrocytes, as confirmed by the measurement of the respective mRNA and protein. Activation of both the P2X7 and P2Y1,2 subtypes identified at astrocytes initiates astrogliosis isolating damaged brain areas from surrounding healthy cells and synthesizing neurotrophins and pleotrophins that participate in neuronal recovery. Astrocytes are considered as cells of high plasticity which may alter their properties in a culture medium. Therefore, recent work concentrates on investigating nucleotide effects at in situ (acute brain slices) and in vivo astrocytes. A wealth of data relates to the involvement of purinergic mechanisms in astrogliosis induced by acute CNS injury such as mechanical trauma and hypoxia/ischemia. The released ATP may act within minutes as an excitotoxic molecule; at a longer time-scale within days it causes neuroinflammation. These effects sum up as necrosis/apoptosis on the one hand and proliferation on the other. Although the role of nucleotides in chronic neurodegenerative illnesses is not quite clear, it appears that they aggravate the consequences of the primary disease. Epilepsy and neuropathic pain are also associated with the release of ATP and a pathologic glia-neuron interaction leading to astrogliosis and cell death. In view of these considerations, P2 receptor antagonists may open new therapeutic vistas in all forms of acute and chronic CNS damage.

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.

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).

Pathophysiology of astroglial purinergic signalling

Astrocytes are fundamental for central nervous system (CNS) physiology and are the fulcrum of neurological diseases. Astroglial cells control development of the nervous system, regulate synaptogenesis, maturation, maintenance and plasticity of synapses and are central for nervous system homeostasis. Astroglial reactions determine progression and outcome of many neuropathologies and are critical for regeneration and remodelling of neural circuits following trauma, stroke, ischaemia or neurodegenerative disorders. They secrete multiple neurotransmitters and neurohormones to communicate with neurones, microglia and the vascular walls of capillaries. Signalling through release of ATP is the most widespread mean of communication between astrocytes and other types of neural cells. ATP serves as a fast excitatory neurotransmitter and has pronounced long-term (trophic) roles in cell proliferation, growth, and development. During pathology, ATP is released from damaged cells and acts both as a cytotoxic factor and a proinflammatory mediator, being a universal "danger" signal. In this review, we summarise contemporary knowledge on the role of purinergic receptors (P2Rs) in a variety of diseases in relation to changes of astrocytic functions and nucleotide signalling. We have focussed on the role of the ionotropic P2X and metabotropic P2YRs working alone or in concert to modify the release of neurotransmitters, to activate signalling cascades and to change the expression levels of ion channels and protein kinases. All these effects are of great importance for the initiation, progression and maintenance of astrogliosis-the conserved and ubiquitous glial defensive reaction to CNS pathologies. We highlighted specific aspects of reactive astrogliosis, especially with respect to the involvement of the P2X(7) and P2Y(1)R subtypes. Reactive astrogliosis exerts both beneficial and detrimental effects in a context-specific manner determined by distinct molecular signalling cascades. Understanding the role of purinergic signalling in astrocytes is critical to identifying new therapeutic principles to treat acute and chronic neurological diseases.

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.

Long-term (trophic) purinergic signalling: purinoceptors control cell proliferation, differentiation and death

The purinergic signalling system, which uses purines and pyrimidines as chemical transmitters, and purinoceptors as effectors, is deeply rooted in evolution and development and is a pivotal factor in cell communication. The ATP and its derivatives function as a 'danger signal' in the most primitive forms of life. Purinoceptors are extraordinarily widely distributed in all cell types and tissues and they are involved in the regulation of an even more extraordinary number of biological processes. In addition to fast purinergic signalling in neurotransmission, neuromodulation and secretion, there is long-term (trophic) purinergic signalling involving cell proliferation, differentiation, motility and death in the development and regeneration of most systems of the body. In this article, we focus on the latter in the immune/defence system, in stratified epithelia in visceral organs and skin, embryological development, bone formation and resorption, as well as in cancer.

Blockade of P2 nucleotide receptors after spinal cord injury reduced the gliotic response and spared tissue

Spinal cord injury (SCI) triggers a sequel of events commonly associated with cell death and dysfunction of glias and neurons surrounding the lesion. Although astrogliosis and glial scar formation have been involved in both damage and repair processes after SCI, their role remains controversial. Our goal was to investigate the effects of the P2 receptors antagonists, PPADS and suramin, in the establishment of the reactive gliosis and the formation of the glial scar. Molecular biology, immunohistochemistry, spared tissue, and locomotor behavioral studies were used to evaluate astrogliosis, in adult female Sprague-Dawley rats treated with P2 antagonists after moderate injury with the NYU impactor device. Semi-quantitative RT-PCR confirmed the presence of P2Y(1,) P2Y(2,) P2Y(4,) P2Y(6,) P2Y(12), and P2X(2) receptors in the adult spinal cord. Immunohistochemistry studies confirmed a significant decrease in GFAP-labeled cells at the injury epicenter as well as a decrease in spared tissue after treatment with the antagonists. Functional open field testing revealed no significant locomotor score differences between treated and control animals. Our work is consistent with studies suggesting that astrogliosis is an important event after SCI that limits tissue damage and lesion spreading.

P2 receptor signaling in neurons and glial cells of the central nervous system

Purine and pyrimidine nucleotides are extracellular signaling molecules in the central nervous system (CNS) leaving the intracellular space of various CNS cell types via nonexocytotic mechanisms. In addition, ATP is a neuro-and gliotransmitter released by exocytosis from neurons and neuroglia. These nucleotides activate P2 receptors of the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. In mammalians, seven P2X and eight P2Y receptor subunits occur; three P2X subtypes form homomeric or heteromeric P2X receptors. P2Y subtypes may also hetero-oligomerize with each other as well as with other G protein-coupled receptors. P2X receptors are able to physically associate with various types of ligand-gated ion channels and thereby to interact with them. The P2 receptor homomers or heteromers exhibit specific sensitivities against pharmacological ligands and have preferential functional roles. They may be situated at both presynaptic (nerve terminals) and postsynaptic (somatodendritic) sites of neurons, where they modulate either transmitter release or the postsynaptic sensitivity to neurotransmitters. P2 receptors exist at neuroglia (e.g., astrocytes, oligodendrocytes) and microglia in the CNS. The neuroglial P2 receptors subserve the neuron-glia cross talk especially via their end-feets projecting to neighboring synapses. In addition, glial networks are able to communicate through coordinated oscillations of their intracellular Ca(2+) over considerable distances. P2 receptors are involved in the physiological regulation of CNS functions as well as in its pathophysiological dysregulation. Normal (motivation, reward, embryonic and postnatal development, neuroregeneration) and abnormal regulatory mechanisms (pain, neuroinflammation, neurodegeneration, epilepsy) are important examples for the significance of P2 receptor-mediated/modulated processes.

