Specific diadenosine pentaphosphate receptor coupled to extracellular regulated kinases in cerebellar astrocytes

Increased Ap4A levels and ecto-nucleotidase activity in glaucomatous mice retina

The pathogenesis of glaucoma involves numerous intracellular mechanisms including the purinergic system contribution. Furthermore, the presence and release of nucleotides and dinucleotides during the glaucomatous damage and the maintenance of degradation machinery through ecto-nucleotidase activity are participating in the modulation of the suitable extracellular complex balance. The aim of this study was to investigate the levels of diadenosine tetraphosphate (Ap₄A) and the pattern of ecto-nucleotidase activity expression in glaucomatous retinas during the progress the pathology. Ap₄A levels were analyzed by HPLC in glaucomatous retinas from the DBA/2J mice at 3, 9, 15, and 23 months of age. For that, retinas were dissected as flattened whole-mounts and stimulated in Ringer buffer with or without 59 mM KCl. NPP1 expression was analyzed by RT-PCR and western blot and its distribution was assessed by immunohistochemistry studies examined under confocal microscopy. Glaucomatous mice exhibited Ap₄A values, which changed in stimulated retinas as long as the pathology progressed varying from 0.73 ± 0.04 (3 months) to 0.170 ± 0.05 pmol/mg retina (23 months). Concomitantly, NPP1 expression was significantly increased (82.15%) in the DBA/2J mice at 15 months. Furthermore, immunohistochemical studies showed that NPP1 labeling was stronger in OPL and IPL labeling tangentially in the vitreal part of the retina and was upregulated at 15 months of age. Our findings demonstrate that Ap₄A decreased levels may be related with exacerbated activity of NPP1 protein in glaucomatous degeneration and in this way contributing to elucidate different mechanisms involved in retinal impairment in glaucomatous degeneration.

P2X7 Nucleotide and EGF Receptors Exert Dual Modulation of the Dual-Specificity Phosphatase 6 (MKP-3) in Granule Neurons and Astrocytes, Contributing to Negative Feedback on ERK Signaling

Extracellular signal-regulated kinases 1 and 2 (ERK1/2) play a central role in the intracellular signaling of P2X7 nucleotide receptors in neurons and glial cells. Fine spatio-temporal tuning of mitogen-activated protein (MAP) kinases is essential to regulate their biological activity. MAP kinase phosphatases (MKPs) are dual specificity protein phosphatases (DUSPs) that dephosphorylate phosphothreonine and phosphotyrosine residues in MAP kinases. This study focuses on how DUSP, DUSP6/MKP3, a phosphatase specific for ERK1/2 is regulated by the P2X7 nucleotide receptor in cerebellar granule neurons and astrocytes. Stimulation with the specific P2X7 agonist, BzATP, or epidermal growth factor (EGF) (positive control for ERK activation) regulates the levels of DUSP6 in a time dependent manner. Both agonists promote a decline in DUSP6 protein, reaching minimal levels after 30 min yet recovering to basal levels after 1 h. The initial loss of protein occurs through proteasomal degradation, as confirmed in experiments with the proteasome inhibitor, MG-132. Studies carried out with Actinomycin D demonstrated that the enhanced transcription of the Dusp6 gene is responsible for recovering the DUSP6 protein levels. Interestingly, ERK1/2 proteins are involved in the biphasic regulation of the protein phosphatase, being required for both the degradation and the recovery phase. We show that direct Ser¹⁹⁷ phosphorylation of DUSP6 by ERK1/2 proteins could be part of the mechanism regulating their cytosolic levels, at least in glial cells. Thus, the ERK1/2 activated by P2X7 receptors exerts positive feedback on these kinase’s own activity, promoting the degradation of one of their major inactivators in the cytosolic compartment, DUSP6, both in granule neurons and astrocytes. This feedback loop seems to function as a common universal mechanism to regulate ERK signaling in neural and non-neural cells.

Prostaglandin E2 Impairs P2Y2/P2Y4 Receptor Signaling in Cerebellar Astrocytes via EP3 Receptors

