Potent desensitization of human P2X3 receptors by diadenosine polyphosphates

The Plasma Membrane Purinoreceptor P2K1/DORN1 Is Essential in Stomatal Closure Evoked by Extracellular Diadenosine Tetraphosphate (Ap4A) in Arabidopsis thaliana

Dinucleoside polyphosphates (NpnNs) are considered novel signalling molecules involved in the induction of plant defence mechanisms. However, NpnN signal recognition and transduction are still enigmatic. Therefore, the aim of our research was the identification of the NpnN receptor and signal transduction pathways evoked by these nucleotides. Earlier, we proved that purine and pyrimidine NpnNs differentially affect the phenylpropanoid pathway in Vitis vinifera suspension-cultured cells. Here, we report, for the first time, that both diadenosine tetraphosphate (Ap4A) and dicytidine tetraphosphate (Cp4C)-induced stomatal closure in Arabidopsis thaliana. Moreover, we showed that plasma membrane purinoreceptor P2K1/DORN1 (does not respond to nucleotide 1) is essential for Ap4A-induced stomata movements but not for Cp4C. Wild-type Col-0 and the dorn1-3 A. thaliana knockout mutant were used. Examination of the leaf epidermis dorn1-3 mutant provided evidence that P2K1/DORN1 is a part of the signal transduction pathway in stomatal closure evoked by extracellular Ap4A but not by Cp4C. Reactive oxygen species (ROS) are involved in signal transduction caused by Ap4A and Cp4C, leading to stomatal closure. Ap4A induced and Cp4C suppressed the transcriptional response in wild-type plants. Moreover, in dorn1-3 leaves, the effect of Ap4A on gene expression was impaired. The interaction between P2K1/DORN1 and Ap4A leads to changes in the transcription of signalling hubs in signal transduction pathways.

Tonic Calcium-Activated Chloride Current Sustained by ATP Release and Highly Desensitizing Human P2X1 Receptors

Extracellular adenosine triphosphate (ATP) participates in maintaining the vascular tone in the CNS, particularly in the retina, via the tonic activity of ligand gated activated P2X1 receptors. P2X1 receptors are characterized by their high affinity for ATP and their strong desensitization to concentrations of ATP that are 200-fold lower than their EC₅₀. The mechanism behind P2X1 tonic activity remains unclear. In this study, we expressed human P2X1 (hP2X1) homomeric receptors in Xenopus oocytes to explore the relationship between ATP release from oocytes at rest, hP2X1, and Ca²⁺-activated Cl^- channels. Our results indicate that Xenopus oocytes release ATP at rest via vesicular exocytosis, and this process is a constitutive phenomenon independent of extracellular Ca²⁺. Our results also indicate that hP2X1 receptors are able to sustain a tonic activity of Ca²⁺-activated Cl^- channels. In the presence of extracellular Ca²⁺ the activity of hP2X1 receptors is greatly amplified by its coupling with Ca²⁺-activated Cl^- channels. Future studies addressing the relationship between hP2X1 receptors and Ca²⁺-activated Cl^- channels in vascular smooth muscle cells should provide information about additional mechanisms that regulate the vascular tone and their potential as pharmaceutical targets. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Stable, synthetic analogs of diadenosine tetraphosphate inhibit rat and human P2X3 receptors and inflammatory pain

BACKGROUND

A growing body of evidence suggests that ATP-gated P2X3 receptors (P2X3Rs) are implicated in chronic pain. We address the possibility that stable, synthetic analogs of diadenosine tetraphosphate (Ap4A) might induce antinociceptive effects by inhibiting P2X3Rs in peripheral sensory neurons.

RESULTS

The effects of two stable, synthetic Ap4A analogs (AppNHppA and AppCH2ppA) are studied firstly in vitro on HEK293 cells expressing recombinant rat P2XRs (P2X2Rs, P2X3Rs, P2X4Rs, and P2X7Rs) and then using native rat brain cells (cultured trigeminal, nodose, or dorsal root ganglion neurons). Thereafter, the action of these stable, synthetic Ap4A analogs on inflammatory pain and thermal hyperalgesia is studied through the measurement of antinociceptive effects in formalin and Hargreaves plantar tests in rats in vivo. In vitro inhibition of rat P2X3Rs (not P2X2Rs, P2X4Rs nor P2X7Rs) is shown to take place mediated by high-affinity desensitization (at low concentrations; IC50 values 100-250 nM) giving way to only weak partial agonism at much higher concentrations (EC50 values ≥ 10 µM). Similar inhibitory activity is observed with human recombinant P2X3Rs. The inhibitory effects of AppNHppA on nodose, dorsal root, and trigeminal neuron whole cell currents suggest that stable, synthetic Ap4A analogs inhibit homomeric P2X3Rs in preference to heteromeric P2X2/3Rs. Both Ap4A analogs mediate clear inhibition of pain responses in both in vivo inflammation models.

