Structural basis of agonist-induced desensitization and sequestration of the P2Y2 nucleotide receptor. Consequences of truncation of the C terminus

Ligand-dependent intracellular trafficking of the G protein-coupled P2Y6 receptor

Endosomal trafficking is intricately linked to G protein-coupled receptors (GPCR) fate and signaling. Extracellular uridine diphosphate (UDP) acts as a signaling molecule by selectively activating the GPCR P2Y₆. Despite the recent interest for this receptor in pathologies, such as gastrointestinal and neurological diseases, there is sparse information on the endosomal trafficking of P2Y₆ receptors in response to its endogenous agonist UDP and synthetic selective agonist 5-iodo-UDP (MRS2693). Confocal microscopy and cell surface ELISA revealed delayed internalization kinetics in response to MRS2693 vs. UDP stimulation in AD293 and HCT116 cells expressing human P2Y₆. Interestingly, UDP induced clathrin-dependent P2Y₆ internalization, whereas receptor stimulation by MRS2693 endocytosis appeared to be associated with a caveolin-dependent mechanism. Internalized P2Y₆ was associated with Rab4, 5, and 7 positive vesicles independent of the agonist. We have measured a higher frequency of receptor expression co-occurrence with Rab11-vesicles, the trans-Golgi network, and lysosomes in response to MRS2693. Interestingly, a higher agonist concentration reversed the delayed P2Y₆ internalization and recycling kinetics in the presence of MRS2693 stimulation without changing its caveolin-dependent internalization. This work showed a ligand-dependent effect affecting the P2Y₆ receptor internalization and endosomal trafficking. These findings could guide the development of bias ligands that could influence P2Y₆ signaling.

Mathematical modeling of intracellular calcium in presence of receptor: a homeostatic model for endothelial cell

Calcium is a ubiquitous molecule and second messenger that regulates many cellular functions ranging from exocytosis to cell proliferation at different time scales. In the vasculature, a constant adenosine triphosphate (ATP) concentration is maintained because of ATP released by red blood cells (RBCs). These ATP molecules continuously react with purinergic receptors on the surface of endothelial cells (ECs). Consequently, a cascade of chemical reactions are triggered that result in a transient cytoplasmic calcium (Ca[Formula: see text]), followed by return to its basal concentration. The mathematical models proposed in the literature are able to reproduce the transient peak. However, the trailing concentration is always higher than the basal cytoplasmic Ca[Formula: see text] concentrations, and the Ca[Formula: see text] concentration in endoplasmic reticulum (ER) remains lower than its initial concentration. This means that the intracellular homeostasis is not recovered. We propose, herein, a minimal model of calcium kinetics. We find that the desensitization of EC surface receptors due to phosphorylation and recycling plays a vital role in maintaining calcium homeostasis in the presence of a constant stimulus (ATP). The model is able to capture several experimental observations such as refilling of Ca[Formula: see text] in the ER, variation of cytoplasmic Ca[Formula: see text] transient peak in ECs, the resting cytoplasmic Ca[Formula: see text] concentration, the effect of removing ATP from the plasma on Ca[Formula: see text] homeostasis, and the saturation of cytoplasmic Ca[Formula: see text] transient peak with increase in ATP concentration. Direct confrontation with several experimental results is conducted. This work paves the way for systematic studies on coupling between blood flow and chemical signaling, and should contribute to a better understanding of the relation between (patho)physiological conditions and Ca[Formula: see text] kinetics.

The P2Y2 Receptor C-Terminal Tail Modulates but Is Dispensable for β-Arrestin Recruitment

The P2Y₂ receptor (P2Y₂R) is a G protein-coupled receptor that is activated by extracellular ATP and UTP, to a similar extent. This allows it to play roles in the cell's response to the (increased) release of these nucleotides, e.g., in response to stress situations, including mechanical stress and oxygen deprivation. However, despite its involvement in important (patho)physiological processes, the intracellular signaling induced by the P2Y₂R remains incompletely described. Therefore, this study implemented a NanoBiT^® functional complementation assay to shed more light on the recruitment of β-arrestins (βarr1 and βarr2) upon receptor activation. More specifically, upon determination of the optimal configuration in this assay system, the effect of different (receptor) residues/regions on βarr recruitment to the receptor in response to ATP or UTP was estimated. To this end, the linker was shortened, the C-terminal tail was truncated, and phosphorylatable residues in the third intracellular loop of the receptor were mutated, in either singly or multiply adapted constructs. The results showed that none of the introduced adaptations entirely abolished the recruitment of either βarr, although EC₅₀ values differed and time-luminescence profiles appeared to be qualitatively altered. The results hint at the C-terminal tail modulating the interaction with βarr, while not being indispensable.

Streptococcus pneumoniae inhibits purinergic signaling and promotes purinergic receptor P2Y2 internalization in alveolar epithelial cells

Bacterial pneumonia is a global health challenge that causes up to 2 million deaths each year. Purinergic signaling plays a pivotal role in healthy alveolar epithelium. Here, we used fluorophore-based analysis and live-cell calcium imaging to address the question of whether the bacterial pathogen Streptococcus pneumoniae directly interferes with purinergic signaling in alveolar epithelial cells. Disturbed purinergic signaling might result in pathophysiologic changes like edema formation and atelectasis, which are commonly seen in bacterial pneumonia. Purine receptors are mainly activated by ATP, mediating a cytosolic calcium response. We found that this purinergic receptor P2Y₂-mediated response is suppressed in the presence of S. pneumoniae in A549 and isolated primary alveolar cells in a temperature-dependent manner. Downstream inositol 3-phosphate (IP₃) signaling appeared to be unaffected, as calcium signaling via protease-activated receptor 2 remained unaltered. S. pneumoniae-induced suppression of the P2Y₂-mediated calcium response depended on the P2Y₂ phosphorylation sites Ser-243, Thr-344, and Ser-356, which are involved in receptor desensitization and internalization. Spinning-disk live-cell imaging revealed that S. pneumoniae induces P2Y₂ translocation into the cytosol. In conclusion, our results show that S. pneumoniae directly inhibits purinergic signaling by inducing P2Y₂ phosphorylation and internalization, resulting in the suppression of the calcium response of alveolar epithelial cells to ATP, thereby affecting cellular integrity and function.

Excessive Extracellular ATP Desensitizes P2Y2 and P2X4 ATP Receptors Provoking Surfactant Impairment Ending in Ventilation-Induced Lung Injury

Stretching the alveolar epithelial type I (AT I) cells controls the intercellular signaling for the exocytosis of surfactant by the AT II cells through the extracellular release of adenosine triphosphate (ATP) (purinergic signaling). Extracellular ATP is cleared by extracellular ATPases, maintaining its homeostasis and enabling the lung to adapt the exocytosis of surfactant to the demand. Vigorous deformation of the AT I cells by high mechanical power ventilation causes a massive release of extracellular ATP beyond the clearance capacity of the extracellular ATPases. When extracellular ATP reaches levels >100 μM, the ATP receptors of the AT II cells become desensitized and surfactant impairment is initiated. The resulting alteration in viscoelastic properties and in alveolar opening and collapse time-constants leads to alveolar collapse and the redistribution of inspired air from the alveoli to the alveolar ducts, which become pathologically dilated. The collapsed alveoli connected to these dilated alveolar ducts are subject to a massive strain, exacerbating the ATP release. After reaching concentrations >300 μM extracellular ATP acts as a danger-associated molecular pattern, causing capillary leakage, alveolar space edema, and further deactivation of surfactant by serum proteins. Decreasing the tidal volume to 6 mL/kg or less at this stage cannot prevent further lung injury.

Current knowledge on the nucleotide agonists for the P2Y2 receptor

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13, and P2Y14). P2Y2 receptors are widely expressed and play important roles in multiple functionalities. Diquafosol tetrasodium, known as INS365, which was the first P2Y2 receptor agonists that had been approved in April 2010 and launched in Japan by Santen Pharmaceuticals. Besides, a series of similar agonists for the P2Y2 receptor are undergoing development to cure different diseases related to the P2Y2 receptor. This article illustrated the structure and functions of the P2Y2 receptor and focused on several kinds of agonists about their molecular structures, research progress and chemical synthesis methods. Last but not the least, we summarized the structures-activity relationship (SAR) of agonists for the P2Y2 receptor and expected more efficient agonists for the P2Y2 receptor.

The P2Y2 receptor mediates uptake of matrix-retained and aggregated low density lipoprotein in primary vascular smooth muscle cells

BACKGROUND AND AIMS

The internalization of aggregated low-density lipoproteins (agLDL) mediated by low-density lipoprotein receptor related protein (LRP1) may involve the actin cytoskeleton in ways that differ from the endocytosis of soluble LDL by the LDL receptor (LDLR). This study aims to define novel mechanisms of agLDL uptake through modulation of the actin cytoskeleton, to identify molecular targets involved in foam cell formation in vascular smooth muscle cells (VSMCs). The critical observation that formed the basis for these studies is that under pathophysiological conditions, nucleotide release from blood-derived and vascular cells activates SMC P2Y2 receptors (P2Y2Rs) leading to rearrangement of the actin cytoskeleton and cell motility. Therefore, we tested the hypothesis that P2Y2R activation mediates agLDL uptake by VSMCs.

