ATP-gated P2X receptors are trimeric ion channels that assemble as homo- or heteromers from seven cloned subunits. Transcripts and/or proteins of P2X subunits have been found in most, if not all, mammalian tissues and are being discovered in an increasing number of non-vertebrates. Both the first crystal structure of a P2X receptor and the generation of knockout (KO) mice for five of the seven cloned subtypes greatly advanced our understanding of their molecular and physiological function and their validation as drug targets. This review summarizes the current understanding of the structure and function of P2X receptors and gives an update on recent developments in the search for P2X subtype-selective ligands. It also provides an overview about the current knowledge of the regulation and modulation of P2X receptors on the cellular level and finally on their physiological roles as inferred from studies on KO mice.

The G protein-coupled P2Y 11 receptor is involved in immune system modulation. In-depth physiological evaluation is hampered, however, by a lack of selective and potent ligands. By screening a library of sulfonic and phosphonic acid derivatives at P2Y 11 receptors recombinantly expressed in human 1321N1 astrocytoma cells (calcium and cAMP assays), the selective non-nucleotide P2Y 11 agonist NF546 [4,4Ј-(carbonylbis(imino-3,1-phenylene-carbonylimino-3,1-(4-methyl-phenylene)carbonylimino))-bis(1,3-xylene-␣,␣Ј-diphosphonic acid) tetrasodium salt] was identified. NF546 had a pEC 50 of 6.27 and is relatively selective for P2Y 11 over P2Y 1 , P2Y 2 , P2Y 4 , P2Y 6 , P2Y 12 , P2X 1 , P2X 2 , and P2X 2 -X 3 . Adenosine-5Ј-O-(3-thio)triphosphate (ATP␥S), a nonhydrolyzable analog of the physiological P2Y 11 agonist ATP, and NF546 use a common binding site as suggested by molecular modeling studies and their competitive behavior toward the nanomolar potency antagonist NF340 [4,4Ј-(carbonylbis(imino-3,1-(4-methyl-phenylene)carbonylimino))bis(naphthalene-2,6-disulfonic acid) tetrasodium salt] in Schild analysis. The pA 2 of NF340 was 8.02 against ATP␥S and 8.04 against NF546 (calcium assays). NF546 was further tested for P2Y 11 -mediated effects in monocyte-derived dendritic cells. Similarly to ATP␥S, NF546 led to thrombospondin-1 secretion and inhibition of lipopolysaccharide-stimulated interleukin-12 release, whereas NF340 inhibited these effects. Further, for the first time, it was shown that ATP␥S or NF546 stimulation promotes interleukin 8 (IL-8) release from dendritic cells, which could be inhibited by NF340. In conclusion, we have described the first selective, non-nucleotide agonist NF546 for P2Y 11 receptors in both recombinant and physiological expression systems and could show a P2Y 11 -stimulated IL-8 release, further supporting the immunomodulatory role of P2Y 11 receptors. Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.

Selective and potent P2Y(11) receptor antagonists have yet to be developed, thus impeding an evaluation of this G protein-coupled receptor mainly expressed on immune cells. Taking suramin with moderate inhibitory potency as a template, 18 ureas with variations of the methyl groups of suramin and their precursors were functionally tested at P2Y(11), P2Y(1), and P2Y(2) receptors. Fluorine substitution of the methyl groups of suramin led to the first nanomolar P2Y(11) antagonist (8f, NF157, pK(i): 7.35). For selectivity, 8f was also tested at various P2X receptors. 8f displayed selectivity for P2Y(11) over P2Y(1) (>650-fold), P2Y(2) (>650-fold), P2X(2) (3-fold), P2X(3) (8-fold), P2X(4) (>22-fold), and P2X(7) (>67-fold) but no selectivity over P2X(1). QSAR studies confirm that residues with favored resonance and size parameters in the aromatic linker region can indeed lead to an increased potency as is the case for 8f. A symmetric structure linking two anionic clusters seems to be required for bioactivity. 8f may be helpful for studies evaluating the physiological role of P2Y(11) receptors.

