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

Purinergic Signalling Mediates Aberrant Excitability of Developing Neuronal Circuits in the Fmr1 Knockout Mouse Model

Neuronal hyperexcitability within developing cortical circuits is a common characteristic of several heritable neurodevelopmental disorders, including Fragile X Syndrome (FXS), intellectual disability and autism spectrum disorders (ASD). While this aberrant circuitry is typically studied from a neuron-centric perspective, glial cells secrete soluble factors that regulate both neurite extension and synaptogenesis during development. The nucleotide-mediated purinergic signalling system is particularly instrumental in facilitating these effects. We recently reported that within a FXS animal model, the Fmr1 KO mouse, the purinergic signalling system is upregulated in cortical astrocytes leading to altered secretion of synaptogenic and plasticity-related proteins. In this study, we examined whether elevated astrocyte purinergic signalling also impacts neuronal morphology and connectivity of Fmr1 KO cortical neurons. Here, we found that conditioned media from primary Fmr1 KO astrocytes was sufficient to enhance neurite extension and complexity of both wildtype and Fmr1 KO neurons to a similar degree as UTP-mediated outgrowth. Significantly enhanced firing was also observed in Fmr1 KO neuron-astrocyte co-cultures grown on microelectrode arrays but was associated with large deficits in firing synchrony. The selective P2Y2 purinergic receptor antagonist AR-C 118925XX effectively normalized much of the aberrant Fmr1 KO activity, designating P2Y2 as a potential therapeutic target in FXS. These results not only demonstrate the importance of astrocyte soluble factors in the development of neural circuitry, but also show that P2Y purinergic receptors play a distinct role in pathological FXS neuronal activity.

Independent association of Lp(a) with platelet reactivity in subjects without statins or antiplatelet agents

The physiological effect of Lp(a) on platelet activity is unclear. Previous studies explored the relationship between Lp(a) and platelet aggregation in patients taking statins and antiplatelet agents, but few was conducted in individuals without the bias of those drugs that either influence Lp(a) or platelet activity. The aim of this study was to assess the relationship between Lp(a) levels and platelet aggregation in subjects not taking statins or antiplatelet drugs. A hospital-based cross-sectional study was conducted to investigate the independent contribution of Lp(a) to platelet activity by controlling the effects of potential confounding factors including lipoprotein-associated phospholipase A2 [Lp-PLA2]. Blood samples were collected from 92 subjects without statins or antiplatelet agents from the Second Xiangya Hospital. The univariate correlation analysis showed a significant correlation between AA-induced average aggregation rate [AAR] and ApoB (r = 0.324, P = 0.002), ApoA1 (r = 0.252, P = 0.015), Lp(a) (r = 0.370, P < 0.001), Lp-PLA2 (r = 0.233, P = 0.025) and platelet counts [PLT] (r = 0.389, P < 0.001). Multivariate regression analysis suggested that Lp(a) contributed independently to AA-induced average aggregation rate (β = 0.023, P = 0.027) after controlling for the effects of ApoB, Lp-PLA2 and platelet counts. Lp(a) is positively associated with platelet aggregation independent of Lp-PLA2, which may partly account for the atherothrombotic effect of Lp(a).

Inorganic Polyphosphate—Regulator of Cellular Metabolism in Homeostasis and Disease

Inorganic polyphosphate (polyP), a simple anionic polymer consisting of even hundreds of orthophosphate units, is a universal molecule present in both simple and complex organisms. PolyP controls homeostatic processes in animals, such as blood coagulation, tissue regeneration, and energy metabolism. Furthermore, this polymer is a potent regulator of inflammation and influences host immune response in bacterial and viral infections. Disturbed polyP systems have been related to several pathological conditions, including neurodegeneration, cardiovascular disorders, and cancer, but we lack a full understanding of polyP biogenesis and mechanistic insights into the pathways through which polyP may act. This review summarizes recent studies that describe the role of polyP in cell homeostasis and show how disturbances in polyP levels may lead to disease. Based on the collected findings, we highlight the possible usage of this polymer as a promising therapeutic tool in multiple pathologies.

Factors Associated with Platelet Activation-Recent Pharmaceutical Approaches

Platelets are at the forefront of human health and disease following the advances in their research presented in past decades. Platelet activation, their most crucial function, although beneficial in the case of vascular injury, may represent the initial step for thrombotic complications characterizing various pathologic states, primarily atherosclerotic cardiovascular diseases. In this review, we initially summarize the structural and functional characteristics of platelets. Next, we focus on the process of platelet activation and its associated factors, indicating the potential molecular mechanisms involving inflammation, endothelial dysfunction, and miRs. Finally, an overview of the available antiplatelet agents is being portrayed, together with agents possessing off-set platelet-inhibitory actions, while an extensive presentation of drugs under investigation is being given.

Astrocyte-mediated purinergic signaling is upregulated in a mouse model of Fragile X syndrome

Fragile X syndrome (FXS) is the leading monogenic cause of intellectual disability and autism spectrum disorders. With increasing investigation into the molecular mechanisms underlying FXS, there is growing evidence that perturbations in glial signaling are widely associated with neurological pathology. Purinergic signaling, which utilizes nucleoside triphosphates as signaling molecules, provides one of the most ubiquitous signaling systems for glial-neuronal and glial-glial crosstalk. Here, we sought to identify whether purinergic signaling is dysregulated within the FXS mouse cortex, and whether this dysregulation contributes to aberrant intercellular communication. In primary astrocyte cultures derived from the Fmr1 knockout (KO) mouse model of FXS, we found that application of exogenous ATP and UTP evoked elevated intracellular calcium responses compared to wildtype levels. Accordingly, purinergic P2Y₂ and P2Y₆ receptor expression was increased in Fmr1 KO astrocytes both in vitro and in acutely dissociated tissue, while P2Y antagonism via suramin prevented intracellular calcium elevations, suggesting a role for these receptors in aberrant FXS astrocyte activation. To investigate the impact of elevated purinergic signaling on astrocyte-mediated synaptogenesis, we quantified synaptogenic protein TSP-1, known to be regulated by P2Y activation. TSP-1 secretion and expression were both heightened in Fmr1 KO vs wildtype astrocytes following UTP application, while naïve TSP-1 cortical expression was also transiently elevated in vivo, indicating increased potential for excitatory TSP-1-mediated synaptogenesis in the FXS cortex. Together, our results demonstrate novel and significant purinergic signaling elevations in Fmr1 KO astrocytes, which may serve as a potential therapeutic target to mitigate the signaling aberrations observed in FXS.

Role of Hippo-YAP1/TAZ pathway and its crosstalk in cardiac biology

The Hippo pathway undertakes a pivotal role in organ size control and the process of physiology and pathology in tissue. Its downstream effectors YAP1 and TAZ receive upstream stimuli and exert transcription activity to produce biological output. Studies have demonstrated that the Hippo pathway contributes to maintenance of cardiac homeostasis and occurrence of cardiac disease. And these cardiac biological events are affected by crosstalk among Hippo-YAP1/TAZ, Wnt/β-catenin, Bone morphogenetic protein (BMP) and G-protein-coupled receptor (GPCR) signaling, which provides new insights into the Hippo pathway in heart. This review delineates the interaction among Hippo, Wnt, BMP and GPCR pathways and discusses the effects of these pathways in cardiac biology.

