title: Functional properties of five Dictyostelium discoideum P2X receptors.
abstract: The Dictyostelium discoideum genome encodes five proteins that share weak sequence similarity with vertebrate P2X receptors. Unlike vertebrate P2X receptors, these proteins are not expressed on the surface of cells, but populate the tubules and bladders of the contractile vacuole. In this study, we expressed humanized cDNAs of P2XA, P2XB, P2XC, P2XD, and P2XE in human embryonic kidney cells and altered the ionic and proton environment in an attempt to reflect the situation in amoeba. Recording of whole-cell membrane currents showed that four receptors operated as ATP-gated channels (P2XA, P2XB, P2XD, and P2XE). At P2XA receptors, ATP was the only effective agonist of 17 structurally related putative ligands that were tested. Extracellular sodium, compared with potassium, strongly inhibited ATP responses in P2XB, P2XD, and P2XE receptors. Increasing the proton concentration (pH 6.2) accelerated desensitization at P2XA receptors and decreased currents at P2XD receptors, but increased the currents at P2XB and P2XE receptors. Dictyostelium lacking P2XA receptors showed impaired regulatory volume decrease in hypotonic solution. This phenotype was readily rescued by overexpression of P2XA and P2XD receptors, partially rescued by P2XB and P2XE receptors, and not rescued by P2XC receptors. The failure of the nonfunctional receptor P2XC to restore the regulatory volume decrease highlights the importance of ATP activation of P2X receptors for a normal response to hypo-osmotic shock, and the weak rescue by P2XB and P2XE receptors indicates that there is limited functional redundancy among Dictyostelium P2X receptors.
authors.affiliation: From the Faculty of Life Sciences and.
authors.forename: Abigail
authors.initials: A
authors.lastname: Baines
authors.affiliation: From the Faculty of Life Sciences and.
authors.forename: Katie
authors.initials: K
authors.lastname: Parkinson
authors.affiliation: Faculty of Medical and Human Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom.
authors.forename: Joan A
authors.initials: JA
authors.lastname: Sim
authors.affiliation: Faculty of Medical and Human Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom.
authors.forename: Laricia
authors.initials: L
authors.lastname: Bragg
authors.affiliation: From the Faculty of Life Sciences and. Electronic address: christopher.thompson@manchester.ac.uk.
authors.forename: Christopher R L
authors.initials: CRL
authors.lastname: Thompson
authors.affiliation: From the Faculty of Life Sciences and; Faculty of Medical and Human Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom. Electronic address: r.a.north@manchester.ac.uk.
authors.forename: R Alan
authors.initials: RA
authors.lastname: North
mesh_terms: D000143:Acids / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D004023:Dictyostelium / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D005110:Extracellular Space / Q000187:drug effects / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D042541:Intracellular Space / Q000187:drug effects / Q000378:metabolism; D015640:Ion Channel Gating / Q000187:drug effects; D007477:Ions / Q000494:pharmacology; D008024:Ligands; D010641:Phenotype; D011188:Potassium / Q000494:pharmacology; D015800:Protozoan Proteins / Q000378:metabolism*; D058469:Receptors, Purinergic P2X / Q000378:metabolism*; D012996:Solutions
chemical_list: D000143:Acids; D007477:Ions; D008024:Ligands; D015800:Protozoan Proteins; D058469:Receptors, Purinergic P2X; D012996:Solutions; D000255:Adenosine Triphosphate; D011188:Potassium
keywords: ATP; Dictyostelium; Ion Channels; Osmoregulation; P2X Receptor; Purinergic Agonists; Purinergic Receptor; Vacuolar Acidification