Developmental expression of metabotropic P2Y(1) and P2Y(2) receptors in freshly isolated astrocytes from rat hippocampus

Regulation of GABAergic neurotransmission by purinergic receptors in brain physiology and disease

Purinergic receptors regulate the processing of neural information in the hippocampus and cerebral cortex, structures related to cognitive functions. These receptors are activated when astrocytic and neuronal populations release adenosine triphosphate (ATP) in an autocrine and paracrine manner, following sustained patterns of neuronal activity. The modulation by these receptors of GABAergic transmission has only recently been studied. Through their ramifications, astrocytes and GABAergic interneurons reach large groups of excitatory pyramidal neurons. Their inhibitory effect establishes different synchronization patterns that determine gamma frequency rhythms, which characterize neural activities related to cognitive processes. During early life, GABAergic-mediated synchronization of excitatory signals directs the experience-driven maturation of cognitive development, and dysfunctions concerning this process have been associated with neurological and neuropsychiatric diseases. Purinergic receptors timely modulate GABAergic control over ongoing neural activity and deeply affect neural processing in the hippocampal and neocortical circuitry. Stimulation of A2 receptors increases GABA release from presynaptic terminals, leading to a considerable reduction in neuronal firing of pyramidal neurons. A1 receptors inhibit GABAergic activity but only act in the early postnatal period when GABA produces excitatory signals. P2X and P2Y receptors expressed in pyramidal neurons reduce the inhibitory tone by blocking GABAA receptors. Finally, P2Y receptor activation elicits depolarization of GABAergic neurons and increases GABA release, thus favoring the emergence of gamma oscillations. The present review provides an overall picture of purinergic influence on GABAergic transmission and its consequences on neural processing, extending the discussion to receptor subtypes and their involvement in the onset of brain disorders, including epilepsy and Alzheimer's disease.

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.

Astrocytes of the early postnatal brain

In the rodent forebrain, the majority of astrocytes are generated during the early postnatal phase. Following differentiation, astrocytes undergo maturation which accompanies the development of the neuronal network. Neonate astrocytes exhibit a distinct morphology and domain size which differs to their mature counterparts. Moreover, many of the plasma membrane proteins prototypical for fully developed astrocytes are only expressed at low levels at neonatal stages. These include connexins and Kir4.1, which define the low membrane resistance and highly negative membrane potential of mature astrocytes. Newborn astrocytes moreover express only low amounts of GLT-1, a glutamate transporter critical later in development. Furthermore, they show specific differences in the properties and spatio-temporal pattern of intracellular calcium signals, resulting from differences in their repertoire of receptors and signalling pathways. Therefore, roles fulfilled by mature astrocytes, including ion and transmitter homeostasis, are underdeveloped in the young brain. Similarly, astrocytic ion signalling in response to neuronal activity, a process central to neuron-glia interaction, differs between the neonate and mature brain. This review describes the unique functional properties of astrocytes in the first weeks after birth and compares them to later stages of development. We conclude that with an immature neuronal network and wider extracellular space, astrocytic support might not be as demanding and critical compared to the mature brain. The delayed differentiation and maturation of astrocytes in the first postnatal weeks might thus reflect a reduced need for active, energy-consuming regulation of the extracellular space and a less tight control of glial feedback onto synaptic transmission.

Astroglial Ca2+-Dependent Hyperexcitability Requires P2Y1 Purinergic Receptors and Pannexin-1 Channel Activation in a Chronic Model of Epilepsy

Astrocytes from the hippocampus of chronic epileptic rats exhibit an abnormal pattern of intracellular calcium oscillations, characterized by an augmented frequency of long lasting spontaneous Ca²⁺ transients, which are sensitive to purinergic receptor antagonists but resistant to tetrodotoxin. The above suggests that alterations in astroglial Ca²⁺-dependent excitability observed in the epileptic tissue could arise from changes in astrocyte-to-astrocyte signaling, which is mainly mediated by purines in physiological and pathological conditions. In spite of that, how purinergic signaling contributes to astrocyte dysfunction in epilepsy remains unclear. Here, we assessed the possible contribution of P2Y₁R as well as pannexin1 and connexin43 hemichannels—both candidates for non-vesicular ATP-release—by performing astroglial Ca²⁺ imaging and dye uptake experiments in hippocampal slices from control and fully kindled rats. P2Y₁R blockade with MRS2179 decreased the mean duration of astroglial Ca²⁺ oscillations by reducing the frequency of slow Ca²⁺ transients, and thereby restoring the balance between slow (ST) and fast transients (FT) in the kindled group. The potential contribution of astroglial pannexin1 and connexin43 hemichannels as pathways for purine release (e.g., ATP) was assessed through dye uptake experiments. Astrocytes from kindled hippocampi exhibit three-fold more EtBr uptake than controls, whereby pannexin1 hemichannels (Panx1 HCs) accounts for almost all dye uptake with only a slight contribution from connexin43 hemichannels (Cx43 HCs). Confirming its functional involvement, Panx1 HCs inhibition decreased the mean duration of astroglial Ca²⁺ transients and the frequency of slow oscillations in kindled slices, but had no noticeable effects on the control group. As expected, Cx43 HCs blockade did not have any effects over the mean duration of astroglial Ca²⁺ oscillations. These findings suggest that P2Y₁R and Panx1 HCs play a pivotal role in astroglial pathophysiology, which would explain the upregulation of glutamatergic neurotransmission in the epileptic brain and thus represents a new potential pharmacological target for the treatment of drug-refractory epilepsy.

Purinergic Receptors in Neurological Diseases With Motor Symptoms: Targets for Therapy

Since proving adenosine triphosphate (ATP) functions as a neurotransmitter in neuron/glia interactions, the purinergic system has been more intensely studied within the scope of the central nervous system. In neurological disorders with associated motor symptoms, including Parkinson's disease (PD), motor neuron diseases (MND), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), Huntington's Disease (HD), restless leg syndrome (RLS), and ataxias, alterations in purinergic receptor expression and activity have been noted, indicating a potential role for this system in disease etiology and progression. In neurodegenerative conditions, neural cell death provokes extensive ATP release and alters calcium signaling through purinergic receptor modulation. Consequently, neuroinflammatory responses, excitotoxicity and apoptosis are directly or indirectly induced. This review analyzes currently available data, which suggests involvement of the purinergic system in neuro-associated motor dysfunctions and underlying mechanisms. Possible targets for pharmacological interventions are also discussed.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

KV 7/M channels as targets for lipopolysaccharide-induced inflammatory neuronal hyperexcitability

KEY POINTS

Neuroinflammation associated with CNS insults leads to neuronal hyperexcitability, which may culminate in epileptiform discharges. Application of the endotoxin lipopolysaccharide (LPS) to brain tissue initiates a neuroinflammatory cascade, providing an experimental model to study the mechanisms of neuroinflammatory neuronal hyperexcitability. Here we show that LPS application to hippocampal slices markedly enhances the excitability of CA1 pyramidal cells by inhibiting a specific potassium current, the M-current, generated by K_V 7/M channels, which controls the excitability of almost every neuron in the CNS. The LPS-induced M-current inhibition is triggered by sequential activation of microglia, astrocytes and pyramidal cells, mediated by metabotropic purinergic and glutamatergic transmission, leading to blockade of K_V 7/M channels by calcium released from intracellular stores. The identification of the downstream molecular target of neuroinflammation, namely the K_V 7/M channel, potentially has far reaching implications for the understanding and treatment of many acute and chronic brain disorders.

