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

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.

Modulation of P2X3 receptors by spider toxins

Recently, the novel peptide named purotoxin-1 (PT1) has been identified in the venom of the spider Geolycosa sp. and shown to exert marked modulatory effects on P2X3 receptors in rat sensory neurons. Here we studied another polypeptide from the same spider venom, purotoxin-2 (PT2), and demonstrated that it also affected activity of mammalian P2X3 receptors. The murine and human P2X3 receptors were heterologously expressed in cells of the CHO line, and nucleotide-gated currents were stimulated by CTP and ATP, respectively. Both PT1 and PT2 negligibly affected P2X3-mediated currents elicited by brief pulses of the particular nucleotide. When subthreshold CTP or ATP was added to the bath to exert the high-affinity desensitization of P2X3 receptors, both spider toxins strongly enhanced the desensitizing action of the ambient nucleotides. At the concentration of 50nM, PT1 and PT2 elicited 3-4-fold decrease in the IC(50) dose of ambient CTP or ATP. In contrast, 100nM PT1 and PT2 negligibly affected nucleotide-gated currents mediated by mP2X2 receptors or mP2X2/mP2X3 heteromers. Altogether, our data point out that the PT1 and PT2 toxins specifically target the fast-desensitizing P2X3 receptor, thus representing a unique tool to manipulate its activity.

Functional up-regulation of P2X3 receptors in dorsal root ganglion in a rat model of bone cancer pain

BACKGROUND

Cancer-induced bone pain remains a clinical challenge due to the poor understanding of the mechanisms. Recent study revealed extracellular adenosine triphosphate (ATP) and P2X receptors may be implicated in nociceptive signalling under cancer pain state. Therefore, here we investigated the potential role of P2X(3) receptor in a rat model of bone cancer pain.

METHODS

Walker 256 tumour cells were inoculated into the left tibia of Wistar rats. The model was verified by X-ray imaging, pathology and behaviour examinations. The expression of P2X(3) receptors in dorsal root ganglia (DRG) was examined. Functional significance of altered P2X(3) receptors was investigated by measuring influx upon α,β-meATP stimulation in acutely dissociated DRG neurons. Moreover, A-317491, an antagonist of P2X(3) receptors, was administrated intrathecally or locally to evaluate its analgesia effect in the cancer pain animals.

RESULTS

The P2X(3) receptor was up-regulated for about 50% in DRG neurons in rats with bone cancer at both protein and mRNA levels and correlated with the pain behaviour in bone cancer rats. A 51.9% increase of α,β-me ATP (10 μM, for 4 s) evoked transient response currents and a higher percentage of neurons responsive to the application of α,β-me ATP was detected in bone cancer rats. Intrathecal or local injection of A-317491 significantly attenuated pain behaviour induced by bone cancer.

CONCLUSIONS

These results suggest that the P2X(3) receptor is functionally up-regulated in DRG in cancer rats. P2X(3) receptor is a promising target for therapeutic intervention in cancer patients for pain management.

Uridine adenosine tetraphosphate induces contraction of airway smooth muscle

Contraction of airway smooth muscle (ASM) plays an important role in the regulation of air flow and is potentially involved in the pathophysiology of certain respiratory diseases. Extracellular nucleotides regulate ASM contraction via purinergic receptors, but the signaling mechanisms involved are not fully understood. Uridine adenosine tetraphosphate (Up(4)A) contains both pyrimidine and purine moieties, which are known to potentially activate P2X and P2Y receptors. Both P2X and P2Y receptors have been identified in the lung, including airway epithelial cells and ASM. We report here a study of purinergic signaling in the respiratory system, with a focus on the effect of Up(4)A on ASM contraction. Up(4)A induced contraction of rat isolated trachea and extrapulmonary bronchi as well as human intrapulmonary bronchioles. Up(4)A-induced contraction was blocked by di-inosine pentaphosphate, a P2X antagonist, but not by suramin, a nonselective P2 antagonist. Up(4)A-induced contraction was also attenuated by α,β-methylene-ATP-mediated P2X receptor desensitization. Several P2X receptors were detected at the mRNA level: P2X1, P2X4, P2X6, and P2X7, and to a lesser extent P2X3. Furthermore, the Up(4)A response was inhibited by removal of extracellular Ca(2+) and by the presence of the L-type Ca(2+) channel blocker, nifedipine, or the Rho-associated kinase inhibitor, H1152. We conclude that Up(4)A stimulates ASM contraction, and the underlying signaling mechanism appears to involve P2X (most likely P2X1) receptors, extracellular Ca(2+) entry via L-type Ca(2+) channels, and Ca(2+) sensitization through the RhoA/Rho-associated kinase pathway. This study will add to our understanding of the pathophysiological roles of extracellular nucleotides in the lung.

