Molecular cloning and functional expression of Xenopus laevis oocyte ATP-activated P2X4 channels

Comparative Embryonic Spatio-Temporal Expression Profile Map of the Xenopus P2X Receptor Family

P2X receptors are ATP-gated cations channels formed by the homo or hetero-trimeric association from the seven cloned subunits (P2X1-7). P2X receptors are widely distributed in different organs and cell types throughout the body including the nervous system and are involved in a large variety of physiological but also pathological processes in adult mammals. However, their expression and function during embryogenesis remain poorly understood. Here, we report the cloning and the comparative expression map establishment of the entire P2X subunit family in the clawed frog Xenopus. Orthologous sequences for 6 mammalian P2X subunits were identified in both X. laevis and X. tropicalis, but not for P2X3 subunit, suggesting a potential loss of this subunit in the Pipidae family. Three of these genes (p2rx1, p2rx2, and p2rx5) exist as homeologs in the pseudoallotetraploid X. laevis, making a total of 9 subunits in this species. Phylogenetic analyses demonstrate the high level of conservation of these receptors between amphibian and other vertebrate species. RT-PCR revealed that all subunits are expressed during the development although zygotic p2rx6 and p2rx7 transcripts are mainly detected at late organogenesis stages. Whole mount in situ hybridization shows that each subunit displays a specific spatio-temporal expression profile and that these subunits can therefore be grouped into two groups, based on their expression or not in the developing nervous system. Overlapping expression in the central and peripheral nervous system and in the sensory organs suggests potential heteromerization and/or redundant functions of P2X subunits in Xenopus embryos. The developmental expression of the p2rx subunit family during early phases of embryogenesis indicates that these subunits may have distinct roles during vertebrate development, especially embryonic neurogenesis.

ATP stimulates pannexin 1 internalization to endosomal compartments

The ubiquitous pannexin 1 (Panx1) ion- and metabolite-permeable channel mediates the release of ATP, a potent signalling molecule. In the present study, we provide striking evidence that ATP, in turn, stimulates internalization of Panx1 to intracellular membranes. These findings hold important implications for understanding the regulation of Panx1 when extracellular ATP is elevated. In the nervous system, this includes phenomena such as synaptic plasticity, pain, precursor cell development and stroke; outside of the nervous system, this includes things like skeletal and smooth muscle activity and inflammation. Within 15 min, ATP led to significant Panx1-EGFP internalization. In a series of experiments, we determined that hydrolysable ATP is the most potent stimulator of Panx1 internalization. We identified two possible mechanisms for Panx1 internalization, including activation of ionotropic purinergic (P2X) receptors and involvement of a putative ATP-sensitive residue in the first extracellular loop of Panx1 (Trp(74)). Internalization was cholesterol-dependent, but clathrin, caveolin and dynamin independent. Detailed analysis of Panx1 at specific endosome sub-compartments confirmed that Panx1 is expressed in endosome membranes of the classical degradation pathway under basal conditions and that elevation of ATP levels diverts a sub-population to recycling endosomes. This is the first report detailing endosome localization of Panx1 under basal conditions and the potential for ATP regulation of its surface expression. Given the ubiquitous expression profile of Panx1 and the importance of ATP signalling, these findings are of critical importance for understanding the role of Panx1 in health and disease.

Molecular and functional properties of P2X receptors--recent progress and persisting challenges

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.

Pharmacological properties and physiological function of a P2X-like current in single proximal tubule cells isolated from frog kidney

Although previous studies have provided evidence for the expression of P2X receptors in renal proximal tubule, only one cell line study has provided functional evidence. The current study investigated the pharmacological properties and physiological role of native P2X-like currents in single frog proximal tubule cells using the whole-cell patch-clamp technique. Extracellular ATP activated a cation conductance (P2X_f) that was also Ca²⁺-permeable. The agonist sequence for activation was ATP = αβ-MeATP > BzATP = 2-MeSATP, and P2X_f was inhibited by suramin, PPADS and TNP-ATP. Activation of P2X_f attenuated the rundown of a quinidine-sensitive K⁺ conductance, suggesting that P2X_f plays a role in K⁺ channel regulation. In addition, ATP/ADP apyrase and inhibitors of P2X_f inhibited regulatory volume decrease (RVD). These data are consistent with the presence of a P2X receptor that plays a role in the regulation of cell volume and K⁺ channels in frog renal proximal tubule cells.