Activation of distinct P2Y receptor subtypes stimulates insulin secretion in MIN6 mouse pancreatic beta cells

Extracellular nucleotides and their receptor antagonists have therapeutic potential in disorders such as inflammation, brain disorders, and cardiovascular diseases. Pancreatic beta cells express several purinergic receptors, and reported nucleotide effects on insulin secretion are contradictory. We studied the effect of P2Y receptors on insulin secretion and cell death in MIN6, mouse pancreatic beta cells. Expression of P2Y(1) and P2Y(6) receptors was revealed by total mRNA analysis using RT-PCR. MIN6 cells were stimulated in the presence of 16.7 mM glucose with or without P2Y(1) and P2Y(6) agonists, 2-MeSADP and Up(3)U, respectively. Both the agonists increased insulin secretion with EC(50) values of 44.6+/-7.0 nM and 30.7+/-12.7 nM respectively. The insulin secretion by P2Y(1) and P2Y(6) agonists was blocked by their selective antagonists MRS2179 and MRS2578, respectively. Binding of the selective P2Y(1) receptor antagonist radioligand [125I]MRS2500 in MIN6 cell membranes was saturable (K(D) 4.74+/-0.47 nM), and known P2Y(1) ligands competed with high affinities. Inflammation and glucose toxicity lead to pancreatic beta cell death in diabetes. Flow cytometric analysis revealed that Up(3)U but not 2-MeSADP protected MIN6 cells against TNF-alpha induced apoptosis. Overall, the results demonstrate that selective stimulation of P2Y(1) and P2Y(6) receptors increases insulin secretion that accompanies intracellular calcium release, suggesting potential application of P2Y receptor ligands in the treatment of diabetes.

Enhanced cerebral expression of MCT1 and MCT2 in a rat ischemia model occurs in activated microglial cells

Monocarboxylate transporters (MCTs) are essential for the use of lactate, an energy substrate known to be overproduced in brain during an ischemic episode. The expression of MCT1 and MCT2 was investigated at 48 h of reperfusion from focal ischemia induced by unilateral extradural compression in Wistar rats. Increased MCT1 mRNA expression was detected in the injured cortex and hippocampus of compressed animals compared to sham controls. In the contralateral, uncompressed hemisphere, increases in MCT1 mRNA level in the cortex and MCT2 mRNA level in the hippocampus were noted. Interestingly, strong MCT1 and MCT2 protein expression was found in peri-lesional macrophages/microglia and in an isolectin B4+/S100beta+ cell population in the corpus callosum. In vitro, MCT1 and MCT2 protein expression was observed in the N11 microglial cell line, whereas an enhancement of MCT1 expression by tumor necrosis factor-alpha (TNF-alpha) was shown in these cells. Modulation of MCT expression in microglia suggests that these transporters may help sustain microglial functions during recovery from focal brain ischemia. Overall, our study indicates that changes in MCT expression around and also away from the ischemic area, both at the mRNA and protein levels, are a part of the metabolic adaptations taking place in the brain after ischemia.

Opposing effects of P2X(7) and P2Y purine/pyrimidine-preferring receptors on proliferation of astrocytes induced by fibroblast growth factor-2: implications for CNS development, injury, and repair

Extracellular nucleotides play important trophic roles in development and central nervous system (CNS) injury, but the functions of distinct purinergic receptors and related signaling pathways have not been fully elucidated. In the present study we identified opposing effects of P2X and P2Y receptors on the ability of FGF2 to induce proliferation in primary cultures of rat cortical astrocytes. Low concentrations of ATP enhanced DNA synthesis induced by FGF2, whereas high concentrations inhibited FGF2-induced proliferation. Comparison of concentration-response experiments with ATP and 2',3'-O-(4-benzoyl)-benzoyl-ATP (BzATP) indicated that the inhibitory effect was mediated by P2X(7) receptors. Interestingly, activation of P2X(7) receptors led to a state of reversible growth arrest rather than cell death. Selectivity studies showed that proliferation evoked by epidermal growth factor and platelet-derived growth factor was also inhibited by P2X(7) receptors, but P2X(1) or P2X(3) receptors did not inhibit proliferation induced by FGF2. A marker of mitosis, phosphohistone-3, was reduced by BzATP and increased by UTP, suggesting that the enhancing effect of ATP on FGF2-induced proliferation was mediated by P2 purine/pyrimidine receptors. Phosphorylation of the growth arrest-related protein kinases p38/MAPK and SAPK/JNK was strongly increased by BzATP but only weakly affected by UTP. We conclude that P2Y purine/pyrimidine receptors enhance proliferation induced by FGF2 in astrocytes, whereas stimulation of P2X(7) receptors inhibits proliferation by shifting cells to a state of reversible growth arrest that may be mediated by protein kinase signaling. These trophic actions of P2X(7) and P2Y purine/pyrimidine receptors may contribute to the regulation of CNS development, adult neurogenesis, and the response of astrocytes to injury.

