Activation of P2X(7) receptors induced [(3)H]GABA release from the RBA-2 type-2 astrocyte cell line through a Cl(-)/HCO(3)(-)-dependent mechanism

Purinergic signaling in cognitive impairment and neuropsychiatric symptoms of Alzheimer's disease

About 10 million new cases of dementia develop worldwide each year, of which up to 70% are attributable to Alzheimer's disease (AD). In addition to the widely known symptoms of memory loss and cognitive impairment, AD patients frequently develop non-cognitive symptoms, referred to as behavioral and psychological symptoms of dementia (BPSDs). Sleep disorders are often associated with AD, but mood alterations, notably depression and apathy, comprise the most frequent class of BPSDs. BPSDs negatively affect the lives of AD patients and their caregivers, and have a significant impact on public health systems and the economy. Because treatments currently available for AD are not disease-modifying and mainly aim to ameliorate some of the cognitive symptoms, elucidating the mechanisms underlying mood alterations and other BPSDs in AD may reveal novel avenues for progress in AD therapy. Purinergic signaling is implicated in the pathophysiology of several central nervous system (CNS) disorders, such as AD, depression and sleep disorders. Here, we review recent findings indicating that purinergic receptors, mainly the A₁, A_2A, and P2X7 subtypes, are associated with the development/progression of AD. Current evidence suggests that targeting purinergic signaling may represent a promising therapeutic approach in AD and related conditions. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

The GABA and GABA-Receptor System in Inflammation, Anti-Tumor Immune Responses, and COVID-19

GABA and GABA_A-receptors (GABA_A-Rs) play major roles in neurodevelopment and neurotransmission in the central nervous system (CNS). There has been a growing appreciation that GABA_A-Rs are also present on most immune cells. Studies in the fields of autoimmune disease, cancer, parasitology, and virology have observed that GABA-R ligands have anti-inflammatory actions on T cells and antigen-presenting cells (APCs), while also enhancing regulatory T cell (Treg) responses and shifting APCs toward anti-inflammatory phenotypes. These actions have enabled GABA_A-R ligands to ameliorate autoimmune diseases, such as type 1 diabetes (T1D), multiple sclerosis (MS), and rheumatoid arthritis, as well as type 2 diabetes (T2D)-associated inflammation in preclinical models. Conversely, antagonism of GABA_A-R activity promotes the pro-inflammatory responses of T cells and APCs, enhancing anti-tumor responses and reducing tumor burden in models of solid tumors. Lung epithelial cells also express GABA-Rs, whose activation helps maintain fluid homeostasis and promote recovery from injury. The ability of GABA_A-R agonists to limit both excessive immune responses and lung epithelial cell injury may underlie recent findings that GABA_A-R agonists reduce the severity of disease in mice infected with highly lethal coronaviruses (SARS-CoV-2 and MHV-1). These observations suggest that GABA_A-R agonists may provide off-the-shelf therapies for COVID-19 caused by new SARS-CoV-2 variants, as well as novel beta-coronaviruses, which evade vaccine-induced immune responses and antiviral medications. We review these findings and further advance the notions that (1) immune cells possess GABA_A-Rs to limit inflammation in the CNS, and (2) this natural "braking system" on inflammatory responses may be pharmacologically engaged to slow the progression of autoimmune diseases, reduce the severity of COVID-19, and perhaps limit neuroinflammation associated with long COVID.

Astrocytic and Oligodendrocytic P2X7 Receptors Determine Neuronal Functions in the CNS

P2X7 receptors are members of the ATP-gated cationic channel family with a preferential localization at the microglial cells, the resident macrophages of the brain. However, these receptors are also present at neuroglia (astrocytes, oligodendrocytes) although at a considerably lower density. They mediate necrosis/apoptosis by the release of pro-inflammatory cytokines/chemokines, reactive oxygen species (ROS) as well as the excitotoxic (glio)transmitters glutamate and ATP. Besides mediating cell damage i.e., superimposed upon chronic neurodegenerative processes in Alzheimer’s Disease, Parkinson’s Disease, multiple sclerosis, and amyotrophic lateral sclerosis, they may also participate in neuroglial signaling to neurons under conditions of high ATP concentrations during any other form of neuroinflammation/neurodegeneration. It is a pertinent open question whether P2X7Rs are localized on neurons, or whether only neuroglia/microglia possess this receptor-type causing indirect effects by releasing the above-mentioned signaling molecules. We suggest as based on molecular biology and functional evidence that neurons are devoid of P2X7Rs although the existence of neuronal P2X7Rs cannot be excluded with absolute certainty.

Astrocytes as Guardians of Neuronal Excitability: Mechanisms Underlying Epileptogenesis

Astrocytes are key homeostatic regulators in the central nervous system and play important roles in physiology. After brain damage caused by e.g., status epilepticus, traumatic brain injury, or stroke, astrocytes may adopt a reactive phenotype. This process of reactive astrogliosis is important to restore brain homeostasis. However, persistent reactive astrogliosis can be detrimental for the brain and contributes to the development of epilepsy. In this review, we will focus on physiological functions of astrocytes in the normal brain as well as pathophysiological functions in the epileptogenic brain, with a focus on acquired epilepsy. We will discuss the role of astrocyte-related processes in epileptogenesis, including reactive astrogliosis, disturbances in energy supply and metabolism, gliotransmission, and extracellular ion concentrations, as well as blood-brain barrier dysfunction and dysregulation of blood flow. Since dysfunction of astrocytes can contribute to epilepsy, we will also discuss their role as potential targets for new therapeutic strategies.

GABA, but Not Bestrophin-1, Is Localized in Astroglial Processes in the Mouse Hippocampus and the Cerebellum

GABA is proposed to act as a gliotransmitter in the brain. Differences in GABA release from astroglia are thought to underlie differences in tonic inhibition between the cerebellum and the CA1 hippocampus. Here we used quantitative immunogold cytochemistry to localize and compare the levels of GABA in astroglia in these brain regions. We found that the density of GABA immunogold particles was similar in delicate processes of Bergman glia in the cerebellum and astrocytes in the CA1 hippocampus. The astrocytic GABA release is proposed to be mediated by, among others, the Ca²⁺ activated Cl^- channel bestrophin-1. The bestrophin-1 antibodies did not show any significant bestrophin-1 signal in the brain of wt mice, nor in bestrophin-1 knockout mice. The bestrophin-1 signal was low both on Western blots and immunofluorescence laser scanning microscopic images. These results suggest that GABA is localized in astroglia, but in similar concentrations in the cerebellum and CA1 hippocampus, and thus cannot account for differences in tonic inhibition between these brain regions. Furthermore, our data seem to suggest that the GABA release from astroglia previously observed in the hippocampus and cerebellum occurs via mechanisms other than bestrophin-1.

