Vertebrate P2X receptors comprise a family of ligand gated ion channels activated by extracellular ATP 1. They form homo or heteromeric trimers with each monomer consisting of intracellular amino and carboxy termini, two transmembrane domains and a large glycosylated extracellular region containing five disulphide bonds 2 and the agonist binding site 3. Mammalian species possess seven distinct P2X subtypes (P2X_1–7) that play important roles in a wide range of physiological processes including neurotransmission, platelet aggregation, smooth muscle contractility and bone formation 45. Many studies have also described potential roles for ATP as an extracellular signalling molecule in a range of invertebrate phyla 6 and plants 78 leading to the assumption that ATP is a primitive signalling molecule and that the emergence of purinergic receptors occurred relatively early in evolution 9. This assumption is supported by the definitive molecular and functional identification of P2X receptors in the slime mould Dictyostelium discoideum 10, the green alga Ostreococcus tauri 11 and the choanoflagellate Monosiga brevicollis 11. However, several non-vertebrate organisms for which full genome data are available lack P2X-like genes, including Anopheles gambiae, Caenorhabditis elegans, Drosophila melanogaster, and Apis mellifera. Arthropods and nematodes are members of the protostome superphylum Ecdysozoa. Given that P2X receptors are present in choanoflagellates, believed to be the sister group to the Metazoa, and in some representatives of the protostome superphylum Lophotrochozoa including Schistosoma mansoni 12 and Lymnaea stagnalis 13, the absence of P2X receptors from the above fully sequenced nematode and arthropod genomes suggests a loss of this class of gene in an ancestor of the Ecdysozoa.

We identified a partial P2X-like sequence in expressed sequence tag (EST) data from the tardigrade Hypsibius dujardini. Tardigrades are microscopic animals around 200 to 500 μm in length that inhabit both marine and fresh water habitats 1415. They possess a fascinating ability to desiccate into a reversible state of metabolic suspension called cryptobiosis allowing them to survive for many decades in harsh environments, such as lack of oxygen, extremes in temperature, and high pressure, before rehydration to an active state within minutes 16. Tardigrades are placed in their own phylum, Tardigrada and share features with both arthropods and nematodes. Like arthropods they possess legs and a distinctly segmented body. However they also have a triradiate pharynx more reminiscent of the nematodes. Indeed, molecular phylogenetic analyses support tardigrades as part of the Ecdysozoa 1718. Functional confirmation that tardigrades possess P2X receptors would expand our emerging knowledge of P2X phylogeny and better estimate the pattern of loss of P2X in nematodes and arthropods. Furthermore, a better understanding of P2X receptor function in simple invertebrate organisms may help shed new light on structure-function aspects of human P2X receptors in health and disease by virtue of conservation of functionally important amino acid residues between evolutionary remote P2X receptors and the identification of novel P2X mediated signalling pathways. The aim of this present work therefore was to determine whether the Hypsibius dujardini EST sequence corresponds to a gene encoding a functional P2X receptor and to determine its pharmacological properties. This was achieved by expression of cRNA encoding the Hypsibius dujardini P2X gene in Xenopus oocytes to enable two electrode voltage clamp recordings of ATP evoked membrane currents. Using this approach we show that the Hypsibius dujardini EST sequence does indeed code a functional P2X receptor with fast activation and desensitisation kinetics and similar to some vertebrate P2X channels, is potentiated by ivermectin and inhibited by zinc and copper.

Despite their absence in several key model organisms such as Saccharomyces cerevisiae, Drosophila melanogaster and Caenorhabditis elegans, functional P2X receptors have now been described in a wide range of non-vertebrate metazoan phyla including platyhelminthes 1227, and molluscs (41), and in representatives of other eukaryotic kingdoms such as dictyostelida 10, prasinophyceae 11 and choanoflagellates 11. Additional partial and predicted P2X-like proteins can be identified in EST and partial genome data from other animals, including additional molluscs, echinoderms 30 and cnidaria. These data provide an emerging phylogenetic understanding of the evolution of ionotropic purinergic signalling. Alignment of P2X proteins from a diverse range of taxa with the mammalian P2X_(1–7) families (Fig. 2) clearly shows that the mammalian P2X receptors are a lineage-specific gene expansion, with ortholog triplets evident in mouse, human and rat. A lineage-specific expansion is also evident in D. discoideum. In the other taxa, the single P2X receptors identified appear to be orthologous to the entire vertebrate expansion. This strongly suggests that the emergence of a family of seven mammalian P2X receptors was a relatively recent evolutionary event that occurred subsequent to the split between vertebrates and invertebrates. The functional confirmation that P2X receptors exist in the tardigrade H. dujardini strongly suggests that the loss of P2X receptors in some nematode (represented by C. elegans) and arthropod (represented by the sequenced hexapod genomes) species occurred independently, as both traditional and molecular phylogenies place the divergence between Nematoda and Arthropoda before their divergence from Tardigrada. Pharmacological evidence for purinergic signalling in some nematode and arthropod species 6 suggests that P2X-like receptors may have been maintained in some members of these phyla. Indeed a partial transcript for a P2X gene from the nematode Xiphinema index 31 suggests that the loss of P2X in C. elegans may have occurred relatively late in nematode evolution, and P2X-like ESTs from the chelicerate Boophilus microplus similarly suggest that the loss of P2X receptors in arthropods may be restricted to Hexapoda.