Functional homomeric and heteromeric ATP-gated P2X receptor channels have been shown to display a characteristic trimeric architecture. Of the seven different isoforms (designated P2X(1)-P2X(7)), P2X(5) occurs in humans primarily as a non-functional variant lacking the C-terminal end of the ectodomain and the outer half of the second transmembrane domain. We show that this truncated variant, which results from the splice-skipping of exon 10, is prone to subunit aggregation because the residual transmembrane domain 2 is too short to insert into the membrane. Alleviation of the negative hydrophobic mismatch by the addition of a stretch of moderately hydrophobic residues enabled formation of a second membrane-spanning domain and strictly parallel homotrimerization. Systematic mutagenesis identified only one transmembrane domain 2 residue, Asp(355), which supported homotrimerization in a side chain-specific manner. Our results indicate that transmembrane domain 2 formation contributes 2-fold to hP2X(5) homotrimerization by tethering the end of the ectodomain to the membrane, thereby topologically restricting conformational mobility, and by intramembrane positioning of Asp(355). While transmembrane domain 2 appears to favor assembly by enabling productive subunit interactions in the ectodomain, Asp(355) seems to assist by simultaneously driving intramembrane helix interactions. Overall, these results indicate a complex interplay between topology, helix-helix interactions, and oligomerization to achieve a correctly folded structure.

Evaluation and optimization of anthraquinone derivatives related to Reactive Blue 2 at P2X2 receptors yielded the first potent and selective P2X2 receptor antagonists. The compounds were tested for inhibition of ATP (10 μM) mediated currents in Xenopus oocytes expressing the rat P2X2 receptor. The most potent antagonists were sodium 1-amino-4-[3-(4,6-dichloro[1,3,5]triazine-2-ylamino)phenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (63, PSB-10211, IC(50) 86 nM) and disodium 1-amino-4-[3-(4,6-dichloro[1,3,5]triazine-2-ylamino)-4-sulfophenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (57, PSB-1011, IC(50) 79 nM). Compound 57 exhibited a competitive mechanism of action (pA(2) 7.49). It was >100-fold selective versus P2X4, P2X7, and several investigated P2Y receptor subtypes (P2Y(2,4,6,12)); selectivity versus P2X1 and P2X3 receptors was moderate (>5-fold). Compound 57 was >13-fold more potent at the homomeric P2X2 than at the heteromeric P2X2/3 receptor. Several anthraquinone derivatives were found to act as positive modulators of ATP effects at P2X2 receptors, for example, sodium 1-amino-4-(3-phenoxyphenylamino)-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (51, PSB-10129, EC(50) 489 nM), which led to about a 3-fold increase in the ATP-elicited current.

Second primary tumours occur frequently in patients with a history of head and neck malignancies. Delays in making an early and correct diagnosis can seriously affect the therapy management and survival. This was a retrospective study of 120 patients with a history of head and neck cancer, presenting with a second primary tumour. Current follow-up strategies and the use of routine sonographic imaging of the head and neck regions were evaluated, and the impact that tumour chronology, the tumour site and the various treatment modalities have on the survival were assessed. Forty-two per cent of patients developed a metachronous second malignancy more than five years after diagnosis of the index tumour. The accuracy of colour-duplex sonography in detection of second primaries in the head and neck was 82.3 per cent. First and second primary tumours located in the larynx were observed to have the highest five-year survival rate. Patients who developed metachronous tumours had a five-year survival rate of 68.9 per cent for the index tumours, and a 26 per cent five-year survival rate with the occurrence of a second neoplasm. With synchronous tumours a mean survival time of 18 months and a five-year survival rate of 11.9 per cent was found (p < 0.0001). Where clinically appropriate an aggressive treatment strategy was employed and yielded the most favourable results with a five-year survival rate of 66.8 per cent and 35.9 per cent for index tumours and second primary malignancies, respectively. Since more than 40 per cent of the metachronous second primaries in patients with a history of head and neck malignancy occur beyond the five-year follow-up period, an extended protocol with individually adjusted close monitoring of high-risk patients seems appropriate. Colour-duplex sonography is a valuable screening investigation for the early detection of second primary tumours. The treatment of a second primary is often less successful than for the same malignancy occurring primarily. The prognosis of synchronous tumours is significantly lower when compared to malignancies of a metachronous nature, despite some encouraging individual results. Only the early implementation of aggressive treatment methods for second primaries is successful in terms of survival.

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