Functional Properties of Five Dictyostelium discoideum P2X Receptors

• Published in: The Journal of biological chemistry 2013-07, Vol.288 (29), p.20992-21000
• Main Authors: Baines, Abigail, Parkinson, Katie, Sim, Joan A., Bragg, Laricia, Thompson, Christopher R.L., North, R. Alan
• Format: Article
• Language: English
• Subjects: Acids - metabolism; Adenosine Triphosphate - pharmacology; Animals; ATP; Cell Biology; Dictyostelium; Dictyostelium - cytology; Dictyostelium - drug effects; Dictyostelium - metabolism; Extracellular Space - drug effects; Extracellular Space - metabolism; HEK293 Cells; Humans; Intracellular Space - drug effects; Intracellular Space - metabolism; Ion Channel Gating - drug effects; Ion Channels; Ions - pharmacology; Ligands; Osmoregulation; P2X Receptor; Phenotype; Potassium - pharmacology; Protozoan Proteins - metabolism; Purinergic Agonists; Purinergic Receptor; Receptors, Purinergic P2X - metabolism; Solutions; Vacuolar Acidification
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M112.445346

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. Background:Dictyostelium discoideum P2X receptors are found on the contractile vacuole. Results: Four of the five receptors (P2XA, P2XB, P2XD, and P2XE) form ATP-activated channels but differ in their optimal ionic conditions. Conclusion: Properties of five P2X receptors correlate with their rescue of an osmoregulatory phenotype in P2XA-deficient Dictyostelium cells. Significance: A P2X receptor is required for normal contractile vacuole operation.