Involvement of extrasynaptic glutamate in physiological and pathophysiological changes of neuronal excitability

Glutamate is the most abundant neurotransmitter of the central nervous system, as the majority of neurons use glutamate as neurotransmitter. It is also well known that this neurotransmitter is not restricted to synaptic clefts, but found in the extrasynaptic regions as ambient glutamate. Extrasynaptic glutamate originates from spillover of synaptic release, as well as from astrocytes and microglia. Its concentration is magnitudes lower than in the synaptic cleft, but receptors responding to it have higher affinity for it. Extrasynaptic glutamate receptors can be found in neuronal somatodendritic location, on astroglia, oligodendrocytes or microglia. Activation of them leads to changes of neuronal excitability with different amplitude and kinetics. Extrasynaptic glutamate is taken up by neurons and astrocytes mostly via EAAT transporters, and astrocytes, in turn metabolize it to glutamine. Extrasynaptic glutamate is involved in several physiological phenomena of the central nervous system. It regulates neuronal excitability and synaptic strength by involving astroglia; contributing to learning and memory formation, neurosecretory and neuromodulatory mechanisms, as well as sleep homeostasis.The extrasynaptic glutamatergic system is affected in several brain pathologies related to excitotoxicity, neurodegeneration or neuroinflammation. Being present in dementias, neurodegenerative and neuropsychiatric diseases or tumor invasion in a seemingly uniform way, the system possibly provides a common component of their pathogenesis. Although parts of the system are extensively discussed by several recent reviews, in this review I attempt to summarize physiological actions of the extrasynaptic glutamate on neuronal excitability and provide a brief insight to its pathology for basic understanding of the topic.

Circadian ATP Release in Organotypic Cultures of the Rat Suprachiasmatic Nucleus Is Dependent on P2X7 and P2Y Receptors

The circadian rhythms in physiological and behavioral functions are driven by a pacemaker located in the suprachiasmatic nucleus (SCN). The rhythms continue in constant darkness and depend on cell-cell communication between neurons and glia. The SCN astrocytes generate also a circadian rhythm in extracellular adenosine 5′-triphosphate (ATP) accumulation, but molecular mechanisms that regulate ATP release are poorly understood. Here, we tested the hypothesis that ATP is released via the plasma membrane purinergic P2X7 receptors (P2X7Rs) and P2Y receptors (P2YRs) which have been previously shown to be expressed in the SCN tissue at transcriptional level. We have investigated this hypothesis using SCN organotypic cultures, primary cultures of SCN astrocytes, ATP bioluminescent assays, immunohistochemistry, patch-clamping, and calcium imaging. We found that extracellular ATP accumulation in organotypic cultures followed a circadian rhythm, with a peak between 24:00 and 04:00 h, and the trough at ~12:00 h. ATP rhythm was inhibited by application of AZ10606120, A438079, and BBG, specific blockers of P2X7R, and potentiated by GW791343, a positive allosteric modulator of this receptor. Double-immunohistochemical staining revealed high expression of the P2X7R protein in astrocytes of SCN slices. PPADS, a non-specific P2 antagonist, and MRS2179, specific P2Y1R antagonist, also abolished ATP rhythm, whereas the specific P2X4R blocker 5-BDBD was not effective. The pannexin-1 hemichannel blocker carbenoxolone displayed a partial inhibitory effect. The P2Y1R agonist MRS2365, and the P2Y2R agonist MRS2768 potentiated ATP release in organotypic cultures and increase intracellular Ca²⁺ level in cultured astrocytes. Thus, SCN utilizes multiple purinergic receptor systems and pannexin-1 hemichannels to release ATP.

Under stressful conditions activation of the ionotropic P2X7 receptor differentially regulates GABA and glutamate release from nerve terminals of the rat cerebral cortex

γ-Aminobutyric acid (GABA) and glutamate (Glu) are the main inhibitory and excitatory neurotransmitters in the central nervous system (CNS), respectively. Fine tuning regulation of extracellular levels of these amino acids is essential for normal brain activity. Recently, we showed that neocortical nerve terminals from patients with epilepsy express higher amounts of the non-desensitizing ionotropic P2X7 receptor. Once activated by ATP released from neuronal cells, the P2X7 receptor unbalances GABAergic vs. glutamatergic neurotransmission by differentially interfering with GABA and Glu uptake. Here, we investigated if activation of the P2X7 receptor also affects [³H]GABA and [¹⁴C]Glu release measured synchronously from isolated nerve terminals (synaptosomes) of the rat cerebral cortex. Data show that activation of the P2X7 receptor consistently increases [¹⁴C]Glu over [³H]GABA release from cortical nerve terminals, but the GABA/Glu ratio depends on extracellular Ca²⁺ concentrations. While the P2X7-induced [³H]GABA release is operated by a Ca²⁺-dependent pathway when external Ca²⁺ is available, this mechanism shifts towards the reversal of the GAT1 transporter in low Ca²⁺ conditions. A different scenario is verified regarding [¹⁴C]Glu outflow triggered by the P2X7 receptor, since the amino acid seems to be consistently released through the recruitment of connexin-containing hemichannels upon P2X7 activation, both in the absence and in the presence of external Ca²⁺. Data from this study add valuable information suggesting that ATP, via P2X7 activation, not only interferes with the high-affinity uptake of GABA and Glu but actually favors the release of these amino acids through distinct molecular mechanisms amenable to differential therapeutic control.