Exogenous NAD+ Stimulates MUC2 Expression in LS 174T Goblet Cells via the PLC-Delta/PTGES/PKC-Delta/ERK/CREB Signaling Pathway

BACKGROUND

MUC2, a major component of the mucus layer in the intestine, is associated with antimicrobial activity and gut immune system function. Currently, mucin is mainly known for its critical function in defense against toxic molecules and pathogens. In this study, we investigated the stimulatory effects of exogenous nicotinamide adenine dinucleotide (NAD+) on the expression of MUC2 in LS 174T goblet cells.

METHODS

Genes related to MUC2 synthesis were measured by quantitative real-time PCR (qPCR). To analyze the gene expression profiles of NAD+-treated LS 174T goblet cells, RNA sequencing was performed. MUC2 expression in the cells and secreted MUC2 were measured by immunocytochemistry (ICC) and ELISA, respectively.

RESULTS

NAD+ significantly stimulated MUC2 expression at mRNA and protein levels and increased the secretion of MUC2. Through RNA sequencing, we found that the expression of genes involved in arachidonic acid metabolism increased in NAD+-treated cells compared with the negative control cells. NAD+ treatment increased phospholipase C (PLC)-δ and prostaglandin E synthase (PTGES) expression, which was inhibited by the appropriate inhibitors. Among the protein kinase C (PKC) isozymes, PKC-δ was involved in the increase in MUC2 expression. In addition, extracellular signal-regulated kinase (ERK)1/2 and cyclic AMP (cAMP) response element-binding protein (CREB) transcript levels were higher in NAD+-treated cells than in the negative control cells, and the enhanced levels of phosphorylated CREB augmented MUC2 expression.

CONCLUSIONS

Exogenous NAD+ increases MUC2 expression by stimulating the PLC-δ/PTGES/PKC-δ/ERK/CREB signaling pathway.

Calcium Signaling in Glioma Cells: The Role of Nucleotide Receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y₁ and P2Y₁₂ nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

Cross-Talk in Nucleotide Signaling in Glioma C6 Cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y₁, P2Y₂, P2Y₁₂, P2Y₁₄ and the ionotropic P2X₇ receptor in glioma C6 cells. P2Y₁ and P2Y₁₂ both respond to ADP, but while P2Y₁ links to PLC and elevates cytosolic Ca²⁺ concentration, P2Y₁₂ negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y₁ receptor strongly decreases and P2Y₁₂ becomes a major player responsible for ADP-evoked signal transduction. The P2Y₁₂ receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y₁ has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X₇ receptor, often responsible for apoptotic fate, is not involved in Ca²⁺elevation in C6 cells. The shift in nucleotide receptor expression from P2Y₁ to P2Y₁₂ during serum withdrawal, the cross talk between both receptors and the lack of P2X₇ activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

Identification of a miRNAs signature associated with exposure to stress early in life and enhanced vulnerability for schizophrenia: New insights for the key role of miR-125b-1-3p in neurodevelopmental processes

Epidemiological and clinical studies have provided evidence for a role of both genetic and environmental factors, such as stressful experiences early in life, in the pathogenesis of Schizophrenia (SZ) and microRNAs (miRNAs) have been suggested to play a key role in the interplay between the environment and our genome. In this study, we conducted a miRNOme analysis in different samples (blood of adult subjects exposed to childhood trauma, brain (hippocampus) of rats exposed to prenatal stress and human hippocampal progenitor cells treated with cortisol) and we identified miR-125b-1-3p as a down-regulated miRNA in all the three datasets. Interestingly, a significant down-regulation was observed also in SZ patients exposed to childhood trauma. To investigate the biological systems targeted by miR-125b-1-3p and also involved in the effects of stress, we combined the list of biological pathways modulated by predicted and validated target genes of miR-125b-1-3p, with the biological systems significantly regulated by cortisol in the in vitro model. We found, as common pathways, the CXCR4 signaling, the G-alpha signaling, and the P2Y Purigenic Receptor Signaling Pathway, which are all involved in neurodevelopmental processes. Our data, obtained from the combining of miRNAs datasets across different tissues and species, identified miR-125b-1-3p as a key marker associated with the long-term effects of stress early in life and also with the enhanced vulnerability of developing SZ. The identification of such a miRNA biomarker could allow the early detection of vulnerable subjects for SZ and could provide the basis for the development of preventive therapeutic strategies.

Role of P2 receptors in normal brain development and in neurodevelopmental psychiatric disorders

The purinergic signaling system, including P2 receptors, plays an important role in various central nervous system (CNS) disorders. Over the last few decades, a substantial amount of accumulated data suggest that most P2 receptor subtypes (P2X1, 2, 3, 4, 6, and 7, and P2Y₁, ₂, ₆, ₁₂, and ₁₃) regulate neuronal/neuroglial developmental processes, such as proliferation, differentiation, migration of neuronal precursors, and neurite outgrowth. However, only a few of these subtypes (P2X2, P2X3, P2X4, P2X7, P2Y₁, and P2Y₂) have been investigated in the context of neurodevelopmental psychiatric disorders. The activation of these potential target receptors and their underlying mechanisms mainly influence the process of neuroinflammation. In particular, P2 receptor-mediated inflammatory cytokine release has been indicated to contribute to the complex mechanisms of a variety of CNS disorders. The released inflammatory cytokines could be utilized as biomarkers for neurodevelopmental and psychiatric disorders to improve the early diagnosis intervention, and prognosis. The population changes in gut microbiota after birth are closely linked to neurodevelopmental/neuropsychiatric disorders in later life; thus, the dynamic expression and function of P2 receptors on gut epithelial cells during disease processes indicate a novel avenue for the evaluation of disease progression and for the discovery of related therapeutic compounds.

Sodium channel TRPM4 and sodium/calcium exchangers (NCX) cooperate in the control of Ca2+-induced mucin secretion from goblet cells

Regulated mucin secretion is essential for the formation of the mucus layer that protects the underlying epithelial cells from foreign particles. Alterations in the quantity or quality of secreted mucins are therefore detrimental to airway and colon physiology. Based on various biochemical assays in several human cell lines, we report here that Na⁺/Ca²⁺ exchanger 2 (NCX2) works in conjunction with transient receptor potential cation channel subfamily M member 4 (TRPM4), and perhaps TRPM5, Na⁺ channels to control Ca²⁺-mediated secretion of both mucin 2 (MUC2) and MUC5AC from HT29-18N2 colonic cancer cells. Differentiated normal bronchial epithelial (NHBE) cells and tracheal cells from patients with cystic fibrosis (CFT1-LC3) expressed only TRPM4 and all three isoforms of NCXs. Blocking the activity of TRPM4 or NCX proteins abrogated MUC5AC secretion from NHBE and CFT1-LC3 cells. Altogether, our findings reveal that NCX and TRPM4/TRPM5 are both required for mucin secretion. We therefore propose that these two proteins could be potential pharmacological targets to control mucus-related pathologies such as cystic fibrosis.