Pharmacological properties of P2 receptors on rat otic parasympathetic ganglion neurons

To elucidate the pharmacological profile of P2X receptors and the probable expression of P2Y receptors in otic ganglion neurons from 17-day-old rats, single neurons were enzymatically isolated and maintained in tissue culture for up to 30 h. Whole-cell voltage-clamp recording was carried out at a holding potential of -60 mV. Most otic ganglion neurons responded to adenosine 5'-triphosphate (ATP), 2-methylthio ATP (2-MeSATP) and alpha,beta-methylene ATP (alphabeta-meATP) with sustained currents and EC(50) values of 19 microM, 47 microM and 94 microM, respectively. 2',3'-O-trinitrophenyl-ATP (TNP-ATP) inhibited the response to alphabeta-meATP and ATP with an IC(50) values of 3.9 nM and 18.3 nM, respectively, which was closed to that observed in nodose neurons. The response to ATP was antagonized by suramin and cibacron blue. The dose-response curve of suramin against ATP response at a pH of 6.5 was shifted to the left compared to that at a pH of 7.4. Diinosine pentaphosphate (Ip(5)I), which blocks P2X(3), but not P2X(2/3)-mediated responses, had no effect on the currents evoked by ATP or alphabeta-meATP. In some neurons, uridine 5'-triphosphate (UTP) induced a tiny, but long-lasting current with a mean amplitude of 0.034+/-0.011 nA. Reverse transcriptase-polymerase chain reaction (RT-PCR) confirmed the expression of mRNAs for P2X(2), P2X(3), P2X(4), P2X(6) and P2X(7), but not for P2X(1) and P2X(5) receptors in otic ganglion. In conclusion, in rat otic ganglion neurons, P2X(2/3) heteromultimer receptors dominate, but P2X(7) and P2Y(2) or P2Y(4) receptors also play roles.

Effect of PPADS on achondroplasic chondrocytes: inhibition of FGF receptor type 3 over-activity

Achondroplasia, results from a mutation in the FGF receptor type 3, leading to receptor hyperactivation and subsequent amplification of FGF receptor type 3 signals. We have tested the ability of pyridoxal-5'-phosphate-6-azophenyl-2', 4'-disulfonate (PPADS) to decrease the overactivation and signalling of FGF receptor type 3 in achondroplasic chondrocytes. PPADS reduced the tyrosine phosphorylation of FGF receptor type 3 triggered by fibroblast growth factor 9 (FGF9) (50% reduction), as well as the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathway. As a consequence of this inhibitory effect on ERK1/2 activity the loss of extracellular matrix was also reversed by PPADS. The action of PPADS seems to be due to a mechanism independent of P2 receptor antagonism.

Conserved lysin and arginin residues in the extracellular loop of P2X(3) receptors are involved in agonist binding

Wild-type human (h) P2X(3) receptors expressed in HEK293 cells responded to the prototypic agonist alpha,beta-methylene ATP (alpha,beta-meATP) with rapidly desensitizing inward currents and an increase in the intracellular Ca(2+) concentration. In contrast to electrophysiological recordings, Ca(2+) microfluorimetry showed a lower maximum of the concentration-response curve of alpha,beta-meATP in the transiently than in the permanently transfected HEK293 cells. However, the concentrations causing 50% of the maximum possible effect (EC(50) values) were identical, when measured with either method. In order to determine the role of certain conserved, positively charged amino acids in the nucleotide binding domains (NBD-1-4) of hP2X(3) receptors for agonist binding, the lysine-63, -65, -176 and -299 as well as the arginine-281 and -295 residues were substituted by the neutral amino acid alanine. We observed no effect of alpha,beta-meATP at the K63A, K176A, R295A, and K299A mutants, and a marked decrease of agonist potency at the K65A and R281A mutants. The P2X(3) receptor antagonist 2',3'-O-trinitrophenyl-ATP (TNP-ATP) blocked the effect of alpha,beta-meATP at the wild-type hP2X(3) receptor with lower affinity than at the mutant K65A, indicating an interference of this mutation with the docking of the antagonist with its binding sites. The use of confocal fluorescence microscopy in conjunction with an antibody raised against the extracellular loop of the hP2X(3) receptor documented the expression of all mutants in the plasma membrane of HEK293 cells. Eventually, we modelled the possible agonist and antagonist binding sites NBD-1-4 of the hP2X(3) subunit by using structural bioinformatics. This model is in complete agreement with the available data and integrates results from mutagenesis studies with geometry optimization of the tertiary structure predictions of the receptor.

Pharmacology of P2X channels

Significant progress in understanding the pharmacological characteristics and physiological importance of homomeric and heteromeric P2X channels has been achieved in recent years. P2X channels, gated by ATP and most likely trimerically assembled from seven known P2X subunits, are present in a broad distribution of tissues and are thought to play an important role in a variety of physiological functions, including peripheral and central neuronal transmission, smooth muscle contraction, and inflammation. The known homomeric and heteromeric P2X channels can be distinguished from each other on the basis of pharmacological differences when expressed recombinantly in cell lines, but whether this pharmacological classification holds true in native cells and in vivo is less well-established. Nevertheless, several potent and selective P2X antagonists have been discovered in recent years and shown to be efficacious in various animal models including those for visceral organ function, chronic inflammatory and neuropathic pain, and inflammation. The recent advancement of drug candidates targeting P2X channels into human trials, confirms the medicinal exploitability of this novel target family and provides hope that safe and effective medicines for the treatment of disorders involving P2X channels may be identified in the near future.

Molecular properties of P2X receptors

P2X receptors for adenosine tri-phosphate (ATP) are a distinct family of ligand-gated cation channels with two transmembrane domains, intracellular amino and carboxy termini and a large extracellular ligand binding loop. Seven genes (P2X(1-7)) have been cloned and the channels form as either homo or heterotrimeric channels giving rise to a wide range of phenotypes. This review aims to give an account of recent work on the molecular basis of the properties of P2X receptors. In particular, to consider emerging information on the assembly of P2X receptor subunits, channel regulation and desensitisation, targeting, the molecular basis of drug action and the functional contribution of P2X receptors to physiological processes.