CONCLUSIONS

Stable, synthetic Ap4A analogs (AppNHppA and AppCH2ppA) being weak partial agonist provoke potent high-affinity desensitization-mediated inhibition of homomeric P2X3Rs at low concentrations. Therefore, both analogs demonstrate clear potential as potent analgesic agents for use in the management of chronic pain associated with heightened P2X3R activation.

Diadenosine tetra- and pentaphosphates affect contractility and bioelectrical activity in the rat heart via P2 purinergic receptors

Diadenosine polyphosphates (Ap(n)As) are endogenously produced molecules which have been identified in various tissues of mammalian organism, including myocardium. Ap(n)As contribute to the blood clotting and are also widely accepted as regulators of blood vascular tone. Physiological role of Ap(n)As in cardiac muscle has not been completely elucidated. The present study aimed to investigate the effects of diadenosine tetra- (Ap4A) and penta- (Ap5A) polyphosphates on contractile function and action potential (AP) waveform in rat supraventricular and ventricular myocardium. We have also demonstrated the effects of A4pA and Ap5A in myocardial sleeves of pulmonary veins (PVs), which play a crucial role in genesis of atrial fibrillation. APs were recorded with glass microelectrodes in multicellular myocardial preparations. Contractile activity was measured in isolated Langendorff-perfused rat hearts. Both Ap4A and Ap5A significantly reduced contractility of isolated Langendorff-perfused heart and produced significant reduction of AP duration in left and right auricle, interatrial septum, and especially in right ventricular wall myocardium. Ap(n)As also shortened APs in rat pulmonary veins and therefore may be considered as potential proarrhythmic factors. Cardiotropic effects of Ap4A and Ap5A were strongly antagonized by selective blockers of P2 purine receptors suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), while P1 blocker DPCPX was not effective. We conclude that Ap(n)As may be considered as new class of endogenous cardioinhibitory compounds. P2 purine receptors play the central role in mediation of Ap4A and Ap5A inhibitory effects on electrical and contractile activity in different regions of the rat heart.

Desensitization properties of P2X3 receptors shaping pain signaling

ATP-gated P2X3 receptors are mostly expressed by nociceptive sensory neurons and participate in transduction of pain signals. P2X3 receptors show a combination of fast desensitization onset and slow recovery. Moreover, even low nanomolar agonist concentrations unable to evoke a response, can induce desensitization via a phenomenon called “high affinity desensitization.” We have also observed that recovery from desensitization is agonist-specific and can range from seconds to minutes. The recovery process displays unusually high temperature dependence. Likewise, recycling of P2X3 receptors in peri-membrane regions shows unexpectedly large temperature sensitivity. By applying kinetic modeling, we have previously shown that desensitization characteristics of P2X3 receptor are best explained with a cyclic model of receptor operation involving three agonist molecules binding a single receptor and that desensitization is primarily developing from the open receptor state. Mutagenesis experiments suggested that desensitization depends on a certain conformation of the ATP binding pocket and on the structure of the transmembrane domains forming the ion pore. Further molecular determinants of desensitization have been identified by mutating the intracellular N- and C-termini of P2X3 receptor. Unlike other P2X receptors, the P2X3 subtype is facilitated by extracellular calcium that acts via specific sites in the ectodomain neighboring the ATP binding pocket. Thus, substitution of serine275 in this region (called “left flipper”) converts the natural facilitation induced by extracellular calcium to receptor inhibition. Given their strategic location in nociceptive neurons and unique desensitization properties, P2X3 receptors represent an attractive target for development of new analgesic drugs via promotion of desensitization aimed at suppressing chronic pain.