ABSTRACT

Acute brain insults and many chronic brain diseases manifest an innate inflammatory response. The hallmark of this response is glia activation, which promotes repair of damaged tissue, but also induces structural and functional changes that may lead to an increase in neuronal excitability. We have investigated the mechanisms involved in the modulation of neuronal activity by acute inflammation. Initiating inflammatory responses in hippocampal tissue rapidly led to neuronal depolarization and repetitive firing even in the absence of active synaptic transmission. This action was mediated by a complex metabotropic purinergic and glutamatergic glia-to-neuron signalling cascade, leading to the blockade of neuronal K_V 7/M channels by Ca²⁺ released from internal stores. These channels generate the low voltage-activating, non-inactivating M-type K⁺ current (M-current) that controls intrinsic neuronal excitability, and its inhibition was the predominant cause of the inflammation-induced hyperexcitability. Our discovery that the ubiquitous K_V 7/M channels are the downstream target of the inflammation-induced cascade, has far reaching implications for the understanding and treatment of many acute and chronic brain disorders.

Purinergic P2Y1 Receptors Control Rapid Expression of Plasma Membrane Processes in Hippocampal Astrocytes

Astrocytes regulate neuronal activity and blood brain barrier through tiny plasma membrane branches or astrocytic processes (APs) making contact with synapses and brain vessels. Several transmitters released by astrocytes and exerting their action on several receptor classes expressed by astrocytes themselves influence their physiology. Here we found that APs are dynamically modulated by purines. In live imaging experiments carried out in rat hippocampal astrocytes, Gq-coupled P2Y1 receptor blockade with the selective antagonist MRS2179 (1 μM) or inhibition of its effector phospholipase C using U73122 (3 μM) produced APs retraction, while stimulation of the same receptor with the selective agonist 2MeSADP (100 μM) increased their number. Since astrocytes, among other transmitters, release ATP by several mechanisms including connexin hemichannels, we used the connexin hemichannel inhibitor carbenoxolone (100 μM) and APs retraction was observed. In our system we then measured expression or function of channels important for modulation of volume transmission and K+ buffering, aquaporin-4, and K+ inward rectifying (Kir) channels, respectively. Aquaporin-4 expression level did not change whereas, in whole-cell patch-clamp recordings performed to measure Kir current, we observed an increase in K+ current in all conditions where APs number was reduced. These data are supporting the idea of a dynamic modulation of astrocytic processes by purinergic signal, strengthening the role of purines in brain homeostasis.

Purinergic signalling during development and ageing

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

Spinal muscular atrophy astrocytes exhibit abnormal calcium regulation and reduced growth factor production

Spinal muscular atrophy (SMA) is a genetic disorder caused by the deletion of the survival motor neuron 1 (SMN1) gene that leads to loss of motor neurons in the spinal cord. Although motor neurons are selectively lost during SMA pathology, selective replacement of SMN in motor neurons does not lead to full rescue in mouse models. Due to the ubiquitous expression of SMN, it is likely that other cell types besides motor neurons are affected by its disruption and therefore may contribute to disease pathology. Here we show that astrocytes in SMAΔ7 mouse spinal cord and from SMA-induced pluripotent stem cells exhibit morphological and cellular changes indicative of activation before overt motor neuron loss. Furthermore, our in vitro studies show mis-regulation of basal calcium and decreased response to adenosine triphosphate stimulation indicating abnormal astrocyte function. Together, for the first time, these data show early disruptions in astrocytes that may contribute to SMA disease pathology.

Activation of UTP-sensitive P2Y2 receptor induces the expression of cholinergic genes in cultured cortical neurons: a signaling cascade triggered by Ca2+ mobilization and extracellular regulated kinase phosphorylation

ATP functions as an extracellular signaling molecule that is costored and coreleased with neurotransmitters at central and peripheral neuronal synapses. Stimulation by ATP upregulates the expression of synaptic genes in muscle-including the genes for nicotine acetylcholine receptor (α-, δ-, and ε-subunits) and acetylcholinesterase (AChE)-via the P2Y receptor (P2YR), but the trophic response of neurons to the activation of P2YRs is less well understood. We reported that cultured cortical neurons and the developing rat brain expressed different types of P2YRs, and among these the UTP-sensitive P2Y2R was the most abundant. P2Y2R was found to exist in membrane rafts and it colocalized with the postsynaptic protein PSD-95 in cortical neurons. Notably, agonist-dependent stimulation of P2Y2R elevated the neuronal expression of cholinergic genes encoding AChE, PRiMA (an anchor for the globular form AChE), and choline acetyltransferase, and this induction was mediated by a signaling cascade that involved Ca(2+) mobilization and extracellular regulated kinases 1/2 activation. The importance of P2Y2R action was further shown by the receptor's synergistic effect with P2Y1R in enhancing cholinergic gene expression via the robust stimulation of Ca(2+) influx. Taken together our results revealed a developmental function of P2Y2R in promoting synaptic gene expression and demonstrated the influence of costimulation of P2Y1R and P2Y2R in neurons.

Purinergic mechanisms and pain--an update

There is a brief summary of the background literature about purinergic signalling. The review then considers purinergic mechanosensory transduction involved in visceral, cutaneous and musculoskeletal nociception and on the roles played by P2X3, P2X2/3, P2X4, P2X7 and P2Y₁₂ receptors in neuropathic and inflammatory pain. Current developments of compounds for the therapeutic treatment of both visceral and neuropathic pain are discussed.

Purinergic signaling in embryonic and stem cell development

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

Minor contribution of ATP P2 receptors to electrically-evoked electrographic seizure activity in hippocampal slices: Evidence from purine biosensors and P2 receptor agonists and antagonists

While the position of adenosine as an endogenous anticonvulsant is well established, it is unclear to what extent its precursor, ATP, contributes to seizure activity via P2 receptors. In this study we have addressed this issue through the use of ATP biosensors and agonists and antagonists of ATP P2 receptors to detect the release and role of ATP, respectively, during electrically-evoked electrographic seizure-like events (eSLEs) in rat hippocampal slices. The broad-spectrum P2 receptor antagonists RB-2 and PPADS (10μM) caused a small ∼30% inhibition of eSLE duration, and a reduction in intensity. This inhibition of eSLEs was partially reproduced with the P2X(1,2/3,3) antagonist NF023 (10μM), but not the P2X(7) antagonist BBG (10μM). However, the P2X receptor agonist α,β-meATP did not enhance eSLEs, but instead reduced their duration. Furthermore, we could discern no role for P2Y(1) receptors in electrically-evoked eSLEs: both the P2Y(1) antagonist MRS2179 (10μM) and the P2Y(1) receptor agonist 2-methylthioADP (10μM) were without effect on eSLEs. Consistent with a minor role for ATP P2 receptors on eSLEs we could detect no ATP release during eSLEs, although appreciable quantities of adenosine were detected, which had a pronounced inhibitory action on eSLEs via A(1) receptors. We conclude that the role of ATP P2 receptors in modulating electrographic seizure activity is limited, at least in models such as this one requiring electrical stimulation of afferent fibres. We further conclude that the presence and action of adenosine under these conditions may primarily reflect direct release of this purine.