Prostaglandin E₂ (PGE₂) is an important bioactive lipid that accumulates after tissue damage or inflammation due to the rapid expression of cyclooxygenase 2. PGE₂ activates specific G-protein coupled EP receptors and it mediates pro- or anti-inflammatory actions depending on the cell-context. Nucleotides can also be released in these situations and they even contribute to PGE₂ production. We previously described the selective impairment of P2Y nucleotide signaling by PGE₂ in macrophages and fibroblasts, an effect independent of prostaglandin receptors but that involved protein kinase C (PKC) and protein kinase D (PKD) activation. Considering that macrophages and fibroblasts influence inflammatory responses and tissue remodeling, a similar mechanism involving P2Y signaling could occur in astrocytes in response to neuroinflammation and brain repair. We analyzed here the modulation of cellular responses involving P2Y₂/P2Y₄ receptors by PGE₂ in rat cerebellar astrocytes. We demonstrate that PGE₂ inhibits intracellular calcium responses elicited by UTP in individual cells and that inhibiting this P2Y signaling impairs the astrocyte migration elicited by this nucleotide. Activation of EP3 receptors by PGE₂ not only impairs the calcium responses but also, the extracellular regulated kinases (ERK) and Akt phosphorylation induced by UTP. However, PGE₂ requires epidermal growth factor receptor (EGFR) transactivation in order to dampen P2Y signaling. In addition, these effects of PGE₂ also occur in a pro-inflammatory context, as evident in astrocytes stimulated with bacterial lipopolysaccharide (LPS). While we continue to investigate the intracellular mechanisms responsible for the inhibition of UTP responses, the involvement of novel PKC and PKD in cerebellar astrocytes cannot be excluded, kinases that could promote the internalization of P2Y receptors in fibroblasts.

Ectonucleotide pyrophosphatase/phosphodiesterase activity in Neuro-2a neuroblastoma cells: changes in expression associated with neuronal differentiation

Neuro-2a (N2a) neuroblastoma cells display an ectoenzymatic hydrolytic activity capable of degrading diadenosine polyphosphates. The Apn A-cleaving activity has been analysed with the use of the fluorogenic compound BODIPY FL guanosine 5'-O-(3-thiotriphosphate) thioester. Hydrolysis of this dinucleotide analogue showed a hyperbolic kinetic with a Km value of 4.9 ± 1.3 μM. Diadenosine pentaphosphate, diadenosine tetraphosphate, diadenosine triphosphate, and the nucleoside monophosphate AMP behaved as an inhibitor of BODIPY FL guanosine 5'-O-(3-thiotriphosphate) thioester extracellular degradation. Ectoenzymatic activity shared the typical characteristics of the ectonucleotide pyrophosphatase/phosphodiesterase family, as hydrolysis reached maximal activity at alkaline pH and was dependent on the presence of divalent cations, being strongly inhibited by EDTA and activated by Zn(2+) ions. Both NPP1 and NPP3 isozymes are expressed in N2a cells, their expression levels substantially changing when cells differentiate into a neuronal-like phenotype. In this sense, it is relevant to point the expression pattern of the NPP3 protein, whose levels were drastically reduced in the differentiated cells, being almost completely absent after 24 h of differentiation. Enzymatic activity assays carried out with differentiated N2a cells showed that NPP1 is the main isozyme involved in the extracellular degradation of dinucleotides in these cells, this enzyme reducing its activity and changing its subcellular location following neuronal differentiation. We described the presence of an ectoenzymatic activity able to hydrolyse diadenosine polyphosphates (ApnA) in N2a cells. This activity displays biochemical features that are typical of the ectonucleotide pyrophosphatase/phosphodiesterase (E-NPP) family members, as demonstrated by the use of the fluorogenic compound BODIPY-FL-GTPγS. Both NPP1 and NPP3 ectoenzymes are expressed in N2a cells, their levels dramatically changing when cells differentiate into a neuronal-like phenotype. Activity assays in differentiated cells showed that the ApnA-hydrolytic activity largely depends on the NPP1 isozyme.

ERK1/2 activation is involved in the neuroprotective action of P2Y13 and P2X7 receptors against glutamate excitotoxicity in cerebellar granule neurons

Cerebellar granule neurons express several types of nucleotide receptors, with the metabotropic P2Y(13) and the ionotropic P2X7 being the most relevant in this model. In the present study we investigated the role of P2Y(13) and P2X7 nucleotide receptors in ERK1/2 signalling. The nucleotidic agonists 2MeSADP (2-methylthioadenosine-5'-diphosphate) for P2Y(13) and BzATP (2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate) for P2X7 receptors were coupled to ERK1/2 activation in granule neurons, being able to increase around two-fold the levels of ERK1/2 phosphorylation. These effects were sensitive to the inhibitory action of the antagonists MRS-2211 and A-438079, specific for P2Y(13) and P2X7 receptors, respectively. Although both receptor subtypes shared the same pattern of transient ERK1/2 phosphorylation, they differed in the intracellular cascades they triggered, being PI3K-dependent for P2Y(13) and calcium/calmodulin kinase II (CaMKII)-dependent for P2X7. These two different ERK-mediated pathways were involved in the neuroprotective effects displayed by both P2Y(13) and P2X7 receptors against apoptosis induced by an excitotoxic concentration of glutamate, in a similar manner to the neurotrophin, BDNF. In addition, P2Y(13) and P2X7 receptor agonists were also able to phosphorylate and activate the ERK-dependent target CREB, which could be involved in their neuroprotective effect. These results indicate that nucleotide receptors share with trophic factors the same survival routes in neurons, such as the ERK signalling route, and therefore, can contribute to the maintenance of granule neurons in conditions in which survival is being compromised.