Adenosine produced by neurons is metabolized to hypoxanthine by astrocytes

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

Purinergic receptor signaling regulates N-cadherin expression in primary astrocyte cultures

Extracellular ATP exerts both short-term and long-term effects in the CNS by stimulating cell-surface purinergic receptors. Here we have examined the effect of purinergic receptor activation on N-cadherin expression, a calcium-dependent cell adhesion molecule involved in many processes, including glia-glia and axon-glia interactions. When primary cultures of rat cortical astrocytes were treated with ATP, N-cadherin protein expression increased in a time- and concentration-dependent manner. In addition, ATP treatment caused an increase in N-cadherin immunoreactivity in both the cytoplasm and on the cell surface membrane. Interestingly, experiments with cycloheximide revealed that relocalization of N-cadherin to the cell surface membrane were independent of protein synthesis. The ATP-induced increase in N-cadherin protein expression was blocked by reactive blue 2 and 8-(p-sulfophenyl)-theophylline, suggesting involvement of both P2 and P1 purinergic receptors, respectively. In addition, N-cadherin expression was partially blocked when signaling from purinergic receptors to extracellular signal regulated protein kinase or Akt was inhibited by 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene or wortmannin, respectively. By using an in vitro model of traumatic CNS injury, we found that N-cadherin expression was increased when astrocytes were subjected to rapid and reversible mechanical strain. The findings presented here demonstrate a role for extracellular ATP, purinergic receptors and protein kinase signaling in regulating N-cadherin expression and suggest a role for this mechanism in cell-cell interactions.

P2 purinoceptors: historical perspective and classification

This article presents an overview that gives some historical perspective to the detailed papers at the cutting edge of P2 purinoceptor research that follow. I consider the proposal, first put forward by Abbracchio & Burnstock (Pharmacol Ther 64:445-475, 1994), that P2 purinoceptors should be regarded as members of two main families: a P2X purinoceptor family consisting of ligand-gated ion channels, and a P2Y purinoceptor family consisting of G protein-coupled receptors. The latest subclasses of these two families (P2X1-4 and P2Y1-5), identified largely on the basis of molecular cloning and expression, are tabled. Finally, I suggest some future directions for P2 purinoceptor research, including studies of the long-term (trophic) actions of purines, the evolution and development of purinoceptors and therapeutic applications.

P2Y2 nucleotide receptors inhibit trauma-induced death of astrocytic cells

Nucleotides as well as other neurotransmitters are known to be released to the extracellular space upon injury. To determine whether nucleotides acting on P2Y(2) nucleotide receptors promote protective or degenerative events after trauma in astrocytic cells, a well-established model of in vitro brain trauma was applied to 1321N1 cells expressing recombinant P2Y(2) nucleotide receptors (P2Y(2)R-1321N1). Cellular death was examined by measuring DNA fragmentation and caspase activation. Fragmented DNA was observed 48 h post-injury in 1321N1 cells, while P2Y(2) nucleotide receptor expressing cells did not show DNA fragmentation. A laddering pattern of fragmented DNA following injury was observed upon inhibition of P2Y(2) nucleotide receptors with suramin. Time-dependent increases of cleaved caspase-9, a mitochondrial-associated caspase, correlated with injury-induced cellular death. A decreased bax/bcl-2 gene expression ratio was observed in P2Y(2)R-1321N1 cells after traumatic injury, while untransfected 1321N1 cells showed a significant time-dependent increase of the bax/bcl-2 gene expression ratio. Activation of protein kinases was assessed to determine the signaling pathways involved in cell death and survival responses following traumatic injury. In P2Y(2)R-1321N1 and 1321N1 cells p38 phosphorylation was stimulated in a time-dependent manner but the phosphatidylinositol 3-kinase-dependent activation of extracellular signal-regulated kinase 1/2 and protein kinase B (PKB)/Akt was only observed in P2Y(2)R-1321N1 cells after injury. The stress-activated protein kinase/c-Jun NH2-terminal kinase (SAPK/JNK) signaling pathway was not activated by traumatic injury in either astrocytic cell line. Inhibition of p38 kinase signaling pathway by treatment with PD1693, a MKK3/6 inhibitor, abolished the expression of cleaved caspase-9, the increase in the bax/bcl-2 gene expression ratio, as well as the fragmentation of DNA that followed injury of 1321N1 cells. Taken together, our results demonstrate a novel role for P2Y(2) nucleotide receptors and extracellular nucleotides in mediating survival responses to glial cells undergoing cellular death induced by trauma.

Extracellular adenosine triphosphate induces glutamate transporter-1 expression in hippocampus

ATP can be significantly released following various brain insults and activates the extracellular signal-regulated protein kinase (ERK) pathway in astrocytes. Glutamate transporter-1 (GLT1) is the major forebrain astroglial glutamate transporter and its expression is stimulated also via ERK1/2 phosphorylation. We thus hypothesized that extracellular ATP could be a signal to GLT1 modulation in hippocampal slices obtained from rat. We indeed observed by western blot analysis that, after 1 mM ATP exposure, GLT1 expression, but not the glutamate-aspartate transporter, was enhanced. At the same time, high ATP induced significant rates of cell death in piramidal and granule cell layers, as shown by propidium iodide uptake, and increased glutamate uptake through GLT1 transporter. Also using confocal laser-scanning microscopy, we observed that ATP induced a vigorous and extensive GLT1-labeling on glial fibrillary acidic protein-positive cells. This stimulation was abolished by purine/pyrimidine nucleotide receptor antagonists and by MEK1/2 inhibitor. The present study demonstrates a novel mechanism of GLT1 regulation by extracellular ATP, reinforcing the evidence of cross talk between glutamatergic and purinergic systems.

P19 embryonal carcinoma cells as in vitro model for studying purinergic receptor expression and modulation of N-methyl-d-aspartate–glutamate and acetylcholine receptors during neuronal differentiation