Astrocytic Actions on Extrasynaptic Neuronal Currents

In the last few decades, knowledge about astrocytic functions has significantly increased. It was demonstrated that astrocytes are not passive elements of the central nervous system (CNS), but active partners of neurons. There is a growing body of knowledge about the calcium excitability of astrocytes, the actions of different gliotransmitters and their release mechanisms, as well as the participation of astrocytes in the regulation of synaptic functions and their contribution to synaptic plasticity. However, astrocytic functions are even more complex than being a partner of the “tripartite synapse,” as they can influence extrasynaptic neuronal currents either by releasing substances or regulating ambient neurotransmitter levels. Several types of currents or changes of membrane potential with different kinetics and via different mechanisms can be elicited by astrocytic activity. Astrocyte-dependent phasic or tonic, inward or outward currents were described in several brain areas. Such currents, together with the synaptic actions of astrocytes, can contribute to neuromodulatory mechanisms, neurosensory and -secretory processes, cortical oscillatory activity, memory, and learning or overall neuronal excitability. This mini-review is an attempt to give a brief summary of astrocyte-dependent extrasynaptic neuronal currents and their possible functional significance.

Involvement of a P2X7 Receptor in the Acrosome Reaction Induced by ATP in Rat Spermatozoa

The acrosome reaction (AR) is the exocytosis of the acrosomal vesicle in response to different physiological and non-physiological stimuli. Particularly in mammals, the AR is needed for sperm to fuse with the oocyte plasma membrane, and it occurs only in capacitated sperm. Previous evidence in the literature indicates that extracellular ATP induces the AR in capacitated human and bovine spermatozoa, but its receptor has not yet been identified. The aim of this work was to define a putative ATP receptor in rat spermatozoa using pharmacological and biochemical approaches. We found that ATP induced the AR only in capacitated rat spermatozoa, which was inhibited in the presence of two general inhibitors of ATP receptors (P2 receptors), Suramin, and oxidized ATP (oATP), and one inhibitor of P2X receptor (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid [PPADS]). In addition, the AR induced by ATP in capacitated rat spermatozoa was inhibited by brilliant blue-G (BB-G) and 17-β-oestradiol, two blockers of P2X7 receptors. Moreover, the ATP analog 2'(3')-O-(4-benzoylbenzoyl) ATP (BzATP) was almost 500 times more potent than ATP to induce the AR, which agrees with the pharmacology of a P2X7 receptor. Here, we show the presence of P2X7 receptor by Western blot and its localization in the tail and acrosome by indirect immunofluorescence. Finally, we quantify the presence of ATP in the rat oviduct during the estrous cycle. We found that the ATP concentration within the lumen of the oviduct is similar to those required to induce acrosome reaction, which agree with its role during in vivo fertilization. Therefore, our results strongly suggest that ATP induces the AR in capacitated rat spermatozoa through a P2X7 receptor, which may be functional during in vivo fertilization.

Extracellular ATP and other nucleotides-ubiquitous triggers of intercellular messenger release

Extracellular nucleotides, and ATP in particular, are cellular signal substances involved in the control of numerous (patho)physiological mechanisms. They provoke nucleotide receptor-mediated mechanisms in select target cells. But nucleotides can considerably expand their range of action. They function as primary messengers in intercellular communication by stimulating the release of other extracellular messenger substances. These in turn activate additional cellular mechanisms through their own receptors. While this applies also to other extracellular messengers, its omnipresence in the vertebrate organism is an outstanding feature of nucleotide signaling. Intercellular messenger substances released by nucleotides include neurotransmitters, hormones, growth factors, a considerable variety of other proteins including enzymes, numerous cytokines, lipid mediators, nitric oxide, and reactive oxygen species. Moreover, nucleotides activate or co-activate growth factor receptors. In the case of hormone release, the initially paracrine or autocrine nucleotide-mediated signal spreads through to the entire organism. The examples highlighted in this commentary suggest that acting as ubiquitous triggers of intercellular messenger release is one of the major functional roles of extracellular nucleotides. While initiation of messenger release by nucleotides has been unraveled in many contexts, it may have been overlooked in others. It can be anticipated that additional nucleotide-driven messenger functions will be uncovered with relevance for both understanding physiology and development of therapy.

Neuroglial Transmission

Neuroglia, the "glue" that fills the space between neurons in the central nervous system, takes active part in nerve cell signaling. Neuroglial cells, astroglia, oligodendroglia, and microglia, are together about as numerous as neurons in the brain as a whole, and in the cerebral cortex grey matter, but the proportion varies widely among brain regions. Glial volume, however, is less than one-fifth of the tissue volume in grey matter. When stimulated by neurons or other cells, neuroglial cells release gliotransmitters by exocytosis, similar to neurotransmitter release from nerve endings, or by carrier-mediated transport or channel flux through the plasma membrane. Gliotransmitters include the common neurotransmitters glutamate and GABA, the nonstandard amino acid d-serine, the high-energy phosphate ATP, and l-lactate. The latter molecule is a "buffer" between glycolytic and oxidative metabolism as well as a signaling substance recently shown to act on specific lactate receptors in the brain. Complementing neurotransmission at a synapse, neuroglial transmission often implies diffusion of the transmitter over a longer distance and concurs with the concept of volume transmission. Transmission from glia modulates synaptic neurotransmission based on energetic and other local conditions in a volume of tissue surrounding the individual synapse. Neuroglial transmission appears to contribute significantly to brain functions such as memory, as well as to prevalent neuropathologies.

Interactions of pannexin 1 with NMDA and P2X7 receptors in central nervous system pathologies: Possible role on chronic pain

Pannexin 1 (Panx1) is a glycoprotein that acts as a membrane channel in a wide variety of tissues in mammals. In the central nervous system (CNS) Panx1 is expressed in neurons, astrocytes and microglia, participating in the pathophysiology of some CNS diseases, such as epilepsy, anoxic depolarization after stroke and neuroinflammation. In these conditions Panx1 acts as an important modulator of the neuroinflammatory response, by secreting ATP, by interacting with the P2X7 receptor (P2X7R), and as an amplifier of NMDA receptor (NMDAR) currents, particularly in conditions of pathological neuronal hyperexcitability. Here, we briefly reviewed the current evidences that support the interaction of Panx1 with NMDAR and P2X7R in pathological contexts of the CNS, with special focus in recent data supporting that Panx1 is involved in chronic pain signaling by interacting with NMDAR in neurons and with P2X7R in glia. The participation of Panx1 in chronic pain constitutes a novel topic for research in the field of clinical neurosciences and a potential target for pharmacological interventions in chronic pain.