Effect of diadenosine polyphosphates in achondroplasic chondrocytes: inhibitory effect of Ap4A on FGF9 induced MAPK cascade

Achondroplasia is characterised by a mutation in the gene that encodes for the FGF receptor type 3 (FGFR3), producing a hyperactivation of this receptor and a subsequent increase in MAPK activity. We have tested the ability of nucleotides to decrease the activation of MAPK in chondrocytes with achondroplasic FGFR3 receptor. Diadenosine tetraphosphate, Ap(4)A, reduced the phosphorylation of pERK1/2 triggered by FGF9 (38% reduction). Ap(4)A diminished the expression of achondroplasic FGFR3 receptor (65% reduction), stimulating FGFR3 receptor degradation. The action of Ap(4)A seems to be mediated by a dinucleotide receptor rather than by any other ATP receptor.

P2Y1and P2X7receptors induce calcium/calmodulin-dependent protein kinase II phosphorylation in cerebellar granule neurons

The activation of nucleotide receptors-- both ionotropic, P2X, and most of metabotropic, P2Y-- increases intracellular calcium concentration, resulting in calcium/calmodulin-dependent protein kinase II (CaMKII) activation. Stimulation of cerebellar granule neurons in culture-- with different P2X and P2Y agonists and their effect on CaMKII phosphorylation-- was studied using immunocytochemical and microfluorimetrical techniques. P2X agonist: 2'-3'-o-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP), alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP) and diadenosine pentaphosphate (Ap(5)A); and P2Y agonists: 2-(methylthyo)-adenosine diphosphate (2MeSADP) and uridine 5'-bisphosphate (UDP); tested induced a CaMKII phosphorylation but with a different immunostaining pattern in each group. Stimulation with 2MeSADP induced a Ca(2+) release from intracellular stores and a significant CaMKII phosphorylation in cell somas and neurites. This agrees with the subcellular distribution of P2Y(1). MRS 2179, a specific P2Y(1) inhibitor, antagonized the 2MeSADP effect. On the other hand, cerebellar granule neuron stimulation with BzATP, in Mg(2+)-free conditions, produced extracellular calcium entrance and, as a result, a significant increase in CaMKII phosphorylation mostly in fibres, which correspond with P2X(7) subdistribution. Immunocytochemical and microfluorimetrical experiments, using Zn(2+) and Brilliant Blue G (BBG), as a specific P2X(7) antagonist, confirmed that BzATP was acting through the P2X(7) receptor. These results indicate that P2Y(1) and P2X(7) produce a significant increase in CaMKII phosphorylation, but show important differences in subcellular distribution and in effect duration. P2X(7) activation in granule neurons is not associated with pore formation, according to the absence of YO-PRO-1 fluorescence. The abundant presence of P2X(7) at the synaptic structures suggests a relevant role played by this receptor in synaptic plasticity.

Dinucleoside polyphosphates and their interaction with other nucleotide signaling pathways

Dinucleoside polyphosphates or Ap(n)A are a family of dinucleotides formed by two adenosines joined by a variable number of phosphates. Ap(4)A, Ap(5)A, and Ap(6)A are stored together with other neurotransmitters into secretory vesicles and are co-released to the extracellular medium upon stimulation. These compounds can interact extracellularly with some ATP receptors, both metabotropic (P2Y) and ionotropic (P2X). However, specific receptors for these substances, other than ATP receptors, have been described in presynaptic terminals form rat midbrain. These specific dinucleotide receptors are of ionotropic nature and their activation induces calcium entry into the terminals and the subsequent neurotransmitter release. Calcium signals that cannot be attributable to the interaction of Ap(n)A with ATP receptors have also been described in cerebellar synaptosomes and granule cell neurons in culture, where Ap(5)A induces CaMKII activation. In addition, cerebellar astrocytes express a specific Ap(5)A receptor coupled to ERK activation. Ap(5)A engaged to MAPK cascade by a mechanism that was insensitive to pertussis toxin and required the involvement of src and ras proteins. Diadenosine polyphosphates, acting on their specific receptors and/or ATP receptors, can also interact with other neurotransmitter systems. This broad range of actions and interactions open a promising perspective for some relevant physiological roles for the dinucleotides. However, the physiological significance of these compounds in the CNS is still to be determined.