ATP induces synaptic gene expressions in cortical neurons: transduction and transcription control via P2Y1 receptors

Studies in vertebrate neuromuscular synapses have revealed previously that ATP, via P2Y receptors, plays a critical role in regulating postsynaptic gene expressions. An equivalent regulatory role of ATP and its P2Y receptors would not necessarily be expected for the very different situation of the brain synapses, but we provide evidence here for a brain version of that role. In cultured cortical neurons, the expression of P2Y(1) receptors increased sharply during neuronal differentiation. Those receptors were found mainly colocalized with the postsynaptic scaffold postsynaptic density protein 95 (PSD-95). This arises through a direct interaction of a PDZ domain of PSD-95 with the C-terminal PDZ-binding motif, D-T-S-L of the P2Y(1) receptor, confirmed by the full suppression of the colocalization upon mutation of two amino acids therein. This interaction is effective in recruiting PSD-95 to the membrane. Specific activation of P2Y(1) (G-protein-coupled) receptors induced the elevation of intracellular Ca(2+) and activation of a mitogen-activated protein kinase/Raf-1 signaling cascade. This led to distinct up-regulation of the genes encoding acetylcholinesterase (AChE(T) variant), choline acetyltransferase, and the N-methyl-d-aspartate receptor subunit NR2A. This was confirmed, in the example of AChE, to arise from P2Y(1)-dependent stimulation of a human ACHE gene promoter. That involved activation of the transcription factor Elk-1; mutagenesis of the ACHE promoter revealed that Elk-1 binding at its specific responsive elements in that promoter was induced by P2Y(1) receptor activation. The combined findings reveal that ATP, via its P2Y(1) receptor, can act trophically in brain neurons to regulate the gene expression of direct effectors of synaptic transmission.

TRP channels are involved in mediating hypercapnic Ca2+ responses in rat glia-rich medullary cultures independent of extracellular pH

The medulla contains central chemosensitive cells important for the maintenance of blood gas and pH homeostasis. To identify the intrinsic chemosensitive cells, we measured responses of intracellular Ca(2+) ([Ca(2+)](i)) and H(+) ([H(+)](i)), and membrane potential of rat primary-cultured medullary cells to 6-s exposure to acidosis. The cells showed transient [Ca(2+)](i) increases to extracellular pH 6.8, which was inhibited by the specific ASIC1a blocker (psalmotoxin-1), but did not respond to pH 7.1 in the HEPES-buffered solution. Isocapnic acidosis induced no changes in [Ca(2+)](i), whereas hypercapnic acidosis induced a remarkable Ca(2+) response and an increase in membrane potential in the HCO(3)(-)-buffered solution (pH 7.1). In glia-rich cultures, intracellular acidification preceded the hypercapnic acidosis-induced Ca(2+) response, and acetazolamide, a carbonic anhydrase inhibitor suppressed these responses. Transient receptor potential (TRP) channel broad-spectrum blockers Ni(2+) and ruthenium red, and a TRPV1- and TRPM8-specific blocker N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)-tetrahydropyrazine-1(2H)-carbox-amide attenuated the hypercapnic acidosis-induced Ca(2+) response. Subpopulations of cells that exhibited the hypercapnic acidosis-induced Ca(2+) response also responded to the application of capsaicin (TRPV1 agonist) and menthol (TRPM8 agonist). These results suggest that the TRP channel family partially mediates the fast hypercapnic acidosis-induced Ca(2+) response via changes in [H(+)](i) and is a candidate of central chemosensing proteins.

Astrocyte heterogeneity: an underappreciated topic in neurobiology

Astrocytes, one of the most numerous types of cells in the central nervous system, are crucial for potassium homeostasis, neurotransmitter uptake, synapse formation, regulation of blood-brain-barrier, and the development of the nervous system. Historically, astrocytes have been studied as a homogeneous group of cells. However, evidence has accumulated that suggests heterogeneity of astrocytes across brain regions as well as within the same brain regions. Astrocytes differ in their morphology, developmental origin, gene expression profile, physiological properties, function, and response to injury and disease. A better understanding of the heterogeneity of astrocytes will greatly aid investigation of the function of astrocytes in normal brain as well as the roles of astrocytes in neurological disorders.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

Astrocytes are a major component of the resident non-neuronal glial cell population of the CNS. They are ubiquitously distributed throughout the brain and spinal cord, where they were initially thought to function in both structural and homeostatic capacities, providing the framework and environment in which neurons performed their parenchymal duties. However, this stroma-like view of astrocytes is no longer satisfactory. Mounting evidence particularly within the last decade indicates that astrocytes do not simply support neuronal activity but directly contribute to it. Congruent with this evolving view of astrocyte function in information processing is the emergent notion that these glial cells are not a homogeneous population of cells. Thus, astrocytes in various anatomically distinct regions of the normal CNS possess unique phenotypic characteristics that may directly influence the particular neuronal activities that define these regions. Remarkably, regional populations of astrocytes appear to exhibit local heterogeneity as well. Many phenotypic traits of the astrocyte lineage are responsive to local environmental cues (i.e., are adaptable), suggesting that plasticity contributes to this diversity. However, compelling evidence suggests that astrocytes arise from multiple distinct progenitor pools in the developing CNS, raising the intriguing possibility that some astrocyte heterogeneity may result from intrinsic differences between these progenitors. The purpose of this review is to explore the evidence for and mechanistic determinants of regional and local astrocyte diversity.

P2Y1 receptor signaling enhances neuroprotection by astrocytes against oxidative stress via IL-6 release in hippocampal cultures

Cell survival is a critical issue in the onset and progression of neurodegenerative diseases and following pathological events including ischemia and traumatic brain injury. Oxidative stress is the main cause of cell damage in such pathological conditions. Here, we report that adenosine 5'-triphosphate (ATP) protects hippocampal astrocytes from hydrogen peroxide (H(2)O(2))-evoked oxidative injury in astrocyte monocultures. The effect of ATP was prevented by a selective antagonist of or siRNAs against P2Y(1)R. Interestingly, in astrocyte-neuron cocultures, ATP also produced neuroprotective effects against H(2)O(2)-evoked neuronal cell death, whereas ATP did not produce any neuroprotective effects in monocultures. The ATP-induced neuroprotection in cocultures was completely inhibited by silencing of astrocytic P2Y(1)R expression, indicating that ATP acts on astrocytes and enhances their neuroprotective functions by activating P2Y(1)R. Furthermore, this neuroprotective effect was mimicked by applying conditioned medium from astrocytes that had been stimulated by ATP, implying an involvement of diffusible factors from astrocytes. We found that, in both purified astrocyte cultures and astrocyte-neuronal cocultures, ATP and the P2Y(1)R agonist 2-methylthioadenosine 5' diphosphate (2MeSADP) induced the release of interleukin-6 (IL-6), but this did not occur in neuron monocultures. Moreover, exogenous IL-6 produced a neuroprotective effect, and the neuroprotection induced by P2Y(1)R-stimulated astrocytes was prevented in the presence of an anti-IL-6 antibody. Taken together, these results suggest that P2Y(1)R-stimulated astrocytes protect against neuronal damage induced by oxidative stress, and that IL-6 is a crucial signaling molecule released from astrocytes. Thus, activation of P2Y(1)R in astrocytes may rescue neurons from secondary cell death under pathological conditions.