METHODS

Primary VSMCs were isolated from aortas of wild type (WT) C57BL/6 and.P2Y2R-/- mice to investigate whether P2Y2R activation modulates LRP1 expression. Cells were transiently transfected with cDNA encoding a hemagglutinin-tagged (HA-tagged) WT P2Y2R, or a mutant P2Y2R that unlike the WT P2Y2R does not bind the cytoskeletal actin-binding protein filamin-A (FLN-A).

RESULTS

P2Y2R activation significantly increased agLDL uptake, and LRP1 mRNA expression decreased in P2Y2R-/- VSMCs versus WT. SMCs, expressing P2Y2R defective in FLN-A binding, exhibit 3-fold lower LDLR expression levels than SMCs expressing WT P2Y2R, while cells transfected with WT P2Y2R show greater agLDL uptake in both WT and P2Y2R-/- VSMCs versus cells transfected with the mutant P2Y2R.

CONCLUSIONS

Together, these results show that both LRP1 and LDLR expression and agLDL uptake are regulated by P2Y2R in VSMCs, and that agLDL uptake due to P2Y2R activation is dependent upon cytoskeletal reorganization mediated by P2Y2R binding to FLN-A.

Computational Model of Ca2+ Wave Propagation in Human Retinal Pigment Epithelial ARPE-19 Cells

Objective

Computational models of calcium (Ca²⁺) signaling have been constructed for several cell types. There are, however, no such models for retinal pigment epithelium (RPE). Our aim was to construct a Ca²⁺ signaling model for RPE based on our experimental data of mechanically induced Ca²⁺ wave in the in vitro model of RPE, the ARPE-19 monolayer.

Methods

We combined six essential Ca²⁺ signaling components into a model: stretch-sensitive Ca²⁺ channels (SSCCs), P₂Y₂ receptors, IP₃ receptors, ryanodine receptors, Ca²⁺ pumps, and gap junctions. The cells in our epithelial model are connected to each other to enable transport of signaling molecules. Parameterization was done by tuning the above model components so that the simulated Ca²⁺ waves reproduced our control experimental data and data where gap junctions were blocked.

Results

Our model was able to explain Ca²⁺ signaling in ARPE-19 cells, and the basic mechanism was found to be as follows: 1) Cells near the stimulus site are likely to conduct Ca²⁺ through plasma membrane SSCCs and gap junctions conduct the Ca²⁺ and IP³ between cells further away. 2) Most likely the stimulated cell secretes ligand to the extracellular space where the ligand diffusion mediates the Ca²⁺ signal so that the ligand concentration decreases with distance. 3) The phosphorylation of the IP₃ receptor defines the cell’s sensitivity to the extracellular ligand attenuating the Ca²⁺ signal in the distance.

Conclusions

The developed model was able to simulate an array of experimental data including drug effects. Furthermore, our simulations predict that suramin may interfere ligand binding on P₂Y₂ receptors or accelerate P₂Y₂ receptor phosphorylation, which may partially be the reason for Ca²⁺ wave attenuation by suramin. Being the first RPE Ca²⁺ signaling model created based on experimental data on ARPE-19 cell line, the model offers a platform for further modeling of native RPE functions.

P2Y receptors in the mammalian nervous system: pharmacology, ligands and therapeutic potential

P2Y receptors for extracellular nucleotides are coupled to activation of a variety of G proteins and stimulate diverse intracellular signaling pathways that regulate functions of cell types that comprise the central nervous system (CNS). There are 8 different subtypes of P2Y receptor expressed in cells of the CNS that are activated by a select group of nucleotide agonists. Here, the agonist selectivity of these 8 P2Y receptor subtypes is reviewed with an emphasis on synthetic agonists with high potency and resistance to degradation by extracellular nucleotidases that have potential applications as therapeutic agents. In addition, the recent identification of a wide variety of subtype-selective antagonists is discussed, since these compounds are critical for discerning cellular responses mediated by activation of individual P2Y receptor subtypes. The functional expression of P2Y receptor subtypes in cells that comprise the CNS is also reviewed and the role of each subtype in the regulation of physiological and pathophysiological responses is considered. Other topics include the role of P2Y receptors in the regulation of blood-brain barrier integrity and potential interactions between different P2Y receptor subtypes that likely impact tissue responses to extracellular nucleotides in the CNS. Overall, current research suggests that P2Y receptors in the CNS regulate repair mechanisms that are triggered by tissue damage, inflammation and disease and thus P2Y receptors represent promising targets for the treatment of neurodegenerative diseases.

Identification of a promising drug candidate for the treatment of type 2 diabetes based on a P2Y(1) receptor agonist

The activation by extracellular nucleotides of pancreatic P2Y receptors, particularly, the P2Y(1)R subtype, increases insulin secretion. Therefore, we developed analogues of the P2Y(1)R receptor agonist 2-MeS-ADP, as potential antidiabetic drugs. Analogue 3A was found to be a potent P2Y(1)R agonist (EC(50) = 0.038 μM vs 0.0025 μM for 2-MeS-ADP) showing no activity at P2Y(2/4/6)Rs. Analogue 3A was stable at pH 1.4 (t(1/2) = 7.3 h) and resistant to hydrolysis vs 2-MeS-ADP by alkaline phosphatase (t(1/2) = 6 vs 4.5 h), human e-NPP1 (4% vs 16% hydrolysis after 20 min), and human blood serum (30% vs 50% hydrolysis after 24 h). Intravenous administration of 3A in naive rats decreased blood glucose from 155 mg/dL to normal values, ca. 87 mg/dL, unlike glibenclamide, leading to subnormal values (i.e., 63 mg/dL). Similar observations were made for streptozotocin (STZ)-treated and db(+)/db(-) mouse models. Furthermore, 3A inhibits platelet aggregation in vitro and elongates bleeding time in mice (iv administration of 30 mg of 3A/kg), increasing bleeding time to 16 vs 9 min for Prasugrel. Oral administration of 30 mg/kg 3A to rats increased tail bleeding volume, similar to aspirin. These findings suggest that 3A may be an effective treatment for type 2 diabetes by reducing both blood glucose levels and platelet aggregation.

Coupling of P2Y receptors to G proteins and other signaling pathways

P2Y receptors are G protein-coupled receptors (GPCRs) that are activated by adenine and uridine nucleotides and nucleotide sugars. There are eight subtypes of P2Y receptors (P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄), which activate intracellular signaling cascades to regulate a variety of cellular processes, including proliferation, differentiation, phagocytosis, secretion, nociception, cell adhesion, and cell migration. These signaling cascades operate mainly by the sequential activation or deactivation of heterotrimeric and monomeric G proteins, phospholipases, adenylyl and guanylyl cyclases, protein kinases, and phosphodiesterases. In addition, there are numerous ion channels, cell adhesion molecules, and receptor tyrosine kinases that are modulated by P2Y receptors and operate to transmit an extracellular signal to an intracellular response.

Neuroprotective roles of the P2Y(2) receptor

Purinergic signaling plays a unique role in the brain by integrating neuronal and glial cellular circuits. The metabotropic P1 adenosine receptors and P2Y nucleotide receptors and ionotropic P2X receptors control numerous physiological functions of neuronal and glial cells and have been implicated in a wide variety of neuropathologies. Emerging research suggests that purinergic receptor interactions between cells of the central nervous system (CNS) have relevance in the prevention and attenuation of neurodegenerative diseases resulting from chronic inflammation. CNS responses to chronic inflammation are largely dependent on interactions between different cell types (i.e., neurons and glia) and activation of signaling molecules including P2X and P2Y receptors. Whereas numerous P2 receptors contribute to functions of the CNS, the P2Y(2) receptor is believed to play an important role in neuroprotection under inflammatory conditions. While acute inflammation is necessary for tissue repair due to injury, chronic inflammation contributes to neurodegeneration in Alzheimer's disease and occurs when glial cells undergo prolonged activation resulting in extended release of proinflammatory cytokines and nucleotides. This review describes cell-specific and tissue-integrated functions of P2 receptors in the CNS with an emphasis on P2Y(2) receptor signaling pathways in neurons, glia, and endothelium and their role in neuroprotection.

