Functional characterization of a P2X receptor from Schistosoma mansoni

The cloning and characterization of a P2X receptor (schP2X) from the parasitic blood fluke Schistosoma mansoni provides the first example of a non-vertebrate ATP-gated ion channel. A number of functionally important amino acid residues conserved throughout vertebrate P2X receptors, including 10 extracellular cysteines, aromatic and positively charged residues involved in ATP recognition, and a consensus protein kinase C site in the amino-terminal tail, are also present in schP2X. Overall, the amino acid sequence identity of schP2X with human P2X(1-7) receptors ranges from 25.8 to 36.6%. ATP evoked concentration-dependent currents at schP2X channels expressed in Xenopus oocytes with an EC50 of 22.1 muM. 2', 3'-O-(4-Benzoylbenzoyl) adenosine 5'-triphosphate (Bz-ATP) was a partial agonist ( maximum response 75.4 +/- 4.4% that of ATP) with a higher potency (EC50 of 3.6 muM) than ATP. Suramin and pyridoxal-phosphate-6- azophenyl- 2', 4'-disulfonic acid blocked schP2X responses to 100 muM ATP with IC50 values of 9.6 and 0.5 muM, respectively. Ivermectin ( 10 muM) potentiated currents to both ATP and Bz-ATP by similar to 60% with a minimal effect on potency (EC50 of 18.2 and 1.6 muM, respectively). The relative permeability of schP2X expressed in HEK293 cells to various cations was determined under bi-ionic conditions. schP2X has a relatively high calcium permeability (P-Ca/P-Na = 3.80 +/- 0.29) and an estimated minimum pore diameter similar to that of vertebrate P2X receptors. SchP2X provides a useful comparative model for the better understanding of human P2X receptor function and may also provide an alternative drug target for treatment of schistosomiasis.