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

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

ATP as a multi-target danger signal in the brain

ATP is released in an activity-dependent manner from different cell types in the brain, fulfilling different roles as a neurotransmitter, neuromodulator, in astrocyte-to-neuron communication, propagating astrocytic responses and formatting microglia responses. This involves the activation of different ATP P2 receptors (P2R) as well as adenosine receptors upon extracellular ATP catabolism by ecto-nucleotidases. Notably, brain noxious stimuli trigger a sustained increase of extracellular ATP, which plays a key role as danger signal in the brain. This involves a combined action of extracellular ATP in different cell types, namely increasing the susceptibility of neurons to damage, promoting astrogliosis and recruiting and formatting microglia to mount neuroinflammatory responses. Such actions involve the activation of different receptors, as heralded by neuroprotective effects resulting from blockade mainly of P2X7R, P2Y1R and adenosine A_2A receptors (A_2AR), which hierarchy, cooperation and/or redundancy is still not resolved. These pleiotropic functions of ATP as a danger signal in brain damage prompt a therapeutic interest to multi-target different purinergic receptors to provide maximal opportunities for neuroprotection.

P2X7 R-mediated Ca(2+) -independent d-serine release via pannexin-1 of the P2X7 R-pannexin-1 complex in astrocytes

D-serine is a coagonist of N-methyl-d-aspartate (NMDA) subtype of glutamate receptor and plays a role in regulating activity-dependent synaptic plasticity. In this study, we examined the mechanism by which extracellular ATP triggers the release of d-serine from astrocytes and discovered a novel Ca(2+) -independent release mechanism mediated by P2X7 receptors (P2X7 R). Using [(3) H] d-serine, which was loaded into astrocytes via the neutral amino acid transporter 2 (ASCT2), we observed that ATP and a potent P2X7 R agonist, 2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate (BzATP), stimulated [(3) H]D-serine release and that were abolished by P2X7 R selective antagonists and by shRNAs, whereas enhanced by removal of intracellular or extracellular Ca(2+) . The P2X7 R-mediated d-serine release was inhibited by pannexin-1 antagonists, such as carbenoxolone (CBX), probenecid (PBN), and (10) Panx-1 peptide, and shRNAs, and stimulation of P2X7 R induced P2X7 R-pannexin-1 complex formation. Simply incubating astrocytes in Ca(2+) /Mg(2+) -free buffer also induced the complex formation, and that enhanced basal d-serine release through pannexin-1. The P2X7 R-mediated d-serine release assayed in Ca(2+) /Mg(2+) -free buffer was enhanced as well, and that was inhibited by CBX. Treating astrocytes with general protein kinase C (PKC) inhibitors, such as chelerythrine, GF109203X, and staurosporine, but not Ca(2+) -dependent PKC inhibitor, Gö6976, inhibited the P2X7 R-mediated d-serine release. Thus, we conclude that in astrocytes, P2X7 R-pannexin-1 complex formation is crucial for P2X7 R-mediated d-serine release through pannexin-1 hemichannel. The release is Ca(2+) -independent and regulates by a Ca(2+) -independent PKC. The activated P2X7 R per se is also functioned as a permeation channel to release d-serine in part. This P2X7 R-mediated d-serine release represents an important mechanism for activity-dependent neuron-glia interaction.

GABA as a rising gliotransmitter

Gamma-amino butyric acid (GABA) is the major inhibitory neurotransmitter that is known to be synthesized and released from GABAergic neurons in the brain. However, recent studies have shown that not only neurons but also astrocytes contain a considerable amount of GABA that can be released and activate GABA receptors in neighboring neurons. These exciting new findings for glial GABA raise further interesting questions about the source of GABA, its mechanism of release and regulation and the functional role of glial GABA. In this review, we highlight recent studies that identify the presence and release of GABA in glial cells, we show several proposed potential pathways for accumulation and modulation of glial intracellular and extracellular GABA content, and finally we discuss functional roles for glial GABA in the brain.

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.

P2X7 receptor mediates activation of microglial cells in prostate of chemically irritated rats

PURPOSE

Evidence shows that adenosine triphosphate (ATP) is involved in the transmission of multiple chronic pain via P2X7 receptor. This study was to investigate the P2X7 and microglial cells in the chronic prostatitis pain.

MATERIALS AND METHODS

Rats were divided into control group and chronic prostatitis group (n = 24 per group). A chronic prostatitis animal model was established by injecting complete Freund's adjuvant (CFA) to the prostate of rats, and the thermal withdrawal latency (TWL) was detected on days 0, 4, 12 and 24 (n = 6 at each time point in each group). Animals were sacrificed and the pathological examination of the prostate, detection of mRNA expression of P2X7 and ionized calcium binding adaptor molecule 1 (IBA-1) and measurement of content of tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) in the dorsal horn of L5-S2 spinal cord were performed on days 0, 4, 12 and 24. In addition, the content of TNF-α and IL-1β in the dorsal horn of L5-S2 spinal cord was measured after intrathecal injection of inhibitors of microglial cells and/or P2X7 for 5 days.

RESULTS

The chronic prostatitis was confirmed by pathological examination. The expression of P2X7 and IBA-1 and the content of TNF-α and IL-1β in rats with chronic prostatitis were significantly higher than those in the control group. On day 4, the expressions of pro-inflammatory cytokines became to increase, reaching a maximal level on day 12 and started to reduce on day 24, but remained higher than that in the control group. Following suppression of microglial cells and P2X7 receptor, the secretion of TNF-α and IL-1β was markedly reduced.

CONCLUSION

In chronic prostatitis pain, the microglial cells and P2X7 receptor are activated resulting in the increased expression of TNF-α and IL-1β in the L5-S2 spinal cord, which might attribute to the maintenance and intensification of pain in chronic prostatitis.

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

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

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.

