CENTRAL P2X(4) AND P2X(6) CHANNEL SUBUNITS COASSEMBLE INTO A NOVEL HETEROMERIC ATP RECEPTOR

Ionotropic ATP receptors are widely expressed in mammalian CNS. Despit e extensive functional characterization of neuronal homomeric P2X rece ptors in heterologous expression systems, the subunit composition of n ative central P2X ATP-gated channels remains to be elucidated. P2X(4) and P2X(6) are major central subunits with highly overlapping mRNA dis tribution at both regional and cellular levels. When expressed alone i n Xenopus oocytes, P2X(6) subunits do not assemble into surface recept ors responsive to ATP applications. On the other hand, P2X(4) subunits assemble into bona fide ATP-gated channels, slowly desensitizing and weakly sensitive to the partial agonist alpha,beta-methylene ATP and t o noncompetitive antagonists suramin and pyridoxal-5-phosphate-6-azoph enyl-2',4'-disulfonic acid. We demonstrate here that the coexpression of P2X(4) and P2X(6) subunits in Xenopus oocytes leads to the generati on of a novel pharmacological phenotype of ionotropic ATP receptors. H eteromeric P2X(4+6) receptors are activated by low-micromolar alpha,be ta-methylene ATP (EC50 = 12 mu M) and are blocked by suramin and by Re active Blue 2, which has the property, at low concentrations, to poten tiate homomeric P2X(4) receptors. The assembly of P2X(4) with P2X(6) s ubunits results from subunit-dependent interactions, as shown by their specific copurification from HEK-293 cells transiently transfected wi th various epitope-tagged P2X channel subunits. Our data strongly sugg est that the numerous cases of neuronal colocalizations of P2X(4) and P2X(6) subunits observed in mammalian CNS reflect the native expressio n of heteromeric P2X(4+6) channels with unique functional properties.