POM-1 inhibits P2 receptors and exhibits anti-inflammatory effects in macrophages

Extracellular nucleotides can modulate the immunological response by activating purinergic receptors (P2Rs) on the cell surface of macrophages, dendritic, and other immune cells. In particular, the activation of P2X7R can induce release of cytokines and cell death as well as the uptake of large molecules through the cell membrane by a mechanism still poorly understood. Polyoxotungstate-1 (POM-1) has been proposed as a potent inhibitor of ecto-nucleotidases, enzymes that hydrolyze extracellular nucleotides, regulating the activity of P2Rs. However, the potential impact of POM-1 on P2Rs has not been evaluated. Here, we used fluorescent dye uptake, cytoplasmic free Ca2+ concentration measurement, patch-clamp recordings, scanning electron microscopy, and quantification of inflammatory mediators to investigate the effects of POM-1 on P2Rs of murine macrophages. We observed that POM-1 blocks the P2YR-dependent cytoplasmic Ca2+ increase and has partial effects on the cytoplasmic Ca2+, increasing dependence on P2XRs. POM-1 can inhibit the events related with ATP-dependent inflammasome activation, anionic dye uptake, and also the opening of large conductance channels, which are associated with P2X7R-dependent pannexin-1 activation. On the other hand, this compound has no effects on cationic fluorescent dye uptake, apoptosis, and bleb formation, also dependent on P2X7R. Moreover, POM-1 can be considered an anti-inflammatory compound, because it prevents TNF-α and nitric oxide release from LPS-treated macrophages.

P2Y1 Receptors - Properties and Functional Activities

In this chapter we try to show a comprehensive image of current knowledge of structure, activity and physiological role of the P2Y₁ purinergic receptor. The structure, distribution and changes in the expression of this receptor are summarized, as well as the mechanism of its signaling activity by the intracellular calcium mobilization. We try to show the connection between the components of its G protein activation and cellular or physiological effects, starting from changes in protein phosphorylation patterns and ending with such remote effects as receptor-mediated apoptosis. The special emphasis is put on the role of the P2Y₁ receptor in cancer cells and neuronal plasticity. We concentrate on the P2Y₁ receptor, it is though impossible to completely abstract from other aspects of nucleotide signaling and cross-talk with other nucleotide receptors is here discussed. Especially, the balance between P2Y₁ and P2Y₁₂ receptors, sharing the same ligand but signaling through different pathways, is presented.

Potent Suppressive Effects of 1-Piperidinylimidazole Based Novel P2X7 Receptor Antagonists on Cancer Cell Migration and Invasion

The P2X7 receptor (P2X7R) has been reported as a key mediator in inflammatory processes and cancer invasion/metastasis. In this study, we report the discovery of novel P2X7R antagonists and their functional activities as potential antimetastatic agents. Modifications of the hydantoin core-skeleton and the side chain substituents of the P2X7R antagonist 7 were performed. The structure-activity relationships (SAR) and optimization demonstrated the importance of the sulfonyl group at the R1 position and the substituted position and overall size of R2 for P2X7R antagonism. The optimized novel analogues displayed potent P2X7 receptor antagonism (IC50 = 0.11-112 nM) along with significant suppressive effects on IL-1β release (IC50 = 0.32-210 nM). Moreover, representative antagonists (12g, 13k, and 17d) with imidazole and uracil core skeletons significantly inhibited the invasion of MDA-MB-231 triple negative breast cancer cells and cancer cell migration in a zebrafish xenograft model, suggesting the potential therapeutic application of these novel P2X7 antagonists to block metastatic cancer.

Regulation of allergic inflammation by the ectoenzyme E-NPP3 (CD203c) on basophils and mast cells

Adenosine 5'-triphosphate (ATP) is released from dying or damaged cells, as well as from activated cells. Once secreted, extracellular ATP induces several immune responses via P2X and P2Y receptors. Basophils and mast cells release ATP upon FcεRI-crosslinking, and ATP activates basophils and mast cells in an autocrine manner. Nucleotide-converting ectoenzymes, such as E-NTPD1, E-NTPD7, and E-NPP3, inhibit ATP-dependent immune responses by hydrolyzing ATP, thereby contributing to immune response regulation. E-NPP3 is a well-known activation marker for human basophils. E-NPP3's physiologic function has recently been disclosed in mice. E-NPP3 is rapidly induced on basophils and mast cells after FcεRI-crosslinking and hydrolyzes extracellular ATP on cell surfaces to prevent ATP-dependent excess activation of basophils and mast cells. In the absence of E-NPP3, basophils and mast cells are overactivated and mice suffer from severe chronic allergic inflammation. Thus, the ATP-hydrolyzing ectoenzymes E-NPP3 has a nonnegligible role in the regulation of basophil- and mast cell-mediated allergic responses.

Expression of purinergic receptor P2Y4 in Schwann cell following nerve regeneration

OBJECTIVE

Emerging evidences suggested an important role of purinergic receptor P2Y4 in nerve system. The present study aims to investigate the localization and possible function of P2Y4 receptor in recurrent laryngeal nerve (RLN) following regeneration.

METHODS

Right RLN of fifty Sprague-Dawley rats was cut and immediately repaired with PLGA/chitosan nerve conduit. Immunofluorescence, real-time qPCR and Western blot were used to detect the expression alterations of P2Y4 receptor.

RESULTS

Weak immunostaining signals of P2Y4 receptor were located on the plasmalemma of Schwann cell (SC) with regular arrangement of axons in normal RLN. On the post-injury 4th day, the sprouting axons regrowed along the degenerated SCs intensively expressing P2Y4 receptor. On the post-injury 7th day, the regenerating axons existed in multicellular cords of P2Y4 receptor-positive SCs occupying the nerve gap. On the post-injury 14th day, the axons grew along the bands of P2Y4 receptor-positive SCs exhibiting the regularly parallel distribution. On the post-injury 30th day, mild immunostaining signals of P2Y4 receptor still existed on SC surface, and the regenerated axons were located inside the remodeled endoneurium tube. In accordance with immunofluorescence findings, the transcription and protein expression levels of P2Y4 were significantly increased after the injury and the peak value appeared on the post-injury 7th day, compared to control group (P < 0.05).

CONCLUSION

Data from the present study suggested a potential role of P2Y4 receptors in functional modulation of SCs in the regeneration of RLN.

P2Y(1) receptor mediated neuronal fibre outgrowth in organotypic brain slice co-cultures

Extracellular purines have multiple functional roles in development, plastic remodelling, and regeneration of the CNS by stimulating certain P2X/Y receptor (R) subtypes. In the present study we elucidated the involvement of P2YRs in neuronal fibre outgrowth in the developing nervous system. We particularly focused on the P2Y1R subtype and the dopaminergic system, respectively. For this purpose, we used organotypic slice co-cultures consisting of the ventral tegmental area/substantia nigra (VTA/SN) and the prefrontal cortex (PFC). After detecting the presence of the P2Y1R in VTA/SN, PFC, and on outgrowing fibres in the border region (e.g. on glial processes) connecting both brain slices, we could show that pharmacological modulation of the receptor influenced neuronal fibre outgrowth. Biocytin-tracing and tyrosine hydroxylase-immunolabelling together with quantitative image analysis revealed a significant increase in fibre growth in the border region of the co-cultures after treatment with ADPβS (P2Y1,12,13R agonist). The observed stimulatory potential of ADPβS was inhibited by pre-treatment with the P2X/YR antagonist PPADS. In P2Y1R knockout (P2Y1R(-/-)) mice, the ADPβS-induced stimulatory effect was absent, while growth was significantly enhanced in the co-cultures of the respective wild-type. This observation was confirmed in entorhino-hippocampal co-cultures, an example of a different projection system, expressing the P2Y1R. Using wortmannin and PD98059 we further showed that PI3K/Akt and MAPK/ERK cascades are involved in the mechanism underlying ADPβS-induced fibre growth. In conclusion, the data of this study clearly indicate that activation of the P2Y1R stimulates fibre growth and thereby emphasises the general role of this particular receptor subtype during development and regeneration.