The in vitro differentiation of P19 murine embryonal carcinoma cells to neurons resembles developmental stages which are encountered during neuronal development. Three days following induction to neuronal differentiation by retinoic acid, most cells of the P19 population lost expression of the stage specific embryonic antigen (SSEA-1) and expressed the neural progenitor cell specific antigen nestin. Beginning from day 4 of differentiation nestin expression was down-regulated, and expression of neuron-specific enolase as marker of differentiated neurons increased. The molecular mechanisms underlying neuronal differentiation are poorly understood. We have characterized the participation of purinergic ionotropic (P2X) and metabotropic (P2Y) receptors at mRNA transcription and protein levels as well as ATP-induced Ca2+ transients during neuronal differentiation of P19 cells. Gene and protein expression of P2X2, P2X6, P2Y2, and P2Y6 receptors increased during the course of differentiation, whereas P2X3, P2X4, P2Y1 and P2Y4 receptor expression was high in embryonic P19 cells and then decreased following induction of P19 cells to differentiation. P2X1 receptor protein expression was only detected on days 2 and 4 of differentiation. Although P2X5 and P2X7 mRNA transcription was present, no protein expression for this receptor subunit could be detected throughout the differentiation process. In undifferentiated cells, mainly ionotropic P2X receptors contributed to the ATP-induced Ca2+-response. In neuronal-differentiated P19 cells, the ATP-induced Ca2+-response was increased and the metabotropic component predominated. Purinergic receptor function is implicated to participate in neuronal maturation, as cholinergic and glutamate-N-methyl-D-aspartate (NMDA) induced calcium responses were affected when cells were differentiated in the presence of purinergic receptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), suramin or reactive blue-2. Our data suggest that inhibition of P2Y1 and possibly P2X2 receptors led to a loss of NMDA receptor activity whereas blockade of possibly P2X2 and P2Y2 purinergic receptors during neuronal differentiation of P19 mouse led to inhibition of cholinergic receptor responses.

Different patterns of Ca²⁺ signals are induced by low compared to high concentrations of P2Y agonists in microglia

Brain-resident macrophages (microglia) are key cellular elements in the preservation of tissue integrity. On the other hand, they can also contribute to the development of pathological events by causing an extensive and inappropriate inflammatory response. A growing number of reports indicate the involvement of nucleotides in the control of microglial functions. With this study on P2Y receptors in rat microglia, we want to contribute to the definition of their expression profile and to the characterisation of their signalling mechanisms leading to Ca²⁺ movements. Endogenous nucleotides, when applied at a concentration of 100 μM, elicited robust Ca²⁺ transients, thanks to a panel of metabotropic receptors comprising mainly P2Y₂, P2Y₆ and P2Y₁₂ subtypes. The involvement of P2Y₁₂ receptors in Ca²⁺ responses induced by adenine nucleotides was confirmed by the pharmacological and pertussis toxin sensitivity of the response induced by adenosine diphosphate (ADP). Beside the G protein involved, Gi and Gq respectively, adenine and uracil nucleotides differed also for induction by the latter of a capacitative Ca²⁺ plateau. Moreover, when applied at low (sub-micromolar) concentrations with a long-lasting challenge, uracil nucleotides elicited oscillatory Ca²⁺ changes with low frequency of occurrence (≤ 1 min⁻), sometimes superimposed to an extracellular Ca²⁺-dependent sustained Ca²⁺ rise. We conclude that different patterns of Ca²⁺ transients are induced by low (i.e., oscillatory Ca²⁺ activity) compared to high (i.e., fast release followed by sustained raise) concentrations of nucleotides, which can suggest different roles played by receptor stimulation depending not only on the type but also on the concentration of nucleotides.

P2 purinergic receptors signal to STAT3 in astrocytes: Difference in STAT3 responses to P2Y and P2X receptor activation

Extracellular ATP, released upon tissue damage to the CNS, can evoke reactive astrogliosis. The released ATP activates P2 purinergic receptors associated with the proliferation of normally quiescent astrocytes, although the underlying mechanisms remain to be fully elucidated. Signal transducer and activator of transcription 3 (STAT3) has been implicated in reactive astrogliosis and plays an important role in cell cycle regulation. Therefore, we investigated whether extracellular ATP and purinergic receptors regulate STAT3 signaling. Using immunoblot analysis, we found that addition of ATP to primary cultures of rat cortical astrocytes increased Ser-727 phosphorylation of STAT3 in a time-sensitive and concentration-dependent manner. ATP-stimulated Ser-727 STAT3 phosphorylation was mediated through P2 receptor activation since suramin, an antagonist of P2 receptors, diminished this response, whereas 8-(para-sulfo-phenyl)-theophylline (8PSTP), an antagonist of P1 receptors, did not. We found that UTP, an agonist of P2Y(2/4/6) receptors, stimulated rapid and robust phosphorylation of Ser727-STAT3, whereas BzATP, an agonist for P2X receptors, exhibited a delayed and weaker response. In contrast, both P2Y and P2X agonists stimulated phosphorylation of Tyr705-STAT3 to a similar extent. P2 receptors can couple to extracellular signal-regulated protein kinases (ERK) and we found that inhibition of ERK signaling blocked phosphorylation of Ser727-STAT3. Further characterization of the Ser727-STAT3 phosphorylation response to P2Y receptor activation supported a role for P2Y2 and P2Y4, but not P2Y6, receptors as well as a partial role for P2Y1 receptors. Because phosphorylation of Ser727-STAT3 can promote DNA transcriptional activity of cell cycle regulatory genes, the differences in phosphorylation of Ser727-STAT3 may contribute to the mechanism by which P2Y receptors promote, whereas P2X receptors inhibit, astrocyte proliferation. In support of this hypothesis, inhibition of STAT3 activation by cucurbitacin I prevented ATP-stimulated mitogenesis. We conclude that P2 receptors stimulate STAT3 activation and suggest that P2 receptor/STAT3 signaling may play an important role in astrocyte proliferation and reactive astrogliosis.