Analysis of intercellular calcium signaling using microfluidic adjustable laminar flow for localized chemical stimulation

The propagation of intercellular calcium signals provides a mechanism to coordinate cell population activity, which is essential for regulating cell behavior and organ development. However, existing analytical methods are difficult to realize localized chemical stimulation of a single cell among a population of cells that are in close contact with one another for studying the propagation of calcium wave. In this work, a microfluidic method is presented for the analysis of contact-dependent propagation of intercellular calcium wave induced by extracellular ATP using multiple laminar flows. Adjacent cells were seeded ∼300 μm downstream the intersection of a Y-shaped microchannel with negative pressure pulses. Consequently, the lateral diffusion distance of the chemical at cell locations was limited to ∼26 μm with a total flow rate of 20 μL min(-1), which prevented the interference of diffusion-induced cellular responses. Localized stimulation of the target cell with ATP induced the propagation of intercellular calcium wave among the cell population. In addition, studies on the spread of intercellular calcium wave under octanol inhibition allowed us to characterize the gap junction mediated cell-cell communication. Thus, this novel device will provide a versatile platform for intercellular signal transduction studies and high throughput drug screening.

Boranophosphate isoster controls P2Y-receptor subtype selectivity and metabolic stability of dinucleoside polyphosphate analogues

Dinucleoside polyphosphates, Np(n)N', exert their physiological effects via P2 receptors (P2Rs). Np(n)N' are attractive drug candidates as they offer better stability and specificity compared to nucleotides, the most common P2R ligands. To further improve the agonist properties of Np(n)N', we synthesized novel isosters of dinucleoside polyphosphates where N and N' are A or U and where the Pα or Pβ phosphate groups are replaced by boranophosphate, denoted as Np(n)(α-B)N' or Np(n)(β-B)N' (n = 3, 4), respectively. The potency of Np(n)(α/β-B)N' analogues was evaluated at tP2Y(1), hP2Y(2), hP2Y(4), and rP2Y(6) receptors. The most potent P2Y(1)R and P2Y(6)R agonists were the Up(4)(β-B)A (A isomer, EC(50) of 0.5 μM vs 0.004 μM for 2-SMe-ADP) and Up(3)(α-B)U (B isomer, EC(50) of 0.3 μM vs 0.2 μM for UDP), respectively. The receptor subtype selectivity is controlled by the position of the borano moiety on the Np(n)N' polyphosphate chain and the type of the nucleobase. In addition, Np(n)(α/β-B)N' proved ∼22-fold more resistant to hydrolysis by e-NPP1, as compared to the corresponding Np(n)N' analogues. In summary, Up(4)(β-B)A and Up(3)(α-B)U are potent, stable, and highly selective P2Y(1) and P2Y(6) receptor agonists, respectively.

A chemical signal generator for resolving temporal dynamics of single cells

To investigate rapid cell signaling, analytical methods are required that can generate repeatable chemical signals for stimulating live cells with high temporal resolution. Here, we present a chemical signal generator based on hydrodynamic gating, permitting flexible stimulation of single adherent cells with a temporal resolution of 20 ms. Studies of adenosine triphosphate (ATP)-induced calcium signaling in HeLa cells were demonstrated using this developed method. Consecutive treatment of the cells with ATP pulses of 20 or 1 s led to an increase of latency, which might be another indicator of receptor desensitization in addition to the decrease in the amplitude of calcium spikes. With increasing duration of ATP pulses from milliseconds to a few seconds, the cellular responses transitioned from single calcium spikes to calcium oscillation gradually. We expected this method to open up a new avenue for potential investigation of rapid cell signaling.

Arsenic alters ATP-dependent Ca²+ signaling in human airway epithelial cell wound response

Arsenic is a natural metalloid toxicant that is associated with occupational inhalation injury and contaminates drinking water worldwide. Both inhalation of arsenic and consumption of arsenic-tainted water are correlated with malignant and nonmalignant lung diseases. Despite strong links between arsenic and respiratory illness, underlying cell responses to arsenic remain unclear. We hypothesized that arsenic may elicit some of its detrimental effects on the airway through limitation of innate immune function and, specifically, through alteration of paracrine ATP (purinergic) Ca²+ signaling in the airway epithelium. We examined the effects of acute (24 h) exposure with environmentally relevant levels of arsenic (i.e., < 4 μM as Na-arsenite) on wound-induced Ca²+ signaling pathways in human bronchial epithelial cell line (16HBE14o-). We found that arsenic reduces purinergic Ca²+ signaling in a dose-dependent manner and results in a reshaping of the Ca²+ signaling response to localized wounds. We next examined arsenic effects on two purinergic receptor types: the metabotropic P2Y and ionotropic P2X receptors. Arsenic inhibited both P2Y- and P2X-mediated Ca²+ signaling responses to ATP. Both inhaled and ingested arsenic can rapidly reach the airway epithelium where purinergic signaling is essential in innate immune functions (e.g., ciliary beat, salt and water transport, bactericide production, and wound repair). Arsenic-induced compromise of such airway defense mechanisms may be an underlying contributor to chronic lung disease.

A novel insulin secretagogue based on a dinucleoside polyphosphate scaffold

Dinucleoside polyphosphates exert their physiological effects via P2 receptors (P2Rs). They are attractive drug candidates, as they offer better stability and specificity compared to nucleotides, the most common P2 receptor ligands. The activation of pancreatic P2Y receptors by nucleotides increases insulin secretion. Therefore, in the current study, dinucleoside polyphosphate analogues (di-(2-MeS)-adenosine-5',5''-P(1),P(4),alpha,beta-methylene-tetraphosphate), 8, (di-(2-MeS)-adenosine-5',5''-P(1),P(4),beta,gamma-methylene-tetraphosphate), 9, and di-(2-MeS)-adenosine-5',5''-P(1),P(3),alpha,beta-methylene triphosphate, 10, were developed as potential insulin secretagogues. Analogues 8 and 9 were found to be agonists of the P2Y(1)R with EC(50) values of 0.42 and 0.46 microM, respectively, whereas analogue 10 had no activity. Analogues 8-10 were found to be completely resistant to hydrolysis by alkaline phosphatase over 3 h at 37 degrees C. Analogue 8 also was found to be 2.5-fold more stable in human blood serum than ATP, with a half-life of 12.1 h. Analogue 8 administration in rats caused a decrease in a blood glucose load from 155 mg/dL to ca. 100 mg/dL and increased blood insulin levels 4-fold as compared to basal levels. In addition, analogue 8 reduced a blood glucose load to normal values (80-110 mg/dL), unlike the commonly prescribed glibenclamide, which reduced glucose levels below normal values (60 mg/dL). These findings suggest that analogue 8 may prove to be an effective and safe treatment for type 2 diabetes.

2-MeS-beta,gamma-CCl2-ATP is a potent agent for reducing intraocular pressure

Extracellular nucleotides can modify the production or drainage of the aqueous humor via activation of P2 receptors and therefore affect the intraocular pressure (IOP). We have synthesized slowly hydrolyzable nucleoside di- and triphosphate analogues, 1, and 8-14. Analogues 8-14 were completely resistant to hydrolysis by alkaline phosphatase over 30 min at 37 degrees C. In human blood serum, analogues 8-14 exhibited high stability, e.g., analogues 9 and 10-14 were only 15% and 0% degraded after 24 h, respectively. Moreover, analogues 8-14 were highly stable at pH 1.4 (t(1/2) 1 h-30 days). Analogues 8-14 were agonists of the P2Y(1) receptor (EC(50) 0.57-9.54 muM). Ocular administration of most analogues into rabbits reduced IOP, e.g., analogue 9 reduced IOP by 32% (EC(50) 95.5 nM). Analogue 9 was more effective at reducing IOP than several common glaucoma drugs and represents a promising alternative to timolol maleate, which cannot be used for the treatment of patients suffering from asthma or cardiac problems.

Modeling species-specific diacylglycerol dynamics in the RAW 264.7 macrophage

A mathematical model of the G protein signaling pathway in RAW 264.7 macrophages downstream of P2Y(6) receptors activated by the ubiquitous signaling nucleotide uridine 5'-diphosphate is developed. The model, which is based on time-course measurements of inositol trisphosphate, cytosolic calcium, and diacylglycerol, focuses particularly on differential dynamics of multiple chemical species of diacylglycerol. When using the canonical pathway representation, the model predicted that key interactions were missing from the current network structure. Indeed, the model suggested that accurate depiction of experimental observations required an additional branch to the signaling pathway. An intracellular pool of diacylglycerol is immediately phosphorylated upon stimulation of an extracellular receptor for uridine 5'-diphosphate and subsequently used to aid replenishment of phosphatidylinositol. As a result of sensitivity analysis of the model parameters, key predictions can be made regarding which of these parameters are the most sensitive to perturbations and are therefore most responsible for output uncertainty.