Based on their kinetic parameters and ligand sensitivity, mammalian P2X receptors can be divided into three groups 32. Group 1 consists of P2X₁ and P2X₃ receptors that have high sensitivity for ATP (EC₅₀ ~1–3 μM) and show rapid channel activation and desensitisation during the continued presence of agonist. Group 2 includes P2X₂, P2X₄, P2X₅, and P2X₆ receptors with a lower sensitivity for ATP (EC₅₀ ~10 μM) and much slower desensitization whilst group 3 is represented by P2X₇ which has a very low sensitivity for ATP (EC ₅₀ ~300 μM) and shows little desensitization. HdP2X displays an unusual phenotype in that it has a relatively low sensitivity for ATP (EC₅₀ ~44.5 μM) but has very fast activation and desensitisation kinetics. This is interesting from a mechanistic point of view regarding the relationship between channel gating and agonist binding since it demonstrates that that fast channel kinetics do not necessarily have to be associated with a high agonist sensitivity. Similar to HdP2X, a P2X₃-like receptor from Zebrafish has also been shown to combine fast current kinetics with low sensitivity to ATP 33. However, unlike HdP2X (Fig. 3A) current amplitudes at the Zebrafish channel never fully recover from the first stimulation. Other lower organism P2X receptors that have been characterised to date have all displayed slow rates of desensitisation making HdP2X the first example of a non vertebrate P2X channel with fast kinetics. Given that it is likely that a fast phenotype evolved independently in P2X_1,3 and HdP2X channels, it will be interesting to determine whether the same factors determine the rates of current activation and desensitisation in these evolutionary remote channels.

HdP2X contains several amino acid residues that are highly conserved throughout the vertebrate P2X receptor family. The ten conserved cysteine residues present in the extracellular loop of all vertebrate P2X receptors are thought to form five disulphide bonds 2 and these residues are also present in HdP2X suggesting conservation of gross extracellular structure constrained by disulphide bond location. It is interesting to note that the P2X receptors from O. tauri and M. brevicollis 11 possess only eight of these ten conserved cysteine residues in their putative extracellular regions and that in each case the two absent cysteines correspond to a proposed disulphide bond pairing in the human P2X₁ receptor 2 (the equivalents of hP2X₁ C217 and C227 are absent in O. tauri and C117 and C165 are absent in M. brevicollis). The agonist binding site of P2X receptors is distinct from other ATP-binding proteins and is thought to be formed from the interaction between adjacent subunits 34. Based on site directed mutagenesis combined with methanethiosulfonate reactivity a core common mode of action of ATP binding at P2X receptors has been proposed consisting of lysine residues adjacent to the transmembrane domains coordinating phosphate binding, an NFR motif binding the adenine/ribose moiety and an FT motif also involved in agonist action 21. The lysine residues, NFR and FT motifs are also present in HdP2X (Fig. 1) suggesting that this mechanism of ATP binding arose relatively early in evolution and is not restricted to mammalian receptors. However, the NFR motif and the equivalent of hP2X₁ K68 are absent in the D. discoideum P2XA receptor 10, the FT motif is YT in S. mansoni P2X 12 and the NFR motif is NIR in the O. tauri 11 receptor suggesting that conservation of the residues involved in ATP binding is not universal to all P2X receptors. Residues at positions D280 and R278 in the rat P2X₄ receptor have also been shown to play an important role in the action of ATP 35. These residues are also conserved in HdP2X, S. mansoni P2X and M. brevicollis P2X suggesting that, despite their incomplete conservation amongst the seven mammalian P2X subtypes, these two residues may also play an important role in the formation of the ATP binding site in some P2X receptor subtypes. Interestingly, the only feature with complete conservation across all vertebrate and lower organism P2X receptors characterised to date is a consensus protein kinase C phosphorylation site in the N-terminal tail of the receptor (Fig. 1). This putative phosphorylation site has been shown to regulate desensitisation in both P2X₁ 36 and P2X₂ 20 receptors however, surprisingly direct phosphorylation of this site has been shown not to occur 3738.