P2X receptors and their roles in astroglia in the central and peripheral nervous system

Astrocytes are a class of neural cells that control homeostasis at all levels of the central and peripheral nervous system. There is a bidirectional neuron-glia interaction via a number of extracellular signaling molecules, glutamate and ATP being the most widespread. ATP activates ionotropic P2X and metabotropic P2Y receptors, which operate in both neurons and astrocytes. Morphological, biochemical, and functional evidence indicates the expression of astroglial P2X(1/5) heteromeric and P2X(7) homomeric receptors, which mediate physiological and pathophysiological responses. Activation of P2X(1/5) receptors triggers rapid increase of intracellular Na(+) that initiates immediate cellular reactions, such as the depression of the glutamate transporter to keep high glutamate concentrations in the synaptic cleft, the activation of the local lactate shuttle to supply energy substrate to pre- and postsynaptic neuronal structures, and the reversal of the Na(+)/Ca(2+) exchange resulting in additional Ca(2+) entry. The consequences of P2X(7) receptor activation are mostly but not exclusively mediated by the entry of Ca(2+) and result in reorganization of the cytoskeleton, inflammation, apoptosis/necrosis, and proliferation, usually at a prolonged time scale. Thus, astroglia detect by P2X(1/5) and P2X(7) receptors both physiological concentrations of ATP secreted from presynaptic nerve terminals and also much higher concentrations of ATP attained under pathological conditions.

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.

Purinergic signaling in the cerebellum: Bergmann glial cells express functional ionotropic P2X7 receptors

Astrocytes constitute active networks of intercommunicating cells that support the metabolism and the development of neurons and affect synaptic functions via multiple pathways. ATP is one of the major neurotransmitters mediating signaling between neurons and astrocytes. Potentially acting through both purinergic metabotropic P2Y receptors (P2YRs) and ionotropic P2X receptors (P2XRs), up until now ATP has only been shown to activate P2YRs in Bergmann cells, the radial glia of the cerebellar cortex that envelopes Purkinje cell afferent synapses. In this study, using multiple experimental approaches in acute cerebellar slices we demonstrate the existence of functional P2XRs on Bergmann cells. In particular, we show here that Bergmann cells express uniquely P2X7R subtypes: (i) immunohistochemical analysis revealed the presence of P2X7Rs on Bergmann cell processes, (ii) in whole cell recordings P2XR pharmacological agonists induced depolarizing currents that were blocked by specific antagonists of P2X7Rs, and could not be elicited in slices from P2X₇R-deficient mice and finally, (iii) calcium imaging experiments revealed two distinct calcium signals triggered by application of exogenous ATP: a transient signal deriving from release of calcium from intracellular stores, and a persistent one following activation of P2X7Rs. Our data thus reveal a new pathway by which extracellular ATP may affect glial cell function, thus broadening our knowledge on purinergic signaling in the cerebellum.

Electrophysiological classification of P2X7 receptors in rat cultured neocortical astroglia

BACKGROUND AND PURPOSE

P2X7 receptors are ATP-gated cation channels mediating important functions in microglial cells, such as the release of cytokines and phagocytosis. Electrophysiological evidence that these receptors also occur in CNS astroglia is rare and rather incomplete.

EXPERIMENTAL APPROACH

We used whole-cell patch-clamp recordings to search for P2X7 receptors in astroglial-neuronal co-cultures prepared from the cerebral cortex of rats.

KEY RESULTS

All the astroglial cells investigated responded to ATP with membrane currents, reversing around 0 mV. These currents could be also detected in isolated outside-out patch vesicles. The results of the experiments with the P2X [alpha,beta-methylene ATP and 2'-3'-O-(4-benzoyl) ATP] and P2Y receptor agonists [adenosine 5'-O-(2-thiodiphosphate), uridine 5'-diphosphate, uridine 5'-triphosphate (UTP) and UDP-glucose] suggested the involvement of P2X receptors in this response. The potentiation of ATP responses in a low divalent cation or alkaline bath, but not by ivermectin, made it likely that a P2X7 receptor is operational. Blockade of the ATP effect by the P2X7 antagonists Brilliant Blue G, calmidazolium and oxidized ATP corroborated this assumption.

CONCLUSIONS AND IMPLICATIONS

Rat cultured cortical astroglia possesses functional P2X7 receptors. It is suggested that astrocytic P2X7 receptors respond to high local ATP concentrations during neuronal injury.

Purinergic signaling: a novel mechanism in immune surveillance

Purinergic receptors and the associated signaling cascades are known to play critical roles in cardiovascular, nervous, respiratory, gastrointestinal and urinogenital systems. Recent studies have also shed light on the importance of nucleotides and purinergic receptors in the regulation of the immune response. With a better understanding of the distribution and the receptor subtypes, the purinoceptors have the potential to become important therapeutic targets in inflammation, chemotaxis and immune-related diseases.

P2X7 pre-synaptic receptors in adult rat cerebrocortical nerve terminals: a role in ATP-induced glutamate release

Although growing evidence suggests that extracellular ATP might play roles in the control of astrocyte/neuron crosstalk in the CNS by acting on P2X(7) receptors, it is still unclear whether neuronal functions can be attributed to P2X(7) receptors. In the present paper, we investigate the location, pharmacological profile, and function of P2X(7) receptors on cerebrocortical nerve terminals freshly prepared from adult rats, by measuring glutamate release and calcium accumulation. The preparation chosen (purified synaptosomes) ensures negligible contamination of non-neuronal cells and allows exposure of 'nude' release-regulating pre-synaptic receptors. To confirm the results obtained, we also carried out specific experiments on human embryonic kidney 293 cells which had been stably transfected with rat P2X(7) receptors. Together, our findings suggest that (i) P2X(7) receptors are present in a subpopulation of adult rat cerebrocortical nerve terminals; (ii) P2X(7) receptors are localized on glutamatergic nerve terminals; (iii) P2X(7) receptors play a significant role in ATP-evoked glutamate efflux, which involves Ca(2+)-dependent vesicular release; and (iv) the P2X(7) receptor itself constitutes a significant Ca(2+)-independent mode of exit for glutamate.