Inosine strongly enhances proliferation of human C32 melanoma cells through PLC-PKC-MEK1/2-ERK1/2 and PI3K pathways

Malignant melanoma is the most deadly type of skin cancer. The lack of effective pharmacological approaches for this tumour can be related to the incomplete understanding of the pathophysiological mechanisms involved in melanoma cell proliferation. Adenosine has growth-promoting and growth inhibitory effects on tumour cells. We aimed to investigate effects of adenosine and its metabolic product, inosine, on human C32 melanoma cells and the signalling pathways involved. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) reduction and bromodeoxyuridine (BrdU) proliferation assays were used to evaluate adenosine, adenosine deaminase and inosine effects, in the absence or presence of adenosine receptor (AR), A3 AR and P2Y1 R antagonists and PLC, PKC, MEK1/2 and PI3K inhibitors. ERK1/2 levels were determined using an ELISA kit. Adenosine and inosine levels were quantified using an enzyme-coupled assay. Adenosine caused cell proliferation through AR activation. Adenosine deaminase increased inosine levels (nanomolar concentrations) on the extracellular space, in a time-dependent manner, inducing proliferation through A3 AR activation. Micromolar concentrations of inosine enhanced proliferation through A3 AR activation, causing an increase in ERK1/2 levels, and P2Y1 R activation via ENT-dependent mechanisms. We propose the simultaneous activation of PLC-PKC-MEK1/2-ERK1/2 and PI3K pathways as the main mechanism responsible for the proliferative effect elicited by inosine and its significant role in melanoma cancer progression.

Differential targeting of cPKC and nPKC decodes and regulates Ca2+ and lipid signalling

Protein kinases C (PKCs) are ubiquitously expressed and play critical roles in a plethora of physiological and pathophysiological processes. Owing to PKCs’ highly conserved phosphorylation consensus sequence, it has been difficult to distinguish the role of individual PKC isoforms. Recently, the identification of novel membrane targeting via subcellularly targeted diacylglycerol production found for novel PKCs (nPKCs), together with a characterization of their putative functions, has shed new light on the specific roles of individual PKCs in cellular processes.

Contribution of TRPC3 to store-operated calcium entry and inflammatory transductions in primary nociceptors

BACKGROUND

Prolonged intracellular calcium elevation contributes to sensitization of nociceptors and chronic pain in inflammatory conditions. The underlying molecular mechanisms remain unknown but store-operated calcium entry (SOCE) components participate in calcium homeostasis, potentially playing a significant role in chronic pain pathologies. Most G protein-coupled receptors activated by inflammatory mediators trigger calcium-dependent signaling pathways and stimulate SOCE in primary afferents. The aim of the present study was to investigate the role of TRPC3, a calcium-permeable non-selective cation channel coupled to phospholipase C and highly expressed in DRG, as a link between activation of pro-inflammatory metabotropic receptors and SOCE in nociceptive pathways.

RESULTS

Using in situ hybridization, we determined that TRPC3 and TRPC1 constitute the major TRPC subunits expressed in adult rat DRG. TRPC3 was found localized exclusively in small and medium diameter sensory neurons. Heterologous overexpression of TRPC3 channel subunits in cultured primary DRG neurons evoked a significant increase of Gd3+-sensitive SOCE following thapsigargin-induced calcium store depletion. Conversely, using the same calcium add-back protocol, knockdown of endogenous TRPC3 with shRNA-mediated interference or pharmacological inhibition with the selective TRPC3 antagonist Pyr10 induced a substantial decrease of SOCE, indicating a significant role of TRPC3 in SOCE in DRG nociceptors. Activation of P2Y2 purinoceptors or PAR2 protease receptors triggered a strong increase in intracellular calcium in conditions of TRPC3 overexpression. Additionally, knockdown of native TRPC3 or its selective pharmacological blockade suppressed UTP- or PAR2 agonist-evoked calcium responses as well as sensitization of DRG neurons. These data show a robust link between activation of pro-inflammatory receptors and calcium homeostasis through TRPC3-containing channels operating both in receptor- and store-operated mode.

CONCLUSIONS

Our findings highlight a major contribution of TRPC3 to neuronal calcium homeostasis in somatosensory pathways based on the unique ability of these cation channels to engage in both SOCE and receptor-operated calcium influx. This is the first evidence for TRPC3 as a SOCE component in DRG neurons. The flexible role of TRPC3 in calcium signaling as well as its functional coupling to pro-inflammatory metabotropic receptors involved in peripheral sensitization makes it a potential target for therapeutic strategies in chronic pain conditions.

Uridine 5'-triphosphate promotes in vitro Schwannoma cell migration through matrix metalloproteinase-2 activation

In response to peripheral nerve injury, Schwann cells adopt a migratory phenotype and modify the extracellular matrix to make it permissive for cell migration and axonal re-growth. Uridine 5′-triphosphate (UTP) and other nucleotides are released during nerve injury and activate purinergic receptors expressed on the Schwann cell surface, but little is known about the involvement of purine signalling in wound healing. We studied the effect of UTP on Schwannoma cell migration and wound closure and the intracellular signaling pathways involved. We found that UTP treatment induced Schwannoma cell migration through activation of P2Y₂ receptors and through the increase of extracellular matrix metalloproteinase-2 (MMP-2) activation and expression. Knockdown P2Y₂ receptor or MMP-2 expression greatly reduced wound closure and MMP-2 activation induced by UTP. MMP-2 activation evoked by injury or UTP was also mediated by phosphorylation of all 3 major mitogen-activated protein kinases (MAPKs): JNK, ERK1/2, and p38. Inhibition of these MAPK pathways decreased both MMP-2 activation and cell migration. Interestingly, MAPK phosphorylation evoked by UTP exhibited a biphasic pattern, with an early transient phosphorylation 5 min after treatment, and a late and sustained phosphorylation that appeared at 6 h and lasted up to 24 h. Inhibition of MMP-2 activity selectively blocked the late, but not the transient, phase of MAPK activation. These results suggest that MMP-2 activation and late MAPK phosphorylation are part of a positive feedback mechanism to maintain the migratory phenotype for wound healing. In conclusion, our findings show that treatment with UTP stimulates in vitro Schwannoma cell migration and wound repair through a MMP-2-dependent mechanism via P2Y₂ receptors and MAPK pathway activation.

Targeted activation of conventional and novel protein kinases C through differential translocation patterns

Activation of the two ubiquitous families of protein kinases, protein kinase A (PKA) and protein kinase C (PKC), is thought to be independently coupled to stimulation of Gαs and Gαq, respectively. Live-cell confocal imaging of protein kinase C fluorescent protein fusion constructs revealed that simultaneous activation of Gαs and Gαq resulted in a differential translocation of the conventional PKCα to the plasma membrane while the novel PKCδ was recruited to the membrane of the endoplasmic reticulum (ER). We demonstrate that the PKCδ translocation was driven by a novel Gαs-cyclic AMP-EPAC-RAP-PLCε pathway resulting in specific diacylglycerol production at the membrane of the ER. Membrane-specific phosphorylation sensors revealed that directed translocation resulted in phosphorylation activity confined to the target membrane. Specific stimulation of PKCδ caused phosphorylation of the inositol-1,4,5-trisphosphate receptor and dampening of global Ca(2+) signaling revealed by graded flash photolysis of caged inositol-1,4,5-trisphosphate. Our data demonstrate a novel signaling pathway enabling differential decoding of incoming stimuli into PKC isoform-specific membrane targeting, significantly enhancing the versatility of cyclic AMP signaling, thus demonstrating the possible interconnection between the PKA and PKC pathways traditionally treated independently. We thus provide novel and elementary understanding and insights into intracellular signaling events.