Functional characterization of P2Y1 versus P2X receptors in RBA-2 astrocytes: elucidate the roles of ATP release and protein kinase C

A physiological concentration of extracellular ATP stimulated biphasic Ca(2+) signal, and the Ca(2+) transient was decreased and the Ca(2+) sustain was eliminated immediately after removal of ATP and Ca(2+) in RBA-2 astrocytes. Reintroduction of Ca(2+) induced Ca(2+) sustain. Stimulation of P2Y(1) receptors with 2-methylthioadenosine 5'-diphosphate (2MeSADP) also induced a biphasic Ca(2+) signaling and the Ca(2+) sustains were eliminated using Ca(2+)-free buffer. The 2MeSADP-mediated biphasic Ca(2+) signals were inhibited by phospholipase C (PLC) inhibitor U73122, and completely blocked by P2Y(1) selective antagonist MRS2179 and protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) whereas enhanced by PKC inhibitors GF109203X and Go6979. Inhibition of capacitative Ca(2+) entry (CCE) decreased the Ca(2+)-induced Ca(2+) entry; nevertheless, ATP further enhanced the Ca(2+)-induced Ca(2+) entry in the intracellular Ca(2+) store-emptied and CCE-inhibited cells indicating that ATP stimulated Ca(2+) entry via CCE and ionotropic P2X receptors. Furthermore, the 2MeSADP-induced Ca(2+) sustain was eliminated by apyrase but potentiated by P2X(4) allosteric effector ivermectin (IVM). The agonist ADPbetaS stimulated a lesser P2Y(1)-mediated Ca(2+) signal and caused a two-fold increase in ATP release but that were not affected by IVM whereas inhibited by PMA, PLC inhibitor ET-18-OCH(3) and phospholipase D (PLD) inhibitor D609, and enhanced by removal of intra- or extracellular Ca(2+). Taken together, the P2Y(1)-mediated Ca(2+) sustain was at least in part via P2X receptors activated by the P2Y(1)-induced ATP release, and PKC played a pivotal role in desensitization of P2Y(1) receptors in RBA-2 astrocytes.

Modulation of astrocyte P2Y1 receptors by the carboxyl terminal domain of the gap junction protein Cx43

Gap junction proteins, connexins, provide intercellular channels that allow ions and small signaling molecules to be transmitted to adjacent coupled cells. Besides this function, it is becoming apparent that connexins also exert channel-independent effects, which are likely mediated by processes involving protein-protein interactions. Although a number of connexin interacting proteins have been identified, only little is known about the functional consequences of such interactions. We have previously shown that deletion of the astrocytic gap junction protein, connexin43 (Cx43) causes a right-ward shift in the dose-response curve to P2Y1R agonists and decreased P2Y1R expression levels. To evaluate whether these changes were due to reduced gap junctional communication or to protein-protein interactions, Cx43-null astrocytes were transfected with full-length Cx43 and Cx43 domains, and P2Y1R function and expression levels evaluated. Results indicate that restoration of P2Y1R function is independent of gap junctional communication and that the Cx43 carboxyl terminus spanning the SH3 binding domain (260-280) participates in the rescue of P2Y1R pharmacological behavior (shifting to the left the P2Y1R dose-response curve) without affecting its expression levels. These results suggest that the Cx43 carboxyl-terminus domain provides a binding site for an intracellular molecule, most likely a member of the c-Src tyrosine kinase family, which affects P2Y1R-induced calcium mobilization. It is here proposed that a nonchannel function of Cx43 is to serve as a decoy for such kinases. Such modulation of P2Y1R is expected to influence several neural cell functions, especially under inflammation and neurodegenerative disorders where expression levels of Cx43 are decreased.

New evidence for purinergic signaling in the olfactory bulb: A2A and P2Y1 receptors mediate intracellular calcium release in astrocytes

Purinergic receptors play a key role in neuron-glia and glia-neuron interactions. In the present study, we have recorded cytosolic Ca(2+) responses using confocal imaging in astrocytes of acute olfactory bulb slices from mice (postnatal days 3-8). By application of agonists and antagonists, we identified two types of receptors, P2Y(1) and A(2A), that mediated Ca(2+) responses attributable to Ca(2+) release from intracellular stores in the astrocytes. Both receptor types were activated by application of ATP and ADP; however, when enzymatic ATP degradation was suppressed by the alkaline phosphatase inhibitor levamisole, ATP only activated MRS2179-sensitive P2Y(1) but not ZM241385-sensitive A(2A) receptors. The dose-response curve for A(2A) receptors activated by adenosine revealed an EC(50) of 0.3 microM, one order of magnitude smaller than the EC(50) of 5 microM determined for P2Y(1) receptors activated by ADP. Electrical stimulation of the olfactory nerve in the presence of glutamate receptor blockers to suppress excitation of postsynaptic neurons evoked Ca(2+) responses in most of the astrocytes, which were inhibited by blocking both P2Y(1) and A(2A) receptors. Our results indicate that olfactory nerve terminals release not only glutamate, but also ATP, which activates P2Y(1) receptors and, after degradation of ATP to adenosine, A(2A) receptors in astrocytes.

P2 receptor expression in the dopaminergic system of the rat brain during development

Extracellular ATP facilitates the release of dopamine via P2 receptor activation in parts of the mesolimbic system. To characterize P2X/Y receptor subtypes in the developing dopaminergic system, their expression in organotypic slice co-cultures including the ventral tegmental area/substantia nigra (VTA/SN) complex and the prefrontal cortex (PFC) was studied in comparison to the receptor expression in 3-5 day-old and adult rats. Reverse transcriptase-polymerase chain reaction (RT-PCR) with specific primers for the P2X(1,2,3,4,6,7) and P2Y(1) receptors in the tissue extracts of organotypic co-cultures revealed the presence of the P2X and P2Y receptor mRNAs investigated. Multiple immunofluorescence labeling of the P2X/Y receptor protein indicated differences in the regional expression in the organotypic co-cultures after 10 days of cultivation (VTA/SN, P2X(1,2,3,4,6,7), P2Y(1,6,12); PFC, P2X(1,3,4,6,7), P2Y(1,2,4,6,12)). At postnatal days 3-5, an immunofluorescence mostly comparable to that of adult rats was observed (VTA/SN and PFC: P2X(1,2,3,4,6,7), P2Y(1,2,4,6,12)). There was one important exception: the P2X(7) receptor immunocytochemistry was not found in adult tissue, suggesting a potential role of this receptor in the development. Only few P2 receptors (e.g. P2X(1), P2Y(1)) were expressed at fibers interconnecting the dopaminergic VTA/SN with the PFC in the organotypic co-cultures. The treatment of the cultures with the ATP analogues 2-methylthio-ATP and alpha,beta-methylene-ATP induced an increase in axonal outgrowth and fiber density, which could be inhibited by pre-treatment with the P2X/Y receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid. The co-localization of the dopamine-(D1) receptor with the P2X(1) receptor in organotypic slice cultures was evident. In the PFC of the co-cultures, and that of young but not adult rats, a number of tyrosine hydroxylase (TH)-positive cells also possessed P2Y(1)-immunoreactivity (IR). Additionally, a strong P2Y(1)-IR was observed on astrocytes. The present results show a time-, region- and cell type-dependent in vitro and in vivo expression pattern of different P2 receptor subtypes in the dopaminergic system indicating the involvement of ATP and its receptors in neuronal development and growth.

Co-localization and functional cross-talk between A1 and P2Y1 purine receptors in rat hippocampus

Adenosine and ATP, via their specific P1 and P2 receptors, modulate a wide variety of cellular and tissue functions, playing a neuroprotective or neurodegenerative role in brain damage conditions. Although, in general, adenosine inhibits excitability and ATP functions as an excitatory transmitter in the central nervous system, recent data suggest the existence of a heterodimerization and a functional interaction between P1 and P2 receptors in the brain. In particular, interactions of adenosine A1 and P2Y1 receptors may play important roles in the purinergic signalling cascade. In the present work, we investigated the subcellular localization/co-localization of the receptors and their functional cross-talk at the membrane level in Wistar rat hippocampus. This is a particularly vulnerable brain area, which is sensitive to adenosine- and ATP-mediated control of glutamatergic transmission. The postembedding immunogold electron microscopy technique showed that the two receptors are co-localized at the synaptic membranes and surrounding astroglial membranes of glutamatergic synapses. To investigate the functional cross-talk between the two types of purinergic receptors, we evaluated the reciprocal effects of their activation on their G protein coupling. P2Y1 receptor stimulation impaired the potency of A1 receptor coupling to G protein, whereas the stimulation of A1 receptors increased the functional responsiveness of P2Y1 receptors. The results demonstrated an A1-P2Y1 receptor co-localization at glutamatergic synapses and surrounding astrocytes and a functional interaction between these receptors in hippocampus, suggesting ATP and adenosine can interact in purine-mediated signalling. This may be particularly important during pathological conditions, when large amounts of these mediators are released.