Unlike the D. discoideum 10 and O. tauri 11 P2X receptors, the general P2 receptor antagonists suramin and PPADS both inhibited HdP2X receptor currents in a concentration dependent manner. PPADS at 100 μM effectively blocked ATP evoked currents. Suramin however was less potent and a component of the current was resistant to antagonism by suramin even at high concentrations. At the highest concentration tested (300 μM) only ~75% of the 100 μM ATP-evoked current was blocked. These properties are opposite to those of the S. mansoni P2X, which was more potently inhibited by suramin and incompletely blocked by PPADS 12. The presence of a lysine residue at position 246 in rat P2X₂ and at equivalent positions in other mammalian subunits has been proposed to be involved in the inhibition of P2X receptors by PPADS, possibly forming a Schiff base with the aldehyde group of PPADS 39. Similarly a lysine residue at position 138 in human P2X₁ has recently been shown to play a key role in the binding of suramin 40. Neither of these lysine residues shown to be involved in PPADS and suramin action at mammalian receptors are conserved in either HdP2X or S. mansoni P2X suggesting that additional residues in the ectodomain also play key roles in the mechanism of antagonist binding.

HdP2X is inhibited by both copper and zinc in a concentration dependent manner (Fig. 5). The effects of these metal ions on vertebrate P2X receptors differ markedly with P2X₁ being inhibited by zinc, P2X₇ inhibited by both zinc and copper, P2X₂ potentiated by both zinc and copper and P2X₄ potentiated by zinc but inhibited by copper (reviewed by 41). The action of these metal ions at P2X receptors has physiological significance since both zinc and copper are stored in presynaptic terminals and released after nerve stimulation 4142. In addition to HdP2X, the modulation of P2X receptors by metal ions is also present in other lower organisms with Dictyostelium P2X inhibited by copper 10 and S. mansoni P2X inhibited by zinc 27. Several mutagenic studies have identified amino acid residues involved in metal binding in P2X_{2, 4 and 7} receptors (reviewed by 41) however, no consensus binding site has emerged showing independent evolution of metal binding sites among the different vertebrate P2X receptor subtypes. Histidine residues appear to be a common feature in metal binding sites in P2X_{2, 4 and 7} receptors (reviewed by41) and in rat P2X₂, H120 and H213 have been shown to form an intersubunit zinc binding site 26. There are three extracellular histidine residues present in HdP2X and we mutated each of these residues, both individually and in combination with each other, to alanine in order to assess their involvement in metal binding. Surprisingly, none of the extracellular HdP2X histidine residues are essential for inhibition by zinc or copper (Fig. 5B). This demonstrates that the mechanisms by which metal ions can influence the properties of P2X receptor function are not restricted to interactions with histidine residues. However, mutation of H306 in combination with either H252 or H232 did produce a significant reduction in copper but not zinc inhibition. This suggests that H306 plays a minor but non critical role in the formation of the copper binding site and that the copper and zinc binding sites in HdP2X are distinct from one another. Distinct zinc and copper binding sites are also likely for P2X₄ which is potentiated by zinc but inhibited by copper 43.

Of the currently identified vertebrate and lower organism P2X receptors only human and rat P2X₄ 29 and S. mansoni P2X 12 are known to be potentiated by the semi synthetic macrocyclic lactone ivermectin. Both P2X₄ and S. mansoni P2X have a slow current phenotype and it is therefore interesting with respect to the mechanism of ivermectin action that HdP2X which has very fast current activation and desensitisation kinetics is also potentiated (Fig. 6). Ivermectin does not affect the kinetics of desensitization of HdP2X currents, a feature also observed in S. mansoni P2X. However, in human P2X₄, ivermectin confers a slower rate of desensitization 29. Ivermectin is thought to have an allosteric mode of action by increasing maximum current when binding to a high affinity site on the receptor to stabilise the open state, whereas binding to a low affinity site is proposed to slow the rate of deactivation 44. More recently it was proposed that during gating TM1 rotates relative to TM2, and ivermectin is able to access the lipid environment and optimally bind to a hydrophobic crevice created by TM1 and TM2 at the protein-lipid interface 45. Interestingly several hydrophobic-nonpolar residues in both TM1 and TM2 that have been shown to be involved in the action of ivermectin at P2X₄ are conserved in S. mansoni P2X 28 and these hydrophobic-nonpolar residues are mostly also present HdP2X. Future investigations comparing the mode of ivermectin action in both naturally fast and slow desensitising P2X channels may enhance our understanding of the process of gating and motions of the channel pore.

The identification and pharmacological characterisation of a P2X receptor from H. dujardini provides further evidence that P2X receptors for ATP emerged early in eukaryotic evolution. Several unusual characteristics of HdP2X including fast current kinetics with low ATP sensitivity, ivermectin potentiation in a channel with fast kinetics and the likelihood of distinct copper and zinc metal binding sites make this channel a useful model for comparative structure-function studies allowing a better understanding of P2X receptors in higher organisms.

PPADS: Pyridoxal-phosphate-6-azophenyl-2',4' disulfonic acid; αβmeATP: α,β-methylene-adenosine 5'-triphosphate; ADP_hex: Hexokinase treated ADP.

SB carried out electrophysiological recordings, wrote the first draft of the manuscript and analysed the data. VAS participated in the design of the study and helped draft the manuscript. MLB conducted the phylogenetic analysis, helped draft the manuscript and generated the HdP2X EST clone. SJE conceived and designed the study, performed the molecular biological aspects of the study and wrote the manuscript. All authors read and approved the final manuscript.