Acid evoked thermal hyperalgesia involves peripheral P2Y1 receptor mediated TRPV1 phosphorylation in a rodent model of thrombus induced ischemic pain

Background

We previously developed a thrombus-induced ischemic pain (TIIP) animal model, which was characterized by chronic bilateral mechanical allodynia without thermal hyperalgesia (TH). On the other hand we had shown that intraplantar injection of acidic saline facilitated ATP-induced pain, which did result in the induction of TH in normal rats. Because acidic pH and increased ATP are closely associated with ischemic conditions, this study is designed to: (1) examine whether acidic saline injection into the hind paw causes the development of TH in TIIP, but not control, animals; and (2) determine which peripheral mechanisms are involved in the development of this TH.

Results

Repeated intraplantar injection of pH 4.0 saline, but not pH 5.5 and 7.0 saline, for 3 days following TIIP surgery resulted in the development of TH. After pH 4.0 saline injections, protein levels of hypoxia inducible factor-1α (HIF-1α) and carbonic anhydrase II (CA II) were elevated in the plantar muscle indicating that acidic stimulation intensified ischemic insults with decreased tissue acidity. At the same time point, there were no changes in the expression of TRPV1 in hind paw skin, whereas a significant increase in TRPV1 phosphorylation (pTRPV1) was shown in acidic saline (pH 4.0) injected TIIP (AS-TIIP) animals. Moreover, intraplantar injection of chelerythrine (a PKC inhibitor) and AMG9810 (a TRPV1 antagonist) effectively alleviated the established TH. In order to investigate which proton- or ATP-sensing receptors contributed to the development of TH, amiloride (an ASICs blocker), AMG9810, TNP-ATP (a P2Xs antagonist) or MRS2179 (a P2Y1 antagonist) were pre-injected before the pH 4.0 saline. Only MRS2179 significantly prevented the induction of TH, and the increased pTRPV1 ratio was also blocked in MRS2179 injected animals.

Conclusion

Collectively these data show that maintenance of an acidic environment in the ischemic hind paw of TIIP rats results in the phosphorylation of TRPV1 receptors via a PKC-dependent pathway, which leads to the development of TH mimicking what occurs in chronic ischemic patients with severe acidosis. More importantly, peripheral P2Y1 receptors play a pivotal role in this process, suggesting a novel peripheral mechanism underlying the development of TH in these patients.

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.

Intestinal alkaline phosphatase inhibits the proinflammatory nucleotide uridine diphosphate

Uridine diphosphate (UDP) is a proinflammatory nucleotide implicated in inflammatory bowel disease. Intestinal alkaline phosphatase (IAP) is a gut mucosal defense factor capable of inhibiting intestinal inflammation. We used the malachite green assay to show that IAP dephosphorylates UDP. To study the anti-inflammatory effect of IAP, UDP or other proinflammatory ligands (LPS, flagellin, Pam3Cys, or TNF-α) in the presence or absence of IAP were applied to cell cultures, and IL-8 was measured. UDP caused dose-dependent increase in IL-8 release by immune cells and two gut epithelial cell lines, and IAP treatment abrogated IL-8 release. Costimulation with UDP and other inflammatory ligands resulted in a synergistic increase in IL-8 release, which was prevented by IAP treatment. In vivo, UDP in the presence or absence of IAP was instilled into a small intestinal loop model in wild-type and IAP-knockout mice. Luminal contents were applied to cell culture, and cytokine levels were measured in culture supernatant and intestinal tissue. UDP-treated luminal contents induced more inflammation on target cells, with a greater inflammatory response to contents from IAP-KO mice treated with UDP than from WT mice. Additionally, UDP treatment increased TNF-α levels in intestinal tissue of IAP-KO mice, and cotreatment with IAP reduced inflammation to control levels. Taken together, these studies show that IAP prevents inflammation caused by UDP alone and in combination with other ligands, and the anti-inflammatory effect of IAP against UDP persists in mouse small intestine. The benefits of IAP in intestinal disease may be partly due to inhibition of the proinflammatory activity of UDP.

Cross-talk in nucleotide signaling in glioma C6 cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y(1), P2Y(2), P2Y(12), P2Y(14) and the ionotropic P2X(7) receptor in glioma C6 cells. P2Y(1) and P2Y(12) both respond to ADP, but while P2Y(1) links to PLC and elevates cytosolic Ca(2+) concentration, P2Y(12) negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y(1) receptor strongly decreases and P2Y(12) becomes a major player responsible for ADP-evoked signal transduction. The P2Y(12) receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y(1) has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X(7) receptor, often responsible for apoptotic fate, is not involved in Ca(2+)elevation in C6 cells. The shift in nucleotide receptor expression from P2Y(1) to P2Y(12) during serum withdrawal, the cross talk between both receptors and the lack of P2X(7) activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

Calcium signaling in glioma cells--the role of nucleotide receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present Chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y(1) and P2Y(12) nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

N-cadherin expression is regulated by UTP in schwannoma cells

Schwann cells (SCs) are peripheral myelinating glial cells that express the neuronal Ca(2+)-dependent cell adhesion molecule, neural cadherin (N-cadherin). N-cadherin is involved in glia-glia and axon-glia interactions and participates in many key events, which range from the control of axonal growth and guidance to synapse formation and plasticity. Extracellular UTP activates P2Y purinergic receptors and exerts short- and long-term effects on several tissues to promote wound healing. Nevertheless, the contribution of P2Y receptors in peripheral nervous system functions is not completely understood. The current study demonstrated that UTP induced a dose- and time-dependent increase in N-cadherin expression in SCs. Furthermore, N-cadherin expression was blocked by the P2 purinoceptor antagonist suramin. The increased N-cadherin expression induced by UTP was mediated by phosphorylation of mitogen-activated protein kinases (MAPKs), such as Jun N-terminal kinase, extracellular-regulated kinase and p38 kinase. Moreover, the Rho kinase inhibitor Y27632, the phospholipase C inhibitor U73122 and the protein kinase C inhibitor calphostin C attenuated the UTP-induced activation of MAPKs significantly. Extracellular UTP also modulated increased in the expression of the early transcription factors c-Fos and c-Jun. We also demonstrated that the region of the N-cadherin promoter between nucleotide positions -3698 and -2620, which contained one activator protein-1-binding site, was necessary for UTP-induced gene expression. These results suggest a novel role for P2Y purinergic receptors in the regulation of N-cadherin expression in SCs.

Novel identified receptors on mast cells

Mast cells (MC) are major participants in the allergic reaction. In addition they possess immunomodulatory roles in the innate and adaptive immune reactions. Their functions are modulated through a number of activating and inhibitory receptors expressed on their surface. This review deals with some of the most recently described receptors, their expression patterns, ligand(s), signal transduction mechanisms, possible cross-talk with other receptors and, last but not least, regulatory functions that the MC can perform based on their receptor expression in health or in disease. Where the receptor role on MC is still not clear, evidences from other hematopoietic cells expressing them is provided as a possible insight for their function on MC. Suggested strategies to modulate these receptors’ activity for the purpose of therapeutic intervention are also discussed.