Diadenosine and diuridine poly(borano)phosphate analogues: synthesis, chemical and enzymatic stability, and activity at P2Y1 and P2Y2 receptors

Dinucleoside polyphosphates, NpnN', exert their physiological effects via P2 receptors. They are attractive drug targets as they offer better stability and specificity compared to nucleotides, the most common P2-receptor ligands. To further improve the properties of NpnN', which are still pharmacologically unsatisfactory, we developed novel boranophosphate isosteres of dinucleoside polyphosphates, denoted as Npn(B)N. These analogues were obtained in a facile and efficient synthesis as the exclusive products in a concerted reaction of two nucleoside phosphorimidazolides and inorganic boranophosphate. This unusual reaction is due to the preorganization of three reactant molecules by the Mg2+ ion. We found that Ap3/5(beta/gamma-B)A analogues were potent P2Y1-R agonists. Ap5(gamma-B)A was equipotent to 2-MeS-ADP (EC50 6.3x10(-8) M), thus making it one of the most potent P2Y1-R agonists currently known. Moreover, Ap5(gamma-B)A did not activate P2Y2-R. In contrast, Up3/5(beta/gamma-B)U analogues were extremely poor agonists of both P2Y1-R and P2Y2-R. Npn(B)N analogues exhibited remarkable chemical stability under physiological conditions. Under conditions mimicking gastric juice, Np3(beta-B)N analogues exhibited a half-life (t1/2) of 1.3 h, whereas Np5(gamma-B)N degraded at a much faster rate (t1/2 18 min). The hydrolysis of Ap3(beta-B)A by human nucleotide pyrophosphatase phosphodiesterases (NPP1 and NPP3) was slowed by 40% and 59%, respectively, as compared to Ap3A. However, this effect of the boranophosphate was position-dependent, as Np5(gamma-B)N was degraded at a rate comparable to that of Np5N. In summary, Ap5(gamma-B)A appears to be a highly potent and selective P2Y1-R agonist, as compared to the parent compound. This promising scaffold will be applied in the design of future metabolically stable analogues.

Cross-talk among epidermal growth factor, Ap(5)A, and nucleotide receptors causing enhanced ATP Ca(2+) signaling involves extracellular kinase activation in cerebellar astrocytes

In previous papers, we reported that ATP calcium responses in cerebellar astrocytes were strongly potentiated by preincubation with nanomolar concentrations of the diadenosine pentaphosphate Ap(5)A. However, the intracellular signaling pathway mediating this effect was not defined. We also showed that stimulation of astrocytes with the dinucleotide led to the activation of extracellular regulated kinases (ERKs). Here, we examined whether ERKs are involved in the potentiating mechanism and intracellular mechanism leading to their activation. Epidermal growth factor (EGF) exactly reproduced the potentiation displayed by the dinucleotide. Moreover, the potentiation of ATP responses by Ap(5)A and EGF was completely abolished by the MAP kinase (MEK) inhibitor U-0126, indicating that ERK activation is a required step for the potentiation event. Our data also indicated that ERK activation and the potentiation of ATP calcium responses were sensitive to the src-like kinase inhibitor herbimycin A, p21(ras) farnesyltransferase inhibitor peptide, and some PKC inhibitors. Taken together, our findings reveal that Ap(5)A triggers the potentiation of ATP calcium responses through an intracellular mechanism that is insensitive to pertussis toxin and that this potentiation requires src protein-mediated ERK activation and the participation of an atypical protein kinase C isoform activated downstream from ERK.