Different effects of ATP on the contractile activity of mice diaphragmatic and skeletal muscles

Apart from acetyl-choline (Ach), adenosine-5'-trisphosphate (ATP) is thought to play a role in neuromuscular function, however little information is available on its cellular physiology. As such, effects of ATP and adenosine on contractility of mice diaphragmatic and skeletal muscles (m. extensor digitorum longa-MEDL) have been investigated in in vitro experiments. Application of carbacholine (CCh) in vitro in different concentrations led to pronounced muscle contractions, varying from 9.15+/-4.76 to 513.13+/-15.4 mg and from 44.65+/-5.01 to 101.46+/-9.11 mg for diaphragm and MEDL, respectively. Two hundred micromolars of CCh in both muscles caused the contraction with the 65% (diaphragm) to 75% (MEDL) of maximal contraction force-this concentration was thus used in further experiments. It was found that application of ATP (100 microM) increased the force of diaphragmatic contraction caused by CCh (200 microM) from 335.2+/-51.4 mg (n=21) in controls to 426.5+/-47.8 mg (n=10; P<0.05), but decreased the contractions of MEDL of CCh from 76.6+/-6.5mg (n=26) in control to 40.2+/-9.0mg (n=8; P<0.05). Application of adenosine (100 microM) had no effect on CCh-induced contractions of these muscles. Resting membrane potential (MP) measurements using sharp electrodes were done at 10, 20 and 30 min after the application of ATP and adenosine. Diaphragm showed depolarization from 75+/-0.6 down to 63.2+/-1.05, 57.2+/-0.96 and 53.6+/-1.1 mV after 10, 20 and 30 min of exposition, respectively (20 fibers from 4 muscles each, P<0.05 in all three cases). Adenosine showed no effect on diaphragmatic MP. Both agents were ineffective in case of MEDL. The effects of ATP in both tissues were abolished by suramin (100 microM), a P2-receptor antagonist, and chelerythrin (50 microM), a specific protein-kinase C (PKC) inhibitor, but were not affected by 1H-[1,2,4]-oxadiazolo-[4,3-alpha]-quinoxalin-1-one (ODQ, 1 microM), a guanylyl-cyclase inhibitor, or by adenosine-3,5-monophosphothioate (Rp-cAMP, 1 microM), a protein-kinase A (PKA) inhibitor. Besides the action on contractile activity, ATP (100 microM) led to a significant (P<0.001) depolarization of diaphragm muscle fibers from 74.5+/-2.3 down to 64+/-2.1, 58.2+/-2.2 and 54.3+/-2.4 mV after 10, 20 and 30 min of incubation, respectively. Incubation of MEDL with the same ATP concentration showed no significant change of MP. Denervation of the muscles for 28 days led to a decrease of CCh-induced contractions of diaphragm down to 171.1+/-34.5mg (n=11, P<0.05), but increased the contractile force of MEDL up to 723.9+/-82.3mg (n=9, P<0.01). Application of ATP elevated the contractility of denervated diaphragm caused by CCh up to normal values (311.1+/-79.7 mg, n=6, P>0.05 versus control), but did not significantly affect of contractility of MEDL, which became 848.1+/-62.7 mg (n=6). These results show that the effects of ATP on both diaphragmatic and skeletal muscles are mediated through P2Y receptors coupled to chelerytrin-sensitive protein-kinase C.

P2Y12 receptor signalling towards PKB proceeds through IGF-I receptor cross-talk and requires activation of Src, Pyk2 and Rap1

Previously it was shown that stimulation of the P2Y12 receptor activates PKB signalling in C6 glioma cells [K. Van Kolen and H. Slegers, J. Neurochem. 89, 442.]. In the present study, the mechanisms involved in this response were further elucidated. In cells transfected with the Gbetagamma-scavenger beta-ARK1/GRK2 or Rap1GAPII, stimulation with 2MeSADP failed to enhance PKB phosphorylation demonstrating that the signalling proceeds through Gbetagamma-subunits and Rap1. Moreover, Rap1-GTP pull-down assays revealed that P2Y12 receptor stimulation induced a rapid activation of Rap1. Treatment of cells with the Ca2+ chelator BAPTA-AM and inhibition of Src and PLD2 with PP2 or 1-butanol, respectively, abrogated P2Y12 receptor-mediated activation of Rap1 and PKB. In addition inhibition of PKCzeta decreased basal and 2MeSADP-stimulated phosphorylation of PKB indicating a role for this PKC isoform in PKB signalling. Although the increased PKB phosphorylation was abolished in the presence of the IGF-I receptor tyrosine kinase inhibitor AG 1024, 2MeSADP did not significantly increase receptor phosphorylation. Nevertheless, phosphorylation of a 120 kDa IGF-I receptor-associated protein was observed. The latter protein was identified by MALDI-TOF/TOF-MS as the proline-rich tyrosine kinase 2 (Pyk2) that co-operates with Src in a PLD2-dependent manner. Consistent with the signalling towards Rap1 and PKB, activation of Pyk2 was abrogated by Ca2+ chelation, inhibition of PLD2 and IGF-I receptor tyrosine kinase activity. In conclusion, the data reveal a novel type of cross-talk between P2Y12 and IGF-I receptors that proceeds through Gbetagamma-, Ca2+-and PLD2-dependent activation of the Pyk2/Src pathway resulting in GTP-loading of Rap1 required for an increased PKB phosphorylation.

Roles of P2 receptors in glial cells: focus on astrocytes

Central nervous system glial cells release and respond to nucleotides under both physiological and pathological conditions, suggesting that these molecules play key roles in both normal brain function and in repair after damage. In particular, ATP released from astrocytes activates P2 receptors on astrocytes and other brain cells, allowing a form of homotypic and heterotypic signalling, which also involves microglia, neurons and oligodendrocytes. Multiple P2X and P2Y receptors are expressed by both astrocytes and microglia; however, these receptors are differentially recruited by nucleotides, depending upon specific pathophysiological conditions, and also mediate the long-term trophic changes of these cells during inflammatory gliosis. In astrocytes, P2-receptor-induced gliosis occurs via activation of the extracellular-regulated kinases (ERK) and protein kinase B/Akt pathways and involves induction of inflammatory and anti-inflammatory genes, cyclins, adhesion and antiapoptotic molecules. While astrocytic P2Y₁ and P2Y_2,4 are primarily involved in short-term calcium-dependent signalling, multiple P2 receptor subtypes seem to cooperate to astrocytic long-term changes. Conversely, in microglia, exposure to inflammatory and immunological stimuli results in differential functional changes of distinct P2 receptors, suggesting highly specific roles in acquisition of the activated phenotype. We believe that nucleotide-induced activation of astrocytes and microglia may originally start as a defence mechanism to protect neurons from cytotoxic and ischaemic insults; dysregulation of this process in chronic inflammatory diseases eventually results in neuronal cell damage and loss. On this basis, full elucidation of the specific roles of P2 receptors in these cells may help exploit the beneficial neuroprotective features of activated glia while attenuating their harmful properties and thus provide the basis for novel neuroprotective strategies that specifically target the purinergic system.