Negative-feedback regulation of ATP release: ATP release from cardiomyocytes is strictly regulated during ischemia

Extracellular ATP acts as a potent agonist on cardiomyocytes, inducing a broad range of physiological responses via P2 purinoceptors. Its concentration in the interstitial space within the heart is elevated during ischemia or hypoxia due to its release from a number of cell types, including cardiomyocytes. However, the exact mechanism responsible for the release of ATP from cardiomyocytes during ischemia is not known. In this study, we investigated whether and how the release of ATP was strictly regulated during ischemia in cultured neonatal rat cardiomyocytes. Ischemia was mimicked by oxygen-glucose deprivation (OGD). Exposure of cardiomyocytes to OGD resulted in an increase in the concentration of extracellular ATP shortly after the onset of OGD (15 min), and the increase was reversed by treatment with blockers of maxi-anion channels. Unexpectedly, at 1 and 2h after the onset of OGD, the blocking of maxi-anion channels increased the concentration of extracellular ATP, and the increase was significantly suppressed by co-treatment with blockers of hemichannels, suggesting that ATP release via maxi-anion channels was involved in the suppression of ATP release via hemichannels during persistent OGD. Here we show the possibility that the release of ATP from cardiomyocytes was strictly regulated during ischemia by negative-feedback mechanisms; that is, maxi-anion channel-derived ATP-induced suppression of ATP release via hemichannels in cardiomyocytes.

Interaction of purinergic receptors with GPCRs, ion channels, tyrosine kinase and steroid hormone receptors orchestrates cell function

Extracellular purines and pyrimidines have emerged as key regulators of a wide range of physiological and pathophysiological cellular processes acting through P1 and P2 cell surface receptors. Increasing evidence suggests that purinergic receptors can interact with and/or modulate the activity of other classes of receptors and ion channels. This review will focus on the interactions of purinergic receptors with other GPCRs, ion channels, receptor tyrosine kinases, and steroid hormone receptors. Also, the signal transduction pathways regulated by these complexes and their new functional properties are discussed.

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.

Purinergic signalling in epithelial ion transport: regulation of secretion and absorption

Intracellular ATP, the energy source for many reactions, is crucial for the activity of plasma membrane pumps and, thus, for the maintenance of transmembrane ion gradients. Nevertheless, ATP and other nucleotides/nucleosides are also extracellular molecules that regulate diverse cellular functions, including ion transport. In this review, I will first introduce the main components of the extracellular ATP signalling, which have become known as the purinergic signalling system. With more than 50 components or processes, just at cell membranes, it ranks as one of the most versatile signalling systems. This multitude of system components may enable differentiated regulation of diverse epithelial functions. As epithelia probably face the widest variety of potential ATP-releasing stimuli, a special attention will be given to stimuli and mechanisms of ATP release with a focus on exocytosis. Subsequently, I will consider membrane transport of major ions (Cl(-) , HCO(3)(-) , K(+) and Na(+) ) and integrate possible regulatory functions of P2Y2, P2Y4, P2Y6, P2Y11, P2X4, P2X7 and adenosine receptors in some selected epithelia at the cellular level. Some purinergic receptors have noteworthy roles. For example, many studies to date indicate that the P2Y2 receptor is one common denominator in regulating ion channels on both the luminal and basolateral membranes of both secretory and absorptive epithelia. In exocrine glands though, P2X4 and P2X7 receptors act as cation channels and, possibly, as co-regulators of secretion. On an organ level, both receptor types can exert physiological functions and together with other partners in the purinergic signalling, integrated models for epithelial secretion and absorption are emerging.

Cyclic AMP-dependent down regulation of ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) in rat C6 glioma

In this communication, we demonstrate that an increase in intracellular cAMP by 1) addition of dibutyrylic cAMP (dbcAMP), a membrane-permeable cAMP-analogue, or 2) activation of the β-adrenoceptor with (-)-isoproterenol, down regulates the levels of ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) mRNA, NPP1 protein and ecto-NPPase activity in rat C6 glioma cells. DbcAMP and (-)-isoproterenol inhibit NPP1 expression in a time and dose-dependent manner. After 48h of stimulation, 1mM dbcAMP or 5μM (-)-isoproterenol decreases the amount of NPP1 protein by 75±3% and 81±1% respectively. Contrary to down regulation of NPP1, we observe an up regulation of glial fibrillary acidic protein (GFAP), a differentiation marker for astrocytic cells. Using specific inhibitors and activators, we have shown that Ca(2+), PKA, PI 3-K/PKB/GSK-3, Epac/Rap1/PP2A and MAP kinase modules are not involved in the inhibition of NPP1 gene expression. The transcription factor c-jun is significantly reduced while c-fos becomes up regulated after cAMP elevation. However an electrophoretic mobility shift assay with the activator protein-1 motif present in the promoter of the rat NPP1 gene indicates that this motif is not involved in the cAMP-dependent inhibition of NPP1 expression. In conclusion, these results indicate that intracellular cAMP levels regulate the expression of NPP1 in rat C6 glioma cells by a signalling pathway that is different from the GFAP signal transduction pathway.

2', 3'-O-(4-benzoylbenzoyl)-ATP-mediated calcium signaling in rat glioma C6 cells: role of the P2Y(2) nucleotide receptor

In this study, we examined the response of glioma C6 cells to 2',3'-O-(4-benzoylbenzoyl)-ATP (BzATP) and showed that the BzATP-induced calcium signaling does not involve the P2X(7) receptor activity. We show here that in the absence of extracellular Ca(2+), BzATP-generated increase in [Ca(2+)](i)via Ca(2+) release from intracellular stores. In the presence of calcium ions, BzATP established a biphasic Ca(2+) response, in a manner typical for P2Y receptors. Brilliant Blue G, a selective antagonist of the rat P2X(7) receptor, did not reduce any of the two components of the Ca(2+) response elicited by BzATP. Periodate-oxidized ATP blocked not only BzATP- but also UTP-induced Ca(2+) elevation. Moreover, BzATP did not open large transmembrane pores. What is more, a cross-desensitization between UTP and BzATP occurred, which clearly shows that in glioma C6 cells BzATP activates most likely the P2Y(2) but not the P2X(7) receptors.

Differential expression of P2Y receptors in the rat cochlea during development

Purinergic signaling has broad physiological significance to the hearing organ, involving signal transduction via ionotropic P2X receptors and metabotropic G-protein-coupled P2Y and P1 (adenosine), alongside conversion of nucleotides and nucleosides by ecto-nucleotidases and ecto-nucleoside diphosphokinase. In addition, ATP release is modulated by acoustic overstimulation or stress and involves feedback regulation. Many of these principal elements of the purinergic signaling complex have been well characterized in the cochlea, while the characterization of P2Y receptor expression is emerging. The present study used immunohistochemistry to evaluate the expression of five P2Y receptors, P2Y(1), P2Y(2), P2Y(4), P2Y(6), and P2Y(12), during development of the rat cochlea. Commencing in the late embryonic period, the P2Y receptors studied were found in the cells lining the cochlear partition, associated with establishment of the electrochemical environment which provides the driving force for sound transduction. In addition, early postnatal P2Y(2) and P2Y(4) protein expression in the greater epithelial ridge, part of the developing hearing organ, supports the view that initiation and regulation of spontaneous activity in the hair cells prior to hearing onset is mediated by purinergic signaling. Sub-cellular compartmentalization of P2Y receptor expression in sensory hair cells, and diversity of receptor expression in the spiral ganglion neurons and their satellite cells, indicates roles for P2Y receptor-mediated Ca(2+)-signaling in sound transduction and auditory neuron excitability. Overall, the dynamics of P2Y receptor expression during development of the cochlea complement the other elements of the purinergic signaling complex and reinforce the significance of extracellular nucleotide and nucleoside signaling to hearing.