Changes in the control of gastric motor activity during metamorphosis in the amphibian Xenopus laevis, with special emphasis on purinergic mechanisms

The stomach of the amphibian Xenopus laevis is subject to extensive remodelling during metamorphosis. We investigated the changes in gastric activity control during this period using in vitro circular smooth muscle preparations mounted in organ baths. The nitric oxide synthase inhibitor L-NAME increased mean force in metamorphic and juvenile frogs but not in prometamorphic tadpoles. Serotonin (5-HT) relaxed stomach muscle prior to metamorphosis but elicited a biphasic response in juveniles consisting of contraction at low concentrations and relaxation at high concentrations. The effects of 5-HT were blocked by methysergide. In the prometamorphic tadpole, ATP elicited relaxation that was blocked by the ectonucleotidase inhibitor ARL67156 and the adenosine A(1) receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), suggesting adenosine as the mediator. Exogenous adenosine and the A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) induced relaxation at all stages. After metamorphosis, the potency of ATP decreased and neither DPCPX nor ARL67156 could block ATP-induced relaxation. Uridine 5'-triphosphate (UTP) induced relaxation prior to metamorphosis, but caused contraction of muscle strips from metamorphosing tadpoles. Single doses of UTP blocked phasic contractions in juveniles in a tetrodotoxin (TTX)-sensitive manner while the simultaneous increase in muscle tension was TTX insensitive. The P2X(1)/P2X(3) receptor agonist alpha-beta-MeATP elicited pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS)-sensitive contractions at all stages investigated. These results indicate the development of an inhibitory nitrergic tonus during metamorphosis and a 5-HT receptor involved in muscle contraction. Also, the development of UTP receptors mediating increased tension and neural UTP receptors decreasing contraction frequency in juveniles is indicated. An adenosine A(1)-like receptor mediating relaxation and a P2X-like receptor mediating contraction is demonstrated at all stages.

Developmental changes of purinergic control of intestinal motor activity during metamorphosis in the African clawed frog, Xenopus laevis

Little is known about the purinergic regulation of intestinal motor activity in amphibians. Purinergic control of intestinal motility is subject to changes during development in mammals. The aim of this study was to investigate purinergic control of intestinal smooth muscle in the amphibian Xenopus laevis and explore possible changes in this system during the developmental phase of metamorphosis. Effects of purinergic compounds on mean force and contraction frequency in intestinal circular muscle strips from prometamorphic, metamorphic, and juvenile animals were investigated. Before metamorphosis, low concentrations of ATP reduced motor activity, whereas the effects were reversed at higher concentrations. ATP-induced relaxation was not inhibited by the P2-receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) but was blocked by the ecto-nucleotidase inhibitor 6-N,N-diethyl-d-beta,gamma-dibromomethylene ATP (ARL67256), indicating that an ATP-derived metabolite mediated the relaxation response at this stage. Adenosine induced relaxation before, during, and after metamorphosis, which was blocked by the A(1)-receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). The stable ATP-analog adenosine 5'-[gamma-thio]-triphosphate (ATPgammaS) and 2-methylthioATP (2-MeSATP) elicited contractions in the circular muscle strips in prometamorphic tadpoles. However, in juvenile froglets, 2-MeSATP caused relaxation, as did ATPgammaS at low concentrations. The P2Y(11)/P2X(1)-receptor antagonist NF157 antagonized the ATPgammaS-induced relaxation. The P2X-preferring agonist alpha-beta-methyleneadenosine 5'-triphosphate (alpha-beta-MeATP) evoked PPADS-sensitive increases in mean force at all stages investigated. This study demonstrates the existence of an adenosine A(1)-like receptor mediating relaxation and a P2X-like receptor mediating contraction in the X. laevis gut before, during, and after metamorphosis. Furthermore, the development of a P2Y(11)-like receptor-mediated relaxation during metamorphosis is shown.

Cloning, tissue distribution and functional characterization of the chicken P2X1 receptor

We describe a new chicken P2X subunit that is an orthologue of the mammalian P2X1 receptor. Functional characterization of chicken P2X1 receptors was performed using the amphotericin B perforated patch configuration to avoid the current run-down observed under whole-cell patch-clamp conditions. Responses to agonists and to the antagonist PPADS (pyridoxal 5-phosphate 6-azophenyl-2',4'-disulfonic acid) were similar to what has been described for mammalian orthologues. However, the antagonists suramin and NF023 were much less potent at chicken P2X1 receptors than at human P2X1 receptors. In embryonic tissues, transcript expression is predominant in lung, liver and skeletal muscle. Overlapping expression with cP2X4 and cP2X5 subunits in several embryonic tissues, including skeletal muscle, indicates that the native embryonic P2X receptors could be heteromultimeric.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.