Distinct clathrin-coated pits sort different G protein-coupled receptor cargo

Upon activation, many G protein-coupled receptors (GPCRs) internalize by clathrin-mediated endocytosis and are subsequently sorted to undergo recycling or lysosomal degradation. Here we observe that sorting can take place much earlier than previously thought, by entry of different GPCRs into distinct populations of clathrin-coated pit (CCP). These distinct populations were revealed by analysis of two purinergic GPCRs, P2Y(1) and P2Y(12), which enter two populations of CCPs in a mutually exclusive manner. The mechanisms underlying early GPCR sorting involve differential kinase-dependent processes because internalization of P2Y(12) is mediated by GPCR kinases (GRKs) and arrestin, whereas P2Y(1) internalization is GRK- and arrestin-independent but requires protein kinase C. Importantly, the beta(2) adrenoceptor which also internalizes in a GRK-dependent manner also traffics exclusively to P2Y(12)-containing CCPs. Our data therefore reveal distinct populations of CCPs that sort GPCR cargo at the plasma membrane using different kinase-dependent mechanisms.

Purinergic P2 receptors as targets for novel analgesics

Following hints in the early literature about adenosine 5'-triphosphate (ATP) injections producing pain, an ion-channel nucleotide receptor was cloned in 1995, P2X3 subtype, which was shown to be localized predominantly on small nociceptive sensory nerves. Since then, there has been an increasing number of papers exploring the role of P2X3 homomultimer and P2X2/3 heteromultimer receptors on sensory nerves in a wide range of organs, including skin, tongue, tooth pulp, intestine, bladder, and ureter that mediate the initiation of pain. Purinergic mechanosensory transduction has been proposed for visceral pain, where ATP released from epithelial cells lining the bladder, ureter, and intestine during distension acts on P2X3 and P2X2/3, and possibly P2Y, receptors on subepithelial sensory nerve fibers to send messages to the pain centers in the brain as well as initiating local reflexes. P1, P2X, and P2Y receptors also appear to be involved in nociceptive neural pathways in the spinal cord. P2X4 receptors on spinal microglia have been implicated in allodynia. The involvement of purinergic signaling in long-term neuropathic pain and inflammation as well as acute pain is discussed as well as the development of P2 receptor antagonists as novel analgesics.

The P2X3 antagonist P1, P5-di[inosine-5'] pentaphosphate binds to the desensitized state of the receptor in rat dorsal root ganglion neurons

P2X3 purinergic receptors are predominantly expressed in dorsal root ganglion (DRG) neurons and play an important role in pain sensation. P2X3-specific antagonists are currently being sought to ameliorate pain in several indications. Understanding how antagonists interact with the P2X3 receptor can aid in the discovery and development of P2X3-specific antagonists. We studied the activity of the noncompetitive antagonist P1, P5-di[inosine-5'] pentaphosphate (IP5I) at the P2X3 receptor, compared with the well studied competitive antagonist TNP-ATP, using a whole-cell voltage-clamp technique in dissociated rat DRG neurons. IP5I blocked alphabeta-methylene ATP (alphabeta-meATP)-evoked P2X3 responses in a concentration-dependent manner (IC50 = 0.6 +/- 0.1 microM). IP5I effectively inhibited P2X3 currents when pre-exposed to desensitized but not unbound receptors. Furthermore, IP5I equally blocked 1 and 10 microM alphabeta-meATP-evoked currents and had no effect on the desensitization rate constant of these currents. This supports the action of IP5I as a noncompetitive antagonist that interacts with the desensitized state of the P2X3 receptor. In contrast, TNP-ATP inhibited the current evoked by 1 microM alphabeta-meATP significantly more than the one evoked by 10 microM alphabeta-meATP. It also significantly slowed down the desensitization rate constant of the current. These results suggest that TNP-ATP acts as a competitive antagonist and competes with alphabeta-meATP at the P2X3 agonist binding site. These findings may help to explain why IP5I acts selectively at the fast-desensitizing P2X1 and P2X3 subtypes of the P2X purinoceptor, while having much less potency at slow-desensitizing P2X2 and P2X(2/3) subtypes that lack the fast desensitized conformational state.

Reciprocal cross-talk between P2Y1 and P2Y12 receptors at the level of calcium signaling in human platelets

Adenosine diphosphate (ADP), an important platelet agonist, acts through 2 G-protein-coupled receptors (GPCRs), P2Y(1) and P2Y(12), which signal through Gq and Gi, respectively. There is increasing evidence for cross-talk between signaling pathways downstream of GPCRs and here we demonstrate cross-talk between these 2 ADP receptors in human platelets. We show that P2Y(12) contributes to platelet signaling by potentiating the P2Y(1)-induced calcium response. This potentiation is mediated by 2 mechanisms: inhibition of adenylate cyclase and activation of phosphatidylinositol 3 (PI 3)-kinase. Furthermore, the Src family kinase inhibitor PP1 selectively potentiates the contribution to the calcium response by P2Y(12), although inhibition of adenylate cyclase by P2Y(12) is unaffected. Using PP1 in combination with the inhibitor of PI 3-kinase LY294002, we show that Src negatively regulates the PI 3-kinase-mediated component of the P2Y(12) calcium response. Finally, we were able to show that Src kinase is activated through P2Y(1) but not P2Y(12). Taken together, we present evidence for a complex signaling interplay between P2Y(1) and P2Y(12), where P2Y(12) is able to positively regulate P2Y(1) action and P2Y(1) negatively regulates this action of P2Y(12). It is likely that this interplay between receptors plays an important role in maintaining the delicate balance between platelet activation and inhibition during normal hemostasis.