Gap junction and purinergic P2 receptor proteins as a functional unit: insights from transcriptomics

Gap junctions and purinergic P2 receptors (P2Rs) can be regarded as belonging to a common functional unit, given that they are involved in the transmission of calcium signals between cells. We have previously shown that deletion of the Gja1 gene alters expression levels of numerous genes encoding proteins with diverse functions, including purinergic receptors (P2Rs), and have found that genes synergistically or antagonistically expressed in wild-type tissues are more prone to be similarly or oppositely regulated in Cx43-nulls. We have now explored the use of coordination analysis of gene expression as a strategy to identify interlinked genes encoding functionally related proteins and pull-downs to evaluate their interlinkage. Our findings indicate that, in brain and in cultured astrocytes, several of these coexpressed genes encode proteins that are components of P2R signal-transduction pathways and/or directly interact with these receptors, including the gap junction protein connexin43 (Cx43) and Cx45 as well as pannexins. It is proposed that coordination analysis of gene expression may provide a novel unbiased strategy for the identification of proteins belonging to supramolecular complexes.

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.

Subepithelial fibroblasts in intestinal villi: roles in intercellular communication

Ingestion of food and water induces chemical and mechanical signals that trigger peristaltic reflexes in the gut. Intestinal villi are motile, equipped with chemosensors and mechanosensors, and transduce signaling to sensory neurons, but the exact mechanisms have not yet been elucidated. Subepithelial fibroblasts located under the villous epithelium form contractile cellular networks via gap junctions. The networks ensheathe lamina propria and are in close contact with epithelium, neural and capillary networks, smooth muscles, and immune cells. Unique characteristics of subepithelial fibroblasts have been revealed by primary cultures isolated from rat duodenal villi. They include rapid reversal changes in cell shape by cAMP reagents and endothelins, cell shape-dependent mechanosensitivity that induces ATP release as a paracrine mediator, contractile ability, and expression of various receptors for vasoactive and neuroactive substances. Herein, we review these characteristics that play a key role in the villi. They serve as a barrier/sieve, flexible mechanical frame, mechanosensor, and signal transduction machinery in the intestinal villi, which are regulated locally and dynamically by rapid cell shape conversion.

Astrocyte calcium waves: what they are and what they do

Several lines of evidence indicate that the elaborated calcium signals and the occurrence of calcium waves in astrocytes provide these cells with a specific form of excitability. The identification of the cellular and molecular steps involved in the triggering and transmission of Ca(2+) waves between astrocytes resulted in the identification of two pathways mediating this form of intercellular communication. One of them involves the direct communication between the cytosols of two adjoining cells through gap junction channels, while the other depends upon the release of "gliotransmitters" that activates membrane receptors on neighboring cells. In this review we summarize evidence in favor of these two mechanisms of Ca(2+) wave transmission and we discuss that they may not be mutually exclusive, but are likely to work in conjunction to coordinate the activity of a group of cells. To address a key question regarding the functional consequences following the passage of a Ca(2+) wave, we list, in this review, some of the potential intracellular targets of these Ca(2+) transients in astrocytes, and discuss the functional consequences of the activation of these targets for the interactions that astrocytes maintain with themselves and with other cellular partners, including those at the glial/vasculature interface and at perisynaptic sites where astrocytic processes tightly interact with neurons.

P2Y receptor-mediated excitation in the posterior hypothalamus

Histaminergic neurons located in the posterior hypothalamus (tuberomamillary nucleus, TMN) project widely through the whole brain controlling arousal and attention. They are tonically active during wakefulness but cease firing during sleep. As a homeostatic theory of sleep involves ATP depletion and adenosine accumulation in the brain, we investigated the role of ATP and its analogues as well as adenosine on neuronal activity in the TMN. We show increased firing of rat TMN neurons by ATP, ADP, UTP and 2meSATP, indicating activation of receptors belonging to the P2Y family. Adenosine affected neither membrane potential nor firing of these cells. Single-cell reverse transcriptase-polymerase chain reaction revealed that P2Y1 and P2Y4 are prevailing receptors in TMN neurons. P2Y1 receptor mRNA was detected with a higher frequency in 2-week-old than in 4-week-old rats; in accordance, 2meSATP was more potent than ATP. Semi-quantitative real-time polymerase chain reaction revealed a developmental downregulation of mRNA levels for P2Y1 and P2Y4 receptors. Immunocytochemistry demonstrated neuronal and glial localization of the P2Y1 receptor protein. Network activity measured with multielectrode arrays in primary cultures made from the posterior hypothalamus was enhanced by UTP and 2meSATP (P2Y4 and P2Y1 agonists, respectively). ATP caused an inhibition of firing, which was reversed in the presence of suramin or gabazine [gamma-aminobutyric acid (GABA)A receptor antagonist], indicating that GABAergic neurons are preferentially activated by ATP in this network. Excitation of the wake-active TMN neurons by nucleotides and the lack of adenosine action may be important factors in sleep-wake regulation.

Signaling proteins in raft-like microdomains are essential for Ca2+ wave propagation in glial cells

The hypothesis that calcium signaling proteins segregate into lipid raft-like microdomains was tested in isolated membranes of rat oligodendrocyte progenitor (OP) cells and astrocytes using Triton X-100 solubilization and density gradient centrifugation. Western blot analysis of gradient fractions showed co-localization of caveolin-1 with proteins involved in the Ca2+ signaling cascade. These included agonist receptors, P2Y1, and M1, TRPC1, IP3R2, ryanodine receptor, as well as the G protein Galphaq and Homer. Membranes isolated from agonist-stimulated astrocytes showed an enhanced recruitment of phospholipase C (PLCbeta1), IP3R2 and protein kinase C (PKC-alpha) into lipid raft fractions. IP3R2, TRPC1 and Homer co-immunoprecipitated, suggesting protein-protein interactions. Disruption of rafts by cholesterol depletion using methyl-beta-cyclodextrin (beta-MCD) altered the distribution of caveolin-1 and GM1 to non-raft fractions with higher densities. beta-MCD-induced disruption of rafts inhibited agonist-evoked Ca2+ wave propagation in astrocytes and attenuated wave speeds. These results indicate that in glial cells, Ca2+ signaling proteins might exist in organized membrane microdomains, and these complexes may include proteins from different cellular membrane systems. Such an organization is essential for Ca2+ wave propagation.

P2 receptor-stimulation influences axonal outgrowth in the developing hippocampus in vitro

Extracellular ATP might act as a trophic factor on growing axons during development of the CNS via P2 receptors. In the present study the postnatal presence of selected P2 receptor subtypes was analyzed and their putative trophic capacity in entorhino-hippocampal slice co-cultures of mouse brain was tested. The effect of the P2 receptor ligands 2-methylthioadenosine-5'-triphosphate (P2X/Y receptor agonist) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (P2X/Y receptor antagonist) on axonal growth and fiber density of biocytin-labeled hippocampal projections was compared both with untreated cultures and with cultures treated with artificial cerebrospinal fluid. After 10 days in vitro, double immunofluorescence labeling revealed the expression of P2X(1), P2X(2), P2X(4) as well as P2Y(1) and P2Y(2) receptors in the examined regions of entorhinal fiber termination. Further, quantitative analysis of identified biocytin-traced entorhinal fibers showed a significant increase in fiber density in the dentate gyrus after incubation of the slices with the P2 receptor agonist 2-methylthioadenosine-5'-triphosphate. This neurite outgrowth promoting effect was completely abolished by the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid. Our in vitro data indicate that ATP via its P2X and P2Y receptors can shape hippocampal connectivity during development.