Key determinants of nucleotide-activated G protein-coupled P2Y(2) receptor function revealed by chemical and pharmacological experiments, mutagenesis and homology modeling

The P2Y(2) receptor, which is activated by UTP, ATP, and dinucleotides, was studied as a prototypical nucleotide-activated GPCR. A combination of receptor mutagenesis, determination of its effects on potency and efficacy of agonists and antagonists, homology modeling, and chemical experiments was applied. R272 (extracellular loop EL3) was found to play a gatekeeper role, presumably responsible for recognition and orientation of the nucleotides. R272 is also directly involved in binding of dinucleotides, which behaved as partial agonists. Y118A (3.37) mutation led to dramatically reduced efficacy of agonists; it is part of the entry channel as well as the triphosphate binding site. While the Y114A (3.33) mutation did not have any effect on agonist activities, the antagonist Reactive Blue 2 (6) was completely inactive at that mutant. The disulfide bridge Cys25-Cys278 was found to be important for agonist potency but neither for agonist efficacy nor for antagonist potency.

Identification of hydrolytically stable and selective P2Y(1) receptor agonists

P2Y nucleotide receptors (P2YRs) are attractive pharmaceutical targets. Most P2YR agonists proposed as drugs consist of a nucleotide scaffold, but their use is limited due to their chemical and enzymatic instabilities. To identify drug candidates, we developed non-hydrolyzable P2YR agonists. We synthesized ATP-beta,gamma-CH(2) analogues 2-4, and evaluated their chemical and metabolic stabilities and activities at P2Y(1,2,4,6) receptors. Analogues 2-4 exhibited t(1/2) values of 14.5-65 h in gastric juice pH. They were completely resistant to alkaline phosphatase for 30 min at 37 degrees C and slowly hydrolyzed in human blood serum (t(1/2) 12.7-71.9 h). In comparison to ATP, analogues 2-4 were barely hydrolyzed by nucleoside triphosphate diphosphohydrolases, NTPDase1,2,3,8 (< 8% hydrolysis), and nucleotide pyrophosphatases, NPP1,3 (< or = 10% hydrolysis). Analogues 2 and 4B were selective agonists of the P2Y(1)R with EC(50)s of 0.08 and 17.2 microM, respectively. These features make analogues 2 and 4B potential therapeutic agents for health disorders involving the P2Y(1)R.

Agonist-selective, receptor-specific interaction of human P2Y receptors with beta-arrestin-1 and -2

Interaction of G-protein-coupled receptors with beta-arrestins is an important step in receptor desensitization and in triggering "alternative" signals. By means of confocal microscopy and fluorescence resonance energy transfer, we have investigated the internalization of the human P2Y receptors 1, 2, 4, 6, 11, and 12 and their interaction with beta-arrestin-1 and -2. Co-transfection of each individual P2Y receptor with beta-arrestin-1-GFP or beta-arrestin-2-YFP into HEK-293 cells and stimulation with the corresponding agonists resulted in a receptor-specific interaction pattern. The P2Y(1) receptor stimulated with ADP strongly translocated beta-arrestin-2-YFP, whereas only a slight translocation was observed for beta-arrestin-1-GFP. The P2Y(4) receptor exhibited equally strong translocation for beta-arrestin-1-GFP and beta-arrestin-2-YFP when stimulated with UTP. The P2Y(6), P2Y(11), and P2Y(12) receptor internalized only when GRK2 was additionally co-transfected, but beta-arrestin translocation was only visible for the P2Y(6) and P2Y(11) receptor. The P2Y(2) receptor showed a beta-arrestin translocation pattern that was dependent on the agonist used for stimulation. UTP translocated beta-arrestin-1-GFP and beta-arrestin-2-YFP equally well, whereas ATP translocated beta-arrestin-1-GFP to a much lower extent than beta-arrestin-2-YFP. The same agonist-dependent pattern was seen in fluorescence resonance energy transfer experiments between the fluorescently labeled P2Y(2) receptor and beta-arrestins. Thus, the P2Y(2) receptor would be classified as a class A receptor when stimulated with ATP or as a class B receptor when stimulated with UTP. The ligand-specific recruitment of beta-arrestins by ATP and UTP stimulation of P2Y(2) receptors was further found to result in differential stimulation of ERK phosphorylation. This suggests that the two different agonists induce distinct active states of this receptor that show differential interactions with beta-arrestins.

Binding of the P2Y2 nucleotide receptor to filamin A regulates migration of vascular smooth muscle cells

The functional expression of the G protein-coupled P2Y(2) nucleotide receptor (P2Y(2)R) has been associated with proliferation and migration of vascular smooth muscle cells (SMCs), two processes involved in atherosclerosis and restenosis. Activation of the P2Y(2)R causes dynamic reorganization of the actin cytoskeleton, which transmits biochemical signals and forces necessary for cell locomotion, suggesting that P2Y(2)Rs may be linked to the actin cytoskeleton. Here, we identified filamin A (FLNa) as a P2Y(2)R-interacting protein using a yeast 2-hybrid system screen with the C-terminal region of the P2Y(2)R as bait. The FLNa binding site in the P2Y(2)R is localized between amino acids 322 and 333. Deletion of this region led to selective loss of FLNa binding to the P2Y(2)R and abolished Tyr phosphorylation of FLNa induced by the P2Y(2)R agonist UTP. Using both time-lapse microscopy and the Transwell cell migration assay, we showed that UTP significantly increased SMC spreading on collagen I (6.8 fold; P < or = 0.01) and migration (3.6 fold; P < or = 0.01) of aortic SMCs isolated from wild-type mice, as compared with unstimulated SMCs. UTP-induced spreading and migration of aortic SMCs did not occur with cells isolated from P2Y(2)R knockout mice. Expression of the full-length P2Y(2)R in SMCs isolated from P2Y(2)R knockout mice restored both UTP-induced spreading and migration. In contrast, UTP-induced spreading and migration did not occur in SMCs isolated from P2Y(2)R knockout mice transfected with a mutant P2Y(2)R that does not bind FLNa. Furthermore, ex vivo studies showed that both ATP and UTP (10 micromol/L) promoted migration of SMCs out of aortic explants isolated from wild-type but not P2Y(2)R knockout mice. Thus, this study demonstrates that P2Y(2)R/FLNa interaction selectively regulates spreading and migration of vascular SMCs.

P2Y2 nucleotide receptor activation up-regulates vascular cell adhesion molecule-1 [corrected] expression and enhances lymphocyte adherence to a human submandibular gland cell line

Sjögren's syndrome (SS) is a chronic inflammatory autoimmune disease that causes salivary and lacrimal gland tissue destruction resulting in impaired secretory function. Although lymphocytic infiltration of salivary epithelium is associated with SS, the mechanisms involved have not been adequately elucidated. Our previous studies have shown that the G protein-coupled P2Y2 nucleotide receptor (P2Y2R) is up-regulated in response to damage or stress of salivary gland epithelium, and in salivary glands of the NOD.B10 mouse model of SS-like autoimmune exocrinopathy. Additionally, we have shown that P2Y2R activation up-regulates vascular cell adhesion molecule-1 (VCAM-1) expression in endothelial cells leading to the binding of monocytes. The present study demonstrates that activation of the P2Y2R in dispersed cell aggregates from rat submandibular gland (SMG) and in human submandibular gland ductal cells (HSG) up-regulates the expression of VCAM-1. Furthermore, P2Y2R activation mediated the up-regulation of VCAM-1 expression in HSG cells leading to increased adherence of lymphocytic cells. Inhibitors of EGFR phosphorylation and metalloprotease activity abolished P2Y2R-mediated VCAM-1 expression and decreased lymphocyte binding to HSG cells. Moreover, silencing of EGFR expression abolished UTP-induced VCAM-1 up-regulation in HSG cells. These results suggest that P2Y2R activation in salivary gland cells increases the EGFR-dependent expression of VCAM-1 and the binding of lymphocytes, a pathway relevant to inflammation associated with SS.

A kinetic model for calcium dynamics in RAW 264.7 cells: 2. Knockdown response and long-term response

This article addresses how quantitative models such as the one proposed in the companion article can be used to study cellular network perturbations such as knockdowns and pharmacological perturbations in a predictive manner. Using the kinetic model for cytosolic calcium dynamics in RAW 264.7 cells developed in the companion article, the calcium response to complement 5a (C5a) for the knockdown of seven proteins (C5a receptor; G-beta-2; G-alpha,i-2,3; regulator of G-protein signaling-10; G-protein coupled receptor kinase-2; phospholipase C beta-3; arrestin) is predicted and validated against the data from the Alliance for Cellular Signaling. The knockdown responses provide insights into how altered expressions of important proteins in disease states result in intermediate measurable phenotypes. Long-term response and long-term dose response have also been predicted, providing insights into how the receptor desensitization, internalization, and recycle result in tolerance. Sensitivity analysis of long-term response shows that the mechanisms and parameters in the receptor recycle path are important for long-term calcium dynamics.