Integration of P2Y receptor-activated signal transduction pathways in G protein-dependent signalling networks

The role of nucleotides in intracellular energy provision and nucleic acid synthesis has been known for a long time. In the past decade, evidence has been presented that, in addition to these functions, nucleotides are also autocrine and paracrine messenger molecules that initiate and regulate a large number of biological processes. The actions of extracellular nucleotides are mediated by ionotropic P2X and metabotropic P2Y receptors, while hydrolysis by ecto-enzymes modulates the initial signal. An increasing number of studies have been performed to obtain information on the signal transduction pathways activated by nucleotide receptors. The development of specific and stable purinergic receptor agonists and antagonists with therapeutical potential largely contributed to the identification of receptors responsible for nucleotide-activated pathways. This article reviews the signal transduction pathways activated by P2Y receptors, the involved second messenger systems, GTPases and protein kinases, as well as recent findings concerning P2Y receptor signalling in C6 glioma cells. Besides vertical signal transduction, lateral cross-talks with pathways activated by other G protein-coupled receptors and growth factor receptors are discussed.

Purinergic signaling induces thrombospondin-1 expression in astrocytes

Thrombospondin (TSP)-1, a multidomain glycoprotein, is secreted from astrocytes and promotes synaptogenesis. However, little is known about the mechanisms regulating its expression and release. In this article, we report that purinergic signaling participates in the production and secretion of TSP-1. Treatment of primary cultures of rat cortical astrocytes with extracellular ATP caused an increase in TSP-1 expression in a time- and concentration-dependent manner and was inhibited by antagonists of P2 and P1 purinergic receptors. Agonist studies revealed that UTP, but not 2',3'-O-(4-benzoyl)benzoyl-ATP, 2-methylthio-ADP, adenosine, or 5'-N-ethyl-carboxamidoadenosine, caused a significant increase in TSP-1 expression. In addition, release of TSP-1 was stimulated by ATP and UTP but not by 2-methylthio-ADP or adenosine. Additional studies indicated that P2Y(4) receptors stimulate both TSP-1 expression and release. P2Y receptors are coupled to protein kinase cascades, and signaling studies demonstrated that blockade of mitogen-activated protein kinases or Akt inhibited ATP- and UTP-induced TSP-1 expression. Using an in vitro model of CNS trauma that stimulates release of ATP, we found that TSP-1 expression increased after mechanical strain and was completely blocked by a P2 receptor antagonist and by inhibition of p38/mitogen-activated protein kinase and Akt, thereby indicating a major role for P2 receptor/protein kinase signaling in TSP-1 expression induced by trauma. We conclude that TSP-1 expression can be regulated by activation of P2Y receptors, particularly P2Y(4), coupled to protein kinase signaling pathways and suggest that purinergic signaling may be an important factor in TSP-1-mediated cell-matrix and cell-cell interactions such as those occurring during development and repair.

Atypical PKCzeta is involved in RhoA-dependent mitogenic signaling by the P2Y12 receptor in C6 cells

When nucleotide hydrolysis is prevented, agonists of the P2Y(12) receptor enhance the proliferation of C6 glioma cells by RhoA-dependent, protein kinase C (PKC)-dependent activation of the extracellular signal-regulated kinase (ERK) pathway [Claes P, Grobben B, Van Kolen K, Roymans D & Slegers H (2001) Br J Pharmacol134, 402-408; Grobben B, Claes P, Van Kolen K, Roymans D, Fransen P, Sys SU & Slegers H (2001) J Neurochem78, 1325-1338]. In this study, we show that ERK1/2 phosphorylation was not affected by transfection of the cells with the Gbetagamma-subunit-scavenging adrenergic receptor kinase peptide [betaARK1-(495-689)] or with Rap1GAPII, indicating that P2Y(12) receptor stimulation enhances ERK1/2 phosphorylation by G(i)alpha subunit-mediated signaling independently of Rap1 activation. Inhibition of the RhoA downstream effector Rho-associated coiled-coil-containing kinase (ROCK) with Y-27632 did not affect the P2Y(12) receptor-induced increase in ERK1/2 phosphorylation but abrogated the mitogenic response. Involvement of growth factor receptor transactivation in the signaling towards ERK phosphorylation could be ruled out by the lack of an effect of PP2, AG1024, AG1296 or SU1498, inhibitors of Src, insulin-like growth factor receptor, platelet-derived growth factor receptor and vascular endothelial growth factor receptor kinase activity, respectively. Experiments with bisindolylmaleimide I and IX indicated the requirement of PKC activity. Classical and novel PKC isoforms could be excluded by treatment of the cells with Gö6976 and calphostin C, whereas addition of a myristoylated PKCzeta pseudosubstrate inhibitor completely abolished P2Y(12) receptor-induced ERK1/2 activation. Moreover, coimmunoprecipitation experiments revealed PKCzeta/Raf1 and PKCzeta/ERK association, indicating the involvement of PKCzeta. From the data presented, we can conclude that the P2Y(12) receptor enhances cell proliferation by a G(i)alpha-dependent, RhoA-dependent PKCzeta/Raf1/MEK/ERK pathway that requires activation of ROCK, which is not involved in ERK1/2 signaling.

Signaling from P2 nucleotide receptors to protein kinase cascades induced by CNS injury: implications for reactive gliosis and neurodegeneration

Gliosis is a hypertrophic and hyperplastic response to many types of central nervous system injury, including trauma, stroke, seizure, as well as neurodegenerative and demyelinating disorders. Reactive astrocytes, a major component of the glial scar, express molecules that can both inhibit and promote axonal regeneration. ATP, which is released upon traumatic injury, hypoxia, and cell death, contributes to the gliotic response by binding to specific cell surface astrocytic P2 nucleotide receptors and evoking characteristic features of gliosis such as increased expression of glial fibrillary acidic protein (GFAP), generation and elongation of astrocytic processes, and cellular proliferation. Here, we review recent studies that demonstrate that (1) metabotropic, P2Y, and ionotropic, P2X, receptors expressed in astrocytes are coupled to protein kinase signaling pathways that regulate cellular proliferation, differentiation, and survival such as ERK and protein kinase B/Akt and (2) these P2 receptor/protein kinase cascades are activated after trauma induced by mechanical strain. We suggest that P2 receptor/protein kinase signaling pathways might provide novel therapeutic targets to regulate the formation of reactive astrocytes and the production of molecules that affect axonal regeneration and neurodegeneration.