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.

The TRPC2 channel forms protein-protein interactions with Homer and RTP in the rat vomeronasal organ

BACKGROUND

The signal transduction cascade operational in the vomeronasal organ (VNO) of the olfactory system detects odorants important for prey localization, mating, and social recognition. While the protein machinery transducing these external cues has been individually well characterized, little attention has been paid to the role of protein-protein interactions among these molecules. Development of an in vitro expression system for the transient receptor potential 2 channel (TRPC2), which establishes the first electrical signal in the pheromone transduction pathway, led to the discovery of two protein partners that couple with the channel in the native VNO.

RESULTS

Homer family proteins were expressed in both male and female adult VNO, particularly Homer 1b/c and Homer 3. In addition to this family of scaffolding proteins, the chaperones receptor transporting protein 1 (RTP1) and receptor expression enhancing protein 1 (REEP1) were also expressed. RTP1 was localized broadly across the VNO sensory epithelium, goblet cells, and the soft palate. Both Homer and RTP1 formed protein-protein interactions with TRPC2 in native reciprocal pull-down assays and RTP1 increased surface expression of TRPC2 in in vitro assays. The RTP1-dependent TRPC2 surface expression was paralleled with an increase in ATP-stimulated whole-cell current in an in vitro patch-clamp electrophysiological assay.

CONCLUSIONS

TRPC2 expression and channel activity is regulated by chaperone- and scaffolding-associated proteins, which could modulate the transduction of chemosignals. The developed in vitro expression system, as described here, will be advantageous for detailed investigations into TRPC2 channel activity and cell signalling, for a channel protein that was traditionally difficult to physiologically assess.

ATP stimulates the proliferation of MCF-7 cells through the PI3K/Akt signaling pathway

We studied the modulation of the PI3K/Akt signaling pathway by ATP in MCF-7 cells. Western blot analysis showed that ATP stimulated the phosphorylation of Akt in a dose- and time-dependent manner. Akt phosphorylation in response to nucleotides followed the potency order ATP=UTP=ATPgammaS>>ADP=UDP>ADPbetaS=adenosine, suggesting participation of P2Y(2/4) receptors. Inhibitors of PI3K, PLC, PKC and Src or Src antisense oligonucleotides prevented ATP-induced phosphorylation of Akt. Incubation of cells with 2-APB or in a nominally Ca(2+)-free medium plus EGTA showed that Akt phosphorylation by ATP depends on intracellular calcium release but is independent of calcium influx. The PI3K inhibitor was not effective in reducing MAPKs phosphorylation by ATP. ATP and UTP stimulated MCF-7 cell proliferation, effect that was inhibited by PI3K, PLC, PKC, Src and MAPKs inhibitors. These findings suggest that ATP modulation of P2Y(2/4) receptors increases MCF-7 cell proliferation by activation of the PI3K/Akt signaling pathway through PLC/IP(3)/Ca(2+), PKC and Src.

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

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

P2 receptor-mediated signaling in mast cell biology

Mast cells are widely recognized as effector cells of allergic inflammatory reactions. They contribute to the pathogenesis of different chronic inflammatory diseases, wound healing, fibrosis, thrombosis/fibrinolysis, and anti-tumor immune responses. In this paper, we summarized the role of P2X and P2Y receptors in mast cell activation and effector functions. Mast cells are an abundant source of ATP which is stored in their granules and secreted upon activation. We discuss the contribution of mast cells to the extracellular ATP release and to the maintenance of extracellular nucleotides pool. Recent publications highlight the importance of purinergic signaling for the pathogenesis of chronic airway inflammation. Therefore, the role of ATP and P2 receptors in allergic inflammation with focus on mast cells was analyzed. Finally, ATP functions as mast cell autocrine/paracrine factor and as messenger in intercellular communication between mast cells, nerves, and glia in the central nervous system.

ATP modulates transcription factors through P2Y2 and P2Y4 receptors via PKC/MAPKs and PKC/Src pathways in MCF-7 cells

In this work, we studied the involvement of PKC and Src in the phosphorylation of ERK1/2, p38 and JNK1 MAPKs and in the modulation of ATF-1, c-Fos, c-Jun and Jun D transcription factors by ATP in MCF-7 breast cancer cells. RT-PCR studies and nucleotide sequence analysis confirmed first the expression of P2Y(2)- and P2Y(4)-receptor subtypes. The use of specific inhibitors and Src antisense oligonucleotides showed that PKC, but not Src, plays a role in the phosphorylation of MAPKs by ATP. ATP stimulated the expression of c-Fos and the phosphorylation c-Jun, Jun D and ATF-1. PKC and Src only participated in c-Fos induction and in ATF-1 phosphorylation. Pharmacological inhibition of MAPKs demonstrated that c-Fos induction and phosphorylation of c-Jun and Jun D, but not of ATF-1, depend on MAPK activation. These results suggest that stimulation of P2Y(2) and P2Y(4) receptors by ATP modulates transcription factors through PKC/MAPKs and PKC/Src pathways in MCF-7 cells.

Modeling and analysis of the molecular basis of pain in sensory neurons

Intracellular calcium dynamics are critical to cellular functions like pain transmission. Extracellular ATP plays an important role in modulating intracellular calcium levels by interacting with the P2 family of surface receptors. In this study, we developed a mechanistic mathematical model of ATP-induced P2 mediated calcium signaling in archetype sensory neurons. The model architecture, which described 90 species connected by 162 interactions, was formulated by aggregating disparate molecular modules from literature. Unlike previous models, only mass action kinetics were used to describe the rate of molecular interactions. Thus, the majority of the 252 unknown model parameters were either association, dissociation or catalytic rate constants. Model parameters were estimated from nine independent data sets taken from multiple laboratories. The training data consisted of both dynamic and steady-state measurements. However, because of the complexity of the calcium network, we were unable to estimate unique model parameters. Instead, we estimated a family or ensemble of probable parameter sets using a multi-objective thermal ensemble method. Each member of the ensemble met an error criterion and was located along or near the optimal trade-off surface between the individual training data sets. The model quantitatively reproduced experimental measurements from dorsal root ganglion neurons as a function of extracellular ATP forcing. Hypothesized architecture linking phosphoinositide regulation with P2X receptor activity explained the inhibition of P2X-mediated current flow by activated metabotropic P2Y receptors. Sensitivity analysis using individual and the whole system outputs suggested which molecular subsystems were most important following P2 activation. Taken together, modeling and analysis of ATP-induced P2 mediated calcium signaling generated qualitative insight into the critical interactions controlling ATP induced calcium dynamics. Understanding these critical interactions may prove useful for the design of the next generation of molecular pain management strategies.