Differential coupling of the P2Y1 receptor to Galpha14 and Galphaq/11 proteins during the development of the rat salivary gland

In rat submandibular gland (SMG), the P2Y1 receptor (P2Y1R) mediates increases in the intracellular calcium concentration, [Ca2+]i that diminish as the animal ages from 1 to 4-6 weeks. However, P2Y1R mRNA levels do not change with age, suggesting that the age-dependent decrease in the [Ca2+]i response to P2Y1R agonists may be due to alterations in the activity of a component of the P2Y1R signalling pathway.

OBJECTIVES

To assess whether the decrease in P2Y1R-mediated intracellular calcium signalling in SMG cells as rats age is due to a decrease in P2Y1R coupling to G proteins or to a decrease in the expression of a cognate G protein.

DESIGN

SMG cells were isolated from Sprague-Dawley rats. P2Y1R function was assessed by measuring 2-MeSADP-induced increases in [Ca2+]i and ERK1/2 activation. P2Y(1)R-mediated activation of G proteins was determined by the [35S]GTPgammaS binding assay. Gq protein expression was determined by RT-PCR, Northern, and Western analysis.

RESULTS

In SMG cells from 1-week-old rats, two bands (52 and 42kDa) were detected using anti-Galpha14 antibody, whereas in SMG cells from 4- to 6-week-old rats only the 42 kDa band was detected. Furthermore, 2-MeSADP-induced GTPgamma35S binding to Galpha14 and Galphaq/11 decreases in SMG cells from 4- to 6-week-old rats as compared to 1-week-old rats.

CONCLUSIONS

These findings suggest that the age-dependent decrease in P2Y1R-mediated intracellular calcium signalling in rat SMG cells is due to a loss of 52 kDa Galpha14 and indicate the differential coupling of the P2Y1R to Galpha14 and Galphaq/11 as the gland develops.

A stochastic two-dimensional model of intercellular Ca2+ wave spread in glia

We describe a two-dimensional stochastic model of intercellular Ca(2+) wave (ICW) spread in glia that includes contributions of external stimuli, ionotropic and metabotropic P2 receptors, exo- and ecto-nucleotidases, second messengers, and gap junctions. In this model, an initial stimulus evokes ATP and UTP release from a single cell. Agonists diffuse and are degraded both in bulk solution and at cell surfaces. Ca(2+) elevation in individual cells is determined by bound agonist concentrations s and by number and features of P2 receptors summed with that generated by IP(3) diffusing through gap junction channels. Variability of ICWs is provided by randomly distributing a predetermined density of cells in a rectangular grid and by randomly selecting within intervals values characterizing the extracellular compartment, individual cells, and interconnections with neighboring cells. Variability intervals were obtained from experiments on astrocytoma cells transfected to express individual P2 receptors and/or the gap junction protein connexin43. The simulation program (available as Supplementary Material) permits individual alteration of ICW components, allowing comparison of simulations with data from cells expressing connexin43 and/or various P2 receptor subtypes. Such modeling is expected to be useful for testing phenomenological hypotheses and in understanding consequences of alteration of system components under experimental or pathological conditions.

Differential Regulation of Ca2+ Signaling and Membrane Trafficking by Multiple P2 Receptors in Brown Adipocytes

Extracellular ATP triggers changes in intracellular Ca(2+), ion channel function, and membrane trafficking in adipocytes. The aim of the present study was to determine which P2 receptors might mediate the Ca(2+) signaling and membrane trafficking responses to ATP in brown fat cells. RT-PCR was used to determine which P2 receptors are expressed in brown fat cells. Responses to nucleotide agonists and antagonists were characterized using fura-2 fluorescence imaging of Ca(2+) responses, and FM 1-43 fluorescence imaging and membrane capacitance measurements to assess membrane trafficking. The pharmacology of the Ca(2+) responses fits the properties of the P2Y receptors for which mRNA is expressed, but the agonist and antagonist sensitivity of the membrane-trafficking response was not consistent with any P2 receptor described to date. Brown adipocytes expressed mRNA for P2Y(2), P2Y(6), and P2Y(12) metabotropic receptors and P2X(1), P2X(2), P2X(3), P2X(4), P2X(5), and P2X(7) ionotropic receptors. The agonists ATP, ADP, UTP, UDP and 2', 3'-(benzoylbenzoyl) ATP (BzATP) increased intracellular Ca(2+), while 100 microM: suramin, pyridoxal-phosphate-6-azophenyl-2' 4'-disulfonic acid (PPADS), or Reactive Blue 2 partially blocked Ca(2+) responses. ATP, but not ADP, UTP, UDP or BzATP activated membrane trafficking. The membrane response could be blocked completely with 1 microM: PPADS but not by the antagonist MRS2179. We conclude that multiple P2 receptors mediate the ATP responses of brown fat cells, and that membrane trafficking is regulated by a P2 receptor showing unusual properties.

Involvement of P2 receptors in the growth and survival of neurons in the CNS

Extracellular adenosine 5'-triphosphate (ATP) has been recognized as a ubiquitous, unstable signalling molecule, acting as a fast neurotransmitter and modulator of transmitter release and neuronal excitability. Recent findings have demonstrated that ATP is a growth factor participating in differentiation, cell proliferation, and survival, as well as a toxic agent that mediates cellular degeneration and death. Potential sources of extracellular purines in the nervous system include neurons, glia, endothelium, and blood. A complex family of ectoenzymes rapidly hydrolyzes or interconverts extracellular nucleotides, thereby either terminating their signalling action or producing an active metabolite of altered purinoceptor selectivity. Most effects are mediated through the 2 main subclasses of specific cell surface receptors, P2X and P2Y. Members of these P2X/Y receptor families are widely expressed in the central nervous system (CNS) and are involved in glia-glia and glia-neuron communications, whereby they play important physiological and pathophysiological roles in a variety of biological processes. After different kinds of "acute" CNS injury (e.g., ischemia, hypoxia, mechanical stress, axotomy), extracellular ATP can reach high concentrations, up to the millimolar range, flowing out from cells into the extracellular space, exocytotically, via transmembrane transport, or as a result of cell damage. In this review, P2 receptor activation as a cause or a consequence of neuronal cell activation or death and/or glial activation is described. The involvement of P2 receptors is also described under different "chronic" pathological conditions, such as pain, epilepsia, toxic influence of ethanol or amphetamine, retinal diseases, Alzheimer's disease (AD), and possibly, Parkinson's disease. The relationship between changes in P2 receptor expression and the specific response of different cell types to injury is extremely complex and can be related to detrimental and/or beneficial effects. The present review therefore considers ATP acting via P2 receptors as a potent regulator of normal physiological and pathological processes in the brain, with a focus on pathophysiological implications of P2 receptor functions.