Immobilized P2X2 purinergic receptor stationary phase for chromatographic determination of pharmacological properties and drug screening

The purinergic receptor signaling system plays an important role in communication between cells in the nervous system and opens new opportunities for screening of potential drugs. Our objective was to explore the pharmacological properties and establish a new methodology for ligand screening for the P2X2 receptor, which has been developed by the combinatorial library approach Systematic Evolution of Ligands by Exponential enrichment (SELEX). To this end, membranes of 1321N1 cells stably transfected with rat P2X2 receptors were resuspended in 2% cholate detergent and subsequently coupled onto an immobilized artificial membrane (IAM). The IAM-cholate-P2X2 mixture was then dialyzed, centrifuged and packed into a FPLC column. Equilibrium binding to the receptor and competition between ATP and the purinergic antagonists suramin and 2'3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) were analyzed by a chromatographic assay using 32P alpha ATP as a radioligand. Our data indicate that suramin does not compete with ATP for the ligand binding site and TNP-ATP is a competitive antagonist, confirming previous studies [C.A. Trujillo, A.A. Nery, A.H. Martins, P. Majumder, F.A. Gonzalez, H. Ulrich, Biochemistry 45 (2006) 224-233]. In addition, we demonstrate that this assay can be used in in vitro selection procedures for RNA aptamers binding to P2X2 receptors. The results demonstrate that the receptor can be immobilized in a stable format and reused over an extended period of time, facilitating the exploration of ligand-receptor interactions and screening of combinatorial pools for possible ligands.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Agonist-induced phosphorylation and desensitization of the P2Y2 nucleotide receptor

Purification of HA-tagged P2Y2 receptors from transfected human 1321N1 astrocytoma cells yielded a protein with a molecular size determined by SDS-PAGE to be in the range of 57-76 kDa, which is typical of membrane glycoproteins with heterogeneous complex glycosylation. The protein phosphatase inhibitor, okadaic acid, attenuated the recovery of receptor activity from the agonist-induced desensitized state, suggesting a role for P2Y2 receptor phosphorylation in desensitization. Isolation of HA-tagged P2Y2 nucleotide receptors from metabolically [32P]-labelled cells indicated a (3.8 +/- 0.2)-fold increase in the [32P]-content of the receptor after 15 min of treatment with 100 microM UTP, as compared to immunoprecipitated receptors from untreated control cells. Receptor sequestration studies indicated that approximately 40% of the surface receptors were internalized after a 15-min stimulation with 100 microM UTP. Point mutation of three potential GRK and PKC phosphorylation sites in the third intracellular loop and C-terminal tail of the P2Y2 receptor (namely, S243A, T344A, and S356A) extinguished agonist-induced receptor phosphorylation, caused a marked reduction in the efficacy of UTP to desensitize P2Y2 receptor signalling to intracellular calcium mobilization, and impaired agonist-induced receptor internalization. Activation of PKC isoforms with phorbol 12-myristate 13-acetate that caused heterologous receptor desensitization did not increase the level of P2Y2 receptor phosphorylation. Our results indicate a role for receptor phosphorylation by phorbol-insensitive protein kinases in agonist-induced desensitization of the P2Y2 nucleotide receptor.

Inhibition mechanism of the recombinant rat P2X(2) receptor in glial cells by suramin and TNP-ATP

P2X receptors play an important role in communication between cells in the nervous system. Therefore, understanding the mechanisms of inhibition of these receptors is important for the development of new tools for drug discovery. Our objective has been to determine the pharmacological activity of the antagonist suramin, the most important antagonist of purinergic receptor function, as well as to demonstrate its noncompetitive inhibition and confirm a competitive mechanism between ATP and TNP-ATP in 1321N1 glial cells stably transfected with the recombinant rat P2X(2) receptor. A radioligand binding assay was employed to determine whether suramin, TNP-ATP, and ATP compete for the same binding site on the receptor. TNP-ATP displaced [alpha-32P]ATP, whereas suramin did not interfere with [alpha-32P]ATP-receptor binding. To determine the inhibition mechanism relevant for channel opening, currents obtained in fast kinetic whole-cell recording experiments, following stimulation of cells by ATP in the presence of suramin, were compared to those obtained by ATP in the presence of TNP-ATP. Supported by a mathematical model for receptor kinetics [Breitinger, H. G., Geetha, N., and Hess, G. P. (2001) Biochemistry 40, 8419-8429], the inhibition factors were plotted as functions of inhibitor or agonist concentrations. Analysis of the data indicated a competitive inhibition mechanism for TNP-ATP and a noncompetitive inhibition for suramin. Taken together, both data support a noncompetitive inhibition mechanism of the rat recombinant P2X(2) receptor by suramin, confirm the competitive inhibition by TNP-ATP, and allow the prediction of a model for P2X(2) receptor inhibition.

P2 receptors: intracellular signaling

P2 receptors for extracellular nucleotides are divided into two categories: the ion channel receptors (P2X) and the G-protein-coupled receptors (P2Y). For the P2X receptors, signal transduction appears to be relatively simple. Upon activation by extracellular ATP, a channel comprised of P2X receptor subunits opens and allows cations to move across the plasma membrane, resulting in changes in the electrical potential of the cell that, in turn, propagates a signal. This regulated flux of ions across the plasma membrane has important signaling functions, especially in impulse propagation in the nervous system and in muscle contractility. In addition, P2X receptor activation causes the accumulation of calcium ions in the cytoplasm, which is responsible for activating numerous signaling molecules. For the P2Y receptors, signal transduction is more complex. Intracellular signaling cascades are the main routes of communication between G-protein-coupled receptors and regulatory targets within the cell. These signaling cascades operate mainly by the sequential activation or deactivation of heterotrimeric and monomeric G proteins, phospholipases, protein kinases, adenylyl and guanylyl cyclases, and phosphodiesterases that regulate many cellular processes, including proliferation, differentiation, apoptosis, metabolism, secretion, and cell migration. In addition, there are numerous ion channels, cell adhesion molecules and receptor tyrosine kinases that are modulated by P2Y receptors and operate to transmit an extracellular signal to an intracellular response. These intracellular signaling pathways and their regulation by P2 receptors are discussed in this review.

Mechanisms for inhibition of P2 receptors signaling in neural cells

Trophic factors are required to ensure neuronal viability and regeneration after neural injury. Although abundant information is available on the factors that cause the activation of astrocytes, little is known about the molecular mechanisms underlying the regulation of this process. Nucleotides released into the extracellular space from injured or dying neural cells can activate astrocytes via P2 nucleotide receptors. After a brief historical review and update of novel P2 receptor antagonists, this article focuses on recent advancements toward understanding molecular mechanisms that regulate G protein-coupled P2Y receptor signaling. Among P2Y receptor subtypes, the heptahelical P2Y2 nucleotide receptor interacts with vitronectin receptors via an RGD sequence in the first extracellular loop, and this interaction is required for effective signal transduction to activate mitogen-activated protein kinases ERK1/2, to mobilize intracellular calcium stores via activation of phospholipase C, protein kinase C isoforms, and to activate focal adhesion kinase and other signaling events. Ligation of vitronectin receptors with specific antibodies caused an inhibition of P2Y2 receptor-induced ERK1/2 and p38 phosphorylation and P2Y2 receptor-induced cytoskeleton rearrangement and DNA synthesis. Structure-function studies have identified agonist-induced phosphorylation of the C-terminus of the P2Y2 receptor, an important mechanism for receptor desensitization. Understanding selective mechanisms for regulating P2Y2 receptor signaling could provide novel targets for therapeutic strategies in the management of brain injury, synaptogenesis, and neurological disorders.

Internalization and desensitization of a green fluorescent protein-tagged P2Y nucleotide receptor are differently controlled by inhibition of calmodulin-dependent protein kinase II

De- and re-sensitization and trafficking of P2Y nucleotide receptors modulate physiological responses of these receptors. Here, we used the rat brain P2Y1 receptor tagged with green fluorescent protein (P2Y1-GFP receptor) expressed in HEK293 human embryonic kidney cells. Ca2+ release was used as a functional test to investigate ATP-induced receptor de- and re-sensitization. By confocal laser scanning microscopy (CLSM), endocytosis of P2Y1-GFP receptor was visualized in live cells. Stimulation of the cells with ATP induced complete receptor endocytosis within 30 min and appearance of the P2Y1 receptor in small vesicles. Removal of the agonist resulted in reappearance of the receptor after 60 min on the plasma membrane. Exposure of the cells to KN-62 and KN-93, inhibitors of the calmodulin dependent protein kinase II (CaMKII), prevented receptor internalization upon stimulation with ATP. However, the receptor which was still present on the plasma membrane was desensitized, seen by decreased Ca2+ response. The decreased Ca2+ response after 30-min exposure to ATP can be attributed to desensitization and is not as a result of depletion of internal stores, as the cells exposed to ATP for 30 min exhibited a normal Ca2+ response upon stimulation with thrombin. However, okadaic acid, an inhibitor of protein phosphatase 2A (PP2A), did not affect ATP-induced P2Y1 receptor endocytosis, but delayed the reappearance of the P2Y1 receptor on the plasma membrane after ATP withdrawal. Consistently, in okadaic acid-treated cells the ATP-induced Ca2+ response observed after the 30-min exposure to ATP recovered only partially. Thus, CaMKII seems to be involved in P2Y1 receptor internalization, but not desensitization, whereas protein phosphatase 2A might play a role in recycling of the receptor back to the plasma membrane.