Agonist-dependence of recovery from desensitization of P2X(3) receptors provides a novel and sensitive approach for their rapid up or downregulation

1. Fast-desensitizing P2X(3) receptors of nociceptive dorsol root ganglion (DRG) neurons are thought to mediate pain sensation. Since P2X(3) receptor efficiency is powerfully modulated by desensitization, its underlying properties were studied with patch-clamp recording. 2. On rat cultured DRG neurons, 2 s application of ATP (EC(50)=1.52 microm), ADP (EC(50)=1.1 microm) or alpha,beta-meATP (EC(50)=1.78 microm) produced similar inward currents that fully desensitized, at the same rate, back to baseline. Recovery from desensitization was much slower after ATP and ADP than after alpha,beta-meATP and, in all cases, it had sigmoidal time course. 3. By alternating the application of ATP and alpha,beta-meATP, we observed complete cross-desensitization indicating that these agonists activated the same receptors. This notion was confirmed by the similar antagonism induced by 2', 3'-O-(2,4,6,trinitrophenyl)-adenosine triphosphate (TNP-ATP). 4. Recovery from desensitization elicited by ATP was unexpectedly shaped by transient application of alpha,beta-methylene-adenosine triphosphate (alpha,beta-meATP), and vice versa. Thus, short-lasting, full desensitization produced by alpha,beta-meATP protected receptors from long-lasting desensitization induced by subsequent ATP applications. ATP and ADP had similar properties of recovery from desensitization. 5. Low nm concentrations of alpha,beta-meATP (unable to evoke membrane currents) could speed up recovery from ATP-induced desensitization, while low nm concentrations of ATP enhanced it. Ambient ATP levels were found to be in the pm range (52+/-3 pm). 6. The phenomenon of cross-desensitization and protection was reproduced by rP2X(3) receptors expressed by rat osteoblastic cell 17/2.8 or human embryonic kidney cell 293 cells, indicating P2X(3) receptor specificity. 7. It is suggested that transient application of an agonist that generates rapid recovery from desensitization, is a novel, powerful tool to modulate P2X(3) receptor responsiveness to the natural agonist ATP.

Characterization of P2X3, P2Y1 and P2Y4 receptors in cultured HEK293-hP2X3 cells and their inhibition by ethanol and trichloroethanol

Membrane currents and changes in the intracellular Ca2+ concentration ([Ca2+]i) were measured in HEK293 cells transfected with the human P2X3 receptor (HEK293-hP2X3). RT-PCR and immunocytochemistry indicated the additional presence of endogenous P2Y1 and to some extent P2Y4 receptors. P2 receptor agonists induced inward currents in HEK293-hP2X3 cells with the rank order of potency alpha,beta-meATP approximately ATP > ADP-beta-S > UTP. A comparable rise in [Ca2+]i was observed after the slow superfusion of ATP, ADP-beta-S and UTP; alpha,beta-meATP was ineffective. These data, in conjunction with results obtained by using the P2 receptor antagonists TNP-ATP, PPADS and MRS2179 indicate that the current response to alpha,beta-meATP is due to P2X3 receptor activation, while the ATP-induced rise in [Ca2+]i is evoked by P2Y1 and P2Y4 receptor activation. TCE depressed the alpha,beta-meATP current in a manner compatible with a non-competitive antagonism. The ATP-induced increase of [Ca2+]i was much less sensitive to the inhibitory effect of TCE than the current response to alpha,beta-meATP. The present study indicates that in HEK293-hP2X3 cells, TCE, but not ethanol, potently inhibits ligand-gated P2X3 receptors and, in addition, moderately interferes with G protein-coupled P2Y1 and P2Y4 receptors. Such an effect may be relevant for the interruption of pain transmission in dorsal root ganglion neurons following ingestion of chloral hydrate or trichloroethylene.