Cell cycle regulation of astrocytes by extracellular nucleotides and fibroblast growth factor-2

Extracellular ATP enhances the mitogenic activity of fibroblast growth factor-2 (FGF2) in astrocytes, but the molecular mechanism underlying this synergistic interaction is not known. To determine whether the potentiating effect of extracellular ATP involves cell cycle control mechanisms, we have measured the expression of cyclins that are induced in different phases of the cell cycle in primary cultures of rat cortical astrocytes. We found that ATP potentiated the ability of FGF2 to stimulate expression of cyclin D1, a regulator of cell cycle entry, as well as cyclin A, a regulator of DNA replication. Because FGF2 and P2 purinergic receptors are coupled to extracellular signal regulated protein kinase (ERK), a key member of a signaling cascade that regulates proliferation, we also investigated the role of ERK in regulating cyclin expression induced by FGF2 and ATP. We found that the potentiating effect of ATP on cyclin expression was significantly reduced by U0126, an inhibitor of MEK, the upstream activator of ERK. P2 receptor agonist studies revealed that UTP enhanced FGF2-induced cyclin expression and mitogenesis whereas 2-methylthioADP was ineffective. By contrast, 2',3'-O-(4-benzoyl)-benzoyl-ATP markedly inhibited FGF2-induced mitogenesis. Consistent with opposing effects of P2Y and P2X receptors on mitogenesis, UTP stimulated a transient activation of ERK whereas BzATP stimulated a more sustained ERK signal. These findings suggest that signaling by P2Y receptors, most likely of the purine/pyrimidine subtype, enhance the ability of FGF2 to stimulate entry into a new cell cycle, as well as DNA replication, by an ERK-dependent mechanism, whereas signaling by P2X receptors, possibly the P2X7 subtype, inhibits FGF2-induced mitogenesis in astrocytes. Interactions between P2Y, P2X and polypeptide growth factor signaling pathways may have important implications for CNS development as well as injury and repair.

The Role of Glial Adenosine Receptors in Neural Resilience and the Neurobiology of Mood Disorders

Adenosine receptors were classified into A1- and A2-receptors in the laboratory of Bernd Hamprecht more than 25 years ago. Adenosine receptors are instrumental to the neurotrophic effects of glia cells. Both microglia and astrocytes release after stimulation via adenosine receptors factors that are important for neuronal survival and growth. Neuronal resilience is now considered as of pivotal importance in the neurobiology of mood disorders and their treatment. Both sleep deprivation and electroconvulsive therapy, two effective therapeutic measures in mood disorders, are associated with an increase of adenosine and upregulation of adenosine A1-receptors in the brain. Parameters closely related to adenosine receptor activation such as cerebral metabolic rate and delta power in the sleep EEG provide indirect evidence that adenosinergic signaling may be associated with the therapeutic response to these measures. Thus, neurotrophic effects evoked by adenosine receptors might be important in the mechanism of action of ECT and perhaps also sleep deprivation.

Regulation of cell-to-cell communication mediated by astrocytic ATP in the CNS

It has become apparent that glial cells, especially astrocytes, not merely supportive but are integrative, being able to receive inputs, assimilate information and send instructive chemical signals to other neighboring cells including neurons. At first, the excitatory neurotransmitter glutamate was found to be a major extracellular messenger that mediates these communications because it can be released from astrocytes in a Ca(2+)-dependent manner, diffused, and can stimulate extra-synaptic glutamate receptors in adjacent neurons, leading to a dynamic modification of synaptic transmission. However, recently extracellular ATP has come into the limelight as an important extracellular messenger for these communications. Astrocytes express various neurotransmitter receptors including P2 receptors, release ATP in response to various stimuli and respond to extracellular ATP to cause various physiological responses. The intercellular communication "Ca(2+) wave" in astrocytes was found to be mainly mediated by the release of ATP and the activation of P2 receptors, suggesting that ATP is a dominant "gliotransmitter" between astrocytes. Because neurons also express various P2 receptors and synapses are surrounded by astrocytes, astrocytic ATP could affect neuronal activities and even dynamically regulate synaptic transmission in adjacent neurons as if forming a "tripartite synapse". In this review, we summarize the role of astrocytic ATP, as compared with glutamate, in gliotransmission and synaptic transmission in neighboring cells, mainly focusing on the hippocampus. Dynamic communication between astrocytes and neurons mediated by ATP would be a key event in the processing or integration of information in the CNS.

Traumatic injury activates protein kinase B/Akt in cultured astrocytes: role of extracellular ATP and P2 purinergic receptors

Protein kinase B/Akt is a key signaling molecule that regulates cell survival, growth, and metabolism, and inhibits apoptosis. Traumatic brain injury (TBI) activates Akt, and Akt has been implicated in neuronal survival after TBI, but little is known about injury-induced Akt activation in astrocytes, cells that exhibit hypertrophic and hyperplastic responses to CNS injury. Here we have investigated the effect of mechanical strain on Akt activation in primary cultures of rat cortical astrocytes growing on deformable Silastic membranes. When astrocytes were subjected to mechanical strain (50 msec; 5-7.5 mm displacement), we observed an increase in phosphorylation of serine 473, a key indicator of Akt activation. Akt phosphorylation was increased at 3 min postinjury, was maximal from 5 to 10 min, and declined gradually thereafter. Akt activation was also dependent on the severity of the injury. Stretch-induced Akt phosphorylation was attenuated by blocking calcium influx and phosphoinositide 3-kinase (PI3K), an upstream activator of Akt. In addition, we found that ATP is rapidly released after mechanical strain and that the P2 purinergic receptor antagonist iso-pyridoxal-5'-phosphate-6-azophenyl-2',5'disulfonate (PPADS) attenuated trauma-induced Akt activation. We conclude that mechanical strain causes activation of Akt in astrocytes via stimulation of P2 receptors. This suggests that P2 receptor/Akt signaling promotes astrocyte survival and growth, and this process may play a role in the generation of reactive gliosis after TBI.