The priming effect of extracellular UTP on human neutrophils: Role of calcium released from thapsigargin-sensitive intracellular stores

P2Y₂ receptors, which are equally responsive to ATP and UTP, can trigger intracellular signaling events, such as intracellular calcium mobilization and mitogen-activated protein (MAP) kinase phosphorylation in polymorphonuclear leukocytes (PMN). Moreover, extracellular nucleotides have been shown to prime chemoattractant-induced superoxide production. The aim of our study was to investigate the mechanism responsible for the priming effect of extracellular nucleotides on reactive oxygen species (ROS) production induced in human neutrophils by two different chemoattractants: formyl-methionyl-leucyl-phenylalanine (fMLP) and interleukin-8 (IL-8). Nucleotide-induced priming of ROS production was concentration- and time-dependent. When UTP was added to neutrophil suspensions prior to chemoattractant, the increase of the response reached the maximum at 1 min of pre-incubation with the nucleotide. UTP potentiated the phosphorylation of p44/42 and p38 MAP kinases induced by chemoattractants, however the P2 receptor-mediated potentiation of ROS production was still detectable in the presence of a SB203580 or U0126, supporting the view that MAP kinases do not play a major role in regulating the nucleotide-induced effect. In the presence of thapsigargin, an inhibitor of the ubiquitous sarco-endoplasmic reticulum Ca²⁺-ATPases in mammalian cells, the effect of fMLP was not affected, but UTP-induced priming was abolished, suggesting that the release of calcium from thapsigargin-sensitive intracellular stores is essential for nucleotide-induced priming in human neutrophils.

Apical membrane P2Y4 purinergic receptor controls K+ secretion by strial marginal cell epithelium

Background

It was previously shown that K⁺ secretion by strial marginal cell epithelium is under the control of G-protein coupled receptors of the P2Y family in the apical membrane. Receptor activation by uracil nucleotides (P2Y₂, P2Y₄ or P2Y₆) leads to a decrease in the electrogenic K⁺ secretion. The present study was conducted to determine the subtype of the functional purinergic receptor in gerbil stria vascularis, to test if receptor activation leads to elevation of intracellular [Ca²⁺] and to test if the response to these receptors undergoes desensitization.

Results

The transepithelial short circuit current (Isc) represents electrogenic K⁺ secretion and was found to be decreased by uridine 5'-triphosphate (UTP), adenosine 5'-triphosphate (ATP) and diadenosine tetraphosphate (Ap4A) but not uridine 5'-diphosphate (UDP) at the apical membrane of marginal cells of the gerbil stria vascularis. The potencies of these agonists were consistent with rodent P2Y₄ and P2Y₂ but not P2Y₆ receptors. Activation caused a biphasic increase in intracellular [Ca²⁺] that could be partially blocked by 2-aminoethoxy-diphenyl borate (2-APB), an inhibitor of the IP3 receptor and store-operated channels. Suramin (100 μM) did not inhibit the effect of UTP (1 μM). The ineffectiveness of suramin at the concentration used was consistent with P2Y₄ but not P2Y₂. Transcripts for both P2Y₂ and P2Y₄ were found in the stria vascularis. Sustained exposure to ATP or UTP for 15 min caused a depression of Isc that appeared to have two components but with apparently no chronic desensitization.

Conclusion

The results support the conclusion that regulation of K⁺ secretion across strial marginal cell epithelium occurs by P2Y₄ receptors at the apical membrane. The apparent lack of desensitization of the response is consistent with two processes: a rapid-onset phosphorylation of KCNE1 channel subunit and a slower-onset of regulation by depletion of plasma membrane PIP₂.

The effects of extracellular purines and pyrimidines on human airway smooth muscle cells

Extracellular ATP and UTP modulate the function of many cell types through the stimulation of specific P2 receptors, and the inhalation of UTP has been proposed as a therapeutic means of increasing mucociliary clearance in cystic fibrosis patients. The aim of this study was to determine whether P2 receptors are present and functional in human airway smooth muscle (HASM) cells. Experiments were conducted on primary cultures of HASM cells. Reverse transcription-polymerase chain reaction and Western blot analysis showed that P2Y(1), P2Y(2), P2Y(4), and P2Y(6) receptor subtypes are expressed. Exposure to extracellular ATP, UTP, ADP, and UDP at concentrations ranging from 10(-6) to 10(-4) M, produced significant increases in intracellular Ca(2+) that peaked to 491 +/- 51 nM (p < 0.001) with ATP 10(-5) M and to 321 +/- 30 nM with UTP 10(-4) M. ATP and UTP also induced HASM cell contraction, decreasing cell length by 9.9 +/- 4.3 and 5.6 +/- 2.0%, respectively. Pretreatment of the cells with UTP for short periods of time (10 and 30 min) enhanced the peak Ca(2+) release to UTP, whereas repeated and prolonged pretreatment with UTP decreased it. These results indicate that several subtypes of P2Y receptors are present and functional in HASM cells. They also show that the response of the receptors is increased after short periods of exposure to UTP and decreased after prolonged and repeated exposure. Considering that ATP and UTP are endogenous mediators and that analogs of UTP could be used as a therapeutic modality, the role of extracellular triphosphate nucleotides in physiological and pathophysiological processes in the airways warrants further investigation.

The P2Y2 nucleotide receptor interacts with alphav integrins to activate Go and induce cell migration

Extracellular ATP and UTP induce chemotaxis, or directed cell migration, by stimulating the G protein-coupled P2Y(2) nucleotide receptor (P2Y(2)R). Previously, we found that an arginine-glycine-aspartic acid (RGD) integrin binding domain in the P2Y(2)R enables this receptor to interact selectively with alpha(v)beta(3) and alpha(V)beta(5) integrins, an interaction that is prevented by mutation of the RGD sequence to arginine-glycine-glutamic acid (RGE) (Erb, L., Liu, J., Ockerhausen, J., Kong, Q., Garrad, R. C., Griffin, K., Neal, C., Krugh, B., Santiago-Perez, L. I., Gonzalez, F. A., Gresham, H. D., Turner, J. T., and Weisman, G. A. (2001) J. Cell Biol. 153, 491-501). This RGD domain also was found to be necessary for coupling the P2Y(2)R to G(o)- but not G(q)-mediated intracellular calcium mobilization, leading us to investigate the role of P2Y(2)R interaction with integrins in nucleotide-induced chemotaxis. Here we show that mutation of the RGD sequence to RGE in the human P2Y(2)R expressed in 1321N1 astrocytoma cells completely prevented UTP-induced chemotaxis as well as activation of G(o), Rac, and Vav2, a guanine nucleotide exchange factor for Rac. UTP also increased expression of vitronectin, an extracellular matrix protein that is a ligand for alpha(v)beta(3)/beta(5) integrins, in cells expressing the wild-type but not the RGE mutant P2Y(2)R. P2Y(2)R-mediated chemotaxis, Rac and Vav2 activation, and vitronectin up-regulation were inhibited by pretreatment of the cells with anti-alpha(v)beta(5) integrin antibodies, alpha(v) integrin antisense oligonucleotides, or the G(i/o) inhibitor, pertussis toxin. Thus, the RGD-dependent interaction between the P2Y(2)R and alpha(v) integrins is necessary for the P2Y(2)R to activate G(o) and to initiate G(o)-mediated signaling events leading to chemotaxis.