Roles of P2X7 receptor in glial and neuroblastoma cells: the therapeutic potential of P2X7 receptor antagonists

Recently, one of the P2 purinergic receptors, the P2X(7) receptor, has been extensively studied in nervous system and important functions have been revealed in both astrocytes and microglia. Stimulation of the receptors induces a sustained and nondesensitized increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). In astrocytes purinergic receptors primarily regulate neurotransmission by inducing gliotransmitters release whereas in microglia the receptors stimulate the processing and release of proinflammation cytokines such as interleukin-1 and are thereby involved in inflammation and neurodegeneration. Thus, P2X(7) receptors are considered not only to exert physiological functions but also mediate cell death. P2X(7) receptors have also been identified in various cancer cells and in neuroblastoma cells. In these cells, the P2X(7) receptor-mediated sustained Ca(2+) signal is important in maintaining cellular viability and growth. Accordingly, these findings not only lead to a better understanding of roles of the receptor but also prompt the development of more potent, selective and safer P2X(7) selective antagonists. These emerging antagonists bring new hope in the treatment of inflammatory-induced neurodegenerative diseases as well as neuroblastoma.

Astrocyte cultures exhibit P2X7 receptor channel opening in the absence of exogenous ligands

P2X7 receptors (P2X7Rs) gate the opening of large channels when activated by ATP or other ligands. P2X7Rs are expressed by astrocytes in culture and by reactive astrocytes in vivo, and astrocytes in culture have been shown to release glutamate and ATP in response to P2X7R activation. However, P2X7Rs are activated by ATP only at concentrations greater than 1 mM. The conditions under which astrocyte P2X7Rs would be activated in vivo are, thus, unclear. Here we show that astrocytes in culture exhibit basal P2X7R activity. Primary mouse astrocytes were found to take up the P2X7R permeant dyes YO-PRO-1 (YP) and propidium iodide in absence of any added ligands. By contrast, cultured rat astrocytes took up very little YP, consistent with their much lower level of P2X7R expression. The uptake by mouse astrocytes was inhibited by oxATP, suramin, KN-62 and brilliant blue G, and by siRNA knock-down of P2X7R. Astrocyte uptake of YP was also inhibited by phenol red at concentrations above 50 muM, suggesting that phenol red present in standard cell culture media may influence P2X7R channel activity. Treatment with apyrase, an enzyme that degrades extracellular ATP, partially decreased YP uptake in astrocytes. Conversely, exposure to the ectonucleotidase inhibitor ARL67156 enhanced YP uptake and astrocytes plated without contiguous neighboring astrocytes showed reduced basal YP uptake. These results suggest that the basal uptake of YP may be due to activation of P2X7R by release of ATP by astrocytes themselves into intercellular spaces.

GABA, a forgotten gliotransmitter

The amino acid gamma-aminobutiric acid (GABA) is a major inhibitory transmitter in the vertebrate central nervous system (CNS) where it can be released by neurons and by glial cells. Neuronal GABAergic signaling is well characterized: the mechanisms of GABA release, the receptors it targets and the functional consequences of their activation have been extensively studied. In contrast, the corresponding features of glial GABAergic signaling have attracted less attention. In this review, we first discuss evidence from the literature for GABA accumulation, production and release by glial cells. We then review the results of recent experiments that point toward functional roles of GABA as a "gliotransmitter".

Activation of P2X(7) receptors decreases glutamate uptake and glutamine synthetase activity in RBA-2 astrocytes via distinct mechanisms

Glutamate clearance by astrocytes is critical for controlling excitatory neurotransmission and ATP is an important mediator for neuron-astrocyte interaction. However, the effect of ATP on glutamate clearance has never been examined. Here we report that treatment of RBA-2 cells, a type-2-like astrocyte cell line, with ATP and the P2X(7) receptor selective agonist 3'-O-(4-benzoylbenzoyl) adenosine 5'-triphosphate (BzATP) decreased the Na+-dependent [3H]glutamate uptake within minutes. Mechanistic studies revealed that the decreases were augmented by removal of extracellular Mg2+ or Ca2+, and was restored by P2X7 selective antagonist , periodate-oxidized 2',3'-dialdehyde ATP (oATP), indicating that the decreases were mediated through P2X(7) receptors. Furthermore, stimulation of P2X7 receptors for 2 h inhibited both activity and protein expression of glutamine synthetase (GS), and oATP abolished the inhibition. In addition, removal of extracellular Ca(2+) and inhibition of protein kinase C (PKC) restored the ATP-decreased GS expression but failed to restore the P2X(7)-decreased [3H]glutamate uptake. Therefore, P2X7-mediated intracellular signals play a role in the down-regulation of GS activity/expression. Activation of P2X7 receptors stimulated increases in intracellular Na+ concentration ([Na+](i)) suggesting that the P2X(7)-induced increases in [Na+](i) may affect the local Na+ gradient and decrease the Na+-dependent [3H]glutamate uptake. These findings demonstrate that the P2X7-mediated decreases in glutamate uptake and glutamine synthesis were mediated through distinct mechanisms in these cells.

P2 receptor-mediated modulation of neurotransmitter release-an update

Presynaptic nerve terminals are equipped with a number of presynaptic auto- and heteroreceptors, including ionotropic P2X and metabotropic P2Y receptors. P2 receptors serve as modulation sites of transmitter release by ATP and other nucleotides released by neuronal activity and pathological signals. A wide variety of P2X and P2Y receptors expressed at pre- and postsynaptic sites as well as in glial cells are involved directly or indirectly in the modulation of neurotransmitter release. Nucleotides are released from synaptic and nonsynaptic sites throughout the nervous system and might reach concentrations high enough to activate these receptors. By providing a fine-tuning mechanism these receptors also offer attractive sites for pharmacotherapy in nervous system diseases. Here we review the rapidly emerging data on the modulation of transmitter release by facilitatory and inhibitory P2 receptors and the receptor subtypes involved in these interactions.

New insights into purinergic receptor signaling in neuronal differentiation, neuroprotection, and brain disorders

Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system and participate in the synaptic process particularly associated with acetylcholine, GABA, and glutamate neurotransmission. As a result of activation, the P2 receptors promote the elevation of free intracellular calcium concentration as the main signaling pathway. Purinergic signaling is present in early stages of embryogenesis and is involved in processes of cell proliferation, migration, and differentiation. The use of new techniques such as knockout animals, in vitro models of neuronal differentiation, antisense oligonucleotides to induce downregulation of purinergic receptor gene expression, and the development of selective inhibitors for purinergic receptor subtypes contribute to the comprehension of the role of purinergic signaling during neurogenesis. In this review, we shall discuss the participation of purinergic receptors in developmental processes and in brain physiology, including neuron-glia interactions and pathophysiology.