P2Y12 receptor stimulation inhibits beta-adrenergic receptor-induced differentiation by reversing the cyclic AMP-dependent inhibition of protein kinase B

Cyclic AMP-dependent induction of differentiation by activation of the beta-adrenergic receptor is correlated with inhibition of protein kinase B activity concomitant with growth arrest and increase in glial fibrillary acidic protein (GFAP) synthesis in rat C6 glioma cells. Costimulation of the beta-adrenergic receptor with purinergic receptors activated by 2-methylthio-adenosine-5'-diphosphate (2MeSADP) increased protein kinase B (PKB) phosphorylation above the level measured in non-stimulated cells and abolished cAMP-dependent differentiation. Transfection of cells with constitutively active PKB confirmed that reactivation of PKB is involved in the 2MeSADP-dependent inhibition of GFAP synthesis. The P2Y(12) and P2Y(13) receptor antagonist AR-C69931MX [N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloro-methylene ATP] decreased PKB phosphorylation to the level in non-stimulated cells, whereas the P2Y(13) antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) and P(1),P(3)-di(adenosine-5') tetraphosphate (Ap(4)A) did not alter the 2MeSADP-induced phosphorylation of PKB, showing that enhanced PKB activity and subsequent phosphorylation of glycogen synthase kinase-3 is due to stimulation of the P2Y(12) receptor. In addition, experiments in the presence of pertussis toxin and phosphatidylinositol 3-kinase (PI 3-K) activity assays demonstrated that the P2Y(12) receptor-mediated increase in PKB phosphorylation is G(i) protein- and PI 3-K-dependent. The presented data demonstrated that a cAMP-dependent inhibition of PKB induces differentiation of C6 glioma cells and that inhibition of adenylate cyclase and reactivation of the PI 3-K/PKB pathway by the P2Y(12) receptor reverses differentiation into enhanced proliferation.

Differential modulation of ATP-induced calcium signalling by A1 and A2 adenosine receptors in cultured cortical astrocytes

1. Despite the accumulating evidence that under various pathological conditions the extracellular elevation of adenine-based nucleotides and nucleosides plays a key role in the control of astroglial reactivity, how these signalling molecules interact in the regulation of astrocyte function is still largely elusive. 2. The action of the nucleoside adenosine in the modulation of the intracellular calcium signalling ([Ca(2+)](i)) elicited by adenosine 5'-triphosphate (ATP)-induced activation of P2 purinoceptors was investigated on neocortical type-1 astrocytes in primary culture by using single-cell microfluorimetry. 3. Astrocyte challenge with ATP (1-10 microm) elicited biphasic [Ca(2+)](i) responses consisting of an initial peak followed by a sustained elevation. The stable adenosine analogue 2-chloroadenosine (2-ClA) potentiated the transient [Ca(2+)](i) rise induced by activation of metabotropic P2Y receptors. Among the various P1 receptor agonists tested, the nonselective agonist 5'-N-ethylcarboxamidoadenosine (NECA) mimicked the 2-ClA action, whereas the selective A1 R(-) N6-(2-phenylisopropyl)-adenosine (R-PIA), the A2A 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS-21680) and A3 1-deoxy-1-(6-[([3-lodophenyl]methyl)-amino]-9H-purin-9-yl)-N-methyl-beta-d-ribofuranuronamide (IB-MECA) agonists were ineffective. 4. Application of R-PIA>NECA>or=2-ClA depressed the [Ca(2+)](i) plateau reversibly. Moreover, in the presence of R-PIA or 2-ClA, the prolonged [Ca(2+)](i) signal was maintained by application of the A1 antagonist 1,3-diethyl-8-phenylxanthine (DPX). Finally, preincubation of the astrocytes with pertussis toxin abrogated the 2-ClA inhibition of the ATP-elicited sustained [Ca(2+)](i) rise without affecting the transient [Ca(2+)](i) potentiation. 5. Taken together, these findings indicate that stimulation of A1 and A2 adenosine receptors mediates a differential modulation of [Ca(2+)](i) signalling elicited by P2 purinoceptors. Since variations in [Ca(2+)](i) dynamics also affect cell proliferation and differentiation, our data suggest that tuning of the extracellular levels of adenosine may be relevant for the control of astrogliosis mediated by adenine nucleotides.

GPR105, a novel Gi/o-coupled UDP-glucose receptor expressed on brain glia and peripheral immune cells, is regulated by immunologic challenge: possible role in neuroimmune function

We have recently shown that UDP-glucose, and some related UDP-sugars, are potent agonists of the novel G protein-coupled receptor GPR105 (recently re-named P2Y(14)). GPR105 is widely expressed throughout many brain regions and peripheral tissues of human and rodents, and couples to a pertussis toxin-sensitive G protein. To further characterise the role of GPR105, we demonstrate by immunohistochemistry with receptor-specific antiserum that GPR105 protein is widely distributed throughout the post mortem human brain where it is localised to glial cells, and specifically co-localises with astrocytes. Using quantitative RT-PCR we also show that GPR105 mRNA exhibits a restricted expression profile in an array of human cell lines and primary cells, with prominent expression detected in immune cells including neutrophils, lymphocytes, and megakaryocytic cells. To investigate the G protein selectivity of GPR105, we used chimeric Galpha subunits (Galpha(qi5), Galpha(qo5), and Galpha(qs5)) and an intracellular Ca(2+) mobilisation assay to demonstrate that GPR105 couples to Galpha subunits of the G(i/o) family but not to G(s) family proteins or to endogenous G(q/11) proteins in HEK-293 cells. Finally, we show that expression of GPR105 mRNA in the rat brain is up-regulated by immunologic challenge with lipopolysaccharide. Based on these observations, we propose that G(i/o)-coupled GPR105 might play an important role in peripheral and neuroimmune function in response to extracellular UDP-sugars.