Role of VASP phosphorylation for the regulation of microglia chemotaxis via the regulation of focal adhesion formation/maturation

Microglia activation and migration are known to play crucial roles for the response to brain injuries. Extracellular ADP was reported to induce microglia chemotaxis and membrane ruffles through P2Y12 receptor. In this study, we examined the role of VASP phosphorylation in ADP-induced microglia chemotaxis and membrane ruffle formation. ADP stimulation transiently increased intracellular cAMP level, VASP phosphorylation at Ser153, membrane ruffle formation, and chemotaxis. PKA inhibitor effectively inhibited VASP phosphorylation and chemotaxis, indicating that P2Y12-mediated activation of PKA and subsequent VASP phosphorylation are involved in the regulation of microglia chemotaxis. Forskolin and okadaic acid induced sustained VASP phosphorylation at a high level, causing a significant reduction of the retraction of membrane ruffles and chemotaxis. In forskolin- or okadaic acid-treated cells, phosphorylated VASP remained at the membrane cortex, and size and number of mature focal adhesions were not increased, indicating that prolonged phosphorylation of VASP could inhibit transformation of focal complexes into focal adhesions. VASP knockdown cells showed markedly reduced frequency and distance of membrane ruffling upon ADP stimulation, reinforcing the idea that VASP is required for the ruffle formation. Cells expressing GFP-VASP(S153A) also showed a significant reduction of protrusion distance during ruffle formation, but the frequency and the distance of retraction were not affected by FSK at all. This result suggests that dephosphorylation of VASP might be required for the growth of adhesion strength during membrane retraction. Our results suggest that VASP phosphorylation by PKA plays an important role in membrane ruffle formation and chemotaxis via the regulation of focal adhesion formation/maturation.

Akt activation is involved in P2Y12 receptor-mediated chemotaxis of microglia

Microglia play a variety of significant roles in the central nervous system (CNS), and in one of those roles they undergo morphological change in response to neural injury and migrate to the injured region. We previously reported that ATP/ADP promotes microglial chemotaxis via the Gi/o-coupled P2Y12 receptor; however, the intracellular signaling underlying P2Y12-receptor-mediated microglial chemotaxis is not fully understood. In this study, we examined the role of phospholipase C (PLC) and calcium signaling in ADP-induced microglial chemotaxis. A PLC inhibitor, U73122, significantly suppressed the chemotaxis and completely blocked the ADP-evoked intracellular calcium response, and a calcium chelator, BAPTA-AM, inhibited the chemotaxis. These results indicate that ADP-induced microglial chemotaxis is regulated by a PLC-mediated calcium pathway. ADP stimulation induced Akt phosphorylation in microglia, and the phosphorylation was inhibited by a P2Y12 receptor antagonist, AR-C69931MX. The Akt phosphorylation was blocked by U73122 and BAPTA-AM as well as by a phosphatidylinositol 3-kinase (PI3K) inhibitor, wortmannin, and inhibition of the Akt activation resulted in failure of chemotaxis. These results indicate that Akt activation is dependent on the PI3K pathway and a PLC-mediated increase in intracellular calcium and suggest that Akt activation is involved in ADP-induced microglial chemotaxis.

Purinergic receptors in the endocrine and exocrine pancreas

The pancreas is a complex gland performing both endocrine and exocrine functions. In recent years there has been increasing evidence that both endocrine and exocrine cells possess purinergic receptors, which influence processes such as insulin secretion and epithelial ion transport. Most commonly, these processes have been viewed separately. In beta cells, stimulation of P2Y(1) receptors amplifies secretion of insulin in the presence of glucose. Nucleotides released from secretory granules could also contribute to autocrine/paracrine regulation in pancreatic islets. In addition to P2Y(1) receptors, there is also evidence for other P2 and adenosine receptors in beta cells (P2Y(2), P2Y(4), P2Y(6), P2X subtypes and A(1) receptors) and in glucagon-secreting alpha cells (P2X(7), A(2) receptors). In the exocrine pancreas, acini release ATP and ATP-hydrolysing and ATP-generating enzymes. P2 receptors are prominent in pancreatic ducts, and several studies indicate that P2Y(2), P2Y(4), P2Y(11), P2X(4) and P2X(7) receptors could regulate secretion, primarily by affecting Cl(-) and K(+) channels and intracellular Ca(2+) signalling. In order to understand the physiology of the whole organ, it is necessary to consider the full complement of purinergic receptors on different cells as well as the structural and functional relation between various cells within the whole organ. In addition to the possible physiological function of purinergic receptors, this review analyses whether the receptors could be potential therapeutic targets for drug design aimed at treatment of pancreatic diseases.

Binding of 5'-O-(2-thiodiphosphate) to rat brain membranes is prevented by diadenosine tetraphosphate and correlates with ecto-nucleotide pyrophosphatase phosphodiesterase 1 (NPP1) activity

The distribution of binding sites for [(35)S]5'-O-(2-thiodiphosphate) ([(35)S]ADPbetaS), a radioligand of P2Y(1,12,13) receptors, and of ecto-nucleotide pyrophosphatase phosphodiesterase activity were analyzed in the rat forebrain. Binding sites for the radilogand are widespreadly distributed in the rat forebrain, showing the highest density in hypothalamus. K(d) values were in the range 1-2 nM. Diadenosine tetraphosphate (Ap(4)A) and diethenoadenosine tetraphosphate, epsilon-(Ap(4)A), displaced the radioligand, indicating dinucleotide binding to ADPbetaS-recognizing P2Y receptors. Activity ecto-nucleotide pyrophosphatase phosphodiesterase 1 (NPP1), able to hydrolyze Ap(4)A and other diadenosine polyphosphates, is also widely distributed through the rat forebrain, with the highest activity in hypothalamus. These results suggests that Ap(4)A signalling mediated by P2Y(1,12,13) receptors and enzymatically regulated by NPP1 activity may be particularly important in hypothalamus and add new support for neurotransmitter/neuromodulatory functions of diadenosine polyphosphates in brain.

Involvement of P2X4 and P2Y12 receptors in ATP-induced microglial chemotaxis

We previously reported that extracellular ATP induces membrane ruffling and chemotaxis of microglia and suggested that their induction is mediated by the Gi/o-protein coupled P2Y(12) receptor (P2Y(12)R). Here we report discovering that the P2X(4) receptor (P2X(4)R) is also involved in ATP-induced microglial chemotaxis. To understand the intracellular signaling pathway downstream of P2Y(12)R that underlies microglial chemotaxis, we examined the effect of two phosphatidylinositol 3'-kinase (PI3K) inhibitors, wortmannin, and LY294002, on chemotaxis in a Dunn chemotaxis chamber. The PI3K inhibitors significantly suppressed chemotaxis without affecting ATP-induced membrane ruffling. ATP stimulation increased Akt phosphorylation in the microglia, and the increase was reduced by the PI3K inhibitors and a P2Y(12)R antagonist. These results indicate that P2Y(12)R-mediated activation of the PI3K pathway is required for microglial chemotaxis in response to ATP. We also found that the Akt phosphorylation was reduced when extracellular calcium was chelated, suggesting that ionotropic P2X receptors are involved in microglial chemotaxis by affecting the PI3K pathway. We therefore tested the effect of various P2X(4)R antagonists on the chemotaxis, and the results showed that pharmacological blockade of P2X(4)R significantly inhibited it. Knockdown of the P2X(4) receptor in microglia by RNA interference through the lentivirus vector system also suppressed the microglial chemotaxis. These results indicate that P2X(4)R as well as P2Y(12)R is involved in ATP-induced microglial chemotaxis.