Association of polymorphisms in P2RX7 and CaMKKb with anxiety disorders

BACKGROUND

There is considerable evidence that genetic factors play an important role in the pathophysiology of affective disorders including bipolar disorder, major depressive disorder and anxiety disorders. Long-term follow up studies as well as drug treatment studies suggest that these clinical conditions share a number of pathophysiological commonalities including genetic variables. One possible candidate region is located on chromosome 12q24.31, originated from previous linkage and association studies with bipolar disorder and unipolar depression. This region contains two candidate genes for purinergic ligand-gated ion channels, P2RX7 and P2RX4, and one gene coding for calmodulin-dependent protein kinase kinase b (CaMKKb).

METHODS

In the present study, we investigated the genetic associations between 15 SNPs in the candidate genes P2RX7, P2RX4 and CaMKKb on chromosome 12q24.31 in 179 patients with anxiety disorders and syndromal panic attacks versus 462 healthy controls.

RESULTS

One nominal case-control association could be detected for a SNP in the 5'UTR region of P2RX4, which did not remain significant after correction for multiple testing. We found, however, a prominent association between severity of panic- and agoraphobia symptoms and an exonic SNP (rs3817190) in the CaMKKb gene and a trend for association with an exonic SNP in P2RX7 (rs1718119) with severity scores in the panic- and agoraphobia scale.

CONCLUSION

The locus 12q24.31 seems to be an important genetic region for anxiety, bipolar and unipolar disorders, suggesting a genetic overlap in the group of affective disorders. The specific contribution of the herein reported gene polymorphisms to the clinical condition is still unclear and warrants further analysis.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Exocytosis of ATP from astrocyte progenitors modulates spontaneous Ca2+ oscillations and cell migration

In the mature central nervous system (CNS) regulated secretion of ATP from astrocytes is thought to play a significant role in cell signaling. Whether such a mechanism is also operative in the developing nervous system and, if so, during which stage of development, has not been investigated. We have tackled this question using cells derived from reconstituted neurospheres, as well as brain explants of embryonic mice. Here, we show that in both models of neural cell development, astrocyte progenitors are competent for the regulated secretion of ATP-containing vesicles. We further document that this secretion is dependent on cytosolic Ca(2+) and the v-SNARE system, and takes place by exocytosis. Interference with ATP secretion alters spontaneous Ca(2+) oscillations and migration of neural progenitors. These data indicate that astrocyte progenitors acquire early in development the competence for regulated secretion of ATP, and that this event is implicated in the regulation of at least two cell functions, which are critical for the proper morphogenesis and functional maturation of the CNS.

The Transmembrane Sodium Gradient Influences Ambient GABA Concentration by Altering the Equilibrium of GABA Transporters

Tonic inhibition is widely believed to be caused solely by “spillover” of GABA that escapes the synaptic cleft and activates extrasynaptic GABAA receptors. However, an exclusively vesicular source is not consistent with the observation that tonic inhibition can still occur after blocking vesicular release. Here, we made patch-clamp recordings from neurons in rat hippocampal cultures and measured the tonic current that was blocked by bicuculline or gabazine. During perforated patch recordings, the tonic GABA current was decreased by the GAT1 antagonist SKF-89976a. Zero calcium solution did not change the amount of tonic current, despite a large reduction in vesicular GABA release. Perturbations that would be expected to alter the transmembrane sodium gradient influenced the tonic current. For example, in zero calcium Ringer, TTX (which can decrease cytosolic [Na+]) reduced tonic current, whereas veratridine (which can increase cytosolic [Na+]) increased tonic current. Likewise, removal of extracellular sodium led to a large increase in tonic current. The increases in tonic current induced by veratridine and sodium removal were completely blocked by SKF89976a. When these experiments were repeated in hippocampal slices, similar results were obtained except that a GAT1- and GAT3-independent nonvesicular source(s) of GABA was found to contribute to the tonic current. We conclude that multiple sources can contribute to ambient GABA, including spillover and GAT1 reversal. The source of GABA release may be conceptually less important in determining the amount of tonic inhibition than the factors that control the equilibrium of GABA transporters.

Oxidized ATP decreases beta-actin expression and intracellular superoxide concentrations in RBA-2 type-2 astrocytes independently of P2X7 receptor

Periodate-oxidized 2',3'-dialdehyde ATP (oxidized ATP) has been used extensively as a selective antagonist at P2X(7) receptors, although P2X(7)-independent actions on pro-inflammatory cytokine release have also been reported. Because P2X(7) receptors in astrocytes have been suggested as potential targets of anti-inflammatory drug therapy, we examined the effect of oxidized ATP on beta-actin expression and superoxide production of RBA-2 type-2 astrocytes known to possess P2X(7) receptors. Oxidized ATP per se decreased beta-actin expression time and dose dependently. Treatment with oxidized ATP for 8 h caused an approximately 50% decrease in beta-actin expression whereas other P2 receptor antagonists, brilliant blue G (BBG), suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), were not effective. In addition, oxidized ATP per se decreased the intracellular superoxide concentration, whereas ATP and the P2X(7) receptor-selective agonist 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) stimulated intracellular superoxide production, an effect inhibited by oxidized ATP. In addition, oxidized ATP neither affected cellular viability nor affected interleukin-1beta, converting enzyme (ICE)-like protease activity in these astrocytes. To further elucidate the mechanism, the effects of oxidized ATP on intracellular superoxide concentration and beta-actin expression were examined in a P2X(7) receptor-negative astrocyte cell line, IA-1g1. Oxidized ATP-induced a time-dependent decrease in intracellular superoxide concentration whereas oxidized ATP had no effect on beta-actin expression. Nevertheless, oxidized ATP altered f-actin cytoskeleton arrangement in IA-1g1 astrocytes. Taken together, these results indicate that oxidized ATP per se caused a cell specific decrease in beta-actin expression in RBA-2 type-2 astrocytes. In addition, oxidized ATP decreased intracellular superoxide concentrations and altered f-actin cytoskeleton arrangement in both P2X(7) receptor-positive and -negative astrocytes. Thus, we conclude from these results that the effects of oxidized ATP on actin and superoxide are mediated through mechanisms that are at least in part, independent of P2X(7) receptors.