P2Y2 nucleotide receptors enhance alpha-secretase-dependent amyloid precursor protein processing

The amyloid precursor protein (APP) is proteolytically processed by beta- and gamma-secretases to release amyloid beta, the main component in senile plaques found in the brains of patients with Alzheimer disease. Alternatively, APP can be cleaved within the amyloid beta domain by alpha-secretase releasing the non-amyloidogenic product sAPP alpha, which has been shown to have neuroprotective properties. Several G protein-coupled receptors are known to activate alpha-secretase-dependent processing of APP; however, the role of G protein-coupled nucleotide receptors in APP processing has not been investigated. Here it is demonstrated that activation of the G protein-coupled P2Y2 receptor (P2Y2R) subtype expressed in human 1321N1 astrocytoma cells enhanced the release of sAPP alpha in a time- and dose-dependent manner. P2Y2 R-mediated sAPP alpha release was dependent on extracellular calcium but was not affected by 1,2-bis(2-aminophenoxy)ethane-N,N,N,-trimethylammonium salt, an intracellular calcium chelator, indicating that P2Y2R-stimulated intracellular calcium mobilization was not involved. Inhibition of protein kinase C (PKC) with GF109203 or by PKC down-regulation with phorbol ester pre-treatment had no effect on UTP-stimulated sAPP alpha release, indicating a PKC-independent mechanism. U0126, an inhibitor of the mitogen-activated protein kinase pathway, partially inhibited sAPPalpha release by UTP, whereas inhibitors of Src-dependent epidermal growth factor receptor transactivation by P2Y2Rs had no effect. The metalloprotease inhibitors phenanthroline and TAPI-2 and the furin inhibitor decanoyl-Arg-Val-Lys-Arg-chloromethylketone also diminished UTP-induced sAPP alpha release. Furthermore, small interfering RNA silencing of an endogenous adamalysin, ADAM10 or ADAM17/TACE, partially suppressed P2Y2R-activated sAPP alpha release, whereas treatment of cells with both ADAM10 and ADAM17/TACE small interfering RNAs completely abolished UTP-activated sAPP alpha release. These results may contribute to an understanding of the non-amyloidogenic processing of APP.

A signal transduction pathway model prototype I: From agonist to cellular endpoint

The postgenomic era is providing a wealth of information about the genes involved in many cellular processes. However, the ability to apply this information to understanding cellular signal transduction is limited by the lack of tools that quantitatively describe cellular signaling processes. The objective of the current studies is to provide a framework for modeling cellular signaling processes beginning at a plasma membrane receptor and ending with a measurable endpoint in the signaling process. Agonist-induced Ca(2+) mobilization coupled to down stream phosphorylation events was modeled using knowledge of in vitro and in vivo process parameters. The simulation process includes several modules that describe cellular processes involving receptor activation phosphoinositide metabolism, Ca(2+)-release, and activation of a calmodulin-dependent protein kinase. A Virtual Cell-based simulation was formulated using available literature data and compared to new and existing experimental results. The model provides a new approach to facilitate hypothesis-driven investigation and experimental design based upon simulation results. These investigations may be directed at the timing of multiple phosphorylation/dephosphorylation events affecting key enzymatic activities in the signaling pathway.

Calcium mobilization and spontaneous transient outward current characteristics upon agonist activation of P2Y2 receptors in smooth muscle cells

A quantitative model is provided that links the process of metabotropic receptor activation and sequestration to the generation of inositol 1,4,5-trisphosphate, the subsequent release of calcium from the central sarcoplasmic reticulum, and the consequent release of calcium from subsarcolemma sarcoplasmic reticulum that acts on large-conductance potassium channels to generate spontaneous transient outward currents (STOCs). This model is applied to the case of STOC generation in vascular A7r5 smooth muscle cells that have been transfected with a chimera of the P2Y(2) metabotropic receptor and green fluorescent protein (P2Y(2)-GFP) and exposed to the P2Y(2) receptor agonist uridine 5'-triphosphate. The extent of P2Y(2)-GFP sequestration from the membrane on exposure to uridine 5'-triphosphate, the ensuing changes in cytosolic calcium concentration, as well as the interval between STOCs that are subsequently generated, are used to determine parameter values in the model. With these values, the model gives a good quantitative prediction of the dynamic changes in STOC amplitude observed upon activation of metabotropic P2Y(2) receptors in the vascular smooth muscle cell line.

A quantitative description of the contraction of blood vessels following the release of noradrenaline from sympathetic varicosities

A model is presented that highlights the principal factors determining the form and extent of contraction in arteries upon stimulation of their sympathetic nerve supply. This model incorporates a previous quantitative model of the process of noradrenaline (NAd) diffusion into the vascular media and reuptake into sympathetic varicosities during nerve stimulation (J. Theor. Biol. 226 (2004) 359). It is also dependent on a model of how the subsequent activation of metabotropic receptors initiates a G-protein cascade, resulting in the production of inositol trisphosphate (IP3) and an increase in intracellular calcium concentration, [Ca2+]i, in the smooth muscle cells (J. Theor. Biol. 223 (2003) 93). In the present work we couple this rise in [Ca2+]i to the increase in phosphorylated myosin bound to actin in the cells and hence determine the force development in arteries due to nerve stimulation. The model accounts for force development as a function of [Ca2+]i and for the rate of change of force as a function of the rate of change of [Ca2+]i in single smooth muscle cells. It also accounts for the characteristic time course of the force developed by the media of the rat-tail artery upon nerve stimulation. This consists of a rapid rise to a transient peak followed by a sustained plateau of contraction during the stimulation period, after which the contraction slowly decays back to baseline at a rate dependent on the strength of the stimulation. The model indicates that the transient peak is primarily due to the partial block of the IP3 receptor by the rise in [Ca2+]i and that the main determinant of the equilibrium condition indicated by the plateau phase is the rate of pumping of calcium into the sarcoplasmic reticulum. The relatively slow decline of contraction at the end of nerve stimulation is primarily a consequence of the slow rates of removal of NAd from the media by diffusion and reuptake into the sympathetic varicosities. The model thus provides a quantitative account of vascular smooth muscle contraction upon sympathetic nerve stimulation.

The NK1 receptor localizes to the plasma membrane microdomains, and its activation is dependent on lipid raft integrity

The spatial targeting of receptors to discrete domains within the plasma membrane allows their preferential coupling to specific effectors, which is essential for rapid and accurate discrimination of signals. Efficiency of signaling is further increased by protein and lipid segregation within the plasma membrane. We have previously demonstrated the importance of raft-mediated signaling in the regulation of smooth and skeletal muscle cell contraction. Since G protein-coupled receptors (GPCRs) are key components in the regulation of smooth muscle contraction-relaxation cycles, it is important to determine whether GPCR signaling is mediated by lipid rafts and raft-associated molecules. Neurokinin 1 receptor (NK1R) is expressed in central and peripheral nervous system as well as in endothelial and smooth muscle cells and involved in mediation of pain, inflammation, exocrine secretion, and smooth muscle contraction. The NK1 receptor was transiently expressed in HEK293 and HepG2 cell lines and its localization in membrane microdomains investigated using biochemical methods and immunofluorescent labeling. We show that the NK1 receptor, similar to the earlier described beta(2)-adrenergic receptor and G proteins, localizes to lipid rafts and caveolae. Protein kinase C (PKC) is one of the downstream effectors of the NK1 activation. Its active form translocates from the cytoplasm to the plasma membrane. Upon stimulation of the NK1 receptor with Substance P, the activated PKC relocated to lipid rafts. Using cholesterol extraction and replenishment assays we show that activation of NK1 receptor is dependent on the microarchitecture of the plasma membrane: NK1R-mediated signaling was abolished after cholesterol depletion of the receptor-expressing cells with methyl-beta-cyclodextrin. Our results demonstrate that reorganization of the plasma membrane has an effect on the activation of the raft-associated NK1R and the down-stream events such as recruitment of protein kinases.

alpha,beta-MeATP augments the UTP contraction of rabbit basilar artery

The mechanism underlying the interaction between alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-MeATP) and uridine 5'-triphosphate (UTP) was investigated using the basilar artery of a rabbit. UTP induced a concentration-dependent contraction, whereas P2X receptor agonists, such as alpha,beta-MeATP and 2-methylthioadenosine 5'-triphosphate (2-MeSATP), did not induce any contraction up to 100 microM. alpha,beta-MeATP augmented the UTP contraction two-fold, immediately and reversibly. This effect was observed with ectonucleotidase inhibition with 1 mM Ni(2+), the removal of extracellular Ca(2+) or Evans blue. The contractile response to adenosine 5'-O-(3-triphosphate) (ATPgammaS), a selective agonist for P2Y(4), was augmented by pretreatment with alpha,beta-MeATP also. ATPgammaS had no additional effect on the UTP contraction fully activated with alpha,beta-MeATP. UTP (100 microM) did not induce an increase in cytosolic Ca(2+) in a rabbit basilar arterial strip; however, in the presence of 1 mM alpha,beta-MeATP, UTP induced a significant increase in cytosolic Ca(2+). These results suggest that alpha,beta-MeATP facilitates the activation by UTP of the P2Y receptor (P2Y(4)) of the rabbit basilar artery through mechanisms other than nucleotidase inhibition, and that it does not do so via a P2X receptor.