Frequent coexpression of the vesicular glutamate transporter 1 and 2 genes, as well as coexpression with genes for choline acetyltransferase or glutamic acid decarboxylase in neurons of rat brain

It is widely believed that expression of the vesicular glutamate transporter genes VGLUT1 and VGLUT2 is restricted to glutamatergic neurons and that the two transporters segregate in different sets of neurons. Using single-cell multiplex RT-PCR (sc-RT-mPCR), we show that VGLUT1 and VGLUT2 mRNAs were coexpressed in most of the sampled neurons from the rat hippocampus, cortex, and cerebellum at postnatal Day (P)14 but not P60. In accordance, changes in VGLUT1 and VGLUT2 mRNA concentrations were found to occur in these and other brain areas between P14 and P60, as revealed by semiquantitative RT-PCR and quantitated by ribonuclease protection assay. VGLUT1 and -2 coexpression in the hippocampal formation is supported further by in situ hybridization data showing that virtually all cells in the CA1-CA3 pyramidal and granule cell layers were highly positive for both transcripts until P14. It was revealed using sc-RT-mPCR that transcripts for VGLUT1 and VGLUT2 were also present in neurons of the cerebellum, striatum, and septum that expressed markers for gamma-aminobutyric acid (GABA)ergic or cholinergic phenotypes, as well as in hippocampal cells containing transcripts for the glial fibrillary acidic protein. Our study suggests that VGLUT1 and VGLUT2 proteins may often transport glutamate into vesicles within the same neuron, especially during early postnatal development, and that they are expressed widely in presumed glutamatergic, GABAergic, and cholinergic neurons, as well as in astrocytes. Furthermore, our study shows that such coexpressing neurons remain in the adult brain and identifies several areas that contain them in both young and adult rats.

A quantitative model of purinergic junctional transmission of calcium waves in astrocyte networks

A principal means of transmitting intracellular calcium (Ca2+) waves at junctions between astrocytes involves the release of the chemical transmitter adenosine triphosphate (ATP). A model of this process is presented in which activation of purinergic P2Y receptors by ATP triggers the release of ATP, in an autocrine manner, as well as concomitantly increasing intracellular Ca2+. The dependence of the temporal characteristics of the Ca2+ wave are shown to critically depend on the dissociation constant (K(R)) for ATP binding to the P2Y receptor type. Incorporating this model astrocyte into networks of these cells successfully accounts for many of the properties of propagating Ca2+ waves, such as the dependence of velocity on the type of P2Y receptor and the time-lag of the Ca2+ wave behind the ATP wave. In addition, the conditions under which Ca2+ waves may jump from one set of astrocytes across an astrocyte-free lane to another set of astrocytes are quantitatively accounted for by the model. The properties of purinergic transmission at astrocyte junctions may determine many of the characteristics of Ca2+ propagation in networks of these cells.

Characteristics of subepithelial fibroblasts as a mechano-sensor in the intestine: cell-shape-dependent ATP release and P2Y1 signaling

Subepithelial fibroblasts form a cellular network just under the epithelium of the gastrointestinal tract. Using primary cultured cells isolated from rat duodenal villi, we previously found that subepithelial fibroblasts reversibly changed cell morphology between flat and stellate-shape depending on intracellular cAMP levels. In this paper, we examined cell-cell communication via released ATP and Ca2+ signaling in the cellular network. Subepithelial fibroblasts were sensitive to mechanical stress such as ;touching' a cell with a fine glass rod and ;stretching' cells cultured on elastic silicone chamber. Mechanical stimulations evoked Ca2+-increase in the cells and ATP-release from the cells. The released ATP activated P2Y receptors on the surrounding cells and propagated Ca2+-waves through the network. Concomitant with Ca2+-waves, a transient contraction of the network was observed. Histochemical, RT-PCR, western blotting and Ca2+ response analyses indicated P2Y1 is a dominant functional subtype. ATP-release and Ca2+ signaling were cell-shape dependent, i.e. they were abolished in stellate-shaped cells treated with dBcAMP, and recovered or further enhanced in re-flattened cells treated with endothelin. The response to ATP also decreased in stellate-shaped cells. These findings indicate cAMP-mediated intracellular signaling causes cell-shape change, which accompanies the changes in mechano- and ATP sensitivities. Using a co-culture system of neuronal cells (NG108-15) with subepithelial fibroblasts, we confirmed that mechanically induced Ca2+-waves propagated to neurons. From these findings we propose that subepithelial fibroblasts work as a mechanosensor in the intestine. Uptake of food, water and nutrients may cause mechanical stress on subepithelial fibroblasts in the villi. The ATP released by mechanical stimulation elicits Ca2+-wave propagation through the network via P2Y1 activation and also activates P2X on terminals of mucosal sensory neurons to regulate peristaltic motility.

Direct excitation of inhibitory interneurons by extracellular ATP mediated by P2Y1 receptors in the hippocampal slice

ATP is an important cell-to-cell signaling molecule mediating the interactions between astrocytes and neurons in the CNS. In the hippocampal slices, ATP suppresses excitatory transmission mostly through activation of adenosine A1 receptors, because the ectoenzyme activity for the extracellular breakdown of ATP to adenosine is high in slice preparations in contrast to culture environments. Because the hippocampus is also rich in the expression of P2 receptors activated specifically by ATP, we examined whether ATP modulates neuronal excitability in the acute slice preparations independently of adenosine receptors. Although ATP decreased the frequency of spontaneously occurring EPSCs in the CA3 pyramidal neurons through activation of adenosine A1 receptors, ATP concurrently increased the frequency of IPSCs in a manner dependent on action potential generation. This effect was mediated by P2Y1 receptors because (1) 2-methylthio-ATP (2meSATP) was the most potent agonist, (2) 2'-deoxy-N6-methyladenosine-3',5'-bisphosphate diammonium (MRS2179) abolished this effect, and (3) this increase in IPSC frequency was not observed in the transgenic mice lacking P2Y1 receptor proteins. Application of 2meSATP elicited MRS2179-sensitive time- and voltage-dependent inward currents in the interneurons, which depolarized the cell to firing threshold. Also, it increased [Ca2+]i in both astrocytes and interneurons, but, unlike the former effect, the latter was entirely dependent on Ca2+ entry. Thus, in hippocampal slices, in addition to activating A1 receptors of the excitatory terminals after being converted to adenosine, ATP activates P2Y1 receptors in the interneurons, which is linked to activation of unidentified excitatory conductance, through mechanisms distinct from those in the astrocytes.

P2Y receptor expression on astrocytes in the nucleus accumbens of rats

The expression of purinoceptor (P2)Y-subtypes on astrocytes in vivo under physiological conditions and after stab wound injury was investigated. Reverse transcriptase-polymerase chain reaction with specific primers for the receptor-subtypes P2Y1,2,4,6,12 in tissue extracts of the nucleus accumbens of untreated rats revealed the presence of all P2Y receptor mRNAs investigated. Double immunofluorescence visualized with laser scanning microscopy indicated the expression of the P2Y1,4 receptors on glial fibrillary acidic protein (GFAP)-labeled astrocytes under physiological conditions. After stab wound injury the additional expression of the P2Y2 and P2Y6 receptors, and an up-regulation of the P2Y1,4 receptor-labeling on astrocytic cell bodies and/or processes was observed. Astrocytes of cortical, in contrast to accumbal areas exhibited P2Y1,2,4,6 receptor-immunoreactivity (IR) under control conditions, which was up-regulated after stab would injury. Labeling for the P2Y12 receptor was not observed on GFAP-positive cortical and accumbal astrocytes under any of the conditions used. For the first time, the co-localization of different P2 receptor-subtypes (e.g. P2Y1 and P2X3) on the same astrocyte was shown immunocytochemically. The up-regulation of P2Y1 receptor-IR on astrocytes and non-glial cells after mechanical injury could be facilitated by microinfusion of the P2Y1,12,13 receptor agonist adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS). Proliferative changes after ADPbetaS-microinjection were characterized by means of double-staining with antibodies against GFAP and 5-bromo-2'-deoxyuridine. The non-selective P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, the P2Y1 receptor antagonist N6-methyl-2'-deoxyadenosine 3',5'-bisphosphate and the P2Y1 receptor-antibody itself inhibited the agonist-induced effects. The data indicate the region-specific presence of P2Y receptors on astrocytes in vivo and their up-regulation after injury as well as the co-localization of P2X and P2Y receptor-subtypes on the same astrocyte. The dominant role of P2Y1 receptors in proliferation and the additional stimulation of non-P2Y1 receptors has been demonstrated in vivo suggesting the involvement of this receptor-type in the gliotic response under physiological and pathological conditions.