Effects of diadenosine polyphosphates on glomerular volume

1. Diadenosine polyphosphates (P(1),P(3)-diadenosine triphosphate, Ap(3)A; P(1),P(4)-diadenosine tetraphosphate, Ap(4)A; and P(1),P(5)-diadenosine pentaphosphate, Ap(5)A) are vasoactive molecules. The experimental model of isolated rat renal glomeruli was used to investigate their effects on glomerular vasculature. We measured the changes of glomerular inulin space (GIS) as a marker of glomeruli contractility. 2. Ap(4)A and Ap(5)A induced concentration- and time-dependent reduction of GIS whereas Ap(3)A had no effect. The effects of Ap(4)A and Ap(5)A (both at 1 microM) were prevented by a nonselective P2 receptor antagonist, that is, suramin (10 microM) and P2Y receptor antagonist - reactive blue 2 (50 microM). However, the antagonist of P1 receptor, that is, theophylline (1 microM) and A(1) receptor 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 microM) did not affect the responses of glomeruli to Ap(4)A or Ap(5)A. 3. Ap(3)A, in contrast to Ap(4)A and Ap(5)A, prevented angiotensin II-induced reduction of GIS in a concentration- and time-dependent manner. This effect was partially prevented by suramin and markedly reduced by reactive blue 2 and the specific antagonist of P2Y(1) receptor - MRS 2179 (10 microM). However, theophylline and the specific antagonist of A(2) receptor - 3,7-dimethyl-1-propargylxanthine (DMPX; 10 microM) - did not affect Ap(3)A action. 4. We indicate that diadenosine polyphosphates changed the glomerular volume via activation of P2 receptors. We suggest that extracellular Ap(4)A and Ap(5)A via P2X and P2Y receptors may decrease and Ap(3)A via, at least in part, P2Y(1) receptors may increase filtration surface, which in turn may modify glomerular filtration rate.

Spatial, temporal, and cellular distribution of the activated extracellular signal regulated kinases 1 and 2 in the developing and mature rat cerebellum

The extracellular signal regulated kinases 1 and 2 (ERK1/2) are important members of an intracellular signaling cascade that is involved in many aspects of the cellular physiology and development of neurons and glia. ERK1/2 are expressed in many brain regions including the cerebellum; however, their role during cerebellar development is poorly understood. Immunohistochemical approaches using phosphorylation-state specific antiserum that recognizes only the activated-ERK1/2 (pERK) were used to characterize the spatial and temporal patterns of activated-ERK in the developing and adult rat cerebellum. The distribution and cell type-specificity of pERK-immunoreactivity (IR) followed an age-related pattern, with the density of pERK-IR Purkinje cells decreasing between P6 and P15 and increasing at later times. Immunopositive granule cell neurons increased from P6 to P12, became decreased during much of late postnatal cerebellar development, and absent in adults. Co-localization of pERK with glial fibrillary acidic protein or the neuronal marker beta-tubulin revealed that activated ERK is present in maturing Purkinje and granule cells, and the soma of Bergmann glia on P4, P10 and P15; pERK was detected in astrocytes on P10 and P15. Associated with weaning, there was a general increase in activated-ERK in all cell types on P22. In adults, pERK-IR was confined to the Purkinje cell layer and scattered cells in the corpus medullare. In summary, a high degree of developmental plasticity was observed in the spatiotemporal distribution of cerebellar pERK-IR suggesting that the ERK-pathway plays a dynamic role in regulating neuronal and glial migration, proliferation and differentiation in the developing cerebellum. In the mature cerebellum, ERK signaling may also mediate postsynaptic information processing.

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.

Coexpression of functional P2X and P2Y nucleotide receptors in single cerebellar granule cells

The present study describes the presence and expression of functional nucleotide receptors, both ionotropic and metabotropic, in highly purified cultures of cerebellar granule neurons. Microfluorimetric experiments have been carried out to record specific [Ca(2+)](i) transients in individual granule neurons after challenge with diverse nucleotides. Although great heterogeneity was found in nucleotide responses in single cells, these responses all became modified during the course of granule cell differentiation, not only at the level of the number of responding cells, but also in the magnitude of the response to nucleotides. These in vitro developmental changes were more significant in metabotropic responses to pyrimidine nucleotides, UTP and UDP, which were down- and upregulated, respectively, during the time in culture. At least two types of ADP-specific receptors seem expressed in different granule cell subpopulations responding to 2MeSADP, as the specific P2Y(1) antagonist MRS-2179 inhibited Ca(2+) responses in only one of these populations. The great diversity of metabotropic responses observed was confirmed by the RT-PCR expression of different types of P2Y receptors in granule cell cultures: P2Y(1), P2Y(4), P2Y(6), and P2Y(12). Similarly, ionotropic nucleotide responses were confirmed by the presence of specific messengers for different P2X subunits, and by immunolabeling studies (P2X(1), P2X(2), P2X(3), P2X(4) and P2X(7)). Immunolabeling reflected great variety in the P2X subunit distribution along the granule neuron cytoarchitecture, with P2X(2), P2X(3) and P2X(4) present at somatodendritic locations, and P2X(1), P2X(7), and P2X(3), located at the axodendritic prolongations. The punctuated labeling pattern obtained for P2X(3) and P2X(7) subunits is particularly notable, as it presents a high degree of colocalization with synaptophysin, a specific marker of synaptic vesicles, suggesting specialized localization and function in granule neurons.