Elucidation of ATP-stimulated stress protein expression of RBA-2 type-2 astrocytes: ATP potentiate HSP60 and Cu/Zn SOD expression and stimulates pI shift of peroxiredoxin II

ATP has been shown to mediate stress responses in the brain. The present study examined the ATP-stimulated stress protein expression of RBA-2 type-2 astrocytes. Our results revealed that ATP stimulated HSP60 expression in a dose- and time-dependent manner. The stimulation requires a minimal ATP concentration of 500 microM and high concentration of extracellular ATP (1 mM) stimulated a significant increase of HSP60 expression from 2 to 24 h. In addition, the ATP-stimulated HSP60 expressions were inhibited by inhibitors for protein kinase C (PKC) and phospholipase D (PLD), and by antioxidants, resveratrol, and catalase. Furthermore, ATP stimulated the expression of Cu/Zn superoxide dismutase (SOD). In addition, ATP and P2X7 receptor selective agonist BzATP also decreased mitochondria membrane potential measured by flow cytometry. To further examine the proteins involving in ATP-mediated stress responses, we conducted proteomic analysis. We found that RBA-2 astrocytes possess abundant peroxiredoxin II (Prx II), an antioxidant enzyme. ATP and exogenous H2O2 stimulated Prx II shifting from oxidized form to reduced form. Thus, we concluded that ATP potentiated the expression of HSP60 and Cu/Zn SOD, and decreased mitochondria membrane potential. In addition, RBA-2 astrocytes expressed Prx II that might also serve as a protective mechanism to control the concentration of reactive oxygen species.

Purinergic signalling in neuron-glia interactions

Activity-dependent release of ATP from synapses, axons and glia activates purinergic membrane receptors that modulate intracellular calcium and cyclic AMP. This enables glia to detect neural activity and communicate among other glial cells by releasing ATP through membrane channels and vesicles. Through purinergic signalling, impulse activity regulates glial proliferation, motility, survival, differentiation and myelination, and facilitates interactions between neurons, and vascular and immune system cells. Interactions among purinergic, growth factor and cytokine signalling regulate synaptic strength, development and responses to injury. We review the involvement of ATP and adenosine receptors in neuron-glia signalling, including the release and hydrolysis of ATP, how the receptors signal, the pharmacological tools used to study them, and their functional significance.

P2 receptors and neuronal injury

Extracellular adenosine 5'-triphosphate (ATP) was proposed to be an activity-dependent signaling molecule that regulates glia-glia and glia-neuron communications. ATP is a neurotransmitter of its own right and, in addition, a cotransmitter of other classical transmitters such as glutamate or GABA. The effects of ATP are mediated by two receptor families belonging either to the P2X (ligand-gated cationic channels) or P2Y (G protein-coupled receptors) types. P2X receptors are responsible for rapid synaptic responses, whereas P2Y receptors mediate slow synaptic responses and other types of purinergic signaling involved in neuronal damage/regeneration. ATP may act at pre- and postsynaptic sites and therefore, it may participate in the phenomena of long-term potentiation and long-term depression of excitatory synaptic transmission. The release of ATP into the extracellular space, e.g., by exocytosis, membrane transporters, and connexin hemichannels, is a widespread physiological process. However, ATP may also leave cells through their plasma membrane damaged by inflammation, ischemia, and mechanical injury. Functional responses to the activation of multiple P2 receptors were found in neurons and glial cells under normal and pathophysiological conditions. P2 receptor-activation could either be a cause or a consequence of neuronal cell death/glial activation and may be related to detrimental and/or beneficial effects. The present review aims at demonstrating that purinergic mechanisms correlate with the etiopathology of brain insults, especially because of the massive extracellular release of ATP, adenosine, and other neurotransmitters after brain injury. We will focus in this review on the most important P2 receptor-mediated neurodegenerative and neuroprotective processes and their beneficial modulation by possible therapeutic manipulations.

P2X7 receptors mediate ATP release and amplification of astrocytic intercellular Ca2+ signaling

Modulation of synaptic transmission and brain microcirculation are new roles ascribed to astrocytes in CNS function. A mechanism by which astrocytes modify neuronal activity in the healthy brain depends on fluctuations of cytosolic Ca2+ levels, which regulate the release of "gliotransmitters" via an exocytic pathway. Under pathological conditions, however, the participation of other pathways, including connexin hemichannels and the pore-forming P2X7R, have been proposed but remain controversial. Through the use of genetically modified 1321N1 human astrocytoma cells and of spinal cord astrocytes derived from neonatal Cx43- and P2X7R-null mice, we provide strong evidence that P2X7Rs, but not Cx43 hemichannels, are sites of ATP release that promote the amplification of Ca2+ signal transmission within the astrocytic network after exposure to low divalent cation solution. Moreover, our results showing that gap junction channel blockers (heptanol, octanol, carbenoxolone, flufenamic acid, and mefloquine) are antagonists of the P2X7R indicate the inadequacy of using these compounds as evidence for the participation of connexin hemichannels as sites of gliotransmitter release.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

A noncanonical release of GABA and glutamate modulates neuronal migration

Immature neurons express GABA and glutamate receptors before synapse formation, and both transmitters are released at an early developmental stage. We have now tested the hypothesis that the ongoing release of GABA and glutamate modulates neuronal migration. Using 5-bromo-2'-deoxyuridine labeling and cocultures of hippocampal slices obtained from naive and green fluorescent protein-transgenic mice, we report that migration is severely affected by GABA(A) or NMDA receptor antagonist treatments. These effects were also present in munc18-1 knock-out slices in which soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent vesicular secretion of transmitters has been deleted. GABA(A) antagonists were more efficient than NMDA antagonists to reduce cell migration, in keeping with the earlier maturation of GABAergic mechanisms. We conclude that GABA and, to a lesser degree, glutamate released in a SNARE-independent mechanism exert a paracrine action on neuronal migration.