Arg333 and Arg334 in the COOH terminus of the human P2Y1 receptor are crucial for Gq coupling

The P2Y(1) ADP receptor activates G(q) and causes increases in intracellular Ca(2+) concentration through stimulation of PLC. In this study, we investigated the role of the amino acid residues in the COOH terminus of the human P2Y(1) receptor in G(q) activation. Stimulation of Chinese hamster ovary (CHO-K1) cells stably expressing the wild-type human P2Y(1) receptor (P2Y(1)-WT cells), P2Y(1)-DeltaR340-L373, or P2Y(1)-DeltaD356-L373 with 2-methylthio-ADP (2-MeSADP) caused inositol phosphate production. In contrast, cells expressing P2Y(1)-DeltaT330-L373, a mutant lacking the entire COOH terminus, completely lost their response to 2-MeSADP. Similar data were obtained by using these cell lines and measuring Ca(2+) mobilization upon stimulation with 2-MeSADP, indicating that the 10 amino acids (330TFRRRLSRAT339) in the COOH terminus of the human P2Y(1) receptor are essential for G(q) coupling. Radioligand binding demonstrated that both the P2Y(1)-WT and P2Y(1)-DeltaT330-L373-expressing cells have almost equal binding of [(3)H]MRS2279, a P2Y(1) receptor antagonist, indicating that COOH-terminal truncation did not drastically affect the conformation of the receptor. CHO-K1 cells expressing a chimeric P2Y(12) receptor with the P2Y(1) COOH terminus failed to elicit G(q) functional responses, indicating that the P2Y(1) COOH terminus is essential but not sufficient for G(q) activation. Finally, cells expressing a double-mutant P2Y(1) receptor (R333A/R334A) in the conserved BBXXB region of the COOH terminus of the G(q)-activating P2Y receptors completely lost their functional ability to activate G(q). We conclude that the two arginine residues (R333R334) in the COOH terminus of the human P2Y(1) receptor are essential for G(q) coupling.

Acidosis abolishes the effect of repeated applications of ATP on pulmonary artery force and [Ca2+]i

The purine nucleotide, ATP, can cause receptor-mediated desensitizing contractions of smooth muscle that may be modulated by pH. We investigated in the rat the effect of acidosis upon the contractile and Ca2+ responses induced by ATP upon intrapulmonary artery (PA) smooth muscle. Four successive applications of ATP (300 microM) at pH 7.4 induced desensitising contractile responses that showed progressively decreasing peak amplitudes that correlated with decreases of [Ca2+]i. Acidosis significantly reduced the peak contractile response to the first application of ATP without modifying the rate or degree of desensitisation in response to ATP and without decreasing the [Ca2+]i. Successive applications of ATP did not further reduce contractile force nor [Ca2+]i. These results demonstrated that acidosis abolishes the effect of repeat applications of ATP on pulmonary artery force and [Ca2+]i via alteration in the desensitization-resensitisation characteristics of ATP receptor. This suggest a potentially important physiological role for changes in external pH in the regulation of ATP-mediated control of the pulmonary circulation.

The P2Y2 nucleotide receptor mediates vascular cell adhesion molecule-1 expression through interaction with VEGF receptor-2 (KDR/Flk-1)

UTP stimulates the expression of pro-inflammatory vascular cell adhesion molecule-1 (VCAM-1) in endothelial cells through activation of the P2Y(2) nucleotide receptor P2Y(2)R. Here, we demonstrated that activation of the P2Y(2)R induced rapid tyrosine phosphorylation of vascular endothelial growth factor receptor (VEGFR)-2 in human coronary artery endothelial cells (HCAEC). RNA interference targeting VEGFR-2 or inhibition of VEGFR-2 tyrosine kinase activity abolishes P2Y(2)R-mediated VCAM-1 expression. Furthermore, VEGFR-2 and the P2Y(2)R co-localize upon UTP stimulation. Deletion or mutation of two Src homology-3-binding sites in the C-terminal tail of the P2Y(2)R or inhibition of Src kinase activity abolished the P2Y(2)R-mediated transactivation of VEGFR-2 and subsequently inhibited UTP-induced VCAM-1 expression. Moreover, activation of VEGFR-2 by UTP leads to the phosphorylation of Vav2, a guanine nucleotide exchange factor for Rho family GTPases. Using a binding assay to measure the activity of the small GTPases Rho, we found that stimulation of HCAEC by UTP increased the activity of RhoA and Rac1 (but not Cdc42). Significantly, a dominant negative form of RhoA inhibited P2Y(2)R-mediated VCAM-1 expression, whereas expression of dominant negative forms of Cdc42 and Rac1 had no effect. These data indicate a novel mechanism whereby a nucleotide receptor transactivates a receptor tyrosine kinase to generate an inflammatory response associated with atherosclerosis.

P2X7 receptors stimulate AKT phosphorylation in astrocytes

1. Emerging evidence indicates that nucleotide receptors are widely expressed in the nervous system. Here, we present evidence that P2Y and P2X receptors, particularly the P2X(7) subtype, are coupled to the phosphoinositide 3-kinase (PI3K)/Akt pathway in astrocytes. 2. P2Y and P2X receptor agonists ATP, uridine 5'-triphosphate (UTP) and 2',3'-O-(4-benzoyl)-benzoyl ATP (BzATP) stimulated Akt phosphorylation in primary cultures of rat cortical astrocytes. BzATP induced Akt phosphorylation in a concentration- and time-dependent manner, similar to the effect of BzATP on Akt phosphorylation in 1321N1 astrocytoma cells stably transfected with the rat P2X(7) receptor. Activation was maximal at 5 - 10 min and was sustained for 60 min; the EC(50) for BzATP was approximately 50 microM. In rat cortical astrocytes, the positive effect of BzATP on Akt phosphorylation was independent of glutamate release. 3. The effect of BzATP on Akt phosphorylation in rat cortical astrocytes was significantly reduced by the P2X(7) receptor antagonist Brilliant Blue G and the P2X receptor antagonist iso-pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulfonic acid, but was unaffected by trinitrophenyl-ATP, oxidized ATP, suramin and reactive blue 2. 4. Results with specific inhibitors of signal transduction pathways suggest that extracellular and intracellular calcium, PI3K and a Src family kinase are involved in the BzATP-induced Akt phosphorylation pathway. 5. In conclusion, our data indicate that stimulation of astrocytic P2X(7) receptors, as well as other P2 receptors, leads to Akt activation. Thus, signaling by nucleotide receptors in astrocytes may be important in several cellular downstream effects related to the Akt pathway, such as cell cycle and apoptosis regulation, protein synthesis, differentiation and glucose metabolism.

Metabotropic receptor activation, desensitization and sequestration-I: modelling calcium and inositol 1,4,5-trisphosphate dynamics following receptor activation

A mathematical account is given of the processes governing the time courses of calcium ions (Ca2+), inositol 1,4,5-trisphosphate (IP(3)) and phosphatidylinositol 4,5-bisphosphate (PIP(2)) in single cells following the application of external agonist to metabotropic receptors. A model is constructed that incorporates the regulation of metabotropic receptor activity, the G-protein cascade and the Ca2+ dynamics in the cytosol. It is subsequently used to reproduce observations on the extent of desensitization and sequestration of the P(2)Y(2) receptor following its activation by uridine triphosphate (UTP). The theory predicts the dependence on agonist concentration of the change in the number of receptors in the membrane as well as the time course of disappearance of receptors from the plasmalemma, upon exposure to agonist. In addition, the extent of activation and desensitization of the receptor, using the calcium transients in cells initiated by exposure to agonist, is also predicted. Model predictions show the significance of membrane PIP(2) depletion and resupply on the time course of IP(3) and Ca2+ levels. Results of the modelling also reveal the importance of receptor recycling and PIP(2) resupply for maintaining Ca2+ and IP(3) levels during sustained application of agonist.

Mechanisms of P2X7 receptor-mediated ERK1/2 phosphorylation in human astrocytoma cells

Astrocytes are involved in normal and pathological brain functions, where they become activated and undergo reactive gliosis. Astrocytes have been shown to respond to extracellular nucleotides via the activation of P2 receptors, either G protein-coupled P2Y receptors or P2X receptors that are ligand-gated ion channels. In this study, we have examined the manner in which activation of the P2X(7) nucleotide receptor, an extracellular ATP-gated ion channel expressed in astrocytes, can lead to the phosphorylation of ERK1/2. Results showed that the P2X(7) receptor agonist 2',3'-O-(4-benzoyl)benzoyl-ATP induced ERK1/2 phosphorylation in human astrocytoma cells overexpressing the recombinant rat P2X(7) receptor (rP2X(7)-R), a response that was inhibited by the P2X(7) receptor antagonist, oxidized ATP. Other results suggest that rP2X(7)-R-mediated ERK1/2 phosphorylation was linked to the phosphorylation of the proline-rich/Ca(2+)-activated tyrosine kinase Pyk2, c-Src, phosphatidylinositol 3'-kinase, and protein kinase Cdelta activities and was dependent on the presence of extracellular Ca(2+). These results support the hypothesis that the P2X(7) receptor and its signaling pathways play a role in astrocyte-mediated inflammation and neurodegenerative disease.