P2Y2 receptors activate neuroprotective mechanisms in astrocytic cells

Mechanical or ischemic trauma to the CNS causes the release of nucleotides and other neurotransmitters into the extracellular space. Nucleotides can activate nucleotide receptors that modulate the expression of genes implicated in cellular adaptive responses. In this investigation, we used human 1321N1 astrocytoma cells expressing a recombinant P2Y2 receptor to assess the role of this receptor in the regulation of anti-apoptotic (bcl-2 and bcl-xl) and pro-apoptotic (bax) gene expression. Acute treatment with the P2Y2 receptor agonist UTP up-regulated bcl-2 and bcl-xl, and down-regulated bax, gene expression. Activation of P2Y2 receptors was also coupled to the phosphorylation of cyclic AMP responsive element binding protein that positively regulates bcl-2 and bcl-xl gene expression. Cyclic AMP responsive element decoy oligonucleotides markedly attenuated the UTP-induced increase in bcl-2 and bcl-xl mRNA levels. Activation of P2Y2 receptors induced the phosphorylation of the pro-apoptotic factor Bad and caused a reduction in bax/bcl-2 mRNA expression ratio. All these signaling pathways are known to be involved in cell survival mechanisms. Using cDNA microarray analysis and RT-PCR, P2Y2 receptors were found to up-regulate the expression of genes for neurotrophins, neuropeptides and growth factors including nerve growth factor 2; neurotrophin 3; glia-derived neurite-promoting factor, as well as extracellular matrix proteins CD44 and fibronectin precursor--genes known to regulate neuroprotection. Consistent with this observation, conditioned media from UTP-treated 1321N1 cells expressing P2Y2 receptors stimulated the outgrowth of neurites in PC-12 cells. Taken together, our results suggest an important novel role for the P2Y2 receptor in survival and neuroprotective mechanisms under pathological conditions.

Mechanisms of secretion of ATP from cortical astrocytes triggered by uridine triphosphate

The mechanisms involved in autocrine ATP release from cultured astrocytes isolated from the rat cortex were investigated using an online bioluminescence technique. Astrocytes released ATP in response to application of 10 microM uridine triphosphate, which was blocked by the non-specific purinergic receptor antagonist suramin. Intracellular pathways of the uridine triphosphate-stimulated ATP release were seen to involve inositol triphosphate and calcium with the assistance of the Golgi-complex and cytoskeleton as the release was inhibited by phospholipase C antagonist lithium, endoplasmic reticulum calcium-dependent ATPase inhibitor thapsigargin, F-actin interruptor cytochalasin D and Golgi-complex interruptor brefeldin A. The uridine triphosphate-stimulated ATP release was also potently blocked by exocytosis inhibitor botulinum toxin A and anion transporter blockers furosemide and glibenclamide. These results suggest that calcium-dependent exocytosis and transportation via anion transporters are the predominant secretion mechanisms for uridine triphosphate-stimulated ATP release from cortical astrocytes.

Cellular expression of P2Y and beta-AR receptor mRNAs and proteins in freshly isolated astrocytes and tissue sections from the CA1 region of P8-12 rat hippocampus

Although almost all GFAP(+) cells in primary astrocyte cultures show functional beta-adrenergic (beta-AR) and metabotropic purinergic (P2Y) receptors, the fewer studies on astrocytes in situ have shown that a much smaller proportion express these same receptor-mediated activities. Here we show, by multiplex single cell RT-PCR, that 44% of freshly isolated, GFAP(+) astrocytes (FIAs) from the CA1 of P8-12 rat hippocampus always co-express beta-adrenergic receptor mRNA subtypes with metabotropic ATP receptor mRNA subtypes (P2Y1, P2Y2 or P2Y4). We also found that beta2 mRNA was the dominant beta-AR subtype expressed. P2Y1 mRNA always co-expresses with either one or two subtypes of P2U-like receptor (P2Y2 or P2Y4) mRNAs. Immunocytochemical studies showed a similar percentage of all FIAs expressed beta-AR and P2Y1 protein (54% and 52%, respectively), as for the mRNAs (46% and 65%, respectively). The staining of hippocampal sections for beta-AR or P2Y1 receptor plus GFAP shows that there are quite numerous, scattered star-shaped GFAP(+) astrocytes in the CA1 region of P9-10 rat hippocampus that stained positive for either of these receptors. These data show that astrocytes in situ express, and to a large extent likely co-express, beta-AR and P2Y receptors.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

The neurobiology of glia in the context of water and ion homeostasis

Astrocytes are highly complex cells that respond to a variety of external stimulations. One of the chief functions of astrocytes is to optimize the interstitial space for synaptic transmission by tight control of water and ionic homeostasis. Several lines of work have, over the past decade, expanded the role of astrocytes and it is now clear that astrocytes are active participants in the tri-partite synapse and modulate synaptic activity in hippocampus, cortex, and hypothalamus. Thus, the emerging concept of astrocytes includes both supportive functions as well as active modulation of neuronal output. Glutamate plays a central role in astrocytic-neuronal interactions. This excitatory amino acid is cleared from the neuronal synapses by astrocytes via glutamate transporters, and is converted into glutamine, which is released and in turn taken up by neurons. Furthermore, metabotropic glutamate receptor activation on astrocytes triggers via increases in cytosolic Ca(2+) a variety of responses. For example, calcium-dependent glutamate release from the astrocytes modulates the activity of both excitatory and inhibitory synapses. In vivo studies have identified the astrocytic end-foot processes enveloping the vessel walls as the center for astrocytic Ca(2+) signaling and it is possible that Ca(2+) signaling events in the cellular component of the blood-brain barrier are instrumental in modulation of local blood flow as well as substrate transport. The hormonal regulation of water and ionic homeostasis is achieved by the opposing effects of vasopressin and atrial natriuretic peptide on astroglial water and chloride uptake. In conjuncture, the brain appears to have a distinct astrocytic perivascular system, involving several potassium channels as well as aquaporin 4, a membrane water channel, which has been localized to astrocytic endfeet and mediate water fluxes within the brain. The multitask functions of astrocytes are essential for higher brain function. One of the major challenges for future studies is to link receptor-mediated signaling events in astrocytes to their roles in metabolism, ion, and water homeostasis.

New roles for astrocytes: redefining the functional architecture of the brain

Astrocytes have traditionally been considered ancillary, satellite cells of the nervous system. However, work over the past decade has revealed that they interact with the vasculature to form a gliovascular network that might organize not only the structural architecture of the brain but also its communication pathways, activation, thresholds and plasticity. The net effect is that astroglia demarcate gray matter regions, both cortical and subcortical, into functional compartments whose internal activation thresholds and external outputs are regulated by single glial cells. The array of these astrocyte-delimited microdomains along the capillary microvasculature allows the formation of higher-order gliovascular units, which serve to match local neural activity and blood flow while regulating neuronal firing thresholds through coordinative glial signaling. By these means, astrocytes might establish the functional as well as the structural architecture of the adult brain.